17. mineral chemistry of primary and ......5r-2, 123-130 5r-3, 125-131 6r-4, 49-54 7r-2, 122-127...

Larson, R. L., Lancelot, Y., et al., 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 129

17. MINERAL CHEMISTRY OF PRIMARY AND SECONDARY PHASES INBASALTIC ROCKS, LEG 1291

G. Rowbotham2 and P. A. Floyd2

ABSTRACT

Primary magmatic phases (spinel, olivine, plagioclase, clinopyroxene, amphibole, and biotite) and secondary Phyllosilicates(smectite, chlorite-smectite, and celadonite) were analyzed by electron microprobe in alkalic and tholeiitic dolerites and basaltsfrom Ocean Drilling Program Sites 800, 801, and 802. Aphyric alkalic dolerite sills (Hole 800A) and basalt flows (Holes 80IBand 801C) share common mineralogical features: matrix feldspars are strongly zoned from labradorite cores to discrete sodicrims of alkali feldspar with a high Or component, which overlaps that of quench microlites in glassy mesostasis; little fractionatedclinopyroxenes are Ti-rich diopsides and augites (with marked aegirine-augite rims at Site 801); rare, brown, Fe3+-rich amphiboleis winchite; and late biotites exhibit variable Ti contents. Alkalic rims to feldspars probably developed at the same time as quenchedmesostasis feldspars and late-stage magmatic biotite, and represent the buildup of K-rich hydrous fluids during crystallization.Phenocryst phases in primitive mid-ocean ridge tholeiites from Hole 801C (Mg numbers about 70) have extreme compositionswith chrome spinel (Cr/Cr + Al ratios about 0.2-0.4), Ni-rich olivine (Fo90), and highly calcic plagioclase (An90). Laterglomerophyric clumps of plagioclase (An75_80) and clinopyroxene (diopside-augite) are strongly zoned and probably reflectrapidly changing melt conditions during upward transport, prior to seafloor quenching. In contrast, phenocryst phases (olivine,plagioclase, and clinopyroxene) in the Hole 802A tholeiites show limited variation and do not have such primitive compositions,reflecting the uniform and different chemical composition of all the bulk rocks.

Replacive Phyllosilicates in both alkalic and tholeiitic basalts include various colored smectites (Fe-, Mg-, and Al-saponites),chlorite-smectite and celadonite. Smectite compositions typically reflect the replaced host composition; glass is replaced by brownFe-saponites (variable Fe/Mg ratios) and olivine by greenish Mg-saponites (or Al-rich chlorite-smectite).

INTRODUCTION

Tholeiitic and alkalic basalts and dolerites were sampled fromthree sites during Ocean Drilling program (ODP) Leg 129 in thewestern Pacific Ocean (Fig. 1). This paper reports the chemistry ofthe dominant primary and secondary phases in selected basementsamples in terms of magma type, petrogenesis, and the subsequenteffects of secondary alteration.

The local acoustic basement at Site 800 in the northern PigafettaBasin is composed of a series of massive alkalic dolerite sills charac-terized by high incompatible element contents and well-fractionatedrare earth element (REE) patterns. They are comparable with well-evolved ocean island basalts (OIB) of predominantly hawaiite com-position and probably represent deep-sea lateral offshoots from nearbyseamounts of similar composition and age (Floyd et al., this volume).The best recovery of igneous basement was from Site 801 in thesouthern Pigafetta Basin, where the oldest segment of ocean crust inthe Pacific Ocean, of Middle Jurassic age, was sampled (Lancelot,Larson, et al., 1990; Matsuoka, 1990) with radiometric ages rangingfrom about 157 to 167 Ma (Pringle, this volume). The lava flows andpillow lavas of Site 801 comprise an upper alkalic basalt sequenceseparated from a lower tholeiitic basalt sequence by a distinctiveyellow hydrothermal deposit (Floyd et al., this volume). The lowertholeiitic sequence is mineralogically and chemically similar to incom-patible element depleted basalts from normal mid-ocean ridge seg-ments (N-MORB) and thus represents ocean crust of Jurassic age(Floyd et al., 1991). The alkalic basalt cap has affinities with typicalOIB and probably represents off-axis volcanism rather than thelast-stage product of highly fractionated axial seamounts. Drilling atSite 802 in the Mariana Basin penetrated acoustic basement, but

55°N

1 Larson, R. L., Lancelot, Y., et al., 1992. Proc. ODP, Sci. Results, 129: CollegeStation, TX (Ocean Drilling Program).

2 Department of Geology, University of Keele, Staffordshire, ST5 5BG, UnitedKingdom.

40

20

130°E 150°E 170°E 170°W

Figure 1. Map of the western Pacific Ocean showing the position of Leg 129Sites 800, 801, and 802 (Lancelot, Larson, et al., 1990). The unshaded arearepresents normal Pacific Ocean crust with magnetic lineation contours, whereasshaded areas represent thickened crustal sections and younger crust to the westof the Pacific subduction zones. Feature abbreviations are as follows: CarolineIslands (Cl), Ontong-Java Plateau (OJP), Marshall Islands (MI), Nauru Basin(NB), Mid-Pacific Mountains (MPM), Shatsky Rise (SR), Hawaiian Ridge(HR), and Emperor Seamounts (ES).

305

G. ROWBOTHAM, P. A. FLOYD

the composition of tholeiitic pillow lavas of Cretaceous age (about115 Ma; Pringle, this volume) was considered to represent an extru-sive event postdating the local underlying basement of Jurassic age(Lancelot, Larson, et al., 1990). Relative to N-MORB, the predomi-nantly olivine-plagioclase-clinopyroxene phyric tholeiites are en-riched in large-ion-lithophile (LIL) elements with flat, unfractionatedREE patterns, and exhibit compositions similar to those of oceanplateau basalts (Floyd, 1989) of similar age in the western Pacific.

In summary, the basic rocks of Sites 800 and 802 are representativeof the widespread mid-Cretaceous volcanic event in the westernPacific (Lancelot, Larson, et al., 1990), with the former composed ofalkalic dolerite sills and the latter of submarine tholeiitic basalts. Site801, on the other hand, is a section through oceanic crust of Jurassicage and is composed of alkalic and tholeiitic sequences.

ANALYTICAL TECHNIQUES

Analyses of all mineral phases were obtained using both Cameca(Camebax) and Cambridge Instruments Geoscan electron microprobesin the Department of Geology, University of Manchester. The operatingconditions were 15 kV and 3.5 × I0"8 A and a spot size of about 3 µm,except in the analysis of hydrous phases, when the beam was defocused.Metals, oxides, and silicate materials were used in calibration; an estimateof the accuracy and precision of the technique employed is given byDunham and Wilkinson (1978), with corrections using standard softwareprogram ZAF4-FLS from Link Analytical. Fe3+ values were determinedvia stoichiometry after the method of Droop (1987) and with referenceto the limitations of the estimates (Schumacher, 1991). Olivine analyses,while in equilibrium with the adjacent glass, contain variable Na and Alcontents that probably reflect some contamination by secondary replace-ment minerals. Determination and calculation of carbonate end-memberscan only be considered estimates.

PETROGRAPHY OF ANALYZED SAMPLES

Mineral phases analyzed by microprobe in basalt and doleritesamples from Holes 800A, 80IB, 801C, and 802A were selected to

show as complete a range of primary and secondary mineral compo-sitions as possible. A checklist of the phases analyzed in different rocksamples is given in Table 1 and the corresponding bulk-rock geo-chemistry is presented elsewhere in this volume (Floyd and Castillo;Floyd et al.). The general petrography of each bulk-rock sample fromthe different holes is outlined in the following.

Hole 800A

Both of the samples selected for phase analysis (129-800A-58R-1,86-91 cm, and 800A-58R-2, 67-73 cm) are from cooling unit 3 ofthe alkalic dolerites and are well evolved mineralogically and chemi-cally. They have a similar primary petrography, although in Sample800A-58R-2, 67-73 cm, the secondary phases are better developed.Olivine is not present in this cooling unit.

In both samples the plagioclase laths (up to 2 mm long) exhibit anoscillatory zoning, although there is a sharp optical discontinuitybetween the core and rim portions of many individual crystals. Albitetwinning, characteristic of the plagioclase cores, is not present at themargins and suggests that the more sodic rims represent a later over-growth, which may in some cases be intergrown with mafic minerals.Most feldspars are relatively fresh, although the cores are invariablyreplaced by secondary opaque material. Mafic minerals are pinkpleochroic clinopyroxene, a pale brown amphibole, and minor biotite,of which the latter may be largely secondary in origin. Clinopyroxenevaries widely in shape and size, and is commonly replaced by a palegreen smectite. The amphibole either replaces clinopyroxene or occursas discrete ragged subhedral crystals and is clearly later in developmentthan either plagioclase or clinopyroxene. Biotite commonly fringestitanomagnetite grains and needles or replaces clays in earlier replace-ment domains. Much of the biotite growth may have been in responseto subsequent contact metamorphism by adjacent sills (Floyd et al., thisvolume). Rare, euhedral pseudohexagonal basal sections of biotiteexhibit a color zonation showing dark brown rims, as well as partialaltered to a green phyllosilicate. Apatite needles are abundant.

The nucleation sites of secondary Phyllosilicates are grain bound-aries and along cleavages, but are particularly well-developed in

Table 1. Mineral phases analyzed (+) in selected samples of dolerites and basalts from Leg 129.

Core, section,interval (cm) Feldspar Pyroxene Olivine Spinel Biotite Amphibole Smectite Celadonite

129-800A-

58R-1,86-9158R-2, 67-73

129-801B-

41R-1, 130-13643R-1,4-944R-2, 140-14544R-3, 28-33

129-801C-

1R-1, 14-202R-5, 101-1065R-2, 123-1305R-3, 125-1316R-4, 49-547R-2, 122-1279R-5, 0-7

129-802 A-

58R-1,96-10058R-1, 99-10258R-3, 90-9559R-1, 138-14259R-2, 40-4462R-2, 53-59

+ + ++ + +

306

MINERAL CHEMISTRY IN BASALTIC ROCKS

primary mafic silicates and interstitial areas, where they form smec-tite-rich alteration domains. Most smectites are fibrous and green incolor, although the more granular-looking varieties are invariablybrownish. At the boundary between primary and secondary phases,Phyllosilicates are commonly a dark green color, which is replacedoutward by birefringent pale green fibers (0.05-0.075 mm long),which are intimately associated with late replacive carbonate.

Site 801

Alkalic Basalts and Microdolerites

The alkalic rocks of Site 801 are distinct from the alkalic doleritesof Hole 800A in being generally finer-grained throughout (basalts andmicrodolerites), extrusive thin flows rather than sills (with quenched-textured matrices and glassy mesostasis in the microdolerites), andolivine bearing (as phenocrysts and in the groundmass). They aregenerally aphyric, although some plagioclase-olivine phyric basaltsoccur toward the base of Hole 801B. On chemical grounds two groupsof alkalic basalts can be recognized (Floyd and Castillo, this volume):a primitive olivine-rich group and a more evolved, olivine-poor groupthat contains some plagioclase-olivine phyric basalts. The dominantflows are intruded by a minor plagioclase-olivine phyric quenchedbasalt of similar mineralogy to the more evolved phyric flows.

The main petrographic differences between the olivine-rich alkalicbasalts (represented by Samples 801B-43R-1,4-9 cm, and 801C-1R-1,14-20 cm) and the remainder is the proportion of olivine and thepresence of green rims to the pinkish clinopyroxenes, minor brownamphibole, and slightly less titanomagnetite; otherwise, they are min-eralogically and texturally similar. The rest of the samples used forprobe work (Table 1) show similar features, apart from differences intexture, as described in the following. Olivine is always pseudomor-phed by greenish smectites but is generally recognizable by its char-acteristic euhedral form. Apart from zoned phenocrysts, there are twogenerations of plagioclase in the matrix: large (maximum length of upto 3 mm), interlocking fresh laths, which are invariably optically zonedand constitute the main phase, and a later, second generation of curvedand radiating microlites (up to 0.5 mm long) developed in the quenchedmesostasis. Other minerals in the mesostasis consist of an admixtureof clay minerals replacing original glass, titanomagnetite granules,biotite flakes, and titanite. These features are well displayed in Samples801B-41R-1, 130-136 cm, 801B-43R-1, 4-9 cm, and 801B-44R-2,140-145 cm. In the coarser flow interiors the euhedral to subhedralclinopyroxenes are lilac to brown in color (up to 5 mm in length) witha faint pleochroism and optically zoned. They are commonly alteredalong the cleavages and around the rims to a pale green smectite.Pseudohexagonal plates of dark brown, strongly pleochroic biotite arecommon as a late primary phase (up to 0.8 mm diameter) and are foundrimming earlier titanomagnetite (for example, Sample 801B-41R-1,130-136 cm). Common accessory phases are skeletal titanomagnetitewith a high length-to-width ratio (20:1) and acicular needles of apatiteup to 0.6 mm in length.

Superimposed upon this primary assemblage is a secondary min-eralogy dominated by pale green and brownish smectites. In particularthey form radiating fibrous patches within the mesostasis of themicrodolerites or throughout the matrix of the quenched basalts.Phyllosilicates also nucleate on the margins of primary silicates suchas clinopyroxene. The occurrence of a number of generations ofsmectite growth has also been recorded, with light green radiatingcrystals apparently representing the recrystallization of pre-existingfine-grained Phyllosilicates.

Tholeiitic Basalts

The tholeiitic rocks below the hydrothermal deposit in Hole 801Care divided into two petrographic-chemical groups: (1) an uppersequence (Cores 801C-1R to 801C-6R, encompassing cooling unitsC9-C20) of chemically primitive olivine-plagioclase-spinel phyric

basalts and (2) a lower sequence (Cores 801C-7R to 801C-12R,encompassing cooling units C21-C32) of chemically evolved (gen-erally) aphyric, vesicular basalts (Floyd and Castillo, this volume).Compared with the majority of selected samples, those from the uppersequence have been extensively altered by circulating hydrothermalfluids with the subsequent deposition of various green clays andcarbonate (especially Samples 801C-5R-2, 123-130 cm, and 801C-5R-3, 125-131 cm).

The primitive phyric basalts are quench-textured throughout, withsamples selected for analysis from glassy flow rims (for example,Sample 801C-5R-2, 123-130 cm) to variolitic and intersertal-tex-tured flow interiors (for example, Sample 129-801C-5R-5, 73-78cm). In most samples plagioclase is relatively fresh and forms bothmegaphenocrysts ( 4 x 3 mm) and either serrated laths in flowinteriors or forked microlites in the glassy rims. In Sample 801C-6R-4, 49-54 cm, however, plagioclase forms zoned glomerocrysts(2.5 × 2.5 mm) that often contain rounded inclusions of glass(original melt) and small spinels and olivines. There is also a secondgeneration of plagioclase microphenocrysts (0.6 x 0.25 mm) that alsocontain spinel octahedra, but no glass inclusions. Olivine (invariablypseudomorphed by smectites) occurs as euhedral microphenocrystsor small grains scattered throughout the matrix. In some samples (forexample, 801C-6R-4, 49-54 cm) relatively large olivine phe-nocrysts (0.8 × 3.0 mm) contain minute spinels and subroundedinclusions of a quenched dendritic phase. Spinel occurs in twogenerations: dark brown microphenocrysts commonly loosely asso-ciated with olivine in the matrix or minute, pale yellow-brownoctahedra within plagioclase and olivine phenocrysts (for example,Samples 8O1C-5R-5, 73-78, and 801C-6R-4, 49-54 cm). Clinopy-roxene is the only major silicate non-phenocrystic phase, but canform two associated textural variants in the coarser flow interiors—variolitic fans, with or without coaxial plagioclase, and groups ofinterspersed granular crystals. Coarser clinopyroxene grains may beoptically zoned with hour-glass structure or early crystallizing subo-phitic groups passing outward into quenched variolitic growths. Al-though opaque oxides are generally partly obscured by dark brownsmectitic material, in reflected light they are skeletal in outline.

The two samples of the evolved chemical group (Samples 801C-7R-2, 122-127 cm, and 801C-9R-5, 0-7 cm) are both slightly phyricwith scattered plagioclase glomerocrysts, none of which containsinclusions of glass or early primary phases, like the primitive groupsamples described previously. The outer parts of the flows are char-acterized by curved and straight plagioclase microlites typically act-ing as nucleation sites for spherulites. Flow interiors are typicallyintersertal and variolitic, again exhibiting the variation in clinopyrox-ene morphology noted previously.

All samples selected for probe work have been affected by altera-tion with the development of various smectites, celadonite, carbonate,pyrite, and rare zeolites. Phyllosilicates were analyzed from a numberof different locations, including zoned vesicles, olivines and coarsevein material. For example, in the chilled margin of Sample 801C-5R-2, 123-130 cm are mineralogically zoned vesicles with walls linedinitially with brown and subsequently various bright green clays. Veinscontain green smectites and coarser grained colorless birefringentPhyllosilicates, together with late replacive carbonate. The habit of thesecondary minerals varies from fine-grained fibers that nucleate on aprecursor phase to large plates, which are late-stage vein infillings.

Hole 802A

The tholeiitic basalts selected from Hole 802A are predominantlyphyric with olivine, plagioclase, and clinopyroxene phenocrysts, com-monly developed in medium-grained glomerophyric groups. Texturalvariability throughout the flows is largely related to cooling history,ranging from fresh glassy rims (Sample 802A-58R-2,99-102 cm) and aspherulitic or variolitic zone to flow interiors with an intersertal textureand subophitic growth centers (Sample 802A-62R-2, 53-59 cm).

307


Plagioclase forms two generations of phenocrysts ranging fromlarge (about 1 mm), magmatically corroded, euhedral crystals tosmaller laths associated with clinopyroxene in glomerophyric clumps.Both of these early generations have either oscillatory, normal, orreverse zoning with sharp optical discontinuities between core and rim.The latest generation of plagioclase represents matrix and is composedof serrated laths that are intimately associated with clinopyroxene invariolitic growths (for example, Samples 802A-59R-1, 138-142 cmand 802A-59R-3, 40-44 cm). Clinopyroxene may also form threegrowth generations: early subhedral glomerocrysts associated withplagioclase laths, granular aggregates dispersed between quenchedsheaf variolites composed of radiate clinopyroxene. In coarser grainedflow interiors (for example, Sample 802A-62R-2, 53-59 cm) thetexture may be sub-ophitic with elongate plagioclase laths (up to 1 mmlong). Olivine always occurs as (smectite-replaced) microphenocrystsscattered throughout both glassy rims and variolitic flow interiors.

Fresh glass preserved at flow margins is rare, but may exhibit allthree phenocryst phases (olivine, plagioclase, and clinopyroxene) thatare commonly used as nucleation centers for dark spherulites (forexample, Sample 802A-58R-2, 99-102 cm). Away from the glassyspherulitic rind the texture is variolitic, but the phenocryst phaseshave similar dimensions to those in the glass: olivine (0.5 mm),clinopyroxene (0.5 mm), and plagioclase (0.75-0.80 mm).

Secondary alteration is confined to smectites, celadonite, pyrite, andcarbonate. Secondary assemblages are well-developed in Sample 802A-58R-2, 99-102 cm, where alteration domains (>5 mm diameter)consist of late carbonate replacing green and brown Phyllosilicates in thematrix. The olivines are always pseudomorphed by brown smectitethat initially developed along original fractures before replacing thewhole phenocryst.

PRIMARY MINERAL PHASE CHEMISTRY

Because of the marked chemical differences and compositionalrange exhibited by alkalic and tholeiitic basic rocks, primary mag-matic phases in the alkalic rocks (Hole 800 A dolerites and Holes 80 IBand 801C basalts) will be described first, followed by the tholeiiticbasic rocks (Hole 801C basalts and Hole 802A basalts). Fresh glassis relatively rare throughout Leg 129 basalts and although a fewanalyses are recorded here (see Table 10) no comment has been madeat this stage on their composition or petrogenesis.

