Osmium isotopic variations in metalliferous sediments from the East Pacific Rise and the Bauer Basin

Geochimica n Cosmochimica Copyright 0 1993 Pergamon

Acla Vol.57: pp. 4301-4310 Press Ltd. Pnntcd in U.S.A.

001&7037/93/s6.00

t .oo

Osmium isotopic variations in metalliferous sediments from the East Pacific Rise and the Bauer Basin GREG RAVZA ’ and GARY M. MCMURTRY’ ‘Depmrtment of Geoiogy and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA %Jepartmentof Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI 96822, USA (Received September 29, 1992; accepted in revised form April 8, 1993)

Abstract-The osmium concentration and isotopic composition of metalliferous sediments from the East Pacific Rise (EPR) and Bauer Basin have been determined. ‘*70s/‘*60s ratios range from 7.9 to 8.6, with EPR samples between 8.4 and 8.6, and Bauer Basin samples between 7.9 and 8.2. The osmium isotopic composition of EPR samples is indistinguishable from the inferred osmium isotopic composition of seawater, requiring that nearly all the osmium in these sediments is hydrogenous in origin. Thii result is used to place broad consent on the behavior of osmium in ridge crest hyd~~e~~ systems. While Bauer Basin samples also contain predominantly hydrogenous osmium, an additional source of osmium with low “‘Os/ l”Os is required to account for the lower measured ratios. Calculation of total osmium burial fluxes in the Bauer Basin show that the background flux of cosmic osmium is adequate to produce the observed depression in Bauer Basin ‘*70s/‘860s ratios. Burial flux calculations also suggest that metalliferous sediment deposition is a significant sink in the marine osmium cycle. HWRODUfflON

OVER THE COURSE OF geologic time, the decay of Is7Re to 18’Os has produced large differences in the osmium isotopic composition of different reservoirs within the Earttt. These differences provide a valuable tool for tracing the exchange of material between reservoirs. In this study, we examine osmium isotopic variations of marine sediments which have been influenced by submarine hydrothermal activity with the aim of determining the magnitude of osmium enrichment, the burial flux of osmium, and the source of osmium contained in these metalliferous sediments. It is only by virtue of the marked contrast between the ‘870s/3860s ratio of recently mantle-derived osmium ( c 1) and the ‘*70s/ ‘*‘@sratio of hydrogenous osmium (8.3-8.8), that inferences regarding the source of osmium buried in these sediments can be made. Chemical and isotopic signatures provide unequivocal evidence that transfer of mantle-derived material from oceanic crust to the ocean accompanies ridge crest hydrothermal activity. Marine sediments strongly influenced by hydrothermal activity commonly exhibit MORB-like Pb isotopic signatures (DASH, 198 I ; BARRETTet al., 1987; GERMANet al., 1993; MILLS et al., 1993), REE patterns with pronounced positive Eu anomalies ( RUHLINand OWEN, 1986; BARRETT and JARVIS,1988; OLIVAREZand OWEN, 1989; GERMAN et al., 1990; GERMAN et al., 1993; MILLS et al., 1993), and neodymium isotopic com~sitions recording mantle influence (MILLS et al., 1993). In addition, each of these studies shows that during transport through ambient seawater, mantlederived chemical signatures can be extensively modified, some more readily than others. This study is the first to employ the osmium isotope system to examine chemical exchanges accompanying hydrothermal processes at mid-ocean ridges. In the case of osmium isotopes, ln70s/ ‘&OS ratios approaching I are indicative of dominance of mantle-derived osmium. ‘*‘Os/ ‘*‘OS ratios significantly

higher than 1 require overprinting by ambient seawater osmium and suggest that dissolved osmium in seawater is reactive toward hydrothermal precipitates. The results presented here are significant in that they permit examination of the sensitivity of the osmium isotope system as a tracer of the interaction between seawater and oceanic crust. In spite of the potential of the osmium isotope system as a tracer of metal sources in the marine environment, neither the isotopic composition nor the concentration of osmium in seawater have been measured directly. This lack of data reflects the extreme difficulty of trace metal analysis in seawater at subpicomolar levels. Consequently, in&rences about the osmium isotopic composition and concentration of seawater are currently based on invest&ions of recent marine sediments. The data presented here, therefore, contribute in a fUndamental way to our knowledge of the reactivity of osmium in the oceanic environment and augment our understanding of the marine geochemical cycle of osmium. SAMPLE MATERIAL Carbonate-rich sediments proximal to the crest of the EPR and me~llifero~ pelagic clays from the Bauer Basin were-analyzed in this study. The locations of the cores sampled for this study form an approximate northwest-southeast transect from near the ridge crest into the Bauer Basin (Fig. I ). Material used in the present study was obtained from cores archived at the Woods Hole Oceanographic Institution and the University of Hawaii. The influence of submarine hydrothermal activity on sediments From this region of the seaf!oor has been extensively documented as part of the Nazca Plate project (KULM etai.,1981). Previous investigations of these cores have shown that they are highly enriched in transition metals, particularly Fe and Mn (MCMURTRYet al., 1981; SAYLESet al., 1975). Such enrichments have been attributed to hydrothermal activity (BENDERet al., 1971; HEATHand DYMOND, 1977;HEATH andD~MOND, 198 I; M~MuRTRY and BURNER, 1975; MCMURTRY et al., 1981;SAYLES et al., 1975) and, in the case of the Bauer Basin, advection of hydrothermal material away from the ridgecrest(HEATH and DYMOND, 1977; HEATHand DYMOND, 1981; SAYLES et al., 1975) (Fig. 1). Bauer Basin and EPR cores were chosen for this investigation because their

430 1

G. Ravizza and G. M. McMurtry

4302

FIG. 1. Base map of the Eastern Equatorial Pacific showing the sites of cores analyzed in this study. BoId arrows indicate prevailing deep water currents believed to be responsible for advection of hydrothermal material from the EPR into the Bauer Basin (HEATH and LWMOND, 1977). In this region, 1degnx of longitude or latitude is approximately 110 km.

composition and accumulation rates have been determined in a previous study ( MCMURTRY et al., 198 I) . Relevant data are given in Table 1. Sediments from the EPR are carbonate-rich and have sedimentation rates of between I and 2 cm/ Ka. Bauer Basin sediments are nearly devoid of calcium carbonate, retlecting deposition beneath the CCD. As a consequence, bulk sediment accumulation rates in the Bauer Basin are ap~oximately five times lower than on the EPR. In order to obtain large sample sizes, composite samples for osmium analysis went taken over a range of depths (see Table I ) . This sampling strategy also allowed ?I&ased accumulation rates, which represent long-term average rates of sediment accumulation, to be used in osmium burial flux calculations. A consequence of this approach was a significant loss of temporal resolution, with sediment ages ranging from the recent to approximately 500 Ka (Table 1) . Sediments from both settings contain only small quantities of terrigenous material, as reflected by their low Al concentrations. Thus, the influence of terrigenous material on the osmium isotopic composition of these samples is minimal. Samples have Fe/AI ratios which range from 3 in the Bauer Basin to. 14 on the EPR, much higher than average crustal material (Fe/Al = 0.5: SHAW et al. 1986). These elevated

Fe/Al ratios are indicative of a significant hydrothe~~ in these sediments. METHODS

