Rare earth elements in old biogenic apatites

Rare earth elements in old biogenic apatites

0016.7037/93/$6.00 + .oO Geochrmrca PI Cosmochimrca Acia Vol. 57. pp. 2507-2514 Copyright 0 1993 Pergamon F’ressLtd. Printed in U.S.A. Rare earth el...

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0016.7037/93/$6.00 + .oO

Geochrmrca PI Cosmochimrca Acia Vol. 57. pp. 2507-2514 Copyright 0 1993 Pergamon F’ressLtd. Printed in U.S.A.

Rare earth elements in old biogenic apatites PATRICIAGRANDJEAN-L~CUYER,I,* RAIMUND FEIST,’ and FRANCISALBARBDE”+ ‘Centre de Recherches PCtrologiqueset G&ochimiques,BP 20, 54501 Vandoeuvre-L&-Nancy, France 2Laboratoire de PaIContologie, Universite de Montpellier II, Place E. Bataillon, 34060 Montpellier, France (Received May 18, 1992; accepted in revised form December 7, 1992)

Abstract-The REE distributions in individual Upper Devonian conodonts have been measured by ion probe. The patterns of all analyzed conodonts are enriched in middle REE (Eu-Gd) and have a weak or no Ce anomaly. Concentrations and La/Yb or La/Sm ratios vary very little within or among individuals from the same zone, which suggests that uptake oflabile REE from sediments was essentially quantitative. Therefore, the REE signature of the primary carriers, probably organic and oxyhydroxides particulates from marine suspensions, was efficiently transferred to biogenic apatites and survived late diagenetic processes. REE patterns of conodonts do not resemble those of present-day seawater and post-Cretaceous biogenic phosphates, which are typically depleted in Nd and Sm with a negative Ce anomaly. Since REE distributions in the modern water column mimic those of nutrients, we assume that, in pre-Cretaceous seawater, they were not controlled by surface biological activity. We assume instead that REE in preCretaceous seawater can be explained by mechanism of desorption-adsorption on particle surfaces. Progressive extraction of LREE from river water by oxyhydroxides leads to precipitates enriched in middle REE. A simple quantitative model was calculated in order to illustrate the proposed mechanism. INTRODUCTION IN MODERNOCEANS,DISSOLVEDREE have nutrient-like behavior. This results in a near-surface depletion more marked for LREE (La, Nd) than for HREE (Er, Yb) ( ELDERFIELD and GREAVES,1982). Investigation of REE in the Chernobyl fallout show that a critical mechanism for removing REE from modem surface seawater is zooplankton grazing on wind blown detrital particulates and sedimentation of faecal pellets (primary carriers of labile REE), which results in a rapid and efficient transfer of the REE from the surface to the sediment ( FOWLERet al., 1987). However, there are alternative mechanisms for controlling the REE which do not depend on surface biological activity. These include formation of polymetallic nodules from seawater ( PIPER, 1974; APLIN, 1984; CALVERTet al., 1987) and precipitates in estuaries and rivers (BOYLEet al., 1977; ASTON, 1978; HOYLE et al., 1984; GOLDSTEIN and JACOBSEN, 1988a,b). These form iron and manganese oxyhydroxide floes from the seawater or river water, a process associated with REE adsorption. Changes in biological activity at the surface of the ocean may be expected to profoundly influence the hydrologic cycle of REE. Major radiations of plankton strongly suggest an increase of oceanic surface biomass from the late Jurassic (TAPPAN and LOEBLICH, 1973; BOLLI et al., 1985). Before this period and particularly in the late Paleozoic, the control of dissolved REE may have resulted from other biological mechanisms with a different food chain or from inorganic processes. Hence, resulting REE patterns for Paleozoic sea-

