14. geochemistry of carbonate nodules and cements … · 2007-02-06 · 14. geochemistry of...

Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 139

14. GEOCHEMISTRY OF CARBONATE NODULES AND CEMENTS AND IMPLICATIONS FORHYDROTHERMAL CIRCULATION, MIDDLE VALLEY, JUAN DE FUCA RIDGE1

Paul A. Baker,2 Scott L. Cross,2 and Stephen J. Burns3

ABSTRACT

Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for their mineralogical,chemical, and isotopic compositions. These diagenetic carbonates include calcite, high-magnesium calcite, and dolomite. Thechemical sources for carbonate precipitation include alteration of basement rocks (calcium and strontium), diffusion from seawater(magnesium), recrystallization of biogenic calcite (calcium and carbon), oxidation of sedimentary organic matter (carbon), andoxidation of thermogenic methane (carbon).

The shallowest diagenetic carbonates from Hole 857C that were analyzed are dolomites. These are succeeded downcore byhigh-magnesium calcites and, finally, low-magnesium calcites. The shallowest diagenetic carbonates from Hole 858D are high-magnesium calcites which are succeeded downcore by low-magnesium calcites. Dolomites also occur in a limited depth zone inHole 858D. These dolomites have a seawater magnesium source, but their calcium must be solely derived from basement-derivedhydrothermal sources.

Temperature gradients calculated from oxygen isotopic compositions of the carbonates are 0.53°C/m in Hole 857C and10.5°C/m in Hole 858D. The distribution of temperatures is consistent with conductive heat transfer over the analyzed sections.

We hypothesize from chemical data that Hole 858D intersected a fault zone at 28 m below the seafloor. The fault zone isbelieved to be a major conduit for the hydrothermal fluids discharging at the nearby vent. The dissolved CO2 in this fluid has acalculated carbon isotopic composition of about -20‰ Peedee belemnite (PDB), implying its source from oxidation of sedimen-tary organic carbon. A small fraction of the hydrothermal fluid advects laterally at a sub-bottom depth of 3 to 7 m. This flow is notrapid enough to significantly affect the heat transport, but it does affect the mineralogical, chemical (iron and manganese contents),and isotopic compositions of the diagenetic carbonates.

INTRODUCTION

Young, sediment-covered seafloor spreading centers have rarelybeen sampled by deep-sea drilling. Systematic efforts to study suchenvironments by deep drilling have been made only on Deep SeaDrilling Project (DSDP) Leg 64 in the Guaymas Basin of the Gulf ofCalifornia, DSDP Leg 23 in the Red Sea, and Ocean Drilling Program(ODP) Leg 139 in Middle Valley of the northern Juan de Fuca Ridge(Fig. 1). Although this type of ridge represents a relatively minor pro-portion of all active spreading centers, the geological importance ofthis setting can hardly be overemphasized; all trailing edge continen-tal margins evolved from sediment-covered spreading centers. Like-wise, many of the world's great petroleum and sulfide ore accumu-lations originated in this type of sedimentary environment.

The primary goals of drilling on Leg 139 were to understand thenature of hydrothermal circulation at a sediment-covered ridge and toattempt to link hydrothermal circulation to the origin of sediment-hosted massive sulfide ore bodies. This chapter addresses severalaspects of both goals. Specifically, we discuss the thermal record pre-served in the sediments and the contributions of various sources to thechemistry and isotopic composition of hydrothermal solutions.

Unfortunately, it is difficult to frame this discussion in a contextof geological history. One of the frustrations of Leg 139 was that boththe magnetostratigraphic and biostratigraphic records were poorlypreserved. Future studies in this region must seek to improve our abil-ity to determine geological ages of important events.

This study relies on analyses of one component of the sediments:alkaline earth carbonates. We have studied these carbonates in three ofthe four sites on ODP Leg 139 (Fig. 2): Site 856, near a fossil sulfide

1 Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994. Proc. ODP, Sci.Results, 139: College Station, TX (Ocean Drilling Program).

2 Duke University, Department of Geology, Durham, NC 27708, U.S.A.3 Universitat Bern, Geologisches Institut, Baltzerstrasse 1, CH-3012, Bern, Switzer-

land.

ore deposit; Site 857, a hydrothermal "reservoir" site; and Site 858,adjacent to or within an active hydrothermal discharge zone. We pre-sent information about the occurrence and mineralogy of diageneticcarbonates, the stable isotopic and strontium isotopic compositions ofthese carbonates, and the major and minor elemental compositions ofthese carbonates. Many of our conclusions are, necessarily, prelimi-nary. Further refinements and modelling await more detailed analysesof the carbonates, associated sediments, and pore waters.

OCCURRENCE OF CARBONATE SEDIMENTS,NODULES, AND CEMENTS

Alkaline earth carbonates (predominantly calcite and dolomite) arean important component of sediments recovered during Leg 139. Theyoccur in a variety of forms. Biogenic calcite, predominantly foramini-fers and coccoliths, is the major source of sedimentary carbonate inmost holes. Because of the high rates of detrital sedimentation, thecarbonate content of sediments at all sites seldom exceeds 10 weightpercent (wt%) as CaCO3 (Davis, Mottl, Fisher, et al., 1992; Baker andCross, this volume). The highest values are found near the tops of holesin sediments of latest Pleistocene or Holocene age which, uniformlyacross Middle Valley, are finer-grained, more pelagic, and less detritalthan older sediments.

Biogenic calcite is rapidly recrystallized to diagenetic calcite down-core as a consequence of the high thermal gradients in Middle Valleysediments. In Hole 855C calcite fossils persist to greater than 100mbsf. Hole 856A is barren of foraminifers and nannofossils belowabout 50 mbsf; Hole 856B is barren below about 40 mbsf; Holes 857Aand 857C have poor calcareous microfossil preservation below 83mbsf; Hole 85 8 A is barren below 111 mbsf; Hole 85 8B is barren below16 mbsf; Hole 85 8C below 41 mbsf; and Hole 85 8D is barren belowabout 29 mbsf (Davis, Mottl, Fisher, et al., 1992). The disappearanceof calcareous microfossils deep in various holes is generally correla-tive with lower present-day thermal gradients in those holes. The mostobvious exceptions are Holes 856A and 856B. Evidence presented

313

P.A. BAKER, S.L. CROSS, S.J. BURNS

52°N

48*

130°W 126° 122°

Figure 1. Location map of Middle Valley, northern Juan de Fuca Ridge.

below suggests that Hole 856A (and probably Hole 856B) had muchhigher thermal gradients in the recent past.

Diagenetic or authigenic carbonates are present in various forms.Spherical or elliptical (elongate parallel to the bedding plane) nodulesor incipient nodules (see Davis, Mottl, Fisher, et al., 1992, figs. 12 and13, p. 303), in which the carbonate cements preexisting sediments,are most common in Holes 856A, 857C, and 858A. The nodules havediameters up to 10 cm. Occasionally, the carbonate nodules are con-centrically zoned with darker rims and lighter interiors. In Holes 857Cand 857D it is often difficult to distinguish between carbonate-cemented beds that have been rounded by drilling and carbonate nod-ules that are simply carbonate-cemented sediments with a roughlyspherical outer margin. Carbonate cements in these holes occur pref-erentially in the coarser-grained basal turbiditic sediments, whereas theoverlying hemipelagic mudstones tend to be noncalcareous. Small, tubu-lar nodules that appear to cement sediments that infilled burrow struc-tures are the most common form of diagenetic carbonate in Hole 85 8Dand also occur in Holes 858A and 858B. These nodules are usually 3to 5 cm long and a few mm in diameter, and can be straight or branched.Small (a few mm in diameter), roughly spherical carbonate nodules arealso common, particularly in Holes 858A, 858C, and 858D.

Several other less abundant forms of carbonate are present. Car-bonate micronodules that have been identified only by X-radiographyoccur in some parts of Holes 858A (see Rigsby et al., this volume).These may be equivalent to the above-mentioned small nodules. Occa-sionally, larger carbonate nodules, embrittled by cementation, are bro-ken and filled with pure carbonate veins (see Davis, Mottl, Fisher, etal., 1992, figs. 12 and 14, pp. 303-304). Unusual crab-shaped, calciticgrowths are found in several sections of Hole 857C (see Davis, Mottl,Fisher, et al., 1992, fig. 15, p. 305). Single crystals of calcite spar, upto 1 cm on edge, occur in parts of Holes 858B and 858C. Disseminated,

128°50W 128°30'

Figure 2. Site locations within Middle Valley.

zoned, euhedral dolomite crystals, about 50 micrometers on edge,occur in short intervals near the tops of Holes 85 8B and 85 8D. Onelimestone breccia, consisting of mud clasts cemented by calcite, occursnear the top of Hole 858C (see Davis, Mottl, Fisher, et al., 1992,fig. 15, p. 450).

