Fish debris record the hydrothermal activity in the Atlantis II Deep sediments (Red Sea)

Fish debris record the hydrother~l

activity in the Atlantis II Deep sediments (Red Sea)

ELISABETHOUDIN' and ALAIN COCHERIE’ ‘Bureau de Recherches G&ologiqueset Mini&es, DAM/GMX, B.P. 6009,45060 Orleans, France *C.R.S.C.M.,GIS: Bureau de RecherchesGeologiques et Mini&es, Centre National de la Rt?chercheScientifique, 1A, Rue de la Wrollerie, 4507 1 Orleans, Cedex 02, France (Received September 10, 1986; accepted in rhsedform October26, 1987)

Abstract-The REE and U, Th, Zr, Hf, SChave been anal* in samples from Atlantis II and Shaban/Jean Charcot Deeps in the Red Sea. The high Zr/Hf ratio in some sediments indicates the presence of fish debris or of finely crystallized apatite. The positive ZREE vs. P24 and ZREE vs. Zr/Hf correlations show that fish debris and finely crystallized apatite are the main REE sink in Atlantis II Deep sediments as in other marine environments. The hydrothermal sediments and the fish debris concentrates have similar REE patterns, characterized by a LREE enrichment and a large positive Eu anomaly. This REE pattern is also observed in E.P.R. hydrothermal solutions. Fish debris from marine environments acquire their REE content and signaturemostlyftom seawaterduring early diagenesis.The hying REE signature of Atlantis II Deep fish debris indicate that they probably record the REE signature of their hydrothermal sedimentation and diageneticenvironment. Tbe difIerentREE signaturesoftbe Sbaban/Jean Cbarcot and Atlantis II Deep hydrotbertnal sediments suggest a seawater-dominated brine in the Shaban/Jean Charcot Deep as opposed to the predominantly hyclrotbermallbrine in Atlantis II Deep. Atlantis II Deep fish debris are also characterized by their high U but low Th contents. Their low Th contents pro~bly reflect the low Th content of the various possible sources (seawater,brine, ~rnen~~. Their U contents are probably controlled by the redox conditions of sedimentation. Previous work by COURTOISand TREUIL (1977), COCHERIE (1981) and THI~SE (1982) show that most Atlantis II sediments have low REE contents (ZREE = 20 ppm) and chondritic normalization curves show an enrichment in light REE and a positive Eu anomaly. However, the REE contents of these sediments vary by two orders of magnitude and this variation is not related to the bulk mineralogy ofthe samples (COCHERIEand OUDIN, in preparation). The aim of the present work is to identify the REE and trace element (U, Th, Zr, Hf, SC) carriers, the factors controlling their concentration and their sources in Atlantis II and Shaban/Jean Charcot Deeps hydrothermal sediments. This study is part of an extensive inv~ti~tion of the geochemistry of REE and other trace elements in the different facies identified by BACKERand RICHTER(1973) in Atlantis II Deep sediments. This paper is based on the results obtained on two hydrothermal sediment samples (core 268, samples 456 and 1028) and three samples from the D.O.P. (core 264, sample 1002 and 1077; core 198, sample 548) from Atlantis II Deep. Although six cores were investigated in detail, fish debris could only be extracted from two samples, one from a predominantly hydrothermal sediment (core 268, sample 456) and the other one from a p~ominantly detrital sediment (core 264, sample. 1002) in quantities large enough to be analyzed hecause of the low solid particle content (average range: 10 to 30%). The finely layered structure of the sediments preclude the use of large sample for fish debris separation. A sample of hy~~e~~ sediment (core KS0 l-604) from the ShahanfJean Charcot Deep, a different Red Sea environment, was analyzed for eomparison.

