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

[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) frac- tion of samples ranging in age from ~ 2 to ~ 27 Ma indicate the existence of two distinct groups of patterns corre- sponding 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 en- riched in light REE (LREE) relative to deep bottom waters influenced by the hydrothermal plume extending west from the EPR at 19 o S. 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 preferen- tially 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 com- ponent, 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. Introduct ion are strongly enriched in Fe, Mn and other tran- sition metals in comparison with normal pe-

Sediments near the East Pacific Rise (EPR) lagic sediments (Revelle, 1944; BostrSm and

0009-2541/88/$03.50 © 1988 Elsevier Science Publishers B.V.

244

30° w ~ 9 o ow and the Gal@agos Hydrothermal Mounds Field

t / ~ A J " ~ ~- i/r ~°°N (Corliss et al., 1978; Hdkinian et al., 1978; Hon- e ..... j ~ 7 , ~ norez et al., 1981; Barrett and Friedrichsen, s7~. 1982; Moorby, 1983; Walter and Stoffers, 1985). sn. ~ ~ Geochemical data are available on ancient

- ~ ~ .... ~SOUTH metalliferous analogs which have been cored by ~ - - the Deep Sea Drilling Project (DSDP) as a ~A ,~A~ou,s~s .... ~,,~u E R~- o MERICA

: ~ , 5 9 7 5 9 8 / ~ -=~ '-~//~ ~ / ~ ~ ~ PLATE near-basal facies both to the east and to the west / P L A T E ~ ~ 8~s,N ~ ~ ~ ~oo s of the EPR (vonder Borch and Rex, 1970; Tra-

// cey et al., 1971; vonder Borch et al., 1971; Hays ~ ~ o - 2 ~ ~ / ~ et al., 1972; Dymond et al., 1973, 1976; van An-

~,~ c~ del et al., 1973; Cronan, 1976; Yeats et al., 1976).

PLATE ~ / NA z c A Recently, Tertiary metalliferous sediments were

/ . ~ ~ / / recovered from Leg 85 at Sites 573 and 574 in ~ the central-eastern Pacific (Mayer et al., 1985),

, o ~ and Leg 92 on the west flank of the EPR (Lei- \ ~ ~ . ~ 3oos nen et al., 1986; Lyle, 1986; Marchig and Erzin-

. get, 1986; Rea and Leinen, 1986). Detailed rare- / earth element (REE) data for metalliferous

Fig. 1. Index map of the eastern Pacific showing the loca- sediment sequences are available only for Legs tion of Leg-92 Sites 597-602 (the East Pacific Rise tran- 70, 85 a n d 92 (Migdasov et al., 1983; Jarvis, sect), together with DSDP Sites 573 and 574 (Leg 85) on the Pacific Plate. Numbers and arrows on spreading axes 1985; Ruhlin and Owen, 1986 ). refer to total rate (cm yr. -z) and direction of spreading DSDP Leg 92 drilled a t f o u r sites along a n (Circurn-Pacific Council for Energy and Mineral Re- east-west transect at 19 o S (Fig. 1 ). These sites sources map, 1982). intersected basement ranging in age from Oli-

gocene to Pliocene ( ~ 28-5 Ma). This area was Peterson, 1966; Bostrbm et al., 1969; Bender et chosen partly because the sediments are essen- al., 1971). It is generally accepted that these tially a mixture of hydrothermal and biogenic metals were leached from the basaltic crust by components, with only a minimal contribution seawater hydrothermal systems and/or subse- of clastic material (Rea and Leinen, 1986). quently scavenged from seawater by authigenic Nearly complete recovery of the overlying sed- Fe-Mn-rich precipitates that formed upon hy- iment was achieved through use of piston cor- drothermal discharge and settled onto the ocean ing techniques. The objectives of the present floor {Corliss, 1971; Bonatti et al., 1972; Dy- study were: (1) determination of REE patterns mond et al., 1973; Piper, 1973; Dymond and for the metalliferous component of sediments Veeh, 1975; Cronan, 1980; Dymond, 1981). ranging from ~26 to 2 Ma in age; (2) assess- Studies of the mineralogy, geochemistry and ment of the role of hydrothermal plumes ema- origin of metalliferous sediments in the eastern nating from the EPR on the REE patterns of Pacific (Fig. 1) have concentrated on recent the younger sediments; and (3) comparison of sediments of the EPR (Sayles and Bischoff, older and younger sediments to determine if any 1973; Heath and Dymond, 1977, 1981; Lyle, age-related variations in REE distributions are 1981; Marchig and Gundlach, 1982; Marchig et present. Leg-92 Sites 598-602, as well as the al., 1982, 1986; B~icker et al., 1985; Walter and Leg-85 sites which lie ~ 20 ° of latitude ( ~-2000 Stoffers, 1985), the Bauer Deep (Dymond and kin) to the north (Fig. 1), were generated at Veeh, 1975; Sayles et al., 1975; Dymond, 1981; the EPR and are presently located on the east- Lyle, 1981; McMurtry et al., 1981; Cole, 1985), ern margin of the Pacific Plate. Site 597, how-

245

ever, was formed at the fossil Mendoza Rise, SITE 59 7 SITE 598 the forerunner of the present E P R (Mammer- c o , 597J A I L ITHOLOGY EPOCH L ~THOLOGY EPOCH

i ckxe ta l . , 1980). o ' ' ~2_---fl2--JL-~J , , ~c-=--~ ~ .... The REE distribution pat terns of recent me- g 2 : 2 . . . . . . -22_-2- Lo., 2 :~__-__-~

talliferous sediments are broadly similar to ~ 2o 3 3 . . . . . -- ....... 2 3

those of deep-ocean seawater (Bender et al., ~ , _~__--~- ~ :\-2\. ...... 1971; Dymond et al., 1973; Piper and Graef, r, S S . . . . . ~ . . . . " . . . . .

1974; Marchig et al., 1982; Fleet, 1983) The ; ~o . . . . . . . . . . . . . ~

sediment pat terns show comparatively little ~: ~ ,,b . . . . variation despite strong fractionation of the .o SITE 6 0 0 REE with depth in the water column (Elder- . ~ ~,,.o~oo~ ~oc. field and Greaves, 1982; de Baar et al., 1985a, S I T E 5 9 9 1-- . . . . . . , ...... b) Shale-normalized pat terns of recent metal- CORE L I I " O L O G Y EPOCH E~'I [2) . . . . . • 5~ 1 8 2 P h ° c

liferous sediments at the E P R are character- ~ 0 1 . . . . . Pho¢

ized by a marked depletion in Ce relative to its ~ 2 -= . . . . S I T E 6 01

ighb _ _ . . . . . . . . . . . ne o u r s L a a n d P r (Bendereta l . , 1971),and ~ ~° 3 ' . . . . . . . CORE -~_- - - - ~ 5011 B LITHOLOGY EPOCH

I- ......