The shallowest occurrence of most of the diagenetic carbonate nod-ules and cements is near the base of the sediment that contains calcare-ous microfossils. In Hole 856A, the shallowest nodules identifiedoccur at about 18 mbsf. Below 60 mbsf in Hole 856A, the only carbon-ate present in the sediments occurs in the form of carbonate nodules.In Hole 856B, no carbonate nodules were reported and the carbonatecontent of sediments below 40 mbsf is negligible. At Site 857 diage-netic carbonate occurs over an interval from about 46 mbsf (in Hole857A) to about 461 mbsf (in Hole 857C). In Hole 858A carbonatenodules are restricted to depths between 11 mbsf and 16 mbsf. In Hole858B carbonate nodules occur only between 3 mbsf and about 5 mbsf.In Hole 85 8C carbonate nodules first appear at less than 1 mbsf, andno carbonate was observed below 50 mbsf. In Hole 858D, carbonatenodules occur at less than 1 mbsf, but were not reported below 23 mbsf.

The close coincidence at most sites of the depths of the zone ofdisappearance of calcareous microfossils and the zone of abundanceof diagenetic carbonates suggests that diagenetic carbonate is par-tially derived from the dissolution of biogenic carbonate.

METHODOLOGY

Approximately 120 samples of carbonate cements and noduleswere extracted from Holes 856A, 857C, 858A, 858B, 858C, and

314

GEOCHEMISTRY OF CARBONATE NODULES AND CEMENTS

85 8D. The largest of the nodules were subsampled as many as fivetimes each. Samples were dried and powdered. All samples were ana-lyzed by powder X-ray diffraction (XRD) in order to determine thecarbonate mineralogy and the approximate magnesium content ofdolomites and calcites. Splits of these powders were also analyzed forstrontium isotopic ratios, stable carbon and oxygen isotopic ratios,and major and minor elemental concentrations.

Strontium isotopic analyses were done on acetic acid leaches ofsample powders. Strontium was isolated using a strontium-specificextraction resin (Horwitz et al., 1991). Samples were run on a VGSector 54 mass spectrometer at the Department of Geology, Universityof North Carolina, Chapel Hill. Strontium isotopic ratios are normal-ized to 86Sr/88Sr = 0.1194. All 87Sr/86Sr ratios are relative to a value of0.710250 for NBS SRM987. External precision is better than ±2 × 10"5.

Elemental concentrations were determined on buffered (pH = 5.0)acetic acid—ammonium acetate leaches. Approximately 100 mg ofsample powder was dissolved for about 2 hr in 10 ml of acid. Aftercentrifugation, the leachate was decanted and diluted for analysis.Elemental determinations were made by flame atomic absorptionspectrophotometry (AAS) using a Perkin-Elmer 5000 AAS. Separatepowder splits were subjected to the entire protocol in order to deter-mine analytical precision. Mean relative errors are 0.46% for calcium,1.29% for magnesium, 1.55% for strontium, 2.27% for iron, and1.63% for manganese.

Carbon and oxygen stable isotopic ratios were analyzed on splitsof the same sample powders. Approximately 10 mg of sample wasreacted in 100% phosphoric acid at 90°C (McCrea, 1950) in an on-line automated preparation system. The resulting CO2 was analyzedon a VG Prism II ratio mass spectrometer at the Geological Insti-tute, University of Bern, Switzerland. Repeated analyses of standardmaterials yielded a reproducibility of better than 0.10 per mil (%e) foroxygen and 0.05%c for carbon. No correction was made for the differ-ence in phosphoric acid fractionation factors between dolomite andcalcite (i.e., all data were adjusted using the phosphoric acid frac-tionation factor for calcite at 90°C). Values in Table 1 are reported rel-ative to the PDB standard.

RESULTS

The results of the chemical, mineralogical, and isotopic analysesare given in Table 1. The most pertinent of these results are discussedin the following sections. It should be noted that all of the sedimentsand the carbonate nodules from Hole 858D, Core 4H (20.2 to 23.04mbsf) are stratigraphically displaced as a result of coring artifacts.These sediments are all derived from higher in the section and mayhave fallen down the hole and been recored. For this reason, we con-sider variations of properties vs. depth for Cores 858D-1H and 858D-2H only, or we utilize the entire data set from Hole 85 8D by consid-ering variations of properties using oxygen isotopic ratios of nodulesas proxy indicator of depth.

MINERALOGY AND MAGNESIUM CONTENT OFDIAGENETIC CARBONATES

The diagenetic carbonates analyzed are composed of either low-magnesium calcite (LMC), high-magnesium (>4 molar percent[mol%]) calcite (HMC), or dolomite (Table 1). Dolomites occur asnodules near the top of Hole 857C, but are not found below 105 mbsfin that hole. Dolomites also occur as disseminated individual rhombiccrystals in a few intervals near the top of Hole 85 8D. These twooccurrences are quite distinct and are discussed separately.

In both holes, the dolomites usually display superstructure reflec-tions typical of well-ordered dolomites. In Hole 857C the dolomitescontain 38.9 to 47.5 mol% magnesium, whereas those in Hole 858Dare, on average, significantly less magnesian.

In Hole 857C, only one sample (857C-9R-1, 9-10 cm) containsboth dolomite and calcite. In this case, the calcite is HMC, containing

about 8 mol% magnesium (determined by XRD) and the dolomite isa protodolomite which lacks superstructure reflections and containsabout 40 mol% magnesium (as determined by XRD). All of the car-bonates at greater burial depths in Hole 857C are calcites. The magne-sium content of these calcites generally decreases downward (Fig. 3).

The inferred paragenesis (i.e., downcore succession) of carbonateminerals at Hole 857C is an interesting one: ordered dolomiteprotodolomite HMC —» LMC. The implied process, dedolomitiza-tion, is not uncommon in nature, but the intermediate phases have toour knowledge never been observed previously. The dedolomitiza-tion reaction can be written as:

MgCa(CO3)2 + Ca2+ = 2CaCO3 + Mg2 (1)

where the dissolved calcium in pore waters is probably supplied byanhydrite dissolution or plagioclase alteration. The magnesium/cal-cium molar ratio in present-day pore waters at the depth of apparentdedolomitization (110 mbsf) is about 0.5 (Davis, Mottl, Fisher, et al.,1992). At the approximate in-situ temperature of 80°C (Davis, Mottl,Fisher, et al., 1992), this ratio is still well inside the hypothesizedstability field for dolomite according to Land (1987). The reason forthis discrepancy is unknown at present.

In Hole 85 8D, dolomites occur as dispersed rhombs within sedi-ments at depths of 12.3 to 13.6 mbsf. The dolomite rhombs oftencoexist with a small amount of HMC or LMC. Molar ratios of magne-sium/calcium in pore waters in this depth range average about 0.28(Davis, Mottl, Fisher, et al., 1992). These solutions are within the dolo-mite stability field (Baker, unpubl. data). At 200°C (approximately 20mbsf), magnesium/calcium ratios in pore waters decrease to values aslow as 0.08. Experimental work has shown that dolomite should be un-stable relative to calcite in the latter solutions (Rosenberg et al., 1967).

In sediments at depths shallower than 12.3 mbsf, HMC is authi-genic. The magnesium content of the calcites decreases with increasingburial depth (Fig. 4). The shallowest carbonate, occurring at 0.46 mbsf,contains 11.3 mol% magnesium. HMC of 11.3 mol% Mg is highlyundersaturated in cold oceanic bottom waters at this water depth (2467m); thus it forms because of some process that maintains a high degreeof supersaturation (relative to LMC or dolomite) of sedimentary porefluids. This process, microbial sulfate reduction, is discussed in moredetail in a following section.

A layer of aragonite was found in Hole 857A-6H-4, 65-66 cm.High pore water alkalinities were reported at this level (Davis,

Mottl, Fisher, et al., 1992, p. 334).

