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Geochemistry of a hydrothermal sediment core from the OBS vent-field, 21°N East Pacific Rise
Chemical Geology 155 Ž1999. 65–75
Geochemistry of a hydrothermal sediment core from the OBS vent-field, 218N East Pacific Rise C.R. German
a,)
, J. Hergt b, M.R. Palmer c , J.M. Edmond
d
a
Southampton Oceanography Centre, Southampton SO14 3ZH, UK School of Earth Sciences, UniÕersity of Melbourne, Melbourne, Australia c Department of Geological Sciences, UniÕersity of Bristol, Bristol, UK Department of Earth, Atmospheric and Planetary Sciences, MIT, Boston, MA, USA b
d
Received 22 September 1997; accepted 10 May 1998
Abstract We report a geochemical investigation of near-vent hydrothermal sediment from the East Pacific Rise, collected in a short Ž11 cm. core by submersible from the OBS vent-site at 218N. The sediment has high concentrations of Fe Ž23–25%., Cu Ž10–22%., Zn Ž4–13%. and S Ž22–30%., but low Mn concentrations Ž108–279 ppm. indicating a much fresher sulfidic input than in previously reported near-vent sediments from the Mid-Atlantic Ridge. Further studies are required to fully characterise near-field hydrothermal sediments, but the data in this study allow the first comparisons with similar near-vent material collected previously from the Mid-Atlantic Ridge as well as ridge-flank data from the EPR and Juan de Fuca Ridge. Compared to ridge-flank sediments the near field sediments exhibit high ratios of FerMn, CurFe and ZnrFe. Unlike Cu and Zn, Pb is not so significantly enriched. Pb isotope compositions are indistinguishable from local MORB indicating a dominant hydrothermal source for this element in these sediments. This may not necessarily be the case for ridge-flank sediments, however, where scavenging from seawater may also be important. REE distributions throughout the core exhibit low concentrations and characteristics typical of vent-fluids, with LREE enrichments and pronounced positive Eu-anomalies. These patterns are similar to those of relatively fresh sulfides, but there is also evidence for progressive oxidation downward into the core by ingressing seawater. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal sediment core; Geochemistry; OBS vent-field; East Pacific Rise
1. Introduction Submarine hydrothermal vents are a widespread feature of the global mid-ocean ridge system. They represent the surface expression of high-temperature convection systems circulating below the seafloor through young oceanic crust. When high-temperature
)
Corresponding author. Fax: q44-1703-596554; E-mail: [email protected]
Žup to 4008C. hydrothermal fluids, which are rich in dissolved metals and H 2 S, erupt into the overlying cold, alkaline water-column precipitation of finegrained polymetallic sulfides and oxides occurs. Some of this material is carried upward in an ascending buoyant plume ŽLupton et al., 1985., but up to 50% can be deposited in near-field hydrothermal sediments Žsee, e.g., review in Mills and Elderfield, 1995a.. Evidence for over 100 sites of active hydrothermal venting have been found in a variety of tectonic
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 4 1 - 7
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C.R. German et al.r Chemical Geology 155 (1999) 65–75
settings worldwide ŽRona and Scott, 1993., but the EPR 218N site remains one of the world’s beststudied hydrothermal areas since its discovery in 1979. Vent fluids from this area have been studied for more than ten years ŽEdmond et al., 1982; Von Damm et al., 1985; Chen et al., 1986; Campbell et al., 1988; Klinkhammer et al., 1994. and detailed studies have been completed on the mineralogy and geochemistry of the chimneys and associated sulfide deposits ŽSpiess and RISE Project Group, 1980; Haymon and Kastner, 1981; Vidal and Clauer, 1981; Haymon, 1983; Woodruff and Shanks, 1988.. To date, however, there has been little study of the geochemistry of near-field hydrothermal sediments at the 218N site or any other East Pacific Rise vent-field. Instead, previous work has concentrated on metal-rich inputs to ridge flank sediments as recorded in modern core-tops ŽBostrom ¨ and Peterson, 1966; Bostrom ¨ et al., 1969; Bender et al., 1971; Dymond, 1981; Shimmield and Price, 1988. and in ancient sediments recovered from deep-sea drilling across the wider Pacific seafloor ŽBarrett et al., 1987, Barrett and Jarvis, 1988.. In recent work on the Mid-Atlantic Ridge ŽMAR. it has been demonstrated that hydrothermal sediments contain two distinct components; Ž1. a nearfield component derived from corrosion and masswasting of seafloor hydrothermal sulfide deposits, and Ž2. a distal plume-related component ŽGerman et
al., 1993; Mills et al., 1993.. In the present study, we have investigated the geochemistry of a short core of near-vent material from the OBS hydrothermal site, 218N. These data are compared and contrasted with ridge-flank sediments from the EPR and JdF as well as near-vent hydrothermal sediments from the MAR.
2. Sampling and methods Sampling was carried out in September 1985 during DSV AlÕin dive 1636 at 218N on the East Pacific Rise. A 30 cm ‘push-corer’ was used to recover a short Ž11 cm. core of hydrothermal sediment at the OBS vent-site ŽFig. 1., about 5 m from the base of the main black smoker chimney. Although the delay between sampling and analysis rendered the method tentative, because any degree of alteration in the core might reflect post-sampling rather than in situ alteration, XRD study indicated that the sediments consisted of sulfide-rich debris and associated sulfates, with no systematic variation in mineralogy down core ŽR. Foster and C. Stanley, unpublished data.. Mineral phases identified included fine-grained pyrrhotite, isocubanite, wurtzitersphalerite, chalcopyrite, pyrite and marcasite, consistent with an origin linked to reworking of fresh chimney material from the same area ŽHaymon and Kastner, 1981; Haymon, 1983..
Fig. 1. Location map of the EPR 218N hydrothermal fields Žafter Von Damm et al., 1985.. Core 1636 was collected from the OBS vent-field X at approx. 20850 N, within 5 m of the base of the main OBS black-smoker chimney.
C.R. German et al.r Chemical Geology 155 (1999) 65–75
Upon recovery, seawater overlying the sediment was removed and the core was sealed airtight. In the laboratory, the core was re-opened and the wet sediment was extruded in 1 cm vertical sections. Samples of each section were dried for 24 h at 608C and homogenised for geochemical analysis by grinding in a tungsten carbide mill. For elemental analysis, weighed sub-samples Ž; 1 g. were digested in aqua regia Ž3:1 HCl:HNO 3 . followed by a mixture of HClO4 , HNO 3 and HF. The digests were evaporated to dryness and dissolved in 100 ml of 1% HNO 3 . 50 ml aliquots of these solutions were diluted 2-fold in 1% HNO 3 for the determination of Fe, Cu, Zn, Mg, Al, Ca, Mn, Co and Pb concentrations by ICP-AES and ICP-MS. REE were concentrated from the remaining 50 ml of digest solution using cation exchange columns ŽJarvis and Jarvis, 1985. and REE concentrations were determined by ICP-MS. For the extraction of S, a further 0.5 g subsample was treated with aqua regia for two hours and then heated to gentle reflux Ž80– 908C. overnight. The resultant leachate was then filtered through 0.4 mm polycarbonate membranes and diluted to 100 ml with high-purity water prior to
67
analysis by ICP-AES. All ICP-AES and ICP-MS measurements were calibrated using matrix-matched standards. The precision of the measurements was "3–5% Ž1 s . in all cases. For isotopic analysis, Pb was extracted from 10–50 mg sub-samples of powdered sediment using standard ion exchange methods. Samples were analysed by mass spectrometer using the double spike technique of Woodhead et al. Ž1994. to ensure optimum precision and accuracy. All sample preparations and mass spectrometry were carried out in the Research School of Earth Sciences ŽANU, Canberra. using a MAT Finnigan 261 multiple collector instrument. With Pb blanks typically 50 pg or less, no blank corrections were required for the data reported here. 3. Results and discussion 3.1. Metal distributions Concentrations of sulfur and the most abundant metals in core 1636 are listed in Table 1 and downcore variations of Fe, Cu, Zn, Co, Mn and Pb are displayed in Fig. 2. Fe concentrations are uniformly
Table 1 Metal and sulfur concentrations in core 1636 Sample
Fe Ž%.
Cu Ž%.
Zn Ž%.
S Ž%.
Mg Ž%.
Al Ž%.
Ca Ž%.
Co Žppm.
0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm
23.2 25.2 24.1 24.5 24.6 24.9 25.3 24.6 24.1 24.2 23.5
9.6 14.2 13.1 13.1 16.5 20.6 22.5 21.6 21.5 17.5 20.4
13.4 6.9 13.4 13.3 5.1 3.8 4.7 9.2 9.4 11.7 8.2
22.6 24.5 24.9 24.9 24.0 24.1 27.5 26.2 27.8 29.0 30.2
2.4 2.1 2.2 1.8 2.5 2.2 1.8 1.8 1.1 1.4 1.5
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0
0.3 0.3 0.2 0.1 0.2 0.2 0.1 0.1 0.2 0.1 0.1
222 290 476 290 286 250 323 466 389 295 250
245 245 216 279 184 169 108 130 118 175 131
348 336 317 531 279 226 216 250 260 260 211
Average TAG Ž2182. DSDP 92 GC88-6 N. Pacific Clays
24.4 31.0 32.7 8.2 6.0
17.3 4.0 0.1 0.0 0.0
9.0 0.2 0.1 0.0 0.0
26.0 – – – –
1.9 0.5 0.8 – –
0.1 0.7 0.8 7.1 9.7
0.2 6.1 2.6 – –
321 – 148 – –
182 1244 95930 4800 3700
294 104 166 56 20
Ža. German et al. Ž1993.. Žb. Barrett et al. Ž1987.. Žc. German et al. Ž1997a.. Žd. Kyte et al. Ž1993..
Mn Žppm.
Pb Žppm.
Ref Ža. Žb. Žc. Žd.