Alkalic Dolerites and Basalts

Feldspars

The main petrographic characteristics of the feldspars (Table 2) inboth the alkali dolerites and basalts are the presence of normal andoscillatory zoning within the albite twinned portions of plagioclaselaths and a lack of multiple twinning in the outmost zones. Many matrixfeldspars (for example, Samples 801B-43R-1,4-9 cm; 801B-44R-2,140-145 cm, and 801C-1R-1, 14-20 cm) exhibit sharp chemicaldiscontinuities between the highly calcic cores and twin-free sodicrims, whereas other crystals show almost continuous zoning. Themost calcic cores of plagioclases from Hole 800A alkaline dolerites(Samples 800A-58R-2, 67-73 cm and 800A-58R-1, 86-91 cm) aresodic labradorites (An55_53Ab44_40Or,_7) (Fig. 2) and are about 15% lessanorthitic than plagioclase cores from Hole 801C alkali basalts, whichare typically labradorites/bytownites (for example, An69Ab290r2 inSample 801C-43R-1,4-9 cm). As seen in Figure 2, the compositionaltrends of the plagioclase feldspars are all towards enrichment in thealbite component, although there are differences in detail whereby thedolerite plagioclases have systematically lower Or contents at thesodic end than the plagioclases from the basalts. For example, asodic plagioclase composition of An16Ab68Or26 from Hole 80IBbasalts (Sample 801B-44R-2, 140-145 cm) is about 20% richer inOr than a corresponding plagioclase in the dolerites. All further sodiccompositions then change toward the alkali feldspars (Fig. 2). The

range of compositions is identical in all of the feldspars from Holes801B and 801C.

Most of the analyzed grains showed normal zoning (with calcic coresand sodic rims), but oscillatory zoning is not uncommon. For example,a plagioclase from Sample 800A-58R-2, 67-73 cm demonstrates thechanges in composition in adjacent zones outward: An56Ab43Or0 (labra-dorite core); An3lAb670r2; An39Ab59Or2; An18Ab76Or6; An2Ab63Or35

(alkali feldspar rim). These compositional changes are quite large andmust reflect rapid changes in the liquid from which these feldsparscrystallized. As illustrated in the preceding case, most of the rims of theHole 800A feldspar laths are true alkali feldspars. The width of the alkalifeldspar zone in the crystals varies from the extreme rim to occupying alarge proportion of the feldspar lath itself. Alkali feldspar rims are alsocommon in the basalts from Holes 80IB and 801C. The most extremealkali feldspar compositions encountered were An0Ab3 (Or69 in an alkalicbasalt (Hole 80IB) and An2Ab450r54 in an alkalic dolerite (Hole 800A).No reversed zoning has been noted in these new compositions.

Quench crystals of alkali feldspars also occur in alkalic basalt mesosta-sis (Site 801) and have compositions that (1) completely overlap those ofthe rims of the matrix laths and (2) show little systematic chemicalvariation across or along individual alkali feldspar microlites, for example,An0Ab43Or57 (at growth point); An3Ab61Or36; An2Ab56Or42; An3Ab630r34

(to extreme tip).One alkali basalt analyzed (Sample 801B-44R-3,28-33 cm) is textur-

ally different and contains both phenocrysts and groundmass plagioclase.The phenocrysts again exhibit the continuous zoning of the cores withsharp transitions to alkali feldspar rims; for example, An69Ab290r2 (core)to An5Ab62Or33 (rim). The groundmass plagioclase do not show suchpronounced alkalic rims and have an average composition of aboutAn56Ab40Or4. The main minor elements in these feldspars are Fe andMg, but their concentrations are quite low, almost exclusively below0.50 cations per formula unit (cfu).

Overall the feldspars analyzed in both the Hole 800A doleritesand Site 801 basalts have similar textural and chemical charac-teristics to those described by Upton et al. (1985) from the TugtutoqOlder Giant Dyke Complex in south Greenland. They also exhibit aunidirectional variation trend (as shown by the Group 1 feldsparsdescribed by Henderson and Gibb, 1983), ranging from plagio-clase to Or-rich compositions, with no indication of convergenceof two compositionally distinct feldspars. The alkali feldspar rimsof both the Greenland samples and those of the present studyextend beyond the projected position of the experimental 5 kbcotectic (Ab50Or50)—a feature that has been interpreted as due to thepresence of an additional phase, nepheline (Upton et al., 1985).Although the alkalic rocks of Leg 129 are nepheline-normative (Floydand Castillo, this volume) no modal nepheline has been observed,including the most evolved samples. This may suggest that the trueposition of the cotectic is closer to the Or apex than indicated byexperimental systems. The other significant feature is that the alkalicrims to the feldspars probably developed at the same time as theabundant biotite and represent the buildup of late aqueous potassicfluids during the final stages of alkalic melt crystallization.

Pyroxenes

The composition of the pyroxenes (Tables 3 and 4) from thealkaline dolerite sills (Hole 800A) straddles the boundary betweendiopside and augite (Morimoto, 1988) and is typical of clinopyrox-enes developed in alkalic sills and dikes (Wilkinson, 1956; Hendersonand Gibb, 1988). They show little systematic variation in terms ofpyroxene quadrilateral components (Fig. 3) with the maximum com-positional variation represented by a zoned crystal from Sample8OOA-58R-1, 86-91 cm, with a core composition of Ca44Mg42Fe14

and rim of Ca42Mg40Fe18. In contrast, clinopyroxenes from the alkalibasalts (Holes 801B and 801C) are all diopsides (Fig. 3), althoughonce again little fractionation is observed between uniform corecompositions (for example, Ca50Mg33Fe17) and slightly Fe-enriched

308

Alkαlic dolerites-800A-58R-1, 86-91 cm+ 800A-58R-2, 67-73 cm


Alkαlic Basalts

•801B-41R-1, 130-136 cm

+ 801B-43R-1, 4-9 cm

°801B-44R-2, 140-145 cm

Na Ca

Alkalic minor intrusive

801B-44R-3, 28-33 cm

B

Alkalic basalts

801C-1R-1, 14-20 cm

801C-2R-5, 101-106 cm

Na Ca Na Ca

Figure 2. Composition of feldspars from alkalic dolerites and basalts, in terms of orthoclase (K), albite (Na) and anorthite (Ca) components. A. Dolerite sills fromHole 800A. B. Basalt flows from Hole 801B. C. Minor basalt intrusive from Hole 801C. D. Basalt flows from Hole 801C.

rims. These calculations have been made on the basis that Ca2+

substitutions are within the field of the quadrilateral components andexclude the small component of a possible Tschermak^ substitution.The ferric iron content of the diopsidic clinopyroxenes (Site 801) hasbeen calculated for a limited number of analyses and is quite variable(0.04-0.14 Fe3+ cfu). There is, however, little systematic increase ordecrease between core and rim.

The dominant minor constituents in all the alkalic clinopyroxenesanalyzed are Al and Ti. The maximum value of Ti is 0.07 cfu and Al0.25 cfu. Both these elements are positively correlated in the clino-pyroxenes with Ti:Al ratios between 1:2 and 1:4 (Fig. 4). Tracy andRobinson (1977) suggested that there was a relationship between theentry of Ti4+ into the pyroxene lattice and substitutions in both theoctahedral and tetrahedral sites of the type: Fe2+ + Ti4+ + 2A13+ = 2Mg2+

+ 2Si4+. The substitution is common within alkali basalt sills in general(Robinson, 1980; Henderson and Gibb, 1988); the validity of thissubstitution has been supported experimentally by Sack and Car-michael (1984). A similar relationship is also recorded in Leg 129intrusive and extrusive alkalic rocks. If this were the sole coupledsubstitution involving Al3+ in the clinopyroxenes, the ratio Ti4+/Al3+

would be 1:2, whereas it varies to below 1:4 (Fig. 4) and suggests thatthe excess Al3+ occurs in the limited substitution of Tschermak'smolecule (CaAlAlSiO6). Other nonquadrilateral components such asNa+ (<0.07 cfu) and K+ (<O.Ol cfu) are extremely low.

In addition to the dominant diopsidic clinopyroxenes of Site 801there are some with strongly alkalic rims. Alkali pyroxenes wereanalyzed (Table 4) in three Samples (801B-43R-1,4-9 cm, 801B-44R-2, 140-145 cm and 801C-1R-1, 14-20 cm) and in all cases showed asharp chemical discontinuity between the diopside core (for example,Di54Hd38Ac8) and the alkali pyroxene rim (for example, Di25Hd48Ac27)(Fig. 5). The alkali pyroxenes zone outward to some highly aegirine-rich compositions, such as Di8Hd10Ac82 (Samples 801B-43R-1, 4-9cm and 801B-44R-2, 140-145 cm). In addition, the alkali pyroxenesare all highly titaniferous (up to 0.15 cfu) and are classified as titanianaegirine-augites and titanian augites (Morimoto, 1988). Naturally occur-ring highly titaniferous alkali pyroxenes have been described fromperalkaline volcanic and plutonic rocks (Schmincke, 1969; Ferguson,1977; Curtis and Gittins, 1979) where they are commonly found as alate-stage magmatic phase after Ti-poor aegirine-augite and growingdiscontinuously on diopsidic pyroxenes. Laverne (1987) has alsoreported occurrences in veins cross-cutting oceanic basalts (Hole504B), but the compositions are more diverse than those reported hereand include aegirine-augite and fassaite. The main difference be-tween the data presented by Laverne (1987) and the present study isthat Leg 129 alkali pyroxenes exhibit a much more extended solidsolution toward the aegirine end-member composition and also littlejadeite substitution, that is, no Al3+ in octahedral coordination sites(Table 4). The entry of Ti4+ into the octahedral sites of these clinopy-

309


Table 2. Feldspar analyses from alkalic dolerites and basalts, Holes 800A, 801B, and 801C.

Hole

Core/section

Interval (cm)

sio2

TIO,

A12O3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2°3NiO

0

TOTAL1/.

800A-

58R-1.

86-91

C

64.85

0.25

22.13

0.12

0.00

0.12

3.46

8.89

1.34

0.11

0.00

0.00

101.26

800A-

58R-1.

86-91

c

60.73

0.10

23.96

0.35

0.09

0.00

6.22

7.64

0.61

0.03

0.00

0.00

99.73

800A-

58R-1.

86-91

R

62.43

0.21

23.23

0.22

0.00

0.02

5.02

8.23

0.68

0.03

0.00

0.00

100.09

800A-

58R-2,

67-73

c

54.88

0.13

27.80

0.61

0.00

0.00

11.28

4.84

0.20

0.03

0.10

0.00

99.87

800A-

58R-2.

67-73

R/C

63.02

0.24

21.48

0.24

0.06

0.00

3.60

8.47

1.11

0.00

0.00

0.00

98.21

800A-

58R-2.

67-73

R

67.23

0.00

17.87

0.86

0.00

0.06

0.06

6.57

7.17

0.02

0.00

0.00

99.86

801 B-

43R-1.

4-9

G

66.86

0.06

19.04

0.29

0.05

0.00

0.36

5.60

8.25

0.00

0.09

0.00

100.59

801B-

43R-1.

4-9

C

51.73

0.15

29.88

0.48

0.00

0.02

13.57

3.55

0.33

0.00

0.00

0.00

99.70

801B-

43R-1.

4-9

R/C

60.86

0.24

23.48

0.46

0.01

0.00

5.57

7.38

1.17

0.00

0.40

0.00

99.57

801B-

43R-1.

4-9

R

65.62

0.09

19.44

0.28

0.00

0.00

1.02

6.58

6.89

0.09

0.22

0.00

100.21

801B-

44R-3.

28-33

C

51.32

0.08

30.74

0.50

0.00

0.00

14.41

3.31

0.23

0.00

0.00

0.00

100.59

Recalculation to 8 (O)

Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

Ca/Ca+Na

2.84

0.01

1.14

0.00

0.00

0.01

0.16

0.75

0.08

0.00

0.00

8.00

4.99

0.18

2.72

0.00

1.26

0.01

0.00

0.00

0.30

0.66

0.04

0.00

0.00

8.00

5.00

0.31

2.77

0.01

1.22

0.01

0.00

0.00

0.24

0.71

0.04

0.00

0.00

8.00

4.99

0.25

2.49

0.01

1.48

0.02

0.00

0.00

0.55

0.43

0.01

0.00

0.00

8.00

4.99

0.56

2.84

0.01

1.14

0.01

0.00

0.00

0.17

0.74

0.06

0.00

0.00

8.00

4.98

0.19

3.02

0.00

0.95

0.03

0.00

0.00

0.00

0.57

0.41

0.00

0.00

8.00

5.00

0.01

2.99

0.00

1.00

0.01

0.00

0.00

0.02

0.49

0.47

0.00

0.00

8.00

4.98

0.03

2.36

0.01

1.61

0.02

0.00

0.00

0.66

0.31

0.02

0.00

0.00

8.00

4.99

0.68

2.73

0.01

1.24

0.02

0.00

0.00

0.27

0.64

0.07

0.00

0.01

8.00

4.99

0.29

2.95

0.00

1.03

0.01

0.00

0.00

0.05

0.57

0.40

0.00

0.01

8.00

5.02

0.08

2.33

0.00

1.64

0.02

0.00

0.00

0.70

0.29

0.01

0.00

0.00

8.00

5.00

0.71

Footnote:

C = Core

R/C = Intermediate core-rim

R = Rim

G = Groundmass

310


Table 2 (continued).

801B-

44R-3.

28-33

R

54.33

0.16

27.78

0.81

0.02

0.04

11.57

4.58

0.58

0.00

0.26

0.00

100.12

8018-

44R-3.

28-33

G

65.61

0.03

19.64

0.33

0.01

0.00

1.12

7.11

5.81

0.02

0.00

0.00

99.69

801B-

44R-3,

28-33

C

52.04

0.19

29.73

0.35

0.04

0.01

13.06

3.96

0.31

0.00

0.00

0.00

99.68

801C-

1R-1,

14-20

R

65.50

0.18

19.87

0.30

0.05

0.00

1.75

7.92

4.18

0.00

0.08

0.00

99.84

801C-

1R-1.

14-20

G

65.44

0.02

18.59

0.98

0.00

0.43

0.03

3.39

11.46

0.00

0.18

0.00

100.53

801C-

1R-1.

14-20

G

63.31

0.05

21.96

0.37

0.00

0.00

3.98

8.12

1.90

0.04

0.00

0.00

99.73

801C-

2R-5.

101-106

R

53.01

0.13

29.55

0.69

0.08

0.00

13.00

3.79

0.33

0.14

0.06

0.00

100.79

801C-

2R-5.

101-106

c

53.01

0.13

29.55

0.69

0.08

0.00

13.00

3.79

0.33

0.14

0.06

0.00

100.79

801C-

2R-5.

101-106

G

66.21

0.08

18.84

0.29

0.00

0.00

0.39

5.99

7.84

0.09

0.02

0.00

99.73

2.47

0.01

1.49

0.03

0.00

0.00

0.56

0.40

0.03

0.00

0.01

8.00

5.00

0 . 5 8

2.95

0.00

1.04

0.01

0.00

0.00

0.05

0.62

0.33

0.00

0.00

8.00

5.01

0.08

2.38

0.01

1.60

0.01

0.00

0.00

0.64

0.35

0.02

0.00

0.00

8.00

5.00

0.65

2.93

0.01

1.05

0.01

0.00

0.00

0.08

0.69

0.24

0.00

0.00

8.00

5.01

0 . 1 1

2.95

0.00

1.00

0.04

0.00

0.03

0.00

0.30

0.66

0.00

0.01

8.00

4.98

0.00

2.83

0.00

1.16

0.01

0.00

0.00

0.19

0.70

0.11

0.00

0.00

8.00

5.00

0.21

2.39

0.00

1.57

0.03

0.00

0.00

0.63

0.33

0.02

0.01

0.00

8.00

4.99

0.65

2.39

0.00

1.57

0.03

0.00

0.00

0.63

0.33

0.02

0.01

0.00

8.00

4.99

0.65

2.99

0.00

1.00

0.01

0.00

0.00

0.02

0.52

0.45

0.00

0.00

8.00

5.00

0.03

roxenes cannot be the same as that outlined earlier in this section as in alkali pyroxenes are usually accompanied by lower Fe3+ and higherthere is insufficient Al in octahedral coordination. Flower (1974) Mg2+ + Fe2+. Leg 129 alkali pyroxenes, however, have insufficientdetermined the phase relations of titan-acmite and suggested a Fe3+to charge-balance the entry of Na+into the structure and there issubstitution of the type TiY4+MZ3+ = MY3+SiZ4+, where Y = octahe- probably a substitution of the type (Ferguson, 1977) NaFe3+Si2O6

dral co-ordination, Z = tetrahedral co-ordination and M = cation. (aegirine) = Na(MgFe2+) 0 5Ti0 5O6 (neptunite).This substitution implies replacement of Si4+ by a trivalent cation, The occurrence of strongly alkalic rims to both the pyroxenes andbut in the Leg 129 alkali pyroxenes there is limited substitution in the feldspars suggests that rim growth was probably broadly synchro-these sites and therefore this cannot be an accepted substitution in nous and occurred during the late-stage hydrous crystallization of thethis particular case. Deer et al. (1978) showed that high Ti4+ contents alkalic rocks along with amphibole and biotite.

311


Table 3. Clinopyroxene analyses from alkalic dolerites and basalts, Holes 800A, 801B, and 801C.

Hole

Core/section

Interval (cm)

SiO

Ti°2

A12O3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2°3NiO

0

TOTAL1/.

800A-

58R-1.

86-91

C

49.12

1.98

4.43

8.67

0.14

14.03

20.29

0.51

0.06

0.30

0.00

0.00

99.56

800A-

58R-1.

86-91

R/C

50.50

1.57

3.28

9.69

0.31

13.80

19.70

0.44

0.04

0.03

0.10

0.00

99.72

800A-

58R-1,

86-91

R

50.85

0.99

1.70

10.79

0.39

13.80

19.34

0.56

0.10

0.00

0.00

0.00

98.43

800A-

58R-2.

67-73

C

49.23

1.73

3.68

9.45

0.28

13.04

21.34

0.51

0.06

0.04

0.10

0.00

99.45

800A-

58R-2.

67-73

R

51.13

1.20

1.93

8.60

0.18

14.65

21.07

0.50

0.00

0.03

0.19

0.00

99.46

801B

41R-1

130-136

C

48.60

2.13

4.50

9.27

0.14

12.14

21.90

0.69

0.00

0.04

0.40

0.00

99.81

801B

41R-1

130-136

C

45.10

3.73

6.31

8.80

0.14

11.15

22.37

0.56

0.06

0.10

0.00

0.00

98.31

801B

41R-1

130-136

R

48.81

1.92

4.01

10.33

0.26

11.60

21.34

0.74

0.07

0.15

0.05

0.00

99.29

801B

43R-1.

4-9

R

49.01

1.72

4.04

9.80

0.25

12.22

21.61

0.67

0.00

0.00

0.24

0.00

99.57

801B

43R-1.

4-9

C

44.28

4.96

7.89

8.79

0.11

10.82

22.24

0.82

0.05

0.10

0.09

0.00

100.15

801B

43R-1.