- - - . . . . . ~ [KC 2C J. P " " ' a distinct enrichment in the heavy REE r, , '2~_:2:2:~ ~, - - 2.222. , ( H R E E ) relative to the light REE (LREE) ~ , . . . . - - 2 . . . . L . . . .

(Piper and Graef, 1974; Marchig et al., 1982) ~ ,o 7~r . . . . . . .~=_--. • 2 - - 1 P l i o c 3 ,

We refer to this type of pat tern in the present ' ~:'-(:(:':: --- i~:';.!:~:": ' '-'-~

paper as "EPR-crest". By contrast, pelagic clays (montmoril lonit ic) from the northern Pacific ~ ,~..o~o,,L ...... ooze ~ ] ................ ~o ~,..,o,,, have flat pat terns with no Ce anomaly, and au- ~ . . . . . . . . . . . [ ~ ~..,. ............. ~.~ thigenic Mn nodules have positive Ce anoma- ~ ......

lies (Piper, 1974a, b; Elderfield et al., 1981). Fig. 2. StratigraphicsurnmaryofsedimentsdriUedatDSDP Recently, REE data have been obtained for sea- Leg-92 Sites 597-601.Although the stratigraphic columns water from hydrothermally active and inactive (based on shipboard results) give the main sediment li- parts of the world ocean, and at varying depths thology as "clayey nannofossil ooze", the clay mineral com-

ponent in the non-carbonate fraction of the samples which in the water column (Elderfield and Greaves, we analyzed is generally insignificant relative to the ox- 1982; Kl inkhammer et al., 1983; de Baar et al., ide-oxyhydroxide component. 1985a, b) . In the present paper, we report sub- tle but distinct variations in the REE pat terns conclusions generally supporting those of of metalliferous sediments from D S D P Leg 92, Ruhlin and Owen (1986). We also make fur- and use the recent data for seawater to assess ther deductions on the basis of data from the the influence of hydrothermal activity on the other sites, phase partit ioning results, and re- pat terns of sediments, cently published information on deep seawater

Geochemical data on L e g - 9 2 sediments, in- REE patterns. eluding an initial set of REE patterns, are given

2 . S a m p l e s a n d m e t h o d s in Barret t et al. (1986). In the present paper, we report a larger set of REE data, together with 2.1. S a m p l e s

estimates of REE mass accumulation rates and information on elemental partitioning• A de- Sediment lithologies and thicknesses for the tailed study of the REE in Leg-92 sediments various sites are shown in Fig. 2. The majority from Site 598 has also been published by Ruhlin of the sediment sequence is upper Oligo- and Owen (1986) . In our paper, we examine cene-Miocene at Site 597 , Miocene at Sites 5 9 8

sediments from all Leg-92 sites, and arrive at and 599, and Pliocene-Pleistocene at Sites 6 0 0

246

and 601. All of these sediments are essentially (made from BDH & Eldrich ® standard solu- calcareous nannofossil oozes with varying ad- tions for atomic absorption), calculated to mixtures (commonly 20-40% ) of a darker fine- match closely the range of values expected for grained metalliferous component. Smear-slide each element in the unknowns. Careful match- examination by onboard scientists revealed this ing of matrices is essential to minimize inter- latter component to consist of Fe-oxides and ference effects. Raw intensity data were -hydroxides, RSO (red-brown to yellow-brown processed on a digital PDP ® 11-34 computer semiopaque oxides), and poorly crystalline utilizing a program developed by I.J. that in- clays. Although the stratigraphic columns in corporates drift and blank correction. In addi- Fig. 2 give the main sediment lithology as clayey tion, the REE data were corrected for nannofossil ooze, X-ray diffractometry (XRD) interelement interferences by Ba, Ca, Sr, Ti and and geochemical data (Barrett et al., 1986) in- Zr to compensate for the small proportion of dicate that the clay component is minor to in- these elements that remain after the cation-ex- significant relative to the oxide-oxyhydroxide change procedure. Analytical precision as de- component. Variations in the proportion of this termined by replicate analyses is better than latter component determine the colour of the -+ 3% for all of the REE present in amounts sediment, which ranges from light-coloured >0.2 ppm. By reference to international stan- carbonate-rich oozes to yellow-brown and very dards, absolute accuracy is judged to be better dark-brown calcareous mudstones. The main than _+5% of the quoted values (Walsh et al., crystalline phase of the metalliferous compo- 1981; Jarvis and Jarvis, 1985). In this paper, nent is goethite, with minor amounts of fine- REE concentrations are normalized relative to grained apatite and, in a few samples, phillip- U.S.G.S. standard SCo-I (Cody Shale), which site. Traces of quartz and clinoptilolite (?) are is considered to be a more representative stan- present in a few samples. Geochemical data dard (JarvisandJarvis, 1985)thanthe average suggest that the amorphous to poorly crystal- North American Shale values generally used line phases, which constitute roughly half of the ( Haskin and Haskin, 1966 ). metalliferous component, contain Fe-Mn-ox- yhydroxides with minor apatite and Fe-smec-

3. Resul t s and d i scuss ion tite. Sedimentological features at each site are summarized in the individual site chapters for

Leg 92 (Leinen et al., 1986). 3.1. REE patterns

2.2. Methods REE data for the carbonate-free fraction of

The REE and Y were extracted from splits of 27 selected samples from Sites 597-601 are carbonate-free powders following HF-HC104 given in Table I [ marked with ( 1 ) ]. This table open digestion, using a cation separation pro- also gives the REE concentrations following a cedure (Jarvis and Jarvis, 1985 ) modified from second leach designed to remove amorphous to Walsh et al. (1981) and Thompson and Walsh poorly crystallized oxide-oxyhydroxide phases (1983).La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, [marked with (2)]. Details of the leach pro- Yb, Lu and Y were determined simultaneously cedures are given in Barrett et al. (1986). The using a Philips ® PV 8210 inductively coupled following comments refer to the total carbon- plasma-atomic emission spectrometer ( ICP- ate-free fraction. AES) at the Department of Geology, King's Leg-92 sediments have XREE contents rang- College, London. Calibration curves were con- ing mostly from 131 to 867 ppm, with a cluster- structed by using artificial composite standards ing between 167 and 529 ppm; four samples have

247

notably higher X R E E values of 920-1984 ppm ,o2 o s o P Leg ~2 (excluding Tb and Tm which were not deter- ~_, S,TE mined ). The patterns have strongly negative Ce .............. ~ - " ~ A 5 9 8

anomalies, where Ce* is defined as follows: ~ ." ~o~

Ce* = log [ Ce~h/{0.5 ( Lash + Prsh) } ]

where Cesh, Lash and Prsh represent the shale- normalized values of each element. Ce* ranges = from - 0.95 to - 0.60, with a clustering of val- °~