OXYGEN ISOTOPIC THERMOMETRY

Oxygen isotopic compositions of diagenetic carbonates are plot-.ted against sample depth for Hole 856A (Fig. 5), Hole 857C (Fig. 6),Hole 858A (Fig. 7), and Hole 858D, Cores 1H and 2H (Fig. 8). In theabsence of reported measurements, we assume δ 1 8 θ = 0% (standardmean ocean water [SMOW]) for the oxygen isotopic composition ofpore waters. Using the temperature dependence of the fractionationfactor for oxygen isotopes between calcite and water (O'Neil et al.,1969), we can calculate temperatures of carbonate precipitation (Tc).Table 2 shows maximum calculated temperatures (minimum valuesof δ 1 8 θ) for each hole and compares those to present-day temper-atures (Tm) determined from in-situ measurements (Davis, Mottl,Fisher, et al., 1992). The generally good agreement between these twotemperature determinations confirms the utility of isotopic geother-mometry in these cores and is also evidence for the origin or rapidrecrystallization of the carbonates at their present-day burial depths.

Information on present-day thermal gradients can be determined byexamining the isotopic temperatures at Holes 857C and 858D. The iso-topic temperatures are shown for all of the calcites from Holes 857C(Fig. 9) and Cores 858D-1H and -2H (Fig. 10). Dolomite temperatureswere calculated using a dolomite-calcite constant low-temperature frac-tionation of 3.1%e (Fritz and Smith, 1970). At both sites, the increaseof temperature with depth is nearly constant, about 10.5°C/m at Hole

315

Table 1. Geochemistry and mineralogy of diagenetic carbonates.

Core, section,interval (cm) Description

Labsample #

139310139311139315-1139315-2139319139320-1139320-2139321139322-1139322-2139323139324139326139327-1139327-2

139007-1139007-2139007-3139010-1139010-3139012-1139012-2139012-3139012-4139012-5139013-1139013-2139013-3139013-4139014-1139014-2139014-3139014-A139016-1139016-2139019-1139019-2139021139022139037-3139037-A139037-B139038-1139038-2139038-3139038-4139039-3139041-2139041-A139043-1139043-2139043-3139043-A139043-B

Depth(mbsf)

14.5321.4648.0248.0258.563.5863.5864.4865.2265.2269.4978.4379.6280.3380.33

76.176.176.186.386.395.295.295.295.295.2

104.9104.9104.9104.9114.6114.6114.6114.6115115143.6143.6144144.8241.4241.4241.4242242

242259.5269.3269.2276.2276.2276.2276.2276.2

δ ' J C‰, PDB

-10.65-7.99

-24.94-26.01-17.34-17.18-19.47-18.12-18.33-25.92-19.64

-5.77-14.97-16.31-16.28

-14.23-12.27-15.62-16.11-15.19-16.02-15.99-15.79-15.77-15.95-15.52-15.82-15.74-15.75-18.09-17.83-18.79-17.1-14.29-15.73-17.39-17.48-16.06-16.21-11.87-12.05-11.89-10.89

-8.9-7.46-6.61

-11.14-7.74-8.24

-14.5-15.26-14.12-14.74-10.33

δ'°O‰, PDB

-8.51-11.99

-9.27-9.33

-18.98-12.97-11.63-10.93-12.21

-9.82-10.65-12.61-11.27-12.32-10.92

0.260.070.41

-0.65-1.24-1.84-1.66-1.49-1.53-1.54-1.64-1.68-1.78-1.77-5.61-3.54-4.31-7.01-8.13-8.79

-10.09-10.01-11.18-11.82-13.15-13.62-13.48-14.6-14.49-15.23-11.21-14.57-14.99-14.82-15.88-16.85-16.32-16.51-14.65

Sr 87/86

0.705823

0.7055270.705467

0.705077

0.708765

0.7087110.7086840.708672

0.7082120.7083050.708393

0.7074280.70755

0.7069

0.707010.7067540.7069050.706298

0.7059130.706981

Mole%Cadolomite

50.5451.6252.5752.1154.3453.0552.3254.5252.7752.8153.2754.8053.8353.89

Mole %Cacalcitc

88.6793.2994.2693.86

95.9495.1796.9797.3793.2092.75

92.8195.5091.11

81.93

67.5687.29

88.2591.0791.5395.2294.89

94.18

96.4795.4795.3994.1994.5693.6094.8194.27

Mole%Mgdolomite

47.5246.5545.4840.4938.0140.1941.6539.8642.2041.8239.8038.8739.7439.63

Mole%Mgcalcite

4.993.883.724.09

2.383.061.921.734.345.26

4.251.905.02

12.81

26.297.43

6.075.645.252.292.95

4.04

2.131.882.063.733.384.123.21

Sr/Ca(mol/mol)

3.32E-043.64E-031.75E-041.83E-04

1.89E-041.84E-041.66E-041.99E-041.57E-042.11E-04

2.01E-041.88E-041.88E-04

5.38E-045.30E-045.35E-043.76E-043.29E-043.31E-043.20E-043.12E-043.41E-043.26E-043.78E-043.61E-043.69E-043.89E-042.07E-04

3.63E-041.73E-04

1.82E-042.24E-041.95E-041.79E-041.91E-04

1.66E-04

2.27E-041.83E-041.85E-042.21E-042.69E-042.08E-042.43E-041.74E-04

Mole%Fecalcite

1.89(>-I.S

0.24

0.04

0.290.39

0.130.080.710.58

1.170.261.30

1.25

2.100.89

1.530.990.930.380.37

1.04

0.550.550.571.211.071.441.101.11

Mole%Fedolomite

1.411.351.375.685.554.744.434.243.X04.015.154.694.724.77

Mole%Mncalcite

4.452.351.772.01

1.391.38

0.811.751.42

1.772.332.57

4.01

4.054.38

4.152.30

2.111.79

0.75

0.852.091.990.860.990.850.88

Mole%Mndolomite

0.530.480.581.722.102.021.611.381.231.361.781.631.711.70

Zn/Ca(mol/mol)

3.O8E-O52.09E-052.67E-052.52E-05

1.52E-051.61E-057.54E-062.09E-053.18E-O51.71E-03

1.80E-051.37E-052.44E-05


4.19E-052.53E-05

2.44E-046.06E-057.22E-053.62E-052.34E-05

2.38E-052.12E-052.03E-054.87E-054.11E-057.98E-053.89E-051.21E-04

X-ray

HL

1.

1.L

LL

D*

D*

D*

D*

1 ID*

HH*

LL

L

L

LL*

139-856A-3H-2, 118-1193H-7, 34-356H-5, 132-1356H-5, 132-1357H-6, 80-858H-3, 88-918H-3, 88-918H-4, 28-318H-4, 102-1068H-4, 102-1068H-CC, 8-109H-7,23-251 OH-1,92-9410H-2, 13-1510H-2, 13-15

139-857C-4R-CC, 3-54R-CC, 3-54R-CC, 3-56R-1,6-76R-17R-17R-17R-7R-1,2-37R-1,2-38R-CC, 4-58R-CC, 4-58R-CC, 4-58R-CC, 4-5

,6-7,2-3,2-31,2-3

9R-9R-9K9R-9R-9R-

, 9-10, 9—10, 9—10, 9-10, 49-50, 49-50

12R-1, 8-912R-1, 8-912R-1, 47-4812R-1, 126-12722R-1, 108-11222R-1, 108-11222R-1, 108-11222R-3, 1—522R-3, 1-522R-3, 1-522R-3, 1-524R-1, 31-3525R-1, 41^4-325R-1, 41—4326R-CC, 1-326R-CC, 1-326R-CC, 1-326R-CC, 1-326R-CC, 1-3

Small indurated noduleSmall indurated noduleIndur. nodule, light gray zoneIndur. nodule, dark gray crystallineLarge noduleLarge poorly indur. nodule, outerLarge poorly indur. nodule, innerLarge nodule, sandyLarge nodule, sandy, gray outer zoneLarge nodule, sandy, dark gray inner zoneSmall indurated nodulesIndur. noduleIndur. noduleIndur. nodule, medium gray zoneIndur. nodule, dark gray zone

