Rare-earth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect

Rare-earth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect

Chemical Geology, 67 (1988) 243-259 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 243 [3] RARE-EARTH ELEMENT GEOCHEMIS...

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Chemical Geology, 67 (1988) 243-259 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

243

[3]

RARE-EARTH ELEMENT GEOCHEMISTRY OF METALLIFEROUS SEDIMENTS FROM DSDP LEG 92: THE EAST PACIFIC RISE TRANSECT T.J. BARRETT and I. JARVIS IREM-MERI, Department of Geological Sciences, McGiU University, Montreal, P.Q. H3A 2A 7 (Canada) School of Geological Sciences, Kingston Polytechnic, Kingston-on-Thames, Surrey KT1 2EE (Great Britain) (Received December 3, 1986; revised and accepted September 23, 1987)

Abstract Barrett, T.J. and Jarvis, I., 1988. Rare-earth element geochemistry of metalliferous sediments from DSDP Leg 92: The East Pacific Rise transect. Chem. Geol., 67: 243-259. DSDP Leg 92 drilled at four sites along an east-west transect at 19 ° S on the western flank of the East Pacific Rise (EPR), in an area where sediments are essentially a mixture of hydrothermal and biogenic components, with only a minimal contribution of clastic material. Rare-earth element (REE) data on the metalliferous (non-carbonate) fraction of samples ranging in age from ~ 2 to ~ 27 Ma indicate the existence of two distinct groups of patterns corresponding to two broad age groups, one ~ 8 Ma, the other ~ 10 Ma. Within each group, REE patterns have characteristics which are near-uniform, despite large variations in total REE abundances. Sediments of the younger group are enriched in light REE (LREE) relative to deep bottom waters influenced by the hydrothermal plume extending west from the EPR at 19 oS. Sediments of the older groups show further relative LREE enrichment and/or heavy REE (HREE) depletion. Surficial sediments deposited beneath the lysocline have high XREE concentrations resulting from slow accumulation rates, and patterns resembling older sediments due to early diagenetic effects. A correlation between the mass accumulation rates (MAR) of XREE and Fe + Mn suggests that ferromanganese particulate matter supplied by the hydrothermal plume scavenges REE; during this process the LREE are preferentially removed from plume seawater. The MAR ofFe + Mn shows a general decrease with age above basement, whereas XREE concentrations in the metalliferous component increase with age above basement. This supports the Ruhlin and Owen model wherein limited scavenging of REE, due to rapid burial of sediment near the palaeo-axis, leads to low concentrations (but high MAR-values) for the REE. Following deposition and burial of the hydrothermal component, further relative flattening of the REE pattern takes place, probably the result of diagenetic reactions over several million years. Phase partitioning data indicate that the proportion of REE residing in more poorly crystalline phases tends to increase with age (from ~ 45 % to 90% of XREE). This suggests that as initial ferromanganese precipitates undergo diagenetic recrystaUization, REE are transferred to the poorly crystalline phases, and/or are scavenged from pore waters by these phases. Because of the various modifications to REE patterns apparently produced both in the water column and post-depositional settings, the REE patterns of metalliferous sediments will not reflect fine-scale REE variations in associated oceanic water masses.

1. I n t r o d u c t i o n Sediments near the East Pacific Rise (EPR) 0009-2541/88/$03.50

are strongly enriched in Fe, Mn and other transition metals in comparison with normal pelagic sediments (Revelle, 1944; BostrSm and

© 1988 Elsevier Science Publishers B.V.

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. / Fig. 1. Index map of the eastern Pacific showing the location of Leg-92 Sites 597-602 (the East Pacific Rise transect), together with DSDP Sites 573 and 574 (Leg 85) on the Pacific Plate. Numbers and arrows on spreading axes refer to total rate (cm yr. -z) and direction of spreading (Circurn-Pacific Council for Energy and Mineral Resources map,

1982).

Peterson, 1966; Bostrbm et al., 1969; Bender et al., 1971). It is generally accepted that these metals were leached from the basaltic crust by seawater hydrothermal systems and/or subsequently scavenged from seawater by authigenic Fe-Mn-rich precipitates that formed upon hydrothermal discharge and settled onto the ocean floor {Corliss, 1971; Bonatti et al., 1972; Dymond et al., 1973; Piper, 1973; Dymond and Veeh, 1975; Cronan, 1980; Dymond, 1981). Studies of the mineralogy, geochemistry and origin of metalliferous sediments in the eastern Pacific (Fig. 1) have concentrated on recent sediments of the EPR (Sayles and Bischoff, 1973; Heath and Dymond, 1977, 1981; Lyle, 1981; Marchig and Gundlach, 1982; Marchig et al., 1982, 1986; B~icker et al., 1985; Walter and Stoffers, 1985), the Bauer Deep (Dymond and Veeh, 1975; Sayles et al., 1975; Dymond, 1981; Lyle, 1981; McMurtry et al., 1981; Cole, 1985),

and the [email protected] Hydrothermal Mounds Field (Corliss et al., 1978; Hdkinian et al., 1978; Honnorez et al., 1981; Barrett and Friedrichsen, 1982; Moorby, 1983; Walter and Stoffers, 1985). Geochemical data are available on ancient metalliferous analogs which have been cored by the Deep Sea Drilling Project (DSDP) as a near-basal facies both to the east and to the west of the EPR (vonder Borch and Rex, 1970; Tracey et al., 1971; vonder Borch et al., 1971; Hays et al., 1972; Dymond et al., 1973, 1976; van Andel et al., 1973; Cronan, 1976; Yeats et al., 1976). Recently, Tertiary metalliferous sediments were recovered from Leg 85 at Sites 573 and 574 in the central-eastern Pacific (Mayer et al., 1985), and Leg 92 on the west flank of the EPR (Leinen et al., 1986; Lyle, 1986; Marchig and Erzinget, 1986; Rea and Leinen, 1986). Detailed rareearth element (REE) data for metalliferous sediment sequences are available only for Legs 70, 85 a n d 92 (Migdasov et al., 1983; Jarvis, 1985; Ruhlin and Owen, 1986 ). DSDP Leg 92 drilled a t f o u r sites along a n east-west transect at 19 oS (Fig. 1 ). These sites intersected basement ranging in age from Oligocene to Pliocene ( ~ 28-5 Ma). This area was chosen partly because the sediments are essentially a mixture of hydrothermal and biogenic components, with only a minimal contribution of clastic material (Rea and Leinen, 1986). Nearly complete recovery of the overlying sediment was achieved through use of piston coring techniques. The objectives of the present study were: (1) determination of REE patterns for the metalliferous component of sediments ranging from ~26 to 2 Ma in age; (2) assessment of the role of hydrothermal plumes emanating from the EPR on the REE patterns of the younger sediments; and (3) comparison of older and younger sediments to determine if any age-related variations in REE distributions are present. Leg-92 Sites 598-602, as well as the Leg-85 sites which lie ~ 20 ° of latitude ( ~-2000 kin) to the north (Fig. 1), were generated at the EPR and are presently located on the eastern margin of the Pacific Plate. Site 597, how-