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C.R. German et al.r Chemical Geology 155 (1999) 65–75
Fig. 2. Profiles of Ža. Fe, Cu and Zn and Žb. Mn, Co and Pb concentrations measured down-core in core 1636, OBS vent-site, 218N EPR.
high down-core with concentrations close to 25%. Cu concentrations are also high, but exhibit a general increase with depth from 10% at the core top to ) 20% at its base ŽFig. 2a.. The Zn profile exhibits greatest variability with highest concentrations in the upper core Žup to 13%., lowest concentrations in mid-core Ž4–5%. and intermediate concentrations in the deepest samples Ž8–12%.. These data are complemented by S concentrations which are high throughout core 1636 but show a progressive increase from 22% in the core-top to 30% at its base ŽTable 1. consistent with progressive downward oxidation into the core Žsee later.. Mg concentrations are significantly lower than Fe, Cu and Zn, and exhibit a general decrease from ; 2.5% near the core top to ; 1.5% at its base ŽTable 1.. Higher Mg concentrations near the top of core 1636 may indicate uptake of Mg from seawater percolating down into locally-heated sediments Žcf. German et al., 1995; Mills et al., 1996.. Concentrations of Al and Ca are low throughout the core with averages of 0.1% and 0.2% respectively ŽTable 1., confirming
that detrital and biogenic inputs to these near-vent sediments are negligible. Co contents are relatively high and variable downcore, while Mn concentrations decrease more regularly from 200–300 ppm in the upper core to values typically - 150 ppm below 7 cm ŽFig. 2b.. Pb concentrations are comparable to those of Mn but show higher concentrations Ž) 300 ppm. in the upper core Ž0–4 cm. decreasing to lower, more uniform concentrations Ž200–250 ppm. below this depth. Previous studies of near-vent sediment cores from the TAG hydrothermal mound, Mid-Atlantic Ridge have used geochemical analyses to distinguish two distinct sources of material: Ž1. near-vent input derived from reworking of metal-rich primary sulfide deposits, and Ž2. vent-distal settling of plume particulates ŽGerman et al., 1993; Mills et al., 1993.. Fe concentrations in core 1636 are similar to the TAG near-field cores, but Cu and Zn concentrations are significantly higher than in the TAG cores which typically contained F 5% Cu and F 1% Zn ŽMetz et al., 1988; German et al., 1993.. Average composi-
C.R. German et al.r Chemical Geology 155 (1999) 65–75
tions of the metalliferous horizons from the near-vent TAG Core 2182 ŽGerman et al., 1993. are listed in Table 1 for comparison. Cu and Zn do not correlate with Fe in either the OBS or TAG samples. At TAG, this was attributed to down-core variability in the mineralogy of the mass wasted deposits. The distributions of Cu and Zn in core 1636 probably reflect similar variability. CurFe ratios in vent fluids at both sites are very similar, but ZnrFe ratios in vent fluids at TAG are almost an order of magnitude lower than at OBS ŽFig. 3.. In comparison, CurFe ratios in core 1636 are almost an order of magnitude higher than core-top samples from TAG, while ZnrFe ratios in OBS sediments show a more than 60-fold enrichment with respect to TAG. Clearly, there is no simple relationship between the relative abundances of Cu, Zn and Fe in vent-fluids and the CurFe and ZnrFe ratios measured in associated near-vent sediments. Instead, the increased abundances of Cu and Zn relative to Fe in core 1636 likely reflect higher abundances of Cu- and Znbearing sulfide minerals in these samples due to less extensive ‘weathering’ of this core. This is certainly consistent with the high S content of the core ŽTable 1.. It is also consistent with the relative positions of the EPR and TAG sediment cores: core 2182 was recovered from the outer flanks of the TAG hydrothermal mound. Thus, this material had been
69
reworked and transported at least 50 m from the nearest source of venting ŽGerman et al., 1993.. By contrast, core 1636 was collected immediately adjacent to the OBS vent-site and within 5 m of the active source. Mn concentrations in core 1636 are highest near the top of the core. Nevertheless, FerMn ratios remain in the range 900–2400 throughout core 1636. These ratios are very similar to those in the uppermost layers of TAG core 2182 ŽFerMns 900–2000. and are significantly higher than typical ridge-flank EPR hydrothermal sediments, which exhibit FerMn ratios of ; 3.5 Že.g., Dymond, 1981; Ruhlin and Owen, 1986; Shimmield and Price, 1988.. This confirms the near-vent provenance of hydrothermal sediments sampled by core 1636. Hydrothermal Fe is almost completely removed from solution by precipitation close to the hydrothermal vents. In contrast, Mn precipitation is relatively slow, so Mn tends to remain in a reduced, dissolved form at significant dilutions throughout the buoyant and neutrallybuoyant hydrothermal plumes Že.g., Klinkhammer and Hudson, 1986; Mottl and McConachy, 1990.. Interestingly, down-core, Pb concentrations mimic those of Mn, despite showing an exclusively MORB-like Pb isotopic composition Žsee below.. Co distributions, by contrast, do not correlate with any other components down-core ŽFig. 2b.. 3.2. Comparison with other hydrothermal sediments
Fig. 3. Bar chart comparing CurFe ratios and ZnrFe ratios in vent fluids and near-vent hydrothermal sediments at the OBS vent-site East Pacific Rise Žthis study. and the TAG hydrothermal mound, 268N, Mid Atlantic Ridge. Sources of data: OBS sediment data are from this study whilst OBS vent fluids are from Von Damm et al. Ž1985.. TAG vent-fluid data are from Edmond et al. Ž1995. and are compared with near vent hydrothermal sediment data from German et al. Ž1993..
Average core 1636 concentrations are compared with various previously reported hydrothermal sediments in Table 1. There are several published studies on the metalliferous, hydrothermal component of sediments from the flanks of the East Pacific Rise Že.g., Bostrom ¨ and Peterson, 1966; Bostrom ¨ et al., 1969; Bender et al., 1971; Dymond, 1981; Barrett et al., 1987, Barrett and Jarvis, 1988; Shimmield and Price, 1988.. Barrett et al. Ž1987. calculated average salt-free carbonate-free compositions for the metalliferous component of sediments from the flank of the EPR, based upon results from DSDP Leg 92 ŽTable 1.. These sediments have transition metal:Fe ratios which are very similar to the values calculated from the data of Dymond Ž1981. for ridge-flank hydrothermal deposits in Nazca Plate core-top sedi-
70
C.R. German et al.r Chemical Geology 155 (1999) 65–75
ments and so are taken here as representative of EPR ridge-flank metalliferous sediments. Also shown in Table 1, however, are more recently analysed ridgeflank data Žcarbonate-free. for hydrothermally influenced sediments collected 8 km off-axis from the Juan de Fuca Ridge ŽJdF, core GC88-6.. These sediments show much lower metal concentrations ŽGerman et al., 1997a. which almost certainly arises because the carbonate-free component from these sediments includes a significant detrital component Žnote the high Al concentration, comparable to N. Pacific Clays. which was not observed in the DSDP Leg 92 metalliferous sediments ŽLyle, 1986.. Core 1636 is very enriched in Cu and Zn relative to EPR and JdF flank-sediments ŽDSDP 92; GC88-6., as a result of its high concentrations of hydrothermal sulfide minerals ŽTable 1.. There is an order of magnitude deficit of Mn in core 1636 with respect to TAG near-field sediments Žcore 2182. and there is also a 50- to 500-fold depletion in Mn with respect to JdF and EPR ridge-flank sediments. This is consistent with decoupling of Fe precipitation and deposition on-axis Žas both sulfides and oxides. from Mn precipitation away from the source of venting, leading to higher Mn concentrations, and concomitantly lower FerMn ratios, in ridge-flank ŽF 300 km offaxis. metalliferous sediments. In contrast to the
deficit in Cu and Zn and pronounced enrichment in Mn, ridge-flank sediments from DSDP Leg 92 and core GC88-6 show high Pb concentrations which are comparable to near-vent concentrations from both the EPR and MAR, suggesting decoupling in the behaviour of Pb from that of the other chalcophile elements. 3.3. Pb isotope compositions. Pb isotope ratios ŽTable 2. are very uniform throughout core 1636: 206 Pbr204 Pb s 18.456– 18.471; 207 Pbr204 Pb s 15.472–15.490; 208 Pbr204 Pb s 37.83–37.89. These values agree closely with those reported for hydrothermal fluids from the OBS vent-site ŽChen et al., 1986. and also fall within the range of values reported previously for both basalts and sulfides from the 218N East Pacific Rise area ŽVidal and Clauer, 1981.. It is clear, therefore, that the Pb in core 1636 represents mantle-derived hydrothermal Pb which has been deposited directly upon oceanic crust. Samples of Nazca Plate surface sediments and the metalliferous component of DSDP Leg 92 sediments typically form roughly linear arrays in plots of 208 Pbr204 Pb vs. 206 Pbr204 Pb and 207 Pbr204 Pb vs. 206 Pbr204 Pb ŽDasch, 1981; Barrett
Table 2 Pb isotope ratios in core 1636 Sample
Pb-206rPb-204
Pb-207rPb-204
Pb-208rPb-204
0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm
18.469 18.469 18.469 18.469 18.463 18.467 18.466 18.461 18.460 18.456 18.471
15.488 15.485 15.488 15.487 15.485 15.486 15.484 15.478 15.481 15.472 15.490
37.89 37.88 37.89 37.89 37.88 37.88 37.88 37.87 37.87 37.83 37.89
Average OBS Fluidsa EPR 218N MORBb EPR 218N Sulfidesb
18.465 18.474 18.338–18.596 18.441–18.494
15.484 15.489 15.476–15.540 15.489–15.508
37.88 37.88 37.78–38.16 37.87–37.98
a b
Data from Chen et al. Ž1986.. Data from Vidal and Clauer Ž1981..