Large well-indur. nodule or sis, outerLarge well-indur. nodule or sis, middleLarge well-indur. nodule or sis, innerWell-indur. sis or large nodule(?), outerWell-indur. sis or large nodule(?), innerLarge well-indur. nodule, outerLarge well-indur. nodule, innerLarge well-indur. nodule, innerLarge well-indur. nodule, outerLarge well-indur. nodule, outerLarge well-indur. nodule or sis/ms, outerLarge well-indur. nodule or sis/ms, middleLarge well-indur. nodule or sis/ms, outerLarge well-indur. nodule or sis/ms, outerLarge well-indur. nodule or sis/ms, outerLarge well-indur. nodule or sis/ms, innerLarge well-indur. nodule or sis/ms, outerLarge well-indur. nodule or sis/ms, outerIndurated sis/fine ssIndurated sis/fine ssNodule, light gray zoneNodule, dark gray zoneIndurated fine ss/sisPoorly indurated ms/ssIndurated laminated sisIndurated laminated sisIndurated laminated sisLarge nodule, color-zoned; innermost zoneLarge nodule, color-zoned; 2nd zoneLarge nodule, color-zoned; 3rd zoneLarge nodule, color-zoned; outermost zoneIndurated laminated sis/ms; light gray bandIndur. sis/ms; light-colored lense, innerIndur. sis/ms; light-colored lense, outerLarge thin nodule, zoned; light gray zone, outerLarge thin nodule, zoned, light gray zone, innerLarge thin nodule, zoned; light gray zone, outerLarge thin nodule, zoned; light gray zone, outerLarge thin nodule, zoned; darker outer zone

Table 1 (continued).

Core, section,interval (cm) Description

Labsample #

Depth(mbsf)

δ " C‰, PDB

δ O Mole%Ca Mole %Ca‰, PDB Sr 87/86 dolomite calcite

Mole%Mg Mole%Mgdolomite calcite

Sr/Ca Mole%Fe Mole%Fe Mole%Mn Mole%Mn(mol/mol) calcite dolomite calcite dolomite

Zn/Ca(mol/mol) X-ray

36R-2,36R-2,37R-1,39R-1,39R-1,39R-2,45R-1,48R-1,50R-1,50R-1,

35-3761-63104-10657-5957-59107-109128-13040-42102-105102-105

139-858A-2H-6, 99-1002H-7, 28-302H-7, 30-312H-7, 30-312H-7, 32-332H-7, 32-332H-7, 32-333H-1,66-685H-2,1-45H-2, 1-45H-4, 63-655H-6, 40-455H-CC, 10-115H-CC, 10-116H-3, 61-639X-1,25-2812X-CC, 13-14

139-858B-2H-2, 24-252H-2, 26-272H-2, 29-30

139-858C-2H-6, 50-522H-7, 33-352H-7, 33-3514X-CC, 8-9

139-858D-1H-1, 46-481H-1, 51-551H-1,92-941H-1, 114-1171H-2, 57-591H-3, 132-1351H-4, 89-911H-5, 13-151H-5, 56-581H-5, 64-671H-5, 76-801H-5, 89-931H-6, 42-451H-6, 57-591H-6, 90-931H-CC, 8-102H-2, 62-64

Well-indurated msWell-indurated msLarge nodule, sand-size grainsNodule(?)Nodule(?); dark crystalline mat'lIndur. sis/fine ss w/dark (organic?) layerIndur. ms/sis w/light-colored lense shaped layerWell-indur. msWell-indur. ms, It. gray cemented zoneWell-indur. ms, It. gray cemented zone

Small nodule w/clam shellSmall noduleSmall nodule, interiorSmall nodule, marginSmall nodule, innerSmall nodule, middleSmall nodule, outerSmall noduleNodule, zoned; inner zoneNodule, zoned; outer zoneSmall nodulesNoduleNodule, zoned; innerNodule, zoned; outerNodulePoorly indurated ms or noduleLarge nodule or well-indur. ms

Silty mud/muddy siltSilty mud/muddy siltSilty mud/muddy silt

Small nodule/indur. ms "lump"; innerSmall nodule/indur. ms "lump"; outerSmall nodules(?)

Poorly indur. clump/nodulePoorly indur. clump/noduleNoduleSmall nodulePoorly indur. clump/noduleNoduleSmall noduleNodule (burrow-filling?)NoduleNodule (burrow-filling)Nodule (burrow-filling)Nodule (burrow-filling)Small nodulesSmall nodulesVery small nodulesVery small nodulesMud clumps/sm nodules(?)

139067-2139068-2139071139075-1139075-2139077139093-2139098-1139104-1139104-2

139138139139139140-1139140-2139141-1139141-2139141-3139142139159-1139159-2139164139167139169-1139169-2139172139180139187

139298139299139300

139215139216-1139216-2139224

139226139227139228139229139231139233

139236139237139239139240139241139242139245139246139247139248139252

347.9348.2351.8361361363395.6409.1419.3419.3

10.911.6811.711.711.7211.7211.7212.632.432.436

40404462.781.9

8.948.%8.99

12.8

84.3

0.460.510.92

1.12.14.35.46.136.566.646.766.897.9

8.18.49.111.4

-13.69-13.14-11.27-17.18-16.59-15.81-13.09-9.68

-11.86-10.42

-26.85-35.41-25.45-26.53-35.46

-35.1-34.89-36.86-9.22

-10.14-18.55-13.31-12.32-8.12

-16.74-16.98-15.45

-17.49-12.84-12.87

-25.08

-16.52

-35.85

-33.56-27.52-31.94-26.08-25.25-16.97-17.54-16.49-18.29-18.07-18.25-18.2-16.6

-17.03-14.02-14.13

-19.51-19.28-18.31-18.77-16.74-19.19-21.63-21.78-21.5-21.69

2.713.032.563.743.362.713.243.84

-10.06-10.95-13.89-11.48-10.79-13.95-13.87-11.97-18.73

-6.1-8.37-8.17

-14.01

-13.09

3.062.862.67

1.14-2.21-7.28-8.65-8.77-9.96-9.83

-10.22-11.06-11.2

-12.35-11.4-14.98

0.706482

0.705894

0.7060720.706108

0.705974

0.7067340.707359

0.706216

0.706143

0.7055640.7054510.707414

0.704631

36.0144.6747.28

96.5295.9995.7694.0595.7995.8697.5095.5396.02

96.6096.4095.0894.34

96.3395.77

94.2994.1893.95

94.1694.2192.41

95.2393.55

88.6190.9594.1992.0190.7091.94

86.3391.2191.37

91.4291.6892.5993.9892.5890.2495.83

63.9955.1852.59

2.242.384.69

2.673.38

1.38:•α..

2.23

3.023.133.594.63

3.12

3.58

4.14

3.162.90

3.69

3.076.25

3.22

4.96

11.338.875.067.627.245.13

4.745.00

4.904.654.722.89

5.056.172.62

3.34E-043.59E-044.90E-042.85E-042.18E-042.74E-041.74E-042.09E-041.59E-04

3.60E-042.59E-043.92E-044.78E-04

2.33E-045.10E-04

2.94E-041.87E-042.03E-04

2.20E-041.62E-043.67E-04

1.98E-031.03E-O38.10E-04

1.57E-042.29E-04

5.23E-044.12E-043.51E-043.84E-042.47E-041.74E-044.20E-042.81E-043.52E-04

2.72E-042.91E-042.17E-042.40E-043.88E-042.29E-041.53E-04

0.770.650.570.410.350.420.650.64

0.190.130.110.12

0.210.03

0.430.760.93

1.100.810.63

0.210.194.38

0.060.030.030.04

1.061.763.052.792.16

2.252.24

1.822.461.692.860.62

0.961.001.210.691.13(Mil0.701.371.11

0.190.331.220.92

0.341

1.141.902.22

1.061.910.71

1.341.31

0.010.150.730.331.001.161.801.261.47

1.431.440.870.670.690.72

1.85E-052.06E-057.15E-051.44E-05

1.66E-052.70E-055.05E-051.75E-05

2.82E-051.93E-054.11E-052.57E-05

2.44E-051.97E-05

3.87E-051.99E-051.93E-05

4.91E-052.82E-052.10E-05

2.36E-04

0.16 9.63E-050.13 1.75E-04

2.69E-055.54E-05

1.75E-051.63E-051.49E-052.72E-052.00E-051.91E-053.14E-053.34E-053.36E-05

2.43E-052.68E-053.28E-053.78E-054.48E-051.68E-042.37E-05

LLLLLI.