245

ever, was formed at the fossil Mendoza Rise, the forerunner of the present E P R (Mammeri c k x e t a l . , 1980). The R E E distribution patterns of recent metalliferous sediments are broadly similar to those of deep-ocean seawater (Bender et al., 1971; D y m o n d et al., 1973; Piper and Graef, 1974; Marchig et al., 1982; Fleet, 1983) The sediment patterns show comparatively little variation despite strong fractionation of the R E E with depth in the water column (Elderfield and Greaves, 1982; de Baar et al., 1985a, b) • Shale-normalized patterns of recent metalliferous sediments at the E P R are characterized by a marked depletion in Ce relative to its ne ighb o u r s L a a n d P r (Benderetal., 1971),and a distinct enrichment in the heavy R E E ( H R E E ) relative to the light R E E ( L R E E ) (Piper and Graef, 1974; Marchig et al., 1982) • We refer to this type of pattern in the present paper as "EPR-crest". By contrast, pelagic clays (montmorillonitic) from the northern Pacific have flat patterns with no Ce anomaly, and authigenic Mn nodules have positive Ce anomalies (Piper, 1974a, b; Elderfield et al., 1981). Recently, R E E data have been obtained for seawater from hydrothermally active and inactive parts of the world ocean, and at varying depths in the water column (Elderfield and Greaves, 1982; Klinkhammer et al., 1983; de Baar et al., 1985a, b). In the present paper, we report subtle but distinct variations in the R E E patterns of metalliferous sediments from D S D P Leg 92, and use the recent data for seawater to assess the influence of hydrothermal activity on the patterns of sediments, Geochemical data on L e g - 9 2 sediments, ineluding an initial set of R E E patterns, are given in Barrett et al. (1986). In the present paper, we report a larger set of R E E data, together with estimates of R E E mass accumulation rates and information on elemental partitioning• A detailed study of the R E E in Leg-92 sediments from Site 5 9 8 has also been published by Ruhlin and Owen ( 1 9 8 6 ) . In our paper, we examine sediments from all Leg-92 sites, and arrive at

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Fig. 2. StratigraphicsurnmaryofsedimentsdriUedatDSDP Leg-92 Sites 597-601.Althoughthe stratigraphic columns (based on shipboard results) give the main sediment lithology as "clayey nannofossil ooze", the clay mineral component in the non-carbonate fraction of the samples which we analyzed is generally insignificant relative to the ox-

ide-oxyhydroxidecomponent. conclusions generally supporting those of Ruhlin and Owen (1986). We also make further deductions on the basis of data from the other sites, phase partitioning results, and recently published information on deep seawater R E E patterns. 2. Samples

and methods

2.1. S a m p l e s

Sediment lithologies and thicknesses for the various sites are shown in Fig. 2. The majority of the sediment sequence is upper Oligocene-Miocene at Site 597, Miocene at Sites 5 9 8 and 599, and Pliocene-Pleistocene at Sites 6 0 0

246 and 601. All of these sediments are essentially calcareous nannofossil oozes with varying admixtures (commonly 20-40% ) of a darker finegrained metalliferous component. Smear-slide examination by onboard scientists revealed this latter component to consist of Fe-oxides and -hydroxides, RSO (red-brown to yellow-brown semiopaque oxides), and poorly crystalline clays. Although the stratigraphic columns in Fig. 2 give the main sediment lithology as clayey nannofossil ooze, X-ray diffractometry (XRD) and geochemical data (Barrett et al., 1986) indicate that the clay component is minor to insignificant relative to the oxide-oxyhydroxide component. Variations in the proportion of this latter component determine the colour of the sediment, which ranges from light-coloured carbonate-rich oozes to yellow-brown and very dark-brown calcareous mudstones. The main crystalline phase of the metalliferous component is goethite, with minor amounts of finegrained apatite and, in a few samples, phillipsite. Traces of quartz and clinoptilolite (?) are present in a few samples. Geochemical data suggest that the amorphous to poorly crystalline phases, which constitute roughly half of the metalliferous component, contain Fe-Mn-oxyhydroxides with minor apatite and Fe-smectite. Sedimentological features at each site are summarized in the individual site chapters for Leg 92 (Leinen et al., 1986).

(made from BDH & Eldrich ® standard solutions for atomic absorption), calculated to match closely the range of values expected for each element in the unknowns. Careful matching of matrices is essential to minimize interference effects. Raw intensity data were processed on a digital PDP ® 11-34 computer utilizing a program developed by I.J. that incorporates drift and blank correction. In addition, the REE data were corrected for interelement interferences by Ba, Ca, Sr, Ti and Zr to compensate for the small proportion of these elements that remain after the cation-exchange procedure. Analytical precision as determined by replicate analyses is better than -+3% for all of the REE present in amounts >0.2 ppm. By reference to international standards, absolute accuracy is judged to be better than _+5% of the quoted values (Walsh et al., 1981; Jarvis and Jarvis, 1985). In this paper, REE concentrations are normalized relative to U.S.G.S. standard SCo-I (Cody Shale), which is considered to be a more representative standard (JarvisandJarvis, 1985)thanthe average North American Shale values generally used ( Haskin and Haskin, 1966 ). 3. R e s u l t s a n d d i s c u s s i o n 3.1. REE patterns

2.2. Methods

The REE and Y were extracted from splits of carbonate-free powders following HF-HC104 open digestion, using a cation separation procedure (Jarvis and Jarvis, 1985 ) modified from Walsh et al. (1981) and Thompson and Walsh (1983).La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu and Y were determined simultaneously using a Philips ® PV 8210 inductively coupled plasma-atomic emission spectrometer (ICPAES) at the Department of Geology, King's College, London. Calibration curves were constructed by using artificial composite standards