I. INTRODUCI‘ION THE ATLANTISII DEEP is the most mineralized Deep situated in the axial rift zone of the Red Sea (DU;ENS and ROSS, 1969). It is tilIed with approximately IO-20 m of hydrothermal sediments overlain by hot (60°C) brines of predominantly hydrothermal origin (BISHOFF,1969a; BACKERand RICHTER, 1973). Active brine venting occurs in the south-west basin (~HOELL and HARTMANN, 1978; H~TMA~, 1980) where our cores were collected (core locations are: 264: lat. 2 1”20,63’N and long. 38‘94,ll’E; 268: lat. 21”20,45’N and long. 38’04,67’E, 198: lat. 21”20,88W and long. 38”04,4SE). Metal-rich hydrothermal sediments consist mainly of oxides, sulphides, silicates, sulphates, and carbonates. A hydrothermally influenced biodetrital unit D.O.P. (detrital oxidic pyritic zone of BACKBRand RICHTER, 1973) is present at the base of some of the cores and correspond to a transition zone between detrital and hydrothermal sedimentation. The scdiments are formed by three main processes: (1) a normal biodetrital sedimentation, (2) a chemical precipitation from the brines overlying the sediments, (3) an epigenetic precip itation of minerals. Transformations related to compaction and lithification or recrystallization of unstable phases which have heen inferred by WEBER-DIE~NBA~H ( 1977) or HOLM ef al. ( 1982) are difficult to identify but important to distinguish from those related to brine venting (epigenetic hydrothermal circulation) through the sediments in the SW. basin (BISHOFF, 1969b; BACKERand RICHTER, 1973; ZIERENBERG and SHANKS, 1983; OUDIN et af., 1984). In contrast, the Shaban/Jean Charcot Deep situated in the northern part of the Red Sea contains brines which are only 2” or 3“C warmer than normal bottom waters (PAUTOT er a!., 1984). The sediments (core location: lat. 25”13’N and long. 35’2 YE) are mainly formed of bikers material with locally abundant volcanic glass fragments and a 4 cm-thick hydrothermal sediment layer (FOURNIER-GERMAIN,1986).

II. SAMPLES AND ANALYTICAL PROCEDURES All samples were analyzed on a dry, salt-free basis. Fish debris were separated by sieving and hand-picking and cleaned by several washing steps in distilled water and alcohol. 171

178

E. Oudin and A. Cocherie

The trace elements were analyzed by instrumental neutron activation analysis (INAA) using the method described by CHAYLA el ai. (1973) and JAFFREZICet a/. (1980). U, Th, Zr, Hf, SC and Tb were counted after epicadmium neutron irradiation, whereas REE except Tb were counted after irradiation under a high thermal and epithermal flux (10“’ n/cm*/+ The Zr, La, and Ce data are corrected to account for the fission of U (MEYER. 1982).

main P-bearing phases in Red Sea sediments (BISCHOFF. 1969a), concentrate the REE. Fish debris were identified in our cores during a systematic micropaleontological investigation (BOURDILLON, 1982a-c). The low REE content (~60 ppm) of samples containing no or rare debris and/or having a low P205 content (< 1500 ppm) as well as samples with a normal Zr/Hf ratio, suggest that only minor amounts of REE can be present in other phases such as Fe oxides and hydroxides or amorphous material which are often abundant in our samples. The Shaban/Jean Charcot sample which has a normal Zr/Hf ratio but a high level of P205 ( 1.2W) will be discussed below. The chemical and mineralogical compositions (Tables I and 2) of the bulk sample 264- 1002 from the D.O.P. hosting fish debris indicate a predominantly detrital origin with a weak hydrothermal influence. The REE pattern of this sediment is very similar to that of shale except for a slight positive Eu anomaly. In contrast, sample 268-456, which also hosts fish debris, is predominantly of hydrothermal origin and belongs to a sulphide-rich unit in a core displaying the typical facies of the hydrothermal venting zone (BACKER and RICH-

III. RESULTS AND DISCUSSION 1. Rare earth elements, zirconium, h&urn. and phosphorus

a. Atlantis ZZDeep. The major and trace element data are listed in Table 1 and chondrite normalized REE patterns are plotted on Fig. la. The mineralogical composition of the samples is presented in Table 2. Fig. 2a and 2b show that the REE content of the samples is related to their P205 content and Zr/Hf ratio. The high Zr/Hf ratio (due to their high Zr content) in some samples is characteristic of phosphates and especially of fossil fish debris (TLIG and LALLIER-VERGES, 198 1; TLIG et al., 1987), while the usual range for the lithosphere is 40-50. This suggests that fish debris, which are the Table

