resolving crustal and mantle contributions to continental flood volcanism, yemen; constraints from...

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 12 PAGES 1805–1820 2000

Resolving Crustal and Mantle Contributionsto Continental Flood Volcanism, Yemen;Constraints from Mineral Oxygen IsotopeData

J. A. BAKER1∗, C. G. MACPHERSON1, M. A. MENZIES1,M. F. THIRLWALL1, M. AL-KADASI2 AND D. P. MATTEY1

1DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM HILL, EGHAM TW20 0EX, UK2DEPARTMENT OF GEOLOGY, UNIVERSITY OF SANA’A, SANA’A, YEMEN

RECEIVED JANUARY 28, 1999; REVISED TYPESCRIPT ACCEPTED MAY 2, 2000

variations are attributed to rapid assimilation of >25% Pan-Oxygen isotope ratios determined by laser fluorination analysis onAfrican continental crust by hot mafic magma during clinopyroxeneolivine, clinopyroxene and plagioclase separated from 31 Oligocenecrystallization. Contamination in this flood basalt province variedflood basalts and rhyolites from Yemen display small but significantfrom combined assimilation and fractional crystallization to rapidvariations (5·1–6·2‰ for olivine; 5·5–6·9‰ for clinopyroxene;assimilation of crust by hot mafic magmas with little fractionation.5·9–6·9‰ for plagioclase). The range in �18O values exceeds:Laser fluorination oxygen isotope analysis of mineral separates(1) the analytical reproducibility of the technique (±0·15‰; 2allows small differences in �18O to be correlated with radiogenicSD); (2) the range expected for minerals that would have crystallizedisotope data and is a powerful tool for evaluating the relative rolesfrom uncontaminated oceanic basalts or primary magmas in equi-of enriched lithospheric mantle and continental crust in suites oflibrium with mantle peridotite; (3) the range in melt values andcontinental flood basalts.equilibrium phenocryst compositions that could be produced by

fractional crystallization of these magmas. Samples with the highest�18O values exhibit increases in 87Sr/86Sr ratio, decreases in 143Nd/144Nd ratio, and increasing Pb isotopic heterogeneity. Samples with

KEY WORDS: oxygen isotopes; phenocrysts; flood volcanism; crustal con-the lowest �18O values have radiogenic isotope ratios that approachtamination; Yementhose inferred for the Afar plume. The oxygen isotope data provide

unequivocal evidence that assimilation of heterogeneous lower andupper Pan-African crust was the primary control on isotopic variationin this continental flood basalt province. Moreover, new radiogenic

INTRODUCTIONand oxygen isotope data for Pan-African crustal samples fromYemen have appropriate crustal isotopic compositions to generate the Continental flood volcanism has in the past producedobserved isotopic variations in the volcanic rocks. A near-primary massive (>1 × 106 km3) outpourings of largely basaltichigh-MgO basalt with low �Nd and extreme Pb isotope ratios magmas in only a few millions of years (Coffin & Eldholm,contains strongly zoned clinopyroxene crystals that range from green 1993). The large size of these volcanic episodes, coupledcores through to greenish brown, brownish green and dark brown or with the rapid rate at which they are produced, has ledblack rims. Handpicked crystals of each colour type display the to the inference that they are directly related to mantlefollowing correlated range in isotope ratios: 87Sr/86Sr= 0·7036– geodynamics, i.e. deep-seated mantle plumes (e.g. White0·7049; 143Nd/144Nd = 0·5129–0·5127; 206Pb/204Pb = & McKenzie, 1989). Moreover, the intense volcanic

activity associated with continental flood volcanism may18·6–17·9; �18O = 5·67–6·86‰. The Sr–Nd–Pb–O isotope

∗Corresponding author. Present address: Danish Lithosphere Centre,Øster Voldgade 10, L, 1350 Copenhagen K, Denmark. Telephone:+45 38 14 26 42. Fax: +45 33 11 08 78. e-mail: [email protected] Oxford University Press 2000

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 12 DECEMBER 2000

have affected the atmosphere and hydrosphere leadingto abrupt mass extinction events (e.g. Officer & Drake,1983).

Continental flood basalts exhibit a wide range in traceelement and isotopic compositions and, whereas someflood basalts clearly have chemical and isotopic signaturesakin to oceanic basalts, many flood basalts have signaturesthat are more typical of continental crust (e.g. Ferrar;Hergt et al., 1989). Even examples of flood volcanismthat have isotope ratios that fall entirely within the rangeof oceanic basalts display substantial isotopic variabilitythat must represent multiple contributions to volcanism[e.g. Yemen; Fig. 1 (Baker et al., 1996b)]. There is general

Fig. 1. Sr–Nd isotope variations in Oligocene flood volcanic rocksagreement that the enriched isotope ratios (e.g. low �Nd) from Yemen compared with local lithospheric mantle (LM), plumeand lithospheric trace element signatures (e.g. high Rb, (Afar plume) and depleted upper-mantle (MORB) sources, and global

oceanic basalt compositions. Sources of data: MORB–Afar plume,K, Ba, Pb and La) of many flood basalts, which dis-Schilling et al. (1992) and Volker et al. (1993); Afar plume, Vidal et al.tinguishes them from oceanic basalts, must reflect a (1991) and Schilling et al. (1992); LM, Henjes-Kunst et al. (1990),

contribution from the continental lithosphere traversed Blusztajn et al. (1995) and Baker et al. (1998); Yemen flood volcanism(herein) and additional data from Baker (1996) and Baker et al. (1996b);by these magmas. To date, those who study flood basaltglobal oceanic basalts (Hofmann, 1997).provinces have failed to reach any consensus on the

general origin of this lithospheric signature, although itneeds to be stressed that the source of this lithosphericsignature need not be the same in each province. How- volcanism has been identified is it possible to test rig-ever, recent studies have reached directly conflicting orously and develop models for flood basalt petrogenesis.conclusions on the major control on isotopic variability in Using primary magma compositions to identify and tem-many individual examples of continental flood volcanism, poral and spatial variability in mantle source com-e.g. Columbia River Basalt Group (Carlson et al., 1981; position(s) and the degree(s) and depth(s) of partial meltingCarlson, 1984; Brandon et al., 1993; Hooper & Hawkes- is a powerful tool for testing and developing geochemicalworth, 1993), Ethiopian–Yemeni Traps (Chazot & Ber- and geodynamic models for flood basalt genesis (e.g.trand, 1993; Deniel et al., 1994; Baker et al., 1996b; Pik White & McKenzie, 1989, 1995; Ellam, 1992; Gallagheret al., 1999); Deccan Traps (Lightfoot et al., 1990a; Peng & Hawkesworth, 1992).et al., 1994); Siberian Traps (Lightfoot et al., 1990b, 1993; Identification of crustal contamination typically reliesArndt et al., 1993; Wooden et al., 1993; Horan et al., on identification of correlations between indices of frac-1995). tionation and chemical or isotopic data. For example,