* Present address: Personnal 12, rue Jean Malo Renault 35000 Rennes, France. +Present address: ENS Lyon, Laboratoire de GCochimie, 46, all&e d’ltalie 69364, Lyon cCdex 07, France. 2507

water could be different from those of present-day oceans, which are influenced by biological surface activity ( FOWLER et al., 1987). In particular, REE patterns from old samples (fish and conodonts older than the Cretaceous) do not resemble the distribution in present-day seawater or in modem biogenic apatites. Rather, they have concave, downwardshaped REE patterns that raise major questions of significance and interpretation (WRIGHT et al., 1984, 1987; GRANDJEAN et al., 1987). Although the broad patterns of REE distributions in conodonts from various geological ages have been well established by WRIGHT et al. ( 1984, 1987), the importance of late diagenetic processes is still a matter of concern for many authors ( ELDERFIELD and PAGETT, 1986; ELDERFIELD and SHOLKOVITZ, 1987). REE analysis on conodont populations obtained by destructive IDMS or INAA methods could conceivably represent some sort of a loosely defined, stratigraphically controlled average resulting for superimposed diagenetic events. In situ REE determination by secondary ion mass spectrometry (SIMS ion probe) permits the population to be broken down into individual fragments and each fragment investigated for potential zonations. REE variability within a conodont fragment and among individuals from a same stratigraphic level places some weak restrictions on the extent of diagenetic interactions ( GRANDJEAN and ALBAR~~DE, 1989). Consequently, this work presents REE data obtained by SIMS on conodonts as a new constraint on the interpretation of REE in ancient biogenic phosphates. The primary aim is an evaluation of major control processes on marine REE during the late Paleozoic. Different mechanisms are examined to account for the REE patterns of conodonts, and finally a model of REE uptake for ancient primary carriers is proposed. Implications for REE use as paleoceanographic indicator before the Cretaceous are also considered.

P. Grandjean-Lkuyer, R. Feist. and F. Albarkde

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RESULTS Distinct individuals from populations of the upper Devonian conodonts were separated from conodont-rich limestone layers 2-3 centimeters thick sampled from the FrasnianFamennian section near Coumiac in the Montagne Noire area of southern France (FEET, 1985; BECKERet al., 1989). Coumiac is a distinct locality from Causse-et-Veyrans, whose conodonts have been studied in a previous work ( GRANDJEAN and ALBAR~DE, 1989). Three individual conodonts were investigated for their potential REE zoning: two samples from the uppermost gigas zone, and one from the lower triangularis zone of the late Devonian standard zonation (e.g., ZIEGLER, 197 1) . All appeared to be unzoned in reflected light. The REE (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb) have been measured in these several hundred micrometer conodonts on a CAMECA ion probe IMS 3f using a technique similar to that described by SHIMIZUet al. ( 1978 ). The complete analytical method is given in GRANDJEAN and ALBAR&DE(1989). As in GRANDJEAN et al. (1987), the calculation method for Ce anomaly differs from the more usual linear method of ELDERFIELDand GREAVES ( 1982). The logarithm of Ce* was chosen instead as the interpolated value, which insures that smooth REE spectra have no Ce anomaly even for strong La/ Nd fractionation. Data are shown in Table 1. Figures 1, 2, and 3 present the REE data on conodonts. All the analyzed Devonian conodonts (Figs. l-3) show a middle REE enrichment within a factor of 2 to 10 of the shales values, a maximum near EuGd, and a weak or absent Ce anomaly. Each zone may be associated with a distinctive pattern (Figs. l-3 ) and specific La/ Yb and La/ Sm ratios (Table 1) . In the uppermost gigas zone (Cou3lGb, Cou31Gc) and lower triangularis zone (Cou32A-Ba), conodonts have nearly constant REE contents (Fig. 2 ) . No significant variation of La/ Yb or La/ Sm ratios is observed from the center to the rim of individual conodonts (Table 1; Fig. 2). Frasnian-Famennian mass extinctions which correspond to uppermost gigas zone (Cou31G) are not accompanied by any particular REE anomaly. DISCUSSION Seawater or Diagenetic Imprint