LL

Lminor L

LLLLL

LLLLLLLI.L

minor D

DD

L*

L

HHHHHHHHHHHHHHHHDL

Table 1 (continued).

Core, section,

interval (cm)

2H-3, 2-32H-3, 5-62H-3, 6-82H-3, 15-182H-3, 18-192H-3, 21-222H-3, 24-252H-3, 29-302H-3, 119-1202H-3, 124-1252H-3, 129-1302H-4, 40-422H-4, 50-522H-4, 55-562H-4, 94-972H-5, 78-802H-5, 82-834H-2, 18-204H-2, 48-504H-2, 52-544H-2, 58-604H-2, 83-844H-2, 94-984H-2, 103-1044H-2, 109-1104H-2, 126-1274H-2, 148-1504H-3, 15-174H-3, 148-1504H-4, 0-2

Description

Silty mud/muddy siltSilty mud/muddy siltSmall nodulesPoorly indur. mud/ large noduleSilty mud/muddy siltSilty mud/muddy siltSilty mud/muddy siltSilty mud/muddy siltSilty mud/muddy siltSilty mud/muddy siltSilty mud/muddy siltSmall poorly indur. nodules/clumped siltSilt/mudNoduleMud/siltClumped silt/mudSilt/mudNodule (burrow fill)NoduleNoduleNoduleNoduleNoduleSmall nodule (burrow fill)Small nodule (burrow fill)Small nodule (burrow fill)NoduleNoduleNoduleSmall nodule (burrow fill)

Labsample #

139301139302139254139255139303139304139305139306139307139308139309139259139260139261139263139266139267139271139272139274139277139278139279139280139281139283139285139286139291139292

Depth(mbsf>

12.3212.3512.3112.3512.4812.5112.5412.5913.4913.5413.5914.214.314.3514.716.0816.1220.220.520.5420.6220.8720.9821.0721.1221.321.52212323.04

δ°C‰, PDB

-11.36-11.72-10.95-10.7-11.57-10.62-10.47-17.5-11.79-12.55-12.19-16.97-16.75-16.84-16.62-19.07-19.2-17.58-27.77-28.02-18.51-18.4-33.96-17.72-19.46-18.7-33.89-19.51-29.47-34.22

δ' 8 O‰, PDB

-7.79-8.19-7.93-7.42-6.4-7.03-6.76-3.24-7.7-9.41-9.3

-17.26-17.04-17.18-16-18.76-19.1-12.54

1.882.55

-7.19-7.31

2.69-10.94-17.58

-7.592.39

-18.782.462.66

Sr 87/86

0.7053310.7050350.7049940.7053520.7051790.7050730.705026

0.704816

0.7051590.704485

0.706089

0.70593

Mole%Cadolomite

63.6371.2363.97

54.8155.7258.7661.4969.4565.1366.21

Mole %Cacalcite

95.7596.1096.6995.9095.4895.7791.2692.66

90.3190.1288.0189.3794.7789.5092.5495.9093.0592.76

Molc%Mgdolomite

32.4325.2032.10

40.2139.6137.4835.0527.6231.6730.57

Molc%Mgcalcite

2.882.641.992.803.112.775.566.41

5.095.47

11.927.023.965.827.352.856.427.12

Sr/Ca(mol/mol)

1.54E-035.21E-046.41E-04

6.70E-047.87E-041.33E-O32.45E-039.90E-041.14E-038.84E-042.65E-042.62E-041.40E-042.37E-041.58E-041.64E-042.35E-043.89E-04

2.41E-042.92E-045.40E-043.11E-042.05E-042.64E-043.45E-041.40E-043.80E-043.54E-04

Mole%Fecalcite

0.400.220.180.230.110.132.200.11

3.253.300.022.410.063.490.020.090.020.03

Mole%Fedolomite

2.902.292.67

3.4X3.3X2.562.462.192.432.40

Mole%Mncalcite

0.971.041.141.071.311.330.990.X1

1.351.110.041.201.211.190.091.170.500.09

Mole%Mndolomite

1.031.281.26

1.501.291.201.000.730.770.81

Zn/Ca(mol/mol)

3.94E-041.07E-049.73E-05


2.15E-052.49E-051.95E-053.12E-051.37E-043.63E-052.13E-053.O8E-O51.68E-051.87E-05

X-ray

minor DD>LD>L

DDDD

minor DD>HD>LD>L

LLLLLLUH

HHHHLHHLHH

Note: All concentrations are reported in units of mole% of the carbonate fraction or as mol/mol ratios (in the case of strontium and zinc). The last column contains the results of qualitative X-ray diffraction of the carbonate phases. H = high-magnesium calcite; L = low-magnesiumcalcite; D = dolomite. An asterisk identifies sub-samples that have been combined into one sample for the purposes of X-ray diffraction.


3 0

20

10

15

10

-5

-10CO

a

CO

O- -15a>a

- 2 0

100 200 300

Depth (mbsf)

Figure 3. Magnesium contents of calcites vs. depth for Hole 857C.

4 0 0

10

Depth (mbsf)

Figure 4. Magnesium contents of calcites vs. depth for Cores 858D-1H and -2H.

10 20 30 40 50 60

Depth (mbsf)

500

m1

I 1

1

—m

——

—

-

ED

ID

O

•

• B % B •

a m oil a 1•

I

20

β

m

1

ü i

i

ü

D

f. 1

1

α

i

α

•

I

ES -

n •

1 1 1

70 80 90 100

Figure 5. Oxygen isotopic composition of calcites vs. depth for Hole 856A. Reported as ‰ relative to the PDB standard.

319


10(P

DB

CO

6

De

l

0

-10

- 2 0

-30

10

Q

α.

O- -10<D

Q

-20

10

S oα.

o_ -10ΦQ

-20

100 200 300 400

Depth (mbsf)

Figure 6. Oxygen isotopic composition of dolomites and calcites vs. depth for Hole 857C.

10 20 30 7040 50 60

Depth (mbsf)

Figure 7. Oxygen isotopic composition of calcites vs. depth for Hole 858A.

10

Depth (mbsf)

Figure 8. Oxygen isotopic composition of calcites vs. depth for Cores 858D-1H and -2H.

5 0 0

90 100

320


ü

<DX3

ΦCX.

EΦ

3 0 0

2 0 0

100

100 200 300

Depth (mbsf)

4 0 0 5 0 0

Figure 9. Calculated temperatures vs. depth for Hole 857C. The line intercepts bottom-water temperature at the sediment-water interface and passes through thehottest values at any given depth. Its slope is 0.53°C/m.

200

Uσ>Φ

Φ^ 100

β

α.EΦ

Figure 10. Calculated temperatures vs. depth for Cores 858D-1H and -2H. The line intercepts bottom-water temperature at the sediment-water interface and passesthrough the hottest values at any given depth. Its slope is 10.5°C/m.

858D and about 0.53°C/m at Hole 857C. These data are consistentwith an interpretation of conductive heat flow within the sedimentcolumn in both holes indicating that the carbonates apparently remainin or near isotopic equilibrium with the pore fluids at the present-dayburial depths. Although there is less adequate data coverage, a signifi-cant number of samples from Hole 85 8 A indicate temperatures hotterthan predicted by the present-day geotherm.

Hole 856A has had a more complicated thermal history than theother analyzed sections. The shallowest carbonate sample analyzed,a small nodule from 14.5 mbsf, records a temperature of 60°C, thelowest temperature calculated for any of the samples from this hole.The present-day temperature at this burial depth is estimated to beabout 10°C (Davis, Mottl, Fisher, et al., 1992). Although the age ofthe sediment at 14.5 mbsf is not known, we conclude that hydrother-mal activity associated with emplacement of mafic sills and, possibly,with formation of the nearby massive sulfide ore body (Davis, Mottl,Fisher, et al, 1992), affected nearly the entire stratigraphic column;therefore alteration was very recent. The lack of a significant down-hole temperature gradient is probably best explained by carbonate

Table 2. Comparison of calculated and measureddownhole temperatures.

Hole

856A857C858A858D

δ 1 8 o(SMOW)

11.348.45

11.6011.22

Depth(m)

58409

8216

T c

(°C)

169222165171

Tm

(°C)

31224155160

Note: δ O (SMOW) is the minimum oxygen isotopic valuefor a carbonate sample at each hole. The sub-bottomdepth of that sample is given in the third column. T c =temperature calculated from the isotopic composition ofthe sample. T m = temperature calculated at that sampledepth from downhole measurements.