" J 1 0 o

ues between - 0.90 and - 0.74. As noted by Barret t et al. (1986) on the basis

of a smaller database, most samples from Sites 599 and 601 have similar pat terns which dis- cal

1 0 " i , . . . . . , , , , i , , i

play a modest but consistent enrichment in the ~ ~ . . . . . s~ ~ ~0 ~ . . . . . . . . . H R E E relative to the L R E E (excluding Ce,

2 10 D S D P Leg 92 discussed separately), whereas most of those from Sites 597 and 598 have flatter H R E E / s,TE

• 5 9 7 LREE trends. However, since age overlap oc- o ~ curs between sites, it is more appropriate to compare samples of on the basis of age group- z '° "-~ ings. Various REE 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 REE pat terns are near-uniform, despite ~b~ variations in X R E E concentrations of up to a ' 0 =1 I . . . . . . . . . . . . . .

factor of 6. Two sub-lysocline samples from the ~ ~ . . . . . ~ ~° ~ . . . . . . . . . . .

younger group are unusually enriched in REE Fig. 3. a. REE patterns for all metalliferous samples younger and have slightly "aberrant" patterns; they are than ~8Ma. Note close similarity of allpatterns except for discussed in Section 3.4. the two REE-enriched near-surface sediments deposited

below the lysocline (core 598-1; see Section 3.4). With respect to the source of REE in the Leg- b. REE patterns for all metalliferous samples older than

92 metalliferous sediments, a direct hydro- ~ 10 Ma. This group of patterns is also internally consis- thermal source is considered unlikely. REE dis- tent, but differs from those in (a) in having a greater en- tributions in 350 ° C hydrothermal solutions richment in the MREE to LREE relative to the HREE.

from the E P R are characterized by large posi- tive Eu anomalies and depletion in the H R E E normal seawater was limited during chemical relative to the LREE, the result of high-tem- precipitation. At the EPR, by contrast, solu- perature subsurface reaction between normal tions discharge freely into a well-circulated seawater and basaltic crust (Michard et al., water mass, and any hydrothermal component 1983). Metalliferous sediments with pat terns to the REE pat tern of chemical precipitates ap- reflecting a hydrothermal component are in fact pears to be quickly diluted to the point of being known from the Red Sea (Courtois and Treuil, unrecognizable. The seawater-like pat tern of 1977), but in this case it is likely that mixing E P R sediments apparently results from the between hydrothermal solutions and overlying rapid precipitation and adsorption of seawater

248

REE onto hydrothermally derived Fe-Mn-ox- 1o yhydroxide colloidal particles, either in the "'°"" ° ' s,~ ..... o ....... S E D i m E N T S ~ S M ~

water column or after the particles have settled ~,o ~ ~ * : onto the sea floor. A primary seawater source ~ ~ _ ~ ~ n - ' - - ~ W - f f ~ for Nd and Sr in metalliferous sediments from ~ ,oO

~ , I ,t,] ],1~

the eastern Pacific is also indicated by their v / 143Nd/144Nd and STSr/S6Sr ratios, which are very ~ ,o. _~ . . . . . . . . (6xlo °)

close to those of seawater (O'Nions et al., 1978; E,R . . . . . . . . 9°s Piepgras et al., 1979; Elderfield et al., 1981; ,:

(a) Goldstein and O'Nions, 1981; Aplin et al., 1986 ) . . . . . . . . . . . . . . . . . . . . . . . . . By contrast, Pb isotopic values of metalliferous sediments from the EPR axis show a strong in- '°' put of basaltic Pb via the hydrothermal plume R . . 9 , of s e d i . . . . p a r t . . . . . SEDIMENTS ~'IOMa

(Bender et al., 1971; O'Nions et al., 1978), ~.o. *

which apparently transports Pb to distances of -i

hundreds of km east (Dasch, 1981) and west ~ '°° (Barret t et al., 1986) of the axis.

Hydrothermal activity thus provides the ~,o' __s . . . . ,,, ~ (~.,o ~) source of the particulate mat ter that scavenges E P ~ . . . . . . . ' 9° s REE in large parts of the east Pacific (Klink- ,~ . . . . . . . . . . . . . . .

hammer et al., 1983 ). It is therefore of interest . . . . . . . . . . . . . . . . . . . . . . . . ~ to examine the REE pat terns of metalliferous Fig. 4. a. Comparison between the REE patterns of sedi-

ments younger than ~ 8 Ma with deep (2500 m) seawater sediment in light of recently published data for from the hydrothermal plume above the crest of the East deep ocean waters. In Fig. 4a, seawater sampled Pacific Rise (Klinkhammer et al., 1983). The plume pat- directly over the EPR axis and within the hy- tern has been scaled up so that the HREE end (ErandYb) drothermal plume at 19 ° S (the latitude of the matches that of the sediments. This demonstrates that the

sediments are enriched in MREE to LREE relative to deep Leg-92 transect) is compared with the younger seawater influenced by the plume. group of sediments. On the basis of mantle-de- b. As in (a) but for sediments older than ~ 10 Ma. rived He anomalies (Lupton and Craig, 1981), this plume is known to extend west of the EPR

As these authors noted, this probably reflects as far as the present-day locations of Sites 598

the longer residence time of HREE in the deep and 597. The pat tern for EPR deep seawater

ocean (Elderfield and Greaves, 1982), which (Kl inkhammer et al., 1983) has been scaled up itself may result from the greater stability of to match the HREE end of one of the metalli-

(dissolved) HREE complexes relative to LREE ferous sediment pat terns (we chose the lower

complexes in seawater (Turner and Whitfield, pat tern for convenience). When this is done, it

1979). Continuous scavenging would gradually becomes apparent that the sediments are en- modify the initial EPR-crest pat tern by in- riched in the middle REE (MREE) to LREE creasing the ratio of LREE to HREE in the par- relative to deep plume seawater at the same lat- ticulates, while having the opposite effect on itude. This observation directly supports the seawater. Scavenging from pore waters follow- inference of Kl inkhammer et al. (1983, p. 188) ing burial conceivably could also continue to in-

that crease the L R E E / H R E E ratio. [ Relative to the

"the dominant effect of hydrothermal circulation on deep waters in the plume at 19°S, the Ce* REE geochemistry in seawater is preferential removal anomaly in the younger group of Leg-92 sedi- of the lighter elements", ments is less negative by ~0.2-0.3 log units.