The complex problem of the significance of REE in biogenie phosphates has been discussed extensively, and different models (adsorption, substitution, diffusion, percolation) and sources (seawater, sedimented particulates, porewater, sediment) for REE uptake have been proposed ( BERNAT, 1975; ELDERFIELD and PAGETT, 1986; ELDERFIELDand SHOLKOVITZ, 1987; GRANDJEANet al., 1987, 1988; SHOLKOVITZ et al., 1989). In the present work, the association of each zone with a distinctive REE pattern and the absence of prominent zoning within individual conodonts (unlike TOYODA and TOKONAMI, 1990) suggest REE uptake involving the bulk of the phosphatic debris homogeneously. It is unlikely to be a result of a progressive enrichment through percolating fluids of presumably variable concentrations. The zonal consistency of the REE distributions would probably not have survived diagenetic exchanges with the surrounding sediments, as

variations in the clay/phosphate ratios with time would obscure any pattern inherited from the marine environment. In addition, the absence of significant REE zoning in conodont individuals argues for phosphate remaining a system essentially closed to late diagenetic exchanges. These results confirm the previous interpretations from a subset of the present data (GRANDJEAN and ALBAR~DE, 1989), which proposed a quantitative uptake of nondetrital REE locally released at the sediment-water interface, considering a simple model of REE enrichment in a closed system (REE uptake from the surrounding nondetrital particulates). This takes place during early diagenesis as a seemingly unique event ( BERNAT, 1975 ) . Studies of crystallinity of sedimentary apatites ( SHEMESH,1990) confirm that biogenic phosphates are pseudomorphs after the original living bones, which probably reflect the sediment-water interface characteristics. Hence, REE in phosphates take the signature of the bottom water overlying the sediment-water interface. This signature results from a mass balance between the flux of REE from decaying organic and oxyhydroxides particulates (primary carriers with seawater signature), the flux of REE from diagenetic fluids expelled from sediments (diagenetic signature), and the flux of REE from rivers (detrital signature). Generally, late diagenetic disturbances remain of marginal importance ( VEEH, 1982; GRANDJEANand ALBAR~DE,1989), but they can be significant in epicontinental environments under regional control where the diagenetic flux and/or continental/river flux of REE are strong ( GRANDJEANet al., 1987; ELDERFIELDet al., 1990). For our samples, the environment corresponds to a distal and pelagic platform without continental detrital influence (BECKERet al., 1989) and argues against a diagenetic contribution for REE in conodonts.

Seawater-Phosphate

REE Fractionation for Old Samples

The “hat-shaped” REE distributions (Figs. 1, 2, and 3) are a constant feature of all analyzed samples and every layer considered, but they do not resemble the present-day distribution of seawater (DE BAAR et al., 1985; PIEPGRASand JACOBSEN, 1992) or of post-Cretaceous phosphates from opensea environments ( BERNAT, 1975; GRANDJEANet al., 1988; see also Fig. 4a). GRANDJEANet al. (1987) propose two hypotheses for explaining the REE patterns in biogenic phosphates before 100 Ma. First, samples could be from epicontinental platform or estuary environments. The REE patterns resemble those of precipitates in these environments formed by mixing between seawater and river water (HOYLE et al., 1984). REE patterns of biogenic phosphates from epicontinental environments are intermediate between those of seawater and those of shale or river water if regional control influences bottom waters whose phosphate REE acquire the signature (GRANDJEAN et al., 1988; ELDERFIELDet al., 1990). But none of these mechanisms appear to be responsible for the REE signature of our conodont samples, which come from a distal platform (BECKER et al., 1989). An alternative hypothesis is that Paleozoic seawater REE chemistry was different from that existing today ( GRANDJEAN et al., 1987 ) . REE patterns of present-day seawater and recent phosphates from open-sea environments are controlled to a