C.R. German et al.r Chemical Geology 155 (1999) 65–75
et al., 1987.. The arrays extend from a non-radiogenic end-member, coincident with N. Pacific MORB, towards the more radiogenic Mn nodule field Ž‘Nod’ in Fig. 4.. This more radiogenic endmember has been taken to represent the isotopic composition of seawater as recorded by Mn nodules ŽReynolds and Dasch, 1971.. Thus, Pacific hydrothermal sediments falling along such arrays are most readily interpreted to represent simple mixing of Pb from two end-member sources: mantle-derived Pb from hydrothermal discharge and more radiogenic Pb derived from seawater ŽDasch, 1981; Bar-
71
rett et al., 1987.. All the samples from core 1636 plot within a more restricted range in the Pb isotope ratios, well within the range of local basaltic rocks ŽFig. 4., indicating that the Pb in these samples is almost exclusively mantlerhydrothermally-derived. From a simple linear mixing model, Barrett et al. Ž1987. concluded that a strong hydrothermal Pb influence persists on ridge-flanks, with up to 80–90% of Pb in the metalliferous sediment component within 250 km of the EPR ridge-crest being of mantle origin. More recently, however, German et al. Ž1997a. have shown that no more than 50% of the Pb in ridge-flank sediments raised from within just 6–8 km of the Juan de Fuca Ridge axis is of a MORB-like origin indicating that, in addition, seawater scavenging of Pb may be important. The same had already been shown to be true on the East Pacific Rise ridge-flank, albeit in more distal samples, up to 1100 km from the ridge-axis, where Barrett et al. Ž1987. had shown that only 20–30% of the total Pb in the metalliferous sediment component was of mantle origin, despite high Pb concentrations of typically 100–200 ppm. These observations are consistent with independent studies on the MidAtlantic Ridge where 210 Pb and stable Pb isotope data from both hydrothermal plume particles and plume-derived sediments indicate strong evidence for scavenging of dissolved Pb from seawater ŽGerman et al., 1991, German et al., 1993; Mills et al., 1993; Godfrey et al., 1994.. 3.4. REE distributions
Fig. 4. Plots of 208 Pbr204 Pb vs. 206 Pbr204 Pb and 207 Pbr204 Pb vs. 206 Pbr204 Pb for core 1636 from the OBS vent-site, 218N EPR. Data are also plotted for: the metalliferous component of DSDP Leg 92 sediments ŽBarrett et al., 1987.; MORB and sulfide samples from the EPR at 218N ŽVidal and Clauer, 1981.; and mean OBS vent-fluids ŽChen et al., 1986.. The oceanic Mn nodule average ŽNod. is from the average of the data reported by Reynolds and Dasch Ž1971. and by Stacey and Kramers Ž1975. as compiled in Barrett et al. Ž1987..
REE concentrations in core 1636 are presented in Table 3, together with average data for OBS ventfluids ŽKlinkhammer et al., 1994.. The concentrations of total REE decrease down-core and at the base of the core Že.g., Nd: 0.29 ppm. they are only slightly higher than fresh sulfides from the TAG hydrothermal field ŽMills and Elderfield, 1995b.. Even at the top of the core, REE concentrations are significantly lower than those in near-vent hydrothermal sediments recovered previously from TAG ŽNd: 3–10 ppm; German et al., 1993.. Studies from the East Pacific Rise have shown that ridge-flank metalliferous sediments contain significantly higher REE concentrations than those observed in this study Že.g.,
C.R. German et al.r Chemical Geology 155 (1999) 65–75
72 Table 3 REE concentrations Žppm. in core 1636 Sample 0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm Average OBS Fluidsa Žppt.: a
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Er
Yb
CerCe )
EurEu )
NdrYb
1.35 1.27 1.15 1.27 1.16 0.94 0.63 0.44 0.42 0.43 0.31
1.80 2.02 1.55 1.63 1.36 1.32 1.10 0.77 0.69 0.62 0.46
0.27 0.31 0.22 0.23 0.19 0.18 0.15 0.11 0.10 0.08 0.06
1.19 1.37 0.96 0.99 0.82 0.80 0.72 0.53 0.45 0.37 0.29
0.30 0.34 0.23 0.22 0.21 0.18 0.19 0.13 0.11 0.09 0.07
0.45 0.47 0.29 0.66 0.47 0.26 0.17 0.12 0.11 0.43 0.19
0.37 0.43 0.30 0.30 0.26 0.23 0.22 0.16 0.14 0.11 0.08
0.31 0.37 0.25 0.24 0.20 0.16 0.16 0.13 0.11 0.08 0.07
0.17 0.21 0.14 0.13 0.11 0.08 0.08 0.06 0.05 0.03 0.03
0.17 0.21 0.14 0.13 0.09 0.05 0.05 0.06 0.04 0.03 0.03
0.68 0.75 0.69 0.67 0.63 0.73 0.83 0.82 0.80 0.76 0.76
4.2 3.8 3.4 7.9 6.2 3.9 2.6 2.6 2.7 13.3 7.8
2.44 2.27 2.39 2.65 3.17 5.57 5.02 3.08 3.92 4.30 3.37
0.85 108
1.21 165
0.17 21.1
0.77 72.3
0.19 13.5
0.33 157
0.24 16.2
0.19 10.6
0.10 4.77
0.09 3.75
0.74 0.75
4.8 30.8
3.47 7.10
Data from Klinkhammer et al., 1994.
Bender et al., 1971; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988; Owen and Olivarez, 1988; Olivarez and Owen, 1989; Halliday et al., 1992.. For example, DSDP Leg 92 metalliferous sediment samples have total REE concentrations in the range 131–301 ppm, with the majority of the data falling between 167–222 ppm ŽBarrett et al., 1986, Barrett and Jarvis, 1988.. This represents an approximately 50-fold increase over the values reported here. REErFe ratios in ridge-flank sediments increase with distance away from the ridge-axis ŽRuhlin and Owen, 1986; Owen and Olivarez, 1988; Olivarez and Owen, 1989. and REE distribution patterns typically exhibit the heavy ŽH.REE enrichments and pronounced negative Ceanomalies that are characteristic of seawater ŽBender et al., 1971; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988; Owen and Olivarez, 1988; Olivarez and Owen, 1989; Halliday et al., 1992.. These observations led the above authors to infer that extensive scavenging of dissolved REE from seawater occurs as hydrothermal Fe–Mn oxyhydroxide phases are dispersed off-axis, either prior to sedimentation or post-deposition during early diagenesis. This has been confirmed by subsequent analysis of hydrothermal plume particles which has shown that, close to an active vent-field, the particles exhibit relatively low REErFe ratios and REE patterns which exhibit both seawater and vent-fluid characteristics Ž negatiÕe
Ce-anomalies and positiÕe Eu-anomalies.. At increasing dispersion, however, particulate REErFe ratios increase significantly and seawater characteristics become dominant ŽGerman et al., 1990, 1997b.. By contrast, the REE patterns for core 1636 sediments show pronounced vent-fluid characteristics with a strong enrichment in the light ŽL.REE, pronounced positive Eu-anomalies and weak negative Ce-anomalies ŽFig. 5.. Nevertheless, these sediments are not entirely vent-fluid dominated. All the samples have much smaller positive Eu-anomalies than the vent fluid from which they evolved ŽFig. 5, Table 3.. In addition, as REE concentrations increase upcore, REE patterns become flatter, indicating relative HREE enrichment Ždecreasing Nd nrYb n values.. Relative to sediments from the TAG mound ŽMills and Elderfield, 1995b. samples from this study have REE concentrations which are: Ž1. higher than those of the freshest sulfide deposits from TAG ŽF 0.01 ppm Nd.; Ž2. comparable to, or lower than, those of the oxidised rims of relatively fresh sulfides from black smokers, white smokers and the TAG talus mound Ž0.3–3.5 ppm Nd.; and 3. lower than the REE concentrations in ochres from the TAG mound ŽNd: 3.6–13.5 ppm.. Similarly, Nd nrYb n ratios throughout core 1636 fall within the range 2.3–5.6, which overlaps that of the oxidised material from the TAG slope and mound ŽNd nrYb n s 2.1–6.0., but
C.R. German et al.r Chemical Geology 155 (1999) 65–75
73
TAG, again suggesting that core 1636 contains relatively fresh mound-derived sulfide material.
4. Summary
Fig. 5. Chondrite-normalized REE distribution patterns for hydrothermal sediments from core 1636. Also shown is the mean vent-fluid composition for the OBS vent-site at 218N on the EPR. Vent-fluid data Ž=10 3 . are from Klinkhammer et al. Ž1994.. Data have been normalised to the chondrite values of Nakamura Ž1974..
represents significant relative HREE enrichment when compared to fresh TAG black smoker sulfides ŽNd nrYb n s 7.0–9.6. and ochres ŽNd nrYb n s 8.2– 15.5.. The oxidised phases at TAG comprise alteration products of original sulfide material which adsorbed seawater-derived REE during oxidation ŽMills and Elderfield, 1995b.. Those workers also showed that the oxide coatings from inactive chimneys at TAG have more pronounced seawater characteristics than those from fresh chimneys, because oxidation of the inactive chimneys is more advanced and REE uptake from seawater more extensive. The simplest explanation for the REE distributions observed in this study, therefore, is that core 1636 is dominated by fresh sulfidic material with low REE concentrations and REE patterns dominated by vent-fluids. Towards the top of the core, however, total REE concentrations increase and the seawater influence also increases. These data are consistent with the depletion in S concentrations and complementary Mg enrichment observed in the upper layers of core 1636, suggesting the occurrence of near-surface oxidation resulting from the downward advection of seawater into this sulfide-dominated core. The highest REE contents in core 1636 resemble those of oxide rims of fresh chimney deposits at
Ži. An 11 cm sediment core collected from within 5 m of the OBS vent-site at 218N on the East Pacific Rise exhibits strong metal and S enrichments with 23–25 wt.% Fe, 10–22 wt.% Cu, 4–13 wt.% Zn and 22–30 wt.% S, but low Mn concentrations Ž108–279 ppm.. These observations are indicative of a relatively fresh sulfidic input to this core. In comparison, a similar submersible-collected push-core, raised from ) 50 m from the active TAG vent-site, exhibited much lower CurFe and ZnrFe ratios, but higher MnrFe ratios—all consistent with progressive aging and oxidation relative to the EPR core studied here. Žii. Core 1636 also exhibits high FerMn ratios when compared to ridge-flank sediments from the East Pacific Rise. The latter typically exhibit FerMn ratios close to 3.5, representing a decrease of 2–3 orders of magnitude from core 1636 ŽFerMn ; 1000.. The EPR near-vent core 1636 also exhibits much more pronounced CurFe and ZnrFe ratios than those witnessed previously in ridge-flank sediments from the East Pacific Rise. Whilst Cu and Zn concentrations in core 1636 are an order of magnitude greater than ridge-flank sediments, however, Pb concentrations are directly comparable. Pb isotopic compositions in core 1636 indicate that this Pb is exclusively mantle-derived. Such need not necessarily be the case in all ridge-flank sediments, however, because it is increasingly being recognised that scavenging of dissolved Pb from seawater may also be important. Žiii. REE concentrations throughout core 1636 are very low and REE patterns most closely resemble those of vent-fluids, exhibiting LREE enrichments and pronounced positive Eu-anomalies. The upper part of the core has slightly higher REE and Mg concentrations than the base of the core, however. In addition, the upper portions of the core also show relative HREE enrichment and more negative Ceanomalies. These patterns compare most closely with previous reports of fresh hydrothermal sulfide samples which have been partially oxidised. Overall, the observed variations in core 1636 are consistent with
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C.R. German et al.r Chemical Geology 155 (1999) 65–75
progressive downward oxidation by ingressing seawater, leading to REE uptake onto the resultant oxidised phases.