Sediment decomposition and osmium preconcentration was achieved by NiS fire assay ( FEHN et al., 1986; ESSER and TUREIUAN, i 988). Sample sizes ranged from 1g to 40g. The osmium blank was dominated by osmium associated with the fusion reagents. The mass ratio of fusion reagents to sample was kept constant for all analyses and carried 1.5 pg osmium blank per gram sample with a ‘870s/ ‘860smium ratio of -9. The quantity of analyte ranged from 5.2 to 0.3 ng. Blank osmium never comprised more than 1.5% of the total osmium analyzed. As the osmium isotopic composition of Hank osmium is not known to high precision and is similar to the isotopic composition of sample osmium, the reported ‘s70s/‘860s ratios are not blank corrected. Separation and purification of osmium following the fire assay follows established methods. The HCI insoluble residue from the NiS bead was dissolved in a 4N sulfuric acid solution with Crq, and

Table 1. Selected major element data and sediment accumulation rate data are from McMurtry et al. 1981 unless o-se noted. 0s concentrations are determined by isotope dilution. Reported uncertainties in ls7Ck/l% ratio are 2a errorsbasedon countingstatistics. Replicate analyses sregivenin parentheses. !bnkpie Fe% AM fTaCO3% yip= ?kd.w IrrpW Sdktd&s

(Ka) (cm/KaI EPR SAMPLES lOO”6OO 2.4 42-250

(cm) KM

1.4

0.1

83

PCW AHs4/29’

2.0 1.3

0.3 0.1

81 85

400-755 I-12

~~a~1372

14.6

3.2

2

Q-15

FCC114 14.0 4.0 2.1 FCcllS 15.8 2.8 1.7 FCC116 16.5 4.0 3.1 PCS 12.2 2.8 9.7 1. Datacull&& by Cl’-AESat WHOI See 2. Sayles et al. 1974 &dimeiM

1.4 -

ACW&&Xl Rate @rftVKa)

285-540 -

BAUER BA%NSAMFLES no.3 30-50

O-90 30-W curet0ponly O-20

0.18 0.24 0.17 0.41

component

O-500 125-375 O-50

Rwipr and Turekkn 1992for details). armmutation rate is from a near by core.)