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REE in Devonian conodonts

TABLE1.Conodont REE data (ppm). Letters in the first column refer to individual conodonts from the same population, whereas numbers refers to different spots in the same conodont. s1ce = CeN/(LaN)2’3/(Nd.v)“3-1,where N refers to shale-normalised values Sample

La

Ce

Nd

Sm

Gd

DY

Er

Yb

n ce

La/Yb

La/Sm

2.33 3.23 1.96 2.86 2.37 3.51

13.1 19.0 10.8 15.4 11.9 19.7

6.88 9.15 4.59 8.48 6.28 9.48

1.83 2.25 1.26 2.12 1.48 2.62

1.15 1.78 0.78 1.78 1.23 2.08

0.077 0.163 0.110 0.180 0.104 0.125

10.1 11.0 11.2 6.85 7.11 7.84

1.41 1.69 1.35 1.34 1.23 1.39

2.30 5.70 4.60 2.42 6.35 3.09 2.91 3.54

12.5 25.8 19.2 12.0 29.0 13.5 15.6 16.8

4.85 14.4 10.6 6.85 14.3 6.06 9.24 11.7

1.43 3.82 3.04 1.68 4.03 1.59 2.64 2.83

0.97 2.4 2.15 1.08 3.00 1.28 1.94 2.10

0.269 0.238 0.200 0.280 0.094 0.257 -0.278 -0.224

3.72 3.66 3.98 3.47 3.73 4.09 2.28 3.07

0.43 0.42 0.54 0.38 0.52 0.51 0.42 0.52

8.00 6.25 6.40 7.00 4.78 5.29 5.04 1.60 2.90 2.19

33.2 31.3 31.2 32.0 20.9 23.8 22.4 8.25 14.6 13.0

16.4 14.0 12.8 14.3 10.7 11.3 11.0 3.95 6.38 7.41

5.69 4.4 3.61 4.12 3.24 2.92 3.08 1.33 1.77 2.35

4.03 3.04 2.63 3.28 2.53 2.29 2.41 0.82 1.29 1.56

-0.370 -0.253 -0.283 -0.189 -0.215 -0.310 -0.270 -0.096 -0.243 -0.297

3.90 3.75 4.49 3.72 3.37 3.40 3.39 3.12 3.33 3.90

0.54 0.49 0.63 0.51 0.57 0.45 0.51 0.39 0.46 0.68

38.5 37.2 37.8 28.6 46.9 53.4

21.8 19.0 20.8 15.0 28.0 29.7

6.94 6.56 6.03 4.56 9.39 8.82

4.01 4.11 3.07 2.06 5.59 4.79

-0.236 -0.232 -0.239 -0.228 -0.296 -0.254

2.18 2.05 2.50 2.98 2.13 2.48

0.28 0.30 0.27 0.28 0.30 0.29

21.1 14.7 14.8 13.4 11.8

12.2 7.41 6.37 6.61 7.23

3.42 2.19 2.78 2.33 2.27

2.04 1.47 1.67 1.22 1.47

-0.335 -0.35 1 -0.35 1 -0.208 -0.474

7.06 6.08 5.96 7.45 7.14

0.86 0.88 0.90 0.91 1.06

16.2 29.1 19.3

6.28 15.2 7.02

1.72 5.14 2.01

1.18 3.66 1.49

0.181 0.117 0.158

0.92 1.25 0.98

0.15 0.23 0.20

Eu

lower gigas zone, upper Frasnian, Cou23-24A :