249

'o~ SEDIMENTS ~:8Ma tom waters not directly influenced by a hy- ! Ra,,e o, s.~ . . . . . . , ..... drothermal plume, from the east Pacific at

\ j ~ - * ~ 18°N,108°W (De Baar et al., 1985a). Fig. 4c ~,o1 ~ shows that the Leg-92 sediments are still some-

= 1o. ~ what enriched in the lighter REE (particularly 5 ~ s . . . . . . . . ~sx,oO~ Ce) relative to this type of bot tom water. Fi-

- East P,o,,,o nally, in Fig. 4d, Leg-92 sediments are com-

'~' pared with deep water from the northeast ,~, (c~ Atlantic, in an area where the distribution of

. . . . . . . . . . . . . . . . . . . . . . . . the REE is considered to be controlled exclu- sively by external terrigenous sources, with no

,o' LEG 92 SEO,MENTS hydrothermal influence (Elderfield and Typical p . . . . . . . 10 M , Greaves, 1982). Interestingly, the pat terns for

~,o' Tyoio,, p . . . . . . . ~ M" the younger group of Leg-92 sediments match o~

"= the Atlantic deep waters closely for all elements . . . . except Eu and Ce, which have variable oxida- S e a w a t e r x 19x10 e)

,o,t~ .... ,,, ..... tion states (the lowest pat terns in each group '°'! are compared for convenience). Thus, metalli-

ferous sediment formed under the influence of .~ (d )

$ O " L . . . . , . . . . . . . . . . . . . . . . . . hydrothermal plumes at the EPR, has a REE

Fig. 4. c. As in (a) but comparison is with deep (3000 m) pat tern similar to bot tom waters in a region of seawater away from the immediate influence of a ridge no hydrothermal activity. It therefore seems plume (de Baar et al., 1985a). unlikely that REE patterns alone can be used d. Comparison between Leg-92 sediments and deep (2500

to demonstrate a hydrothermal influence on m) seawater not influenced by a hydrothermal plume ( Eld- erfield and Greaves, 1982). Plot also shows that sediments authigenic sedimentation in open-ocean older than ~ 10 Ma have comparable HREE patterns but environments. are enriched in MREE to LREE relative to sediments younger than ~ 8 Ma. The seawater patterns in (c) and (d) have been scaled up 3.2. Mass accumulat ion rates (MAR) to match the HREE end ( Er and Yb) of Leg-92 sediments.

Amongst the LREE, this represents the great- Table I lists MAR-values for Z R E E based on est enrichment relative to the plume water (Fig. the accumulation rates of the bulk sediments 4a), presumably because of preferential incor- poration of Ce 4+ into particulates. ] (Leinen et al., 1986), the total concentration

Fig. 4b shows that for the older group of sed- of REE in the carbonate-free fraction and the iments ( ~ 2 7 - 1 0 Ma) , the discrepancy be- measured carbonate content of the analyzed tween the sediments and E P R deep seawater is samples. The MAR of any individual REE can even greater. This is the result of further in- be calculated from the Z R E E value simply by crease in the sediment in the proportion of multiplying it by the ratio of the concentration MREE and LREE relative to the H R E E (again, of that element to Z R E E ( Table I ). Also given Ce shows slightly anomalous enr ichment) , in Table I are MAR-values for combined Possible causes, involving diagenetic modifi- Fe + Mn (Barre t t et al., 1987), which is repre- cation of the sediments, are discussed in Sec- sentative of the metalliferous component. tion 3.4. With respect to REE concentrations, a plot

For comparison, we have also considered bot- of Z ' R E E / ( F e + Mn) vs. age above basement

250

TABLE I

Concentra t ion of the rare-ear th elements in metalliferous sediments from DSDP Leg 92, on a carbonate-free and salt-free basis

DSDP sample No. La Ce Pr Nd Sm Eu Gd Dy Ho

597A-1-1:66-68 cm (2) 250 78.7 54.5 247 53.0 14.2 59.0 65.9 14.6 597A-2-2:66-68 cm (1) 257 114.0 46.3 223 44.5 12.1 51.3 49.1 10.2 597A-2-2 (2) 29.2 22.4 6.2 27.7 6.0 1.7 6.9 7.5 1.7 597A-4-4:66-68 cm (1) 219 69.7 34.2 166 33.3 9.4 39.2 38.2 9.1 597A-4-4 (2) 35.4 16.8 6.9 30.6 6.4 1.9 7.1 7.6 1.7 597A-5-5:66-68 cm (1) 64.0 19.6 10.2 49.6 10.0 3.0 12.0 12.6 2.9 597A-5-6: 6 6 - 6 8 c m ( I ) 137 41.5 19.6 97.4 19.7 6.1 24.8 23.8 5.3 597A-5-6 (2) 29.0 11.7 4.2 18.4 3.8 1.1 4.3 4.7 1.0 597-6-2: 6 6 - 7 1 c m (1) 77.0 15.7 11.2 54.8 10.8 3.3 13.3 13.8 3.4 597-6-2 (2) 25.4 6.4 4.2 17.7 3.6 1.1 4 . 1 4.9 1.1 598-1-1:121-123 cm (1) 305 152 60.4 295 61.8 16.4 72.3 75.8 17.9 598-1-2:121-123 cm (1) 596 187 99.6 501 101 27.4 127 130 31.0 598-1-2 (2) 264 90.0 50.1 234 49.9 13.6 59.9 65.9 15.7 598-2-5:119-121 cm (1) 258 92.7 40.2 222 41.0 11.6 52.1 52.7 11.9 598-2-5 (2) 28.0 17.5 5.6 25.5 5.6 1.7 7.0 8.1 1.8 598-4-2:121-123 cm (1) 189 52.2 25.0 122 24.6 7.1 31.6 31.6 7.3 598-4-2 (2) 61.8 12.0 10.7 49.2 10.3 3.1 13.1 13.6 3.1 598-5-1:121-123 cm (1) 107 24.8 15.3 72.3 14.3 4.1 17.0 17.8 4.2 598-5-4:121-123 cm (1) 129 34.3 18.7 93.6 19.2 5.7 23.9 24.6 6.0 598-5-4 (2) 12.4 5.2 2.3 10.2 2.2 0.7 2.6 2.9 0.7 598-5-5:121-123 cm (1) 88.3 27.7 13.4 64.0 13.2 3.8 15.8 16.7 4.0 599-2-2:121-123 cm (1) 61.5 14.8 8.6 42.5 8.8 2.8 10.9 13.5 3.3 599-2-2 (2) 33.4 6.5 4.9 22.9 4.9 1.6 6.0 7.4 1.8 599-2-6:121-123 cm (1) 52.8 12.9 7.7 38.6 8.1 2.6 10.2 12.4 3.1 599-3-1:121-123 cm (1) 94.5 21.0 11.6 59.0 12.7 4.4 17.4 20.2 5.2 599-3-1 (2) 27.3 5.0 3.4 16.9 3.7 1.3 5.1 6.2 1.7 599-3-2:121-123 cm (1) 139 27.4 17.5 88.5 18.1 5.7 24.4 26.2 7.0 599-3-2: (2) 62.0 10.2 9.0 44.2 9.5 3.1 12.3 14.1 3.7 599-3-4:121-123 cm (1) 63.1 17.0 8.9 44.4 9.3 2.9 11.8 13.7 3.5 599-4-I: 121-123 cm (1) 97.8 22.7 12.6 64.5 13.6 4.5 18.2 20.0 5.4 599-4-1 (2) 45.0 8.0 6.6 32.5 7.0 2.4 9.5 10.8 2.9 599-4-2:121-123 cm (2) 45.2 9.0 7.0 32.9 7.0 2.3 8.5 10.6 2.5 599-4-5:121-123 cm (1) 63.0 13.2 8.3 41.0 8.5 2.7 10.9 12.8 3.3 599B-2-3:121-123 cm (1) 126 27.8 16.0 81.2 16.5 5.3 22.2 24.9 6.7 599B-2-3 (2) 44.7 8.9 6.9 33.7 7.6 2.4 9.9 11.7 3.1 599B-2-6:121-123 cm (1) 50.8 13.8 7.2 35.8 8.1 2.6 10.2 13.6 3.2 599B-2-6 (2) 24.6 6.3 3.7 17.1 3.9 1.3 4.7 6.2 1.5 600C-1-5:121-123 cm ( t ) 42.2 9.7 5.8 29.1 6.1 2.1 7.9 9.7 2.5 600C-1-6:121-123 cm (1) 79.1 16.5 13.3 65.8 13.9 4.2 18.2 21.2 5.4 600C-1-6 (2) 38.2 9.0 5.9 27.4 6.0 2.0 7.9 9.7 2.5 601-2-2:121-123 cm (1) 180 38.1 24.2 123 25.0 7.5 33.5 36.0 8.2 601-2-2 (2) 72.0 15.2 9.0 46.5 9.6 3.2 13.0 14.9 4.0 601-2-5:121-123 cm (1) 54.9 13.6 7.5 37.4 7.9 2.5 10.2 12.0 3.0