321

P.A. BAKER, S.L. CROSS, SJ. BURNS

0

-5

(PD

B

ü

Del

-10

-15

20100 200 300

Depth (mbsf)

Figure 11. Carbon isotopic composition of dolomites and calcites vs. depth for Hole 857C.

400 500

recrystallization. We hypothesize that original oxygen isotopic com-positions of carbonates precipitated prior to hydrothermal activitywere reset to varying degrees during this recent event.

CARBON ISOTOPES AND CARBON SOURCES

One of the important results of this study is identification of thevarious carbon sources to the authigenic carbonates. To the extent thatthese carbonates are a product of hydrothermal activity, the carbonsources of the nodules reflect the carbon sources of the hydrothermalfluid. Because carbon is one of the most abundant redox-sensitive orredox-controlling elements in the hydrothermal fluids of Middle Val-ley, its chemistry is of great interest. Discussion will focus on the twoholes with the most complete data sets.

Hole 857C (Fig. 11) displays features typical of authigenic carbon-ates from many organic-rich sedimentary profiles previously analyzed(e.g., Claypool and Kaplan, 1974; Kelts and McKenzie, 1982; Bakerand Burns, 1985). Carbon is buried in the form of biogenic calcite(δ13C O‰ PDB) and organic carbon δ13C -20‰ PDB). Throughoutthe upper 50 to 100 mbsf of the hole, microbial sulfate reduction resultsin an input to the pore waters of oxidized organic carbon, mostly in theform of HCO3 (δ13C -20‰ PDB). Because of the steep thermalgradients at this site, microbial sulfate reduction is probably succeededby thermocatalytic sulfate reduction over the depth range of 100 to 200mbsf. Over the same depth range, it is likely that pore water remainsnear equilibrium with respect to anhydrite (Davis, Mottl, Fisher, et al.,1992). Sulfate reduction is complete (below approximately 200 mbsf)when dissolved sulfate is almost totally depleted in pore fluids andwhen anhydrite is totally dissolved from the sediments. Again, becauseof the relatively high thermal gradient, thermal maturation of sedimen-tary organic carbon becomes important at these relatively shallowburial depths and thermocatalytic decarboxylation results in an inputof isotopically depleted (δ13C -20%e dissolved CO2 to pore fluids.

One unusual aspect of this pattern of organic diagenesis is theabsence of a zone of abundant biogenic methane. In previously ana-lyzed continental-margin sedimentary successions, methanogenesisoften imparts a positive carbon isotopic signature to authigenic car-bonates. In the present sediments, most of the methane in the porefluids appears to be derived from thermal cracking.

When the temperature-dependent fractionation factors for carbonisotopes between dolomite and calcite (Sheppard and Schwarcz, 1970)and between calcite and dissolved bicarbonate (Mook et al., 1974 andMalinin et al. reported by Friedman and O'Neil, 1977) are applied, we

calculate that the carbon isotopic composition of dissolved bicarbon-ate at Hole 857C is surprisingly uniform throughout the entire hole,ranging from about -3‰ to -13%e, but overwhelmingly -10 ± 2‰.This suggests a roughly equal contribution of carbon to the pore fluidsfrom the oxidation of organic carbon and from the recrystallization ofbiogenic calcite.

The depth variation of δ13C in authigenic carbonates in Hole 858D(Cores 1H and 2H) is shown in Figure 12. Applying the appropriateequilibrium fractionation factors, we conclude that pore water bicar-bonate in equilibrium with the authigenic carbonates has δ13C -20%oPDB between 5 and 20 mbsf and decreases to values of about -38‰PDB just below the sediment-water interface. Thus, the deeper porewaters in this hole, which resemble modern vent fluids in many of theirchemical characteristics (Butterfield et al., this volume), contain car-bon that is almost completely derived from the oxidation of sedimen-tary organic carbon. At depths less than 5 mbsf, methane oxidationduring microbial sulfate reduction results in a more negative carbonisotopic composition of dissolved bicarbonate. As noted above, meth-ane oxidation maintains the high degree of carbonate supersaturationin the pore waters necessary to allow the formation of HMC. Verynegative carbon isotopic signals were also observed in carbonates fromthe upper 13 mbsf of Hole 858A, at 12.8 mbsf in Hole 858C, and inrecrystallized LMC samples at 48 mbsf in Hole 856A. Goodfellow andBlaise (1988) and Al-Aasm and Blaise (1991) have previously re-ported similar isotopic compositions of carbonates recovered by tradi-tional piston coring within the bounds of the active vent field.

STRONTIUM AND STRONTIUM ISOTOPES

Strontium in carbonates in the Middle Valley comes from four mainsources: seawater (87Sr/86Sr ~ 0.7091); biogenic marine carbonate sedi-ments (87Sr/86Sr ~ 0.709); detrital sediments (strontium isotopic ratiosnot yet measured); and oceanic basement rocks (87Sr/86Sr ~ 0.703).

In Hole 857C the 87Sr/86Sr ratios of the carbonates decrease withincreasing burial depth (Fig. 13). We observe a better correlation withδ 1 8 θ than with burial depth (this is expected, because most of thesesamples formed at a somewhat shallower burial depth than the oneat which they presently occur). The shallowest carbonates analyzedapproach marine values of 87Sr/86Sr, indicating a probable marine-carbonate source of shallow-pore-water strontium. Linear extrapola-tion of the trend in Figure 13 leads to an isotopic ratio of 0.703 at adepth of about 600 mbsf. This is nearly coincident with the depthinterval (610 to 615 mbsf) at which high permeabilities were inferred


-10

CO

Q -20Q.

CO

I

O

Φ -30

- 4 0

mm

mmm

n Bü _.

E3

a

El

1

m •

m

10

Depth (mbsf)

Figure 12. Carbon isotopic composition of dolomites and calcites vs. depth for Cores 858D-1H and -2H.

20

CO00

co

wCO

0.709

0.708

0.707

0.706

0.705100 200 300 400

Depth (mbsf)

Figure 13. Strontium isotopic composition of calcites and dolomites vs. depth for Hole 857C.

500

from packer experiments (Davis, Mottl, Fisher, et al., 1992, p. 373).Our interpretation of these data is that pore water at Hole 857C con-tains strontium that is a mixture (by the mechanism of diffusion) ofseawater strontium and basaltic strontium. Furthermore, the porefluids within basement in the permeable zone are a pure basaltic end-member with respect to their strontium isotopic composition.

Many of the igneous rocks recovered at Site 857 are heavily alteredas is indicated by an increase in wt% MgO (see Davis, Mottl, Fisher,et al., 1992, p. 345). In almost all cases, alteration also involves anequivalent loss of calcium and, often, strontium (see Davis, Mottl,Fisher, et al., 1992, table 26, p. 371). This alteration is the likely sourceof basaltic strontium to basement pore fluids.

In Hole 858D, Cores 1H and 2H, 87Sr/86Sr ratios (Fig. 14) varywithin the range of 0.7062 in the shallowest sample (0.5 mbsf) to0.7045 (14.2 mbsf). In the deeper samples, it appears that up to 75% ofthe strontium is of basaltic origin and the remainder is of marine origin.Similar values are also found in carbonates from Holes 856A and858A. In Hole 858A the three samples analyzed display increasingstrontium isotopic compositions with increasing depth. We tentativelyattribute the higher isotopic ratios to the downward or lateral advection

of seawater along the flanks of hydrothermal discharge zones and par-tial entrainment of these fluids as they heat and mix. This hypothesismerits further investigation.

The strontium concentration of diagenetic carbonates is a functionof the strontium-to-calcium ratio of pore water solutes and the distribu-tion coefficient (k,r) of strontium in calcite (or dolomite, if appropriate).The distribution coefficients are somewhat temperature-dependent(Baker, unpubl. data), and may also be dependent on rate of precipita-tion (e.g., Lorens, 1981) and the magnesium content of the calcite (e.g.,Mucci and Morse, 1983).