Proponents of crustal contamination models argue that correlations between MgO (negative) or SiO2 (positive)crustal contamination is inevitable during the storage contents with 87Sr/86Sr ratios are taken as evidence forand transfer of mantle-derived melts through the litho- progressive contamination of magmas while fractionationsphere and, moreover, that the lithospheric mantle (LM) was taking place in lithospheric magma chambers. Inis too cold and refractory and does not have the ap- principle, however, this approach has a number of lim-propriate composition to generate large volumes of flood itations: (1) such correlations are not always observedbasalt (Arndt & Christensen, 1992; Menzies, 1992). How- when reconnaissance suites of samples are consideredever, others have argued that addition of small amounts that do not represent a coherent magmatic system (e.g.of volatiles to the LM can depress solidus temperatures, Baker et al., 1996b); (2) similar correlations can be pro-allowing large amounts of melting in the presence of a duced during fractionation of magmas within the LM,thermal anomaly when coupled with lithospheric ex- accompanied by a progressive addition of melts fromtension (Gallagher & Hawkesworth, 1992). Presumably, enriched LM (Ellam & Cox, 1991); (3) contaminationhydration and introduction of subducted sediment into processes are thought to vary considerably from com-the LM by ancient subduction zone processes produces bined assimilation and fractional crystallization (AFC;a crustal-like signature in the LM. Recent studies have DePaolo, 1981) through to assimilation of crust by hotalso suggested that plume-derived carbonatitic me- mafic magmas with little or no concomitant fractionationtasomatism of the LM could impart a similar signature (Thirlwall & Jones, 1983; Cox & Hawkesworth, 1985;(Hauri et al., 1993; Baker et al., 1998). Huppert & Sparks, 1985; Devey & Cox, 1987; Kerr et

What remains beyond question is that only when al., 1995); (4) MgO and SiO2 are not always usefulindicators of fractionation, e.g. only small changes inthe primary chemical and isotopic signature of flood

1806

BAKER et al. CONTINENTAL FLOOD VOLCANISM, YEMEN

MgO are generated by fractionation of a plagioclase- SAMPLESdominated gabbroic assemblage. The samples that form the basis of this study are flood

The now well-defined and generally restricted oxygen volcanic rocks that were erupted from 31 to 29 Maisotope ratios of crustally uncontaminated oceanic basalts, in Yemen (Baker et al., 1996a) and form part of theand mantle peridotites (Ito et al., 1987; Mattey et al., Ethiopian–Yemeni large igneous province. The volcanic1994; Eiler et al., 1997), provide a powerful and un- rocks comprise a diverse variety of rock types, including aequivocal method to test the relative roles of crust and spectrum of basaltic compositions (basanite, trachybasalt,LM in the petrogenesis of continental flood basalts. basalt, basaltic andesite and basaltic trachyandesite) andAssimilation of crustal materials that typically, but not also rhyolitic rocks (Baker, 1996). Mafic samples arealways, have elevated �18O relative to mantle-derived variably olivine + clinopyroxene phyric and moremagmas should generate 18O enrichment in the con- evolved mafic samples may also be plagioclase± Fe–Titaminated basalts with predictable correlated changes in oxide phyric. Rhyolitic samples contain phenocrysts ofradiogenic isotope ratios. To date, oxygen isotope study sodic clinopyroxene+ alkali feldspar+ Fe–Ti oxide±of continental flood basalts has been hampered by the amphibole ± quartz.often altered nature of these rocks and, in some cases, The volcanic rocks can be divided into two suites on

the basis of field relationships, chemistry and isotopetheir sparsely porphyritic nature. Susceptibility of thedata. Rocks erupted through the western part of theoxygen isotope system to post-eruptive alteration largelyprovince display limited and distinct isotopic variationsprecludes the use of whole-rock oxygen isotope data incompared with those erupted through the eastern partevaluating the relative roles of crust and mantle in floodof the province (Baker, 1996; Baker et al., 1996b; see thebasalts (e.g. Hergt et al., 1989) and perhaps volcanic rocks‘Origin of isotopic variations’ section, below) and, asin general (e.g. Eiler et al., 1995, 1997).such, this is used to separate the samples into two groupsHerein, we present laser fluorination oxygen isotopein this paper. Rhyolitic pyroclastic rocks were also erupteddata on milligram-sized phenocrysts separated from floodfrom the western part of the province at sites now markedbasalts and rhyolites that were erupted between 31 andby the presence of granite plutons, which are unroofed26 Ma in Yemen, which form part of the Oligocenecaldera centres exhumed in the last 25 my.Ethiopian flood basalt province (Baker et al., 1996a).

These rocks have radiogenic isotope ratios that extendout of the fields defined by local LM, mantle plume (Afarplume) and depleted upper mantle [DMM or mid-ocean ANALYTICAL TECHNIQUESridge basalt (MORB) mantle] compositions, although the

Oxygen isotope analysesisotopic variation is less than that exhibited by oceanicApproximately 1–2 mg of clean olivine, clinopyroxene,basalts (Fig. 1). Baker et al. (1996b) concluded that cor-feldspar (where no mafic silicates were available forrelations between MgO and SiO2 and isotope data, theanalysis) or Fe–Ti oxide phenocrysts were handpickedprobable availability of appropriate crustal components,from coarsely crushed whole-rock volcanic samples toand unavailability of appropriate enriched LM com-avoid inclusion-rich and altered material. One maficponents required much of the chemical and isotopicflood basalt ( JB172) contains strongly colour-zoned clino-variability in the Yemen flood basalts to result frompyroxene crystals that vary from apple-green cores,contamination of plume-derived magmas with regionallythrough slightly brownish green, greenish brown and

heterogeneous crustal components. Whereas the evidence pale brown zones to black or dark brown crystal rims.presented by Baker et al. (1996b) supporting crustal con- The large size of these crystals (1–3 mm) allowed thetamination is largely circumstantial and equivocal, the O–Sr–Nd isotope ratios of the differently coloured zonesmineral oxygen isotope data presented herein show sub- in this sample to be determined, along with the Pb isotopestantial variability that exceeds, and extends to higher ratios of the core material. Bulk separates of each colourvalues than, that observed in oceanic basalts and mantle fraction of the clinopyroxene from JB172 were preparedperidotites. Thus, the oxygen isotope data provide un- and homogenized, and these were then split into portionsequivocal evidence for crustal contamination in this flood for radiogenic isotope analysis and replicate oxygen iso-basalt province and also confirm that contamination tope analyses. A number of mineral phases (amphibole,mechanisms were variable. We also present new radio- biotite, quartz, garnet) were separated from two Pan-genic and oxygen isotope data for crustal basement African crustal samples for oxygen isotope analysis and,samples from Yemen that, in some cases, have precisely in one case, a fine-grained chip of a whole-rock samplethe compositions required to generate some of the mixing (plagioclase + amphibole) was also analysed.arrays exhibited by the volcanic rocks in Sr–Nd–Pb–O Oxygen isotope analyses were carried out using the

laser fluorination technique of Mattey & Macphersonisotopic space.