REE data for the carbonate-free fraction of 27 selected samples from Sites 597-601 are given in Table I [ marked with ( 1 ) ]. This table also gives the REE concentrations following a second leach designed to remove amorphous to poorly crystallized oxide-oxyhydroxide phases [marked with (2)]. Details of the leach procedures are given in Barrett et al. (1986). The following comments refer to the total carbonate-free fraction. Leg-92 sediments have XREE contents ranging mostly from 131 to 867 ppm, with a clustering between 167 and 529 ppm; four samples have

247

notably higher X R E E values of 920-1984 p p m (excluding Tb and T m which were not determined ). The patterns have strongly negative Ce anomalies, where Ce* is defined as follows:

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Ce* = log [ Ce~h/{0.5 ( Lash + Prsh) } ] where Cesh, Lash and Prsh represent the shalenormalized values of each element. Ce* ranges from - 0.95 to - 0.60, with a clustering of values between - 0.90 and - 0.74. As noted by Barrett et al. (1986) on the basis of a smaller database, most samples from Sites 599 and 601 have similar patterns which display a modest but consistent enrichment in the H R E E relative to the L R E E (excluding Ce, discussed separately), whereas most of those from Sites 597 and 598 have flatter H R E E / L R E E trends. However, since age overlap occurs between sites, it is more appropriate to compare samples of on the basis of age groupings. Various R E E data hereafter discussed for our extended set of samples suggest that the sediments can be considered in terms of two broad age groups, one ~ 8 Ma, the other ~ 10 Ma ( we have no intervening samples ). Within each group (Fig. 3a and b) the characteristics of the R E E patterns are near-uniform, despite variations in X R E E concentrations of up to a factor of 6. Two sub-lysocline samples from the younger group are unusually enriched in R E E and have slightly "aberrant" patterns; they are discussed in Section 3.4.

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Fig. 3. a. REE patterns for all metalliferous samples younger than ~8Ma. Note close similarity of allpatterns except for the two REE-enriched near-surface sediments deposited below the lysocline (core 598-1; see Section 3.4). b. REE patterns for all metalliferous samples older than ~ 10 Ma. This group of patterns is also internally consistent, but differs from those in (a) in having a greater enrichment in the MREE to LREE relative to the HREE.

from the E P R are characterized by large positive Eu anomalies and depletion in the H R E E relative to the L R E E , the result of high-temperature subsurface reaction between normal seawater and basaltic crust (Michard et al., 1983). Metalliferous sediments with patterns reflecting a hydrothermal component are in fact known from the Red Sea (Courtois and Treuil, 1977), but in this case it is likely that mixing between hydrothermal solutions and overlying

normal seawater was limited during chemical precipitation. At the EPR, by contrast, solutions discharge freely into a well-circulated water mass, and any hydrothermal component to the R E E pattern of chemical precipitates appears to be quickly diluted to the point of being unrecognizable. The seawater-like pattern of E P R sediments apparently results from the rapid precipitation and adsorption of seawater

248 REE onto hydrothermally derived Fe-Mn-oxyhydroxide colloidal particles, either in the water column or after the particles have settled onto the sea floor. A primary seawater source for Nd and Sr in metalliferous sediments from the eastern Pacific is also indicated by their 143Nd/144Nd and STSr/S6Sr ratios, which are very close to those of seawater (O'Nions et al., 1978; Piepgras et al., 1979; Elderfield et al., 1981; Goldstein and O'Nions, 1981; Aplin et al., 1986 ) . By contrast, Pb isotopic values of metalliferous sediments from the E P R axis show a strong input of basaltic Pb via the hydrothermal plume (Bender et al., 1971; O'Nions et al., 1978), which apparently transports Pb to distances of hundreds of km east (Dasch, 1981) and west (Barrett et al., 1986) of the axis. Hydrothermal activity thus provides the source of the particulate matter that scavenges REE in large parts of the east Pacific (Klinkh a m m e r et al., 1983 ). It is therefore of interest to examine the REE patterns of metalliferous sediment in light of recently published data for deep ocean waters. In Fig. 4a, seawater sampled directly over the E P R axis and within the hydrothermal plume at 19 ° S (the latitude of the Leg-92 transect) is compared with the younger group of sediments. On the basis of mantle-derived He anomalies (Lupton and Craig, 1981), this plume is known to extend west of the E P R as far as the present-day locations of Sites 598 and 597. The pattern for E P R deep seawater ( K l i n k h a m m e r et al., 1983) has been scaled up to match the H R E E end of one of the metalliferous sediment patterns (we chose the lower pattern for convenience). W h e n this is done, it becomes apparent that the sediments are enriched in the middle REE ( M R E E ) to L R E E relative to deep plume seawater at the same latitude. This observation directly supports the inference of K l i n k h a m m e r et al. (1983, p. 188) that "the dominant effect of hydrothermal circulation on REE geochemistryin seawater is preferential removal of the lighter elements",

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Fig. 4. a. Comparisonbetween the REE patterns of sediments youngerthan ~ 8 Ma with deep (2500m) seawater from the hydrothermalplume above the crest of the East PacificRise (Klinkhammeret al., 1983). The plume pattern has been scaledup so that the HREEend (ErandYb) matches that of the sediments.This demonstrates that the

sedimentsare enrichedin MREEto LREErelativeto deep seawaterinfluencedby the plume. b. As in (a) but for sediments olderthan ~ 10 Ma. As these authors noted, this probably reflects the longer residence time of H R E E in the deep ocean (Elderfield and Greaves, 1982), which itself may result from the greater stability of (dissolved) H R E E complexes relative to LREE complexes in seawater ( T u r n e r and Whitfield, 1979). Continuous scavenging would gradually modify the initial EPR-crest pattern by increasing the ratio of L R E E to H R E E in the particulates, while having the opposite effect on seawater. Scavenging from pore waters following burial conceivably could also continue to increase the L R E E / H R E E ratio. [ Relative to the deep waters in the plume at 19°S, the Ce* anomaly in the younger group of Leg-92 sediments is less negative by ~0.2-0.3 log units.