Acknowledgements Hydrothermal research at SOC has been supported by NERC through the BRIDGE Community Research Programme. We thank the Captain and Crew of RV Atlantis II and in particular the skill and dexterity of the AlÕin pilots in collection of material for this work. Acknowledgements are also due to the reviewers of this manuscript, Prof. J. Boulegue and an anonymous reviewer, for their helpful comments.
References Barrett, T.J., Taylor, P.N., Jarvis, I., Lugowski, J., 1986. Pb and Sr isotope and rare earth element composition of selected metalliferous sediments from Sites 597 to 601, Deep Sea Drilling Project Leg 92. Init. Repts. DSDP 92, 355–370. Barrett, T.J., Taylor, P.N., Lugowski, J., 1987. Metalliferous sediments from DSDP Leg 92, the East Pacific Rise transect. Geochim. Cosmochim. Acta 51, 2241–2253. Barrett, T.J., Jarvis, I., 1988. Rare earth element geochemistry of metalliferous sediments from DSDP Leg 92, the East Pacific Rise transect. Chem. Geol. 67, 243–259. Bender, M., Broecker, W., Gornitz, V., Middel, U., Kay, R., Sun, S.-S., Biscaye, P., 1971. Geochemistry of three cores from the East Pacific Rise. Earth Planet. Sci. Lett. 12, 425–433. Bostrom, K., Peterson, M.N.A., 1966. Precipitates from hy¨ drothermal exhalations of the East Pacific Rise. Econ. Geol. 61, 1258–1265. Bostrom, ¨ K., Peterson, M.N.A., Joensuu, O., Fisher, D.E., 1969. Aluminum-poor ferromanganoan sediments on active oceanic ridges. J. Geophys. Res. 74, 3261–3270. Campbell, A.C., Bowers, T.S., Measures, C.I., Falkner, K.K., Khadem, M., Edmond, J.M., 1988. A time series of vent fluid compositions from 218N, East Pacific Rise Ž1979, 1981, 1985. and the Guaymas Basin, Gulf of California Ž1982, 1985.. J. Geophys. Res. 93, 4537–4549. Chen, J.H., Wasserburg, G.J., Von Damm, K.L., Edmond, J.M., 1986. U–Th–Pb systematics in hot springs on the East Pacific Rise at 218N and Guaymas Basin. Geochim. Cosmochim. Acta 50, 2467–2479. Dasch, E.J., 1981. Lead isotopic composition of metalliferous seidments from the Nazca plate. Geol. Soc. Am. Mem. 154, 199–209. Dymond, J., 1981. Geochemistry of Nazca Plate surface sediments: An evaluation of hydrothermal, biogenic, detrital and hydrogenous sources. Geol. Soc. Am. Mem. 154, 133–170.
Edmond, J.M., Von Damm, K.L., McDuff, R.E., Measures, C.I., 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature 297, 187–191. Edmond, J.M., Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., German, C.R., Edmonds, H.N., Elderfield, H., Thompson, G., Rona, P., 1995. Time series studies of vent fluids from the TAG and MARK sites Ž1986, 1990. Mid-Atlantic Ridge, a new solution chemistry model and a mechanism for CurZn zonation in massive sulfide orebodies. Geol. Soc. Spec. Publ. 87, 77–86. German, C.R., Klinkhammer, G.P., Edmond, J.M., Mitra, A., Elderfield, H., 1990. Hydrothermal scavenging of rare earth elements in the ocean. Nature 345, 516–518. German, C.R., Fleer, A.P., Bacon, M.P., Edmond, J.M., 1991. Hydrothermal scavenging at the Mid-Atlantic Ridge: Radionuclide distributions. Earth Planet. Sci. Lett. 105, 170–181. German, C.R., Higgs, N.C., Thomson, J., Mills, R., Elderfield, H., Blusztajn, J., Fleer, A.P., Bacon, M.P., 1993. A geochemical study of metalliferous sediment from the TAG hydrothermal X mound, 26808 N, Mid-Atlantic Ridge. J. Geophys. Res. 98, 9683–9692. German, C.R., Barreiro, B.A., Higgs, N.C., Nelsen, T.A., Ludford, E.M., Palmer, M.R., 1995. Seawater metasomatism in hydrothermal sediments ŽEscanaba Trough, northeast Pacific.. Chem. Geol. 119, 175–190. German, C.R., Bourles, ` D.L., Brown, E.T., Hergt, J., Colley, S., Higgs, N.C., Ludford, E.M., Nelsen, T.A., Feely, R.A., Raisbeck, G., Yiou, F., 1997a. Hydrothermal scavenging on the Juan de Fuca Ridge: 230 Th xs , 10 Be and REE in ridge-flank sediments. Geochim. Cosmochim. Acta 61, 4067–4078. German, C.R., Ludford, E.M., Palmer, M.R., O’Brien, J.D., Patching, J.W., Floch’lay, G., Appriou, P., Barriga, F., Miranda, M., Fouquet, Y., Bougault, H., 1997b. Hydrothermal plumes on the Mid-Atlantic Ridge ŽAzores.: Particle geochemistry and mineralogy—MARFLUXrATJ. Terra Nova 9, 539. Godfrey, L.V., Mills, R., Elderfield, H., Gurvich, E., 1994. Lead behaviour at the TAG hydrothermal vent field, 268N, MidAtlantic Ridge. Mar. Chem. 46, 237–254. Halliday, A.N., Davidson, J.P., Holden, P., Owen, R.M., Olivarez, A.M., 1992. Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents. Geophys. Res. Lett. 19, 761–764. Haymon, R.M., 1983. Growth history of hydrothermal black smoker chimneys. Nature 301, 695–698. Haymon, R.M., Kastner, M., 1981. Hot spring deposits on the East Pacific Rise at 218 N, preliminary description of mineralogy and genesis. Earth Planet. Sci. Lett. 53, 363–381. Jarvis, I., Jarvis, K.E., 1985. Rare-earth element geochemistry of standard sediments, a study using inductively coupled plasma spectrometry. Chem. Geol. 53, 335–344. Klinkhammer, G., Hudson, A., 1986. Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer. Earth Planet. Sci. Lett. 79, 241–249. Klinkhammer, G.P., Elderfield, H., Edmond, J.M., Mitra, A., 1994. Geochemical implications of rare earth element patterns in hydrothermal fluids from mid-ocean ridges. Geochim. Cosmochim. Acta 58, 5105–5113.
C.R. German et al.r Chemical Geology 155 (1999) 65–75 Kyte, F.T., Leinen, M., Heath, G.R., Zhou, L., 1993. Cenozoic sedimentation history of the central North Pacific: Inferences from the elemental geochemistry of core LL44-GPC3. Geochim. Cosmochim. Acta 57, 1719–1740. Lupton, J.E., Delaney, J.R., Johnson, H.P., Tivey, M.K., 1985. Entrainment and vertical transport of deep ocean water by buoyant hydrothermal plumes. Nature 316, 621–623. Lyle, M., 1986. Major element composition of Leg 92 sediments. Init. Reps. DSDP 92, 355–370. Metz, S., Trefry, J.H., Nelsen, T.A., 1988. History and geochemistry of a metalliferous sediment core from the Mid-Atlantic Ridge at 268N. Geochim. Cosmochim. Acta 52, 2369–2378. Mills, R.A., Elderfield, H., 1995a. Hydrothermal activity and the geochemistry of metalliferous sediment. Geophys. Monogr. ŽAGU. 91, 392–407. Mills, R.A., Elderfield, H., 1995b. Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, Mid-Atlantic Ridge. Geochim. Cosmochim. Acta 59, 3511– 5324. Mills, R.A., Elderfield, H., Thomson, J., 1993. A dual origin for the hydrothermal component in a metalliferous sediment core from the Mid-Atlantic Ridge. J. Geophys. Res. 98, 9671–9678. Mills, R.A., Alt, J.A., Clayton, T., 1996. Low-temperature fluid flow through sulfidic sediments from TAG: modification of fluid chemistry and alteration of mineral deposits. Geophys. Res. Lett. 23, 3495–3498. Mottl, M.J., McConachy, T.F., 1990. Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 218N. Geochim. Cosmochim. Acta 54, 1911–1927. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta 38, 757–775. Olivarez, A.M., Owen, R.M., 1989. REErFe variation in hydrothermal sediments, Implications for the REE content of seawater. Geochim. Cosmochim. Acta 53, 757–762. Owen, R.M., Olivarez, A.M., 1988. Geochemistry of rare earth
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elements in Pacific hydrothermal sediments. Mar. Chem. 25, 183–196. Reynolds, P.H., Dasch, E.J., 1971. Lead isotopes in marine Mn nodules and the ore-lead growth curve. J. Geophys. Res. 76, 5124–5129. Rona, P.A., Scott, S.D., 1993. A special issue on sea-floor hydrothermal mineralization, new perspectives. Economic Geology 88, 1935–1976. Ruhlin, D.E., Owen, R.M., 1986. The rare earth element geochemistry of hydrothermal sediments from the East Pacific Rise, examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta 50, 393–400. Shimmield, G.B., Price, N.B., 1988. The scavenging of U, 230 Th and 231 Pa during pulsed hydrothermal activity at 208S, East Pacific Rise. Geochim. Cosmochim. Acta 52, 669–677. RISE Project Group, Spiess, F.N., 1980. East Pacific Rise: Hot springs and geophysical experiments. Science 207, 1421–1433. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotopes evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Vidal, P., Clauer, N., 1981. Pb and Sr isotopic systematics of some basalts and sulfides from the East Pacific Rise at 218N Žproject RITA.. Earth Planet. Sci. Lett. 55, 237–246. Von Damm, K.L., Edmond, J.M., Grant, B., Measures, C.I., Walden, B., Weiss, R.F., 1985. Chemistry of submarine hydrothermal solutions at 218N, East Pacific Rise. Geochim. Cosmochim. Acta 49, 2197–2220. Woodhead, J.D., Volker, F., McCulloch, M.T., 1994. Routine Pb isotope determinations using a 207 Pby204 Pb double spike, a long-term assessment of analytical precision and accuracy. Analyst 120, 35–39. Woodruff, L.G., Shanks, W.C. III, 1988. Sulfur isotope chemistry of chimney materials and vent fluids from 218N, East Pacific Rise, hydrothermal sulfur sources and disequilibrium sulfate reduction. J. Geophys. Res. 93, 4562–4572.