1807


(1993) at Royal Holloway University of London. Mineral isotope analyses, have been presented by Baker et al.separates were heated with a Nd–YAG laser in the (1996b). Sr–Nd–Pb isotope ratios were determined onpresence of ClF3. To optimize oxygen yield, laser power whole-rock powders and are reported here as initialand the quantity of ClF3 used during the fluorination ratios; these isotopic data, along with measured isotopewere varied according to the phase being analysed. ratios and chemical data for the same samples, wereFeldspar, which is transparent to the Nd–YAG laser, was presented by Baker et al. (1996b). Sr–Nd–Pb isotope ratiosmixed with a mafic silicate phase of known oxygen isotope for the rhyolitic samples were obtained on acid-leachedcomposition (San Carlos mantle olivine; SCOL) to assist mineral separates [anorthoclase–sanidine (Sr–Pb), clino-fluorination reactions. Using a simple mixing equation pyroxene (Sr–Nd) and amphibole (Sr–Nd)]. The feldsparinvolving the relative weight proportions of feldspar to Pb isotope data can be considered to be initial data givenolivine in the reaction mixture, their respective oxygen the low U/Pb and Th/Pb ratio of feldspar. However,concentrations, the oxygen isotope ratio of the olivine all the remaining radiogenic isotope data presented hereinand the oxygen isotope ratio of the mixture, the isotopic for volcanic samples have been age-corrected using 40Ar/composition of the feldspar could be devolved. After 39Ar ages (31–26 Ma) from Baker et al. (1996a) and traceclean-up with KBr, the liberated O2 was converted to element data from Baker (1996) and Baker et al. (1996b),CO2 by reaction with hot graphite. CO2 was analysed including Rb–Sr isotope dilution data determined on theon a VG PRISM mass spectrometer. Oxygen isotope rhyolitic mineral separates.ratios are reported as the per mil deviation from Vienna Sr–Nd–Pb isotope data on the zoned clinopyroxeneStandard Mean Ocean Water in the standard � notation. phenocrysts from mafic flood basalt JB172 were de-

Analytical precision can be judged from the mean termined on bulk separates (0·02–0·1 g) on splits ofvalues for in-house olivine (SCOL) and garnet (Beni similarly coloured parts of the zoned phenocrysts as usedBousera peridotite massif; GP143) standards during the for oxygen isotope analysis. Clinopyroxene was leachedperiod of analysis: SCOL = 4·90 ± 0·15‰ (n = 8; 2 in hot HCl and repeatedly rinsed in deionized waterSD); GP143 = 7·23 ± 0·20 (n = 19; 2 SD). During before dissolution, conventional chemical separation andthe period of analysis, and in the 2 years before this, the subsequent mass spectrometric analysis as described byfollowing values were determined on these two in-house Baker et al. (1996b). Only sufficient material from thestandards: SCOL = 4·86 ± 0·16‰ (n = 245; 2 SD); cores was available to perform Pb isotope analyses. ToGP143 = 7·21 ± 0·20 (n = 106; 2 SD). Replicate assess isotopic homogeneity between the same-colouredanalyses of standards or unknowns on the same day splits used for oxygen and radiogenic isotope analyses,typically show enhanced reproducibility (<±0·1‰; 2 the fluoride residues of two differently coloured parts ofSD) compared with these long-term averages. As such, the zoned clinopyroxene crystal were recovered afterreproducibility of �18O values for analysed mafic silicates oxygen isotope analysis and analysed for Sr–Nd isotopes.with fluorination yields >95% can be taken to be better The measured Sr–Nd isotope ratios were within analyticalthan ±0·15‰ (2 SD). Yields for all unknowns and

uncertainty of those determined on the bulk separates ofstandards were 100 ± 2%.

the similarly coloured parts of the zoned clinopyroxeneFeldspar oxygen isotope analyses are inherently lessphenocrysts, implying that the colour fractions used forprecise than those of mafic phases because of the needradiogenic and oxygen isotope analysis are isotopicallyto mix samples with a mafic silicate to induce fluorinationhomogeneous on the scale of the analytical proceduresreactions. However, the reliability of the feldspar analysesused in this study.can be tested by replicate analyses of NBS28 quartz

New Sr–Nd–Pb isotope ratios have also been de-samples by the same technique, fluxing with the SCOLtermined on five crustal basement samples (whole-rockstandard, which yielded a mean value of 9·57± 0·36‰powders) that were collected from the eastern (two(n = 17; 2 SD; D. Lowry, unpublished data, 1996); thissamples) and western (three samples) parts of Yemen.compares favourably with the recommended value for

External precision and reproducibility of Sr–Nd isotopethis NBS standard, i.e. 9·66‰. NBS-30 biotite standardratios are better than ±0·000018 and ±0·000013 (nanalysed in this laboratory gives �18O= 5·03± 0·16‰>50; 2 SD), and ratios are reported relative to values of(2 SD; n = 80) compared with the recommended value0·710250 for SRM987 and 0·511424 for an in-houseof 5·1‰.laboratory Nd standard. This 143Nd/144Nd ratio cor-responds to 0·511860 and 0·512638 for the internationalstandards La Jolla and BCR-1, respectively. External

Sr–Nd–Pb isotope analyses precision or reproducibility of SRM981 shows that thereproducibility of sample Pb isotope ratios is>±0·010,Details of the analytical techniques used to obtain Sr–±0·012 and ±0·030 (2 SD) for 206Pb/204Pb, 207Pb/204PbNd–Pb isotope ratios for host basaltic samples (MgO

>3 wt %), from which minerals were separated for oxygen and 208Pb/204Pb, respectively.

1808


al., 1997; Thirlwall et al., 1997; Macpherson et al., 1998).RESULTSThese fractionations are also broadly consistent withOxygen isotope analyses of 66 mineral separates from 31the mineral phases being in equilibrium at magmaticvolcanic samples are presented in Tables 1 and 2. Tabletemperatures (1200–1000°C; Chiba et al., 1989; Zheng,1 lists mineral oxygen isotope data, host volcanic rock1993).MgO contents and initial Sr–Nd–Pb isotope ratios, along

Figure 2 shows raw mineral �18O values plotted vs hostwith five radiogenic and limited mineral oxygen isotoperock MgO contents. A very general trend to higheranalyses of Pan-African crustal basement samples frommineral �18O values, of each mineral type, with decreasingYemen. Table 2 lists O–Sr–Nd–Pb isotope data forhost rock MgO may be evident. However, more im-differently coloured parts of the strongly zoned clino-portantly, there is much scatter in these possible trendspyroxene phenocrysts from mafic flood basalt ( JB172),and samples with any MgO contents may have elevatedalong with oxygen isotope data for olivine phenocrystsmineral �18O values; for example, olivine and clino-from the same rock.pyroxene from JB172, clinopyroxene from JB82 andolivine from JB282.

In summary, ranges in �18O values for each phase (1)Oxygen isotope data exceed the maximum analytical reproducibility of the

technique (±0·15‰) and (2) extend to higher valuesOlivine �18O values vary from 5·1 to 6·2‰ and a numberthan those recorded in either minerals (olivine and clino-of samples have values (5·1–5·3‰) that overlap the rangepyroxene) separated from crustally uncontaminateddefined by olivine separated from (1) a wide variety ofmantle-derived oceanic basalts (Eiler et al., 1997) andmantle peridotite xenoliths, largely from the sub-mantle peridotites (Mattey et al., 1994) or mineral com-continental LM (5·18 ± 0·28‰; n = 76; Mattey et al.,positions (plagioclase) that can be calculated to have been1994), and (2) MORB and ocean island basalts (OIB)in equilibrium with MORB melts. The lowest �18O values(for MORB, 5·16 ± 0·18‰, n = 6; for OIB, 5·17 ±obtained for each mineral phase overlap values for these0·49‰, n = 62; Eiler et al., 1997) (Fig. 2).phases in oceanic basalts, mantle peridotite xenoliths, orClinopyroxene �18O values also vary widely (5·5–6·9‰)values that would characterize minerals that crystallizedand, again, a number of samples from the lower end ofat basaltic temperatures from melts in equilibrium withthis range have �18O values that closely overlap the rangemantle peridotites. These observations are independentdefined by clinopyroxenes from mantle peridotites (5·57of corrections to the raw mineral �18O values described± 0·32‰; n = 57; Mattey et al., 1994). Four clino-below (in ‘Correlations between mineral oxygen andpyroxene separates from rhyolitic volcanic rocks havehost rock radiogenic data’) to convert them to valuesrestricted �18O values of 5·9–6·1‰, which are actuallyequivalent to clinopyroxene or melt compositions so thatlower than those observed in some of the basaltic samples.all the data can be collectively compared with whole-The differently coloured parts of the zoned clinopyroxenerock Sr–Nd–Pb isotope ratios.crystals separated from mafic flood basalt JB172 have

highly variable �18O values (5·67–6·86‰) and this sampleis discussed in more detail in the next section.