4-9

R

49.45

1.74

3.98

9.47

0.31

12.70

21.80

0.52

0.00

0.08

0.45

0.00

100.49

R e c a l c u l a t i o n t o 6 ( 0 )

Si

Ti

A l

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

1.84

0.06

0.20

0.27

0.00

0.79

0.82

0.04

0.00

0.00

0.01

6.00

4.02

1 .89

0.04

0.15

0.30

0.01

0.77

0.80

0.03

0.00

0.00

0.00

6.00

4.01

1.94

0.03

0.08

0.33

0.01

0.78

0.79

0.04

0.00

0.00

0.00

6.00

4.00

1.86

0.05

0.16

0.30

0.01

0.74

0.87

0.04

0.00

0.00

0.00

6.00

4.03

1.92

0.03

0.09

0.27

0.01

0.82

0.85

0.04

0.00

0.00

0.01

6.00

4.02

1.84

0.06

0.20

0.29

0.00

0.68

0.89

0.05

0.00

0.00

0.01

6.00

4.03

1.74

0.11

0.29

0.28

0.01

0.64

0.92

0.04

0.00

0.00

0.00

6.00

4.03

1.86

0.06

0.18

0.33

0.01

0.66

0.87

0.06

0.00

0.01

0.00

6.00

4.03

1.86

0.05

0.18

0.31

0.01

0.69

0.88

0.05

0.00

0.00

0.01

6.00

4.03

1.68

0.14

0.35

0.28

0.00

0.61

0.90

0.06

0.00

0.00

0.00

6.00

4.04

1.86

0.05

0.18

0.30

0.01

0.71

0.88

0.04

0.00

0.00

0.01

6.00

4.03

Footnote:

C = Core


R = Rim

312



44R-2,

140-145

R

44.54

4.46

•7.43

9.27

0.10

10.63

22.24

0.78

0.00

0.11

1.02

0.00

100.58

801B

44R-2.

140-145

R/C

45.59

3.60

6.76

9.49

0.18

10.65

22.07

0.69

0.00

0.07

0 . 2 8

0.00

99.38

801B

44R-2.

140-145

C

49.71

1.52

3.06

11.72

0.17

11.25

21.31

0.72

0.06

0 . 0 0

0 . 0 0

0.00

99.51

801C

1R-1.

14-20

C

42.23

5.91

9.26

8.94

0.22

10.61

22.16

0.85

0.00

0.02

0 . 0 0

0 . 0 0

1 0 0 . 2 0

801C

1R-1.

14-20

R

50.59

1.43

3.25

8.57

0.26

13.55

21.82

0 . 6 8

0 . 0 0

0 . 0 0

0 . 0 0

0 . 0 0

1 0 0 . 1 3

801C

1R-1.

14-20

c

51.31

1.30

2.60

8.94

0.34

12.91

22.07

0.78

0.03

0.17

0 . 0 0

0.00

100.46

801C

2R-5.

101-106

C

45.17

4.00

7.64

9.55

0.16

10.67

22.67

0.57

0.03

0.10

0 . 0 0

0.00

100.55

801C

2R-5.

101-106

R

44.89

3.97

7.28

9.53

0.06

11.00

21.44

0.64

0.10

0.13

0.00

0.00

99.59

801C

2R-5.

101-106

C

44.43

4.06

7.20

9.69

0.04

10.66

22.27

0.72

0.02

0.06

0.61

0.00

99.76

Amphiboles

1.69

0.13

0.33

0 . 2 9

0.00

0 . 6 0

0 . 9 0

0 . 0 6

0 . 0 0

0 . 0 0

0 . 0 3

6 . 0 0

4 . 0 4

1 .74

0 . 1 0

0 . 3 0

0 . 0 3

0 . 0 1 *

0.61

0 . 9 0

0 . 0 5

0 . 0 0

0 . 0 0

0 . 0 1

6 . 0 0

3.76

1.90

0.04

0.14

0.37

0.01

0.64

0.87

0 . 0 5

0 . 0 0

0 . 0 0

0 . 0 0

6 . 0 0

4 . 0 2

1 .61

0 . 1 7

0 . 4 2

0 . 2 8

0 . 0 1

0 . 6 0

0 . 9 0

0 . 0 6

0 . 0 0

0 . 0 0

0 . 0 0

6 . 0 0

4 . 0 5

1 . 8 9

0 . 0 4

0 . 1 4

0 . 2 7

0 . 0 1

0 . 7 5

0 . 8 7

0 . 0 5

0 . 0 0

0 . 0 0

0 . 0 0

6 . 0 0

4 . 0 2

1 .91

0 . 0 4

0 . 1 1

0 . 2 8

0 . 0 1

0 . 7 2

0 . 8 8

0 . 0 6

0 . 0 0

0 . 0 1

0 . 0 0

6 . 0 0

4 . 0 2

1 .71

0 .11

0 . 3 4

0 . 3 0

0.01

0 . 6 0

0 . 9 2

0 . 0 4

0 . 0 0

0 . 0 0

0 . 0 0

6 . 0 0

4 . 0 3

1 .71

0 . 1 1

0 . 3 5

0 . 3 0

0 . 0 0

0 . 6 2

0 . 8 7

0 . 0 5

0 .01

0 . 0 0

0 . 0 0

6 . 0 0

4 . 0 3

1 .70

0 . 1 2

0 . 3 3

0 . 3 1

0 . 0 0

0.61

0.91

0.05

0.00

0.00

0.02

6.00

4.05

cation content of 0.34 cations/23 oxygens in the

Brown magmatic amphiboles (Table 5) occur in minor proportionsin both Hole 800A dolerites and the coarser interiors of the olivine-rich microdolerites of Site 801. Only samples from Hole 801 havebeen analyzed and are not characteristic of common magmatic calcif-erous amphiboles (Fig. 6). They are aluminum-poor with a maximum

which is allocated to the tetrahedral sites in the cfu recalculation(Table 5). Calculation of the ferric iron content (method of Droop,1987) yielded a range of Fe3+ values between 0.65 and 1.28 cfu. Bothof these features are unusual in igneous clinoamphiboles (Hawthorne,1981), and following the nomenclature of Leake (1978) these miner-als are winchite-ferriwinchite in composition (CaNa (MgFe2+)4Fe3+

313


Table 4. Alkali pyroxene analyses from alkalic basalts, Holes 801B and 801C.

Hole

Core/section

Interval (cm)

801 B-

43R-1.

4-9

8018-

43R-1.

4-9

801 B-

43R-1.

4-9

801C-

1R-1.

14-20

801C-

1R-1.

14-20

801C-

1R-1,

14-20

801C-

1R-1.

14-20

801C-

1R-1.

14-20

801C-

1R-1.

14-20

801C-

1R-1.

14-20

801 C-

1R-1,

14-20

s i o 2

TIO,

A l2°3

FeO

MnO

MgO

CaO

R

5 2 . 8 4

3 . 8 3

0 . 4 6

2 3 . 1 5

4 . 0 6

0 . 3 3

0.81

2.98

12.22

0.10

100.78

R

53.07

3.81

0.78

24.57

2.79

0.20

0.76

2.67

12.75

0.00

101.39

R

52.83

3.56

0.72

22.70

4.74

0.23

0.66

2.60

12.27

0.00

100.30

R

50.27

3.74

0.22

14.03

11.88

0.40

0.11

10.22

8.12

0.08

99.08

R

50.04

2.71

0.00

8.50

17.29

0.98

0.14

14.45

5.44

0.00

99.55

E/R

50.71

4.79

0.20

12.08

13.36

0.63

0.09

9.44

8.33

0.02

99.65

R

50.51

4.74

0.47

11.38

13.65

0.66

0.16

9.21

8.25

0.00

99.03

R

50.08

4.52

0.47

11.61

13.11

0.32

0.15

8.82

8.40

0.00

97.47

E/R

50.96

4.90

0.31

13.98

11.90

0.30

0.10

7.85

9.24

0.02

99.55

R

51.10

5.10

0.47

11.60

14.00

0.59

0.20

9.85

8.22

0.00

100.69

R

50.88

5.11

0.21

11.76

13.54

0.40

0.10

10.08

8.28

0.00

100.36

Recalculation to 6 (0)

Si

Ti

AL

Fe3 +

Fe2*

Mn

Mg

Ca

Na

K

0

Total

Footnote:

R • Rim

E/R = Extreme

2.00

0.11

0.02

0.66

0.13

0.01

0.05

0.12

0.90

0.01

6.00

4.00

rim

1.99

0.11

0.04

0.69

0.09

0.01

0.04

0.11

0.93

0.00

6.00

4.00

2.01

0.03

0.10

0.65

0.15

0.01

0.04

0.11

0.91

0.00

6.00

4.00

1 .99

0.11

0.01

0.42

0.39

0.01

0.01

0.43

0.62

0.00

6.00

4.00

2.00

0.08

0.00

0.26

0.58

0.03

0.01

0.62

0.42

0.00

6.00

4.00

1 .99

0.14

0.01

0.36

0.44

0.02

0.01

0.40

0.64

0.00

6.00

4.00

2.00

0.14

0.02

0.34

0.45

0.02

0.01

0.39

0.63

0.00

6.00

4.00

2.00

0.14

0.02

0.35

0.44

0.01

0.01

0.38

0.65

0.00

6.00

4.00

1.99

0.14

0.01

0.41

0.39

0.01

0.01

0.33

0.70

0.00

6.00

4.00

1.99

0.15

0.02

0.33

0.46

0.02

0.01

0.41

0.62

0.00

6.00

4.00

1.99

0.15

0.01

0.35

0.44

0.01

0.01

0.42

0.63

0.00

6.00

4.00

Si8O22(OH)2 end-member composition) with limited solid-solutiontoward katophorite (NaCaNa[MgFe2+] 4Fe3+Si7A1022 [OH]2). Theseend members imply the two coupled substitutions:

and

Ca (MgFe2+)tremolite-actinolite

(vacancy A site) + Si =winchite

Na Fe3+

winchite-ferriwinchite

NaA + Al2

katophorite

In addition to these dominant substitutions, Fe-Mg replacementis limited (Mg/Mg + Fe2 + ratio = 0.44-0.64). The rims of some ofthe crystals have higher Fe3+/Fe2+ ratios, which may reflect increas-ing f θ 2 as hydrous crystallization proceeded. Other chemical factorsexhibited by the amphiboles, such as low Ti concentrations (0.09-0.17 cfu) which incompletely filled the A sites in the lattice andlimited replacement of Si by Al in the tetrahedral sites, are generallythought to reflect crystallization at relatively low magmatic tempera-tures (Helz, 1973; 1980). Petrographically they are late in develop-ment and herald a change to more hydrous and oxidizing conditionsof crystallization.

314



801 C-

1R-1.

14-20

801C-

1R-1 .

14-20

801C-

1R-1 .

14-20

801C-

1R-1 .

14-20

801C-

1R-1.

14-20

801B-

44R-2.

140-145

801B-

44R-2.

140-145

E/R

5 0 . 5 7

4 . 4 6

0 . 4 5

19.27

6.75

0.29

0.23

6.51

10.48

0.03

99.04

1.97

0.13

0.02

0.57

0.22

0.01

0.01

0.27

0.79

0.00

6.00

4.00

R

50.44

4.28

0.64

10.99

14.24

0.81

0.35

11.16

7.37

0.00

100.28

1.98

0.13

0.03

0.32

0.47

0.03

0.02

0.47

0.56

0.00

6.00

4.00

R

50.44

4.28

0.64

10.99

14.24

0.81

0.35

11.16

7.37

0.00

100.28

1.98

0.13

0.03

0.32

0.47

0.03

0.02

0.47

0.56

0.00

6.00

4.00

E/R

5 1 . 1 0

4 . 4 7

0 . 6 4

14.14

11.70

0.43

0.19

8.52

9.00

0.00

100.19

1.99

0.13

0.03

0.41

0.38

0.01

0.01

0.36

0.68

0.00

6.00

4.00

E/R

50.97

4.87

0.69

1 3 . 4 7

11.83

0.52

0.17

8.33

9.05

0.01

99.91

1 .99

0.14

0.03

0.40

0.39

0.02

0.01

0.35

0.68

0.00

6.00

4.00

R

5 0 . 7 4

5.11

2 . 0 6

20.67

4.74

0.30

1.80

2.01

11.72

0.03

99.18

1.94

0.15

0.09

0.60

0.15

0.01

0.10

0.08

0.87

0.00

6.00

4.00

E/R

50.88

3.97

2.39

22.65

3.32

0.25

2.20

1.90

11.75

0.00

99.31

1.94

0.11

0.11

0.65

0.11

0.01

0.13

0.13

0.08

0.87

6.00

4.00

Biotites

In the Hole 800A dolerites, biotite (Table 6) developed both as alate-stage magmatic phase, replacing brown amphibole, as well aspost-magmatically in response to contact metamorphism, nucleatingon titanomagnetite and replacing smectite-altered interstitial areas.Site 801 basalts contain magmatic biotite more closely associatedwith primary minerals, occurring as discrete euhedral crystals, as wellas nucleating on opaque phases.

Chemically, the very late-stage biotites (Hole 800A) are charac-terized by relatively low Ti4+ (0.49-0.68 cfu) and Al3+ contents (1.43-2.19 cfu), deficiencies in the tetrahedral sites (Si + Al <8.00 cfu) andto a lesser extent in the interlayer sites, and restricted Mg/Mg + Feratios (0.53-0.70) (Fig. 7). On the other hand, the magmatic biotitesfrom Holes 80IB and 801C have greater Ti contents (up to 1.0 cfu)in the octahedral sites, and have tetrahedral sites completely occupiedby Si4+ and Al3+, and a wider range of Mg/Mg + Fe values. In all theSite 801 samples there is minor replacement of K+ by Na+ in theinterlayer site (up to 0.36 cfu) which is not seen to the same extent in

the Hole 800A samples. In addition, there seems to be an inverserelationship between Ti4+ and total Fe* in the Site 801 biotites.Roberts (1976) suggested that increased entry of Ti4+ into the biotitestructure was favored by higher temperatures, and this interpretationis supported by the observations in the present study. Biotites fromSite 801 have the highest Ti4+ contents and appear to be magmatic indevelopment, whereas the Hole 800A biotites were probably devel-oped at lower temperatures post-magmatically and/or very late in thecrystallization sequence.

Tholeiitic Basalts

Plagioclases

Relative to the feldspars from the alkalic rocks described pre-viously, the plagioclases from the tholeiites of Holes 801C and 802Ado not exhibit the same compositional range and lack the devel-opment of alkali feldspar rims (Table 7). In broad terms they aremainly bytownite-calcic labradorite in composition (An90An57), al-though the rims of individual crystals may be quite sodic (An24).

Plagioclases in the geochemically primitive, plagioclase-olivine-spinel phyric tholeiites of Hole 801C (Mg* = 72-60; Floyd andCastillo, this volume) characteristically occur in three generations:glomerophyric clusters, microphenocrysts, and quenched varioliticintergrowths with clinopyroxene (for example, Sample 801C-6R-4,49_54 c m , Table 6). The glomerocrysts are the most anorthitic(An90Abi0Or0), and some possess normal zoning out to the rims withAn50, although zoning is by no means common in these early crystals.There is broad compositional overlap between the glomerocrysts and themicrophenocrysts, although the latter with chrome spinel inclusions haveslightly lower An contents (An75_80). Variolitic plagioclase is distinctlymore sodic and exhibits a greater range (An^.^). Groundmass microlitesand serrated laths are also systematically more albitic than the microphe-nocrysts and contain higher contents of Fe and Mg. There does not seemto be any correlation between An content and minor element abundancesas documented for some submarine basalts (Bence et al., 1975).

Both microphenocrystic and groundmass plagioclases within theHole 802A tholeiites show marginally more albitic compositions thanthose in Hole 801C, although once again, with few exceptions, Orcontents are minimal. The most anorthitic compositions are bytown-ites (up to An86), which can range down to calcic andesine (An50_47)in rim zones (Fig. 8). In general, microphenocryst and quench crystalsoverlap in composition with both commonly exhibiting values >An70.One important petrographic characteristic is the presence in all ana-lyzed plagioclase microphenocrysts of complex zoning patterns (os-cillatory and reversed), irrespective of whether they occur in glassymatrix, quench-textured groundmass, or intersertal-textured matrix.For example, a plagioclase microphenocryst analyzed from Sample802A-59R-3, 40-44 cm, showed the following variation: fromAn84 8Ab145Or00 (core) to An79 5Ab20.5Or0 0 (intermediate zone) toAn74 9Ab24 4Or0 7 (rim). The minor change in An content (of about 5%An) between adjacent zones in the crystal is typical of many Hole802A plagioclases.

Another feature of plagioclases from the Hole 802A tholeiites isthe apparent lack of a correlation between An content and minorcomponents, such as Fe and Mg in the feldspar lattice. Many primitivemid-ocean ridge basalts (MORB) containing highly calcic plagioclasemay display increasing Mg abundances with decreasing An content(Bence et al., 1975), whereas Nauru Basin plagioclases from evolvedtholeiites show a positive correlation between Mg and An (Floyd andRowbotham, 1986). On the other hand, Kempton et al. (1985) noteda positive correlation of Mg with An content for phenocrysts,xenocrysts, and groundmass feldspar only when the compositionswere below An79 and had a complex relationship in compositionsmore An-rich than this figure. The compositions of the Hole 802Aplagioclases are generally more An-rich than those of the NauruBasin, between An75 and An85, and fall into the area of more complexrelationships as noted by Kempton et al. (1985).

315


CaAlkalic dolerites

800A-58R-2, 67-73 cm800A-58R-1, 86-91 cm

Alkalic basalts"801B-43R-1, 4-9 cm*801B-41R-1, 130-136 cm

Mg Fe Mg Fe

CaAlkalic basalt

801B-44R-2, 140-145 cm

CaAlkalic basalts

801C-2R-5. 101-106 cm801C-1R-1, 14-20 cm

Mg Fe Mg Fe

Figure 3. Composition of clinopyroxenes from alkalic dolerites and basalts, in terms of Ca, Fe, and Mg. A. Dolerite sills from Hole 800A. B, C, and D. Variousbasalt flows from Site 801.

Pyroxenes

The pyroxenes in the tholeiitic basalts of Holes 801C and 802Aare extremely variable in terms of both quadrilateral and minorelement components (Table 8). However, with one exception, corecompositions of Hole 802A clinopyroxenes are augite and form a tightcluster around the composition Ca375Mg5 lFel, 5 (Fig. 9). They are verysimilar to MORB clinopyroxene compositions (for example, Bence etal., 1975), although in this case the limited compositional range reflectsthe narrow chemical composition exhibited by the bulk rocks, whichhave essentially uniform FeO*/MgO ratios (Floyd et al., this volume).However, composite pyroxene grains from Sample 802A-62R-2, 53-59 cm do not fit this homogeneous pattern and exhibit wide variationsin composition between optically distinguishable segments. The coresof the grains are Ca-poor (for example, Ca100Mg535Fe365), whereasthe rims are more Ca-rich (for example, Ca308Mg377Fe3i5). The dis-tribution of the spot analyses (Fig. 9) indicates substitution of the typeCa = Mg, Ca = Fe, and Mg = Fe, with trends among the quadrilateralcomponents characteristic of compositional variation due to quench-ing. In terms of the nonquadrilateral components Ti, Cr, and Al(Fig. 10), the Hole 802A clinopyroxenes are extremely variable com-pared to those of the alkalic rocks described previously. Ti abundances

are low (0.01 cfu) with Ti/Al ratios of approximately 1:10, althoughwith the exception of Sample 802A-62R-2, 53-59 cm, within-samplevariation is limited. The Ti and Cr contents are similar to those of theNauru Basin clinopyroxenes (Floyd and Rowbotham, 1986) andgenerally reflect the low Ti and Cr contents of the bulk rocks.