Core

1 - Major and trace elements in Red Sea samples. on a dry, salt-free basis. Cores 260, 264 and core KS 01 from the Shaban/Jean Charcot Deep. except for Cu, Pb, Zr, Hf. IJ, Th and the AEE 268

no

Sawt@,)depth

1028 a

268

268

264

456 =

456

1002

Predominantly hydrothermal sediments 7.9 .7

SiO; Al203 FezOs*

43 14

cao

Pi 60 PzO5 Zn CU Pb 5 u Th Zr Hf SC LEl Ce Nd

sm EU Gd lb Yb LU EREE La/Yb Eu/Eu* Zr/Hf

aAlso>, and s

3.8 .03 .04 .07 .04 4500 30 20.3 18.0 .25 ; 10 .2 .7 2.5 3.6 ".a. .30 .23 n.a. .044 .08 .013 9.4 31.3 2.5
,130

n.a.

“.8.

“.a.

3.5 n.a. ".a. ".a. ".8. n.a. 1.07 n.a. ".a. n.a. 237 2.51 400 < .2 .82 154 325 ".a. 32 22.8

2.6 ".a. ".a. ".8. ".B. n.a. .06 n.i. n.a. n.a. 66 3.58 4785 2.9 91,3 1016 1572 “.a.

105 69

3.8 5.9 ".a. 789 26.1 2.6 >2000

KzO, MnO,

264

11::. 37 7.8 3875 27.5 2.0 1650

264

Predominantly detrital sediment 39.0 7.4 20.9 7.5 3.6 2.2 1.88 .35 .39 934 106 “.a.

2.7 3.78 187 2.9 20.3 17 30 “.a.

3.5 1.3 .50 1.6 .27 82.2 10.6 1.2 64

198

1077 b

1002 b

“.8.

“.8.

CaO, HgO, K,O, MnO,

Fe,O,, CeO, NgO, Al,O,, GERHAIN, 1986). Fe,O, been

“.B.

KS 01

548 a

13.5 2.9 59.2 3.1 1.8 2.6 .53 1.73 .13 2506 Cl0 n.a. 3.7 1.33 1492 .B 9.9 183 382 176 38.3 27.2 28.1 4.20 8.6 1.47 1023 21.3 2.6 1865

604 ' Hydrothermal

Predminsntly hydrotharmnl sediments

sediment 37.5 6.7 34.7 1.1 2.2 1.5 .04 1.20 .Ol 25 ,,.a.

3.16 1.3 52.5 5.7 1.9 .15 2.74 1.82 c.01 4440 46 15.9 18.7 3.14 402 .26 3.4 124 291 n.a. 43 18.9

“.8.

.71 1.84 214 4.5 13.2 7.5 11.7 ".a. 1.6 .42 n.a. .28 1.0 .15 35.4 7.5 _^ /Y 49'

E; 10.0 “.a.

776 12.4 1.6 1546

Zn, Cu, Pb analyzed

by

ICP, SiO, by calorimetry

P.05,

Zn, Cu, Pb analyzed

by

ICP, SiOz by calorimetry

P,Os,

Zn, Cu analyzed

Fe,O,, CaO, MgO, KzO, MnO, PzOs, by titration (THISSE, 1982).

b Al,O>, FeaO,, (this work). ’

20.8 2.3 40.4 2.2 1.9 .55 .a3 .22 6.26 7700 1400 7.8 6.6 .73 26 < .2 1.1
Fish debris

These samples have been analyzed 198 are from Atlantis II Deep and All analyses are expressed in % (in ppm).

and Zn in fish debris samples analyzed by NAA (this work).

and REE,

by X-ray

fiimrescence

(FOMNIER-

Zr, Hf, U, Th and SC in all samples

Z REE = total REE content wlthinterpolatedvalues, et al. (1972). Eu* I interpolated values of Eu. Fe,D,r total Fe BS Fe,O,. ".8. q not analyzed.

chondritic

values

have

are from PHILPOTTS

179

Red Sea sediments: REE source

bJJJJ LaCe

Nd ‘3nEuGdTb

Yb Lu

Late

JJJJJJJJ’J Nd

SmEuGdTbOy Er

YbLu

FIG. 1. Chondrite normalized REE patterns (PHtLPoTls et al.. 1972). (a) Samples from the Red Sea: Atlantis II Deep SW. basin and Shaban/Jean Charcot Deep. (b) Fish debris from Pacific deep sea marine sediments (BERNAT, 1975). Solutions from East Pacific Rise hydrothermal vent field (MICHARDet al., 1983). Composite of 40 North American Shales (HASKINand HASKIN,1966).Hydrothermal sediments from Galapagos (BONN• T-COURTOIS, 1981).