Plagioclase �18O values range from 5·9 to 6·9‰. As aOxygen and radiogenic isotope variationsresult of the scarcity of plagioclase-facies mantle rocks,in a mafic flood basalt ( JB172)no mantle feldspar �18O values are available. However,Sample JB172 is a magnesian basalt containingit is notable, given the relatively small oxygen isotopic10·95 wt % MgO (mg-number 0·68). Its whole-rockfractionation between melt and plagioclase at magmaticpowder is characterized by high 87Sr/86Sr (0·70467) andtemperatures (�melt-plag=−0·1 to−0·3‰; Kalamarides,low 143Nd/144Nd (0·51272) ratios, and the lowest 206Pb/1986; Kyser, 1986, 1990), that the lower feldspar �18O204Pb ratio (17·9) and almost the highest �7/4 (+14·3)values (6·0± 0·1‰) would be in equilibrium with meltsand �8/4 (+101·9) values of the flood volcanic samplesthat have �18O > 5·7–5·8‰ (Table 1). Such melt �18Ofrom Yemen [� notation from Hart (1984)].values are characteristic of fresh MORB glasses (5·71±

From core to rim the �18O values of the clinopyroxene0·34‰, Ito et al., 1987; 5·81 ± 0·10‰, Macpherson,increase systematically from 5·7 to 6·9‰ (Table 2; Fig.1995).3), and �18O correlates with changes in 87Sr/86Sr andOxygen isotope fractionations observed between143Nd/144Nd ratios (Fig. 3). The cores of the crystals havedifferent mineral phases separated from the same rocklow 87Sr/86Sr (0·70356) and high 143Nd/144Nd (0·51290)are: �ol-cpx = −0·20 to −0·35‰; �ol-plag = −0·86‰;ratios, which are amongst the most depleted compositions�mt-plag = −1·86‰. These mineral–mineral frac-measured for the western Yemen flood basalts, andtionations are consistent with those measured in studiessimilar to those inferred for the Afar plume (Vidal et al.,of phenocrysts from other volcanic rocks (Anderson et

al., 1971; Singer et al., 1992; Macpherson, 1995; Eiler et 1991; Schilling et al., 1992; Baker et al., 1996b, 1997).

1809


Tab

le1

:O

–Sr

–N

d–P

bis

otop

eda

tafo

rm

iner

alph

enoc

ryst

s(O

)an

dho

stflo

odvo

lcan

icro

cks

(Sr–

Nd–

Pb)

,an

dcr

usta

lba

sem

ent

(Sr–

Nd–

Pb–

O)

sam

ples

Sam

ple

no

.�18

O(‰

)�18

O�18

OM

gO

87S

r/86

Sr

143 N

d/14

4 Nd

206 P

b/20

4 Pb

207 P

b/20

4 Pb

208 P

b/20

4 Pb

(rel

.cp

x)(r

el.

mel

t)(w

t%

)

Flo

od

bas

alts

eru

pte

dth

rou

gh

the

wes

tern

par

to

fth

evo

lcan

icp

rovi

nce

inYe

men

JB2

bas

alt

5·61

(cp

x)5·

615·

96·

600·

7039

90·

5128

6118

·510

15·5

4938

·380

JB63

bas

alt

5·57

(cp

x)5·

575·

89·

910·

7037

80·

5128

6518

·399

15·5

3738

·135

JB66

bas

alt

5·64

(cp

x)5·

645·

96·

640·

7037

60·

5128

6818

·294

15·5

3838

·077

JB74

bas

alt

6·39

(cp

x)6·

396·

75·

460·

7038

00·

5127

9418

·519

15·5

6438

·270

JB76

bas

alt

6·13

(cp

x)6·

136·

59·

920·

7037

20·

5128

1518

·598

15·5

6738

·271

JB80

bas

alt

5·77

(cp

x)5·

776·

14·

510·

7047

60·

5127

9319

·258

15·5

8838

·392

JB82

bas

alt

5·74

,5·

91(c

px)

5·83

6·0

12·1

20·

7039

30·

5128

1519

·064

15·5

8638

·605

JB84

bas

alt

5·60

,5·

65(c

px)

5·63

5·9

5·40

0·70

424

0·51

2799

18·8

2615

·567

38·4

56JB

259

bas

alt

6·37

(pla

g)

6·02

6·1

5·82

0·70

400

0·51

2881

19·0

6515

·560

38·4

43JB

260

bas

alt

5·90

(pla

g)

5·55

5·6

6·18

0·70

396

0·51

2857

19·0

2915

·568

38·4

93JB

261

bas

alt

6·13

(pla

g)

5·78

5·8

5·25

0·70

403

0·51

2875

19·0

9115

·559

38·4

40JB

262

bas

alt

6·85

(pla

g)

6·50

6·5

3·98

0·70

393

0·51

2799

18·3

3015

·566

38·2

53JB

279

bas

alt

6·02

,6·

06(p

lag

)5·

695·

74·

620·

7038

60·

5129

0018

·969

15·5

6238

·523

JB28

0b

asal

t5·

19(o

l),

6·05

(pla

g)

5·59

,5·

705·

9,5·

86·

500·

7039

60·

5129

0919

·049

15·5

7738

·534

JB28

1b

asal

t5·

12(o

l)5·

525·

713

·49

0·70

373

0·51

2894

18·9

4215

·562

38·5

78JB

282

bas

alt

5·85

,5·

93(o

l)6·

296·

68·

500·

7038

90·

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1810


Table 2: O–Sr–Nd–Pb isotope data for strongly zoned clinopyroxene phenocrysts and O

isotope data for olivine phenocrysts from a mafic flood basalt ( JB172)

Mineral 87Sr/86Sr 143Nd/144Nd �18O (‰) �18O (‰)

Individual data Mean ± 2 SD

Core cpx

Green cpx∗ 0·70356 0·512899 5·60, 5·74, 5·63, 5·61, 5·77 5·67±0·15

Brownish green cpx 0·70365 0·512891 5·73, 5·79 5·76±0·08

Pale brownish green cpx 0·70386 0·512845 5·90, 6·04, 6·15, 5·96 6·01±0·22

Pale brown cpx 0·70432 0·512755 6·52, 6·68, 6·69 6·63±0·19

Black cpx 0·70474 0·512693 6·67, 6·83, 6·73 6·74±0·16

Dark brown cpx† 0·70487 0·512696 6·91, 6·91, 6·73, 6·86, 6·88 6·86±0·15

Rim cpx

Olivine — — 6·15, 6·14, 6·17 6·15±0·03

Whole rock 0·70465 0·512741

∗206Pb/204Pb = 18·586, 207Pb/204Pb = 15·563, 208Pb/204Pb = 38·737.†Whole-rock ratios (equivalent to groundmass or rim compositions) 206Pb/204Pb = 17·920, 207Pb/204Pb = 15·577, 208Pb/204Pb =38·316.

5), and are similar to other whole-rock samples with low87Sr/86Sr and high 143Nd/144Nd ratios.