The Hole 801C clinopyroxenes occur as microphenocrysts, spo-radically with hour-glass structure, as variolitic fans and, rarely, as cleardendrites within olivine (for example, in Sample 801C-6R-4, 49-54cm). In terms of quadrilateral components (Fig. 9), there is a greaterrange of variability from sample to sample that reflects wider chemicaldifferences in bulk-rock composition and textural type than inHole 802A. Microphenocrysts are diopsides and augites, with somecrystals showing a marked Fe enrichment toward grain rims. Sample801C-7R-2, 122-127 cm exhibits hour-glass structure, with all seg-ments being augite with fairly constant Fe contents. Clinopyroxenesin variolites have been analyzed both along the length and acrossindividual fibers and have a fairly constant composition in terms ofquadrilateral components (for example, Ca44Mg43Fe [ 3). The small groupsof granular intersertal clinopyroxenes in flow interiors are commonlyzoned with cores of Ca43Mg41Fe16 (comparable to the variolitic clinopy-roxenes) to extremely Fe-richrims of Ca38Mg12Fe50. The clinopyroxenesof the Hole 801C tholeiites have marginally greater amounts of the

316


0.20i

0.15-

0.105u

0.05

0.0

Acmite

« 800A-58R-1, 86-91 cm• 800A-58R-2, 67-73 cm-801B-41R-1, 130-136 cm•801B-43R-1, 4-9 cm• 801B-44R-2, 140-145 cm• 801C-1R-1, 14-20 cm•801C-2R-5, 101-106 cm

.0 0.1 0.2 0.3 0.4Al Cations

0.5

Figure 4. Minor components, Ti and Al, in clinopyroxenes from alkalic dolerite

sills and basalt flows, Holes 800A, 801B, and 801C.

minor, non-quadrilateral components Ti and Al compared to Hole802A, with a greater spread of Ti/Al ratios (Fig. 10). Clinopyroxenesanalyzed in Sample 801C-5R-5,73-78 cm have the highest Ti content(up to 0.18 cfu) and Ti/Al ratios similar to the alkalic basalts (1:3).The Cr contents are similar to that found in Hole 802A clinopyroxenes.

Within a number of olivine crystals in the Hole 801C basalts arerare, highly birefringent, spherical patches composed of small py-roxene dendrites. Probe analyses recalculated to a pyroxene compo-sition give the following average composition: (Ko.003Naoo24Cao.919Mgo.536M no.oo5F eo. 182Tio.o36Alo i i .598 The dendritesare highly aluminous with Al present at high values and commonly insimilar proportions in both octahedral and tetrahedral sites suggestinga high proportion of Ca Tschermak's molecule with R2+ + SiIV = A1VI

+ A1IV substitution. Pyroxenes of this type are of fassaite compositionand usually associated with high-pressure conditions (Lovering andWhite, 1969).

Olivines

Olivine is commonly replaced completely by alteration products,although rare pristine crystals have been analyzed from two samples(Table 9) in tholeiitic basalts from Hole 801C (cooling unit C20) andHole 802A (cooling unit A5). Sample 801C-6R-4, 49-54 cm is aprimitive group tholeiite with MORB characteristics and containsolivine microphenocrysts that are exceedingly magnesian-rich, reach-ing compositions of Fo90. Typical across-grain variation is from Fo80

to Fo8g (Fig. 11). Cores are invariably Ni-rich (maximum content 0.72wt% NiO), grading to lower values in the rims (minimum content of0.28 wt% NiO).

Sample 802A-58R-2,99-102 cm is an olivine microphyric tholei-ite with euhedral olivines embedded in the fresh glass of a pillow rim.There is no detectable chemical zonation in the olivines involvingmajor oxide components, and all have uniform compositions ofF°79-8o• The nickel content, however, does vary between 1.0 and 0.2wt% NiO, although there is no systematic variation between core andrim. The olivines were in equilibrium with the original melt, nowrepresented by the enclosing glass. Olivine compositions of Fo80 withiron-magnesium distribution coefficients of 0.30 should be in equi-librium with liquids that have FeO/FeO + MgO ratios of about 0.47(Roeder, 1974; Longhi et al., 1978), which is close to that calculated(0.48) from the adjacent glass compositions (Table 10).

Spinels

Spinel occurs as either minute light brown octahedral inclusionswithin plagioclase or olivine phenocrysts or as larger (0.5-0.7 mm)

801B-43R-1, 4-9 cm801B-44R-2, 140-145 cm

+ 801C-1R-1, 14-20 cm

Diopside Hedenbergite

Figure 5. Composition of alkali pyroxenes from alkalic basalts, in terms ofacmite, diopside, and hedenbergite components, Holes 801B and 801C.

euhedral microphenocrysts associated with olivine in primitive grouptholeiites of Hole 801C (for example, Samples 801C-5R-5,73-78 cmand 801C-6R-4, 49-54) (Table 11). Opaque spinel inclusions alsooccur within the plagioclase phenocrysts of the alkalic intrusive unit(cooling unit B14) at the base of Hole 801B (Sample 801B-44R-3,28-33). Spinel paragenesis indicates that they formed at the earlieststages of crystallization.

The opaque spinels within plagioclase crystals from Sample801B-44R-3,28-33 cm (alkalic basalt) possess highly variable Al(2.46-5.05 cfu), and Ti (1.95-3.56 cfu) contents, with smallervariations in total Fe (6.11-8.28 cfu) and Cr (1.67-3.03 cfu); Mg isrelatively constant (about 3.20 cfu). Many of these variations areinter- rather than intracrystalline, making it difficult to assess anysystematic zoning within individual crystals. Sigurdsson and Schil-ling (1976) subdivided spinels from the Mid-Atlantic Ridge intothree types: (1) magnesiochromite with Cr/Cr + Al = 0.4-0.5, (2)titaniferous magnesiochromite, and (3) chrome spinel with Cr/Cr +Al = 0.23-0.27. On this basis the opaque inclusions in Sample801B-44R-3,28-33 cm are highly titaniferous spinels and reflect thegenerally Ti-rich alkalic composition of the initial melt (and bulk rock).The spinels also show strong negative correlations between Ti andMg/(Mg + Fe2+)(Fig. 12).

The brown spinels in the Hole 801C tholeiites have a chemistryquite different from those preceding. They are all Ti-poor chromespinels (criteria of Sigurdsson and Schilling, 1976) with Cr/(Cr + Al)ratios between 0.191 and 0.380, Mg/(Mg + Fe2+) ratios of 0.567-0.848, and Ti <0.5 cfu. Unlike the opaque spinel inclusions describedpreviously, Ti shows a tendency to increase with Mg/(Mg + Fe2+)(Fig. 12). There are systematic changes in chemistry between spinelinclusions in plagioclase and olivine and relative to micropheno-crysts. Similar variations between spinels in different parageneseshave been recorded in MORB younger than the oceanic basaltsconsidered here (Furuta and Tokuyama, 1983; Allan et al., 1988). Forexample, the Cr contents of microphenocryst spinels are similar tothose of inclusions in plagioclase (5.00-5.55 cfu), but both are richerin Cr than the inclusions in the olivine microphenocrysts (cf. Furutaand Tokuyama, 1983). The Cr/(Cr + Al) and Mg/(Mg + Fe2+) ratiosfor these spinels are similar to chromian spinels from the Mid-AtlanticRidge picrites (Sigurdsson and Schilling, 1976) and the East PacificRise Lamont Seamount chain (Allan et al., 1988), although the Alcontent exceeds that of many MORB spinels. These features probablyreflect the generally primitive nature of the Hole 801C tholeiites,some of which exhibit high Mg numbers (about 70) not dissimilar to

317


Table 5. Amphibole analyses from alkalic dolerites, Holes 800A and 802A.

Hole

Core/section

Interval (cm)

sio2

TiO2

A12O3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2O3

NiO

0

TOTALX

800A-

58R-1.

86-91

R

50.95

0.96

1.12

22.00

0.36

9.88

5.65

4.65

0.59

0.12

0.00

0.00

96.29

800A-

58R-1.

86-91

R

50.77

1.28

1.13

23.70

0.55

8.77

6.37

4.54

0.64

0.02

0.18

0.00

97.96

800A-

58R-1.

86-91

C

51.33

1.18

0.85

24.76

0.47

8.20

5.25

4.55

0.39

0.00

0.29

0.00

97.28

800A-

58R-1.

86-91

C

51.19

1.50

0.16

24.81

0.37

7.94

5.89

4.44

0.66

0.00

0.22

0.00

98.63

800A-

58R-1.

86-91

c

52.50

1.22

1.16

20.57

0.40

10.71

6.33

4.62

0.75

0.08

0.00

0.00

98.34

800A-

58R-1.

86-91

R

51.96

1.09

1.12

21.31

0.53

10.84

5.87

4.59

0.63

0.00

0.00

0.00

97.94

800A-

58R-1.

86-91

C

52.29

1.07

0.77

22.36

0.35

10.14

5.75

4.52

0.47

0.14

0.17

0.00

98.03

800A-

58R-1.

86-91

R

50.83

1.10

1.43

24.57

0.39

8.83

5.90

4.71

0.68

0.06

0.00

0.00

98.49

800A-

58R-1,

86-91

R

50.60

0.30

1.73

26.76

0.59

8.48

7.94

2.05

0.28

0.00

0.00

0.00

97.74

800A-

58R-1.

86-91

R

50.94

0.25

1.94

26.23

0.41

8.21

8.28

2.06

0.28

0.00

0.34

0.00

98.92

800A-

58R-1.

86-91

C

52.27

0.83

1.16

21.62

0.56

10.92

5.58

4.41

0.51

0.07

0.00

0.00

97.93


Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

Fe3+

Mg/Mg+Fe2+

C = Core

R = Rim

7.66

0.11

0.20

1.80

0.05

2.22

0.91

1.36

0.13

0.02

0.00

23.00

15.38

0.97

0.55

7.64

0.15

0.20

2.25

0.07

1.97

1.03

1.32

0.12

0.00

0.02

23.00

15.47

0.74

0.47

7.71

0.13

0.15

2.04

0.06

1.84

0.85

1.33

0.08

0.00

0.04

23.00

15.25

1.07

0.47

7.64

0.17

0.28

2.29

0.47

1.77

0.94

1.29

0.13

0.00

0.03

23.00

15.35

0.81

0.44

7.73

0.14

0.20

1.93

0.05

2.35

1.00

1.32

0.14

0.01

0.00

23.00

15.46

0.61

0.55

7.64

0.12

0.17

1.60

0.07

2.38

0.93

1.31

0.12

0.00

0.00

23.00

15.35

1.02

0.60

7.72

0.12

0.13

1.76

0.04

2.23

0.91

1.29

0.09

0.02

0.02

23.00

15.29

1.00

0.56

7.56

0.12

0.25

2.05

0.05

1.96

0.94

1.36

0.13

0.01

0.00

23.00

15.43

1.01

0.49

7.51

0.03

0.30

1.77

0.07

1.88

1.26

0.59

0.05

0.00

0.00

23.00

14.91

1.43

0.52

7.53

0.03

0.34

1.97

0.05

1.81

1.31

0.59

0.05

0.00

0.04

23.00

14.96

1.28

0.48

7.63

0.09

0.20

1.36

0.07

2.38

0.87

1.25

0.10

0.00

0.00

23.00

15.22

1.28

0.64

318



802A-

58R-3.

90-95

C

49.68

0.04

31.45

0.85

0.00

0.03

15.86

2.28

0.01

0.06

0.35

0.00

100.59

802A-

58R-3.

90-95

R/C

48.32

0.00

32.08

0.65

0.00

0.00

16.71

2.01

0.02

0.10

0.18

0.00

100.07

802A-

58R-3.

90-95

R

49.44

0.03

30.55

0.65

0.00

0.08

15.40

2.44

0.09

0.00

0.00

0.00

98.68

802A-

59R-1.

138-142

C

49.04

0.07

30.90

0.93

0.00

0.12

15.60

2.52

0.04

0.00

0.00

0.00

99.23

802A-

59R-1.

138-142

R/C

49.45

0.09

31.00

0.79

0.00

0.13

15.72

2.62

0.00

0.04

0.12

0.00

99.94

802A-

59R-1.

138-142

R

47.95

0.02

32.68

0.67

0.09

0.22

17.04

1.81

0.08

0.01

0.17

0.00

100.72

802A-

62R-2.

53-59

C

51.11

0.08

30.18

0.77

0.00

0.20

14.43

3.17

0.00

0.08

0.52

0.00

100.53

802A-

62R-2.

53-59

R/C

53.36

0.02

28.80

1.03

0.00

0.28

12.79

4.24

0.02

0.00

0.00

0.00

100.53

802A-

62R-2.

53-59

R

56.83

0.00

25.14

1.22

0.11

0.02

9.85

5.35

0.13

0.06

0.00

0.00

98.70

2.267

0.001

1.691

0.032

0.000

0.002

0.775

0.201

0.001

0.002

0.013

8.000

4.986

0.79

2.221

0.000

1.738

0.025

0.000

0.000

0.823

0.179

0.001

0.004

0.007

8.000

4.998

0.82

2.292

0.001

1.670

0.025

0.000

0.006

0.765

0.219

0.006

0.000

0.000

8.000

4.984

0.78

2.268

0.002

1.685

0.036

0.000

0.009

0.773

0.226

0.003

0.000

0.000

8.000

5.001

0.77

2.271

0.003

1.678

0.030

0.000

0.009

0.774

0.233

0.000

0.001

0.004

8.000

5.003

0.77

2.194

0.001

1.762

0.026

0.003

0.015

0.835

0.161

0.004

0.000

0.006

8.000

5.007

0.84

2.327

0.003

1.620

0.029

0.000

0.013

0.704

0.280

0.000

0.003

0.019

8.000

4.999

0.72

2.415

0.001

1.536

0.039

0.000

0.019

0.620

0.372

0.001

0.000

0.000

8.000

5.003

0.63

2.596

0.000

1.353

0.047

0.004

0.001

0.482

0.474

0.007

0.002

0.000

8.000

4.967

0.50

primitive Seamount compositions (Floyd and Castillo, this volume). Thisis also substantiated by the inclusion of the spinels in highly magnesianolivines and calcic plagioclases. Overall the Fe3+ contents of the spinelsare low and probably indicate that the primitive melts from which thespinels crystallized were extremely reducing (low fθ2). Following theexperimental work of Fisk and Bence (1980), a possible sequence ofevents for the growth of the Hole 801C spinels might be as follows: (1)

the individual microphenocrysts of chromian spinel, with some of thehighest Cr/(Cr + Al) ratios, represent the earliest stage of crystallizationand reflect the most primitive melt compositions which have the highestMg/(Mg + Fe2+) ratios; (2) as olivine crystallizes the Cr/(Cr + Al) ratiodecreases in oli vine-enclosed spinels, possibly due to a change in fθ 2 thateffects the valency state of the Cr, and (3) as plagioclase begins tocrystallize next there is competition for Al in the melt, such that the Cr/(Cr

319


2.5-

CD 2 . 0 -

δ

1.Q

+800A-58R-2, 67-73 cm

"800A-58R-1, 86-91 cm

+ 801C-1R-1, 14-20 cm-801B-43R-1, 4-9 cm

A

2.0 2.5Cations Mg

3.0 Mg Fe

Figure 6. Composition of brown amphiboles from alkalic dolerites and basalts in terms of octahedral components, Al, Mg, and Fe. A. Hole 800A. B. Holes 801Band 801C.

•800A-58R-2, 67-73 cm•B00A-58R-1, 86-91 cm-801C-2R-5, 101-106 cm*801C-1R-1, 14-20 cm-801B-43R-1, 4-9 cm

3.0

2.5

o

2.0

1.0-

Fe Mg0.5

•800A-58R-2, 67-73 cm'800A-58R-1, 86-91 cm"801C-2R-5, 101-106 cm+801C-1R-1, 14-20 cm-801B-43R-1, 4-9 cm

B

2.0 3.0 4.0 5.0Cations Mg

Figure 7. Composition of biotites from alkalic dolerites and basalts in terms of Al, Mg, and Fe components. A. Holes 800A and 801B. B. Hole 801C.

6.0

+ Al) ratio increases again in spinel inclusions within plagioclase hosts,as the Mg/(Mg + Fe2+) ratio decreases.

SECONDARY MINERAL PHASE CHEMISTRY

The majority of the secondary minerals developed in the basalticand doleritic rocks of Leg 129 are Phyllosilicates (smectites andceladonites), together with pyrite, rare zeolite(s), and late replacivecarbonate, all occurring within both the bulk rock and veins.

In this section we will be concerned with only the chemicalcomposition of the Phyllosilicates (Tables 12-14), which were recal-culated on the basis of 22 oxygens in the structural formula. In theaccompanying diagrams and tables no account is taken of the presenceof ferric iron, as according to Velde (1985) there is no unambiguousmethod of allocating Fe3+ between octahedral and tetrahedral sites inclay mineral Phyllosilicates. Adamson (1981), however, in his study

on clay minerals from DSDP Sites 501, 504, and 505, used a recal-culation method that determined the Fe3 + content based uponcharge-balance requirements in the following formula: (EX+)e(Al,Fe3+)m(Mg,Fe2+)(4 + 2t _ m)Si8 _ sAlsO20(OH)4.

In the present study, with no Fe3+ estimation, the total cationaddition will be a maximum for that particular analysis. One usefulfacet (Adamson, 1981) is that by using the total of Si + Al + Mg +Fe* atoms (SAMF), dioctahedral clays will have a SAMF value of12, trioctahedral clays 14, and mica Phyllosilicates 15. As most claysare mixtures of di- and trioctahedral species, values between the twoextremes (12-14) are a function of the interlayering between di- andtrioctahedral clays. A recalculation of the structural formula of chlo-rite from the basis of 28(O) to that of 22(O) would give a SAMF of15.7 for a prochlorite composition. As no X-ray powder diffractionanalysis was performed on these rocks, there is no independentevidence for the presence of chlorite sensu stricto in these samples.

320

•801C-9R-5. 0-7 cm'801C-7R-2. 122-127 cm"801C-6R-4, 49-54 cm+ 801C-5R-5, 73-78 cm«801C-5R-2, 123-130 cm


• 802A-62R-2, 53-59 cm• 802A-59R-3, 40-44 cm• 802A-59R-1, 138-142 cm° 802A-58R-3, 90-95 cm• 802A-58R-2, 99-102 cm- 802A-58R-1, 96-100 cm

Figure 8. Composition of feldspars from tholeiitic basalt flows, in terms of orthoclase (K), albite (Na), and anorthite (Ca) components. A. Hole 801C.B. Hole 802A.

* 801C-9R-5, 0-7 cm» 801C-7R-2. 122-127 cm- 801C-6R-4, 49-54 cm° 801C-5R-5, 73-78 cm

• 802A-62R-2, 53-59 cm• 802A-59R-3, 40-44 cm• 802A-59R-1, 138-142 cm• 802A-58R-3, 90-95 cm+ 802A-58R-2, 99-102 cm• 802A-58R-1, 96-100 cm

Mg Fe Mg

Figure 9. Composition of clinopyroxenes from tholeiitic basalt flows, in terms of Ca, Fe, and Mg. A. Hole 801C. B. Hole 802A.

Fe

Phyllosilicates in Alkalic Dolerites and Basalts

The Phyllosilicates analyzed in the Hole 800A alkaline doleritesoccur either as a replacement phase of pseudohexagonal plates of verylate biotite (bright green coloration) or as alteration patches (brownisholive green colored masses) within the matrix and mesostasis. Thebright green variety is relatively Al- and Fe-rich with Fe/Mg ratios upto 0.825 and an SAMF value of about 15. The brownish olive greenspecies with lower Fe/Mg ratios of 0.306 has an SAMF value of about14. The interlayer cations of both varieties have Na > K < Ca (Fig.13B). Using the criteria of Adamson (1981), the bright green phyl-losilicate is a chlorite-smectite rather than a true chlorite, as it pos-sesses significant interlayer cation occupancy. The brownish olivegreen variety is a smectite with significant Fe-Mg occupancy and isa member of the saponite-ferrosaponite series of trioctahedral smec-tites. True chlorites, together with di- and trioctahedral smectites, havebeen recorded in deeper oceanic basement sequences such as at Sites501, 504, and 505 (Adamson, 1981; Alt et al., 1985).

The alteration pattern in the alkalic basalts of Holes 80IB and 801Cis similar to that in Hole 800A dolerites. In Sample 801C-1R-1, 14-20cm, for example, there are three varieties of phyllosilicate: a fine-grainedamorphous brown variety found as interstitial patches throughout therock, a green fibrous mineral that nucleates on either microfractures inprimary minerals or earlier clay minerals, and platelets that pseudomorpholivine. All three varieties have a different chemistry, which in part reflectsthe composition of the host phase. The mineral commonly replacingolivine is a Mg-saponite close to the boundary with the talc-pyrophylliteseries that is Al-poor (0.34-0.48 cfu) with low Fe/Mg ratio (0.356) andan SAMF of about 14.00. The brown amorphous clay and the green fibershave lower Fe/Mg ratios (0.310 and 0.247, respectively) and higher, butdifferent, Al contents (0.80 and 1.52 cfu, respectively). All three have anSAMF of about 14.00 and are therefore saponites with variable propor-tions of Al in tetrahedral coordination. Other greenish clays, however (forexample, Sample 801B-43R-1,4-9 cm), are highly aluminous (3.64 cfu),with low Fe/Mg (0.33) and SAMF values of about 15.00, and arechlorite-smectite, similar to those in the alkalic dolerites (Table 12).