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tom waters not directly influenced by a hydrothermal plume, from the east Pacific at 18°N,108°W (De Baar et al., 1985a). Fig. 4c shows that the Leg-92 sediments are still somewhat enriched in the lighter R E E (particularly Ce) relative to this type of bottom water. Finally, in Fig. 4d, Leg-92 sediments are compared with deep water from the northeast Atlantic, in an area where the distribution of the R E E is considered to be controlled exclusively by external terrigenous sources, with no hydrothermal influence (Elderfield and Greaves, 1982). Interestingly, the patterns for the younger group of Leg-92 sediments match the Atlantic deep waters closely for all elements except Eu and Ce, which have variable oxidation states (the lowest patterns in each group are compared for convenience). Thus, metalliferous sediment formed under the influence of hydrothermal plumes at the E P R , has a R E E

Fig. 4. c. As in (a) but comparison is with deep (3000 m) seawater away from the immediate influence of a ridge plume (de Baar et al., 1985a). d. Comparison between Leg-92 sediments and deep (2500 m) seawater not influencedby a hydrothermal plume (Elderfield and Greaves, 1982). Plot also shows that sediments older than ~ 10 Ma have comparable HREE patterns but are enriched in MREE to LREE relative to sediments younger than ~ 8 Ma. The seawater patterns in (c) and (d) have been scaled up to match the HREE end ( Er and Yb) of Leg-92sediments.

pattern similar to bottom waters in a region of no hydrothermal activity. It therefore seems unlikely that R E E patterns alone can be used to demonstrate a hydrothermal influence on authigenic sedimentation in open-ocean environments.

Amongst the LREE, this represents the greatest enrichment relative to the plume water (Fig. 4a), presumably because of preferential incorporation of Ce 4+ into particulates. ] Fig. 4b shows that for the older group of sediments ( ~ 2 7 - 1 0 M a ) , the discrepancy between the sediments and E P R deep seawater is even greater. This is the result of further increase in the sediment in the proportion of M R E E and L R E E relative to the H R E E (again, Ce shows slightly anomalous enrichment), Possible causes, involving diagenetic modification of the sediments, are discussed in Section 3.4. For comparison, we have also considered bot-

Table I lists MAR-values for Z R E E based on the accumulation rates of the bulk sediments

3.2. M a s s accumulation rates ( M A R )

(Leinen et al., 1986), the total concentration of R E E in the carbonate-free fraction and the measured carbonate content of the analyzed samples. The M A R of any individual R E E can be calculated from the Z R E E value simply by multiplying it by the ratio of the concentration of that element to Z R E E ( Table I ). Also given in Table I are MAR-values for combined Fe + Mn (Barrett et al., 1987), which is representative of the metalliferous component. With respect to R E E concentrations, a plot of Z ' R E E / ( F e + M n ) vs. age above basement

250 TABLE I Concentration of t h e rare-earth elements in metalliferous sediments from D S D P Leg 92, on a carbonate-free a n d salt-free basis D S D P sample No.

La

Ce

Pr

Nd

Sm

Eu

Gd

Dy

Ho

597A-1-1:66-68 cm (2) 597A-2-2:66-68 cm (1) 597A-2-2 (2) 597A-4-4:66-68 cm (1) 597A-4-4 (2) 597A-5-5:66-68 cm (1) 597A-5-6: 6 6 - 6 8 c m ( I ) 597A-5-6 (2) 597-6-2: 6 6 - 7 1 c m (1) 597-6-2 (2) 598-1-1:121-123 cm (1) 598-1-2:121-123 cm (1) 598-1-2 (2) 598-2-5:119-121 cm (1) 598-2-5 (2) 598-4-2:121-123 cm (1) 598-4-2 (2) 598-5-1:121-123 cm (1) 598-5-4:121-123 cm (1) 598-5-4 (2) 598-5-5:121-123 cm (1) 599-2-2:121-123 cm (1) 599-2-2 (2) 599-2-6:121-123 cm (1) 599-3-1:121-123 cm (1) 599-3-1 (2) 599-3-2:121-123 cm (1) 599-3-2: (2) 599-3-4:121-123 cm (1) 599-4-I: 121-123 cm (1) 599-4-1 (2) 599-4-2:121-123 cm (2) 599-4-5:121-123 cm (1) 599B-2-3:121-123 cm (1) 599B-2-3 (2) 599B-2-6:121-123 cm (1) 599B-2-6 (2) 600C-1-5:121-123 cm ( t ) 600C-1-6:121-123 cm (1) 600C-1-6 (2) 601-2-2:121-123 cm (1) 601-2-2 (2) 601-2-5:121-123 cm (1)

250 257 29.2 219 35.4 64.0 137 29.0 77.0 25.4 305 596 264 258 28.0 189 61.8 107 129 12.4 88.3 61.5 33.4 52.8 94.5 27.3 139 62.0 63.1 97.8 45.0 45.2 63.0 126 44.7 50.8 24.6 42.2 79.1 38.2 180 72.0 54.9

78.7 114.0 22.4 69.7 16.8 19.6 41.5 11.7 15.7 6.4 152 187 90.0 92.7 17.5 52.2 12.0 24.8 34.3 5.2 27.7 14.8 6.5 12.9 21.0 5.0 27.4 10.2 17.0 22.7 8.0 9.0 13.2 27.8 8.9 13.8 6.3 9.7 16.5 9.0 38.1 15.2 13.6

54.5 46.3 6.2 34.2 6.9 10.2 19.6 4.2 11.2 4.2 60.4 99.6 50.1 40.2 5.6 25.0 10.7 15.3 18.7 2.3 13.4 8.6 4.9 7.7 11.6 3.4 17.5 9.0 8.9 12.6 6.6 7.0 8.3 16.0 6.9 7.2 3.7 5.8 13.3 5.9 24.2 9.0 7.5

247 223 27.7 166 30.6 49.6 97.4 18.4 54.8 17.7 295 501 234 222 25.5 122 49.2 72.3 93.6 10.2 64.0 42.5 22.9 38.6 59.0 16.9 88.5 44.2 44.4 64.5 32.5 32.9 41.0 81.2 33.7 35.8 17.1 29.1 65.8 27.4 123 46.5 37.4

53.0 44.5 6.0 33.3 6.4 10.0 19.7 3.8 10.8 3.6 61.8 101 49.9 41.0 5.6 24.6 10.3 14.3 19.2 2.2 13.2 8.8 4.9 8.1 12.7 3.7 18.1 9.5 9.3 13.6 7.0 7.0 8.5 16.5 7.6 8.1 3.9 6.1 13.9 6.0 25.0 9.6 7.9