@I

367

osPo8

~~~ pg/anVKa

ppt

1.08iz0.16

152

0.69 f 0.09 -

(128 138 130 (128

8.384f 0.011 8.420f O.oOQ) 8.501f 0.003 8.59Qf0.018 8.551*0.015)

164 138 95 -

= 0.06

;E6

,‘zEo~

0.05f 0.01 0.05f 0.01 0.04f0.01 0.18 f 0.06

581 481 553 199

8.093f 8.156i 7.968f 8.100f

21 22 29 24 22 36

0.030 0.014 0.018 0.025

4303

Isotope geochemistry of 0s in metalliferous sediment osmium was distilled as the volatile osmium tetroxide (LUCK, 1982). Final purification of osmium prior to loading on a Pt filament was accomplished by ion exchange on a single resin bead ( REISBERG et al., 1991) . Isotopic analyses of osmium were made by negative ther-

AverageContinental crustalMaterial

Recmtly h4antlP derivedMaterial

mal ion mass spectrometry (NTIMS) on NIMA-B at WHOI. The details of the mass spectrometry are presented in HAURIand HART, ( 1993). In order to compare NTIMS results with previous osmium isotope analyses performed by secondary ion mass spectrometry (SIMS), two organic-rich sediment samples previously analyzed by SIMS were reanalyzed by NTIMS. In addition one metalliferous clay and one calcareous ooze from the EPR were also analyzed by both methods. All remaining analyses of EPR and Bauer Basin samples were made by NTIMS only.

7.0 8.0 82 8.4 8.6 LB 9.0

Marine Organic-rich Sediments UWizza & Turekian 1992)

RESULTS EPRSediments

SIMS / NTIMS Comparison A significant fraction of existing osmium isotopic data has been collected using SIMS following chemical isolation of osmium (for example: LUCK and ALLEGRE, 1983; LUCK and TUREKIAN, 1983; MARTIN, 1991; PALMER and TUREKIAN, 1986; RE~SBERGet al., 199 I ; ESSER and TUREKIAN, 1993 ) . Recent work has demonstrated that osmium isotopic analyses made by NTIMS (CREASER et al., 199 1; VOLKENING et al., 199 1) yield lower detection limits and higher precisions than are attainable with SIMS. Data presented in Table 2 provide a basis for comparison of the two methods. While SIMS data are less precise than NTIMS data, no systematic discrepancy between NTIMS and SIMS data is apparent. Agreement between these two independent methods of osmium isotopic analysis suggests that both produce results which are accurate to within the limits of uncertainty associated with the SIMS analyses ( - +2%). These results demonstrate that samples previously analyzed by SIMS do not require reanalysis unless enhanced precision is required. Although the precision associated with NTIMS osmium isotope ratio measurements can be better than 0.19’0,sample reproducibility in this data set is at the fOS% level. Differences in ‘*‘Ck/ lwOs ratio and osmium concentration associated with replicate analyses of CHNlOO/ 137 may reflect true sample heterogeneity. This is considered further in the discussion. Reproducibility of osmium concentration measurements made by NTIMS is not significantly better than reproducibility obtained with SIMS. This may be an indication that reproducibility and accuracy of osmium concentration determinations can be limited by incomplete spikesample equilibration during the fire assay. The primary concern is that traces of spike or sample osmium may be volatile during the initial stages of the sample fusion. Osmium radiotracer studies did not yield evidence of osmium loss during Table 2. SIMS / NTIMSdata comparison. Sample ‘8105/~8605 01 ppt NTIMS NTIMS

‘%s/‘@Qs

Dal ppt

SIMS 6.98f 0.07

SIMS 290*5

6.977 f 0.023

307f4

f 0.015 JellyFishLake2 8.478

151f2

8.71f 0.15 8.55f 0.10

172f4 173f3

8.599 f 0.017 8.551 f 0.015

130f2 128f2

8.84f 0.22

l32f4

7.926 f 0.022 350f4 8.166 f 0.019 366f4 1. SIMSdatafromRmiwaetal.(1991). 2 SIMSdata 3. NTfMS data also given inTable 1.

7.88f 0.12

330f5

Blacksea 14’

AI154/293 cl+~ loo/l373

from Ravizza and

Turekian (1992).

J

FIG. 2. A schematic plot illustrating that EPR and Bauer Basin sediments span only a narrow range of “‘Os/ ‘@jOsratios relative to major osmium reservoirs. This figure also shows that there is no resolvable difference between the osmium isotopic composition of EPR sediments (this study) and recent organic-rich sediments.

the NiS fire assay procedure ( E!SSER,199 1) . However, without independent evidence of sample homogeneity, it is extremely difficult to resolve the effects of incomplete spike-sample equilibration from inhomogeneous sample powders. EPR and Bauer Basin Results Results of the bulk sediment analyses of samples from the EPR and Bauer Basin are given in Table 1. Measured osmium concentrations exceed the estimated average osmium concentration of continental crustal material (0.05 ppb: I&ER and TUREKIAN, 1993). All measured ‘*70s/u’60s ratios fall in a restricted range (7.93-8.60) which is between estimated average crustal values ( IO- 11: ESSER and TUREKIAN, 1993) and mantle values ( = 1: ALLEGRE and LUCK, 1980; WALKER et al., 1989; MARTIN, 1991). Sediments from the EPR have osmium isotopic compositions which are similar to modem organic-rich sediments ( ‘870s/‘860s ratios between 8.2 and 8.9: RAVIZZA and TUREKIAN, 1992), while sediments from the Bauer Basin have ‘870s/‘860s ratios which are slightly but distinctly lower (Fig. 2). DISCUSSION Osmium Enrichments in EPR and Bauer Basin Sediments The paucity of Al in these sediments (Table 1) requims that only a small fraction of the osmium associated with these sediments can be carried by terrigenous material. If the detrital material is basaltic, its influence on the osmium isotopic composition of the sediments should be negligible due to the extremely low osmium concentration of MOR basalts (MARTIN, I99 1). If the detrital material has an average crustal composition ( ESSER and TUREKIAN, 1993)) up to 4% of the osmium in the Bauer Basin sample with the highest Al concentration could be terrigenous. The influence of tenigenous osmium on the EPR sample is insignificant due to the ex-

4304

G. Ravizza and G. M. McMurtry

tremely low Al concentrations (Table 1). We interpret the elevated osmium concentrations in both Batter Basin and EPR sediments as a result of the same processes which have produced extreme enrichments of other metals such as Fe, Mn, Cu, Zn, Co, and Ni in these sediments. Previous studies have employed factor analysis (HEATH and DYMOND, 198 1; LEINEN and PISIAS, 1984; MCMURTRY et al., 198 1) as a means of demonstrating that different metals have different sources. Close to the ridge Fe, Mn, Cu, and Zn enrichments are generally attributed a hydrothermal origin, implying that these elements were ultimately derived from the alteration of oceanic crust. Off-axis enrichments of Ni, Cu, and Mn are attributed in part to scavenging of these metals from ambient seawater by hydrothermal precipitates. Metals incorporated into sedimentary deposits in this manner are commonly referred to as hydrogenous. The fact that the 1870s/‘860s ratios of all the sediment samples are markedly higher than 1 requires that the majority of osmium in these sediments is hydrogenous rather than mantle-derived. This result provides unequivocal evidence that these sediments are acting as sinks in the marine geochemical cycle of osmium. The available data do not allow the mechanism of osmium removal from seawater to be identified with certainty. Based on the strong affinity of numerous dissolved trace elements for freshly precipitated Feoxides and oxyhydroxides in hydrothermal plumes ( TROCINE and TREFRY, 1988; TREFRY and METZ, 1989; FEELY et al., 1990; GERMAN et al., 1990; GERMAN et al., 199 1)) we hypothesize that osmium enrichments observed in sediments from the EPR and Bauer Basin result from scavenging of osmium by hydrothermal Fe-oxides. Incorporation of hydrogenous osmium into metal-rich sediments may continue after sediment deposition. Similar behavior is inferred for REEs (RUHLIN and OWEN, 1986; BARRETT and JARVIS, 1988; GERMAN et al., 1990), as well as Ni, Co, and MO ( MARCHIG et al., 