SCo-1 29.6 60.0 6.2 24.7 4.7 1.2 4.0 3.3 0.7

( 1 ) ppm after first leach to remove the carbonate fraction; (2) ppm after second acid-reducible leach to remove the amor- phous to poorly crystallized oxyhydroxide phases. (All samples contain a small component of f ine-grained insoluble clastic material, generally less t han several percent of the carbonate-free fraction.) *12:REE is for measured elements and does not include T m and Tb, which can be es t imated by interpolat ion of flanking REE values. *2Ce* is defined in the text. *3This ratio is based on ppm values normalized to SCo-l .

251

Er Yb Lu XREE* ' Y Ce *(.2~ Y b / P r .3 CaCO3 .4 MAR Age (%) (Ma)

bulk *~ ( F e ÷ M n ) .6 X R E E .7

38.5 38.9 5.7 920 53.4 - 0 . 8 1 7 2.35 28.9 26.2 4.1 867 322 - 0 . 6 2 8 1.88 87.59 650 3.6 70 17.3

5.1 5.2 0.8 121 53.4 - 0 . 4 2 6 2.77 25.2 24.1 3.6 671 266 - 0 . 7 4 4 2.33 87.40 690 38.3 58.3 24.9

4.8 4.6 0.7 125 65.6 - 0 . 6 1 3 2.18 8.0 6.9 1.1 200 97.2 - 0 . 7 6 5 2.31 78.43 690 29.7 26.5

14.9 14.4 2.1 407 189 - 0 . 7 5 0 2.41 79.39 690 50.6 58.9 26.7 2.6 2.7 0.4 84 41.6 - 0.628 2.20 8.9 8.3 1.3 222 114 - 0 . 9 2 5 2.44 67.06 690 35.1 27.2 2.9 2.8 0.4 75 50.8 - 0.859 2.30

47.0 44.8 7.0 1,155 615 - 0 . 5 9 7 2.45 81.96 70 14.6 2.1 85.3 86.0 13.1 1,984 1,150 - 0 . 7 6 3 2.86 63.24 70 7.3 51.1 3.6 43.4 43.1 6.1 936 663 - 0 . 7 5 2 2.84 32.7 32.8 4.9 853 403 - 0 . 6 9 1 2.68 83.38 114 14.8 16.2 10.0

5.0 5.3 0.9 112 66.1 - 0 . 4 9 9 3.08 1.36 20.3 3.2 534 254 - 0 . 7 7 7 2.79 85.78 546 26.9 41.4 14.0 8.5 8.3 1.2 195 175 - 0 . 9 7 8 2.55

11.5 11.1 1.7 301 136 - 0 . 8 6 6 2.36 79.17 505 50.7 31.7 15.0 16.6 15.7 2.6 390 185 - 0 . 8 0 9 2.80 58.49 505 108 62 15.7

1.8 2.0 0.3 43 26.2 - 0 . 6 5 4 2.86 10.5 10.3 1.6 269 126 - 0 . 7 4 5 2.50 79.87 505 30.1 15.9 11.1 10.3 1.6 190 119 - 0 . 8 4 5 3.93 72.70 657 55.8 34 4.6

5.0 5.3 0.8 100 110 - 0 . 9 4 6 3.43 9.0 8.6 1.5 167 107 - 0 . 8 4 7 3.83 63.68 657 39.8 5.3

14.4 14.9 2.5 278 183 - 0 . 8 5 9 4.16 64.20 657 118 65.3 5.6 4.5 4.7 0.7 81 98.3 - 0 . 9 4 3 4.53

20.1 19.9 3.1 397 240 - 0 . 9 1 5 3.79 72.59 657 76.1 71.4 5.7 11.2 10.4 1.9 192 215 - 1.018 3.85

9.7 9.9 1.6 196 124 - 0 . 7 9 8 3.71 74.11 657 37.2 6.1 15.6 15.2 2.4 292 186 - 0 . 8 4 8 4.05 66.58 657 97.8 64.1 6.7

8.4 8.4 1.3 143 167 - 0 . 9 8 3 4.17 7.2 7.4 1.1 141 87.9 - 0 . 9 4 5 3.64 9.2 9.6 1.5 184 118 - 0 . 8 9 5 3.92 63.36 657 44.3 6.9

19.4 18.7 2.9 368 229 - 0 . 8 6 7 3.81 76.49 657 71.5 56.8 7.4 8.6 8.8 1.3 147 170 - 0 . 9 4 7 4.28