Olivine phenocrysts, which on petrographic groundscrystallized after the green clinopyroxene cores but beforethe clinopyroxene rims, have �18O > 6·1‰. With ourtechnique it is impossible to assess if there is any zonalheterogeneity with respect to Sr–Nd–Pb–O isotope ratiosin the olivine crystals, but olivine �18O values of 6·1‰would be in equilibrium with clinopyroxenes with �18O> 6·4–6·5‰. This is intermediate between the valuesdetermined for the end-member core and rim zones of theclinopyroxene and in agreement with the petrographicevidence for the order of crystallization of these phases.

The correlated changes in Sr–Nd–O isotope ratiosfrom core to rim in clinopyroxene crystals from JB172Fig. 2. Raw �18O mineral data vs host rock MgO contents. The

fields for olivine and clinopyroxene separated from mantle peridotites suggest that a magnesian, near-primary magma with(1 SD; Mattey et al. (1994) are also shown. The diamonds show depleted Sr–Nd isotope ratios and Pb isotope ratiosthe hypothetical effects of fractional crystallization on melt and

approaching those of the Afar plume, and with mantle-equilibrium clinopyroxene �18O values using the modelling describedin Table 3. like �18O values, progressively, and rapidly, acquired

more enriched Sr–Nd isotope ratios and higher �18Oduring clinopyroxene crystallization.

Interestingly, two mafic flows intercalated with the flowThe rims have high 87Sr/86Sr (0·70487) and low 143Nd/from which JB172 was sampled are offset to slightly144Nd (0·51273) ratios that are similar to values measuredhigher 87Sr/86Sr and lower 143Nd/144Nd ratios than theon the whole-rock powder, where the analysed Sr andSr–O and Nd–O mixing arrays defined by the JB172Nd would largely have been derived from the ground-analyses (Fig. 3). In these rocks ( JB166 and JB171) themass, which is evidently in equilibrium with the clino-radiogenic isotope ratios were determined on whole-rockpyroxene rims. Pb isotope ratios of the greenpowders and the oxygen isotope ratios were measuredclinopyroxene cores are completely different from theon large clinopyroxene phenocrysts. It is therefore pos-measured whole-rock ratios (Table 2, footnote), falling

close to the inferred Afar plume composition (Figs 4 and sible that these two samples, like JB172, also record some

1811


Fig. 3. Sr–Nd–O isotope variations in strongly zoned clinopyroxenephenocrysts separated from a mafic flood basalt ( JB172). Also annotated Fig. 4. �18O values (relative to clinopyroxene values) vs host rockin these figures are data for two flows immediately above and below Sr–Nd–Pb isotope ratios. Field for the Afar plume taken from referencesJB172 ( JB171 and JB166) and the inferred fields for the Afar plume. cited for Fig. 1 and assuming ‘normal’ mantle clinopyroxene �18OMixing arrays are bulk mixing curves between components with: (1) values (Mattey et al., 1994). Symbols in this figure reflect geographical87Sr/86Sr = 0·7035, 143Nd/144Nd = 0·5129, Sr = 476 ppm, Nd = groupings of samples not mineral type:Φ, Yemen flood basalts erupted32 ppm, �18O = 5·6‰ and (2) 87Sr/86Sr = 0·7086, 143Nd/144Nd = through the western part of the volcanic province;Β, rhyolites erupted0·5120, Sr = 476 ppm, Nd = 26 ppm, �18O = 10·6‰. Component through the western part of the volcanic province; Ε, flood basalts(1) is inferred to be that of a primary magma produced from the Afar erupted through the eastern part of the volcanic province. �8/4 valuesplume before mixing with the high-�18O component (2), and uses the for analysed crustal lithologies (Table 1) are illustrated in the Pb–Oisotope ratios inferred for the Afar plume (same references as for Fig. 1) isotope plot. Crustal �8/4 values from herein (Table 1), G. Chazot &and trace element concentrations measured for JB172 in the modelling J. A. Baker (unpublished data, 1998) and Brueckner et al. (1995).calculations (Baker et al., 1996b). Component (2) is inferred to becontinental crust and uses radiogenic isotopic ratios from an averageof the eastern crustal basement samples from Yemen. The whole-rock compared with the JB172 clinopyroxenes precluded fur-oxygen isotope ratio for this sample was inferred to be similar to an

ther investigation of this hypothesis, in the absence of inanalysis of a similar sample of Archaean gneiss from Sudan (Davidson& Wilson, 1989). Mineral oxygen isotope ratios determined herein on situ analytical techniques.Yemen crust suggest that non-modal melting of Yemen gneiss F27 (i.e.biotite being preferentially melted) would also have resulted in additionof crustal melts with �18O = 10–11‰ to the basaltic magmas.

Correlations between mineral oxygen andhost rock radiogenic dataisotopic disequilibrium between phenocryst phases and

groundmass, i.e. the clinopyroxenes are isotopically The different phenocryst phases present in the volcanicrocks precluded analysis of a common mineral phase inzoned. Unfortunately, the smaller size of the clino-

pyroxene phenocrysts and lack of colour zoning every sample. To compare the oxygen isotope ratios from

1812


�melt-cpx fractionation at lower melt temperatures (e.g.Kalamarides, 1986; Fig. 2; Table 3).

The following mineral fractionation factors were usedto convert the raw olivine, plagioclase and Fe–Ti oxideoxygen isotope data into equilibrium clinopyroxene iso-tope data: �cpx-ol =+0·4‰; �cpx-plag =−0·35‰; �cpx-mt

= +1·6‰. These fractionation factors are consistentwith those measured in basaltic and andesitic rocks(Anderson et al., 1971; Singer et al., 1992; Macpherson,1995; Thirlwall et al., 1997; Macpherson et al., 1998) andalso the fractionations predicted from experimental workand theoretical considerations between these phases atmagmatic temperatures (1000–1200°C; Chiba et al.,1989; Zheng, 1993).

Actual or calculated equilibrium clinopyroxene �18Ovalues, with JB172 represented by the analysis of crystalrims, are plotted vs host rock Sr–Nd–Pb isotope ratios(Fig. 4). Although there is considerable scatter in theseplots, �18O values increase with: (1) increasing 87Sr/86Sr ratios and also with little change in 87Sr/86Sr; (2)decreasing 143Nd/144Nd ratios; (3) high �8/4 values (suchas JB172), and also with little change in �8/4 (such asJB282), and possibly at low �8/4 values (such as JB259)(i.e. increasing Pb isotopic heterogeneity). When thehigher �18O samples are considered, it is notable that thesamples erupted through the western part of the volcanicprovince display more limited Nd isotopic variationsand trend towards different Pb isotopic compositionscompared with those erupted through the eastern partof the volcanic province.

Samples with the lowest �18O values tend to approachisotope ratios of 87Sr/86Sr = 0·7035, 143Nd/144Nd =0·5129, 206Pb/204Pb = 19·0 and �8/4 > 40, which aresimilar to those inferred for the Afar plume (Fig. 4).