Sea Water (H&~DAHLet al., 1968).

hydrothermal sample (268with the predominantly detrital sediment. Although our fish debris were not scrupulously clean as indicated by their FezOr contents (2.8% and 3.5%) which suggests some contamination by the host sediments, their REE content and signature is not modified by this contamination, since both fish debris samples are considerably enriched in REE compared to their host sediments. The fish debris/sediment REE content ratio is distinctly higher for the hydrothermal sample (> 158) compared to the detrital sample (47). Both fish debris samples exhibit the characteristic REE pattern of Atlantis II Deep hydrothermal sediments: they are enriched in LREE and depleted in HREE and have a positive Eu anomaly. The REE hydrothermal signature of fish debris from the predominantly detrital sediment (264-1002) suggests that the fish debris have not acquired their REE pattern from the main detrital constituents of the sediments. The REE signature of these fish debris is related to the minor hydrothermal fraction of the sediment and/or to the hydrothermal brine impregnating the sediment and therefore to the hydrothermal event which occurred during and/or after sedimentation. Fish debris are only preserved in sulphide-rich zones but are absent from oxidedominated samples (BOURDILLONand ABU GIDEIRI, 1983) and numerous dissolution features of biogenic material are TER, 1973). The predominantly

456) is distinctly depleted in REE in comparison

described by BOURDILLON(1982a-c) especially in the sulphide-oxide transition zones. Two samples (264-1077 and 198-548) containing high Pz05 contents (1.7%) have high REE concentrations and a typical hydrothermal REE pattern (Fig. 1) in agreement with their predominantly hydrothermal composition. They contain no or extremely rare fish debris. Instead, finely crystallized phosphatic phases (apatite) were identified by TEM analyses (Fig. 3a). These data suggest that the microcristalline apatite of these two samples may have formed by dissolution of fish debris. Both samples have high Zr/Hf ratio in good agreement with TLIG et al. (1987). All samples including oxide-dominated samples, have the same ZREE-Pro5 and Zr/Hf correlations indicating the presence of phosphates with a similar REE enrichment factor. It has been shown by ELDERFIELDand PAGE?T (1986) and WRIGHT et al. (1987) that in marine sediments the Ce anomaly reflects the redox potential of the sedimentation environment. According to COURTOISand TREUIL (1977) the REE pattern of Atlantis II Deep sediments cannot be induced by the physical and chemical conditions of the hydrothermal sedimentation but reflects fluid-rock interaction at depth; both oxidized and reduced sediments have a similar REE pattern and the huge positive Eu anomaly is not dependent on the redox conditions of sedimentation. This and the fact that apatite included in either oxidized or reduced sediments also

180

E. Oudin and A. Cocherie Table 2 - Hinsralogicsl

composition

X-ray analysis Core Nunber Atlantis

(and

of the ssdimsnta.

in polished

section

Minerals wsrs identified

by

for sample 1028).

Sample Depth (cm)

Mineralogy

II Deep

268

456

sp (P)

; cy

260

1028

Py (A)

;

264

1002

Plagio (A) ; Qz (F) : Ksol (F) i Mont (F) i Ce ? (F) ; Py (P) ; Sid (P) ; Ill (I) i knp CT)i

264

1077

Sp (T)

198

Chercot

KS 01

Pyr

(T)

Coe

(A)

91

; Ill (F)

(F)

; Chl

Qz (R)

:

(P)

: Iso

;

Hem (P) ; Ma (R)

; Sm(P) ;

; POCT).

; Ca (R)

Sid (1).

i

i

Deep

604

Abbreviations

Py

; Anh (A) (T)

; Lep CT). ; rep (F) ; Hem (F) ; Sid (P) ; Ma(f’) i Micro ? (P) ; Pyr CT). Py (A) ; Sid (A) ; Ma (F) ; Cy (F) i th (R) i A”,, ? (R) ; COS (I’) ; Ca (1) : ‘h (1) i 111 ? CT) Chl ? (T) : Amp 7 (1).