321


Table 6. Biotite analyses from alkalic dolerites and basalts, Holes 800A, 801B, and 801C.

Hole

Core/sect ion

Interval (cm)

800A-

58R-1.

86-91

800A-

58R-1.

86-91

800A-

58R-1.

86-91

800A-

58R-1.

86-91

800A-

58R-1.

67-73

800A-

58R-1.

67-73

800A-

58R-1.

67-73

800A-

58R-1.

67-73

801 C-

1R-1.

14-20

801C-

1R-1,

14-20

801C-

1R-1.

14-20

FeO

MnO

MgO

CaO

Cr2°3

N i O

R

4 0 . 0 2

5 .09

11 .65

1 2 . 8 7

0 .00

16 .65

0 .08

1.22

8 .67

0 .04

0 .00

9 6 . 3 0

R

39.67

5.23

11.07

1 3 . 3 2

0 . 0 0

16.08

0.00

1.11

8.51

0.01

0.00

95.02

C

39.42

5.07

10.96

15.52

0.13

14.99

0.00

0.86

8.42

0.00

0.23

95.59

C

39.01

5.12

11.50

16 .04

0 .15

14 .81

0 .33

1.00

8 .48

0 .00

0 .10

96 .53

R

4 0 . 4 7

3 .08

8 . 1 2

21 .10

0.10

13 .26

0 .00

0 .69

8 .12

0 .07

0.17

95 .17

C

4 1 . 8 1

2 . 9 9

9 . 5 7

1 7 . 8 9

0.00

15 .35

0.01

0.85

8 .67

0 .08

0 .49

9 7 . 7 0

C

4 1 . 9 0

3 .02

1 0 . 2 2

1 4 . 3 0

0 . 0 5

17.40

0.08

1.04

8.15

0.02

0.18

96.35

R

37.83

4.82

12.18

19.15

0.13

12.40

0.12

0.85

8.67

0.10

0.28

96.53

R

36.61

7.38

13.77

11.26

0.01

15.65

0.14

1.20

8.17

0.03

0.33

94.52

C

36.39

5.09

13.78

14.14

0.32

15.02

0.34

0.89

7.92

0.00

0.00

93.88

c

36.75

7.36

14.38

13.86

0.06

14.85

0.18

1.02

7.67

0.05

0.00

96.17

Recalculation to 22(0)

Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

Mg/Mg+Fe

Footnote:

C

R

5.84

0.56

2.01

1.57

0.00

3.63

0.01

0.34

1.62

0.01

0.00

22.00

15.57

0.70

5.88

0.58

1.93

1.65

0.00

3.55

0.00

0.32

1.61

0.00

0.00

22.00

15.53

0.68

Core

Rim

5.87

0.57

1.93

1.93

0.02

3.33

0.00

0.25

1.60

0.00

0.03

22.00

15.49

0.63

5.78

0.57

2.01

1.99

0.02

3.27

0.05

0.29

1.60

0.00

0.01

22.00

15.58

0.62

6.21

0.36

1.47

2.71

0.01

3.03

0.00

0.20

1.59

0.01

0.02

22.00

15.57

0.53

6.14

0.33

1.66

2.20

0.00

3.36

0.00

0.24

1.63

0.01

0.06

22.00

15.56

0.60

6.11

0.33

1.76

1.75

0.01

3.78

0.01

0.29

1.52

0.00

0.02

22.00

15.56

0.68

5.71

0.55

2.17

2.42

0.02

2.79

0.02

0.25

1.67

0.01

0.03

22.00

15.57

0.54

5.43

0.82

2.41

1.40

0.00

3.46

0.02

0.35

1.55

0.00

0.04

22.00

15.44

0.71

5.50

0.58

2.46

1.79

0.04

3.38

0.05

0.26

1.53

0.00

0.00

22.00

15.59

0.65

5.39

0.81

2.49

1.70

0.01

3.25

0.03

0.29

1.44

0.01

0.00

22.00

15.40

0.66

Bright green celadonite occurs with smectites in Sample 801C-2R-5, 101-106 cm, replacing mesostasis. The interlayer charge is about1.80, Ex+ is largely K+, tetrahedral sites are filled by Si, and octahedralcation proportions are Al > Mg > Fe, which is unusual for celadonites.The smectite that occurs with this celadonite has a similar octahedralsite occupancy and Fe/Mg ratio (0.555, compared to 0.672 in theceladonite), and an SAMF of about 13.00, denoting that the interlay -

ering is approximately 50:50 di- to trioctahedral smectite. Similarity inoctahedral cation occupancy appears to be a characteristic of coexistingsmectites and celadonites in basaltic rocks and may be a function ofthe replacement of early smectites by later celadonites. Fig. 13A(modified from Velde, 1985) shows that divalent cations dominate theoctahedral sites (3R2+) with K-rich celadonite compositions separatedfrom smectites that display highly variable Na and Ca distributions.

322



801C-

1R-1.

U-20

801C-

1R-1.

U-20

801C-

1R-1.

U-20

801C-

1R-1.

U-20

801C-

1R-1.

U-20

801B-

44R-2.

140-145

801 B-

44R-2.

140-145

E/R

50.57

4.46

0.45

19.27

6.75

0.29

0.23

6.51

10.48

0.03

99.04

1.97

0.13

0.02

0.57

0.22

0.01

0.01

0.27

0.79

0.00

6.00

4.00

R

50.44

4.28

0.64

10.99

14.24

0.81

0.35

11.16

7.37

0.00

100.28

1.98

0.13

0.03

0.32

0.47

0.03

0.02

0.47

0.56

0.00

6.00

4.00

R

50.44

4.28

0.64

10.99

14.24

0.81

0.35

11.16

7.37

0.00

100.28

1.98

0.13

0.03

0.32

0.47

0.03

0.02

0.47

0.56

0.00

6.00

4.00

E/R

51.10

4.47

0.64

14.14

11.70

0.43

0.19

8.52

9.00

0.00

100.19

1.99

0.13

0.03

0.41

0.38

0.01

0.01

0.36

0.68

0.00

6.00

4.00

E/R

50.97

4.87

0.69

13.47

11.83

0.52

0.17

8.33

9.05

0.01

99.91

1.99

0.14

0.03

0.40

0.39

0.02

0.01

0.35

0.68

0.00

6.00

4.00

R

50.74

5.11

2.06

20.67

4.74

0.30

1.80

2.01

11 .72

0.03

99.18

1.94

0.15

0.09

0.60

0.15

0.01

0.10

0.08

0.87

0.00

6.00

4.00

E/R

50.88

3.97

2.39

22.65

3.32

0.25

2.20

1.90

11.75

0.00

99.31

1.94

0.11

0.11

0.65

0.11

0.01

0.13

0.13

0.08

0.87

6.00

4.00

Phyllosilicates in Tholeiitic Basalts

There are many similarities between the chemistry of the secon-dary Phyllosilicates in the alkali basalts (Fig. 13) and those in thetholeiitic basalts (Figs. 14 and 15) with both smectites (of variablecomposition) and celadonites present (Tables 13 and 14, respec-tively). Although smectite analyses from the same sample tend togroup close together, part of the spread reflects the nature of thereplaced host, for example, olivine relative to glassy mesostasis.

Color variation in smectites from Site 801 tholeiites is common,although most of the yellow-brown and dark brown varieties replac-ing interstitial material are intermediate tri- and dioctahedral smec-tites with SAMF values of about 13. The dark brown varieties areinvariably more Mg- and Al-rich than the yellow-brown species, with

distinct Fe/Mg ratios (0.720 and 1.050 respectively). The dominantgreenish Phyllosilicates of Hole 801C below the hydrothermal depositare Fe-rich saponites and celadonites (Tables 13 and 14).

Phyllosilicates analyzed in the Hole 802A hypocrystalline tholeiiticbasalts are almost exclusively interstitial and represent the replacementof original glass, although smectite pseudomorphs after olivine occur inSample 802A-58R-2,99-102 cm. The smectites are all highly siliceous(the majority of the analyses yield Si >7.00 cfu; Table 10) and recal-culate to a saponite-ferrosaponite formula (Guven, 1988), that is,Ex+(Mg - Fe)6(Si8 - JCAI) 020(OH)4, where Ex+ represents the interlayercation. Figure 15B demonstrates the mutual replacement of Mg by Fe inthe smectites, and the clustering of data for specific samples suggeststhat their composition is influenced by bulk-rock Fe/Mg ratios. Ingeneral, saponite Fe/Mg ratios from replaced glass are lower and morevariable (about 0.43-0.65) than those replacing olivine (0.77). Al substi-tution for Si in the tetrahedral sites is extremely limited (Fig. 15)where the Si values tightly group between 7.0 and 7.5 cfu. The excessof Al over that required to fill the tetrahedral sites must indicate that thereis limited solid solution either toward chlorite compositions or di-octa-hedral smectites of beidellite composition. The preponderance of divalentover trivalent cations is demonstrated in Figure 15A, where a tightclustering of smectite compositions is again observed. The total inter-layer cation content of the saponites never exceeds 0.60 cfu, of whichthe dominant interlayer cation is Ca (usually 0.2-0.3 cfu) with smallercontents of Na and K (Fig. 15C). All the analyses have an SAMF of13-14 and total interlayer cations <0.6, and are therefore dominantlytrioctahedral smectites or saponites.

The Hole 802A celadonites (Fig. 15 and Table 14) occur ininterstitial patches and have recalculated Si values >8.00 and Alabout 0.50 cfu. In comparison to the saponite smectites the Fe/Mgratio in the celadonites is higher and more varied, ranging between1.569 (Sample 802A-58R-2, 99-102 cm) and 3.896 (Sample 802A-59R-1, 138-142 cm). The interlayer cation is almost exclusively Kwith only minor amounts of Na and Ca.

Carbonates in Tholeiitic Basalts

Carbonates, intimately associated with Phyllosilicates, are presentin many of the alkalic and tholeiitic basalts, replacing matrix and indi-vidual phases, and also commonly occur as a fracture vein infilling. InHole 801C, vein carbonate associated with celadonite (for example,Sample 801C-5R-3, 125-131 cm) exhibits layers deposited in a se-quential manner with granular carbonate material adjacent to the wallsand coarser rhombic crystals in the vein interior. Individual layers showconsiderable variation in terms of the molecular proportions of Ca, Mg,and Fe carbonate end-members (Table 15) and imply that the fluidspassing through the vein had highly variable compositions, initiallyMg and Fe-rich relative to Ca. The carbonates vary from ferroandolomites to sideritic carbonates to calcite, with the latter forming thelast phase to crystallize in the vein interior.

SUMMARY AND CONCLUSIONS

In this section we briefly summarize the main compositionalvariations of the primary and secondary phases in Leg 129 basalticrocks and relate systematic chemical changes in the major magmaticsilicates to simple petrogenetic schemes.

Alkalic Dolerites and Basalts

The alkalic dolerite sills (Hole 800A) and alkalic basalt flows(Site 801) are similar petrographically, but differ slightly in theirrelative degrees of chemical fractionation and levels of incompatibletrace elements. Both the sills and lava flows have evolved basiccompositions (commonly hawaiites), which are also reflected in thecomposition of the feldspars (with labradorite cores), presence ofabundant titanomagnetite, and late-stage hydrous phases.

323


Table 7. Plagioclase analyses from tholeiitic basalts, Holes 801C and 802A.

Hole

Core/section

Interval (cm)

SiO

TiO2

A1 2O 3

FeO

MnO

MgO

CaO

Na20

K20

Cr 2O 3

NiO

0

TOTAL'/.

801C-

5R-2.

123-130

C

47.98

0.08

33.14

0.47

0.09

0.00

17.14

1.84

0.01

0.00

0.02

0.00

100.78

801C-

5R-2.

123-130

R

49.04

0.00

31.87

0.47

0.00

0.00

16.07

2.32

0.00

0.16

0.00

0.00

99.94

801C-

5R-2.

123-130

G

52.52

0.13

27.45

1.91

0.00

0.75

12.78

3.61

0.27

0.10

0.22

0.00

99.75

801C-

6R-4,

49-54

C

44.92

0.09

34.10

0.35

0.00

0.28

18.02

1.10

0.00

0.00

0.25

0.00

99.10

801C-

6R-4,

49-54

S

50.87

0.21

28.10

1.24

0.01

0.33

12.94

3.95

0.00

0.00

0.11

0.00

97.74

801C-

6R-4,

49-54

S

61.81

0.19

22.42

1.39

0.00

0.00

4.87

8.29

0.13

0.06

0.09

0.00

99.26

801C-

6R-4.

49-54

C

46.20

0.03

33.55

0.20

0.00

0.08

17.78

1.28

0.03

0.01

0.00

0.00

99.14

801C-

9R-5,

0-7

C

52.32

0.06

29.33

0.82

0.00

0.14

13.45

3.79

0.00

0.09

0.12

0.00

100.11

801C-

9R-5,

0-7

R

60.66

0.10

23.14

1.02

0.00

0.19

6.47

7.38

0.06

0.00

0.15

0.00

99.19

802A-

58R-2.

99-102

C

51.43

0.00

30.23

0.79

0.00

0.32

14.60

3.25

0.00

0.08

0.00

0.00

100.71

802A-

58R-2.

99-102

R

49.27

0.06

31.01

0.81

0.10

0.20

15.80

2.63

0.05

0.03

0.16

0.00

100.11


Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

Ca/Ca+Na

Footnote:

C

R

= Core

= Rim

2.19

0.00

1.78

0.02

0.00

0.00

0.84

0.16

0.00

0.00

0.00

8.00

5.00

0.84


G

S

= Groundmass

= Spheruli tic groundmass

2.25

0.00

1.72

0.02

0.00

0.00

0.79

0.21

0.00

0.01

0.00

8.00

4.99

0.79

2.41

0.00

1.49

0.07

0.00

0.05

0.63

0.32

0.02

0.00

0.01

8.00

5.01

0.66

2.10

0.00

1.87

0.01

0.00

0.02

0.90

0.10

0.00

0.00

0.01

8.00

5.01

0.90

2.38

0.01

1.55

0.05

0.00

0.02

0.65

0.36

0.00

0.00

0.00

8.00

5.02

0.64

2.78

0.01

1.19

0.05

0.00

0.00

0.24

0.73

0.01

0.00

0.00

8.00

4.99

0.24

2.14

0.00

1.84

0.01

0.00

0.01

0.88

0.12

0.00

0.00

0.00

8.00

4.99

0.88

2.38

0.00

1.57

0.03

0.00

0.01

0.66

0.33

0.00

0.00

0.00

8.00

4.99

0.66

2.73

0.00

1.23

0.04

0.00

0.01

0.31

0.64

0.00

0.00

0.01

8.00

4.98

0.33

2.33

0.00

1.62

0.03

0.00

0.02

0.71

0.29

0.00

0.00

0.00

8.00

5.00

0.71

2.26

0.00

1.68

0.03

0.00

0.01

0.78

0.23

0.00

0.00

0.01

8.00

5.01

0.77



802A-

58R-3,

90-95

802A-

58R-3.

90-95

802A-

58R-3.

90-95

802A-

59R-1.

138-142

802A-

59R-1.

138-142

802A-

59R-1,

138-142

802A-

62R-2.

53-59

802A-

62R-2.

53-59

802A-

62R-2.

53-59

c

49.68

0.04

31.45

0.85

0.00

0.03

15.86

2.28

0.01

0.06

0.35

0.00

R/C

48.32

0.00

32.08

0.65

0.00

0.00

16.71

2.01

0.02

0.10

0.18

0.00

100.59 100.07

R

49.44

0.03

3 0 . 5 5

0 . 6 5

0 . 0 0

0 . 0 8

15.40

2.44

0.09

0.00

0.00

0.00

98.68

C

49.04

0.07

30.90

0.93

0.00

0.12

15.60

2.52

0.04

0.00

0.00

0.00

99.23

R/C

49.45

0.09

31.00

0.79

0.00

0.13

15.72

2.62

0.00

0.04

0.12

0.00

99.94

R

47.95

0.02

32.68

0.67

0.09

0.22

17.04

1.81

0.08

0.01

0.17

0.00

100.72

C

51.11

0.08

3 0 . 1 8

0 . 7 7

0 . 0 0

0 . 2 0

14.43

3.17

0.00

0.08

0.52

0.00

R/C

53.36

0.02

28.80

1.03

0.00

0.28

12.79

4.24

0.02

0.00

0.00

0.00

1 0 0 . 5 3 1 0 0 . 5 3

R

56.83

0.00

25.14

1.22

0.11

0.02

9.85

5.35

0.13

0.06

0.00

0.00

98.70

2.27

0.00

1.69

0.03

0.00

0.00

0.78

0.20

0.00

0.00

0.01

8.00

4.99

0.79

2.22

0.00

1.74

0.03

0.00

0.00

0.82

0.18

0.00

0.00

0.01

8.00

5.00

0.82

2.29

0.00

1.67

0.03

0.00

0.01

0.77

0.22

0.01

0.00

0.00

8.00

4.98

0.78

2.27

0.00

1.69

0.04

0.00

0.01

0.77

0.23

0.00

0.00

0.00

8.00

5.00

0.77

2.27

0.00

1.68

0.03

0.00

0.01

0.77

0.23

0.00

0.00

0.00

8.00

5.00

0.77

2.19

0.00

1.76

0.03

0.00

0.02

0.84

0.16

0.00

0.00

0.01

8.00

5.01

0.84

2.33

0.00

1.62

0.03

0.00

0.01

0.70

0.28

0.00

0.00

002

8.00

5.00

0.72

2.42

0.00

1.54

0.04

0.00

0.02

0.62

0.37

0.00

0.00

0.00

8.00

5.00

0.63

2.60

0.00

1.35

0.05

0.00

0.00

0.48

0.47

0.01

0.00

0.00

8.00

4.97

0.50

Although erupted under different conditions, both groups of alka-lic rocks show a number of similar chemical features.

1. The feldspars are commonly zoned with calcic cores (com-monly labradorite in Hole 800A; labradorite-bytownite at Site 801)to sodic rims that invariably have a high orthoclase component, suchthat many rim zones are essentially alkali feldspars (maximum up toOr69). Apart from normal zoning, oscillatory zoning may be present(again labradorite core to alkali feldspar rim), reflecting rapid changesin melt composition.

2. Quench feldspar microlites in mesostasis are alkali feldspars withcompositions that overlap that of the alkalic rims of the matrix laths.

3. The pinkish clinopyroxenes are titaniferous diopsides/augites(Hole 800A) or exclusively diopsides (Site 801) with high non-quad-rilateral component contents. They show little fractionation and rimzones are not markedly Fe-rich relative to cores. One major differencewas noted here in that some of the Site 801 diopsides displayed alkalipyroxene (aegirine-augite) rims separated by a sharp chemical dis-continuity from the rest of the host pyroxene.

4. Brown amphibole was a rare hydrous silicate in both groupsand proved to be a low-temperature, Al-poor, winchite-ferriwinchite.

5. Biotite was by far the most abundant hydrous phase in bothalkalic groups and developed either very late in the magmatic sequence(Site 801) or during and after final consolidation (Hole 800A). The

325


Table 8. Clinopyroxene analyses from tholeiitic basalts, Holes 801C and 802A.

Hole

Core/section

Interval (cm)

s io 2

TiO2

AL2°3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2O3

NiO

0

TOTAL1/.

801C-

6R-4,

49-54

D

40.99

1.75

17.50

6.97

0.16

8 . 5 4

2 3 . 2 2

0 .54

0 .08

0 .00

0 .00

0 .00

99 .73

801C-

6R-4.