Clinopyroxene separates from the rhyolite samples fallon the same trends as the basaltic samples eruptedthrough the western part of the volcanic province, but

Fig. 5. Sr–Nd–Pb isotope variations in Oligocene flood volcanic rocks are displaced to higher 87Sr/86Sr ratios at the same �18Ofrom Yemen compared with local lithospheric mantle, plume (Afar values compared with most of these samples. Like all theplume) and depleted upper-mantle (MORB) and continental crust basaltic samples erupted through the western part of thecompositions. Same symbols as for Fig. 4; Χ, analyses of continental

volcanic province, the rhyolites exhibit rather limitedcrust. Sources of data: same as for Fig. 1, but with additional data forcontinental crust from herein (Table 1), G. Chazot & J. A. Baker radiogenic isotope variations compared with basalts erup-(unpublished data, 1998) and Brueckner et al. (1995). ted through the eastern part of the volcanic province.

samples with different analysed phases we have chosenCrustal Sr–Nd–Pb–O isotope datato correct our mineral data to values that are equivalent

to clinopyroxene values. We have taken this approach Sr–Nd–Pb isotope ratios for five crustal basement samplesas: (1) clinopyroxene is the phase we have the most data display substantial variations (Table 1; Fig. 5): 87Sr/for; (2) conversion to melt �18O values is precluded by a 86Sr = 0·704–0·710; 143Nd/144Nd = 0·51286–0·5119;lack of knowledge of mineral–melt fractionation factors 206Pb/204Pb = 17·3–19·9. There is a clear distinctionand adequate thermometric data for the Yemen flood between the basement underlying the western and easternbasalts; (3) although melt �18O values are likely to increase parts of the volcanic province. Samples from the westduring fractionation of olivine + clinopyroxene ± pla- have limited isotopic variability and are akin to juvenile,

Late Proterozoic, Pan-African basement from elsewheregioclase ± Fe–Ti oxides, this is offset by increasing

1813


Table 3: Modelling the hypothetical effects of fractional crystallization on evolving melt and equilibrium

clinopyroxene phenocryst oxygen isotope compositions

MgO range Fraction of melt Mean T �18O melt �18O melt �18O cpx �18O cpx

modelled remaining (F ) (°C) start (‰) finish (‰) start (‰) finish (‰)

(wt %)

13·5–8·5, a 0·69 1200 5·70 5·86 5·49 5·57

(1250–1150)

8·5–4·0, b 0·75 1100 5·86 5·96 5·57 5·59

(1150–1050)

4·0–0·5, c 0·32 1000 5·96 6·34 5·59 5·89

(1050–950)

Modelling uses: (1) the equation of Taylor & Sheppard (1986): �18O = [�18O0 + 1000] × [F�−1] − 1000, where 1000 × ln�i−j = �i−j (i, melt; j, fractionating assemblage); (2) the following fractionating mineral assemblages: a, 45% olivine, 55%clinopyroxene; b, 20% olivine, 15% clinopyroxene, 50% plagioclase, 15% Fe–Ti oxides; c, 9% olivine, 9% clinopyroxene, 62%plagioclase, 20% Fe–Ti oxides (Chazot & Bertrand, 1995; Baker, 1996); (3) average mineral–melt fractionation factors in eachstep calculated from the empirical equations of Kalamarides (1986).

in the Arabian shield (e.g. Duyvermann et al., 1982; their very low TiO2, P2O5, Sr, Ba and V contents requireMcGuire & Stern, 1993; Brueckner et al., 1995). Samples further extensive fractionation of feldspar, Fe–Ti oxidesfrom basement underlying the eastern part of the volcanic and apatite (Chazot & Bertrand, 1995; Baker, 1996).province display more extreme isotope ratios and are Modelling the effects of fractional crystallization onexamples of the Late Archaean crust that has been melt and equilibrium mineral oxygen isotope com-recently identified in this part of Yemen (Windley et al., positions is non-trivial, given the relatively poor con-1996). straints on mineral–melt fractionation factors and melt

Oxygen isotope data for two crustal samples from these temperatures, and the fact that mineral–melt frac-two crustal provinces also exhibit marked differences. A tionation factors are temperature dependent and so willsample from the western part of the volcanic province change in an evolving magma. However, in Table 3 wehas relatively low �18O, comparable with values of the crudely illustrate the effects of fractional crystallizationflood basalts. However, an Archaean(?) gneiss from the for three crystallization steps with changing crystallizingeastern part of the province has much higher �18O assemblages and mineral–melt fractionation factors in(9·6–13·3‰) for all its mineral phases compared with the each step (MgO = 13·5–8·5, 8·5–4·0, 4·0–0·5 wt %).flood basalts. These values are almost identical to whole- The modelling uses a starting magma with �18O =rock �18O values reported by Davidson & Wilson (1989) 5·70‰ and calculates an average mineral–melt frac-for similar Archaean gneisses from Sudan. tionation factor over the crystallization interval using the

relative proportions of fractionating phases obtained fromleast-squares mixing modelling of major and trace ele-ment variations (Chazot & Bertrand, 1993; Baker, 1996).

DISCUSSION Mineral–melt fractionation factors are calculated fromOrigin of isotopic variations the equations of Kalamarides (1986) assuming tem-Effects of magma fractionation perature varies as shown in Table 3. It is then finally

necessary to recalculate equilibrium clinopyroxene com-Few of the Yemen flood basalts and clearly none of thepositions that would have been in equilibrium with therhyolites are primary magmas. Petrographic observationsevolving melts as �melt-cpx increases with decreasing MgOalong with qualitative and quantitative modelling of(i.e. temperature).fractional crystallization processes (Baker, 1996) indicate

These simple calculations show that fractionation ofthat the main fractionating phases in the basaltic rocksolivine+ clinopyroxene and subsequent minor amounts(MgO >7 wt %) were olivine + clinopyroxene withof plagioclase + Fe–Ti oxides within the basaltic spec-minor amounts of plagioclase and Fe–Ti oxides alsotrum of rock types (MgO = 13·5–4·0 wt %) producesjoining the fractionating assemblage at lower MgO con-only small increases in melt �18O (<0·3‰) and that thetents (MgO= 7–3 wt %). If the rhyolites are considered

the product of fractionation from basaltic parents then increasing �melt-cpx fractionation factor with decreasing

1814


temperature (or MgO) means that an increase of only Crustal provinciality from isotopic data0·1‰ in clinopyroxene �18O values is likely to result from Sr–Nd–Pb–O isotope variations require the involvementfractional crystallization (Table 3; Fig. 2). Production of of at least three crustal components (C1, C2 and C3) inthe rhyolitic rocks from basaltic parents requires extensive the petrogenesis of the Yemen flood basalts and rhyolitesfractional crystallization (>85%) and the significant (Figs 4 and 5).amounts of fractionating Fe–Ti oxides result in a modest Basalts erupted through the eastern part of the volcanicincrease in melt �18O values (0·4–0·6‰). Clinopyroxene province are contaminated by a component with un-in equilibrium with such rhyolitic melts produced by radiogenic Pb isotope ratios and elevated �7/4 and �8/protracted differentiation of basaltic parents would have 4 values, and high 87Sr/86Sr and low 143Nd/144Nd ratios�18O > 5·9‰, similar to the lowest values measured in (C1) (Fig. 5). This component has an isotopic compositionrhyolites in this study. remarkably similar to analyses of Late Archaean crustal