588

Shaban/Jean

i

(P)

Hsm (A) ; cy (T)

Amp : Cy :

;

amphibole

; Gas Fe1

;

Anh = anhydrits

;

chalcopyrite

(R)

Do1

(F) (R)

; Ce (A)

;

Lep

; Cs =

calcite

dolomite ; Fe1 :

=

;

Do1 (RI

;

(1).

feldspar

;

Chl

= chlorite

;

;

; Gy I gypaum ; Hem = hematite ; Ill : illite : 180: isocubanite : Kaol = keolinite ; Lep = lepidwrocite : Ma E magnetite ; Micro microcline ; Mont : mont~rillonite ; Plagio = plagioclaae : Py E pyrite ; Pyr = pyroxsne ; Qz = quartz Sid : siderite ; Sm: amectite ; Sp I sphslsrite. Gee : go&hits

q

Abundance

cods

: A = abundant, T :

trace

F = frequent,

REk (PPfl

I

P = present,

amounts.

negative cerium anomaly during sedimentation and early diagenesis (ARRHENIUS and BONATTI, 1965; BERNAT, 1975; DYMOND and EKLUND, 1978; WRIGHT et al., 1984; ELDERFIELD and PAGETT, 1986; WRIGHT et al. 1987; TLIG et al., 1987) (Fig. 1). Recent work of ELDERFIELD and PAGETT (1986) and TLIG et al. (1987) show that other factors could also influence their REE distribution. WRIGHT ef al. (1984), SHAW and WASSERBURG (1985), STAUMGEL et al. (1985) and GRANDJEAN et al. (1987) Nd isotope studies of fossil biogenic apatite provide further arguments in favour of a

has a positive Eu anomaly suggest that the positive Eu anomaly in Atlantis II Deep fish debris is also not controlled by the redox potential of sedimentation but reflects a characteristic of the hydrothermal fluid. Modem fishes have low REE concentrations (WRIGHT et al., 1984; ELDERFIELD and PAGETT, 1986). Modem biogenic apatite was also found to have low Nd concentration (< 150 ppb) by SHAW and WASSERBURG (1985). In deepsea sediments fish debris are enriched in REE and acquire a REE pattern similar to that of seawater (Fig. lb) with a distinct

looo- =

R = rare,

;

I

0

800600MO200-

FIG. 2. Correlationsof the total REE content with PzO$(a) and with the Zr/Hf ratio (b) in AtlantisII (cirbs) and Shaban/JeanCharcot (asterisk) Deeps hydrothermal sediments. Three typical ranges of error are mported on these

&u-es.

Red Sea sediments: REE source

FIG. 3. (a) TEM microphotograph (Xl 12000) of apatite crystal (arrow) associated with finely crystallized goethite in sample 2641077.(b) TEM microphotograph(X112000)of phosphaticphase (Ca, P) associated with clay material in sample KSOl-604.

predominant seawater source for their REE and even suggest that it could be a way to determine Nd isotope signatures in ancient oceans. The similarity of the REE pattern of Atlantis II Deep hydrothermal sediments and East Pacific Rise (E.P.R.) hydrothermal solutions suggests that REE have been scavenged from the hydrothermal fluid with little or no fractionation during the formation of the Atlantis II Deep hydrothermal sediments (MICHARD et al., 1983). These data suggest that Atlantis II Deep fish debris acquire most of their REE content and signature during sedimentation and after their burial in the brine-impregnated sediments. However, REE uptake in fish debris could start in the brine, because, although the sediments have a fast accumulation rate due to the high productivity of the hydrothermal brines, particle settling (especially large porous particles such as fish debris) is hindered