11.1 9.7 1.8 168 130 - 0 . 7 9 6 4.33 54.63 931 137 71 8.2 4.5 4.9 0.7 79 94.3 - 0 . 8 3 8 4.41 7.2 7.4 1.1 131 95.5 - 0 . 8 6 5 4.19 70.92 1,024 131 39 3.8

14.8 14.4 2.1 269 167 - 0 . 9 4 2 3.67 76.84 1,024 110 63.8 3.9 7.4 7.4 1.1 125 133 - 0 . 8 7 3 4.13

24.3 25.2 3.8 529 315 - 0 . 8 9 5 3.44 94.39 825 7.9 24.5 3.9 11.1 11.3 1.9 212 233 - 0 . 8 8 6 4.08

9.0 9.3 1.5 169 118 - 0 . 8 2 9 4.08 84.49 825 48.4 21.6 4.3

1.9 1.9 0.3 138

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

252

~° main control on REE accumulation rates; de- . / / / ,o " " creases in MAR-values of these precipitates and

~+ ~ ~o . . "~" of the REE away from the axis are generally - ' o / accompanied by increase in REE concentra- ~°~ / " , ~,o . / tions in the metalliferous component. /

. ./. ~ r.o~, ~,o - . ~ ." r~O~ The MAR-values of individual REE in most '° i ~ $ bulk samples are nevertheless 2-3 orders of

, • co) • ~ magnitude greater than their authigenic (non- o , i i o . . , , . . . . , . . . .

. . . . . . . . . . . . hydrothermal) fluxes from seawater into abys- A g e { M a ) above basement MAR Fe + Mn

sal sediments (Thomson et al., 1984). This Fig. 5. a. Plot of ZREE per unit Fe + Mn vs. age above base- clearly indicates the importance of hydrother- ment. The data suggest an increase in the proportion of mal precipitates in providing an efficient mech- REE to metalliferous component with distance from pa- anism for the removal of REE from s e a w a t e r . laeo-axis. b. Plot of MAR of XREE vs. MAR of Fe + Mn. The data Such removal apparently o c c u r s n o t only in the suggest that the accumulation rate of the metalliferous near-axis plume, but also during transport in component controls that of the .FREE. the water column away from the axis, and dur-

ing post-burial scavenging. (Fig. 5a) shows a positive trend (r=0.91, n = 16), although we have only four samples > 7

3.3. Element partitioning Ma. This suggests that REE concentrations in the metalliferous component increase with dis- tance from the palaeo-axis. When the MAR of Sixteen of the samples were additionally XREE is plotted against the MAR of combined treated with a second leach designed to remove Fe + Mn (Fig. 5b), a positive trend also emerges the amorphous to poorly crystalline phases. The (r = 0.75, n = 16). However, the MAR of com- results are given in Table II, and selected sam- bined Fe + Mn falls off with distance above pies are plotted in Fig. 6 together with the pat- basement ( r = - 0 . 7 3 , n=17) . Similar rela- terns prior to the second leach. The main tions (usingFerather than F e + M n ) were doc- features of the data are: (1) the proportion of umented by Ruhlin and Owen (1986) at Site each individual REE removed by the second 598 on the basis of a large sample set, and at- leach is near-constant for some samples, al- tributed to limited scavenging of REE (produc- though small differences exist between the ing low 2:REE) due to rapid burial of sediment LREE and HREE in others; (2) the amount of (producing high MAR) near the palaeo-axis. REE removed from different samples by the With recession of a site from the hydrothermal leach ranges from ~ 45 to 90% of total REE in source, precipitates would have travelled fur- the sample; and (3) the proportion of REE re- ther and scavenged more REE before setting moved shows a general relation to age of the from the water column. Ruhlin and Owen sediment. (1986) also found decreasing MAR-values for The difference between older and younger the REE with increasing distance above base- metalliferous sediments is summarized in Fig. ment. This trend, which is similar to the up- 7a, using the Yb/Pr ratio as a measure of HREE wards decrease in MAR-values of Fe, Mn and to LREE enrichment (these relationships are various transition metals at each site (Lyle et not as obvious in Fig. 3 because of the log scale al., 1986; Barrett et al., 1987), is consistent with used there). Fig. 7b shows how the proportion recession from an axial hydrothermal source, of REE removed by the second leach, i.e. bound Thus, the MAR of hydrothermally supplied fer- up in the poorly crystalline fraction of the sed- romanganese precipitates appears to be the iments, correlates roughly with sediment age

253

TABLE II

Percent of each rare-earth element removed by the second leach (relative to the total non-carbonate fraction)

Sample No. Age La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu (Ma)

597A-2-2:66-68 cm 15.8 88.6 80.4 86.6 87.6 86.5 86.1 86.5 84 .7 83 .0 82.2 80.0 81.3 597A-4-4: 66-68cm 26.5 83.9 75.9 79.9 81.6 80.7 80.0 81 .8 80.1 81 .0 80.8 80.7 80.8 597A-5-6:66-68 cm 26.5 78.9 71.9 78.7 81.1 80.6 81.1 82 .6 80.3 81.1 82.3 81.4 82.8 597-6-2:66-71 cm 26.5 67.0 59.5 62.7 67.6 66.6 65.8 69.3 64 .8 68 .6 67.1 65.9 70.5 598-1-2:121-123 cm 2.6 55.7 51.9 49.7 53.3 50.6 50.3 52 .9 49.3 49 .4 49.1 49.9 53.1 598-2-5:119=121 cm 10.5 89.2 81.1 86.2 88.5 86.3 85.8 86 .6 84.6 84 .7 84.6 83.8 81.1 598-4-2: 121=123cm 13.9 67.3 77.0 57.3 59.7 58.1 57.1 58.6 57.1 57 .6 57.6 58.9 61.0 598-5-4: 121-123cm 15.5 90.4 84.7 87.8 89.1 88.6 88.6 89.3 88 .0 88 .4 89.0 87.5 87.6 599-2-2: 121=123cm 4.6 45.7 55.8 42.2 46.0 44.4 44.1 45 .4 45.5 45 .0 54.7 49.0 50.1 599-3-1: 121=123cm 6.8 71.2 76.0 70.4 71.4 70.8 70.3 70.6 69.1 68.0 68.7 68.5 71.8 599-3-2: 121-123cm 6.8 55.4 62.7 48.3 50.0 47.3 46.1 49 .7 46.1 47 .4 44.2 47.5 38.3 599-4-1:121-123 cm 6.8 54.0 64.6 47.9 49.7 48.1 46.6 47.5 45 .8 47.2 46.3 44.9 45.9 599B-2-3:121-123 cm 6.8 64.5 68.1 57.0 58.5 53.9 55.1 55 .4 53.0 54 .4 55.8 53.2 54.6 599B-2-6:121-123 cm 8.4 51.5 54.7 48.0 52.3 52.5 51.0 53.9 54.5 52 .6 59.9 49.3 59.8 600C-1-6: 121=123cm 4.2 51.8 45.5 55.7 58.3 56.6 53.1 56.5 54.0 52 .8 49.9 48.9 46.6 601-2-2: 121-123cm 4.2 60.0 60.2 62.7 62.2 61.7 58.1 61.2 58 .7 51.7 54.3 55.0 50.0

