Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments

Rare earth element geochemistry of oceanic ferromanganese and associated sediments

nodules

H. ELDERFIELD,C. J. HAWKESWORTH*and M. J. GREAVES Department of Earth Sciences. University of Leeds, Leeds LS2 9JT. U.K and S. E. CALVERT Department of Oceanography, University of British Columbia. Vancouver. B.C.. Canada V6T 1W5 (Receiaed 21 July 1980: acwpptrd

in rel:ised,form

27 Noremher

1980)

Abstract-Analyses have been made of REE contents of a well-characterized suite of deep-sea (> 4000 m.) principally todorokite-bearing ferromanganese nodules and associated sediments from the Pacific Ocean. REE in nodules and their sediments are closely related: nodules with the largest positive Ce anomalies are found on sediments with the smallest negative Ce anomalies; in contrast. nodules with the highest contents of other rare earths (3 + REE) are found on sediments with the lowest 3 + REE contents and vice versa. ‘43Nd/‘44Nd ratios in the nodules (-0.51244) point to an original seawater source but an identical ratio for sediments in combination with the REE patterns suggests that diagenetic reactions may transfer elements into the nodules. Analysis of biogenic phases shows that the direct oontribution of plankton and carbonate and siliceous skeletal materials lo REE contents of nodules and sediments is negligible. Inter-element relationships and leaching tests suggest that REE contents are controlled by a P-rich phase with a REE pattern similar to that for biogenous apatite and an Fe-rich phase with a pattern the mirror image of that for sea water. It is proposed that 3 + REE concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate. Processes which favour the enrichment of transition metals in equatorial Pacific nodules favour the depletion of 3 + REE in nodules and enrichment of 3 + REE in associated sediments. In contrast, Ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions.

INTRODU~ION

direct precipitation of the trivalent rare-earth hydroxwith particulate iron colloids or other nodular phases (GOLDBERG, 1954; GOLDBERGer 01.. 1963; GLASBY.1973; PIPER, 1974a) or by an unspecified mechanism (ADDY, 1979). (2) The REE have a seawater source but are incorporated by coprecipitation after their release from biogenic carrier phases in abyssal depths, either calcite (PIPER. 1974a) or fishbone apatite (ARRHENIUSet al., 1957; ARRHENKJSand BONATTI,1965) or indeed from inorganic particles (FOMINA.1966). (3) The REE are derived from the underlying sediments, the host phases being inorganic detrital minerals. and are incorporated into the nodules by surface exchange processes (EHRLICH.1968: BENDER.1972). ides (GLASBY,1973). by coprecipitation

elements (REE) have been used ‘in several recent studies of oceanic ferromanganese deposits in order to identify possible sources of the elements and, specifically, to assess seawater contributions of metal during their formation. For example, the close similarity of the REE pattern in ridge-crest ferromanganese sediments to that of seawater has been used as evidence for the scavenging of the REE from seawater by hydrothermal oxyhydroxides (BENDERet a/., 1971; PIPER and GRAEF, 1974) whereas the similar REE patterns in a seamount ironstone deposit and in oceanic basalt led PIPER et al. (1975) to

THE RARE earth

conclude that both Fe and the REE had originated from a hydrothermal solution. The REE geochemistry of oceanic ferromanganese nodules has been investigated most recently by GLASBY (1973), PIPER (1974a), ADDY (1979) and RAN-

KIN and GLASBY(1979). The possible sources of the REE and the mechanisms of their incorporation into the nodules, based on this and earlier work, can be used to erect the following three hypotheses: (1) The REE have a direct seawater source. being incorporated into the nodular oxyhydroxides by * Present address: Department of Earth Sciences. The Open University, Milton Keynes, MK7 6AA. U.K.

PIPER (1974a) also proposed a polygenetic origin whereby Pacific nodules shallower than 3000-3500 m. (the approximate depth of the lysocline) gained their REE directly from sea water whereas deeper nodules gained their REE from dissolving biogenous calcite. Many of these proposed mechanisms are speculative since they rely almost exclusively on the shapes of REE patterns, whereby the individual REE contents are normalized to average chondrite (see MASUDA, 1962; CORYELLet ul.. 1963) or average shale (see PIPER, 3974b). Little information is available on the relationships between the individual REE concen-

513

H. ELDERFIELD et al

514

nations and other nodule components; furthermore. information on the distribution of the REE in marine sediments, with which we could assess the role (if any) of the associated sediments in the accumulation of REE in nodules. is lacking. In this paper, we report the concentrations of La Ce, Nd, Sm. Eu. Gd. Dy. Er and Yb in a well-characterized suite of deep sea I > 4000 m) ferromanganese nodule and associated sediment samples from the Pacific Ocean. We pay particular attention to samples from a small 230 km2 area within the siliceous clay belt of the northern equatorial Pacific. For comparative purposes. we also report the REE contents of bulk plankton, and siliceous and calcitic biogenous debris from some pelagic deposits. In addition. we have a!so made measurements of the ‘43Nd/144Nd and *‘Sr/*‘%r isotopic ratios of the nodules and sediments. Because of the marked fractionation of Sm/Nd between continental and oceanic crust and the alpha decay of 14’Srn to 143Nd, the source of the REE in the nodules and the genetic relationship between these elements in the nodules and the sediments can be established using the ‘43Nd/‘44Nd ratios. The differences in 87Sr!86Sr ratios of seawater ( -0.709) and detrital matter of continental origin ( -0.716) and of volcanic origin ( -0.703) allow us to comment upon the presence of mineral detritus in these samples.