14.2 12.1 1.7 9.4 1.9 3.0 6.1 1.1 3.3 1.1 16.4 27.4 13.6 11.6 1.7 7.1 3.1 4.1 5.7 0.7 3.8 2.8 1.6 2.6 4.4 1.3 5.7 3.1 2.9 4.5 2.4 2.3 2.7 5.3 2.4 2.6 1.3 2.1 4.2 2.0 7.5 3.2 2.5

59.0 51.3 6.9 39.2 7.1 12.0 24.8 4.3 13.3 4.1 72.3 127 59.9 52.1 7.0 31.6 13.1 17.0 23.9 2.6 15.8 10.9 6.0 10.2 17.4 5.1 24.4 12.3 11.8 18.2 9.5 8.5 10.9 22.2 9.9 10.2 4.7 7.9 18.2 7.9 33.5 13.0 10.2

65.9 49.1 7.5 38.2 7.6 12.6 23.8 4.7 13.8 4.9 75.8 130 65.9 52.7 8.1 31.6 13.6 17.8 24.6 2.9 16.7 13.5 7.4 12.4 20.2 6.2 26.2 14.1 13.7 20.0 10.8 10.6 12.8 24.9 11.7 13.6 6.2 9.7 21.2 9.7 36.0 14.9 12.0

14.6 10.2 1.7 9.1 1.7 2.9 5.3 1.0 3.4 1.1 17.9 31.0 15.7 11.9 1.8 7.3 3.1 4.2 6.0 0.7 4.0 3.3 1.8 3.1 5.2 1.7 7.0 3.7 3.5 5.4 2.9 2.5 3.3 6.7 3.1 3.2 1.5 2.5 5.4 2.5 8.2 4.0 3.0

29.6

60.0

6.2

24.7

4.7

1.2

4.0

3.3

0.7

SCo-1

( 1 ) p p m after first leach to remove the carbonate fraction; (2) p p m after second acid-reducible leach to remove the amorphous to poorly crystallized oxyhydroxide phases. (All samples contain a small c o m p o n e n t of fine-grained insoluble clastic material, generally less t h a n several percent of the carbonate-free fraction.) *12:REE is for measured elements a n d does n o t include T m and Tb, which can be estimated by interpolation of flanking R E E values. *2Ce* is defined in the text. *3This ratio is based on p p m values normalized to SCo-l.

251

Er

Yb

Lu

XREE*'

Y

Ce *(.2~

Y b / P r .3

CaCO3 .4 (%)

MAR bulk *~

38.5 28.9 5.1 25.2 4.8 8.0 14.9 2.6 8.9 2.9 47.0 85.3 43.4 32.7 5.0 1.36 8.5 11.5 16.6 1.8 10.5 11.1 5.0 9.0 14.4 4.5 20.1 11.2 9.7 15.6 8.4 7.2 9.2 19.4 8.6 11.1 4.5 7.2 14.8 7.4 24.3 11.1 9.0

38.9 26.2 5.2 24.1 4.6 6.9 14.4 2.7 8.3 2.8 44.8 86.0 43.1 32.8 5.3 20.3 8.3 11.1 15.7 2.0 10.3 10.3 5.3 8.6 14.9 4.7 19.9 10.4 9.9 15.2 8.4 7.4 9.6 18.7 8.8 9.7 4.9 7.4 14.4 7.4 25.2 11.3 9.3

5.7 4.1 0.8 3.6 0.7 1.1 2.1 0.4 1.3 0.4 7.0 13.1 6.1 4.9 0.9 3.2 1.2 1.7 2.6 0.3 1.6 1.6 0.8 1.5 2.5 0.7 3.1 1.9 1.6 2.4 1.3 1.1 1.5 2.9 1.3 1.8 0.7 1.1 2.1 1.1 3.8 1.9 1.5

920 867 121 671 125 200 407 84 222 75 1,155 1,984 936 853 112 534 195 301 390 43 269 190 100 167 278 81 397 192 196 292 143 141 184 368 147 168 79 131 269 125 529 212 169

1.9

1.9

0.3

138

53.4 322 53.4 266 65.6 97.2 189 41.6 114 50.8 615 1,150 663 403 66.1 254 175 136 185 26.2 126 119 110 107 183 98.3 240 215 124 186 167 87.9 118 229 170 130 94.3 95.5 167 133 315 233 118

-0.817 -0.628 -0.426 -0.744 -0.613 -0.765 -0.750 - 0.628 -0.925 - 0.859 -0.597 -0.763 -0.752 -0.691 -0.499 -0.777 -0.978 -0.866 -0.809 -0.654 -0.745 -0.845 -0.946 -0.847 -0.859 -0.943 -0.915 - 1.018 -0.798 -0.848 -0.983 -0.945 -0.895 -0.867 -0.947 -0.796 -0.838 -0.865 -0.942 -0.873 -0.895 -0.886 -0.829

2.35 1.88 2.77 2.33 2.18 2.31 2.41 2.20 2.44 2.30 2.45 2.86 2.84 2.68 3.08 2.79 2.55 2.36 2.80 2.86 2.50 3.93 3.43 3.83 4.16 4.53 3.79 3.85 3.71 4.05 4.17 3.64 3.92 3.81 4.28 4.33 4.41 4.19 3.67 4.13 3.44 4.08 4.08

Age (Ma) ( F e ÷ M n ) .6

87.59

650

3.6

87.40

690

78.43 79.39

690 690

67.06

690

81.96 63.24

70 70

83.38

X R E E .7

70

17.3

38.3

58.3

24.9

50.6

29.7 58.9

26.5 26.7

35.1

27.2

7.3

14.6 51.1

2.1 3.6

114

14.8

16.2

10.0

85.78

546

26.9

41.4

14.0

79.17 58.49

505 505

50.7 108

31.7 62

15.0 15.7

79.87 72.70

505 657

55.8

30.1 34

15.9 4.6

63.68 64.20

657 657

39.8 65.3

5.3 5.6

72.59

657

76.1

71.4

5.7

74.11 66.58

657 657

97.8

37.2 64.1

6.1 6.7

63.36 76.49

657 657

71.5

44.3 56.8

6.9 7.4

54.63

931

137

71

8.2

70.92 76.84

1,024 1,024

131 110

39 63.8

3.8 3.9

94.39

825

7.9

24.5

3.9

84.49

825

48.4

21.6

4.3

118

"4CACO3 (in % ) as determined by proportion of sediment removed in the first leach ( B a r r e t t et al., 1987). *SMAR of bulk sediment is mass accumulation rate in mg cm -2 k a - ' . MAR-values a n d sediment ages are from Rea and Leinen (1986, table 3). *6MAR of F e ÷ M n is mass accumulation rate in mg cm -2 ka -1, on a bulk sediment basis ( B a r r e t t et al., 1987). *TMAR of X R E E is mass accumulation rate in/~g c m - 2 k a - ' , on a bulk sediment basis.