Geochemistry of a hydrothermal sediment core from the OBS vent-field, 218N East Pacific Rise C.R. German
a,)
, J. Hergt b, M.R. Palmer c , J.M. Edmond
d
a
Southampton Oceanography Centre, Southampton SO14 3ZH, UK School of Earth Sciences, UniÕersity of Melbourne, Melbourne, Australia c Department of Geological Sciences, UniÕersity of Bristol, Bristol, UK Department of Earth, Atmospheric and Planetary Sciences, MIT, Boston, MA, USA b
d
Received 22 September 1997; accepted 10 May 1998
Abstract We report a geochemical investigation of near-vent hydrothermal sediment from the East Pacific Rise, collected in a short Ž11 cm. core by submersible from the OBS vent-site at 218N. The sediment has high concentrations of Fe Ž23–25%., Cu Ž10–22%., Zn Ž4–13%. and S Ž22–30%., but low Mn concentrations Ž108–279 ppm. indicating a much fresher sulfidic input than in previously reported near-vent sediments from the Mid-Atlantic Ridge. Further studies are required to fully characterise near-field hydrothermal sediments, but the data in this study allow the first comparisons with similar near-vent material collected previously from the Mid-Atlantic Ridge as well as ridge-flank data from the EPR and Juan de Fuca Ridge. Compared to ridge-flank sediments the near field sediments exhibit high ratios of FerMn, CurFe and ZnrFe. Unlike Cu and Zn, Pb is not so significantly enriched. Pb isotope compositions are indistinguishable from local MORB indicating a dominant hydrothermal source for this element in these sediments. This may not necessarily be the case for ridge-flank sediments, however, where scavenging from seawater may also be important. REE distributions throughout the core exhibit low concentrations and characteristics typical of vent-fluids, with LREE enrichments and pronounced positive Eu-anomalies. These patterns are similar to those of relatively fresh sulfides, but there is also evidence for progressive oxidation downward into the core by ingressing seawater. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal sediment core; Geochemistry; OBS vent-field; East Pacific Rise
1. Introduction Submarine hydrothermal vents are a widespread feature of the global mid-ocean ridge system. They represent the surface expression of high-temperature convection systems circulating below the seafloor through young oceanic crust. When high-temperature
)
Corresponding author. Fax: q44-1703-596554; E-mail: [email protected]
Žup to 4008C. hydrothermal fluids, which are rich in dissolved metals and H 2 S, erupt into the overlying cold, alkaline water-column precipitation of finegrained polymetallic sulfides and oxides occurs. Some of this material is carried upward in an ascending buoyant plume ŽLupton et al., 1985., but up to 50% can be deposited in near-field hydrothermal sediments Žsee, e.g., review in Mills and Elderfield, 1995a.. Evidence for over 100 sites of active hydrothermal venting have been found in a variety of tectonic
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 4 1 - 7
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C.R. German et al.r Chemical Geology 155 (1999) 65–75
settings worldwide ŽRona and Scott, 1993., but the EPR 218N site remains one of the world’s beststudied hydrothermal areas since its discovery in 1979. Vent fluids from this area have been studied for more than ten years ŽEdmond et al., 1982; Von Damm et al., 1985; Chen et al., 1986; Campbell et al., 1988; Klinkhammer et al., 1994. and detailed studies have been completed on the mineralogy and geochemistry of the chimneys and associated sulfide deposits ŽSpiess and RISE Project Group, 1980; Haymon and Kastner, 1981; Vidal and Clauer, 1981; Haymon, 1983; Woodruff and Shanks, 1988.. To date, however, there has been little study of the geochemistry of near-field hydrothermal sediments at the 218N site or any other East Pacific Rise vent-field. Instead, previous work has concentrated on metal-rich inputs to ridge flank sediments as recorded in modern core-tops ŽBostrom ¨ and Peterson, 1966; Bostrom ¨ et al., 1969; Bender et al., 1971; Dymond, 1981; Shimmield and Price, 1988. and in ancient sediments recovered from deep-sea drilling across the wider Pacific seafloor ŽBarrett et al., 1987, Barrett and Jarvis, 1988.. In recent work on the Mid-Atlantic Ridge ŽMAR. it has been demonstrated that hydrothermal sediments contain two distinct components; Ž1. a nearfield component derived from corrosion and masswasting of seafloor hydrothermal sulfide deposits, and Ž2. a distal plume-related component ŽGerman et
al., 1993; Mills et al., 1993.. In the present study, we have investigated the geochemistry of a short core of near-vent material from the OBS hydrothermal site, 218N. These data are compared and contrasted with ridge-flank sediments from the EPR and JdF as well as near-vent hydrothermal sediments from the MAR.
2. Sampling and methods Sampling was carried out in September 1985 during DSV AlÕin dive 1636 at 218N on the East Pacific Rise. A 30 cm ‘push-corer’ was used to recover a short Ž11 cm. core of hydrothermal sediment at the OBS vent-site ŽFig. 1., about 5 m from the base of the main black smoker chimney. Although the delay between sampling and analysis rendered the method tentative, because any degree of alteration in the core might reflect post-sampling rather than in situ alteration, XRD study indicated that the sediments consisted of sulfide-rich debris and associated sulfates, with no systematic variation in mineralogy down core ŽR. Foster and C. Stanley, unpublished data.. Mineral phases identified included fine-grained pyrrhotite, isocubanite, wurtzitersphalerite, chalcopyrite, pyrite and marcasite, consistent with an origin linked to reworking of fresh chimney material from the same area ŽHaymon and Kastner, 1981; Haymon, 1983..
Fig. 1. Location map of the EPR 218N hydrothermal fields Žafter Von Damm et al., 1985.. Core 1636 was collected from the OBS vent-field X at approx. 20850 N, within 5 m of the base of the main OBS black-smoker chimney.
C.R. German et al.r Chemical Geology 155 (1999) 65–75
Upon recovery, seawater overlying the sediment was removed and the core was sealed airtight. In the laboratory, the core was re-opened and the wet sediment was extruded in 1 cm vertical sections. Samples of each section were dried for 24 h at 608C and homogenised for geochemical analysis by grinding in a tungsten carbide mill. For elemental analysis, weighed sub-samples Ž; 1 g. were digested in aqua regia Ž3:1 HCl:HNO 3 . followed by a mixture of HClO4 , HNO 3 and HF. The digests were evaporated to dryness and dissolved in 100 ml of 1% HNO 3 . 50 ml aliquots of these solutions were diluted 2-fold in 1% HNO 3 for the determination of Fe, Cu, Zn, Mg, Al, Ca, Mn, Co and Pb concentrations by ICP-AES and ICP-MS. REE were concentrated from the remaining 50 ml of digest solution using cation exchange columns ŽJarvis and Jarvis, 1985. and REE concentrations were determined by ICP-MS. For the extraction of S, a further 0.5 g subsample was treated with aqua regia for two hours and then heated to gentle reflux Ž80– 908C. overnight. The resultant leachate was then filtered through 0.4 mm polycarbonate membranes and diluted to 100 ml with high-purity water prior to
67
analysis by ICP-AES. All ICP-AES and ICP-MS measurements were calibrated using matrix-matched standards. The precision of the measurements was "3–5% Ž1 s . in all cases. For isotopic analysis, Pb was extracted from 10–50 mg sub-samples of powdered sediment using standard ion exchange methods. Samples were analysed by mass spectrometer using the double spike technique of Woodhead et al. Ž1994. to ensure optimum precision and accuracy. All sample preparations and mass spectrometry were carried out in the Research School of Earth Sciences ŽANU, Canberra. using a MAT Finnigan 261 multiple collector instrument. With Pb blanks typically 50 pg or less, no blank corrections were required for the data reported here. 3. Results and discussion 3.1. Metal distributions Concentrations of sulfur and the most abundant metals in core 1636 are listed in Table 1 and downcore variations of Fe, Cu, Zn, Co, Mn and Pb are displayed in Fig. 2. Fe concentrations are uniformly
Table 1 Metal and sulfur concentrations in core 1636 Sample
Fe Ž%.
Cu Ž%.
Zn Ž%.
S Ž%.
Mg Ž%.
Al Ž%.
Ca Ž%.
Co Žppm.
0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm
23.2 25.2 24.1 24.5 24.6 24.9 25.3 24.6 24.1 24.2 23.5
9.6 14.2 13.1 13.1 16.5 20.6 22.5 21.6 21.5 17.5 20.4
13.4 6.9 13.4 13.3 5.1 3.8 4.7 9.2 9.4 11.7 8.2
22.6 24.5 24.9 24.9 24.0 24.1 27.5 26.2 27.8 29.0 30.2
2.4 2.1 2.2 1.8 2.5 2.2 1.8 1.8 1.1 1.4 1.5
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0
0.3 0.3 0.2 0.1 0.2 0.2 0.1 0.1 0.2 0.1 0.1
222 290 476 290 286 250 323 466 389 295 250
245 245 216 279 184 169 108 130 118 175 131
348 336 317 531 279 226 216 250 260 260 211
Average TAG Ž2182. DSDP 92 GC88-6 N. Pacific Clays
24.4 31.0 32.7 8.2 6.0
17.3 4.0 0.1 0.0 0.0
9.0 0.2 0.1 0.0 0.0
26.0 – – – –
1.9 0.5 0.8 – –
0.1 0.7 0.8 7.1 9.7
0.2 6.1 2.6 – –
321 – 148 – –
182 1244 95930 4800 3700
294 104 166 56 20
Ža. German et al. Ž1993.. Žb. Barrett et al. Ž1987.. Žc. German et al. Ž1997a.. Žd. Kyte et al. Ž1993..
Mn Žppm.
Pb Žppm.
Ref Ža. Žb. Žc. Žd.