These calculations show that fractionation of the ob- gneisses from eastern Yemen presented herein (Fig. 5).served mineral phases in the Yemen flood basalts cannot Basalts and rhyolites erupted through the western partgenerate the range in mineral �18O values observed in of the volcanic province have been contaminated by atthese rocks (>1‰). However, the marginally elevated least two components. One component (C2) is difficult�18O values measured on clinopyroxenes from the rhyo- to identify on the basis of radiogenic isotopes as it haslitic rocks can be largely accounted for by extensive an isotopic composition relatively close to that of thefractional crystallization from basaltic parents. It is vital uncontaminated basalts compared, for example, with C1.to note that relatively large �18O variations observed in This component (C2) has Pb isotope ratios that fall closeindividual mineral phases from samples with a limited to the Northern Hemisphere Reference Line and therange in MgO contents preclude fractional crystallization MORB–Afar plume array, with slightly unradiogenic Pbcontrol on much of the oxygen isotopic variability in this

and slightly higher 87Sr/86Sr and lower 143Nd/144Nd ratiosdataset.compared with the uncontaminated basalts. This com-position is inferred to be Late Proterozoic Pan-Africanlower crust which is not old enough (or particularlyMantle heterogeneity or crustal contamination?chemically evolved enough) to have developed exoticVolcanic rocks from Yemen have radiogenic isotopeisotope ratios compared with mantle-derived basalts.ratios that exhibit considerable variation, but still fallAnalyses of lower-crustal granulite-facies gneisses fromwithin the field of oceanic basalts (Figs 1 and 5), andthe Zabargad Island massif (Brueckner et al., 1995) andthese correlations correlate with mineral oxygen isotopealso such xenoliths entrained in Late Cenozoic volcanismdata (Figs 3 and 4). However, oxygen isotope variationsthroughout Saudi Arabia and Yemen (McGuire & Stern,(1·5‰) in minerals separated from the flood basalts1993; G. Chazot & J. A. Baker, unpublished data, 1998)exceed that observed in mantle peridotites (or mineralshave precisely the required radiogenic isotope com-that would have crystallized from melts in equilibriumpositions to generate the observed mixing arrays inwith mantle peridotites), including a large number ofSr–Nd–Pb isotopic space (Fig. 5). The second crustalperidotites from the continental LM (Mattey et al., 1994).component (C3) contaminating the basalts (and rhyolites)Oceanic basalts with a wide range in radiogenic isotopeerupted through the western part of the volcanic provinceratios exhibit limited oxygen isotope variations whenhas higher 87Sr/86Sr ratios than, but similar 143Nd/144Ndanalyses of mineral phases from these rocks are consideredratios to, C2 and a distinctive Pb isotope signature marked(Eiler et al., 1997). The previous discussion has shownby radiogenic 206Pb/204Pb isotope ratios with negativethat the mineral oxygen isotope variations are too large�8/4 values. This component is clearly distinct from theto be the product of fractional crystallization. We con-MORB–Afar plume mixing array in Pb isotope spaceclude that assimilation of continental crust, with elevatedand analyses of crustal basement from the western part�18O relative to mantle-derived magmas, rather than aof the volcanic province are a suitable crustal end-contribution from enriched LM or subcontinental mantlemember.is responsible for the isotopic and also much of the

Figure 6 illustrates the multi-element patterns of twoincompatible trace element heterogeneity of the floodrelatively uncontaminated basalts with low �18O eruptedvolcanic rocks from Yemen.through the western ( JB281) and eastern ( JB231II) partsThe oxygen isotope data confirm previous assertionsof the volcanic province in Yemen. These samples arethat crustal contamination was an important process inmarked by relatively smooth multi-element patterns withthis flood volcanic province (Baker et al., 1996b) anda marked negative K anomaly. Figure 6 also illustratescast further doubt on the frequently inferred role of anexamples of basalts contaminated by each of the inferredenriched LM contribution to flood volcanism at the Afro-crustal contaminants. Samples contaminated by C1 andArabian triple junction (Hart et al., 1989; Vidal et al.,

1991; Chazot & Bertrand, 1993; Deniel et al., 1994). C2 show smaller K anomalies and enrichments in Ba

1815


large amounts of lower-crustal material (C2), e.g. JB282.More evolved samples either show evidence for nofurther contamination or have a signature of con-tamination by upper continental crust (C3). Therelatively limited isotopic contrast between the primaryisotopic composition of the flood basalts and the young,Late Proterozoic, crustal components (C2 and C3) inthe western part of the volcanic province meansthat the radiogenic and oxygen isotope effects ofcontamination are less obvious in this suite of samplesthan in the samples erupted through the eastern partof the volcanic province. It also makes it more difficultto model the amounts of contamination. However, giventhe ubiquitous ‘lithospheric’ trace element signature ofcontinental crust, regardless of its age, the trace elementFig. 6. Multi-element plot of uncontaminated ( JB281 and JB231II)

and contaminated ( JB259, JB172 and JB282) flood basalts illustrating effects of contamination are similar in both suites ofthe different effects on trace element signatures produced by lower- basalts (Fig. 6).and upper-crustal contamination. Normalizing values for primitive

Thus, contamination processes in Yemen varied frommantle are from Sun & McDonough (1989).early lower-crustal contamination of hot mafic magmaswith little or no concomitant fractionation (r >1) through

and Pb relative to Rb, Th and U, which is consistent with to combined assimilation and fractional crystallizationthese samples having assimilated Rb–Th–U-depleted, typically within upper-crustal magma chambers. Thegranulite-facies, lower crust. Samples contaminated by most extreme result of the decline in the rate of as-C3 show enrichments in all the large ion lithophile similation to fractional crystallization is probably rep-elements (LILE—Rb, Ba, K and Pb), Th and U relative resented by the rhyolites, which require extensiveto Nb. These features are consistent with C3 being an fractionation with only limited amounts of contamination.upper-crustal component. The switch from lower- to upper-crustal contamination

Scattered arrays of data in Figs 1, 4 and 5 are a natural reflects establishment of magmatic plumbing systems;consequence of crustal contamination with isotopically early intrusion of magmas into the lower crust resultedheterogeneous crustal components, variable distribution in hot mafic magmas removing the fusible parts of thecoefficients for Sr during assimilation and fractional crys- lower crust and subsequent establishment of upper-crustaltallization, and differences in primary magma in- magma chambers produced more evolved magmas with acompatible trace element abundances rendering different variable upper-crustal overprint. Plagioclase fractionationsamples’ isotopic systems to have had different sus- in the most evolved and upper crustally contaminatedceptibilities to a finite amount of contamination. Semi- samples is consistent with a switch in the depth ofquantitative modelling of contamination processes has fractionation and assimilation from deep to shallowalready been presented by Baker et al. (1996b) and in crustal levels.Fig. 3. These calculations show that the Yemen flood Identification of crustal contamination taking placebasalts and rhyolites have variably assimilated 0–25% in the lower crust when hot mafic magmas arecrustal material. introduced into the lithosphere has long been postulated

as being important in flood basalt provinces (Thirlwall& Jones, 1983; Cox & Hawkesworth, 1985; HuppertMechanisms of contamination& Sparks, 1985; Devey & Cox, 1987; Kerr et al., 1995;Mafic magmas erupted through the eastern part of theMartinez et al., 1996). This has now been verified byvolcanic province have, in some cases, assimilated largeoxygen isotope studies of flood basalts from both theamounts (>20–25%) of a Late Archaean silicic lower-Deccan Traps and Yemen (Peng et al., 1994; herein).crustal component (C1) with little concomitant frac-Presumably, assimilation is facilitated by the limitedtionation, e.g. JB172 and JB129. Although assimilationheat input required to melt already hot lower crust.clearly involved little accompanying fractionation, theWhat is also important to note is that magmasoccurrence of this lower-crustal component at the surfacecontaminated at this stage also have near-primary (i.e.today (crustal samples F25 and F27) means it is equivocallow) incompatible trace element contents renderingas to whether or not this assimilation took place at lower-them highly susceptible to small amounts of con-crustal levels.tamination and acquisition of lithospheric-like traceMafic magmas erupted through the western part of

the volcanic province have also commonly assimilated element and isotopic signatures.