(for Yb, r=0.62, n=16 ; other REE are simi- 3.4. Diagenes i s

lar) . Possible explanations are: (1) the REE are gradually expelled from recrystallizing Surflcial metalliferous sediments near the

phases during diagenesis, and incorporated into crest of the E P R are distinctly depleted in the remaining poorly crystalline phases; {2) the LREE relative to s tandard shale (Piper and lat ter phases preferential ly scavenge REE from Graef, 1974; Marchig et al., 1982). Relatively pore waters long after burial; (3) both pro- young metalliferous sediments ( ~ 8 - 4 Ma) cesses operate, from the present study display a similar degree

Ce anomalies af ter the second leach are gen- of depletion. We infer tha t these pat terns were erally less negative for the older group of sam- established through early reaction of particu- ples, but about the same or more negative for late Fe-Mn-oxyhydroxides with seawater in the the younger group (Fig. 7c). The former rela- hydrothermal plumes. These reactions prefer- tion may reflect preferential re tent ion of Ce in entially remove L R E E from seawater, leaving

better-crystall ized phases during diagenesis, in the reacted (or scavenged) seawater of the contrast to the other (initial) REE which partly plume even more LREE depleted ( Klinkham- may transfer into the poorly crystalline frac- mer et al., 1983). Leg-92 sediments older than

tion (cf. Fig. 6). ~ 10 Ma display fur ther enr ichment of the Five of the samples ~ 10 Ma in age show a M R E E - LREE, leading to overall f lat ter pat-

positive change in the Yb /Pr ratio with leach- terns relative to the younger samples (Fig. 3a ing (Fig. 7d). This suggests tha t a greater pro- and b) . We interpret this enr ichment as a re- port ion of the L R E E are in the poorly sult of cont inued post-depositional diagenetic crystalline fraction of these samples. In the 5 reactions. Such reactions may involve scaveng- samples older than ~ 14 Ma, there is no signif- ing from pore waters, which may be expected to icant change in REE pa t te rn following the sec- increase the proport ion of L R E E to H R E E in ond leach, which may indicate the end of REE the sediment (cf. Tu rne r and Whitfleld, 1979), redistribution related to diagenetic effects, and/or recrystallization of the original Fe- and

254

5 90 ~o L E A C H R E S U L T S •

•

S e d i m e n t s ~ 8 Ma 4. so • • •

x~ c

, ~ , o ' ~ ~ ._= " .

W • n , "

Vo O~ 11 - :. ~ Leach pairs 2 • 50 I1=11• mm

--I 10 ¢ • 5 9 9 B - 2 - 6 (1 ) (o) • (b)

• 5 9 9 B - 2 - 6 (2 / ,o 20 30 40 Jo 20 3o

601 - 2 - 2 (1) A g e ( M a ) Age{Ma )

6 0 1 - 2 - 2 (`2) o . s - -

~o" ( a ) La Ce Pr Nd Sm Eu Gd Oy Ho Er Yb Lu • . ~ 0 2 " • •

<11 •

--Q 0.1- Samples>~ lOMa • • •

,,0 10~ t'~] S e d i m e n t s ~ ' 1 0 Ma

L E A C H R E S U L T S ¢~ oo o • m ' • •

o i Samples ~< 8 Ma ~ ~ o •

~Io ~ , (I) (I} ~ "°-o.2 -

CC (c) (d]

Leach pa i rs - o 3 , , . , , - - , , , , . o~ ,o 2~ 30 '0 20 3O

o Age (Ma) Age (Ma) . . i ' ° o • 5 9 7 A - 2 - 2 ( 1 )

• 5 9 7 A - 2 " 2 (2 ) Fig. 7. a. Plot of Yb/Pr (normalized) vs. age for metallifer- • sgs-s-4 (I) ous (carbonate-free) fraction. This ratio is representative • sgo-s-4 (2) of the HREE/LREE characteristics of older and younger

, o ' groups of sediments. La Ce Pr Nd SmEu Gd Dy Ho E, Yb Lu ( b )

b. Effect of second leach on the proportion of Yb removed Fig. 6. Comparison of REE patterns before (I) and after vs. age of the sediment (other REE elements plot (2) treatment with a second leach to remove poorly crys- analogously). talline phases: (a) sediments younger than ~ 8 Ma; and c. Effect of second leach on the mA=~mitude of the Ce anom- (b) sediments older than ~ 10 Ma. Vertical lines connect aly. JCe* is the relative change in the calculated anomaly leach pairs; sample number suffixes as in Table I. 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 (normal-

P-bearing phases. The leach data in Fig. 7d sug- ized) ratio with leaching. gest that such reactions, which lead to fraction- ation of the REE between different fractions of originally present in the sediment acquires REE the sediment, may be mostly completed within during diagenesis (Dymond and Eklund, 1978; ~ 10-15 Ma of deposition. Jonasson et al., 1985). Elderfield et al. (1981) As an example of recrystallization of original found that the concentrations of REE (exclud-

phases, at least some of the original Fe-oxyhy- ing Ce) in northern equatorial Pacific sedi- droxide phases in certain metalliferous sedi- ments are correlated closely with P content, and ments undergo a transition to Fe-smectite as a inferred that the REE are associated with phos- result of reaction with biogenic opal (Heath and phatic fish debris. They also found that the P- Dymond, 1977; Hein et al., 1979; Cole, 1985; rich phase is slightly depleted in the HREE Jarvis, 1985 ). Because the large ionic radii of (relative to a flat pattern). These authors sug- the REE inhibit their incorporation in either gested that where a diagenetic smectite-form- the tetrahedral or octahedral sites of the smec- ing reaction is involved, some of the REE tite structure, some are released into solution originally associated with Fe-oxyhydroxides or lost to other phases during the formation of undergo intrasediment transferral to the phos- the Fe-smectite. In contrast, biogenic apatite phatic phase. Another probable example of this