MATERIALS AND METHODS Samples of nodules and sediments were obtained from the Scripps Institution of Oceanography. They were subsamples of the powdered material used for studies of the major and minor element geochemistry of the deposits by CAL~RT and PRICE (1977a) and CAGVERTPI al. (1978). Sample locations and data on sample characteristics are given in Table 1. The REE were analysed by mixed-solvent ion exchange and mass spectrometric isotope dilution in a manner similar to that described by H~OKER et al. (1975). Isotope analyses were performed on an A.E.I. MS5 mass spectrometer with online computer facilities. Approximately 20 sets of measurements per element were made on each sample. Precisions were generally +2”,, (and often k lo,). except for La ( f5”,). Eight replicate analyses of USGS standard rock BCR-I gave the following mean values Table

1.

Description

Sample Number

WAH 13F8 WAH 18Fl WAH 18F8 WAH 24F2 wm 24~6 WAH 2PG DOD0 20C/20PG TRIP 9G JYN II 8~ AMPH 85~~

and locations

Principal Xn mineral phase of nodule todoroki todoroki todorokite todoroki todoroki todoroki todoroki todoroki S-Mno~ s-Mnoz

te te te te te te te

(ppm): La. 23.X: Ce. 53.7: Nd. 29.2; Sm. 6.56; Eu. 1.93; Gd. 6.63: Dy. 6.31; Er. 3.67; Yb, 3.34 (see, for comparison, FLANAGAN. 1973: HOOKER et al., 1975). “‘Nd/‘44Nd ratios were measured on a VG Micromass 30 mass spectrometer with online computer facilities following a chemical separation procedure similar to that outlined by O’Nro~s rt al. (1977). with a total blank of 4-7 x lo-” g. Samples were loaded onto triple filaments and run as metal species; 800-1200 ratios were measured on each sample and normalized to a ‘4hNd/‘44Nd ratio of 0.7219. In the absence of an accepted Nd isotopic standard, samples of BCR-1 were run. yielding a mean ratio of 0.51266 + 2. “Sria6Sr ratios were measured on the same mass spectrometer following conventional ion-exchange separations. 25&350 ratios were measured on each sample and normalized to a ?ir~?Sr ratio of 0.1194. Measurements of the Eimer and Amend and NBS-987 standards have given mean ratios of 0.70807 i I and 0.71026 k I and I.A.P.S.O. standard seawater has given 0.70916 + 2. Errors in the ratios are quoted at 95’; confidence limits (2a). A single nodule sample (WAH 18F8) was used to measure the REE composition of separate nodule phases. The powdered sample was leached with 0.1 M HCI at a ratio of 1:4 by weight at room temperature for 140 days after which the leachate and the residue were separately analvzed. In addition, determinations of Fe. Mn. Cu. Ni and -Co (atomic absorption spectrophotometry) and P (calorimetry) were made on the leachate. The accuracy of the atomic absorption analyses was assessed by reanalyzing bulk nodule sample WAH 13F8 with the following results [XRF results of CALVERTet al. (1978) given in parentheses]: Fe, 4.879, (4.97”;); Mn, 24.84; (26.79;); Cu. 1.33”; (1.27”;); Ni. 1.27’,;, (1.05”,), Co, 0.129”b (0.11”;). Repeated checks showed that the P content of this sample reported by CALVERTet al. (1978) is too high and we have used the calorimetrically determined value in this paper. RESULTS The rare earth element concentrations of the nodules and their associated sediments, the isotopic data the REE compositions of biogenic phases and the results of the element partition studies arq given in Tables l-6. Methods

of‘ interpretation

In order to evaluate the results. it will be necessary to consider similarities in the behaviour of the REE in the nodules, sediments and other marine phases before discussing the difirences between individual of

nodule

Sediment description siliceous ooze siliceous ooze siliceous ooze siliceous ooze siliceous ooze siliceous ooze siliceous ooze pelagic clay glacial siit ridge-f Lank clay

and sediment

Depth (m) 5125

5180 5034 5038 5082 5221 5280 $175 4250 5338

samples

Lat.

Long.

8O20’N S”:O’N 8”?O’Y _ L 8O70’1’ ao;o’N LlO51’N LO~OO’Y 20058’1: GO029’N Ll”35’S

153”OO’W 153”OO’W 153”OO’W 153~OO’W 153OOO’W 152”57’W 167O5O’W 118~01’W 172O33’E L5803L’w

Rare earth element geochemistry elements and .between the different samples. Almost all previous studies of the REE in the marine environment have been concerned with the patterns which have been obtained by normalizing the results to a standard on an element-by-element basis. Recent studies (PIPER, 1974a; ADDY, 1979) and reviews (PIPER, 1974b; CALVERT, 1978) have used average shale as the standard, this having similar REE contents to most marine phases, thereby allowing small differences in the fractionation of one REE from another to be identified graphically. In this paper we shall use measured concentrations of REE to examine their relationship to the major element composition of the nodules and sediments. Where necessary, chondrite normalized patterns will be used since they emphasize the coherence of the REE group as a whole. In addition they provide a visual appraisal of the accuracy of the data from the ‘smoothness’ of the plots (apart from Ce and Eu) which cannot be obtained from shale-normalized patterns. After examining the behaviour of the REE as a group and their enrichment in the different nodular phases, we shall discuss the fractionation of the REE one from another using shale-normalized patterns and by the more sensitive method of normalizing nodule compositions to those of their associated sediments. REE enrichments The mean REE contents of nodules and sediments are compared with the contents of average shale, continental and oceanic crust and chondrite in Fig. 1.

Ce Gd

Fig. 1. Average concentrations of REE in the nodules and sediments each normalized to concentrations in average shale (PIPER, 1974b). Normalized concentrations in chondrite, ocean crust and continental crust (TAYLOR. 1972)are also shown.