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basalts with the lowest �18O values preclude these rocksOrigin of rhyolitic volcanismbeing substantially crustally contaminated (p5%) andClinopyroxene separated from four rhyolitic rocks hashaving primary �18O values significantly lower than their�18O = 5·9–6·1‰. These values are lower than thosemeasured values. Moreover, LM xenoliths meta-observed in some of the more contaminated flood basalts,somatized by the Afar plume (Baker et al., 1998) also doincluding samples contaminated by crustal rocks in thenot have low �18O values (Chazot et al., 1997).western part of the volcanic province through which the

The final point concerning the oxygen isotope signaturerhyolites were erupted. Moreover, as discussed above, theof the least contaminated Yemen flood basalts is thatslightly elevated �18O values of the rhyolite clinopyroxenesbasanites and basalts sensu stricto have similar oxygenmay largely be accounted for by extensive crystal frac-(and radiogenic) isotope ratios. Major and trace elementtionation.differences between the basanite and basalt rocks areOxygen isotope data for the rhyolite samples mightreadily explained in terms of depth and degree of partialseem to preclude an origin for the rhyolites by extensivemelting, with the basanites being the product of smallercrustal melting in this province, and be more consistentaverage degrees of partial melting generated at greaterwith a model for rhyolite petrogenesis involving pro-depths than the basalts within the Afar plume. As such,tracted fractionation with relatively limited assimilationthere is apparently no partial melting control on oxygenfrom flood basalt parent magmas (Chazot & Bertrand,isotope ratios in the Yemen flood basalt suite—at least1995). However, some Pan-African crust clearly has lowwithin the resolution of the laser fluorination technique.�18O, which requires examination of other chemical

and isotopic data to determine the petrogenesis of therhyolites. Moreover, currently available chemical andisotope data are not sufficient to test an alternative

CONCLUSIONShypothesis that the rhyolites might have been generatedOxygen isotope heterogeneity of minerals separated fromby melting of underplated basaltic material related to theOligocene flood volcanic rocks from Yemen exceedsflood volcanic episode (e.g. Harris & Erlank, 1991). Eitheranalytical reproducibility and also that observed inof the latter two hypotheses has important implicationsoceanic basalts and mantle peridotites. High �18O valuesfor melt generation in this volcanic province—largeare correlated with changes in radiogenic isotope ratios,amounts of unerupted mafic material equivalent to severalsuch as increasing 87Sr/86Sr. Samples with high �18Otimes the volume of erupted rhyolite must be present invalues include primitive and evolved compositions. Thethe lithosphere. Given that the basalt:rhyolite ratio isoxygen isotope data place the following constraints on>1:1 in Yemen and Ethiopia, the volume of (basaltic)magma genesis during flood volcanism in Yemen:melt produced and the rate at which it was produced is

(1) assimilation of heterogeneous Pan-African crust isat least three times greater than that currently inferredunequivocally confirmed to have been the major controlfrom study of the preserved volcanic volumes.on isotopic and much trace element variability in thisflood volcanic province.

(2) The assimilated crustal components include bothThe primary oxygen isotope signature upper and lower crust of variable age. Assimilation of

lower crust was a particularly important process in theBaker et al. (1996b) noted that the primary trace elementand isotopic signature of the Yemen flood basalts is Yemen flood volcanic province.

(3) Crustal contamination processes can be dem-characterized by LILE depletion relative to the high fieldstrength elements and a depleted Sr–Nd isotopic signature onstrated to have varied from combined assimilation and

fractional crystallization through to rapid assimilation ofthat approaches that of HIMU or PREMA mantle (Figs5 and 6). However, Pb isotope ratios are not as radiogenic crust by hot mafic magmas with little or no concomitant

fractionation.as HIMU mantle proper. These features are similar tothose exhibited by magmas produced by the Iceland (4) Rhyolitic magmas are not characterized by ex-

cessively high �18O values and/or crustal radiogenicplume and have been explained in terms of recentrecycling of oceanic lithosphere, i.e. the Afar and Iceland isotope ratios, which seems to preclude their origin by

wholesale crustal anatexis. The �18O data suggest theymantle plumes are immature HIMU mantle plumes(Thirlwall et al., 1994; Baker et al., 1996b). are probably either the product of extensive assimilation

with small amounts of crustal contamination from basalticIt is noteworthy that, to date, the least crustally con-taminated flood basalt samples from Yemen do not share parents or the result of melting underplated flood basalt

material.the particularly low �18O values that have been observedin some rocks from Hawaii, Iceland and some other (5) Least contaminated samples with low �18O have

radiogenic isotope ratios that approach those inferred forHIMU ocean islands (Eiler et al., 1997). Low K/Nb andBa/Nb ratios (<150 and <5, respectively) in Yemen flood the Afar plume, and these samples have �18O values

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isotopic composition of the Picture Gorge Basalt of the Columbiawithin the range of other crustally uncontaminatedRiver Basalt Group. Contributions to Mineralogy and Petrology 114,plume-related magmas, i.e. ocean island basalts.452–464.Laser fluorination oxygen isotope analysis of mineral

Brueckner, H. K., Elhaddad, M. A., Hamelin, B., Hemming, S.,separates is a powerful tool for examining the relative Kroner, A., Reisberg, L. & Seyler, M. (1995). A Pan-African originroles of crust and mantle in suites of continental volcanic and uplift for gneisses and peridotites of Zabargad Island, Red Sea:rocks. Future combined oxygen and osmium isotope a Nd, Sr, Pb and Os isotope study. Journal of Geophysical Research 100,

22283–22297.studies, along with in situ mineral multi-collector–Carlson, R. W. (1984). Isotopic constraints on Columbia River floodinductively coupled plasma mass spectrometry studies,

basalt genesis and the nature of the subcontinental mantle. Geochimicashould finally resolve the debate regarding the relativeet Cosmochimica Acta 48, 2357–2372.contributions of crust and LM to flood basalt magmas. Carlson, R. W., Lugmair, J. & Macdougall, J. D. (1981). ColumbiaRiver volcanism: the question of mantle heterogeneity or crustalcontamination. Geochimica et Cosmochimica Acta 45, 2483–2499.

Chazot, G. & Bertrand, H. (1993). Mantle sources and magma–ACKNOWLEDGEMENTS continental crust interaction during early Red Sea–Aden rifting in

southern Yemen: elemental and Sr–Nd–Pb isotope evidence. JournalAbdulkarim Al-Subbary is thanked for assistance withof Geophysical Research 98, 1819–1835.fieldwork in Yemen. Gerry Ingram helped with radio-

Chazot, G. & Bertrand, H. (1995). Genesis of silicic magmas duringgenic isotope analyses. The British Council, Royal SocietyTertiary continental rifting in Yemen. Lithos 36, 69–83.and the Industrial Association at RHUL supported this

Chazot, G., Lowry, D., Menzies, M. & Mattey, D. (1997). Oxygenresearch. The radiogenic and stable isotope laboratories isotopic compositions of hydrous and anhydrous mantle peridotites.at RHUL are University of London Intercollegiate facil- Geochimica et Cosmochimica Acta 61, 161–169.ities. Constructive reviews by John Eiler, Richard Carlson Chiba, H., Chacko, T., Clayton, R. N. & Goldsmith, J. R. (1989).

Oxygen isotopic fractionation involving diopside, magnetite, andand Chris Harris, which improved this manuscript, arecalcite: application to geothermometry. Geochimica et Cosmochimicagratefully acknowledged.Acta 53, 2985–2995.

Coffin, M. & Eldholm, O. (1993). Scratching the surface: estimatingthe dimensions of large igneous provinces. Geology 21, 515–518.

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