19.6 11.6

66.8 37.4

45.0 28.2

11.6 8.23

C

8.77 12.2 16.3 8.75

30.0 43.4 55.0 31.1

23.4 30.5 40.1 26.6

6.48 9.07 11.7 7.10

d 5:

upper gigas zone, upper Frasnian, Cou24B a b

3.61 8.79

18.7 44.4

22.4 54.4

8.30 20.5

: e f

3.75 8.56 11.2 5.23 4.42 6.45

20.7 39.3 42.4 23.1 13.0 19.5

27.4 43.7 42.3 20.7 27.2 34.7

15.7 9.73 21.2 10.1 10.5 12.2

g h

uppermost gigas zone, uppermost Frasnian, Cou3 1G :,

11.4 15.7

32.4 35.4

65.6 57.3

23.0 28.7

bz br Cl

11.8 12.2 8.53 7.79 8.16 2.56 4.30 6.09

32.0 35.2 24.0 19.8 21.9 8.68 12.4 13.9

58.2 50.1 35.8 35.5 35.7 12.3 21.7 19.0

18.7 23.5 14.8 17.3 16.1 6.48 9.31 8.93

C2 C3

d e f

lower triangularis zone, lowermost Famennian, Cou32A-B al a2 a3

b :

8.14 8.44 7.67 6.14 11.9

28.2 26.9 23.8 19.0

60.1 55.0 47.3 36.0

30.3 27.9 28.0 21.4

35.5 32.9

69.4 65.7

40.9 38.7

7.92 7.18 1.18 6.19 11.5 10.6

middle triangularis zone, Famennian, Cou32C : :

14.4 8.94 9.09 9.96

31.2 18.6 23.3 20.4

45.3 26.9 28.1 28.5

10.1 16.6 11.0 9.89

4.98 2.92 2.83 3.04

e

10.5

16.7

26.4

9.9

2.69

upper triangularis zone, Famennian, Cou34 :

4.59 1.09

20.7 6.18

27.5 11.0

19.6 7.27

5.94 3.14

C

1.46

7.52

11.7

7.13

3.51

large extent by surface biological activity: directly for seawater and primary carriers of REE, such as organic and oxyhydroxides particulates in marine suspensions identified by RISLER ( 1984); MURPHY and DYMOND ( 1984); FISCHER et al. ( 1988); and indirectly for phosphates through the medium of primary carriers (GRANDJEAN and ALBAR~DE, 1989). The rapidity and efficiency of REE transfer by primary carriers from sea-surface to sediment (on a time scale of a

few days per hundred meters) are clearly demonstrated by studies of the Chernobyl fallout in the Mediterranean sea ( FOWLERet al., 1987). Hence, the concave downward pattern of conodonts and other ancient fish remains could well be an undocumented feature of the Panthalassa seawater. Chemical constituents in the biological cycle of the ocean (e.g., oxygen, carbonate, nitrate, phosphate, heavy metals) must have experienced the effects of evolution in the major

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P.

Grandjean-LCcuyer, R. Feist, and F. Albar6de 100

Sample REE Shales

Sample REE Shales

t

.l

-O1,LaCe.Nd

SmEuGd’Dy’

Er ‘Yb’

.Dl

Lace’

Cou24B-29B

Nd’SmEuGd

Dy

Er

Yb

._

FIG. 1. Shale-normalized (GROMETet al., 1984) REE distributions in upper Frasnian conodonts from the lower gigas (Cou23-24A) and upper gigas (Cou24B-29B) zones. The analyzed REE are shown in abscissa. Each pattern corresponds to different conodonts from the same limestone layer.

living groups. Plankton in its present form, as the start of the food chain, is apparently a recent feature of biological activity. TAPPAN and LOEBLICH ( 1973) and BOLLI et al. ( 1985) showed that phytoplankton has existed since the middle Precambrian but with little extension in the open ocean. Major evolution radiations of phytoplankton (dino-flagellata, calcareous and siliceous nannoplankton) and zooplankton (Polycystine, Radiolaria, Tintinnida), held by FOWLER et al. ( 1987) to control REE transfer from the detrital to the labile

loo

phases, began only in the late Jurassic. During the Paleozoic, surface biomass was not well developed. Moreover, benthic foraminifera existed during almost all of the Paleozoic, and some groups were eliminated during upper Paleozoic extinctions, including the major crisis at the Frasnian-Famennian boundary dealt with in the present work. Foraminifera fauna diversified rapidly and reached pelagic environments from Trias-Jurassic only. Hence, the effects of surface biological activity in REE in Paleozoic oceans combined with a food

loo

Cou32A-B

FIG. 2. Shale-normalized REE distributions in uppermost Frasnian-Famennian conodonts from the uppermost gigas (Cou3 1G ) and lower triangularis (Cou32A-B) zones. For each of Cou3 1Gb (open circles), Cou3 1Gc (closed circles), and Cou32A-Ba (open squares), the patterns have been obtained on the same conodont.