181

by important convective circulations, agitation at the brine interfaces and the high density of the brines and may be relatively slow. The sediment-brine interface is not sharp: the upper part of the sediment column (3-4 m) is liquid (i.e. brine-dominated) and contains only 4% to 8% solid particles (BACKERand RICHTER, 1973) and also contribute to longer exposure of the sediment particles to the brines. In Atlantis II Deep, the sediment accumulation rate is high (at least 0.2 to 1.5 mm&r according to KU, 1969 and SHANKS and BISHOFF, 1980). This implies a minimum age of 3000 years for our most recent Atlantis II Deep fish debris, while sample 264- 1002 is approximately 10,000 years old, as indicated by micropaleontological analyses (BOURDILLON, 1982a) and contains approximately five times the REE concentration of the younger fish debris. The abundance of the detrital fraction in 264- 1002 compared to 268-456, indicates that the former has a slower accumulation rate than the latter in agreement with ELDERFIELDand PAGETT(1986) data who show that fish debris in the more slowly accumulating sediments have higher REE contents. WRIGHT et al. (1987) have shown that the sedimentation rate is correlated with the REE concentration in approximately half of the samples from two cores collected in the Guaymas basin and the Peru margin and deduced from those observations that other factors must be involved in the enrichment process. The higher REE content of our older Atlantis II Deep fish debris could also be related to their longer exposure to interstitial brines in agreement with RUHLIN and OWEN(1986) who found that phases within hydrothermal sediments from the E.P.R. become enriched in REE with longer exposure to seawater and pore fluids. According to BACKERand RICHTER (1973) brine deposition did not occur during D.O.P. accumulation and hydrothermal deposits formed by direct mixing of hydrothermal fluids and seawater. Another possibility is that the hydrothermal minerals in sample 264-1002 was formed during epigenetic fluid circulation which lead to the formation of a brine pool at a later stage. In the absence of a brine pool fish debris from the detrital environment would first acquire a seawater-influenced pattern. Thereafter over a maximum time span of approximately 10,000 yrs fish debris would have to reequilibrate with the interstitial brines and acquire a REE hydrothermal signature similar to that of fish debris deposited in an hydrothermal environment. ELDERFIELDand PAGE-IT ( 1986) have already suggested that the REE pattern and enrichment of fish debris from deep-sea sediments could be modified by interaction with pore waters. However it seems unlikely that the similar REE pattern of the two fish debris accumulated in different depositional environments could be the result of two very different processes. Most authors following BERNAT( 1975) agree that fish debris acquire high REE contents during early diagenesis and are therefore rather impervious of further diagenetic changes. Therefore, it seems more probable that both fish debris were accumulated in an hydrothermal environment and that a brine pool started forming during D.O.P. accumulation in agreement with HACKETT and BISCHOFF(1973), who suggest that brine deposition started during the formation of the thin sulphide zone situated directly above the uppermost lithified pteropod zone where sample 264-1002 is taken from. b. Shaban/Jean Charcot Deep. The Shaban/Jean Charcot

182

E.

Oudin and A. Cocherie

hydrothermal sediment has a REE pattern which is quite distinct from that of Atlantis II Deep hydrothermal sediments. The Shaban/Jean Charcot sediment REE pattern is characterized by negative Ce and Eu anomalies and a rather flat HREE pattern. Hydrothermal sediments from the Galapagos have similar REE concentrations and patterns with a negative Ce anomaly generally less pronounced than in seawater (CORLISSet al., 1978; BONNOT-COURTOIS,198 1). BONNOTCOURTOIS (198 1) has interpreted this REE pattern as the result of a mixture of basaltic and seawater components. The predominant biodetrital composition of the Shaban/Jean Charcot core (FOURNIER-GERMAIN,1986), the mineralogy, the chemical composition and REE concentration and pattern of the hydrothermal intercalated layer (KS0 l-604) are therefore consistent with a deposition from brines essentially formed by seawater with local hydrothermal input. Stable isotope studies of Red Sea brines suggest a seawater or paleoseawater origin (CRAIG, 1969; SCHOELLand FABER, 1978) and the high salinities are attributed to interaction with the thick Miocene evaporites. Strontium and lead isotope studies (FAURE and JONES, 1969; ZIERENBERGand SHANKS, 1986; COOPER and RICHARD, 1969; DELEVAUXand DOE, 1974) indicate that the brines can further react with the basalt from the rifi zone leading to the formation of base-metal rich sediments as in the Atlantis II Deep. This is also supported by the similarity of the REE pattern of Atlantis II Deep sediments and E.P.R. hydrothermal solutions (MICHARD et al., 1983) which are also the result of seawater-basalt interaction. The hydrothermal sediment from the Shaban Deep (KS0 l604) contains high levels of PIOs (1.2%) but is depleted in REE (ZREE = 37 ppm) and has a “normal” Zr/Hf ratio (49). Extremely rare phosphatic phases identified by TEM analyses have a different facies (Fig. 3b) from that observed in samples 264-1077 and 198-548 from the Atlantis II Deep which are thought to be derived from fish debris. The scarcity of the Shaban/Jean Charcot phosphatic phases cannot account for the high P205 content of the Shaban/Jean Charcot sample. Finer phosphatic phase particles might be present but below detection limits or a major fraction of P might be adsorbed by the poorly crystallized oxides and hydroxides (or clay) as suggested by ELDERFIELDet al. (198 1) in ferromanganese nodules and by BERNER( 1973), FROELICHet al. (1977) and BLOCH(1978) for P of seawater origin in E.P.R. metalliferous sediments. In a similar way, P in the Shaban/Jean Charcot sediment could be scavenged from the brine during the precipitation of clay and oxides and hydroxides. It is possible that a minor fraction of P may also be derived from the brine in Atlantis II Deep. The Pro5 vs. ZREE regression line intercept at 0.13% P20s on Fig. 2a could be significant and could suggest another source of phosphorus in addition to fish debris in Atlantis II Deep sediments. These phosphatic phases would have to be a less efficient sink for REE and Zr than fish debris and their recrystallization products. ELDERFIELD et al. ( 198 1) have also suggested the presence of two types of phosphatic phases in ferromanganese nodules, a Prich phase composed of fish debris and/or recrystallized biogenie apatite and a Fe-rich phase where phosphate is chemisorbed by hydrous Fe-oxides; the P-rich and Fe-rich phases are characterized by a distinct REE pattern but have similar REE contents. The low REE content of the Shaban/Jean Charcot sediment probably reflects the low REE content of