515

Like several other minor elements which have been studied extensively (Cu, Co, Ni. As, Pb, Zn), the REE are. on average. enriched in deep-sea ferromanganese nodules compared with pelagic sediments which are themselves enriched in these elements compared with average shale (and nearshore sediments). These enrichments, alone, make it extremely unlikely that the REE content of pelagic sediments is controlled simply by the presence of landderived detritus, as suggested by EHRIJCH (1968). Studies of the geochemistry of deep-sea sediments (see ELDERFIELD, 1977, for a recent review) have shown that those minor elements which are predominantly of continental origin have similar concentrations in deep-sea and near-shore sediments and are not enriched in nodules, whereas those elements which are enriched in deepsea sediments contain a substantial ‘hydrogenous’ component (GOLDBERG,1954) derived either directly from seawater or from metal-rich colloids supplied by rivers or. in some instances, derived via marine hydrothermal activity. As shown previously by GOLDBERGet al. (1963) and PIPER (1974a), the behaviour of Ce is different from that of the other REE in ferromanganese nodules (Fig. 1). It is markedly enriched compared with its REE neighbours. which can be attributed to the mechanism originally suggested by GOLDBERG(1961) whereby Ce 3* is oxidized to tetravalent CeOl and separated from the trivalent members of the group (3 + REE). Although the absolute concentrations of each of the 3 + REE in the nodules studied here differ from sample to sample (Table 2; Fig. 2c), the concentrations of one REE relative to another, in the equatorial nodules, are virtually constant (Fig. 2a). This observation also applies to the sediments (Fig. 2b) and contrasts strongly with the behaviour of Ce. In the nodules, there is a positive Ce anomaly (compared with the value obtained by interpolation between normalized La and Nd) but of variable magnitude. On the other hand, there is a negatioe Ce anomaly in the sediments, also of variable magnitude. A comparison of the relative enrichment of Ce compared with the 3 + REE in nodules and sediments also shows distinct differences. The absolute concentrations of Ce in the nodules are always higher, by factors of 3 to 4. than in the associated sediment (Table 2). On the other hand, although the contents of the 3 + REE are. on average, higher in the nodules, the contrast is much less constant; for example, the samples include one nodule/sediment pair where the concentrations in the sediment are higher than those in the nodule (Fig. 2~). Furthermore, there appears to be a regular relationship between the 3 + REE content in the nodules and sediments which is not found with Ce (Fig. 3). Nodules with the highest 3 + REE contents are found on sediments with the lowest 3 + REE contents and vice versa. Although there is no relationship between the Ce contents of the nodules and sediments, the magnitudes of the Ce anomalies in the REE patterns of the

Table

2.

concentrations

at

rare

earth

elements

cppml

3nd

Fe.

Yn ~“a

P (%I

in

nooules

and

associated

sediments

Sample

number

TYPe

Fe

!ln

P

?l!l,‘Fe

Fe!P

La

Ce

ia

18* 90.3

102 121

23.3 31.:

5.97 8. ~-

23.0 33.0

450 103

lb5 94.0

51.3 23.9

9.5: 5.77

107 101

23.9 25.3

IUXilJle sediment

q.97 4.55

‘6.7~ 0.31

0.I, 0.30

5.38 0.37

35.5 !5.1

66.3 27.6

WAH 18Fl

nodule sediment

9.7-l 4.51

-i.u_ 0.3i

0.16 0.21

2.h0 0.08

Si.8 ‘1.5

112 6u.5

WAH 18F8

nodule

5.74

26.~3

0.14

i.00

il.0

78.3

sediment

4.48

0.43

0.25

0.10

li.Y

77.9

218 90.2

nudule sediment

6.3: 3.2_?

25.5; 1.05

u.15 0.23

i.01 0.33

42.5 14.0

85.6 61.8

JO1 91.3

WAN 13F8

WAH 24F2

Fr

Yb

72.6 31.2

12.1 17.8

11.0 li.l

36.1 25.8

35.1 24.0

18.6 14.i

17.1 13.6

5.59 b.:i

24.3 24.9

22.2 25.3

12.1 14.9

11.0 12.3

28.0 21.9

6.84 i.34

27.3 22.:

24.5 21.8

12.8 12.x

11.5 12.0

135 103

32.; 25.0

8.02 6.23

31.9 26.0

30.2 26.0

L5.7 14.7

lq.3 14.2

210

48.6

22.8

20.:

Sm

I:5 38.+

EU

Gd

OY

zodule

6.93

26.58

0.15

3.8,

46.2

95.5

sediment

3.74

0.58

0.26

0.16

1h.a

b4.3

113 90.1

nodule sediment

11.80 4.90

18.10 0.57

0.21 0.19

I.53 0.12

56.1 25.8

152 52.3

49.6

i2.3

93.0

72.4

16.7

i. 14

18.7

16.7

9.89

9.13

WDU ?OC/POPC

noaule sediment

Y.bO 3.70

17.00 0.58

0.19 0.17

i.?’ O.:b

50.5 11.8

119 i7.3

660 78.5

170 hr.8

39.8 16.0

J.il i.a9

38.9 -

36.5 16.0

‘0.h 9.49

19.4 8.10

TRIP

nodule sediment

a.50 5.40

21.70 1.90

0.13 0.10

2.55 0.35

56.: 54.0

LLi8 41.9

529 150

156 65.8

37.6 15.4

X.85 3.5;

35.1 14.3

29.1 13.1

14.4 7.23

:3.2 6.54

nodule sediment

11.40 1.60

13.20 0.0*

0.15 0.11

1.16 0.03

76.0 14.6

:04 18.:

521 35.0

li5 22.2

43.0 4.85

10.0 1.13

+O.M 4.35

32.2 4.09

15.1 2.19

13.0 1.92

nodule

16.40 10.90

lb.00 7.10

0.33 0.43

0.98 0.65

49.7 25.4

161 105

1098 465

189 161

9.03 8.05

*2.3 39.3

40.1 36.7

23.5 22.4

21.6 19.:

WAH 2SF6 WAR 2PG

9G

JYN II

UT’H

8G

85PG

sediment

823

. I l

3 ’

I

La

WAH

:1.1

40.6 35.9

13F8

WAH 18F1 WA” 18F8 WAM24F2 WAH24F6 KAH2PG DODO2OC

PG

I

Ce

Nd

Sm

Eu

Gd

DY

Er

Yb

Fig. 2. REE patterns of equatorial Pacitic nodules and associated sediments. (a) and (b) show patterns normalized to chondrite and to Er = I: (c, show chondrite-normalized patterns for samples wth highest and lowest REE contents of this group.