252

. /// ~

~+

~° ,o ~o

" ..

-'o

" "~"

/

~°~

/", . ./. ~ '° i ~ $ , • o

,

Age {Ma)

r.o~,

/

~,o . / ~,o - . ~

."

r~O~

co) • ~ . . . . . . . . . . . . i

above b a s e m e n t

io

.

.

,

,

.

.

.

.

,

.

.

.

.

MAR Fe + M n

Fig. 5. a. Plot of Z R E E per unit Fe + Mn vs. age above basement. The data suggest an increase in the proportion of REE to metalliferous component with distance from palaeo-axis. b. Plot of MAR of XREE vs. MAR of Fe + Mn. The data suggest that the accumulation rate of the metalliferous component controlsthat of the .FREE.

(Fig. 5a) shows a positive trend (r=0.91, n = 16), although we have only four samples > 7 Ma. This suggests that REE concentrations in the metalliferous component increase with distance from the palaeo-axis. When the MAR of XREE is plotted against the MAR of combined Fe + Mn (Fig. 5b), a positive trend also emerges (r = 0.75, n = 16). However, the MAR of combined Fe + Mn falls off with distance above basement ( r = - 0 . 7 3 , n = 1 7 ) . Similar relations (usingFerather than F e + M n ) were documented by Ruhlin and Owen (1986) at Site 598 on the basis of a large sample set, and attributed to limited scavenging of REE (producing low 2:REE) due to rapid burial of sediment (producing high MAR) near the palaeo-axis. With recession of a site from the hydrothermal source, precipitates would have travelled further and scavenged more REE before setting from the water column. Ruhlin and Owen (1986) also found decreasing MAR-values for the REE with increasing distance above basement. This trend, which is similar to the upwards decrease in MAR-values of Fe, Mn and various transition metals at each site (Lyle et al., 1986; Barrett et al., 1987), is consistent with recession from an axial hydrothermal source, Thus, the MAR of hydrothermally supplied ferromanganese precipitates appears to be the

main control on REE accumulation rates; decreases in MAR-values of these precipitates and of the REE away from the axis are generally accompanied by increase in REE concentrations in the metalliferous component. The MAR-values of individual REE in most bulk samples are nevertheless 2-3 orders of magnitude greater than their authigenic (nonhydrothermal) fluxes from seawater into abyssal sediments (Thomson et al., 1984). This clearly indicates the importance of hydrothermal precipitates in providing an efficient mechanism for the removal of REE from s e a w a t e r . Such removal apparently o c c u r s n o t only in the near-axis plume, but also during transport in the water column away from the axis, and during post-burial scavenging.

3.3. Element partitioning Sixteen of the samples were additionally treated with a second leach designed to remove the amorphous to poorly crystalline phases. The results are given in Table II, and selected sampies are plotted in Fig. 6 together with the patterns prior to the second leach. The main features of the data are: (1) the proportion of each individual REE removed by the second leach is near-constant for some samples, although small differences exist between the LREE and HREE in others; (2) the amount of REE removed from different samples by the leach ranges from ~ 45 to 90% of total REE in the sample; and (3) the proportion of REE removed shows a general relation to age of the sediment. The difference between older and younger metalliferous sediments is summarized in Fig. 7a, using the Yb/Pr ratio as a measure of HREE to LREE enrichment (these relationships are not as obvious in Fig. 3 because of the log scale used there). Fig. 7b shows how the proportion of REE removed by the second leach, i.e. bound up in the poorly crystalline fraction of the sediments, correlates roughly with sediment age

253 TABLE II Percent of each rare-earth element removedby the second leach (relative to the total non-carbonate fraction) Sample No.

Age La (Ma)

Ce

Pr

Nd

Sm

Eu

Gd

Dy

Ho

Er

Yb

Lu

597A-2-2:66-68 cm 597A-4-4: 66-68cm 597A-5-6:66-68 cm 597-6-2:66-71 cm 598-1-2:121-123 cm 598-2-5:119=121 cm 598-4-2: 121=123cm 598-5-4: 121-123cm 599-2-2: 121=123cm 599-3-1: 121=123cm 599-3-2: 121-123cm 599-4-1:121-123 cm 599B-2-3:121-123 cm 599B-2-6:121-123 cm 600C-1-6: 121=123cm 601-2-2: 121-123cm