REE in Devonian conodonts

2511

loo

Sample REE Shales

Cou32C

.aCe’Nd’SmEuGd’Dy.Er

‘Yb’

FIG. 3. Shale-normalized REE distributions in Famennian conodonts from the middle triangularis (Cou32C) and upper triangularis (Cou34) zones. Each pattern corresponds to different conodonts from the same limestone layer.

chain different from that in the present-day ocean is expected to cause the behavior of the REE in Paleozoic oceans to be different from today. This deficiency of surface biomass in the past suggests that inorganic chemical processes are a probable alternative for creating REE patterns similar to the conodont patterns measured in the present work.

Our measurements

,,/,I/

I

III

are consistent with neutron activation

IIll

-LCr

Nd

SmhCd

(4

100

I

I

I

I

I

II

I

I

III

I

1

Sample.!Sbala

SamplelShalu

Dy Er Yb

O.O1 bk-% :b)



FIG. 4. (a) Comparison between shale-normalized REE patterns of conodont (bold line: this work), present-day seawater (plain line: average (600-2500 m) of North Atlantic seawater* 10’; see ELDERFIELD and GREAVES, 1982), and post-Cretaceous phosphates (black squares: Tertiary teeth from North Morocco; see GRANDJEAN et al., 1988). (b) Comparison between shale-normalized REE patterns of conodont (bold line: this work), Pacific ferromanganese. hydroxides (plain line: crusts; see APLIN, 1984; DE CARLOand MCMLJRTRY, 1992), Atlantic micronodule (black triangles: ADDY, 1979), concretion from the Barents Sea (diamonds: INGRIand PONTER,1987), river water (open circles: ~12 Mm; see HOYLEet al., 1984), and flocculatesin estuary (closed circles: Tamar estuary; see ELDERF-IELD et al., 1990), or from laboratory experiment (closed circles: salinity = 2,3%0;see HOYLEet al., 1984).

analyses by WRIGHT et al. ( 1984, 1987) on conodonts from the Cambrian to the Trias of North America and China. These include a middle REE enrichment and a weak Ce anomaly. The only difference is a maximum near Nd-Sm in their samples, which appears near Eu-Gd in our samples. Unfortunately, no Gd data are given by these authors to permit a closer comparison. There is no reason to suspect an overestimation of Eu and Gd in our samples because REE distributions obtained with the ion microprobe and by isotope dilution were mutually consistent in GRANDJEANand AL BARBDE(1989). Nevertheless, samples of WRIGHT et al. ( 1984, 1987) came from Atlantic and Pacific platforms. REE patterns of Paleozoic conodonts are also similar to these of ante-Cretaceous selacian teeth and bones (GRANDJEAN et al., 1987) and other ichthyoliths from Devonian to Trias (WRIGHT et al., 1987). Hence, this type of REE pattern seems to be a characteristic of all biogenic phosphates before 100 Ma, and the mechanism which controlled seawater REE seems to have had worldwide importance. For WRIGHT et al. ( 1987), REE in apatite probably substitute for Ca*+ with the most favorable cases for Nd3+, Ce3+, and Sm3” because of similar ionic radii. The concave, dbwnward shape of REE patterns of fossil material may be controlled partially by the size distribution of the REE and crystal chemistry. A difference of facies or provenance or amount of detrital component could explain the variations observed in shapes of REE patterns between fossil and modem samples. According to SHANNON( 1976)) the most favorable cases to substitute for Ca’+ (ionic radius = 1,12 %,in eight-fold coordination) are probably Nd (ionic radius = 1,109 8, for CN = 8) and Sm (ionic radius = 1,079 8, for CN = 8), but a combination of charge and size imbalances during the substitution (REE3+ vs Ca*+) must restrict the entry of REE into apatite. Ce3+ (ionic radius = 1,143 .,&for CN = 8) is difficult to consider for cationic exchanges with Ca’+. Moreover, these considerations cannot explain the REE patterns of our samples presenting a maximum near Eu3+ (ionic radius