517

Rare earth element geochemistry

200

loo

50 . -0

s e .E E :: 20 .

10 -

5-

5

10

20 ppmm sediment

50

loo

Fig. 3. Relationships between contents of REE (other than Ce) in equatorial nodules and contents in associated sediments.

nodules and sediments are related (Fig. 4). Thus, nodules with the largest Ce anomaly are found on sediments with the smallest negative Ce anomaly. Although all nodules show a positive anomaly and all sediments have a negative anomaly, the positive atiomaly in the nodule is largest where the anomaly in the sediment is most positive.

-120

It is not strictly correct to consider inter-relationships of REE in nodules and sediments from the nonequatorial samples alongside those of Figs 3 and 4 since the sediments each have distinct mineralogical compositions (see CAL~ERTand PRICE, 1977a; CALVERT et al., 1978) which differ from that for the equatorial Wahine area. Neverthdess, it is interesting to

-30 -90 -60 Cs anomaly in sediment tppm)

0

Fig. 4. Relationship between Ce anomaly in REE pattern or equatorial nodules and anomaly in pattern of associated sediments.

H. ELDERFIELD
51x Table

3.

5d

2nd

St' Isotope

rat:"s

noauie

O.jl.!iL

l

;

<>.7091L

+ 5

LIAH lYF5

sediment

~).jl?i3

;

3

o.:109ii

;

4

WAN JYS

nadule

0.51?.+38-+

8

0.70911

;

5

sediment

0.512&38

28

O.XI9i3

+ 5 _

WIH

18FX LPG 11 %

Z

note that the samples from 21 N {TRIP 9G). where the nodule contains todorokite, show a close fit to the 3 + REE relationships found for the equatorial nodule,/sediment pairs (where the nodules also contain todorokite) whereas the other non-equatorial samples. where the mineralogy is 6-Mn02. do not.

MECHANISM

OF INCORPORATION

OF REE INTO FERROMANGANESE NODULES

The clear relationship between the REE contents of ferromanganese nodules and their associated sediments places some constraints on the possible mechanisms of incorporation of the REE into the nodules. The mechanism of direct precipitation from seawater is dearly over-simplistic and the surface transfer of the REE from continental detritus and:or the occfusion of detrital material into the nodules are equally unlikely. The s’Sr,%r ratios in the nodules (Table 3) are close to that of seawater. showing that they contain only small amounts of detrital material. Because of the much lower REE contents of continental and oceanic crust as compared with nodules (Fig. 1). it is impossible for the REE contents of the nodules to be determined by the presence of occluded detritus. in the nodules are The i*ANd.:‘J4 Nd ratios -0.51244. an identicai value to that found in two previous studies (O’NIONS er ul.. 1978: PIEPC~AS PI LII.. 1979). The similarity of this ratio to that of metalliferous sediments, where the REE are commonly assumed to have a seawater source (BENDER zr al.. 1971: PIPER and CRAEF. 1974). led O’NIONS er ~1. (1978) to argue that it also reflected the isotopic composition of seawater and that the REE in nodules are derived from seawater. PIEPCXAS ef trl. 11979) compared their nodule compositions with that for Pacific seawater and. finding good agreement. concluded that the REE are derived by precipitation from seawater. The data in Table 3 show that the REE in sediments of variable mineralogical and Sr isotopic composition. as well as in the nodules. have a seawater source. Hence. the Nd isotopic compositions point to an or& inul seawater source for the REE in both nodules and sediments: but we suspect that the relationships between the nodule and sediment REE compositions are best understood by dingenetic reactions in the sediments which are responsible for the transfer of these elements into the nodules. In understanding of

this process requires the identification of the carrier phase or phases in the sediments and the nodules. Since the REE in the sediments and nodules are derived from seawater. they most probably reside in authigenic phase, or phases. or are adsorbed on a reactive surface. GLASBYt 1973) and PIPER (1974a) suggested that the REE reside in the basal layers of Fe,Mn minerals in the nodules but there is no supporting evidence for such a suggestion. Information on the nature of the phases responsible for REE uptake in nodules and sediments can be obtained by examining the relationship between the concentrations of the REE and the major elements in the deposits and by measuring the REE contents of separated. operationally defined. authigenic phases. We have used both approaches. Comparisons of the REE and major element concentrations strongly imply that two phases are important in controlling the REE geochemistry of the equatorial nodules and sediments. The phase controlling the trivalent REE in the sediments is ~~~)s~~~ur~~. as is shown by the significant positive correlations between the individual REE and P concentrations (Fig. 5). The regression lines all have zero intercept, showing that a single phase controls the contents of the 3 + REE. This also confirms the suggestion made earlier that trivial portions of the REE are associated with alumina-silicates in these particular sediments. An additional phase which couid affect the REE geochemistry of the sediments is the mineral barite. PIPER (1974b) has shown that barite from Pacific sediments may contain up to 2OOppm La and. since barite may be abundant in some sediments, it could locally contribute a signi~cant fraction of this REE to peiagic sediments. In the present case. Ba levels in the sediments range from 1580 to 8OOOppm. equivalent to 0.5-I#,, barite. The maximum possible conIribution of La would therefore be only 2.7 ppm and can be eliminated as an important source. In contrast to the behaviour of the 3 f REE. Ce and P concentrations are poorly correlated in the sediments IFig. 5). In the nodules. on the other hand. Ce and Fe concentrations are significantly positively correlated (Fig. 6) and the data for the sediments fall on the same regression line. Hence. Ce is probably present m an iron phase in the sediments which may have a similar composttion to that in the nodules. Note that the regression line in Fig. 6 does not have a zero intercept, so that this relationship is a complex one. The distribution of the 3 + REE in the nodules appears to be mow complex than that in the sediments Their concentrations correlate both with P and with Fe (Fig. 7). Moreover, nerther regression line has zero intercept. These relationships are consistent with the presence of two REE-carrying phases in the nodules. a phosphatic phase. possibly like that in the sediments. and an iron phase. possibly an oxyhydror;ide which would provtde an stlicient scavenger of such species in seawater.