15.8 26.5 26.5 26.5 2.6 10.5 13.9 15.5 4.6 6.8 6.8 6.8 6.8 8.4 4.2 4.2

80.4 75.9 71.9 59.5 51.9 81.1 77.0 84.7 55.8 76.0 62.7 64.6 68.1 54.7 45.5 60.2

86.6 79.9 78.7 62.7 49.7 86.2 57.3 87.8 42.2 70.4 48.3 47.9 57.0 48.0 55.7 62.7

87.6 81.6 81.1 67.6 53.3 88.5 59.7 89.1 46.0 71.4 50.0 49.7 58.5 52.3 58.3 62.2

86.5 80.7 80.6 66.6 50.6 86.3 58.1 88.6 44.4 70.8 47.3 48.1 53.9 52.5 56.6 61.7

86.1 80.0 81.1 65.8 50.3 85.8 57.1 88.6 44.1 70.3 46.1 46.6 55.1 51.0 53.1 58.1

86.5 81.8 82.6 69.3 52.9 86.6 58.6 89. 3 45.4 70.6 49.7 47.5 55.4 53.9 56.5 61.2

84.7 80.1 80.3 64.8 49. 3 84.6 57.1 88.0 45.5 69.1 46.1 45.8 53.0 54.5 54.0 58.7

83.0 81.0 81.1 68.6 49.4 84.7 57.6 88.4 45.0 68.0 47.4 47.2 54.4 52.6 52.8 51.7

82.2 80.8 82.3 67.1 49.1 84.6 57.6 89.0 54.7 68.7 44.2 46.3 55.8 59.9 49.9 54.3

80.0 80.7 81.4 65.9 49.9 83.8 58.9 87.5 49.0 68.5 47.5 44.9 53.2 49.3 48.9 55.0

81.3 80.8 82.8 70.5 53.1 81.1 61.0 87.6 50.1 71.8 38.3 45.9 54.6 59.8 46.6 50.0

88.6 83.9 78.9 67.0 55.7 89.2 67.3 90.4 45.7 71.2 55.4 54.0 64.5 51.5 51.8 60.0

(for Yb, r = 0 . 6 2 , n = 1 6 ; ot her R E E are similar). Possible explanations are: (1) the R E E are gradually expelled from recrystallizing phases during diagenesis, and incorporated into remaining poorly crystalline phases; {2) the latter phases preferentially scavenge R E E from pore waters long after burial; (3) both processes operate, Ce anomalies a f t e r the second leach are generally less negative for the older group of samples, but about the same or more negative for the younger group (Fig. 7c). T h e former relation may reflect preferential r et ent i on of Ce in better-crystallized phases during diagenesis, in contrast to the other (initial) R E E which partly may transfer into the poorly crystalline fraction (cf. Fig. 6). Five of the samples ~ 10 Ma in age show a positive change in the Y b / P r ratio with leaching (Fig. 7d). Th i s suggests t h a t a greater proportion of the L R E E are in the poorly crystalline fraction of these samples. In the 5 samples older t h a n ~ 14 Ma, there is no significant change in R E E p a t t e r n following the second leach, which may indicate the end of R E E redistribution related to diagenetic effects,

3.4. D i a g e n e s i s

Surflcial metalliferous sediments near the crest of the E P R are distinctly depleted in the L R E E relative to st andard shale ( P i p e r and Graef, 1974; Marchig et al., 1982). Relatively young metalliferous sediments ( ~ 8 - 4 Ma) from the present study display a similar degree of depletion. We infer t h a t these p a t t e r n s were established through early reaction of particulate Fe-Mn-oxyhydroxides with seawater in the h y d r o t h e r m a l plumes. T hese reactions preferentially remove L R E E from seawater, leaving the reacted (or scavenged) seawater of the plume even more L R E E depleted ( K l i nkham mer et al., 1983). Leg-92 sediments older t h a n ~ 10 Ma display furt her e n r i c h m e n t of the M R E E - L R E E , leading to overall flatter patterns relative to the younger samples (Fig. 3a and b). We i nt erpret this e n r i c h m e n t as a result of continued post-depositional diagenetic reactions. Such reactions may involve scavenging from pore waters, which may be expected to increase the proport i on of L R E E to H R E E in the sediment (cf. T u r n e r and Whitfleld, 1979), a n d / o r recrystallization of the original Fe- and

254 5 ~o

LEACH

90

RESULTS

• •

S e d i m e n t s ~ 8 Ma x~

4.

so



••

c

,~,o'

~ ~

._=

"

.

W



n,"

Vo O~

11

-

Leach

:. ~

--I 10 ¢

pairs

2



599B-2-6

(1)



599B-2-6

(2 /

601- 2-2

(1)

601- 2-2

(`2)

~o"



I1=11• mm

50

(o) ,o

20

• 30

40

(b) Jo

Age(Ma)

20

3o

Age{Ma )

o . s - -

(a) La Ce Pr Nd

Oy Ho Er

Sm Eu Gd

Yb Lu

.~02"





LEACH ,,0 10~ t'~]

--Q 0.1¢~ o o

RESULTS

Sediments

~'10

Samples>~lOMa • o

(I)



Ma

~ ~

(I}

• • m'•



Samples ~<8 Ma

o i

~ I o ~,





<11

~ "°-o.2-

o



CC

(c)

Leach

o~

pairs

o

-o3

,

, ,o

.

Age (Ma)

..i ' ° o



597A-2-2

(1)



597A-2"2

(2)

• sgs-s-4 (I) • sgo-s-4 (2) ,o' La Ce

Pr

Nd

S m E u Gd

Dy Ho

E,

Yb Lu

(b)

Fig. 6. Comparison of R E E patterns before (I) and after (2) treatment with a second leach to remove poorly crystalline phases: (a) sediments younger than ~ 8 Ma; and (b) sediments older than ~ 10 Ma. Vertical lines connect leach pairs; sample number suffixesas in Table I.

P-bearing phases. The leach data in Fig. 7d suggest that such reactions, which lead to fractionation of the REE between different fractions of the sediment, may be mostly completed within ~ 10-15 Ma of deposition. As an example of recrystallization of original phases, at least some of the original Fe-oxyhydroxide phases in certain metalliferous sediments undergo a transition to Fe-smectite as a result of reaction with biogenic opal (Heath and Dymond, 1977; Hein et al., 1979; Cole, 1985; Jarvis, 1985 ). Because the large ionic radii of the REE inhibit their incorporation in either the tetrahedral or octahedral sites of the smectite structure, some are released into solution or lost to other phases during the formation of the Fe-smectite. In contrast, biogenic apatite

, 2~

,

(d] 30

--

,

, '0

,

, 20

.

3O

A g e (Ma)

Fig. 7. a. Plot of Yb/Pr (normalized) vs. age for metalliferous (carbonate-free) fraction. This ratio is representative of the H R E E / L R E E characteristics of older and younger groups of sediments. b. Effect of second leach on the proportion of Yb removed vs. age of the sediment (other R E E elements plot analogously). c. Effect of second leach on the mA=~mitude of the Ce anomaly. JCe* is the relative change in the calculated anomaly with leaching. d. Effect of second leach on the Yb/Pr ratio vs. age of the sediment, zJYb/Pr is the relative change in the (normalized) ratio with leaching.

originallypresent in the sediment acquires REE during diagenesis (Dymond and Eklund, 1978; Jonasson et al.,1985). Elderfield et al. (1981) found that the concentrations of REE (excluding Ce) in northern equatorial Pacific sediments are correlated closely with P content, and inferred that the REE are associated with phosphatic fish debris. They also found that the Prich phase is slightly depleted in the HREE (relative to a flat pattern). These authors suggested that where a diagenetic smectite-forming reaction is involved, some of the REE originally associated with Fe-oxyhydroxides undergo intrasediment transferral to the phosphatic phase. Another probable example of this