1986). Osmium enrichments in marine Mn nodules are also well documented (LUCK and TUREKIAN, 1983; PALMERand TUREKIAN, 1986; ESSERand TUREKIAN, 1988). Therefore, scavenging of osmium by Mn-oxides may also be important further off axis. Osmium Burial Flux Comparisons Osmium burial fluxes given in Table 1 are calculated as the product of sediment mass accumulation rate and osmium concentration. Burial fluxes in EPR sediments range from 90-l 50 pg Os/cm’/Ka, while osmium in Bauer Basin sediments accumulates 3-5 times more slowly (20-30 pg OS/ cm’/ Ka) . Hydrogenous osmium burial is = 10 times more rapid in the Bauer Basin and x40 times more rapid on the EPR than in more typical pelagic clays from the DOMES sites ( ESSER and TUREKIAN, 1988). In spite of the high osmium concentrations of Bauer Basin sediments, osmium burial is more rapid near the EPR than in the Bauer Basin. This indicates that sediment accumulation rate exerts a major control on osmium burial fluxes. These calculations also suggest that deposition of metalliferous sediment constitutes a significant sink in the marine osmium budget. It is noteworthy that osmium burial fluxes associated with organic-rich sediment deposition ( RAVIZZA and TUREKIAN, 1992) are 1-2

orders of magnitude larger than osmium burial fluxes associated with deposition at the EPR. Thus, in spite of the limited extent of organic-rich sediment deposition, it is probabte that the magnitude of the hydrothermal sediment osmium sink is subsidiary to the organic-rich sediment sink. The Osmium Isotopic Signature in EPR Sediments The “‘OS/ ‘860s ratios of samples deposited closest to the ridge crest (8.60-8.38) are indistinguishable from the osmiummium isotopic composition of seawater, inferred from analyses of recent modern organic-rich sediments (“‘OS/ lg60s ratios between 8.3-8.8; RAVIZZA and TUREKIAN, 1992). This similarity in osmium isotopic composition indicates that between 95 and 100% of the osmium in the EPR samples is hydrogenous in origin. The predominance of hydrogenous osmium in these sediments is significant in two respects. First, in spite of the proximity of these samples to the EPR, they do not contain a significant component of mantle-derived osmium. This indicates that ridge-crest hydrothermal activity is not exerting a strong control on the osmium isotopic composition of seawater proximal to midocean ridges. Second, the prevalence of hydrogenous osmium signatures in recent EPR carbonates suggests that analogous ancient sediments may provide a record of the osmium isotopic composition of seawater through time. Hydrothermally mediated exchange of Pb, Nd, and Sr between oceanic crust and seawater is better constrained than the behavior of osmium in hydrothermal systems. Comparing the isotopic signatures which other heavy isotope tracers impart on hydrothermally influenced sediments provides a useful context in which the osmium isotopic data can be interpreted. For this reason the Pb, Nd, and Sr isotopic signatures in hydrothermal deposits are reviewed in the following text. Lead isotopic signatures in high temperature sulfides (BREVART et al., 1981; O’NIONS et al., 1978; VIDAL and CLAUER, 198 1) and metalliferous sediments very close to the ridge crest ( DASCH, 198 I ; GERMAN et al., 1993; O’NIONS et al., 1978) are indistinguishable from the lead isotopic signature of MORB. Although sediments deposited further from the source of hydrothermal venting show the evidence of gradual overprinting of mantle-derived Pb by hydrogenous Pb ( DASCH, 198 I), evidence of mantle-derived Pb can be found in sediments far removed from the ridge crest. The persistence of mantle-like isotopic signatures reflects the very high concentration ofPb in vent fluids (300 nM: VON DAMM, 1990) relative to the Pb concentration of ambient seawater ( 50 pM: SHERRELLand BOYLE, 1992). Like Pb, high temperature hydrothermal fluids are enriched in Nd relative to ambient seawater approximately 10 to 400 fold ( MICHARD et al., 1983; PIEPGRAS and WASSERBURG, 1985). In contrast to Pb, isotopic evidence of mantle-derived Nd is limited to deposits which are intimately associated with high temperature venting (MILLS et al., 1993). More distal sites of metalliferous sediment deposition have neodymium isotopic signatures which are indistinguishable from ambient seawater ( O’NIONS et al., 1978; HALLIDAYet al., 1992). The limited extent of mantle-influenced neodymium isotopic signatures most likely results from rapid and efficient scavenging

Isotope geochemistry of 0s in metalliferous sediment

of Nd, and the REEs in general, by hy~~~~

Fe-oxides

(GERMAN et al., 1990; HALLIDAYet al., 1992; OLIVAREZ

and OWEN, 1989; RUHLINand OWEN, 1986). Highly efficient scavenging of REE has two consequences. First, vent fluid derived Nd is removed from solution very rapidly, thereby limiting its influence on the neodymium isotopic composition of seawater. Second, scavenging of REEs derived from ambient seawater, both prior to and after deposition, obscures evidence of mantle-derived Nd. Similar inferences have been made based on the dissipation of positive Eu anomalies and the evolution of negative Ce anomalies with increasing REE/ Fe ratios in me~life~us sediments (BARRETTand JARVIS, 1988; GERMANet al., 1993; RUHLIN and OWEN, 1986). The behavior of Sr in submarine hydrothermal environments differs radically from Nd and Pb, principally because Sr has a very long residence time in seawater (-2.5 Ma). Upon mixing of high temperature vent fluid with seawater, the behavior of strontium is essentially conservative. The low “Sr/*%r ratios carried by the hydrothermal endmember are rapidly overprinted by seawater strontium, reflecting the similar strontium concentrations of seawater (90 pm/kg: PALMERand EDMOND, 1989) and vent fluids ( 126 .um/kg: PALMERand EDMOND,1989). Thus, although app~ximately 20% of the strontium flux into the oceans is supplied by hydrothermal activity (PALMERand EDMOND,1989), evidence of the influence of mantle-derived strontium can be found only by direct analysis of high temperature vent fluids (PALMERand EDMOND,1989; PIEPCRASand WASSERBURG, 1985 ) or, in rare instances, by analysis of direct precipitates from high temperature vent solutions ( ALBAREDEet al., I98 1; VIDALand CLAUER,198 1). The fact that metalliferous carbonates from the EPR carry a purely hydrogenous osmium isotopic signature suggests that osmium exchange in hyd~~e~~ systems is more analogous to strontium or Nd than to Pb. Seawater osmium isotopic signatures observed in Fe and Mn-rich sediments from the EPR do not preclude the possibility that ridge crest hydrothermal activity supplies a significant quantity of mantlederived osmium to the oceans. Just as sediments from the flanks of the EPR do not record mantle-like Nd signatures, it is possible that mantle-derived osmium is efficiently scavenged from seawater in close proximity to the source of vent fluids. Alternatively, osmium may be similar to strontium in that the osmium concentration of hydrothermal fluids may be comparable to ambient seawater. In this case, any mantlederived osmium isotopic signature would he rapidly overprinted by mixing with seawater, even in the absence of highly efficient scavenging processes. Thus, these data do not preclude the possibility that deposits more closely associated with high temperature venting than the samples used in this study will exhibit more mantle-like osmium isotopic signatures, as is the case for Nd. Nor do these data indicate that ridge crest hydrothermal activity cannot play an important role in controlling the osmium isotopic composition of the ocean as a whole. With respect to residence time, we believe osmium is more similar to strontium than Nd. Three lines of evidence lead us to this interpretation. First, speciation calculations indicate that at seawater pH and pE osmium is stable as an oxy-anion