Rare

earth element

geochemistry

0.2

0.1

519

0.3

wt % Phosphorus

Fig. 5. Relationships between REE and P in equatorial Pacific sediments.

The existence of two separate carrier phases in the nodules is supported by two further lines of evidence. Using the P contents of the nodules and the relationship between the 3 + REE and P in the sediments, the concentrations of the 3 + REE associated with the phosphatic phase can be calculated. When these concentrations are subtracted from the total REE contents, the concentrations of the remaining 3 + REE are positively correlated with Fe, with zero intercept (Fig. 7~). Thus, the 3 + REE geochemistry of each nodule can be described simply by the relative proportions of these two phases. Both phases are important in the suite of equatorial nodules analyzed (Table 4). Nodules with higher REE contents (Table 1) appear to have a greater proportion of these elements in theFe-phase compared with nodules having lower REE contents (Table 4). This is a consequence of the high Fe/P ratios of the nodules, even though the concentration of the 3 + REE in the phosphatic phase is higher than that in the iron-phase (Nd/P -0.042 compared with Nd/Fe -0.001). The second piece of evidence for the existence of two carrier phases in the nodules comes from a leaching experiment. Although chemical partition studies often yield equivocal results, the method used here appears to have partially separated the two phases (Table 5). The acid leachate is strongly depleted in Fe and enriched in P compared with the residue. The REE contents of the leachate are very similar to those of the phosphate phase inferred from the inter-element relationships (Fig. 5). Table 5 shows that slightly more than half the 3 + REE are present in the

leached component (on average 64 f 5% compared with an average of 58 + 6% derived by calculation) whereas very little of the Ce is removed with the phosphatic component. Evidently, the phosphatic phase is chemically relatively more labile than the Fe phase. CALVERTand PRICE(1970. 1977ab) have previously suggested that Fe and P are closely related in ferromanganese nodules, perhaps as a ferriphosphate phase. However, the variation in the REE with that of the Fe/P ratio argues for the presence of a discrete phosphatic phase. An obvious candidate is fish debris, which was first suggested as a major carrier of the REE in marine sediments by ARRHENKJS et al. (1957). Published analyses of fish debris show that it contains appropriate concentrations of REE to account for the relationships observed here. For example, DYMOND and EKLUND (1978) report concentrations of 2500 ppm La in fish debris from Bauer Basin sediments which, at a P20S content of 35%, yields a La/P ratio within a factor of 2 of that inferred from Fig. 5. In addition, BERNAT(1975) reports Sm contents of around 1000 ppm in fish debris from equatorial Pacific sediments; no phosphorus contents are quoted but a level of 35% PZ05 yields a similar REE/P ratio to that inferred from Fig. 5. We do not wish to imply, however. that the phosphatic phase in all ferromanganese deposits might be a discrete one. Both hydrothermal sediments and shallow-water nodules show significant Fe-P correlations with zero intercepts (BERNER,1973; CAL~ERT and PRICE, 1977b); these results have been explained by scavenging of P from seawater by hydrothermal Fe

H. ELDERFIELD rr ui.

1250 -

. equatorial - equatorial

nodule sediment

‘! non-equatorial .‘.’ 1000

nodule

,’

: E 750 .3 ;; 0 E

-

:: 500

-

:’ c

,<’ : ? I’ : ,?’ /

2

,: i. l

c

: :

3

-4 , 250

.J /

J 1: todorokite

3$

‘. 6 -MnOn

1

o0

5

I

I

10

15

20

wt 46 Iron

Fig. 6. Relationship between Ce and Fe in the nodules and sediments (including data for additional d-Mn02-rich nodules from CALVERT and PRICE.1977a).

oxides and by ferriphosphate formation, respectively. However, the Fe-P correlation in oceanic ferromanganese nodules (CALVERTand PRICE. 1977a. Fig. 6) is less well defined than that for the shallow water varieties. Moreover, the Fe/P ratio for equatorial Pacific nodules ranges between 36 and 58 (Table 2; CALVERT et al., 1978), lower than the average ratio of -60 for Pacific ferromanganese nodules in general. These observations support the idea that a discrete P phase is present in the equatorial samples. However, it is also realistic to argue that P is associated in part with Fe. Thus, Fe and P covary in the nodules giving a Fe/P ratio of 68 and an intercept value of 0.0439; P. Possibly the P correlated to Fe is present as adsorbed phosphate and the residue is present as a discrete phosphatic phase. A further factor of importance is suggested when the relationships between the concentrations of the REE and the major elements in the non-equatorial nodules are considered. Only the data for the nodules containing todorokite as the principal manganese mineral phase fall on the regression lines (Figs 6 and 7) found for the equatorial nodules (which themselves contain todorokite). In contrast, the nodules which contain 6-Mn02 as the principal manganese phase show different relationships to the variations in Fe and P. The Ce and Fe contents (using additional data from CAL~ERTand PRICE, 1977a) do not covary and these nodules are depleted in Ce compared with todorokite-bearing nodules at equivalent iron concentrations. The limited results we have available for the 3 + REE suggest that Fe and P contents exert much

in their concentrations control over b-MnOz-bearing nodules than in nodules containing todorokite. For example. in contrast to the results in Fig. 7. the P contents of the two 8-MnOz-bearing nodules analyzed here differ by a factor of 2 yet their contents of intermediate REE (Sm, Eu, Gd) are almost identical. The presence of todorokite in oceanic ferromanganese nodules has been associated with diagenetic remobilization processes in the associated sediments whereas &MnO? is thought to form by direct precipitation from seawater (PRICE and CALVERT, 1970). Thus. it is only in nodules where a diagenetic influence has been suggested that distinct relationships between REE and P and/or Fe are seen and where, as discussed earlier. 3 + REE contents in

less

nodules and sediments

are related.