49-54

D

43.02

1.54

17.10

5.39

0.19

9.10

23.15

0.83

0.00

0.20

0.04

0.00

100.54

8Q1C-

6R-4.

49-54

S

45.96

0.92

1.35

28.27

1.05

3.95

16.48

0.45

0.07

0.14

0.00

0.00

98.63

801C-

6R-4.

49-54

S

48.17

1.15

1.72

20.72

0.58

8.60

17.56

0.38

0.00

0.00

0.13

0.00

98.85

801C-

6R-4.

49-54

C

50.14

1.23

2.79

13.08

0.31

13.66

18.58

0.53

0.00

0.06

0.10

0.00

100.47

801C-

6R-4.

49-54

R

48.26

1.21

1.27

23.88

0.83

7.54

15.74

0.34

0.00

0.00

0.22

0.00

99.28

801C-

7R-2.

122-127

G

51.09

1.13

4.03

7.68

0 .22

15 .64

19 .83

0.44

0 .03

0 .35

0 .29

0 .00

100 .72

801C-

7R-2,

122-127

H

53.80

0.41

1.91

8.10

0.18

19.46

15.80

0 .43

0 .00

0 . 2 9

0 .30

0 .00

1 0 0 . 6 6

801C-

7R-2.

122-127

H

50.49

1.58

4.16

11.31

0.25

15.92

16.27

0.28

0.00

0 .16

0 .00

0 .00

100 .42

801C-

9R-5.

0-7

C

51.05

0.52

3.41

6.45

0.01

16.46

19.61

0.49

0.04

0.82

0.00

0.00

98.86

801C-

9R-5,

0-7

R

52.14

0.62

2.92

6.80

0.12

16.50

20.41

0.23

0.04

0.38

0.22

0.00

100.39


Si

T i

A l

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

T o t a l

1.54

0 .05

0 . 7 8

0 .22

0.01

0.48

0.94

0.04

0.00

0.00

0.00

6.00

3.77

1.57

0.04

0.74

0.17

0.01

0.50

0.92

0.06

0.00

0.01

0.00

6.00

19.31

1.91

0.03

0.07

0.98

0.04

0.25

0.73

0.04

0.00

0.00

0.00

6.00

99.14

1.92

0.04

0 .08

0 .68

0.02

0.51

0.75

0.03

0 .00

0 .00

0.00

6 .00

1 .89

0.04

0.12

0.41

0.01

0.77

0.75

0.04

0.00

0.00

0.00

6.00

1.93

0.04

0.06

0.80

0.03

0.45

0.68

0.03

0.00

0.00

0.01

6.00

2.57 129.06 123.27

1.88

0.03

0.17

0.24

0.01

0.86

0.78

0.03

0.00

0.01

0.01

6.00

2.78

1.95

0.01

0.08

0.25

0.01

1.05

0.61

0.03

0.00

0.01

0.01

6.00

0.23

1.87

0.04

0.18

0.35

0.01

0.88

0.65

0.02

0.00

0.01

0.00

6.00

2.12

1.90

0.01

0 .15

0.20

0.00

0.91

0.78

0.04

0.00

0.02

0.00

6.00

1.67

1.91

0.02

0.13

0.21

0.00

0.90

0.80

0.02

0.00

0.01

0.01

6.00

0.00

Footnote:

D

S

H

Dendr i t ic inclusions

Spheru l i t i c groundmass

Hour-glass zoned c rys ta l



802A-

58R-3,

90-95

802A-

58R-3.

90-95

802A-

58R-3.

90-95

802A-

59R-3.

40-44

802A-

59R-3.

40-44

802A-

59R-3.

40-44

802A-

62R-2.

53-59

802A-

62R-2.

53-59

802A-

62R-2.

53-59

R

5 3 . 1 3

0 . 3 2

2 . 0 8

7 . 0 3

0 . 2 2

1 7 . 8 4

19.14

0.19

0.01

0.17

0 . 1 2

0 . 0 0

1 0 0 . 2 5

C

5 2 . 1 3

0 . 6 7

6 . 3 0

8 . 2 9

0 . 2 3

1 4 . 3 0

16.61

0.79

0.08

0.16

0.00

0.00

99.54

C

52.71

0.26

2.54

7 . 1 0

0.19

17.81

1 8 . 5 6

0 . 3 7

0 . 0 3

0 . 4 0

0 . 0 5

0 . 0 0

1 0 0 . 0 0

c

5 3 . 1 0

0 . 2 6

2 . 0 5

6 . 5 2

0 . 2 7

1 7 . 6 8

1 9 . 4 9

0.11

0.03

0 . 5 0

0 . 3 8

0 . 0 0

1 0 0 . 3 7

c

5 3 . 5 5

0 . 2 2

1.88

6 . 8 6

0 . 1 5

18.69

18.42

0.31

0.00

0 . 4 1

0 . 3 2

0 . 0 0

100.81

R

52.69

0.29

1.95

6.28

0.34

17.41

19.55

0.11

0.03

0.36

0.22

0.00

99.21

C

52.73

0.44

0.91

2 2 . 4 3

0 . 4 1

18.46

4.78

0.36

0.00

0.09

0.28

0.00

100.88

R

49.73

0.82

2.05

18.90

0.45

12.69

14.42

0.39

0.07

0.12

0.43

0.00

100.05

R

50.32

0.82

1.65

22.77

0.55

12.03

11.45

0.35

0.00

0.08

0.00

0.00

100.02

1.94

0.01

0.09

0.22

0.01

0.97

0.75

0.01

0.00

0.01

0.00

6.00

800.88

1.91

0.02

0.27

0.25

0.01

0.78

0.65

0.06

0.00

0.01

0.00

6.00

15.51

1.93

0.01

0.11

0.22

0.01

0.97

0.73

0.03

0.00

0.01

0.00

6.00

4.01

1.94

0.01

0.09

0.20

0.01

0.96

0.76

0.01

0.00

0.01

0.01

6.00

4.01

1.94

0.01

0.08

0.21

0.01

1.01

0.72

0.02

0.00

0.01

0.01

6.00

4.02

1.95

0.01

0.09

0.19

0.01

0.96

0.77

0.01

0.00

0.01

0.01

6.00

4.00

1.98

0.01

0.04

0.70

0.01

1.03

0.19

0.03

0.00

0.00

0.01

6.00

4.00

1.92

0.02

0.09

0.61

0.02

0.73

0.60

0.03

0.00

0.00

0.01

6.00

4.03

1.95

0.02

0.08

0.74

0.02

0.69

0.48

0.03

0.00

0.00

0.00

6.00

4.00

late-magmatic biotites were distinctive in having higher Ti contentsand Mg/Fe ratios, indicative of higher temperatures of formation.

In both alkalic groups, minor olivine and plagioclase phenocrystswere the first phases to crystallize, although most units are aphyric incharacter. A broadly similar petrogenetic scheme can be consideredcommon to both sills and lava flow interiors. The strongly zoned matrixfeldspar has cores of similar composition to plagioclase phenocrystsand crystallized under nonequilibrium conditions that gradually be-came more sodic. Elongate prisms of titaniferous diopside crystallizedat about the same time or slightly later, but did not undergo major

internal fractionation (uniform Fe/Mg ratios) due to the precipitationof abundant titanomagnetite that reduced the Fe and Ti content of theremaining melt. As anhydrous crystallization proceeded, the watercontent of the melt built up and fθ 2 increased, allowing the growth ofminor brown amphibole (with high Fe3+). Any melt remaining was nowK-rich and produced alkalic rims to the plagioclases and precipitatedmagmatic biotite that nucleated on the opaques. Na-rich fluids alsodeveloped alkali pyroxene rims to the diopsides in Site 801 basalt flowinteriors. The final melt was quenched as glassy mesostasis withinwhich alkali feldspar microlites rapidly grew.

327

G. ROWBOTHAM, P.A.FLOYD

Table 9. Olivine analyses from tholeiitic basalts, Holes 801C and 802A.

Hole

Core/sect ion

Interval (cm)

sio2

TiO2

A12O3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2O3

NiO

TOTAL'/.

801C-

6R-4.

49-54

C

39.34

0.04

0.00

11.58

0.28

45.62

0.28

0.16

0.00

0.04

0.23

97.57

801C-

6R-4,

49-54

C

39.79

0.07

0.00

9.76

0.13

46.43

0.36

0.00

0.00

0.25

0.38

97.17

801C-

6R-4.

49-54

C

38.64

0.00

0.00

13.61

0.00

44.48

0.44

0.45

0.00

0.05

0.00

97.67

801C-

6R-4,

49-54

C

39.19

0.07

0.00

12.79

0.22

44.79

0.53

0.21

0.00

0.00

0.28

98.14

801C-

6R-4.

49-54

C

38.59

0.00

0.00

13.66

0.16

43.52

0.39

0.35

0.05

0.12

0.21

97.07

801C-

6R-4.

49-54

R

39.64

0.02

0.00

10.64

0.07

45.81

0.27

0.13

0.02

0.00

0.38

96.97

801C-

6R-4.

49-54

R

39.00

0.12

0.00

12.95

0.21

44.60

0.26

0.25

0.06

0.00

0.03

97.48

801C-

6R-4.

49-54

C

38.46

0.06

0.00

15.33

0.35

42.98

0.32

0.00

0.09

0.00

0.14

97.73

801C-

6R-4.

49-54

R

39.28

0.00

0.00

9.46

0.24

46.26

0.36

0.08

0.00

0.11

0.63

96.42

801C-

6R-4,

49-54

C

40.40

0.10

0.39

10.66

0.27

46.92

0.25

0.33

0.01

0.01

0.34

99.67

801 C-

6R-4.

49-54

c

39.31

0.10

0.00

9.58

0.04

45.81

0.34

0.00

0.00

0.09

0.33

95.60


Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

Mg/Mg+Fe

1.00

0.00

0.00

0.25

0.01

1.73

0.01

0.01

0.00

0.00

0.00

4.00

3.00

1.01

0.00

0.00

0.21

0.00

1.75

0.01

0.00

0.00

0.01

0.01

4.00

2.99

0.99

0.00

0.00

0.29

0.00

1.70

0.01

0.02

0.00

0.00

0.00

4.00

3.02

1.00

0.00

0.00

0.27

0.01

1.70

0.02

0.01

0.00

0.00

0.01

4.00

3.02

1 .00

0.00

0.00

0.30

0.00

1.72

0.01

0.02

0.00

0.00

0.00

4.00

3.05

1.01

0.00

0.00

0.23

0.00

1.74

0.01

0.01

0.00

0.00

0.01

4.00

3.00

1.00

0.00

0.00

0.28

0.00

1.70

0.01

0.01

0.00

0.00

0.00

4.00

3.01

0.99

0.00

0.00

0.33

0.01

1.66

0.01

0.00

0.00

0.00

0.00

4.00

3.00

1.00

0.00

0.00

0.20

0.01

1.76

0.01

0.00

0.00

0.00

0.01

4.00

3.00

1.00

0.00

0.01

0.22

0.01

1.73

0.01

0.02

0.00

0.00

0.01

4.00

3.00

1.01

0.00

0.00

0.21

0.00

1.75

0.01

0.00

0.00

0.00

0.01

4.00

2.99

0.88 0.90 0.85 0.86 0.85 0.89 0.86 0.83 0.90 0.89 0.90

Footnote .-

C = Core

R = Rim

328



801C- 802A- 802A- 802A- 802A- 802A- 802A- 802A-

6R-4. 58R-2. 58R-2, 58R-2. 58R-2. 58R-2. 58R-2. 58R-2.

49-54 99-102 99-102 99-102 99-102 99-102 99-102 99-102

R

39.02

0.00

0.10

14.19

0.28

44.01

0.39

0.20

0.00

0.10

0.17

98.52

1.00

0.00

0.00

0.30

0.01

1.67

0.01

0.01

0.00

0.00

0.00

4.00

3.01

C

39.93

0.00

0.26

18.12

0.38

42.14

0.32

0.16

0.02

0.00

0.68

101.99

1.00

0.00

0.01

0.38

0.01

1.57

0.01

0.01

0.00

0.00

0.01

4.00

3.00

C

39.90

0.00

0.39

19.13

0.25

42.25

0.36

0.14

0.02

0.03

0.47

102.92

0.99

0.00

0.01

0.40

0.01

1.57

0.01

0.01

0.00

0.00

0.01

4.00

3.00

R

39.92

0.00

0.25

18.46

0.06

42.03

0.29

0.36

0.02

0.00

0.83

102.22

1.00

0.00

0.01

0.39

0.00

1.57

0.01

0.02

0.00

0.00

0.02

4.00

3.01

C

39.64

0.00

0.28

18.38

0.34

42.04

0.35

0.17

0.00

0.00

0.40

101.59

1.00

0.00

0.01

0.39

0.01

1.58

0.01

0.01

0.00

0.00

0.01

4.00

3.00

R

39.78

0.00

0.29

18.71

0.26

42.33

0.33

0.24

0.01

0.07

1.04

103.05

0.99

0.00

0.01

0.39

0.01

1.57

0.01

0.01

0.00

0.00

0.02

4.00

3.01

C

39.63

0.05

0.32

18.79

0.28

42.48

0.36

0.43

0.00

0.00

0.60

102.94

0.99

0.00

0.01

0.39

0.01

1.58

0.01

0.02

0.00

0.00

0.01

4.00

3.02

c

39.52

0.08

0.15

18.10

0.23

41.35

0.36

0.00

0.03

0.13

0.41

100.35

1.01

0.00

0.00

0.39

0.01

1.57

0.01

0.00

0.00

0.00

0.01

4.00

2.99

0.85 0.81 0.80 0.80 0.80 0.80 0.80 0.80

Tholeiitic Basalts

The relatively uniform chemical composition of the pillow lavas fromHole 802A is reflected in the major silicate phases. Phenocrystic olivine(Fo80) and low-Ti augite show no variation in Fe/Mg ratios, althoughminor components can be highly variable (for example, Cr, Ti, and Al inclinopyroxene). Plagioclase megaphenocrysts do show some minorzoning (An86 to An50), although there is general compositional overlapbetween microphenocrysts and quenched laths in the matrix.

More varied chemistry is exhibited by the major phases in Site 801lavas, especially between mineral species that crystallized at differentstages in the early glomerophyric groups, as microphenocrysts, andfinally as quenched variolites. The primitive group of tholeiitic lavas ischaracterized by extreme compositions of the main phenocrystic phases:chrome spinels (Cr/Cr + Al = 0.2-0.4), very Ni-rich olivines (Fo^), andhighly calcic plagioclase (AI^Q). The compositional range of the phe-nocrysts is typical of some types of MORB and, in particular, charac-teristic of very primitive melt compositions. Clinopyroxene is a minor

329


* 801C-9R-5, 0-7 cm0 801C-7R-2, 122-127 cmΔ801C-6R-4, 49-54 cmQ801C-5R-5, 73-78 cm

0.4 0.6Al cations

0.05

0.04-

* 802A-62R-2, 53-59 cmo 802A-59R-3, 40-44 cm

8 0 2 A 5 9 R 1 1 3 8 1 4 2802A-59R-1, 138-142 cm802A-58R-3, 90-95 cm

99-102 cm96-100 cm

802A-58R-2,802A-58R-1

LO 0.1 0.2 0.3 0.4 0.5Al cations

Figure 10. Minor components, Ti and Al, in clinopyroxenes from tholeiitic basalt flows. A. Hole 801C. B. Hole 802A.

phase in these basalts, but is present in the chemically evolved groupas zoned diopside-augite microphenocrysts. Spinel occurs both asmicro-phenocrysts (with high Cr) and also as inclusions within oli-vine and plagioclase which display chemical variation depending onthe precipitation of other major silicates. Compositional variationrelative to crystallization history is exhibited by plagioclase: individualmegacrysts and those in glomerophyric clumps are the most calcic (An90;some zoned to An50) and microphenocrysts are marginally more sodic(An75^80), whereas plagioclase microlites and serrated laths in variolitesshow the greatest variation (An65_24).

Crystallization history for the primitive tholeiites starts with thegrowth of euhedral Cr-rich spinel microphenocrysts, closely followed byolivine and then calcic plagioclase. However, during olivine precipitationfurther spinel growth occurred but with a lower Cr content (possibly dueto an increase in fθ2). Renewed spinel growth also occurred duringplagioclase precipitation when competition for Al once again increasedspinel Cr/Cr + Al ratios. A possible earlier phase of high-pressure

0.4i

co'σθ.31o

CD

0.

801C-6R-4, 49-54 cm

.5 1.6 1.7Mg Cations

1.8

crystallization may be recorded by high-Al pyroxene (fassaite)-bear-ing spheroids enclosed in the olivine microphenocrysts. After precipi-tation of the major primitive phenocryst compositions (in a magmachamber) further microphenocrystic growth in glomerophyric clumpsmay have taken place nearer the seafloor during upward transport.Most microphenocrysts of plagioclase and clinopyroxene are stronglyzoned and indicative of rapidly changing melt conditions. Quenchingof the remaining melt on extrusion produced clinopyroxene andplagioclase variolites with the most evolved chemical compositions.

It has been suggested that chemical subgroups within the evolvedtholeiitic group of Hole 801 are a consequence of magma mixingbetween fractionated compositions and a series of more primitive meltinputs (Floyd and Castillo, this volume). The evolved tholeiites aregenerally aphyric or poorly olivine and plagioclase-clinopyroxeneglomerophyric and apparently do not contain resorbed and stronglyzoned megaphenocrysts (for example, plagioclase) of primitive com-position similar to those in the primitive group tholeiites. Also,phenocryst phases have not been analyzed in detail to confirm if theyhave core compositions more primitive than expected for their hostmelt composition, although some matrix plagioclases in flow interiorsare zoned with bytownite cores. At present, magma mixing is basedlargely on bulk rock-chemical features, rather than strong petro-graphic evidence and phase analysis.

Table 10. Glass analyses from tholeiitic basalt Sample 129-802A-58R-2,99-102 cm.

Figure 11. Compositional variation in olivines from Sample 129-801C-6R-4,49-54 cm, in terms of Mg and Fe components.

Analysis

SiO2

TiO2

A1 2 O 3

FeOMnOMgOCaONa2OK2O

Total

MgO/MgO + FeO

1

50.881.16

13.2911.830.256.89

11.832.240.16

98.53

0.37

2

51.391.17

13.3711.550.136.75

12.152.210.13

98.85

0.37

3

51.771.22

13.4011.800.216.92

11.262.200.14

98.92

0.37

4

51.621.21

13.6611.370.236.92

11.452.350.16

98.97

0.38

5

51.651.22

13.5511.700.326.78

11.382.230.13

98.96

0.37

6

51.621.19

13.4811.890.136.98

11.372.160.17

98.99

0.37

330


4.0-1

3.5-

3.0-

2.5-

2.0-

1.

801B-44R-3, 28-33 cm

.2 0.3 0.4Mg/Mg + Fe2+

0.5

0.10-1

0.08-

0.06-

0.04-

0.02-

0.0

B801C-6R-4, 49-54 cm801C-5R-5, 73-78 cm

%.5 0.6 0.7 0.8 0.9 1.0Mg/Mg + Fe2+

0.50-]

0.45-

<

0 0.40-

ò0.35-

0.3.2

0.55i

(D0.45-

u^0.351

Φ

0.2

0.3 0.4Mg/Mg + Fe2+

0.5

.25 0.30 0.35 0.40Mg/Mg + Fe2+

0.45

0.5i

0.4-I

<

CJ

o

0.2-

0.10.5 0.6 0.7 0.8 0.9 1.0

Mg/Mg + Fe2+

o

CD

0.16i

0.12-

0.08-

0.04-

0.0.5 0.6 0.7 0.8 0.9

Mg/Mg + Fe2+1.0

Figure 12. Composition of spinels from Holes 80IB and 801C in terms of Ti content, and Cr/(Cr + Al) and Fe3+/(Fe3+ + Cr + Al) ratios.A. Titaniferous spinels from alkalic intrusive, Sample 129-801B-44R-3, 28-33 cm. B. Chrome spinels from primitive tholeiites, Samples129-801C-5R-5, 73-78 cm and 801C-6R-4, 40-54 cm.