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C.R. German et al.r Chemical Geology 155 (1999) 65–75
Fig. 2. Profiles of Ža. Fe, Cu and Zn and Žb. Mn, Co and Pb concentrations measured down-core in core 1636, OBS vent-site, 218N EPR.
high down-core with concentrations close to 25%. Cu concentrations are also high, but exhibit a general increase with depth from 10% at the core top to ) 20% at its base ŽFig. 2a.. The Zn profile exhibits greatest variability with highest concentrations in the upper core Žup to 13%., lowest concentrations in mid-core Ž4–5%. and intermediate concentrations in the deepest samples Ž8–12%.. These data are complemented by S concentrations which are high throughout core 1636 but show a progressive increase from 22% in the core-top to 30% at its base ŽTable 1. consistent with progressive downward oxidation into the core Žsee later.. Mg concentrations are significantly lower than Fe, Cu and Zn, and exhibit a general decrease from ; 2.5% near the core top to ; 1.5% at its base ŽTable 1.. Higher Mg concentrations near the top of core 1636 may indicate uptake of Mg from seawater percolating down into locally-heated sediments Žcf. German et al., 1995; Mills et al., 1996.. Concentrations of Al and Ca are low throughout the core with averages of 0.1% and 0.2% respectively ŽTable 1., confirming
that detrital and biogenic inputs to these near-vent sediments are negligible. Co contents are relatively high and variable downcore, while Mn concentrations decrease more regularly from 200–300 ppm in the upper core to values typically - 150 ppm below 7 cm ŽFig. 2b.. Pb concentrations are comparable to those of Mn but show higher concentrations Ž) 300 ppm. in the upper core Ž0–4 cm. decreasing to lower, more uniform concentrations Ž200–250 ppm. below this depth. Previous studies of near-vent sediment cores from the TAG hydrothermal mound, Mid-Atlantic Ridge have used geochemical analyses to distinguish two distinct sources of material: Ž1. near-vent input derived from reworking of metal-rich primary sulfide deposits, and Ž2. vent-distal settling of plume particulates ŽGerman et al., 1993; Mills et al., 1993.. Fe concentrations in core 1636 are similar to the TAG near-field cores, but Cu and Zn concentrations are significantly higher than in the TAG cores which typically contained F 5% Cu and F 1% Zn ŽMetz et al., 1988; German et al., 1993.. Average composi-
C.R. German et al.r Chemical Geology 155 (1999) 65–75
tions of the metalliferous horizons from the near-vent TAG Core 2182 ŽGerman et al., 1993. are listed in Table 1 for comparison. Cu and Zn do not correlate with Fe in either the OBS or TAG samples. At TAG, this was attributed to down-core variability in the mineralogy of the mass wasted deposits. The distributions of Cu and Zn in core 1636 probably reflect similar variability. CurFe ratios in vent fluids at both sites are very similar, but ZnrFe ratios in vent fluids at TAG are almost an order of magnitude lower than at OBS ŽFig. 3.. In comparison, CurFe ratios in core 1636 are almost an order of magnitude higher than core-top samples from TAG, while ZnrFe ratios in OBS sediments show a more than 60-fold enrichment with respect to TAG. Clearly, there is no simple relationship between the relative abundances of Cu, Zn and Fe in vent-fluids and the CurFe and ZnrFe ratios measured in associated near-vent sediments. Instead, the increased abundances of Cu and Zn relative to Fe in core 1636 likely reflect higher abundances of Cu- and Znbearing sulfide minerals in these samples due to less extensive ‘weathering’ of this core. This is certainly consistent with the high S content of the core ŽTable 1.. It is also consistent with the relative positions of the EPR and TAG sediment cores: core 2182 was recovered from the outer flanks of the TAG hydrothermal mound. Thus, this material had been
69
reworked and transported at least 50 m from the nearest source of venting ŽGerman et al., 1993.. By contrast, core 1636 was collected immediately adjacent to the OBS vent-site and within 5 m of the active source. Mn concentrations in core 1636 are highest near the top of the core. Nevertheless, FerMn ratios remain in the range 900–2400 throughout core 1636. These ratios are very similar to those in the uppermost layers of TAG core 2182 ŽFerMns 900–2000. and are significantly higher than typical ridge-flank EPR hydrothermal sediments, which exhibit FerMn ratios of ; 3.5 Že.g., Dymond, 1981; Ruhlin and Owen, 1986; Shimmield and Price, 1988.. This confirms the near-vent provenance of hydrothermal sediments sampled by core 1636. Hydrothermal Fe is almost completely removed from solution by precipitation close to the hydrothermal vents. In contrast, Mn precipitation is relatively slow, so Mn tends to remain in a reduced, dissolved form at significant dilutions throughout the buoyant and neutrallybuoyant hydrothermal plumes Že.g., Klinkhammer and Hudson, 1986; Mottl and McConachy, 1990.. Interestingly, down-core, Pb concentrations mimic those of Mn, despite showing an exclusively MORB-like Pb isotopic composition Žsee below.. Co distributions, by contrast, do not correlate with any other components down-core ŽFig. 2b.. 3.2. Comparison with other hydrothermal sediments
Fig. 3. Bar chart comparing CurFe ratios and ZnrFe ratios in vent fluids and near-vent hydrothermal sediments at the OBS vent-site East Pacific Rise Žthis study. and the TAG hydrothermal mound, 268N, Mid Atlantic Ridge. Sources of data: OBS sediment data are from this study whilst OBS vent fluids are from Von Damm et al. Ž1985.. TAG vent-fluid data are from Edmond et al. Ž1995. and are compared with near vent hydrothermal sediment data from German et al. Ž1993..
Average core 1636 concentrations are compared with various previously reported hydrothermal sediments in Table 1. There are several published studies on the metalliferous, hydrothermal component of sediments from the flanks of the East Pacific Rise Že.g., Bostrom ¨ and Peterson, 1966; Bostrom ¨ et al., 1969; Bender et al., 1971; Dymond, 1981; Barrett et al., 1987, Barrett and Jarvis, 1988; Shimmield and Price, 1988.. Barrett et al. Ž1987. calculated average salt-free carbonate-free compositions for the metalliferous component of sediments from the flank of the EPR, based upon results from DSDP Leg 92 ŽTable 1.. These sediments have transition metal:Fe ratios which are very similar to the values calculated from the data of Dymond Ž1981. for ridge-flank hydrothermal deposits in Nazca Plate core-top sedi-
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ments and so are taken here as representative of EPR ridge-flank metalliferous sediments. Also shown in Table 1, however, are more recently analysed ridgeflank data Žcarbonate-free. for hydrothermally influenced sediments collected 8 km off-axis from the Juan de Fuca Ridge ŽJdF, core GC88-6.. These sediments show much lower metal concentrations ŽGerman et al., 1997a. which almost certainly arises because the carbonate-free component from these sediments includes a significant detrital component Žnote the high Al concentration, comparable to N. Pacific Clays. which was not observed in the DSDP Leg 92 metalliferous sediments ŽLyle, 1986.. Core 1636 is very enriched in Cu and Zn relative to EPR and JdF flank-sediments ŽDSDP 92; GC88-6., as a result of its high concentrations of hydrothermal sulfide minerals ŽTable 1.. There is an order of magnitude deficit of Mn in core 1636 with respect to TAG near-field sediments Žcore 2182. and there is also a 50- to 500-fold depletion in Mn with respect to JdF and EPR ridge-flank sediments. This is consistent with decoupling of Fe precipitation and deposition on-axis Žas both sulfides and oxides. from Mn precipitation away from the source of venting, leading to higher Mn concentrations, and concomitantly lower FerMn ratios, in ridge-flank ŽF 300 km offaxis. metalliferous sediments. In contrast to the
deficit in Cu and Zn and pronounced enrichment in Mn, ridge-flank sediments from DSDP Leg 92 and core GC88-6 show high Pb concentrations which are comparable to near-vent concentrations from both the EPR and MAR, suggesting decoupling in the behaviour of Pb from that of the other chalcophile elements. 3.3. Pb isotope compositions. Pb isotope ratios ŽTable 2. are very uniform throughout core 1636: 206 Pbr204 Pb s 18.456– 18.471; 207 Pbr204 Pb s 15.472–15.490; 208 Pbr204 Pb s 37.83–37.89. These values agree closely with those reported for hydrothermal fluids from the OBS vent-site ŽChen et al., 1986. and also fall within the range of values reported previously for both basalts and sulfides from the 218N East Pacific Rise area ŽVidal and Clauer, 1981.. It is clear, therefore, that the Pb in core 1636 represents mantle-derived hydrothermal Pb which has been deposited directly upon oceanic crust. Samples of Nazca Plate surface sediments and the metalliferous component of DSDP Leg 92 sediments typically form roughly linear arrays in plots of 208 Pbr204 Pb vs. 206 Pbr204 Pb and 207 Pbr204 Pb vs. 206 Pbr204 Pb ŽDasch, 1981; Barrett
Table 2 Pb isotope ratios in core 1636 Sample
Pb-206rPb-204
Pb-207rPb-204
Pb-208rPb-204
0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm
18.469 18.469 18.469 18.469 18.463 18.467 18.466 18.461 18.460 18.456 18.471
15.488 15.485 15.488 15.487 15.485 15.486 15.484 15.478 15.481 15.472 15.490
37.89 37.88 37.89 37.89 37.88 37.88 37.88 37.87 37.87 37.83 37.89
Average OBS Fluidsa EPR 218N MORBb EPR 218N Sulfidesb
18.465 18.474 18.338–18.596 18.441–18.494
15.484 15.489 15.476–15.540 15.489–15.508
37.88 37.88 37.78–38.16 37.87–37.98
a b
Data from Chen et al. Ž1986.. Data from Vidal and Clauer Ž1981..
C.R. German et al.r Chemical Geology 155 (1999) 65–75
et al., 1987.. The arrays extend from a non-radiogenic end-member, coincident with N. Pacific MORB, towards the more radiogenic Mn nodule field Ž‘Nod’ in Fig. 4.. This more radiogenic endmember has been taken to represent the isotopic composition of seawater as recorded by Mn nodules ŽReynolds and Dasch, 1971.. Thus, Pacific hydrothermal sediments falling along such arrays are most readily interpreted to represent simple mixing of Pb from two end-member sources: mantle-derived Pb from hydrothermal discharge and more radiogenic Pb derived from seawater ŽDasch, 1981; Bar-
71
rett et al., 1987.. All the samples from core 1636 plot within a more restricted range in the Pb isotope ratios, well within the range of local basaltic rocks ŽFig. 4., indicating that the Pb in these samples is almost exclusively mantlerhydrothermally-derived. From a simple linear mixing model, Barrett et al. Ž1987. concluded that a strong hydrothermal Pb influence persists on ridge-flanks, with up to 80–90% of Pb in the metalliferous sediment component within 250 km of the EPR ridge-crest being of mantle origin. More recently, however, German et al. Ž1997a. have shown that no more than 50% of the Pb in ridge-flank sediments raised from within just 6–8 km of the Juan de Fuca Ridge axis is of a MORB-like origin indicating that, in addition, seawater scavenging of Pb may be important. The same had already been shown to be true on the East Pacific Rise ridge-flank, albeit in more distal samples, up to 1100 km from the ridge-axis, where Barrett et al. Ž1987. had shown that only 20–30% of the total Pb in the metalliferous sediment component was of mantle origin, despite high Pb concentrations of typically 100–200 ppm. These observations are consistent with independent studies on the MidAtlantic Ridge where 210 Pb and stable Pb isotope data from both hydrothermal plume particles and plume-derived sediments indicate strong evidence for scavenging of dissolved Pb from seawater ŽGerman et al., 1991, German et al., 1993; Mills et al., 1993; Godfrey et al., 1994.. 3.4. REE distributions
Fig. 4. Plots of 208 Pbr204 Pb vs. 206 Pbr204 Pb and 207 Pbr204 Pb vs. 206 Pbr204 Pb for core 1636 from the OBS vent-site, 218N EPR. Data are also plotted for: the metalliferous component of DSDP Leg 92 sediments ŽBarrett et al., 1987.; MORB and sulfide samples from the EPR at 218N ŽVidal and Clauer, 1981.; and mean OBS vent-fluids ŽChen et al., 1986.. The oceanic Mn nodule average ŽNod. is from the average of the data reported by Reynolds and Dasch Ž1971. and by Stacey and Kramers Ž1975. as compiled in Barrett et al. Ž1987..