the brine (related to the minor hydrothermal contribution) as well as the difference in efficiency of both types of phosphatic phases as sinks for REE. 2. Uranium, thorium and scandium in fish debris and phosphates According to KU (1969) an important fraction of U in Atlantis II Deep sediments is probably scavenged out of the overlying normal Red Sea water through co-precipitation with iron hydroxides and silica. The U concentration of our Red Sea fish debris (166 and 237 ppm) is very high compared to the Red Sea hydrothermal sediments (maximum U concentration: 30 ppm, KU, 1969). The U enrichment factor (ratio of U in fish debris/U in sediment) is higher in the detrital sample (62) than in the hydrothermal sample (36), although the latter has a higher U content. The two samples containing no fish debris but having a high Zr/Hf ratio have different U contents. Sample (198-548) is a relatively reduced sediment (see Table 1 and 2) and has a rather high U content (18.7 ppm). The other sample (264-1077) is an oxidized sediment having a low U content (3.7 ppm). The ?&&an/Jean Charcot hydrothermal sample is also an oxidized sediment with a low U content (0,71 ppm). This suggests that U concentration of fish debris and sediments is related to the oxidation state of the sediment. DEGENS et al. (1977), who studied the U enrichment of coccoliths in modem Black Sea sediments and of diatom-dominated plankton living in a lake in central Ontario which is polluted by discharge from a U mine, also concluded that reducing conditions in the depositional environment are only essential for the preservation of U-rich detritus and not for the fixation. Fish debris from most marine sediments (ARRHENIUSand BONATTI, 1965; BERNAT, 1975; DYMOND and EKLUND, 1978; BATURIN et al., 1971) and fossil environments (DAVIDSONand ATIUN, 1953; BOWIE and ATKIN, 1956) are also enriched in U and especially in Th. The Atlantis II Deep fish debris are depleted in Th. According to BERNAT (1975), Th in the fish debris of some Pacific sediments is derived from the host sediments, because the Th contents of fish debris and sediments are positively correlated. TLIG et al. (1987) have also analyzed fish debris from deep-sea sediments and argue that the low U and Th contents of fish debris reflect the low U and Th contents of the sediments. BOWIE and ATKIN (1956) have shown that most U is concentrated in bone phosphate while Th is associated with the organic matter present in the bone structure. Therefore, the low Th concentration of our Red Sea fish debris may reflect the low Th content of the possible sources including sediments, brines and seawater and/or their low organic carbon content (SAXBY, 1972; SHANKSand BISHOFF, 1980). Fish debris from the detrital sediment (264-1002) are enriched in Sc (9 1.3 ppm) and contain approximately four times the SC concentration of its host-sediment. Fish debris from the hydrothermal sediment are SC-poor (0.82 ppm) and contain the same concentration as its host-sediment. Fish debris from the detrital sediment are therefore enriched in SC compared to fish debris hosted in the hydrothermal sediment. Fossil fish bones associated with sulphide-rich layers in the Maikop beds (Caucasus) also contain high levels of REE (KOCHENOV and ZONOV’EV, 1960) and Sc (BORISENKO,