REE CONTENTS OF BIOGENOUS COMPONENTS

We have analyzed some samples of plankton and skeletal detritus from pelagic sediments for the rare earths in order to define the possible contributions of such elements from biogenous sources. Bulk plankton. which we collected by net from tropical Pacific surface waters (5’S 105-W). contains extremely low levels of REE (La, 0.135 ppm: Ce. 0.229 ppm; Nd. 0.030 ppm and Dy. 0.008 ppm) with a tendency to be relatively enriched in the light rare earths when compared on a chondrite-normalized basis. The concentrations reported above are for bulk planktonic material dried at 60’ C. and are therefore significantly

Rare earth element geochemistry

2” __-

H. ELDERFIELD

Table

i.

Partition of 3+REE between Fe-rich and P-rich the REE-P relationships in the sediments and Fe/P

Sample WAK 18 Fi (Nax.Fe/P

ratio)

average 01 WAHINE nodules WAH 13 FH (min. Fe/P ratio)

Iable

5.

Distribution by leaching

Sample unleached nodule residue aftor leaching* leachare**

% of element leached ? of

et al.

58

ppm in Fe-rich phase ppm in P-rich phase 4 in Fe-rich phase

$9

% in

36

ppm in Fe-rich phase ppm in P-rich phase % in Fe-rich phase

Fe-rich

of REE, transition with O.lM HC1

Nd

Sm

Eu

Gd

Dy

46.3 45.7 59

98.7 66.3 60

26.6 14.7 64

6.18 3.34 65

18.7 17.G 52

25.2 14.9 58

jj

53

ij

53

i:

i9

26.3 40.0 38

44.0 58.0 43

10.4 12.9 45

3.05 2.92 51

7.8 15.2 34

9.6 13.0 42

and major

Yb

i.20

1.11

78.3

218

107

23.9

5.59

24.3

22.2

12.:

11.0

1.33

0.97

37.1

257

55.4

ir..

3.:0

Ii.1

10.2

5.04

4.99

0.17

a.50

17.2

14.5

59.2

Lb.2

3.:i

15.6

15.0

a.44

7.6J

0.20

66

0.16

lea

-

WAH 18FtJ as determined

ET

0.14

0.56

4.05 6.95 37

Dy

0. li

0.01

4.02 8.08 33

Cd

26.4

O.00022

44

Eu

3.?L

0.090

&2

Sm

CU

0 .ObO

9.16 7.94 54

Nd

Co

26.7

9.37 9.23 50

Cr

P

0.147

in nodule

Yb

La

Mn

6.34

alemencs

Er

Ni

Fe

0.00077

derived from the nudules

La

phase

metals

phases ot noduies the P contents of

14.2

45.1

60

6.7

55

59

66

64

68

70

69

_

_

51

-

54

54

52

61

60

6i

63

eirmenc

in P-rich phase from inter-element relacianships

0

Concentrarions of REE in ppm. other concns. in vt.%.Fe, %I, P, Co, Cu. Ni data for unlsached nodule from CALVERT et al. (1978).* conLents as fracrlons of WC. of residue: ** conrxn~s normalized to wt. of nodule ii.@, concn. in leach soln. x WC. of ieach soln./ut. of nodul+.

lower than the values reported by TUREKIAN et ul. (1973) which were for ashed material. Moreover, our results are consistent with the measurements of the fission-product nuchdes r4’Ce and ‘“‘Pm in zooplankton reported by SUGIHARA and BOWEN (1962). The analyses of separated plates of the diatom Ethmodiscus rex from an Indian Ocean core sample and of cleaned foraminifera from an eastern Pacific core sample are given in Table 6. The diatoms and foraminifera have markedly lower REE concentrations compared with the sediments studied here. Note that the foraminifera analyzed were carefully cleaned to remove adhering iron-rich staining, this probably accounting for the much lower contents than those obtained by SPIRN (1965). A radiolaria-enriched fraction separated from a sample of the Wahine area sediments was also analyzed and found to show no

;abie

6.

Sample

ioraminirrra iiatom

Rare

earfh

elements

(ppm)

in

blownow

RARE EARTH ELEA%~ENT ~RA~ONA~ON The fractionation among the rare earth series in different marine phases has proven useful for identifying likely sources of these elements in such phases and the technique used, as discussed previously. has been the evaluation of normalized-REE patterns, We shall discuss only the trivalent REE eiements here in view

marine

Location Pa&ii< Ocean irId sn ocean

enrichment of REE as compared with the bulk sediment. We conclude, therefore. both from the concentrations of the REE in these biogenous ph,ases, and their chondrite-normalize patterns, that the contribution of the REE to sediments and nodules by direct skeletal or planktonic input can be ruled out.

La n”j9.9’S107’00.S’U* ::*?l.i’S6ba2i.?‘E’

* hana-picked from O-5 cm gectlon of stained tests rejected - plates or rchnad~scus rex separated (tl
graviry irsm

1.28 5.7: core +a-!

SX1599 diatom-ricn

phases ce 0.355 5.21

Nd

Sm

1.00 5.71

0.199 1.56

(ELDERFIELD, band

ac

EU O.ObOi 3. a*

1976). 120

cm ai

cleaned graicy

Cd

DY

Er

Yb

0.29

0.33 0.17

o.ra

;.?s

!.69

0.8S9

ultrasonically
CC:0

0.998 and

Rare earth element geochemistry

LaGa

WC4

ml!