4305

(PALMER et al., 1988). This led PALMERet al. (1988) to suggest that the residence time of osmium in seawater should be long relative to the mixing time of the oceans. Second, osmium in recent, organic-rich marine sediments (RAVlZZA and TUREKIAN, 1992), osmium in EPR sediments (this study), and leachable osmium in pelagic clays (EASERand TUREKIAN, 1988) from widely distributed regions of the world’s oceans all have 1s70s/‘860s ratios in the range 8.38.8. This isotopic homogeneity, relative to the range of ln70s/ ‘860s ratios observed in crustal rocks ( 1O- 11 in average continental crust: ESSERand TUREK~AN,1993; = 1 in ocean island basalts: MARTIN, 1991; PEGRAMet al., 1992; HAURI and HART, 1993) provides empirical evidence supporting the inference that osmium has a long residence time in seawater. Finally, preliminary analyses of the osmium isotopic composition of metalliferous sediments from the base of the TAG hydrothermal mound, a site of active high temperature venting, yield 1870s/‘s%s ratios similar to EPR sediments (G. Ravizza and C. R. German, unpubl. data). This result suggests that, even at the source of high temperature venting, seawater-like osmium isotopic signatures predominate in metalliferous sediments. Unlike Sr, osmium appears to be actively scavenged by hydrothermal precipitates. Osmium removal in association with hydrothermal precipitates is not incompatible with the inference that osmium has a relatively long residence time in the oceans. Other transition metals, such as MO and V, believed to exist as oxy-anions in oxic seawater (MoO;* and VOr( OH);*), exhibit similar behavior. Both have fairly long oceanic residence times f MO = 800,000 yrs: EMERSONand HUESTED, 199 1 and V fij 100,000 yrs: SHILLERand BOYLE, 1987). Vanadium and molybdenum are known to be actively cycled with Mn oxides in sediment porewaters ( SHAW et al., 1990). Moly~enum en~chments in ridge crest sediments have also been observed (MARCHIG et al., 1986; MARCHIC and GUNDLACH, 1982; TUREKIAN and BERTINE, 197 1). Recent work has shown that scavenging by hydrothermal precipitates also plays an important role in the marine geochemical cycle of V ( TREFRY and METZ, 1989). In addition, osmium (RAVI~~A and TUREKIAN, 1992) shares with MO ( BERTINE and TUREKIAN, 1972) and V (EMERSON and HUESTED, 199 1) an affinity for burial in association with organic-rich sediments. In all three cases enrichment of these elements in anoxic sediments is attributed to reduction to a less soluble species under low oxygen conditions. Thus, as suggested by speciation calculations (PALMER et al., 1988), osmium may be most analogous in its marine chemistry to oxy-anions like MO and V. Drawing this analogy is compatible with scavenging of osmium by hydrothermal precipitates, osmium burial in association with anoxic sediments, and also the inference of a fairly long oceanic residence time for osmium. Osmium Isotopic Variations in Bauer Basin Metalliferous CIays The measured r870s/‘%s ratios of samples from the Bauer Basin have osmium isotopic compositions which are distinct from EPR samples. Six samples from the Bauer Basin have

4306 Tabk3.

G. Ravizza and G. M. McMurtry Caku~ted

sample

cosmic

96Cosmic ‘8605’

OS fhxes

to Batter Basin sediments.

Totd=Q Burial Flu:

cosmic’wo6

CcSmic 0s FIUX

FlUX

2. Total ‘8605

burial flux is calculated ao the product of OS burial flux (horn Table 1). the abundance of 1%~ in the sample, and the ratio of tbr isotope mtu of 1%~ to the atomic weight of Sample 05.

3. Gwnic

I%

flux is calculated as the prcduct of the first and second columns.

4. lhe total flux of msmic 0s is calculated using a 167C5/‘%s

ratio of 1.1 for cosmic 0s

‘e70s/1860s ratios between 7.93 and 8.17. While these “‘OS/ lR60s ratios are significantly lower than samples from the EPR f 8.60-8.38), the signature is still p~ominan~y an ambient seawater signature. The difference in the osmium isotopic composition of Bauer Basin and EPR sediments requires addition of a small Fraction of nonradiogenic osmium to Bauer Basin sediments. We consider two possible sources of osmium with low ‘870s/1860s ratios: osmium supplied to the sediment in association with cosmic dust and mantle-derived osmium. Each of these is discussed in turn. it is well established that small part&s of extraterrestrial origin are continually impinging on the surface of the Earth (BROWNLEE, 1985) and that this material can be isolated ~om~~cclays( B~o~LEE,~~~~;BRo~LEE~~~., 1984; MURRELL et al., 1980). The osmium isotopic composition of slowly accumulating sediments is extremely sensitive to

the presenne of cosmic dust because the osmium concentration of extraterrestrial material is three to four orders of magnitude greater than bulk sediment osmium concentrations (EBIHARA and ANDERS, 1982).The smaller magnitude of the osmium burial flux in the Bauer Basin sediments requires that osmium supplied by the background flux of cosmic dust will constitute a larger faction of the total osmium flux than in the EPR sediments. The greater proportion of cosmic osmium, with its low “‘OS/ 18aOsratio (w 1 ), would decrease the 1s70s/‘860s ratio of Bauer Basin sediments relative to the more rapidly accumulating EPR sediments. This hypothesis can be evaluated quantitatively using a simplified version of a mixing model developed by ESSERand TIJREKIAN (1988).We assume two endmember mixing between cosmic osmium ( 1870s/ Is60s = 1.1) and terrestrial osmium ( ‘870s/ ‘*‘OS = 8.6; assuming a purely hydrogenous 0s signature) controls the osmium isotopic composition of Bauer Basin sediments, and use the 1s70s/“%s ratio of the bulk sediments to calculate the fraction of cosmic osmium. The flux of cosmic osmium to Bauer Basin sediments (Table 3) can then be calculated from the osmium burial flux (Table 1 ), the sediment mass accumulation rate (Table 1), and the calculated fraction of cosmic osmium (Table 3 ). Figure 3 graphically depicts calculation of the cosmic osmium flux. It is analogous to two-component mixing models where isotopic composition is plotted against the reciprocal of concentration. In this case, however, endmembers are defined in terms of osmium burial flux rather than osmium concentration. The low ‘870s/‘860s endmember corresponds to a hiatus in deposition of terrestrial osmium. Under these conditions the total osmium flux to the sediment is equal to the background flux of cosmic osmium, and the ‘870s/‘860s

10 DOMES

User

Batter Basin

9

and Turekian 1988) (This study)

EPR (This Shldy) Organic-rich sediments (Ravizza and Turekian 19921

8

10

20

30

40

50

1/['860s Burial Flux Cpg/an2/Ka)l FIG. 3. This mixing plot illustrates the sensitivity of cosmic osmium flux calculations to the ‘*‘O~/‘~~osmiurnratio chosen for the terrestrial endmember, which corresponds to the Y-intercept. The intersection of the less steep pair of mixing lines with the line ‘*‘Os/‘%s = I. 1corresponds to the range of cosmic osmium flux calculated from the Bauer Basin data ( f.3-2.2 pg Os/cm’/Ka: see Table 3). The steeper pair of heavy mixing lines are based on North Pa&c pelagic clay data. The intersection of these lines with the line ‘s70s/‘*60s = 1.1 define the range of cosmic osmium flux estimates recently revised by E!SSER and TUREKIAN ( 1993: 2.9-3.8 pg Os/cm*/Ka). See text for further discussion.