331


Table 11. Spinel analyses from tholeiitic basalts, Holes 801B and 801C.

Hole

Core/section

Interval (cm)

s*o2

TiO2

Al 2 0 3

FeO

MnO

MgO

CaO

Na 0

K20

Cr2°3

NiO

0

TOTAL"/.

801B-

44R-3.

28-33

P

1.03

12.50

11.91

51.01

0.39

8.65

0.27

0.46

0.02

13.24

0.46

0.00

99.93

801B-

44R-3.

28-33

P

0.71

12.58

11.69

51.04

0.13

8.36

0.32

0.51

0.00

13.61

0.00

0.00

98.94

801B-

44R-3.

28-33

P

0.57

15.29

8.73

54.16

0.33

7.94

0.28

0.31

0.00

11.82

0.50

0.00

99.92

801B-

44R-3.

28-33

P

0.51

16.67

8.24

55.11

0.43

7.90

0.17

0.25

0.00

9.11

0.91

0.00

99.29

801B-

44R-3.

28-33

P

0.83

17.54

8.24

55.85

0.26

7.32

0.22

0.29

0.09

7.84

0.35

0.00

98.82

801B-

44R-3,

28-33

P

0.64

17.14

8.25

55.29

0.16

7.71

0.14

0.39

0.03

8.94

0.44

0.00

99.11

801 C-

5R-5.

73-78

0

0.43

0.15

39.76

12.04

0.27

18.96

0.00

0.00

0.00

28.07

0.86

0.00

100.53

801C-

5R-5.

73-78

0

0.33

0.28

40.61

13.78

0.14

19.00

0.08

0.05

0.00

26.56

0.66

0.00

101.49

801C-

5R-5.

73-78

P

0.24

0.32

40.68

13.80

0.20

18.83

0.11

0.09

0.00

26.20

1.16

0.00

101.63

801C-

5R-5,

73-78

p

0.46

0.37

41.56

13.94

0.23

18.84

0.07

0.00

0.00

25.47

0.71

0.00

101.66

801 C-

6R-4,

49-54

P

0.45

0.24

35.21

16.54

0.62

16.52

0.18

0.60

0.00

29.70

0.30

0.00

100.34


Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

F e 3 +

Cr/Cr+Al

Fe + / Fe 3 + +Cr+Al

Mg/Mg+Fe2+

C r / C r + A U F e 3 +

0.27

2.44

3.65

6.63

0.09

3.35

0.08

0.23

0.01

2.72

0.10

32.00

24.00

4.45

0.43

0.41

0.34

0.25

0.18

2.49

3.62

6.76

0.03

3.28

0.09

0.26

0.00

2.83

0.00

32.00

24.00

4 .47

0.44

0.41

0.33

0.26

0.15

3.05

2.73

7.49

0.07

3.14

0.08

0.15

0.00

2.48

0.11

32.00

24.00

4.53

0.48

0.47

0.30

0.25

0.14

3.36

2.60

7.74

0.10

3.15

0.05

0.13

0.00

1.93

0.20

32.00

24.00

4.61

0.43

0.50

0.29

0.21

0.22

3.56

2.62

8.28

0.06

2.94

0.06

0.15

0.03

1.67

0.08

32.00

24.00

4.33

0.39

0.50

0.26

0.19

0.17

3.46

2.61

7.95

0.04

3.08

0.04

0.20

0.01

1.90

0.10

32.00

24.00

4.45

0.42

0.50

0.28

0.21

0.10

0.03

10.34

1.69

0.05

6.23

0.00

0.00

0.00

4 .89

0.15

32.00

24.00

0.53

0.32

0.03

0.79

0.31

0.07

0.05

10.44

1.74

0.03

6.17

0.02

0.02

0.00

4 .58

0.12

32.00

24.00

0.77

0.31

0.05

0.78

0.29

0.05

0.05

10.45

1.65

0.04

6.11

0.03

0.04

0.00

4.51

0.20

32.00

24.00

0.87

0.30

0.06

0.79

0.29

0.10

0.06

10.65

1.88

0.04

6.10

0.02

0.00

0.00

4 .38

0.12

32.00

24.00

0.66

0.29

0.04

0.77

0.28

0.10

0.04

9.38

1.84

0.12

5.57

0.05

0.27

0.00

5.31

0.06

32.00

24.00

1.29

0.36

0.08

0.75

0.33

Footnote:

P = Inclusion in plagioclase

M = Microphenocryst

0 = Inclusion in olivine

332



801 C-

6R-4.

49-54

P

0.34

0.38

34.73

16.85

0.26

16.63

0.30

0.50

0.00

29.59

0.31

0.00

99.89

0.08

0.07

9.28

1.83

0.05

5.69

0.07

0.22

0.00

5.30

0.06

32.00

24.00

1.36

0.36

0.09

0.76

0.33

801C-

6R-4.

49-54

0

0.15

0.16

44.35

12.00

0.10

19.40

0.03

0.51

0.07

23.28

0.47

0.00

100.49

0.03

0.03

11.26

1.26

0.02

6.23

0.01

0.21

0.02

3.96

0.08

32.00

24.00

0.90

0.26

0.06

0.83

0.25-

801C-

6R-4.

49-54

M

0.21

0.42

34.85

14.81

0.22

16.97

0.00

0.02

0.08

31 .84

0.43

0.00

99.85

0.05

0.07

9.38

2.16

0.04

5.77

0.01

0.01

0.02

5.75

0.08

32.00

24.00

0.67

0.38

0.04

0.73

0.36

801C-

6R-4.

49-54

M

0.61

0.50

36.58

15.96

0.00

17.14

0.04

0.40

0.00

30.30

0.04

0.00

101.55

0.14

0.09

9.60

2.18

0.00

5.69

0.01

0.17

0.00

5.34

0.01

32.00

24.00

0.80

0.36

0.05

0.72

0.34

801C-

6R-4,

49-54

0

0.21

0.32

48.38

11.46

0.09

19.87

0.05

0.05

0.00

19.16

0.38

0.00

99.96

0.05

0.05

12.18

1.64

0.02

6.33

0.01

0.02

0.00

3.24

0.07

32.00

24.00

0.41

0.21

0.03

0.79

0.20

801C-

6R-4,

49-54

0

0.00

0.17

47.44

11.43

0.24

19.67

0.00

0.45

0.06

18.80

0.38

0.00

98.64

0.00

0.03

12.06

1.18

0.04

6.33

0.00

0.02

0.02

3.21

0.07

32.00

24.00

0.88

0.21

0.06

0.84

0.20

801C-

6R-4.

49-54

0

0.46

0.21

45.24

12.24

0.17

19.97

0.05

0.51

0.09

22.75

0.24

0.00

101.93

0.10

0.03

11.30

1.28

0.03

6.31

0.01

0.21

0.02

3.78

0.04

32.00

24.00

0.89

0.25

0.06

0.83

0.24

801C-

6R-4.

49-54

0

0.51

0.20

44.17

12.33

0.00

19.67

0.00

0.54

0.06

23.99

0.17

0.00

101.63

0.11

0.03

11.10

1.38

0.00

6.25

0.00

0.22

0.02

4.04

0.03

32.00

24.00

0.82

0.27

0.05

0.82

0.25

801 C-

6R-4.

49-54

0

0.17

0.09

43.64

11.79

0.15

19.32

0.06

0.33

0.00

23.60

0.53

0.00

99.68

0.04

0.02

11.20

1.37

0.03

6.27

0.01

0.14

0.00

4.06

0.09

32.00

24.00

0.78

0.27

0.05

0.82

0.25

Secondary Phases and Alteration

Phyllosilicates dominate secondary assemblages in both the alka-lic and tholeiitic basalts and include various colored smectites (Fe-,Mg-, and Al-saponites), chlorite-smectite, and celadonite. Smectitecompositions commonly reflect the nature of the replaced host, such

that greenish Mg-saponites replace primary mafic minerals, althougholivine can also be pseudomorphs by either Al-rich saponites withhigh Fe/Mg ratios or Al-rich chlorite-smectite. Interstitial areas, es-pecially glassy mesostasis, and pillow rims are replaced by variousbrownish Fe-saponites with variable Fe/Mg ratios (and proportionsof APV), that may reflect the composition of early quenched (parental)

333

G. ROWBOTHAM, P.A.FLOYD

Table 12. Smectite analyses from alkalic dolerites and basalts, Holes 800A, 801B, and 801C.

Hole

Core/section

Interval (cm)

sio2

TiO2

A1 2O 3

FeO

MnO

MgO

CaO

Na2O

K20

Cr2°3

NiO

0

TOTAL'/.

800A-

58R-1.

86-91

A

33.30

0.32

12.41

21.33

0.07

18.52

0.25

2.13

0.30

0.00

0.00

0.00

88.74

800A-

58R-1.

86-91

A

34.18

0.22

12.47

21.41

0.22

19.09

0.24

0.43

0.15

0.00

0.00

0.00

88.70

800A-

58R-1.

86-91

I

41.00

0.02

7.42

8.60

0.16

17.26

1.69

0.71

0.43

0.00

0.00

0.00

77.83

800A-

58R-1.

86-91

I

37.41

0.03

6.97

7.57

0.00

13.96

0.56

1.05

1.09

0.00

0.00

0.00

68.82

800A-

58R-2.

67-73

B/G

30.15

0.06

11.35

30.52

0.27

14.31

0.20

0.34

0.15

0.00

0.00

0.00

87.36

800A-

58R-2.

67-73

B/G

39.19

0.00

11.10

12.26

0.04

17.97

1.32

0.83

0.13

0.00

0.10

0.00

82.94

800A-

58R-2.

67-73

B/G

42.05

0.00

8.05

10.35

0.00

19.56

1.21

1.13

0.30

0.01

0.18

0.00

82.86

800A-

58R-2.

67-73

B/G

31.85

0.08

10.53

26.91

0.32

16.61

0.17

0.66

0.04

0.00

0.10

0.00

87.26

801B-

41R-1,

130-136

I

54.96

0.17

13.45

10.22

0.03

4.73

0.86

1.47

1.45

0.00

0.26

0.00

87.59

801B-

41R-1,

130-136

I

55.61

0.16

14.00

8.92

0.00

4.52

0.71

1.27

1.19

0.00

0.02

0.00

86.40

801B-

43R-1.

4-9

A

33.15

0.02

18.79

12.75

0.19

19.71

0.57

0.39

0.34

0.06

0.00

0.00

85.97


Si

Ti

Al

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

0

Total

5.34

0.04

2.35

2.86

0.01

4.43

0.04

0.66

0.06

0.00

0.00

22.00

15.79

5.44

0.03

2.34

2.85

0.03

4.53

0.04

0.13

0.03

0.00

0.00

22.00

15.41

6.86

0.00

1.47

1.20

0.02

4.31

0.30

0.23

0.09

0.00

0.00

22.00

14.49

7.05

0.01

1.55

1.19

0.00

3.92

0.11

0.38

0.26

0.00

0.00

22.00

14.47

5.17

0.01

2.30

4.38

0.04

3.66

0.04

0.11

0.03

0.00

0.00

22.00

15.74

6.27

0.00

2.10

1.64

0.01

4.29

0.23

0.26

0.03

0.00

0.01

22.00

14.81

6.66

0.00

1.50

1.37

0.00

4.62

0.21

0.35

0.06

0.00

0.02

22.00

14.77

5.35

0.01

2.09

3.78

0.05

4.16

0.03

0.22

0.01

0.00

0.01

22.00

15.69

7.90

0.02

2.28

1.23

0.00

1.01

0.13

0.41

0.27

0.00

0.00

22.00

13.25

7.99

0.02

2.37

1.07

0.00

0.97

0.11

0.35

0.22

0.00

0.00

22.00

13.09

5.17

0.00

3.46

1.66

0.02

4.58

0.10

0.12

0.07

0.01

0.00

22.00

15.19

Footnote:

A

B/G

I

0

M

Replacement of biotite

Bright green matrix replacement

Interstitial smectite

Replacement of olivine

Matrix smectite

334



801B-

4 3 R - 1 .

4 - 9

A

32.83

0.02

19.97

12.12

0.31

19.34

0.57

0.41

0.34

0.00

0.00

0.00

85.93

5.11

0.00

3.66

1.58

0.04

4.48

0.10

0.12

0.07

0.00

0.00

22.00

15.16

801B-

4 3 R - 1 .

4 - 9

I

44 .08

0.06

5.16

8.11

0.00

18.06

0.47

1.04

0.82

0.00

0.00

0.00

77.81

7.29

0.01

1.01

1.12

0.00

4.45

0.08

0.33

0.17

0.00

0.00

22.00

14.46

801B-

4 3 R - 1 .

4 - 9

I

44.03

0.07

5.33

8.73

0.04

16.67

0.45

0.65

1.00

0.04

0.00

0.00

77.01

7.36

0.01

1.05

1.22

0.01

4.16

0.08

0.21

0.21

0.01

0.00

22.00

14.31

801B-

44R-2.

140-145

I

51.68

0.07

10.78

11.81

0.00

6.47

1.45

2.00

1.90

0.06

0.00

0.00

84.43

7.84

0.01

1.93

1.50

0.00

1.46

0.24

0.06

0.37

0.01

0.00

22.00

13.40

801B-

44R-2.

140-145

I

52.91

0.15

11.32

10.93

0.00

5.60

1.33

0.29

1.50

0.11

0.19

0.00

84.35

7.95

0.02

2.00

1.37

0.00

1.25

0.21

0.09

0.29

0.01

0.02

22.00

13.18

801C-

1 R - 1 .

14-20

B/G

40.88

0.13

11.83

10.26

0.12

19.93

1.54

0.00

0.44

0.00

0.34

0.00

85.48

6.27

0.02

2.14

1.32

0.02

4 .56

0.25

0.00

0.09

0.00

0.04

22.00

14.65

801C-

1 R - 1 .

14-20

0

55.17

0.01

2.20

11.98

0.00

20.98

0.52

0.18

0.61

0.00

0.52

0.00

92.19

7.73

0.00

0.36

1.40

0.00

4 .38

0.08

0.05

0.11

0.00

0.06

22.00

14.11

801C-

1 R - 1 .

14-20

0

53.04

0.06

2.75

12.93

0.00

18.74

0.71

0.38

0.70

0.07

0.05

0.00

89.42

7.70

0.01

0.47

1.57

0.00

4.06

0.11

0.11

0.13

0.01

0.01

22.00

14.16

801C-

1 R - 1 .

14-20

M

48.50

0.04

4.18

11.04

0.00

18.86

0.57

0.45

1.49

0.14

0.41

0.00

85.68

7.40

0.01

0.75

1.41

0.00

4.29

0.09

0.13

0.29

0.02

0.05

22.00

14.36

melts vs. late interstitial, Fe-rich melts. Very bright green claysreplacing late magmatic biotite in the alkalic rocks are Al- and Fe-richchlorite-smectites. Celadonite may also be associated with alteredinterstitial areas and generally has higher and more varied Fe/Mgratios than the associated smectites.

Some carbonates infilling veins may show a zonation from fine-grained granular ferroan dolomites to coarse rhombic calcite in the centers.

ACKNOWLEDGMENTS

The analyses were performed in the electron microprobe facilityin the Geology Department, University of Manchester, United King-dom, and we are grateful to Professor C. D. Curtis for permission touse the instruments. We thank Tim Hopkins, Dave Plant, and DaveWright for keeping the microprobes in such good working order. Mike

335


Table 13. Smectite analyses from tholeiitic basalts, Holes 801C and 802A.

Hole 801C-

Core/section 5R-5,

Interval (cm) 73-78

801C-

5R-5.

73-78

801C-

5R-5.

73-78

801C-

6R-4.

49-54

801C-

6R-4.

49-54

801C-

6R-4.

49-54

801C- 801C- 801C- 801C- 801C-

6R-4. 7R-2. 7R-2. 7R-2. 9R-5.

49-54 122-127 122-127 122-127 0-7

SiO2

TiO2

Al2°3FeO

MnO

MgO

CaO

Na2O

K20

Cr2O3

NiO

0

TOTALX

P

43.32

0.06

6.41

8.70

0.00

18.73

1.03

0.47

1.01

0.05

0.23

0.00

80.01

P

43.19

0.05

6.52

9.33

0.07

18.61

1.09

0.49

0.89

0.00

0.00

0.00

80.24

0

50.80

0.02

1.32

14.64

0.13

16.23

0.66

0.35

0.74

0.05

0.36

0.00

85.32

H

26.16

0.43

4.69

26.65

0.16

12.99

1.09

0.10

0.89

0.00

0.00

0.00

73.15

H

25.86

0.41

4.05

31.25

0.07

9.71

2.06

1.41

0.61

0.00

0.00

0.00

75.43

V

36.96

0.00

3.48

21.93

0.18

16.08

0.82

0.48

1.05

0.00

0.00

0.00

80.98

V

33.20

0.00

2.68

23.20

0.38

15.37

0.70

0.40

1.12

0.00

0.00

0.00

77.04

B

53.03

0.29

6.28

16.88

0.01

12.78

1.65

0.56

0.55

0.07

0.20

0.00

92.29

Y/B

51.41

0.07

4.62

16.47

0.00

7.85

1.39

0.64

0.59

0.03

0.20

0.00

83.25

B

54.78

0.54

8.49

14.64

0.03

11.75

1.92

0.36

0.78

0.00

0.35

0.00

93.64

B

50.52

0.30

2.92

18.38

0.08

16.35

0.89

0.64

0.54

0.00

0.13

0.00

90.75


Si

Ti

Al

Fe

Hn

Mg

Ca

Na

K

Cr

Ni

0

Total

7.02

0.01

1.22

1.18

0.00

4.53

0.18

0.15

0.21

0.01

0.03

22.00

14.53

7.00

0.01

1.24

1.26

0.01

4.49

0.19

0.15

0.18

0.00

0.00

22.00

14.54

7.85

0.00

0.24

1.89

0.02

3.74

0.11

0.11

0.15

0.01

0.05

22.00

14.15

5.49

0.07

1.16

4.67

0.03

4.06

0.24

0.04

0.24

0.00

0.00

22.00

16.00

5.46

0.07

1.01

5.52

0.01

3.06

0.47

0.58

0.17

0.00

0.00

22.00

16.34

6.53

0.00

0.72

3.24

0.03

4.23

0.16

0.16

0.24

0.00

0.00

22.00

15.31

6.14

0.00

0.60

3.69

0.06

4.36

0.14

0.15

0.27

0.00

0.00

22.00

15.41

7.59

0.03

1.06

2.02

0.00

2.73

0.25

0.16

0.10

0.01

0.02

22.00

13.97

8.12

0.01

0.86

2.18

0.00

1.85

0.24

0.20

0.12

0.00

0.03

22.00

13.59

7.61

0.06

1.39

1.70

0.00

2.43

0.29

0.10

0.14

0.00

0.04

22.00

13.76

7.46

0.03

0.51

2.28

0.01

3.61

0.14

0.18

0.10

0.00

0.02

22.00

14.33

P = Plates of smectite

0 = Replacement of olivine

H = Matrix smectite

V = Smectite in vescicle

B = Brown matrix smectite

Y/B = Yellow-brown matrix smectite

336



801C- 802A- 802A- 802A- 802A- 802A- 802A- 802A- 802A-

9R-5. 58R-2. 58R-2. 58R-2. 58R-2. 59R-1. 59R-1. 59R-3. 59R-3.