L”

(ia

D”

cr

70

Fig. 8. Shale-normalized REE patterns, (a) Nodules, (b) Sediments, (c) Foraminifera, (d) Seawater (X10’; concentrations using data from H~~DAHLet al., 1968).

of the marked fractionation

of Ce from its neighbours in marine deposits. PIPER (1974a), in an important study, showed that the shale-normalized patterns of oceanic ferromanganese nodules appeared to fall into two groups: deepwater nodules (>3OOOm) showed heavy REE depletions relative to the intermediate REE and shallow-water nodules (~3000 m) showed heavy REE enrichments. The fractionation of the light REE was less clear-cut; most deep-water nodules were also depleted in light REE so that the patterns were ‘convex’. On the other hand, some shallow-water nodules showed ‘concave’ patterns where the intermediate

523

REE were depleted relative to the light and heavy members of the series. The shale-normalized patterns for the nodules studied here fall, appropriately, into Piper’s deep-water group, with ‘convex’ patterns (Fig. 8a). Moreover, the same pattern is exhibited by the sediments (Fig. 8b). This is at variance with the view of GLASBY(1973) that there is no similarity between the REE distribution in nodules and in pelagic sediments. Like PIPER(1974a), we are impressed by the mirrorimage relationship between the REE patterns of deepwater nodules and seawater. This has led to the suggestion that the fractionation of the rare earths during their incorporation into the nodules may be chiefly responsible for the seawater pattern. However, the mechanisms for the fractionations suggested by PIPER(1974a) are not supported by our data. Because the depth at which the REE patterns of nodules change is close to the foraminiferal lysocline and because the foraminiferal shells analyzed by SPIRN (1965) showed a depletion of the heavy REE, PIPER (1974a) proposed that the release of REE from dissolving biogenous tests and their coprecipitation with Fe and Mn phases may explain the REE patterns of the deep-water nodules. However, the shalenormalized pattern of biogenous calcite obtained here (Fig. 8c), which we emphasize was scrupulously cleaned, does not show a depletion in heavy relative to intermediate REE. On the contrary, the calcite pattern is indistinguishable from that of seawater (Fig. 8d). We conclude, therefore, that the REE pattern in forams merely reflects the seawater pattern and that the dissolution of calcite tests cannot be responsible for the pattern observed in nodules or in seawater. From the earlier discussion about the probable phases controlling the REE geochemistry of nodules and associated sediments, we suggest that the REE patterns of principally todorokite-bearing ferromanganese nodules merely reflect the proportions of the Fe-rich and P-rich carrier phases in the nodules. Figure 9 shows the patterns of these two phases obtained from the inter-element relationships (Table 4) and from the leaching experiment (Table 5). Although both patterns are ‘convex’, there are significant differences. The phosphatic phase shows a strong depletion in the light REE and only a weak depletion in the heavy REE. This pattern is similar to that of fish debris (Fig. SC).In contrast, the Fe-phase shows a much stronger depletion in the heavy REE. Consequently, the REE distribution pattern of the nodules relative to the sediments shows a distinctive, linear pattern with a strong depletion in the heavy REE (Fig. 9d). Since the REE in the sediments are associated exclusively with P, the gradient shown reflects the proportions of the Fe-rich and P-rich phases in the nodules (Fig. 9e). Note also that FOMINA (1966) found that abyssal nodules were enriched in light REE leaving the associated pelagic oozes relatively enriched in heavy REE. Also, in relation to the codicil

that the P may be present as adsorbed- and apatite-

H. ELDERFIELD er al

08-

20

r

1: --===T--p: t 07L

1 La

Nd

Sm

Eu

Gd

f-h

d

e

J Er

Yb

Fig. 9. REE patterns. (a) Shale-normalized patterns of Fe-rich and P-rich phases deduced from mterelement relationships (Table 4). (b) Shale-normalized patterns of Fe-rich and P-rich phases inferred from leaching experiments (Table 5). (c) Shale-normalized patterns of fishbone apatite (BERNAT,1975). (d) average content of nodules normalized to average content of sediments. (e) average content of Fe-rich phase normalized to average content of P-rich phase (using results of Table 4).

phosphate. it follows that the sites binding REE in nodules may be of two types: (a) a phosphatic phase composed of fish debris and/or recrystallized REErich biogenic apatite occluded by the Fe-Mn oxide matrix where. because the heavy REE have smaller ionic radii than the light REE, they more easily substite for Ca. and (b) a surface layer of phosphate chemisorbed by hydrous Fe-oxides which then interacts with REE with a weaker binding for the heavy REE than for the light REE because of steric effects or because of the greater stability of dissolved complexes of heavy REE. The REE pattern for seawater is characterized by a strong enrichment in heavy REE (Fig. 8d). From the

patterns shown in Fig. 9, we conclude that it is the Fe-rich phase rather than the P-rich phase of the nodules which produces an inverted seawater pattern. In this connection, it is possible that the higher REE concentrations and the pattern for foraminifera obtained by PIPER (1974a) from the analyses of SPIRN (1965). showing a depletion in heavy REE. are caused by the presence of iron-rich floes in the calcite tests. which could be incorporated into nodules when the tests dissolve. The REE pattern of the Fe-rich phase in nodules (Fig. 9) is quite unlike that of iron-rich hydrothermal sediments (see BENDER er al.. 1971) which resembles the pattern for seawater. .Apparently. the pattern for

Rare earth element geochemistry

hydrothermal iron oxyhydroxides reflects that of seawater whereas the pattern for the iron phase of nodules may define that of seawater.