P. Grandjean-L&uyer, R. Feist, and F. Albarede

2512

= 1,066 A for CN = 8 ) and Gd (ionic radius = 1,053 A for CN = 8) that is less favorable for substitutions. Middle REE enrichment is a feature commonly observed in some modern deposits (Fig. 4b), like Fe-rich nodules and encrustations from the Pacific (PIPER, 1974; ELDERF~ELDet al., 1981; APLIN, 1984; DE CARLOand MCMURTRY, 1992), nodules and micronodules ( ADDY, 1979) and Fe-Mn coatings (PALMERand ELDERFIELD,1986) from the southern Atlantic Ocean, and coatings from the Barents Sea ( INGRI and PONTER, 1987). HOYLE et al. ( 1984) report middle REE enrichment in the flocculant and precipitates from river water mixing with seawater (Fig. 4b). Therefore, precipitation of oxyhydroxides and adsorption onto these substances seem to have the capability of promoting “hat-shaped” patterns in oxyhydroxide that can be passed to biogenic phosphates during the earliest phase of diagenesis. In estuaries, the LREE enrichment relative to the solution observed in precipitates (GOLDSTEINand JACOBSEN,1988b) is associated with a corresponding river water depletion that increases with the seawater mixing ratio (GOLDSTEIN and JACOBSEN, 1988b). Such a preferential incorporation of LREE into the solid was also found experimentally by KOEPPENKASTROPand DE CARLO ( 199 1) and by KOEPPENKASTROP et al. ( 199 1) in a study of REE fractionation between seawater, on the one hand, and phosphates and Fe and Mn oxyhydroxides, on the other hand. This rather simple behavior of REE in the modem environment suggests that modelling REE uptake through inorganic processes may illustrate some critical aspects of ancient ocean chemistry. The model which will be developed assumes that “hat-shaped” REE distributions in inorganic precipitates result from ( 1) precipitate-solution fraction coefficients smaller for HREE and (2) LREE depletion in the solution increasing with the extent of entrainment. Let us assume for simplicity that inorganic precipitate is mostly iron hydroxide. Mass balance of one element with concentration C’ at the ith step of entrainment by iron hydroxide can be written as follows: Ci = (1 - X,+i)Ci+’ + X;+iOC;“, where the subscript I refers to seawater, Xi+, stands for the fraction of iron removed at the i + lth step, and D for the floe/seawater fractionation coefficient of the element. An alternative expression is Ci+’ -= C(

1

( 1 - Xi+, 1 + X+ID ’ Values for relative concentrations in seawater and floes resulting from successive steps of entrainment relative to the initial concentration Cp can be obtained by combining an appropriate number of these expressions. As precipitation increments get smaller and smaller, a distillation (Rayleigh) equation is obtained that reads c, = cp( 1 - #-I. Application of these equations has been made, assuming that REE distribution in ancient seawater results from riverine input with a shale-like REE pattern modified by oxyhydroxide entrainment. We have assumed that Log D varies linearly with the atomic number; and DLa = 1000, and &, = 10. Figures 5a,b show that even minute extents of iron subtraction