183

Red Sea sediments: REE source

196 1; BLOKHand KOCHENOV,1964; both cited by ALTSHULER, 1973). There are some crystallochemical similarities between SCand some of the lanthanides and the association of SC with REE-bearing phosphate has been documented (F~ONDEL, 1970), but the geochemistry of SC in hydrothermal-sedimentary environments is not well known. The two samples containing phosphates derived from fish debris dissolution and the Pro,-rich Shaban/Jean Charcot sample have low SC concentrations. IV. CONCLUDING REMARKS Atlantis II Deep fish debris and phosphatic phases thought to be derived from their dissolution, although representing less than 1% of the sediment mass, can act as a major carrier of the REE as suggested by ARRHENIUSet af. (1957) for marine sediments. Fish debris from marine sediments have a REE signature similar to that of seawater. Fish debris from Atlantis II Deep have a REE signature similar to that of Atlantis II Deep hydrothermal sediments and E.P.R. hydrothermal solutions. These data suggest that Atlantis II Deep fish debris record their interaction with the hydrothermal fluids and that their REE enrichment is a diagenetic process. The REE content of fish debris is probably related to the sedimentation rate as in marine sediments but other enrichment factors are also possibly involved. Our data also suggest the early existence of a brine pool during the hydrothermally influenced detrital sedimentation underlying the hydrothermal sediments. The positive Eu anomaly of the hydrothermal sediments, fish debris and apatite does not appear to be controlled by the redox conditions of the sedimentation environment but reflects the positive Eu anomaly of the hydrothermal fluids which is, as in E.P.R. solutions, not well explained. The Eu anomaly and the LREE enrichment could possibly correspond to feldspar dissolution when the hot hydrothermal fluids react with the underlying magmatic rocks before their emergence in the Deep. Further studies are needed in sediments from Atlantis II Deep and other Red Sea environments to determine the source and the factors controlling the fixation process, stability and d&genetic behaviour of REE and other trace elements in fish debris and their possible use as tracers of past or discrete hydrothermal events. The U concentrations of Red Sea fish debris may depend on the redox conditions of sedimentation and diagenesis (or epigenetic hydrothermal circulation). It could therefore provide important information about control on the U migration in the sediment, one of the main problems in the use of U series isotopes for dating marine sediments. The low Th content of fish debris reflect the low Th content of the environment. The REE content and pattern of one hydrothermal sediment from the Shaban/Jean Charcot Deep in the northern part of the Red Sea is similar to REE contents and pattern of hydrothermal sediments from the Galapagos areas and imply a predominant seawater component for the brine with minor hydrothermal imput. In summary, this study of the geochemistry of fish debris in Red Sea Atlantis II Deep sediments provide important

data concerning their role as a sink for various trace elements including REE, U, Zr and sometimes SC, their conditions of sedimentation and the influence of diagenesis in an hydrothermal environment. Acknowledgements-We are grateful to the Saudi-Sudanese Red 8ea

Commission for authorization to work on these samples and to puhlish the results.We are also gratefulto M. Treuil, Director of “Grcupe des Sciences de la Terre” from P. Stie Laboratory (Saclay, France), where trace element determinations were perfomml by A.C. We wish to thank P. Guennoc for giving us samples from the Shaban/ Jean Charcot Deep. We also wishto thank C. Bcurdillon, J. R. Disnar, J. P. Barusseauand W. Burnett for important and fruitful discussions as well as A. M. Gallas and F. Pillard and J. L. Boulmier for X-ray and TEM analyses and mineral separations. This Paper would not have been written without the support of H. M. Dunlop who also improved the English. H. Elderlield, H. Staudigel, R. M. Owen and three anonymous persons reviewed this paper and considerably improved it by their suggestions and comments. Editorial handling: H. Elderfield

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