Isotope geochemistry of 0s in metalliferous sediment ratio of the sediment is determined by the ‘*70s/‘860s ratio of incoming cosmic dust (w 1.1). The high ‘r70s/ ‘860s endmember corresponds to a situation in which the burial flux of terrigenous osmium is large enough to render the background flux of cosmic osmium negligible. In this case, the *870s/‘860s ratio is determined by the relative proportions and osmium isotopic compositions of the terrigenous and hydrogenous osmium supplied to the sediment. Ideally data from sediments deposited under intermediate conditions in which mixtures of cosmic and terrestrial osmium are being buried will define a linear array of points between the two endmembers. The less steep of the two pairs of lines in Fig. 3 shares the same Y-intercept, which is fixed by the “‘OS/ ‘saOs ratio chosen for the terrestrial endmember and is defined by the measured ‘870s/‘860s ratio and osmium burial flux associated with samples PC8 and FCC 115. The intersection of these lines with the line ‘870s/ ‘860s = I. 1, the osmium isotopic composition of the cosmic osmium endmember, corresponds to the reciprocal of maximum and minimum cosmic ‘8aOs flux estimates given in Table 3. The fact that data from the EPR and Bauer Basin do not define a single co-linear array of points demonstm~s that two component mixing of terrestrial and cosmic osmium is an imperfect description of the processes controlling the osmium isotopic composition of these sediments. Variability in the osmium isotopic composition of the terrestrial endmember is a likely cause of this scatter. Although we favor the interpretation that osmium has a refatively long oceanic residence time, the osmium isotopic composition of modern seawater has not been measured directly and it remains possible that subtle variations in the ‘870s/‘860s ratio of seawater exist. In addition, PEGRAM et al. ( 1992) have interpreted variations in the isotopic composition of leachable osmium from a North Pacific pelagic clay sequence as evidence of temporal variation in the ‘*‘Os/ “Qs ratio of seawater, possibly on time scales as short as 100 Ka. Thus, the ‘*‘OS/ ls60s ratio of the hydrogenous endmember may be variable due to small temporal and/ or spatial variations in the ‘870s/‘860s ratio of seawater. Because of the large extrapolation required to estimate the bac~und flux of cosmic osmium, small shifts in the value of the hydrogenous endmember give rise to significant differences in the estimated value of the cosmic osmium flux. This phenomenon may contribute to producing the range of values of cosmic osmium flux reported in Table 3. The solid circles in Fig. 3 represent data from Northern Equatorial Pacific pelagic clay samples (EASER and TUREKIAN,1988 ). Recently ESSERand TUREKIAN ( 1993) reported an average “‘OS J ‘86Os ratio for continental crust, and used this value to revise an earlier estimate of the background flux ofcosmic osmium ( ESSERand TUREIUAN, 1988). The steeper pair of lines passing through these data are mixing lines based on Esser and Turekian’s estimate of the ‘*‘OSJ’%s ratio of terrestrial osmium in the samples. The influence of tenigenous osmium, assumed to have an average crustal osmium isotopic composition, causes the termstrial endmember used by ESSERand TUREKIAN ( 1993) to have a higher IB70sJ’860s ratio than the terrestrial endmember used in the Bauer Basin calculation. The cosmic osmium flux derived from the pelagic

4307

clay data is roughly twice as large as the Bux calculated from the Batter Basin data. Estimates of cosmic osmium flwr based on these two data sets can be forced to agree by raising the 1870sJ’860s ratio of the terrestrial endmember used in the Bauer Basin calculation or by lowering ‘s70sf ‘%s ratio of the terrestrial endmember used in the pelagic clay calculation. Making such shifts in the Bauer Basin caIculation would require invoking an additional source of nomadiogenic osmium in the EPR samples or a radiogenic detrital component in the Bauer Basin samples, Shifting the terrestrial endmember toward a lower ‘*‘Os/ ‘%Os ratio in the pelagic clay calculation would require the terrigenous eminent to have a 18’Osf “%s ratio lower than average crustal material. Without additional data it is not possible to better constrain the endmember osmium isotopic compositions. In spite of uncertainties associated with the osmiumm isotopic composition of the terrigenous endmember, these two independent estimates of cosmic osmium flux compare quite well. The range of values of the cosmic osmium flux obtained from the Bauer Basin data is 1.3-2.2 pg/cm2 JKa, and corresponds to a global chondrite flux of 1.3-2.2 X lo4 tonnes/ y, based on the osmium concentration ofchondritic material ( EBIHARA and ANDERS, 1982). This estimate compares favorably with cosmic dust flux estimates based on Ir in Antarctic aerosols (0.6-1.1 X IO4 tonnes/y; TUNCEL and ZOLLER, 1987) and direct observations of micrometeoritic debris (1.6 X lo4 tonnes/y; HUGHES, 1978), but is two to six times lower than estimates based on Ir in pelagic clays f KYTE and WASSON, 1986), and Ir in Antarctic Ice (GANAPATHY, 1983). We suspect higher cosmic dust flux estimates based on Ir data from ice and pelagic clays are a consequence of underestimation of the influence of terrestrially derived Ir. In light of the uncertainties inherent in osmium-based estimates of cosmic dust flux, the Bauer Basin results are consistent with previous estimates of cosmic dust flux. Therefore, it is a viable hypothesis that the background fIux of cosmic osmium is responsible for the depression of “‘OS/ ls60s ratios in the Batter Basin sediments relative to EPR sediments. An alternative means of accounting for the low ‘87~/‘~~ ratios of Bauer Basin sediments is to invoke a source of mantle-derived osmium within the Bauer Basin. Although there is no direct evidence of hydrothermal activity within the Bauer Basin, there are indirect indications of hydrothermal activity, including elevated metal ac~m~ation rates ( MCMURTRY and BURNE’IT, 1975), measurements of anomalous heat flow (ANDERSONet al., 1978), and oxygen isotopic data indicating clay mineral formation at slightly elevated temperatures ( MCMURTRY and YEH, 198 1) . Other investigators have attributed elevated metal accumulation rates to advection of ridge crest derived hydro~e~~ material (HEATH and DYMOND, 1977; HEATH and DYMOND, 1981). Additional oxygen isotope data failed to yield further evidence of elevated fluid temperatures (COLE&1985). As a result of these conflicting data sets, disagreement regarding the role of hydrothermal activity in the Bauer Basin persists. To invoke a source of mantle-derived osmium in the Bauer Basin to explain the low “‘0s J ls60s ratios observed is unwarranted. Sediments from the EPR, which are known to be

6. Ravizza and G. M. McMurtry

4308

proximal to hydrothermal activity, do not record any osmium isotopic evidence of hydrothermal activity. This indicates that the osmium isotopic system is not a sensitive indicator of hydrothermal activity. To suggest that mantle-derived osmium is supplied by hydrothermal fluids in the Bauer Basin requires invoking the presence of an osmium-bearing fluid without additional supporting evidence from known hydrothermal settings. In addition, the observed depression of ‘s70s/‘s60s ratios is readily accounted for by the background flux of osmium supplied by cosmic dust. If mantle-derived osmium was an important source of osmium in Bauer Basin sediments, estimates of cosmic osmium flux from this region should be anomalously high. Instead, calculated cosmic osmium fluxes are slightly lower than many other independent estimates. Taken together, the lack of direct evidence that hydrothermal processes carry significant quantities of mantlederived osmium, and the concordance of Bauer Basin-based estimates of cosmic dust flux with independent estimates, make us wary of suggesting that rnan~~e~v~ osmium has influenced the isotopic composition of Bauer Basin sediments. Replicate analyses of sample CHNlOO/ 137 yield Is70s/ lWOs ratios which differ by 3%, well outside the limits a.lIowed by the precisions given in Table 1. Replicate analyses of two EPR talc-oozes agree to within 0.5% or better. Therefore, we interpret the variation in 1870s/1860s associated with replicated analyses of CHNlOO/ 137 as evidence of sample