In Hole 857C, the strontium content of calcites increases downcoreto a maximum at about 350 mbsf and then decreases in deeper samples(Fig. 15). This peak is similar to the peak in dissolved strontium con-centration that spans an interval from about 275 to 300 mbsf (Wheatand Mottl, this volume). The offset between the solid peak and thesolute peak may attest to the "memory" of carbonate nodules as theysuccessively accrete carbonate from pore waters of varying composi-tion (assuming a steady-state distribution of dissolved strontium).

In Hole 858D, the strontium contents of calcites (Fig. 16) are notclosely related to equilibrium values predicted from pore water chem-

323

P.A. BAKER, S.L. CROSS, SJ. BURNS

0.707

0.706

0.705

0.704

m m

mn

n

iE H

u

ü

10

Depth (mbsf)

20

Figure 14. Strontium isotopic composition of calcites and dolomites vs. depth for Hole 858D. Dolomites are outlined by the box. Their elevated values relative tocalcites are believed to be due to retention of strontium during dolomitization of pre-existing calcite.

0.0006

mo

l)(m

ol/

CO

ü

0.

0.

0.

0.

0,

0.

0005

0004

0003

0002

0001

0000100 200 300

Depth (mbsf)

400 500

Figure 15. Strontium/calcium ratios of calcites vs. depth for Hole 857C. The squares connected by the line are the calculated equilibrium strontium/calcium ratiosusing the pore water strontium/calcium ratios from Wheat and Mottl (this volume) and distribution coefficients for strontium in calcite determined experimentallyby Baker (unpubl. data). The latter were determined in solutions of similar composition and temperature as the pore fluids.

l/m

ol)

(mo

/Ca

CO

0

0

0

0

0

.0006

.0005

.0004

.0003

.0002

0.0001

0.0000

ü

m

-

-

mED

ü

E3

m m

3

@l Ell H

ü •

IDB _ _ ^ a —

u m

10

Depth (mbsf)

20

Figure 16. Strontium/calcium ratios of calcites vs. depth for Cores 858D-1H and -2H. The line is the calculated equilibrium strontium/calcium ratio (see captionfor Fig. 15).

324

GEOCHEMISTRY OF CARBONATE NODULES AND CEMENTS/m

ol)

(mo

l

β

üi—

CO

0

0

0

0

0

.0006

.0005

.0004

.0003

.0002

0.0001

0.0000

• I aH %

B m

I . I I ! I I I I I I

0 1 2 3 4 5 6 7 8 9 10 11 12

Mg

Figure 17. Strontium/calcium ratios of calcite vs. magnesium contents ofcalcites from Hole 858D.

istry (Wheat and Mottl, this volume) and temperature. There is, how-ever, a good positive correlation (r = 0.85, n = 35) between the stron-tium and magnesium contents of the calcites (Fig. 17). This correla-tion suggests that the strontium contents of calcites in Hole 85 8D arecontrolled by rates of calcite precipitation: high rates of precipitationproduce high magnesium contents and also permit high strontiumcontents (e.g., Mucci and Morse, 1983; Morse and Bender, 1990;Carpenter and Lohmann, 1992). Different controls on the distributionof strontium in calcites from the two holes may be due to more rapidrates of precipitation of calcites in Hole 858D.

MANGANESE AND IRON

Manganese and iron are abundant substituent ions in the authi-genic dolomites and calcites. In Hole 857C, we observe generallyhigher values of manganese in calcites than in dolomites and highervalues of iron in dolomites than in calcites (Table 1). These observa-

tions can be rationalized by the ionic radii of these cations whichdecrease in the order Ca > Mn > Fe > Mg. Hence, manganese substi-tutes preferentially for calcium in calcite and iron substitutes prefer-entially for magnesium in dolomite.

There is a general trend toward decreasing values of both iron andmanganese in calcites with increasing burial depth in Hole 857C (Fig.18). The trend is amplified when depth is replaced by the oxygen iso-topic composition of the carbonate. We speculate that decreasing con-centrations of iron and manganese with decreasing δ 1 8 θ are a reflec-tion of the decreased availability of iron and manganese at higher tem-peratures. Labile iron or manganese, originally associated with thehemipelagic and turbiditic sediments, is released into pore waters inhigher quantities during early diagenesis than during deeper burial.

The iron and manganese concentrations in calcites in Hole 858Dare more coherent in their behavior and help to clarify the nature ofongoing hydrothermal activity. Figures 19 and 20 show that: (1) ironconcentrations in calcites range to a maximum of about two times theconcentrations of manganese in calcites; (2) calcites formed at tem-peratures less than 200°C and at burial depths less than 2 mbsf havelow concentrations of both iron and manganese; (3) calcites thatformed at the highest temperatures (about 200°C at about 20 mbsf)have low concentrations of iron, but relatively high concentrations ofmanganese; and (4) the highest concentrations of both iron and man-ganese are found in calcites from about 3 to 7 mbsf, the same inter-val as a pore-water chlorinity maximum (Davis, Mottl, Fisher, et al.,1992). These calcites were precipitated at temperatures between about30° and 70°C.

The plot of calcitic iron and manganese against δ13C shows addi-tional separation of these metals during very shallow burial (Fig. 21).Manganese varies linearly with δ13C from low concentrations in surfi-cial, methanogenic calcites (shallowest occurrence of 0.46 mbsf) tohigh concentrations in deeper calcites (up to 5.40 mbsf) with a δ13C of-16.97%e. The high-manganese calcites are believed to be precipitatingin the sedimentary pore waters that most closely resemble the present-day vent fluids; hence, a carbon isotopic value of about -17‰ isbelieved to be the most characteristic of the hydrothermal source.

The pore-water manganese and iron concentration—depth profilesat Hole 85 8D (Wheat and Mottl, this volume) resemble the profiles inthe calcites. Pore-water iron is nearly zero at the sediment surface andnear the bottom of the core; it attains a maximum value at about 5 mbsf.Pore-water manganese is very low in the uppermost sediments, attainsa maximum value at about 5 mbsf, decreases to a minimum value

o

ΦU. 1

πH D

• Fβ (mol%)D Mn (mol%)

100 400200 300

Depth (mbsf)

Figure 18. Iron and manganese contents of calcites vs. depth for Hole 857C.

5 0 0

325


I 2

ii Fe (mol%)π Mn (mol%)

0

lü

O1 E

10

Depth (mbsf)

Figure 19. Iron and manganese contents of calcites vs. depth for Cores 858D-1H and -2H.

20

I 2

D Fe (mol%)r: Mn (mol%)

H üD

D

D

0

G

-20 -10 0 10

Del O-18 (PDB)

Figure 20. Iron and manganese contents of calcites vs. oxygen isotopic compositions of calcites for Hole 858D.

between 12 and 20 mbsf, and then attains an intermediate value nearthe base of the hole. The pore-water measurements of these metals arequite noisy, however, a likely result of sampling artifacts. Furthermore,pore-water data between 19.8 and 23.04 mbsf are unreliable because,as the isotopic data in this work show, this section is fall-in.

Some of the aforementioned differences between iron and manga-nese concentrations in calcites are likely due to downhole variabilityin the concentration of dissolved sulfide. Where dissolved sulfide ishigh, dissolved iron should be precipitated as iron sulfide and ironconcentrations in calcite should be low. Because manganese is not asreadily incorporated into sulfide minerals as is iron, the concentrationof dissolved sulfide does not significantly affect manganese concen-trations in pore fluids or in diagenetic calcites. This is clearly illus-trated by calcites from the upper 2 mbsf of Hole 85 8D. In this depthrange microbial sulfate reduction is an active process. Dissolved ironis removed as iron sulfides and little reduced iron is present forincorporation into diagenetic carbonates (Fig. 19). We also believe

that at temperatures above 100°C, thermochemical sulfate reductionby dissolved or gaseous methane (Krouse et al., 1988) is an activeprocess, producing dissolved sulfide. Thus, the shallowest and thedeepest pore fluids at Hole 85 8D have little dissolved iron and theassociated calcites are non-ferroan. These observations have impor-tant implications for the shallow circulation of hydrothermal fluids atSite 858 (see Fig. 22). We propose that the maximum values of ironand manganese, coincident with the chlorinity maximum, are due tolateral advection of fluid within a depth range of about 3 to 7 mbsf.The lateral flow is not rapid enough, however, to transport significantheat. Heat flow in Hole 85 8D, as stated previously in this paper, isprobably satisfactorily modeled as purely conductive (albeit two- orthree-dimensional). Linear extrapolation of the data in Figure 9 to thetemperature of present-day vent fluids (276°C) yields a depth of 28mbsf. We propose that 28 mbsf is the depth at which Hole 85 8D inter-sects one of the faults that serves as an upward conduit for ventingfluids. Additional evidence for this proposal includes poor core re-

326

3 —

• Fβ (mol%)c Mn (mol%)


üD

Ik-

o1 E

-40 -30 -20 -10

Del C-13 (PDB)

Figure 21. Iron and manganese contents of calcites vs. carbon isotopic compositions of calcites for Hole 858D.