255

process occurs in the Leg-85 metalliferous sed- have REE patterns that are similar to those of iment, which formed under the equatorial belt surficial Bauer Basin ferromanganese crusts of opal production ~ 2000 km north of the Leg- (Elderfield and Greaves, 1981 ) from ~ 750 km 92 sediments (Jarvis, 1985). In the case of the east of the EPR [i.e. HREE-depleted relative Leg-85 sediments, Jarvis found a good correla- to an EPR-crest pattern; see also Piper and tion between REE and P contents, and an ad- Graef (1974) for similar HREE-depleted pat- ditional poorer one between REE and Fe (the terns several hundred km east of the EPR at latter largely in Fe-smectite ) ; both phases ap- 39 ° S ]. In the Bauer Basin, surficial diagenetic parently contributed to the relatively flat REE effects occur owing to the slow rate of accumu- patterns, lation of the sediment (Sayles and Bischoff,

With respect to the Leg-92 samples, HREE 1973; Dymond and Veeh, 1975; Cole, 1985). It depletion through the smectite-forming reac- is of interest that in both of these sub-lysocline tion is unlikely to be important, as Fe-smectite and ridge-removed areas, there is evidence for is only a minor phase. HREE depletion is more some hydrothermal input into the sediments, likely to be controlled by the apatitic compo- as indicated by Pb isotope data (Dasch, 1981; nent of the sediment. Although XRD data sug- Barrett et al., 1986 ). It appears that in surficial gest that apatite constitutes ~ 5% of the sediments that are accumulating at a very slow metalliferous component, P analyses for five of rate (but not nodules or crusts), diagenetic ef- our REE samples indicate a positive correla- fects may obliterate any initial depletion of tion with XREE. Hence, even a limited com- LREE relative to HREE resulting from the ponent of apatite may play a role in producing presence of ridge-derived precipitates. Never- a depletion in the HREE. By contrast, a nega- theless, a distinct negative Ce anomaly is main- tive correlation between Mn and ZREE, based tained, as is a component of hydothermal Pb. on 16 samples, indicates that the REE are not

significantly associated with manganiferous 4. Conc lus ions phases.

The two REE-enriched samples from Site 598 are shown in Fig. 3a; a similarly enriched pat- The present study has shown that small-scale tern was found, following the second leach, for differences in the REE patterns of deep-sea a surficial sample from Site 597. These sam- metalliferous sediments, which were previously pies, which are within several metres of the sur- not recognized, can now be resolved and inter- face of the ocean floor, come from distances of preted. With the emergence of REE data on

800 and ~ 1200 km west of the EPR. Depo- seawater from different parts of the world ocean, sition occurred below the present lysocline for it is apparent that the concept of a "typical sea- this part of the ocean, which has resulted in the water REE pattern" is only useful in the broad- observed slow MAR-values. Low MAR-values est sense. To make more meaningful apparently lead to appreciable diagenetic ef- interpretations of the REE patterns of deep-sea fects in near-surface sediments, and presum- sediments, comparisons with deep seawater are ably are the cause of the anomalously high REE required. In the case of the Leg-92 metallifer- contents (Table II) and atypical mineralogical ous sediments, which were deposited under the assemblages in these samples. REE contents are influence of an extensive hydrothermal plume in the order of 1000 ppm, a factor of ~2-5 emanating up to 1000 km west of the EPR, greater than for the other sediments analyzed, LREE enrichments are present in accordance and significant proportions of phillipsite, which with predictions based on observed was not detected in the other, deeper sedi- L R E E - d e p l e t e d seawater anomalies in that ments, are also present. The surficial samples plume at its source (Klinkhammer et al., 1983).

256

Furthermore, analyzed sediments older than used with caution when assessing fine-scale ~ 10 Ma are more LREE-enriched and/or variations in the REE composition of seawater. HREE-depleted than those younger than ~ 8 Ma. This is compatible with continued LREE A c k n o w l e d g e m e n t s scavenging from normal pore waters and/or diagenetic reactions resulting in preferential We express our gratitude to Dr. J.N. Walsh release of the HREE to solutions, for providing use of the ICP-AES spectrometer

A correlation between the mass accumula- at King's College, London. Additional facilities were kindly provided by the Department of Ge- tion rates (MAR) of I R E E and Fe + Mn sug-

gests that ferromanganese particulate matter ology, City of London Polytechnic. T.J.B. gratefully acknowledges research support pro- supplied by the hydrothermal plume scavenges vided by grants from the Natural Sciences and REE. Although the MAR of Fe + M n shows a

general decrease with age above basement, Engineering Research Council of Canada, and I R E E concentrations in the metalliferous from the University of Toronto. Various stages component increase with age above basement, of sample preparation were carried out by J.

Lugowski and K.E. Jarvis. Nigel Higgs, Insti- Scavenging of REE appears to be limited near tute of Oceanographic Sciences, kindly sup- the palaeo-axis due to rapid burial of sediment,

but increases with lateral transport from the plied XRD data. We are indebted to the Deep axis and possibly after burial as well. Following Sea Drilling Project for providing the Leg-92 deposition of the hydrothermal component, sample material, and thank the co-chief scien- further relative flattening of the REE pattern tists, Drs. M. Leinen and D.K. Rea, for their takes place, probably the result of diagenetic cooperation and assistance.

reactions requiring up to several million years. R e f e r e n c e s Phase partitioning data indicate that with

age, progressively more REE reside in more Aplin, A., Miehard, A. and Albarbde, F., 1986. 143Nd/144Nd poorly crystalline phases. This suggests that as in Pacific ferromanganese encrustations and nodules. initial ferromanganese precipitates undergo Earth Planet. Sci. Lett., 81: 7-14.

B~icker, H., Lange, J. and Marchig, V., 1985. Hydrothermal diagenetic recrystallization, REE are t rans- activity and sulphide formation in axial valleys of the ferred to the poorly crystalline phases and/or East Pacific Rise crest between 18 and 22°S. Earth are scavenged from pore waters by these phases. Planet Sci. Lett., 72: 9-22. Sub-lysocline surficial sediments can have pat- Barrett, T.J. and Friedrichsen, H., 1982. Elemental and

isotopic compositions of some metalliferous and pelagic terns resembling older sediments due to early sediments from the Galapagos Mounds area, DSDP Leg diagenetic effects resulting from slow accumu- 70. Chem. Geol., 36: 275-298. lation rates. However, they are significantly en- Barrett, T.J., Taylor, P.N., Jarvis, I. and Lugowski, J., 1986. riched in REE as a result of the slow Pb and Sr isotope and rare earth element composition

of selected metalliferous sediments from Sites 597 to 601, accumulation rates. Comparisons between sed- Deep Sea Drilling Project Leg 92. In: M. Leinen, D.K. iment REE patterns should be made between Rea et al. (Editors), Initial Reports of the Deep Sea samples deposited consistently above the pres- Drilling Project, Vol. 92: Washington (U.S. Gov. Print. ent (or past) lysocline, or consistently Off., Washington, D.C., pp. 391-407. below it. Barrett, T.J., Taylor, P.N. and Lugowski, J., 1987. Metal-

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