heterogeneity rather than analytical artifact. Observation of sample heterogeneity with respect to osmium is not surprising in slowly accumulating sediments. As previously noted above, particulate extraterrestrial material, bearing high concentrations of noble metaIs, occur in pelagic clays (BONTE et d., 1987 ) . Basedon isotopic data, we have suggested that osmium of cosmic origin comprises between 5 and 10% of the osmium in Bauer Basin sediments. If the burden of cosmic osmium is carried by relatively few particles which have osmium concentrations three to four orders of m~itude larger than the bulk sediment, obtaining truly homogeneous splits of sample powders would be extremely difficult. However, the differences in CHN 100/ I37 replicates cannot be explained completely by this scenario because the lower of the two measured is70s/‘860s ratios is not accompanied by an elevated osmium concentration, as would be expected if particulate cxtraterrestrial material were responsible for lowering the measured lE70s/ Is608 ratio. Therefore, heterogeneous distribution of osmium in other sedimentary components is required to fully explain the variation between re#cate analyses. For example, hydrogenous osmium concentrated in discrete phases during diagenesis may contribute to sample heterogeneity. More careful investigation of the mineralogical residence of osmium in these sediments is required to resolve this problem. INFUSIONS The ‘*‘OS/ *%Ckratios of metalliferous sediments from the Bauer Basin and the EPR range from 7.9 to 8.6. This range requires that in excess of 90% of the osmium in these deposits is hydrogenous in origin. The lack of mantle-derived osmium isotopic signatures indicates that the osmium system is not a particularly sensitive tracer of seafloor hydrothermal processes. However, the dominance of hydrogenous osmium

signatures in metalliferous sediments does not preclude the possibility that ridge crest hydrothermal activity plays an important role in controlling the osmium isotopic com~sition of seawater. Rates of hydrogenous osmium burial in the Bauer Basin (20-30 pg/cm2/Ka) and on the EPR (90-150 pg/cm’/Ka) are Seven to fifty times greater than observed rates in typical pelagic clays. This indicates that osmium buriai in association with metiliferous sediments constitutes a significant sink in the marine osmium cycle. Based on analogies to other trace elements in hydrothermaily influenced sedimentary environments, scavenging of osmium onto hydrothermal Fe-oxides is likely to be responsible for enhanced osmium burial in these sediments. The slight depression of ‘870s/‘860s ratios of Bauer Basin sediments relative to EPR sediments is interpreted as a consequence of the influence of cosmic osmium in these sediments. Because the total osmium burial flux in these sediments is small compared to EPR sediments, the expound flux of osmium associated with cosmic dust constitutes a larger fraction of the osmium in Bauer Basin sediments than in EPR sediments. Estimates of cosmic dust flux based on Bauer Basin sediments are compatible with independent estimates of cosmic dust flux, indicating that the cosmic OSmium hypothesis provides a viable explanation for the depression of ‘87Os/ lWOs ratios of Bauer Basin samples relative to EPR samples, and that the presence of a local source of mantle-derived osmium in the Bauer Basin is not required. are indebted to Stan Hart who generously provided access to his clean laboratory and NIMA-B for osmium separations and isotopic analysis, respectively.Mike Bacon aho IdndIy

~ckflowledgments-We

allowed use of his laboratory facilities for this work. The WHOI core library is supported by NSF grant OCE 8800693. Jureck Blusztajn, Erik Hauri and Jon Snow all provided assistance with osmium analyses on NIMA-B. Ken Burrhus provided valuable technical help with NIMA-B. Jureck Blusztajn, Debbie Colodner, Chris Getman, and Kathleen Ruttenberg provided helpful comments on earlier drafts of this manuscript. C. Martin, B. Bach, and an anonymous reviewer provided constructive criticism which improved the manuscript. This work was supported by NSF grant OCE-9 1I5253 to G. Ravizza, S. Hart, and M. Bacon. This is WHO1 contribution no. 8348 and SOEST contribution no. 3232. Editorial handling: S. M. McLennan

REFERENCES ALBAREDEE, MICHARDA., MINSTERJ. F., and MICHARDG. f 1981)

*‘Sr/%r ratios in hydrothe~al waters and deposits from the East Pacific Rise at 21 ON. Earth Planet. Sci. Lett. 55,229-236. ALLEGREC.-J. and LUCK J.-M. ( 1980) Osmium isotopes as petrogenetic and geological tracers. Earth Planet. Sci. Lett. 48, 148154. ANDERSONR. N., HOBARTM. A., VON HERZENR. P., and FORNARI

D. J. ( 1978) Geophysical surveys on the East Pacific Rise-Galapagos Rise system. Geo&ys. J. R. As&r.Sot. 54,141-166. BARRY T. J. and JARVISI. f 1988) Ranxarth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect. Chem. Geol. 67,243-259. BARRETTT. J., TAYLOR P. N., and LUG~~KI J. ( 1987) Metalliferous sediments from DSDP leg 92: The East Pacific Rise. Geochim. Cosmoch~m. Acta 51,2241-2253. BENDER M. L., BROECKERW., G~RNITZ G., MIDREL U., KAY R., SUN S. S., and BECAVE P. ( 1971) The geochemistry of three cores

from the East Pacific Rise. Earth Planet. Sci. Lett. 12,425-433.

Isotope geochemistry of 0s in metalliferous sediment BERTINEK. K. and TUREKIANK. K. ( 1972) Molyhdenum in marine deposits. Geochim. Cosmochim. Acta 34, 1415-1434. BONTEP., JEHANNOC., MAURETTEM., and BROWNLEED. E. ( 1987) Platinum metals and microstructure in magnetic deep sea cosmic spherules. J. Geophys. Rex B4, E641-E648. BREVART0.. DUPRE B., and ALLEGREC.-J. ( 198 1) Metallogenesis at spreading centers: Lead isotope systematics for sulfides, manganese-rich crusts, basalts and sediments from the CYAMAX and ALVIN areas (East Pacific Rise). Econ. Geol. 76, 1205-l 210. BROWNLEED. E. ( 1985) Cosmic dust: Collection and research. Ann. Rev. Earth Planet. Sci. 13, 147-173. BROWNLEED. E., BATESB. A., and WHEELCCKM. ( 1984) Extmterrestrial platinum group nuggets in deep sea sediments. Nature 309,693-695. COLE T. G. ( 1985) Composition, oxygen isotope geochemistry, and origin of smectite in the metaliferous sediments of the Bauer Deep, southeast Pacific. Geochim. Cosmochim. Acta 49,221-235. CREASERR. A., PAPANASTASSIOU D. A., and WASSERBURGG. J. ( 199 1) Negative thermal ion mass spectrometry of osmium, rhenium, and iridium. Geochim. Cosmochim. Acta 55, 397-40 1. DASCHJ. E. ( 198 1) Lead isotopic composition of metalliferous sediments from the Nazca plate. Geol. Sot. Amer. Mem. 154, 199209. EB~HARAM., WOLF R., and ANDERS E. ( 1982) Are Cl chondrites chemically fractionated? A trace element study. Geochim. Cosmochim. Acta 46, 1849- 186 1. EMERSONS. R. and HUESTEDS. S. ( 1991) Ocean anoxia and the concentrations of molybdenum and vanadium in seawater. Mar. Chem. 34, 177-196. EASERB. K. ( 199 1) Osmium isotope geochemistry of terrigenous and marine sediments. Ph.D. dissertation, Yale Univ. ESSER B. K. and TUREKIANK. K. ( 1988) Accretion rate of extraterrestrial particles determined from the osmium isotope systematics of Pacific pelagic clays and manganese nodules. Geochim. Cosmochim. Acta 52, 1383- 1388. ESXR B. K. and TUREKIAN K. K. ( 1993) The osmium isotopic composition of the continental crust. Geochim. Cosmochim. Acta, 57, 3093-3 104. FEELY R. A., MAS~OTHG. J., BAKER E. T., COWENJ. P., LAMB M. F., and KROGSLUNDK. ( 1990) The effect of hydrothermal processes in midwater phosphorous distributions in the northeast Pacific. Earth Planet. Sci. Left. 96, 305-318. FEHN U., TENG R., ELMORED., and KUBIK P. W. ( 1986) Isotopic composition of osmium in terrestrial samples determined by accelerator mass spectrometry. Nature 323, 707-7 10. GANAPATHYR. ( 1983) The Tungguska explosion of 1908: Discovery of meteoritic debris at the explosion site and at the South Pole. Science 220, 1158- 116 