REE concentrations in core 1636 are presented in Table 3, together with average data for OBS ventfluids ŽKlinkhammer et al., 1994.. The concentrations of total REE decrease down-core and at the base of the core Že.g., Nd: 0.29 ppm. they are only slightly higher than fresh sulfides from the TAG hydrothermal field ŽMills and Elderfield, 1995b.. Even at the top of the core, REE concentrations are significantly lower than those in near-vent hydrothermal sediments recovered previously from TAG ŽNd: 3–10 ppm; German et al., 1993.. Studies from the East Pacific Rise have shown that ridge-flank metalliferous sediments contain significantly higher REE concentrations than those observed in this study Že.g.,
C.R. German et al.r Chemical Geology 155 (1999) 65–75
72 Table 3 REE concentrations Žppm. in core 1636 Sample 0–1 cm 1–2 cm 2–3 cm 3–4 cm 4–5 cm 5–6 cm 6–7 cm 7–8 cm 8–9 cm 9–10 cm 10–11 cm Average OBS Fluidsa Žppt.: a
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
Er
Yb
CerCe )
EurEu )
NdrYb
1.35 1.27 1.15 1.27 1.16 0.94 0.63 0.44 0.42 0.43 0.31
1.80 2.02 1.55 1.63 1.36 1.32 1.10 0.77 0.69 0.62 0.46
0.27 0.31 0.22 0.23 0.19 0.18 0.15 0.11 0.10 0.08 0.06
1.19 1.37 0.96 0.99 0.82 0.80 0.72 0.53 0.45 0.37 0.29
0.30 0.34 0.23 0.22 0.21 0.18 0.19 0.13 0.11 0.09 0.07
0.45 0.47 0.29 0.66 0.47 0.26 0.17 0.12 0.11 0.43 0.19
0.37 0.43 0.30 0.30 0.26 0.23 0.22 0.16 0.14 0.11 0.08
0.31 0.37 0.25 0.24 0.20 0.16 0.16 0.13 0.11 0.08 0.07
0.17 0.21 0.14 0.13 0.11 0.08 0.08 0.06 0.05 0.03 0.03
0.17 0.21 0.14 0.13 0.09 0.05 0.05 0.06 0.04 0.03 0.03
0.68 0.75 0.69 0.67 0.63 0.73 0.83 0.82 0.80 0.76 0.76
4.2 3.8 3.4 7.9 6.2 3.9 2.6 2.6 2.7 13.3 7.8
2.44 2.27 2.39 2.65 3.17 5.57 5.02 3.08 3.92 4.30 3.37
0.85 108
1.21 165
0.17 21.1
0.77 72.3
0.19 13.5
0.33 157
0.24 16.2
0.19 10.6
0.10 4.77
0.09 3.75
0.74 0.75
4.8 30.8
3.47 7.10
Data from Klinkhammer et al., 1994.
Bender et al., 1971; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988; Owen and Olivarez, 1988; Olivarez and Owen, 1989; Halliday et al., 1992.. For example, DSDP Leg 92 metalliferous sediment samples have total REE concentrations in the range 131–301 ppm, with the majority of the data falling between 167–222 ppm ŽBarrett et al., 1986, Barrett and Jarvis, 1988.. This represents an approximately 50-fold increase over the values reported here. REErFe ratios in ridge-flank sediments increase with distance away from the ridge-axis ŽRuhlin and Owen, 1986; Owen and Olivarez, 1988; Olivarez and Owen, 1989. and REE distribution patterns typically exhibit the heavy ŽH.REE enrichments and pronounced negative Ceanomalies that are characteristic of seawater ŽBender et al., 1971; Ruhlin and Owen, 1986; Barrett and Jarvis, 1988; Owen and Olivarez, 1988; Olivarez and Owen, 1989; Halliday et al., 1992.. These observations led the above authors to infer that extensive scavenging of dissolved REE from seawater occurs as hydrothermal Fe–Mn oxyhydroxide phases are dispersed off-axis, either prior to sedimentation or post-deposition during early diagenesis. This has been confirmed by subsequent analysis of hydrothermal plume particles which has shown that, close to an active vent-field, the particles exhibit relatively low REErFe ratios and REE patterns which exhibit both seawater and vent-fluid characteristics Ž negatiÕe
Ce-anomalies and positiÕe Eu-anomalies.. At increasing dispersion, however, particulate REErFe ratios increase significantly and seawater characteristics become dominant ŽGerman et al., 1990, 1997b.. By contrast, the REE patterns for core 1636 sediments show pronounced vent-fluid characteristics with a strong enrichment in the light ŽL.REE, pronounced positive Eu-anomalies and weak negative Ce-anomalies ŽFig. 5.. Nevertheless, these sediments are not entirely vent-fluid dominated. All the samples have much smaller positive Eu-anomalies than the vent fluid from which they evolved ŽFig. 5, Table 3.. In addition, as REE concentrations increase upcore, REE patterns become flatter, indicating relative HREE enrichment Ždecreasing Nd nrYb n values.. Relative to sediments from the TAG mound ŽMills and Elderfield, 1995b. samples from this study have REE concentrations which are: Ž1. higher than those of the freshest sulfide deposits from TAG ŽF 0.01 ppm Nd.; Ž2. comparable to, or lower than, those of the oxidised rims of relatively fresh sulfides from black smokers, white smokers and the TAG talus mound Ž0.3–3.5 ppm Nd.; and 3. lower than the REE concentrations in ochres from the TAG mound ŽNd: 3.6–13.5 ppm.. Similarly, Nd nrYb n ratios throughout core 1636 fall within the range 2.3–5.6, which overlaps that of the oxidised material from the TAG slope and mound ŽNd nrYb n s 2.1–6.0., but
C.R. German et al.r Chemical Geology 155 (1999) 65–75
73
TAG, again suggesting that core 1636 contains relatively fresh mound-derived sulfide material.
4. Summary
Fig. 5. Chondrite-normalized REE distribution patterns for hydrothermal sediments from core 1636. Also shown is the mean vent-fluid composition for the OBS vent-site at 218N on the EPR. Vent-fluid data Ž=10 3 . are from Klinkhammer et al. Ž1994.. Data have been normalised to the chondrite values of Nakamura Ž1974..
represents significant relative HREE enrichment when compared to fresh TAG black smoker sulfides ŽNd nrYb n s 7.0–9.6. and ochres ŽNd nrYb n s 8.2– 15.5.. The oxidised phases at TAG comprise alteration products of original sulfide material which adsorbed seawater-derived REE during oxidation ŽMills and Elderfield, 1995b.. Those workers also showed that the oxide coatings from inactive chimneys at TAG have more pronounced seawater characteristics than those from fresh chimneys, because oxidation of the inactive chimneys is more advanced and REE uptake from seawater more extensive. The simplest explanation for the REE distributions observed in this study, therefore, is that core 1636 is dominated by fresh sulfidic material with low REE concentrations and REE patterns dominated by vent-fluids. Towards the top of the core, however, total REE concentrations increase and the seawater influence also increases. These data are consistent with the depletion in S concentrations and complementary Mg enrichment observed in the upper layers of core 1636, suggesting the occurrence of near-surface oxidation resulting from the downward advection of seawater into this sulfide-dominated core. The highest REE contents in core 1636 resemble those of oxide rims of fresh chimney deposits at
Ži. An 11 cm sediment core collected from within 5 m of the OBS vent-site at 218N on the East Pacific Rise exhibits strong metal and S enrichments with 23–25 wt.% Fe, 10–22 wt.% Cu, 4–13 wt.% Zn and 22–30 wt.% S, but low Mn concentrations Ž108–279 ppm.. These observations are indicative of a relatively fresh sulfidic input to this core. In comparison, a similar submersible-collected push-core, raised from ) 50 m from the active TAG vent-site, exhibited much lower CurFe and ZnrFe ratios, but higher MnrFe ratios—all consistent with progressive aging and oxidation relative to the EPR core studied here. Žii. Core 1636 also exhibits high FerMn ratios when compared to ridge-flank sediments from the East Pacific Rise. The latter typically exhibit FerMn ratios close to 3.5, representing a decrease of 2–3 orders of magnitude from core 1636 ŽFerMn ; 1000.. The EPR near-vent core 1636 also exhibits much more pronounced CurFe and ZnrFe ratios than those witnessed previously in ridge-flank sediments from the East Pacific Rise. Whilst Cu and Zn concentrations in core 1636 are an order of magnitude greater than ridge-flank sediments, however, Pb concentrations are directly comparable. Pb isotopic compositions in core 1636 indicate that this Pb is exclusively mantle-derived. Such need not necessarily be the case in all ridge-flank sediments, however, because it is increasingly being recognised that scavenging of dissolved Pb from seawater may also be important. Žiii. REE concentrations throughout core 1636 are very low and REE patterns most closely resemble those of vent-fluids, exhibiting LREE enrichments and pronounced positive Eu-anomalies. The upper part of the core has slightly higher REE and Mg concentrations than the base of the core, however. In addition, the upper portions of the core also show relative HREE enrichment and more negative Ceanomalies. These patterns compare most closely with previous reports of fresh hydrothermal sulfide samples which have been partially oxidised. Overall, the observed variations in core 1636 are consistent with
74
C.R. German et al.r Chemical Geology 155 (1999) 65–75
progressive downward oxidation by ingressing seawater, leading to REE uptake onto the resultant oxidised phases.