10

-10 —

αΦ

•σ

Eo5 -20

3CO

-30 -

-40

Vent site

858D

10Q

20Q

28

-40 -30 -20 -10 0

Distance from vent (m)

- 0

10

-10 Λ

a.Φ

εO

-20 £

-30

-40

10

Figure 22. Schematic diagram showing inferred flow paths (arrows), fault zone (solid diagonal line), and isotherms (dashed lines, labelled with temperature, °C)in the vicinity of Hole 858D.

covery, decreased penetration rate, a large jump in wet-bulk density,and a large decrease in porosity at about 28 mbsf (Davis, Mottl,Fisher, et al., 1992, table 41, p. 540). There is also some evidence thatrocks within this depth range are highly silicified (Davis, Mottl,Fisher, et al., 1992, tables 20 and 22). Below 28 mbsf it is likely thatthermal gradients decrease.

CONCLUSIONS

1. Carbonate nodules and cements are abundant throughout theupper portions of the sediment columns in Middle Valley.

2. These carbonates are of a mixed origin: they form partiallyfrom the recrystallization of biogenic calcite and partially from the

327


direct precipitation of carbonate from pore fluids. In the latter case,calcium ions are sourced from plagioclase alteration in the sedimentsor basement rocks; magnesium ions (in the dolomites and HMC) aresourced from seawater; and carbon is supplied variously from sea-water, from oxidation of sedimentary organic matter, or from oxida-tion of thermogenic methane. Strontium contained within the carbon-ates is a mixture of marine-derived (seawater or biogenic calcite) andbasement-derived.

3. The carbonates recrystallize relatively rapidly, thus maintainingcompositions nearly in thermodynamic and isotopic equilibrium withadjacent pore fluids in sites with prograde histories. In Hole 857C, thecalculated thermal gradient is 0.53°C/m. In Hole 858D, the calculatedthermal gradient is 10.5°C/m. In both holes, thermal gradients are con-sistent with conductive heat transport. In Hole 856A, the calculatedtemperatures are much higher than present-day temperatures.

4. We believe the high temperatures are a result of hydrothermalactivity associated with sill emplacement and, possibly, formation ofthe massive sulfide ore body. Because the thermal signature is foundin the uppermost sample analyzed in Hole 856A from 14.5 mbsf, webelieve the event was recent, perhaps within the last 10,000 or 20,000years. In Hole 858A, somewhat higher than present-day temperaturesare also recorded by several samples.

5. We believe that Hole 858D penetrated through a fault zone atabout 28 mbsf. This fault zone is a high-permeability conduit for thehydrothermal vent fluids that discharge a few tens of meters away. Thefault zone is silicified, perhaps because of depressurization, tempera-ture drop, pH increase, or salinity decrease during the rapid ascent ofhydrothermal fluid and mixing with neighboring pore fluids. Althoughmost of the hydrothermal fluid is discharged at the seafloor, some of itadvects laterally through the sediments. In Hole 858D this lateral flowtakes place at a depth of about 3 to 7 mbsf. Maximum iron and man-ganese concentrations are found in calcites precipitated within thisdepth range.

ACKNOWLEDGMENTS

This work would not have been possible without the help of theentire crew and scientific party of the JOIDES Resolution. We alsothank the shore-based personnel at ODP in College Station for theirefforts on our behalf. This paper has benefitted from the reviews ofJohn Compton, Mike Mottl, an anonymous reviewer, and the ODPeditorial staff. Funding for this work was provided by JOI/USSSP.This support was invaluable. We are also grateful to Ms. DebbieGooch for her help in preparation of the manuscript.

REFERENCES*

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Baker, P.A., and Burns, S.J., 1985. The occurrence and formation of dolomitein organic-rich continental margin sediments. AAPGBull., 69:1917-1930.

Carpenter, S.J., and Lohmann, K.C., 1992. Sr/Mg ratios of modern marinecalcite: empirical indicators of ocean chemistry and precipitation rate.Geochim. Cosmochim. Acta, 56:1837-1849.

Claypool, G.E., and Kaplan, I.R., 1974. The origin and distribution of methanein marine sediments. In Kaplan, I.R. (Ed.), Natural Gases in MarineSediments: New York (Plenum), 99-139.

Davis, E.E., Mottl, M.J., Fisher, A.T., et al., 1992. Proc. ODP, Init. Repts., 139:College Station, TX (Ocean Drilling Program).

Friedman, I., and O'Neil, J.R., 1977. Compilation of stable isotope fractiona-tion factors of geochemical interest. In Fleischer, M. (Ed.), Data ofGeochemistry (6th Ed.). Geol. Surv. Prof. Pap. U.S., 440-KK:l-12.

Fritz, P., and Smith, D.C.W., 1970. The isotopic composition of secondarydolomites. Geochim. Cosmochim. Acta., 34:1161-1173.

Goodfellow, W.D., and Blaise, B., 1988. Sulfide formation and hydrothermalalteration of hemipelagic sediment in Middle Valley, northern Juan de FucaRidge. Can. Mineral, 26:675-696.

Horwitz, E., Dietz, M., and Fisher, D., 1991. Separation and preconcentrationof strontium from biological, environmental and nuclear waste samples byextraction chromatography using a crown ether. Anal. Chem., 63:522-525.

Kelts, K., and McKenzie, J.A., 1982. Diagenetic dolomite formation in Qua-ternary anoxic diatomaceous muds of Deep Sea Drilling Project Leg 64,Gulf of California. In Curray, J.R., Moore, D.G., et al., Init. Repts. DSDP,64 (Pt. 2): Washington (U.S. Govt. Printing Office), 553-569.

Krouse, H.R., Viau, C.A., Elink, L.S., Ueda, A., and Halas, S., 1988. Chemicaland isotopic evidence of thermochemical sulphate reduction by lighthydrocarbon gases in deep carbonate reservoirs. Nature, 333:415-519.

Land, L.S., 1987. The major ion chemistry of saline brines in sedimentarybasins. Am. Inst. Phys. Conf. Proc, 154:160-179.

Lorens, R.B., 1981. Sr, Cd, Mn and Co distribution coefficients in calcite as afunction of calcite precipitation rate. Geochim. Cosmochim. Acta., 45:553-561.

McCrea, J.M., 1950. Isotopic chemistry of carbonates and a paleotemperaturescale. J. Chem. Phys., 18:849-857.

Mook, W.G., Bommerson, J.C., and Staverman, W.H., 1974. Carbon isotopefractionation between dissolved bicarbonate and gaseous carbon dioxide.Earth Planet. Sci. Lett., 22:169-176.

Morse, J.W., and Bender, M.L., 1990. Partition coefficients in calcite: exami-nation of factors influencing the validity of experimental results and theirapplication to natural systems. Chem. Geol., 82:265-277.

Mucci, A., and Morse, J.W., 1983. The incorporation of Mg2+ and Sr2"1" intocalcite overgrowths: influence of growth rate and solution composition.Geochim. Cosmochim. Acta, 47:217-233.

O'Neil, J.R., Clayton, R.N., and Mayeda, T.K., 1969. Oxygen isotope frac-tionation in divalent metal carbonates. J. Chem. Phys., 51:5547-5558.

Rosenberg, P.E., Burt, D.M., and Holland, H.D., 1967. Calcite-dolomite-mag-nesite stability relations in solutions: the effect of ionic strength. Geochim.Cosmochim. Acta, 31:391-396.

Sheppard, S.M.F., and Schwarcz, H.P., 1970. Fractionation of carbon andoxygen isotopes and magnesium between coexisting metamorphic calciteand dolomite. Contrib. Mineral. Petrol, 26:161-198.

Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

Date of initial receipt: 3 March 1993Date of acceptance: 1 October 1993Ms 139SR-215

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