1. GERMAN C. R., KLINKHAMMERG. P., MITRA J. M. A., and ELDERFIELDH. ( 1990) Hydrothermal scavenging of rare-earth elements in the ocean. Nature 345, 5 16-5 18. GERMAN C. R., FLEER A. P., BACON M. P., and EDMOND J. M. ( 199 1) Hydrothermal Scavenging at the Mid-Atlantic Ridge. Earth Planek Sci. Left. 105, 17 I- 18 1. GERMAN C. R., M1~t.s R., BLUSZTAJNJ., FLEER A. P., BACON M. P., HIGGS N. C., ELDERFIELDH., and THOMSONJ. ( 1993) A geochemical study of metalliferous sediment from the TAG hydrothermal mound, 26”08’N, MAR. J. Geophys. Res. 98,96839692. HALLIDAYA. N., DAVIDSONJ. P., HOLDEN P., OWEN R. M., and OLIVAREZ A. M. ( 1992) Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents. Geophys. Res. Lett. 19, 761-764. HAURI E. and HART S. R. ( 1993) Re-0s isotope systematics of HIMU and EMII oceanic island basalts from the South Pacific Ocean. Earth Planet. Sci. Lett. 114, 253-371. HEATH G. R. and DYMONDJ. ( 1977) Genesis and transformation of metalliferous sediments from the East Pacific Rise, Bauer Deep, and Central Basin, northwest Nazca plate. Geol. Sot. Amer. Bull. 88, 723-733. HEATH G. R. and DYMONDJ. ( 198 1) Metalliferous sediment deposition in time and space: East Pacific Rise and Bauer Deep, northern Naxca plate. Geol. Sot. Amer. Mem. 154, 175-197.

4309

HUGHES D. W. (1978) Meteors. In Cosmic Dust. (cd. J. A. M. MCDONNELL); pp. 123-185. J. Wiley and Sons. KULM L. D., DYMONDJ., DASCHE. J., and HUSSONGD. M. ( 1981) Nazca Plate: Crustal Formation and Andean Convergence. Geol. Sot. Amer. Mem. 154, l-824. KYTE F. T. and WASSONJ. T. ( 1986) Accretion rate of extratenesuial matter: Iridium deposited 33 to 67 million years ago. Science 232, 1225-1229. LEINENM. and PISIASN. ( 1984) An objective technique for determining endmemher compositions and for partitioning sediments according to their sources. Geochim. Cosmochim. Acta 48,47-62. LUCK J.-M. ( 1982) Gkochimie du Rhenium-Osmium: M&ode et applications. Ph.D. dissertation, Univ. Paris VII. LUCK J.-M. and ALLBGREC. J. ( 1983) is’Re-“‘0s systematics in meteorites and cosmochemical consequences. Nature 302, 130132. LUCK J.-M. and TUREKIAN K. K. (1983) Osmium-187/Osmium186 in manganese nodules and the Cretaceous-Tertiary boundary. Science 222,6 13-6 15. MARCHIGV. and GUNDLACHH. ( 1982) Iron-rich metalliferous sediments on the East Pacific Rise: Prototype of undifferentiated metalliferous sediments on divergent plate boundaries. Earth Planet. Sci. Lett. 58, 361-382. MARCHIGV., ERZINGERJ., and HEINZEP.-M. ( 1986) Sediments in the black smoker area of the East Pacific Rise ( 18.5’S). Earth Planet. Sci. Lett. 79,93-106. MARTIN C. E. ( 199 1) OS isotopic characteristics of mantle derived rocks. Geochim. Cosmochim. Acta 55, 142 I- 1434. MCMURTRY G. M. and BURNETTW. C. ( 1975) Hydrothermal metallogenesis in the Bauer Deep of the Southeastern Pacific. Nature 254,42-44. MCMURTRY G. M. and YEH H.-W. ( 198 1) Hydrothermal clay mineral formation of the East Pacific Rise and Bauer Basin sediments. Chem. Geol. 32, 189-205. MCMURTRY G. M., VEEH H. H., and MOSER C. ( 1981) Sediment accumulation rate patterns of the northwest Nazca plate. Geof. Sot. Amer. Mem. 154,21 l-249. MICHARD A., ALBAREDEF., MICHARD G., MINSTER J. F., and CHARLOUJ. L. ( 1983) Rare-earth elements and uranium in high temperature solutions from the East Pacific Rise hydrothermal vent field ( 13”N). Nature 303, 795-797. MILLS R., ELDERFIELDH., and THOMSONJ. ( 1993) A dual origin for the hydrothermal component in a metalliferous sediment core from the Mid-Atlantic ridge. J. Geophys. Res. 98, 967 l-968 1. MURRELLM. T., DAVISP. A., and NISHIIZUMIK. ( 1980) Deepsea sperules from Pacific clay: Mass distribution and inlux rate. Geochim. Cosmochim. Acta 44,2067-2074. OLIVAREZA. M. and OWEN R. M. ( 1989) REE/Fe variations in hydrothermal sediments: Implications for the REE content of seawater. Geochim. Cosmochim. Acta 53, 757-762. O’NIONS R. K., CARTERS. R., COHEN R. S., EVENSENN. M., and HAMILTONP. J. ( 1978) Pb, Nd, and Sr isotopes in oceanic ferromanganese deposits and ocean floor basalts. Nature 273, 435438. PALMERM. R. and EDMONDJ. M. ( 1989) The strontium budget of the modern ocean. Earth Planet. Sci. Lett. 92, 1l-26. PALMERM. R. and TUREKIANK. K. ( 1986) L870s/1*60s in marine manganese nodules and the constraints on the crustal geochemistries of rhenium and osmium. Nature 319, 216-220. PALMERM. R., FALKNER,K. KENISON,TUREKIANK. K., and CAL VERTS. E. ( 1988 ) Sources of osmium isotopes in manganese nodules. Geochim. Cosmochim. Acta 52, 1197- 1202. PEGRAMW. J. and ALLEGREC.-J. (1992a) Osmium isotopic compositions from oceanic hasalts. Earth Planet. Sci. Lett. 111, 5968.

PEGRAM W. J., KRISHNASWAMIS., RAVIZZA G., and TUREKIAN K. K. ( 1992b) The record of seawater ‘*‘Os/‘%s variation through the Cenozoic. Earth Planet. Sci. Lett. 113, 569-576. PIEPGRASD. J. and WASSERBURGG. J. ( 1985) Strontium and neodymium isotopes in hot springs on the Past Pacific Rise and Guaymas Basin. Earth Planet. Sci. Lett. 72, 341-356. RAVIZZAG. and TUREKIANK. K. ( 1992) The osmium isotopic co-

4310

G. Ravizza and G. M. McMurtry

position of organic-rich marine sediments. Earth Plan& Sci. Lett. 110, 1-6. RAVE% G., TUREK~AN K. K., and HAY B. J. ( 1991) The geochem-

istry of rhenium and osmium in recent sediment from the Black Sea. Geochitn. Cosm~him. Acta 55,3741-3752. REISBERG L. C., ALL&GRE C. J., and LUCKJ. M. ( 1991) The Re-0s systematics of the Rhonda Ultramafic Complex of southern Spain. Earth Planet. Sci. Lett. 105, 196-2 13. RUHLIND. E. and OWEN R. M. ( 1986) The rare-earth element geochemistry of hydrothe~~ sediments from the East Pacific Rise: Examination of seawater scavenging mechanism. Geochim. Cosmochim. Acta So, 393-400.

SAYLESF. L., Ku T. L., and BOWKERP. C. (1975) Chemistry of fe~ornan~n~ sediment from the Batter Deep. Geol. Sot. Amer. Bull. 86, 1423-1431.

SHAWD. M., CRAMERJ. J., HIGGINSM. D., and TRU~COTTM. G. ( 1986) Composition of the Canadian Precambrian shield and the continental crust of the earth. In The Nature ofthe Lower Continental Crust. (ed. DAWSON, J. B., CARSWELL D. A., HALL,J., and WEDEPOHL K. H.); pp. 275-282. Blackwell Sci. Pub]. SHAWT. J., GIIESKES J. M., and JAHNKER. A. ( 1990)Early diigenesis in differing depositional environments: The response of transition metais in porewater. Geochim. Cosm~him. Acta S4, 1233-1246. S~ERRELLR. M. and &YLE E. A. ( 1992) Isotopic equilibration between dissolved and suspended lead in the Atlantic Ocean: Evidence from ?b and stable Pb isotopes. J. Geophys. Res. 97, 11,257-l 1,268.

SHILLERA. M. and BOYLEE. A. (1987) Dissolved vanadium in rivers and estuaries. Earth Planet. Sri. Lett. 86, 2 14-224. TREFRYJ. H. and METZS. ( 1989) Role ofhydrothermal precipitates in the geochemical cycling of vanadium. Nature 342,53 l-533. TROCINER. P. and TREFRYJ. H. ( 1988) D~~~~tion and chemistry of suspended particles from an active hydrothermal vent site on the Mid-Atlantic Ridge at 26”N. Earth Planet. Sci. Lett. 88, l15. TUNCELG. and ZOLLERW. H. ( 1987) Atmospheric iridium at the south pole as a measure of the meteor&ccomponent. Nature 329, 703-705. TUREKIANK. K. and BERTINEK. K. ( I97 1) Deposition of molybdenum and uranium along the major ocean ridge systems. Na6ure 229,250-25 1. VIDALP. and CLAUERN. ( 1981) Pb and Sr isotopic systematics of some basalts and sulfidesfrom the East PacificRise at 2 1“N (project RITA). Earth Planet. Sci. Lett. 55,237-246. VOLKENING J., WALCZYK T., and HEUMANN K. G. ( 1991) Osmium isotope ratio d~e~inatjons by ne@ive thermal ion mass spectrometty. Intl. .i. Mass Spec. Ion Proces. 105, 147-159. VON DAMM K. L. (1990) Seafloor hvdrothermal activitv: Black Smoker chemistry‘and chomneys. knn. Rev. Earth Plartet.Sci. 18, 173-204.

WALKERR. J., CARLSONR. W., SHIREYS. B., and BOYD F. R. ( 1989) OS, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle. Geochim. Cosmochim.Acta 53,1583-1595.