are capable of producing hat-shaped REE patterns in floes resembling those of ancient phosphates. The D values used in this model were selected for illustration purpose. They are smaller and more fractionated than the experimental values obtained by KOEPPENKASTROPand DE CARLO ( 1992) and KOEPPENKASTROPet al. ( 1991) for iron and manganese oxides in the conditions of the modem ocean. Actually, the D values are expectedly dependent on pH conditions (GOLDSTEINand JACOBSEN,1988b) and the concentration of some ligands (e.g., carbonates) that enhance the solubility of these element in seawater. Cerium is not considered in this model because the cycle of dissolved oxygen in the water column of ancient seawater, which is the most important factor affecting the oxidation state of Ce, is not well known. Once such a dependence is better known, the distribution of REE in biogenic phosphates should become a potential source of information on the chemistry of ancient oceans. Y. G. Liu and R. A. Schmitt (pers. commun.) have determined REE patterns in several hundred CaC03 oozes, chalks, and limestones deposited over the past 500 Ma in the world’s oceans. All of them exhibit similar REE patterns. Correcting for the REEs present in North American shale composite-like detritus, generally >95% of the total REEs in most intermediate and deep-sea-deposited carbonates were precipitated (or absorbed) from seawater on Fe-Mn oxyhydroxide coatings of carbonate minerals, on the minuscule phosphate grains in the detritus, and directly on the detrital clay particulates. A rather uniform REE pattern of relative abundances (La/Sm/Yb N 1.00/0.20/O. 10) was precipitated from seawater over the past 500 Ma onto the carbonate sediments. Particularly, for fourteen (380-470 Ma) limestones deposited in shallow seawater platforms (China and USA), a similar average REE pattern of La / Sm/ Yb x 1.OO/ 0. I8 / 0.10 precipitated out of seawater. An average of 78% of the total REEs precipitated out of seawater onto shallow seawater deposited carbonates. These observations suggest that the REE patterns in seawater have not changed significantly over the past 500 Ma, and that arguments for inorganic precipitation discussed above are strengthened (Y. G. Liu and R. A. Schmitt, pers. commun.). CONCLUSIONS Frasnian-Famennian conodonts show typical middle REEenriched patterns. The homogeneous REE concentration in conodonts argue against a progressive enrichment through percolating diagenetic fluids of presumably variable concentrations. REE patterns of ancient conodonts do not resemble the present-day distribution of seawater because REE distributions in the water column before the Cretaceous were probably not controlled by surface biological activity but seem to have been governed by inorganic processes. A remobilisation mechanism, such as a desorption-adsorption event on particle surfaces that depends on iron precipitation, represents a possible explanation. A quantitative model of cumulative extraction of REE from river water is proposed for creating hatshaped REE pattern. A better knowledge of chemistry and biology of ante-Cretaceous seawater is required to refine the interpretations of phosphate data.

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REE in Devonian conodonts

100

ia

4

water 2

Discontinuous

Continuous

Fe precipitation

Nd Sm Gd Dy

Er

Yb

(a)

‘O’

La

Fe precipitation

Nd Sm Gd

Dy Er Yb

@I

FIG. 5. (a) Fractionation model for two steps of iron subtraction with REE enrichment factors relative to the water concentrations before precipitation of iron. Numbers refer to the cumulated fraction. The initial shale-normalized REE pattern of the solution corresponds to river water. As a first approximation, we take shale-like relative concentrations. Details of calculations are &en in the text. (b) Continuous fractionation model with REE enrichment factors of floe expressed for different fraciions of iron subtracted.

Acknowledgments-The authors are grateful to A. Michard from the University of Aix Marseille III and to R. M. Owen, A. Halliday, J. R. O’Neil, and J. Schuffert from the University of Michigan for the comments and suggestions they offered on a review of an earlier version of this paper. Reviews of the manuscript by S. Goldstein, H. Elderfield, and E. H. De Carlo were also helpful. This work was supported by a grant from the “Programme Dynamique et Bilan de la Terre de I’Institut National des Sciences de 1’Univers.” Editorial handling: R. A. Schmitt

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