0-7 99-102 99-102 99-102 99-102 136-142 136-U2 40-4* 40-44

B

50.37

0.33

2.81

17.95

0.05

16.15

0.68

0.55

0.56

0.12

0.09

0.00

89.66

0

46.50

0.55

7.56

19.82

0.02

12.81

2.32

0.41

1.02

0.01

0.25

0.00

91.25

0

46.49

0.46

7.41

20.03

0.00

12.32

2.31

0.53

1.05

0.13

0.07

0.00

90.81

0

47.74

0.08

5.94

17.46

0.03

15.68

1.67

0.21

0.87

0.01

0.08

0.00

89.75

0

48.25

0.14

5.78

17.13

0.00

15.86

1.66

0.16

0.98

0.00

0.17

0.00

90.14

70

53.60

0.49

6.61

11.82

0.00

15.69

1.71

0.29

0.95

0.06

0.33

0.00

91.54

?0

50.32

0.21

4.37

13.12

0.07

18.24

1.67

0.30

0.55

0.06

0.26

0.00

89.15

H

45.31

0.05

5.77

14.26

0.00

14.54

1.78

0.62

0.86

0.01

0.27

0.00

83.47

M

50.11

0.26

4.48

13.48

0.00

18.13

1.82

0.29

0.53

0.01

0.02

0.00

89.12

7.53

0.04

0.50

2.24

0.01

3.60

0.11

0.16

0.11

0.01

0.01

22.00

14.29

6.98

0.06

1.34

2.49

0.00

2.87

0.37

0.12

0.20

0.00

0.03

22.00

14.45

7.02

0.05

1.32

2.53

0.00

2.77

0.37

0.15

0.20

0.02

0.01

22.00

14.44

7.16

0.01

1.05

2.19

0.00

3.51

0.27

0.06

0.17

0.00

0.01

22.000

14.42

7.19

0.02

1.02

2.14

0.00

3.53

0.27

0.05

0.19

0.00

0.02

22.000

14.40

7.562

0.052

1.098

1.395

0.000

3.299

0.258

0.080

0.171

0.006

0.037

22.000

13.96

7.400

0.023

0.758

1.613

0.009

3.999

0.263

0.085

0.102

0.006

0.030

22.000

14.29

7.235

0.006

1.086

1.904

0.000

3.461

0.304

0.191

0.175

0.001

0.035

22.000

14.40

7.378

0.028

0.778

1.659

0.000

3.980

0.287

0.083

0.099

0.001

0.002

22.000

14.30

Stead, David Emley, and David Kelsall, Geology Department, Uni-versity of Keele, are gratefully acknowledged for their consummateskill in linking disparate computing systems and maintaining a senseof humor. Thanks are also due to the reviewers who tightened up thepresentation of the analytical data.

REFERENCES

Adamson, A. C , 1981. Chemistry of alteration minerals from Deep SeaDrilling Project Sites 501, 504, and 505. In Cann, J. R., Langseth, M. G.,Honnorez, J., Von Herzen, R. P., White, S. M., et al., Init. Repts. DSDP,69: Washington (U.S. Govt. Printing Office), 551-563.

Allen, J. F., Sack, R. O., and Batiza, R., 1988. Cr-rich spinels as petrogeneticindicators: MORB-type lavas from the Lamont Seamount chain, easternPacific. Am. Mineral., 73:741-753.

Alt, J. C , Laverne, C , and Muehlenbachs, K., 1985. Alteration of the upperoceanic crust: mineralogy and processes in DSDP Hole 504B, Leg 83. InAnderson, R. N., Honnorez, J., Becker, K., et al., Init. Repts. DSDP, 83:Washington (U.S. Govt. Printing Office), 217-247.

Bence, A. E., Papike, J. J., and Ayuso, R. A., 1975. Petrology of submarinebasalts from the central Caribbean: DSDP Leg 15. J. Geophys. Res.,80:4775-4804.

Curtis, L. W., and Gittins, J., 1979. Aluminous and titaniferous clinopyroxenesfrom regionally metamorphosed agpaitic rocks in central Labrador. J.Petrol., 20:165-186.

337


Deer, W. A., Howie, R. A., and Zussman, J., 1978. Rock Forming Minerals(Vol. 2A): Single Chain Silicates: London (Longman).

Droop, G.T.R., 1987. A general equation for estimating Fe 3 + concentrationsin ferromagnesian silicates and oxides from microprobe analysis, usingstoichiometric criteria. Mineral. Mag., 51:431-437.

Ferguson, A. K., 1977. The natural occurrence of aegirine-neptunite solidsolution. Contrib. Mineral. Petrol., 60:247-253.

Fisk, M. R., and Bence, A. E., 1980. Experimental crystallization of chromespinel in FAMOUS basalt 527-1-1. Earth Planet. Sci. Lett., 48:111-123.

Flower, M.JP., 1974. Phase relations of titanacmite in the system Na2O-Fe2θ3-Al2θ3-TiO2-Siθ2 at 1000 bars total water pressure. Am. Mineral.,59:536-548.

Floyd, P. A., 1989. Geochemical features of intraplate oceanic plateau basalts.In Saunders, A. D., and Norry, M. J. (Eds.), Magmatism in the OceanBasins. Geol. Soc. Spec. Publ. London, 42:215-230.

Floyd, P. A., Castillo, P. R., and Pringle, M, 1991. Tholeiitic and alkali basaltsof the oldest Pacific ocean crust. Terra Nova, 3:257-265.

Floyd, P. A., and Rowbotham, R., 1986. Chemistry of primary and secon-dary phases in intraplate basalts and volcaniclastic sediments, Deep SeaDrilling Project Leg 89. In Moberly, R., Schlanger, S. O., et al., Init.Repts. DSDP, 89: Washington (U.S. Govt. Printing Office), 459-470.

Furuta, T., and Tokuyama, H., 1983. Chromium spinels in Costa Rica basalts,Deep Sea Drilling Project Site 505—a preliminary interpretation of elec-tron microprobe analyses. In Cann, J. R., Langseth, M. G., Honnorez, J.,Von Herzen, R. P., White, S. M., et al., Init. Repts, DSDP, 69: Washington(U.S. Govt. Printing Office), 805-810.

Guven, N., 1988. Smectites. In Bailey, S. W. (Ed.), Hydrous Phyllosilicates.Mineral. Soc. Am., Rev. Mineral., 19:497-550.

Hawthorne, F. C, 1981. The crystal chemistry of amphiboles. In Veblen, D. R.,and Ribbe, P. H. (Eds.), Amphiboles and Other Hydrous Pyriboles. Min-eral. Soc. Am., Rev. Mineral., 9A: 1-102.

Helz, R. T, 1973. Phase relations of basalts in their melting at P H o = 5 Kb.Part II: Melt compositions. J. Petrol, 17:139-193.

, 1980. Crystallization history of Kilauea Iki lava lake as seen in drillcore recovered in 1967-1979. Bull. Volcanol, 43:675-701.

Henderson, C.M.B., and Gibb, F.G.F., 1983. Felsic mineral crystallizationtrends in differentiating alkaline basic magmas. Contrib. Mineral. Petrol.,84:355-364.

, 1988. The petrology of the Lugar Sill, S.W. Scotland. Trans. R. Soc.Edinburgh, 77:325-347.

Kempton, P. D., Autio, L. K., Rhodes, J. M., Holdaway, M. J., Dungan, M.A., and Johnson, P., 1985. Petrology of basalts from Hole 504B, DeepSea Drilling Project, Leg 83. In Anderson, R. N., Honnorez, J., Becker,K., et al., Init. Repts. DSDP, 83: Washington (U.S. Govt. Printing Office),165-183.

Lancelot, Y., Larson, R. L., et al., 1990. Proc. ODP, Init. Repts., 129: CollegeStation, TX (Ocean Drilling Program).

Laverne, C, 1987. Unusual occurrences of aegirine-augite, fassaite and melan-ite in oceanic basalts (DSDP 504B). Lithos, 20:135-151.

Leake, B. E., 1978. Nomenclature on amphiboles (for subcommittee onamphiboles, IMA). Mineral. Mag., 42:533-563.

Longhi, J., Walker, D., and Hayes, J. F., 1978. The distribution of Fe and Mgbetween olivine and lunar basaltic liquids. Geochim. Cosmochim. Acta,42:1545-1558.

Lovering, J. F, and White, A.J.R., 1969. High-Al fassaites in eclogite inclusionsof nephelinite pipes in New South Wales, Australia. Contrib. Mineral.Petrol, 21:9-52.

Matsuoka, A., 1990. The oldest (Middle Jurassic) radiolarians from the west-ern Pacific. Proc. 3rd Int. Symp. "Shallow Tethys," Tokyo, Japan.

Morimoto, N., 1988. Nomenclature of pyroxenes (Commission on new min-erals and mineral names, IMA). Am. Mineral, 73:1123—1133.

Roberts, J. L., 1976. Ti solubility in synthetic phlogopite solid solutions. Chem.Geol, 17:213-227.

Robinson, P., 1980. The composition space of terrestrial pyroxenes—internaland external limits. In Prewitt, C. T. (Ed.), Pyroxenes. Mineral. Soc. Am.,Rev. Mineral., 7:419-494.

Roeder, P. L., 1974. Activity of iron and olivine solubility in basaltic liquids.Earth Planet. Sci. Lett., 23:397-410.

Sack, R. O., and Carmichael, I.S.E., 1984. Fe=Mg and TiAl2=MgSi2 exchangereactions between clinopyroxenes and silicate melts. Contrib. Mineral.Petrol, 85:103-115.

Schmincke, H. U., 1969. Ignimbritic sequences of Gran Canaria. Bull Vol-canol, 33:1199-1218.

Schumacher, J. C , 1991. Empirical ferric iron corrections: necessity as-sumptions, and effects on selected geothermobarometers. Mineral. Mag.,55:3-18.

Sigurdsson, H., and Schilling, J.-G., 1976. Spinels in Mid-Atlantic Ridgebasalts: chemistry and occurrence. Earth Planet. Sci. Lett., 20:7-20.

Tracy, R. J., and Robinson, P., 1977. Zoned titanian augite in alkali olivine basaltfrom Tahiti and the nature of titanium substitution in augite. Am. Mineral,62:634-645.

Upton, B.G.J., Stephenson, D., and Martin, A. R., 1985. The Tugtutoq older giantdyke complex: mineralogy and geochemistry of an alkali gabbro-augitesyenite-foyaite association in the Gardar Province of south Greenland. Min-eral. Mag., 49:623-642.

Velde, B., 1985. Clay Minerals: A Physico-chemical Explanation of TheirOccurrence: Amsterdam (Elsevier), Dev. in Sedimentol. Ser., 40.

Wilkinson, J.F.G., 1956. Clinopyroxenes of alkali basalt magma. Am. Mineral,41:724-743.

Date of initial receipt: 3 June 1991Date of acceptance: 2 March 1992Ms 129B-127

338

M+R: • 801C-1R-1, 14-20 cm•801B-44R-3, 28-33 cm• 801B-44R-2, 140-145 cm* 801B-43R-1, 4-9 cm•801B-41R-1, 130-136 cm* 800A-58R-2, 67-73 cm-800A-58R-1. 86-91 cm


K

3R'

5

4-

§3-DO

CU 2

1 -

00

0 +

C

6 15-

c

O

1 -

00

D

2 4 6 8 10Si Cations

2 4 6Mg Cations

Figure 13. Composition of Phyllosilicates from alkalic dolerite sills and basalt lava flows, Holes 800A, 801B, and 801C. A. M+R3+ - 3R2+ - 2R3+ components(modified after Velde, 1985). B. Interlayer cation variation. C. Fe vs. Mg contents. D. Total Al vs. Si contents.

339


Table 14. Celadonite analyses from Leg 129 dolerites and basalts.

Hole 801C- 801C- 801C-

Core/section 2R-5, 2R-5. 2R-5.

Interval (cm) 101-106 101-106 101-106

801C- 801C- 801C- 801C- 801C- 801C- 801C- 802A-

2R-5. 5R-2. 5R-2, 5R-2, 5R-2, 5R-5. 5R-5. 58R-3.

101-106 123-130 123-130 123-130 123-130 73-78 73-78 90-95

SiO 2

T i O 2

A12O3

FeO

MnO

MgO

CaO

Na.O

K20

Cr 2 O 3

NiO

0

TOTAL%

58.01

0 .11

9.89

7.97

0.05

7.03

0.05

0.33

0.10

9.61

0.00

0.00

93.09

57.78

0.15

9.72

8.21

0 .00

7 . 1 2

0 .26

0.25

9.74

0.08

0.00

0.00

93.30

58.32

0.12

9.55

8.74

0.00

7.28

0.19

0.15

9.91

0.12

0.51

0.00

94.87

57.51

0.05

9.33

8.53

0.00

7.14

0.17

0.11

9.79

0.09

0.40

0.00

93.12

57.11

0.01

7.17

16.65

0.00

5.58

0.59

0.23

7.66

0.00

0.26

0.00

95.30

56.76

0.14

8.62

14.81

0.18

5.99

0.47

0.14

8 .38

0 .00

0 .22

0 .00

95 .70

5 5 . 4 2

0 .13

6 . 0 8

17 .95

0 .00

5 .22

0.71

0.25

7.30

0.23

0.40

0.00

93.68

56.53

0.12

6.60

18.06

0.00

4.98

0.80

0.02

6.86

0.00

0.00

0.00

93.97

57.61

0.12

2.98

19.15

0.00

5.07

0.45

0.17

7 . 5 9

0.05

0.25

0 .00

93.45

55.53

0.02

3.64

19.81

0.00

5.21

0.43

0.14

8.07

0.00

0.43

0.00

93.27

53.05

0.00

2.73

16.92

0.00

9.06

0.54

0.25

7.41

0.03

0.00

0.00

89.98


Si

Ti

AL

Fe

Mn

Mg

Ca

Na

K

Cr

Ni

O

Total

8.11

0.01

1.63

0.93

0.01

1.46

0.05

0.03

1.71

0 .00

0 .00

2 2 . 0 0

13 .94

8 .08

0 .02

1.60

0 .96

0.00

1.48

0 .04

0.07

1.74

0.01

0 .00

2 2 . 0 0

14.00

8 . 0 7

0 .01

1 .56

1.01

0.00

1.50

0 .03

0 .04

1.75

0.01

0.06

22.00

14.03

8.09

0.01

1.55

1 .00

0.00

1.50

0.03

0.03

1.76

0.01

0.05

2 2 . 0 0

14 .02

8 .10

0.01

1.20

1 .97

0 .00

1.18

0 .09

0 .06

1.39

0.00

0 .03

2 2 . 0 0

13 .99

7 .97

0.02

1.43

1.74

0 .02

1.26

0 .07

0 .04

1.50

0.00

0.03

2 2 . 0 0

14.04

8 . 0 8

0 .01

1.05

2 . 1 9

0 .00

1.14

0 .11

0.07

1.36

0.03

0.05

2 2 . 0 0

14 .01

8 .15

0 .01

1.12

2 . 1 8

0 .00

1 .07

0 .12

0 .01

1.26

0.00

0 .00

2 2 . 0 0

1 3 . 9 2

8 .45

0.01

0 .52

2 .35

0 .00

1.11

0 .07

0 .05

1.42

0.01

0 .03

22.00

14.01

8.26

0.00

0.64

2.46

0.00

1.16

0.07

0.04

1.53

0.00

0.05

22.00

14.21

8.10

0.00

0.49

2.16

0.00

2.06

0.09

0.07

1.44

0.00

0.00

22.00

14.41

All celadonites are interstitial.

340



802A- 802A- 802A- 802A- 802A- 802A- 802A- 802A- 802A-

58R-3. 58R-3. 58R-3. 59R-1. 59R-1. 59R-3. 59R-3. 59R-3, 59R-3.

90-95 90-95 90-95 136-142 136-142 40-44 40-44 40-44 40-44

51.85

0.12

1.13

17.83

0.00

6.34

3.99

0.15

9.47

0.07

0.00

0.00

90.93

53.33

0.29

2.44

16.57

0.00

6.17

0.00

0.27

9.87

0.04

0.00

0.00

88.99

54.31

0.00

2.25

17.48

0.05

6.35

0.00

0.23

10.13

0.00

0.00

0.00

90.80

51.79

0.09

2.46

25.47

0.03

3.50

0.64

0.27

7.06

0.00

0.70

0.00

92.00

51.78

0.13

2.70

24.61

0.00

3.45

0.58

0.15

6.96

0.00

0.20

0.00

90.55

52.18

0.09

3.26

22.83

0.01

6.33

0.89

0.20

5.88

0.00

0.00

0.00

91.66

52.15

0.10

2.32

25.39

0.00

3.63

0.52

0.37

7.27

0.17

0.00

0.00

91.94

50.33

0.06

2.08

25.42

0.00

3.75

0.40

0.39

7.12

0.09

0.00

0.00

89.65

51.37

0.10

1.82

25.78

0.00

3.39

0.56

0.31

7.52

0.02

0.00

0.00

90.87

8.10

0.01

0.21

2.33

0.00

1.48

0.67

0.05

1.89

0.01

0.00

22.00

14.74

8.32

0.03

0.45

2.16

0.00

1.43

0.00

0.08

1.96

0.01

0.00

22.00

14.45

8.33

0.00

0.41

2.24

0.01

1.45

0.00

0.07

1.98

0.00

0.00

22.00

14.49

8.10

0.01

0.45

3.33

0.00

0.82

0.11

0.08

1.41

0.00

0.09

22.00

14.41

8.16

0.02

0.50

3.24

0.00

0.81

0.10

0.05

1.40

0.00

0.03

22.00

14.30

7.99

0.01

0.59

2.92

0.00

1.45

0.15

0.06

1.15

0.00

0.00

22.00

14.31

8.14

0.01

0.43

3.31

0.00

0.85

0.09

0.11

1.45

0.02

0.00

22.00

14.41

8.10

0.01

0.39

3.42

0.00

0.90

0.07

0.12

1.46

0.01

0.00

22.00

14.48

8.16

0.01

0.34

3.43

0.00

0.80

0.10

0.10

1.53

0.00

0.00

22.00

14.46

341


B* 801C-9R-5, 0-7 cm« 801C-7R-2, 122-127 cm* 801C-6R-4, 49-54 cm» 801C-5R-5, 73-78 cm* 801C-5R-3, 125-131 cm* 801C-5R-2, 123-130 cm

3 FT Na Ca

C 8

U)co

ü

2\

0

B β

***

0 1 2 3 4 5Mg cations

2.0

cO

σo

<

1.5-

1.0-

0.5-

0.0.

Δ Smectites

• Celadonites5

5.5 6.5 7.5 8.5 9.5Si Cations

Figure 14. Composition of Phyllosilicates from tholeiitic basalt flows, Hole 801C. A. M+R3 + - 3R2+ - 2R3+ components (modified after Velde, 1985). B. Interlayer

cation variation. C. Fe vs. Mg contents. D. Total Al vs. Si contents.

Table 15. Analyses of carbonates from a traverse across the infill of afracture in Sample 129-801C-5R-3,125-131 cm.

Zone

123456

Position

Margin

Center

Margin

Ca carbonate

94.617.560.495.218.160.1

Mg carbonate

1.821.721.8

1.619.221.2

Fe carbonate

2.852.716.42.4

55.817.3

Mn carbonate

0.98.01.40.77.01.4

342

M+R3 +

• 802A-62R-2, 53-59 cm« 802A-59R-3, 40-44 cm•802A-59R-1, 138-142 cm° 802A-58R-3, 90-95 cm* 802A-58R-2, 99-102 cm- 802A-58R-1, 96-100 cm


K

Na Ca

5 i

4-

00

o

σuCD

2-

1 -

0

am *

0 1 *0 V 0

0 1 2 3 4Mg Cations

c

2.0i

1.5-1

cO

O

<

0.5-

0.0

0

Smectites

Celadoπites a D

6.0 6.5 7.0 7.5 8.0 8.5Si Cations

Figure 15. Composition of Phyllosilicates from tholeiitic basalt flows, Hole 802A. A. M+R3+ - 3R2+ - 2R3+ components (modified after Velde, 1985). B. Interlayer

cation variation. C. Fe vs. Mg contents. D. Total Al vs. Si contents.

343

17. mineral chemistry of primary and ......5r-2, 123-130 5r-3, 125-131 6r-4, 49-54 7r-2, 122-127...

Documents