Acknowledgements Hydrothermal research at SOC has been supported by NERC through the BRIDGE Community Research Programme. We thank the Captain and Crew of RV Atlantis II and in particular the skill and dexterity of the AlÕin pilots in collection of material for this work. Acknowledgements are also due to the reviewers of this manuscript, Prof. J. Boulegue and an anonymous reviewer, for their helpful comments.
References Barrett, T.J., Taylor, P.N., Jarvis, I., Lugowski, J., 1986. Pb and Sr isotope and rare earth element composition of selected metalliferous sediments from Sites 597 to 601, Deep Sea Drilling Project Leg 92. Init. Repts. DSDP 92, 355–370. Barrett, T.J., Taylor, P.N., Lugowski, J., 1987. Metalliferous sediments from DSDP Leg 92, the East Pacific Rise transect. Geochim. Cosmochim. Acta 51, 2241–2253. Barrett, T.J., Jarvis, I., 1988. Rare earth element geochemistry of metalliferous sediments from DSDP Leg 92, the East Pacific Rise transect. Chem. Geol. 67, 243–259. Bender, M., Broecker, W., Gornitz, V., Middel, U., Kay, R., Sun, S.-S., Biscaye, P., 1971. Geochemistry of three cores from the East Pacific Rise. Earth Planet. Sci. Lett. 12, 425–433. Bostrom, K., Peterson, M.N.A., 1966. Precipitates from hy¨ drothermal exhalations of the East Pacific Rise. Econ. Geol. 61, 1258–1265. Bostrom, ¨ K., Peterson, M.N.A., Joensuu, O., Fisher, D.E., 1969. Aluminum-poor ferromanganoan sediments on active oceanic ridges. J. Geophys. Res. 74, 3261–3270. Campbell, A.C., Bowers, T.S., Measures, C.I., Falkner, K.K., Khadem, M., Edmond, J.M., 1988. A time series of vent fluid compositions from 218N, East Pacific Rise Ž1979, 1981, 1985. and the Guaymas Basin, Gulf of California Ž1982, 1985.. J. Geophys. Res. 93, 4537–4549. Chen, J.H., Wasserburg, G.J., Von Damm, K.L., Edmond, J.M., 1986. U–Th–Pb systematics in hot springs on the East Pacific Rise at 218N and Guaymas Basin. Geochim. Cosmochim. Acta 50, 2467–2479. Dasch, E.J., 1981. Lead isotopic composition of metalliferous seidments from the Nazca plate. Geol. Soc. Am. Mem. 154, 199–209. Dymond, J., 1981. Geochemistry of Nazca Plate surface sediments: An evaluation of hydrothermal, biogenic, detrital and hydrogenous sources. Geol. Soc. Am. Mem. 154, 133–170.
Edmond, J.M., Von Damm, K.L., McDuff, R.E., Measures, C.I., 1982. Chemistry of hot springs on the East Pacific Rise and their effluent dispersal. Nature 297, 187–191. Edmond, J.M., Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., German, C.R., Edmonds, H.N., Elderfield, H., Thompson, G., Rona, P., 1995. Time series studies of vent fluids from the TAG and MARK sites Ž1986, 1990. Mid-Atlantic Ridge, a new solution chemistry model and a mechanism for CurZn zonation in massive sulfide orebodies. Geol. Soc. Spec. Publ. 87, 77–86. German, C.R., Klinkhammer, G.P., Edmond, J.M., Mitra, A., Elderfield, H., 1990. Hydrothermal scavenging of rare earth elements in the ocean. Nature 345, 516–518. German, C.R., Fleer, A.P., Bacon, M.P., Edmond, J.M., 1991. Hydrothermal scavenging at the Mid-Atlantic Ridge: Radionuclide distributions. Earth Planet. Sci. Lett. 105, 170–181. German, C.R., Higgs, N.C., Thomson, J., Mills, R., Elderfield, H., Blusztajn, J., Fleer, A.P., Bacon, M.P., 1993. A geochemical study of metalliferous sediment from the TAG hydrothermal X mound, 26808 N, Mid-Atlantic Ridge. J. Geophys. Res. 98, 9683–9692. German, C.R., Barreiro, B.A., Higgs, N.C., Nelsen, T.A., Ludford, E.M., Palmer, M.R., 1995. Seawater metasomatism in hydrothermal sediments ŽEscanaba Trough, northeast Pacific.. Chem. Geol. 119, 175–190. German, C.R., Bourles, ` D.L., Brown, E.T., Hergt, J., Colley, S., Higgs, N.C., Ludford, E.M., Nelsen, T.A., Feely, R.A., Raisbeck, G., Yiou, F., 1997a. Hydrothermal scavenging on the Juan de Fuca Ridge: 230 Th xs , 10 Be and REE in ridge-flank sediments. Geochim. Cosmochim. Acta 61, 4067–4078. German, C.R., Ludford, E.M., Palmer, M.R., O’Brien, J.D., Patching, J.W., Floch’lay, G., Appriou, P., Barriga, F., Miranda, M., Fouquet, Y., Bougault, H., 1997b. Hydrothermal plumes on the Mid-Atlantic Ridge ŽAzores.: Particle geochemistry and mineralogy—MARFLUXrATJ. Terra Nova 9, 539. Godfrey, L.V., Mills, R., Elderfield, H., Gurvich, E., 1994. Lead behaviour at the TAG hydrothermal vent field, 268N, MidAtlantic Ridge. Mar. Chem. 46, 237–254. Halliday, A.N., Davidson, J.P., Holden, P., Owen, R.M., Olivarez, A.M., 1992. Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents. Geophys. Res. Lett. 19, 761–764. Haymon, R.M., 1983. Growth history of hydrothermal black smoker chimneys. Nature 301, 695–698. Haymon, R.M., Kastner, M., 1981. Hot spring deposits on the East Pacific Rise at 218 N, preliminary description of mineralogy and genesis. Earth Planet. Sci. Lett. 53, 363–381. Jarvis, I., Jarvis, K.E., 1985. Rare-earth element geochemistry of standard sediments, a study using inductively coupled plasma spectrometry. Chem. Geol. 53, 335–344. Klinkhammer, G., Hudson, A., 1986. Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer. Earth Planet. Sci. Lett. 79, 241–249. Klinkhammer, G.P., Elderfield, H., Edmond, J.M., Mitra, A., 1994. Geochemical implications of rare earth element patterns in hydrothermal fluids from mid-ocean ridges. Geochim. Cosmochim. Acta 58, 5105–5113.
C.R. German et al.r Chemical Geology 155 (1999) 65–75 Kyte, F.T., Leinen, M., Heath, G.R., Zhou, L., 1993. Cenozoic sedimentation history of the central North Pacific: Inferences from the elemental geochemistry of core LL44-GPC3. Geochim. Cosmochim. Acta 57, 1719–1740. Lupton, J.E., Delaney, J.R., Johnson, H.P., Tivey, M.K., 1985. Entrainment and vertical transport of deep ocean water by buoyant hydrothermal plumes. Nature 316, 621–623. Lyle, M., 1986. Major element composition of Leg 92 sediments. Init. Reps. DSDP 92, 355–370. Metz, S., Trefry, J.H., Nelsen, T.A., 1988. History and geochemistry of a metalliferous sediment core from the Mid-Atlantic Ridge at 268N. Geochim. Cosmochim. Acta 52, 2369–2378. Mills, R.A., Elderfield, H., 1995a. Hydrothermal activity and the geochemistry of metalliferous sediment. Geophys. Monogr. ŽAGU. 91, 392–407. Mills, R.A., Elderfield, H., 1995b. Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, Mid-Atlantic Ridge. Geochim. Cosmochim. Acta 59, 3511– 5324. Mills, R.A., Elderfield, H., Thomson, J., 1993. A dual origin for the hydrothermal component in a metalliferous sediment core from the Mid-Atlantic Ridge. J. Geophys. Res. 98, 9671–9678. Mills, R.A., Alt, J.A., Clayton, T., 1996. Low-temperature fluid flow through sulfidic sediments from TAG: modification of fluid chemistry and alteration of mineral deposits. Geophys. Res. Lett. 23, 3495–3498. Mottl, M.J., McConachy, T.F., 1990. Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 218N. Geochim. Cosmochim. Acta 54, 1911–1927. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta 38, 757–775. Olivarez, A.M., Owen, R.M., 1989. REErFe variation in hydrothermal sediments, Implications for the REE content of seawater. Geochim. Cosmochim. Acta 53, 757–762. Owen, R.M., Olivarez, A.M., 1988. Geochemistry of rare earth
75
elements in Pacific hydrothermal sediments. Mar. Chem. 25, 183–196. Reynolds, P.H., Dasch, E.J., 1971. Lead isotopes in marine Mn nodules and the ore-lead growth curve. J. Geophys. Res. 76, 5124–5129. Rona, P.A., Scott, S.D., 1993. A special issue on sea-floor hydrothermal mineralization, new perspectives. Economic Geology 88, 1935–1976. Ruhlin, D.E., Owen, R.M., 1986. The rare earth element geochemistry of hydrothermal sediments from the East Pacific Rise, examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta 50, 393–400. Shimmield, G.B., Price, N.B., 1988. The scavenging of U, 230 Th and 231 Pa during pulsed hydrothermal activity at 208S, East Pacific Rise. Geochim. Cosmochim. Acta 52, 669–677. RISE Project Group, Spiess, F.N., 1980. East Pacific Rise: Hot springs and geophysical experiments. Science 207, 1421–1433. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotopes evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Vidal, P., Clauer, N., 1981. Pb and Sr isotopic systematics of some basalts and sulfides from the East Pacific Rise at 218N Žproject RITA.. Earth Planet. Sci. Lett. 55, 237–246. Von Damm, K.L., Edmond, J.M., Grant, B., Measures, C.I., Walden, B., Weiss, R.F., 1985. Chemistry of submarine hydrothermal solutions at 218N, East Pacific Rise. Geochim. Cosmochim. Acta 49, 2197–2220. Woodhead, J.D., Volker, F., McCulloch, M.T., 1994. Routine Pb isotope determinations using a 207 Pby204 Pb double spike, a long-term assessment of analytical precision and accuracy. Analyst 120, 35–39. Woodruff, L.G., Shanks, W.C. III, 1988. Sulfur isotope chemistry of chimney materials and vent fluids from 218N, East Pacific Rise, hydrothermal sulfur sources and disequilibrium sulfate reduction. J. Geophys. Res. 93, 4562–4572.