255 process occurs in the Leg-85 metalliferous sediment, which formed under the equatorial belt of opal production ~ 2000 km north of the Leg92 sediments (Jarvis, 1985). In the case of the Leg-85 sediments, Jarvis found a good correlation between REE and P contents, and an additional poorer one between REE and Fe (the latter largely in Fe-smectite ) ; both phases apparently contributed to the relatively flat REE patterns, With respect to the Leg-92 samples, HREE depletion through the smectite-forming reaction is unlikely to be important, as Fe-smectite is only a minor phase. HREE depletion is more likely to be controlled by the apatitic component of the sediment. Although XRD data suggest that apatite constitutes ~ 5% of the metalliferous component, P analyses for five of our REE samples indicate a positive correlation with XREE. Hence, even a limited component of apatite may play a role in producing a depletion in the HREE. By contrast, a negative correlation between Mn and ZREE, based on 16 samples, indicates that the REE are not significantly associated with manganiferous phases. The two REE-enriched samples from Site 598 are shown in Fig. 3a; a similarly enriched pattern was found, following the second leach, for a surficial sample from Site 597. These sampies, which are within several metres of the surface of the ocean floor, come from distances of 800 and ~ 1200 km west of the EPR. Deposition occurred below the present lysocline for this part of the ocean, which has resulted in the observed slow MAR-values. Low MAR-values apparently lead to appreciable diagenetic effects in near-surface sediments, and presumably are the cause of the anomalously high REE contents (Table II) and atypical mineralogical assemblages in these samples. REE contents are in the order of 1000 ppm, a factor of ~ 2 - 5 greater than for the other sediments analyzed, and significant proportions of phillipsite, which was not detected in the other, deeper sediments, are also present. The surficial samples

have REE patterns that are similar to those of surficial Bauer Basin ferromanganese crusts (Elderfield and Greaves, 1981 ) from ~ 750 km east of the EPR [i.e. HREE-depleted relative to an EPR-crest pattern; see also Piper and Graef (1974) for similar HREE-depleted patterns several hundred km east of the E P R at 39 ° S ]. In the Bauer Basin, surficial diagenetic effects occur owing to the slow rate of accumulation of the sediment (Sayles and Bischoff, 1973; Dymond and Veeh, 1975; Cole, 1985). It is of interest that in both of these sub-lysocline and ridge-removed areas, there is evidence for some hydrothermal input into the sediments, as indicated by Pb isotope data (Dasch, 1981; Barrett et al., 1986 ). It appears that in surficial sediments that are accumulating at a very slow rate (but not nodules or crusts), diagenetic effects may obliterate any initial depletion of LREE relative to HREE resulting from the presence of ridge-derived precipitates. Nevertheless, a distinct negative Ce anomaly is maintained, as is a component of hydothermal Pb. 4. C o n c l u s i o n s

The present study has shown that small-scale differences in the REE patterns of deep-sea metalliferous sediments, which were previously not recognized, can now be resolved and interpreted. With the emergence of REE data on seawater from different parts of the world ocean, it is apparent that the concept of a "typical seawater REE pattern" is only useful in the broadest sense. To make more meaningful interpretations of the REE patterns of deep-sea sediments, comparisons with deep seawater are required. In the case of the Leg-92 metalliferous sediments, which were deposited under the influence of an extensive hydrothermal plume emanating up to 1000 km west of the EPR, LREE enrichments are present in accordance with predictions based on observed L R E E - d e p l e t e d seawater anomalies in that plume at its source (Klinkhammer et al., 1983).

256

Furthermore, analyzed sediments older than ~ 10 Ma are more LREE-enriched and/or HREE-depleted than those younger than ~ 8 Ma. This is compatible with continued LREE scavenging from normal pore waters and/or diagenetic reactions resulting in preferential release of the HREE to solutions, A correlation between the mass accumulation rates (MAR) of I R E E and Fe + Mn suggests that ferromanganese particulate matter supplied by the hydrothermal plume scavenges REE. Although the MAR of Fe + M n shows a general decrease with age above basement, I R E E concentrations in the metalliferous component increase with age above basement, Scavenging of REE appears to be limited near the palaeo-axis due to rapid burial of sediment, but increases with lateral transport from the axis and possibly after burial as well. Following deposition of the hydrothermal component, further relative flattening of the REE pattern takes place, probably the result of diagenetic reactions requiring up to several million years. Phase partitioning data indicate that with age, progressively more REE reside in m o r e poorly crystalline phases. This suggests that as initial ferromanganese precipitates undergo diagenetic recrystallization, REE are t r a n s ferred to the poorly crystalline phases and/or are scavenged from pore waters by these phases. Sub-lysocline surficial sediments can have patt e r n s resembling older sediments due to early

diagenetic effects resulting from slow accumulation rates. However, they are significantly enriched in REE as a result of the slow accumulation rates. Comparisons between sediment REE patterns should be made between samples deposited consistently above the present (or past) lysocline, or consistently below it. I n detail, Leg-92metalliferous sediments do not match the REE patterns of deep oceanic waters from which they were apparently precipitated. With ageing of the sediments, pat-

terns diverge further. REE patterns of metalliferous sediments therefore should be

used with caution when assessing fine-scale variations in the REE composition of seawater. Acknowledgements We express our gratitude to Dr. J.N. Walsh for providing use of the ICP-AES spectrometer at King's College, London. Additional facilities were kindly provided by the Department of Geology, City of London Polytechnic. T.J.B. gratefully acknowledges research support provided by grants from the Natural Sciences and Engineering Research Council of Canada, and from the University of Toronto. Various stages of sample preparation were carried out by J. Lugowski and K.E. Jarvis. Nigel Higgs, Institute of Oceanographic Sciences, kindly supplied XRD data. We are indebted to the Deep Sea Drilling Project for providing the Leg-92 sample material, and thank the co-chief scientists, Drs. M. Leinen and D.K. Rea, for their cooperation and assistance. References Aplin, A., Miehard, A. and Albarbde, F., 1986. 143Nd/144Nd in Pacific ferromanganese encrustations and nodules. Earth Planet. Sci. Lett., 81: 7-14. B~icker, H., Lange, J. and Marchig, V., 1985. Hydrothermal activity and sulphide formation in axial valleys of the East Pacific Rise crest between 18 and 22°S. Earth Planet Sci. Lett., 72: 9-22. Barrett, T.J. and Friedrichsen, H., 1982. Elemental and isotopic compositions of some metalliferous and pelagic sediments from the Galapagos Mounds area, DSDP Leg 70.

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