525

0

lo-

BO-

DlAGENETlC CYCLING OF REE IN NODULES AND SEDIMENTS The REE geochemistry of ferromanganese nodules and associated sediments is highly complex. As deduced by other workers. the evidence presented here points to a seawater source. It is unlikely, however, that the REE are directly precipitated from seawater since seawater is highly undersaturated with respect to likely trivalent REE salts (TURNER and WHITFIELD,1979). Consequently, the primary input of REE to the sea floor is probably by association with settling particles, a conclusion supported by the evidence of SUGIHARAand BOWEN(1962). Little information is available on the nature of the particles carrying the REE, but iron oxyhydroxide coatings and floes associated with biogenous debris would provide an effective conveyor. The settled biogenous debris undergoes further reaction and dissolution during its relatively long residence time on the surfaces of slowly-accumulating pelagic sediments. The associated REE may then be scavenged by other sediment components, including Fe oxyhydroxides and biogenous apatite. In the latter case, the REE could become fixed by Ca” e REE3+ ion exchange and it can be simply shown by calculation that such a process will lead to an enrichment of the 3 + REE of the magnitude actually observed in fish debris. Such a reaction is the type envisaged by TURNER and WHITFIELD(1979) to control the REE concentrations in seawater. The relationships between the REE contents in the nodules and associated sediments provide further information on possible mechanisms controlling their uptake by the nodules. Thus, considering the suite of nodules from the WAHINE survey area, which have compositions controlled by the pattern of sedimentation (CALVERTet al.. 1978). nodules with high REE concentrations occur on sediments with low REE concentrations and vice versa. Since two phases, one Fe-rich and one P-rich, host the REE in the nodules, any process which affects the relative proportions of these phases will control the REE geochemistry of the nodules. This is probably connected with diagenesis in the surface sediments. In the Wahine area, nodules with high Mn/Fe ratios are considered to represent those precipitates which formed on sediment which had more of the reactive Fe oxyhydroxide phase removed by reaction with biogenous Si to form smectite. thereby leaving less for incorporation into the nodules (CALVERTer al.. 1978). Such sediments evidently accumulate more slowly than those where there is less reaction and consequently more Fe available for uptake by the associated nodules. The same nodules with high Mn/Fe ratios have low REE contents (Fig. 10a) and this is consistent with their reten-

lo-

io -

ob

-

“Od”k

-___

sedlmenl

v . WAH 0 TRIP

5-

DOD0

c, JYN v

/

todorokw

/

.>-MnO.

AMPH

4-

3-

2-

1

I

o-

oo-

c

v

00.

oo-

oo-

o-

0

I

MlVFe

4

c 1

Fig. 10. Relationships between Mn/Fe ratio of nodules and, (a) Concentrations of 3 + REE in nodules and sediments, (b) Ratio of Ce to 3 + REE in nodules, (c) Ce anomaly of nodules. Nd is used as an example of the 3 + REE which all show similar relationships to Mn/Fe ratio.

tion in the more abundant phosphate component of the more slowly accumulating sediments. Note also that the contrast between the REE contents of the nodules and sediments is greater at low Mn/Fe ratios where the nodules have a smaller REE contribution from the phosphatic phase. ARRHENIUSet 01. (1957) first invoked microcrystalline biogenous apatite as a scavenger for rare earths and suggested that it dissolves upon long exposure at the sea floor whereas under higher accumulation rates the fish debris is largely preserved. We interpret our results to indicate,

however. that this material is concentrated

in slowly

526

H. ELDERFIELD YItrl.

accumulating sediments and is also preserved within ferromanganese nodules. possibly by stabilization through recrystallization of insoluble REE-rich apatite. Hence. we suggest that the mode of incorporation of the REE in nodules is governed by surface reactions, involving two competing phases. The relative importance of these phases is controlled. in turn. by the availability of reactive surfaces and this depends on the pattern of sedimentation; low sediment accumulation rates lead to a concentration of reactive biogenous apatite in surface horizons and this serves to immobilize the REE. Nodules growing in this environment are therefore impoverished in REE. HORN et al.(1972), GREENSLATE of al.(1973), CALVERT and PRICE (1977a) and BURNS and BURNS (1978) have suggested that nodules in the equatorial Pacific are enriched in transition metals because of their supply by and release from biogenous particulate material; LYLE ef al. (1977) have specifically proposed that the reaction of siliceous particles with Fe oxyhydroxides to form smectite favours the remobilization of these metals. Hence. the dissolution and reaction processes, which, according to this earlier work. enrich nodules in Cu and Ni. and which proceed under conditions of slow sedimentation. tend to deplete them in the REE because iron is immobilized in the sediment and because more phosphatic surfaces are available for adsorption. Therefore. paradoxically, the same processes which cause enrichment of first row transition metals in nodules also favour enrichment of REE in sediments. This is illustrated by a significant negative correlation between. for example, Cu and the 3 + REE in these nodules (Fig. IIA). Nevertheless, it must be recognized that this mechanism for mobilization/fixation of elements in nodules is based upon two separate and loosely-formulated hypotheses and other possible mechanisms cannot yet

be rejected. For example, the role of smectite formation could be replaced by a scenario based upon differential remobilization of Mn and Fe. Fe fixation in a minor &MnOz phase or continuous independent uptake of Fe and REE although each is less compatible with all of the analytical data than the one we have proposed. The behaviour of Ce in the nodules and associated sediments contrasts strongly with that of the other rare earths. More Ce is present in the nodules where more is present in the sediments (Fig. 4). This is consistent with a process which adds Ce to nodules and sediments directly from seawater: diagenetic remobilization is probably not involved here. SUGIHARA and BOWEN (1962) suggest that ld4Ce is associated with particles sinking at lower rates than those carrying the 3 + REE. Cerium might be preferentially associated with Fe oxyhydroxide floes which could be directly incorporated into nodules and not undergo significant reaction in the surface sediments. Hence. nodules being influenced to a greater extent by seawater sources have low MniFe ratios and high Cei3 + REE ratios and consequently large Ce anomalies and cite wrw (Fig. lob and c). This could explain the relationship claimed between the Ce anomalies and redox conditions (GLASBY. 1973; PIPER.1974a): it seems. by contrast, most unlikely that the processes governing such reactions can be explained using conventional thermodynamic arguments. In the equatorial Pacific. the Ce content of the nodules covaries linearly with the Fe content (Fig. 6) and the 3 + REE contents covary linearly both with Fe and P. so that the magnitude of the Ce anomaly is a function of the relative amounts of Fe and P present. Cerium is one of a small group of trace elements for which removal from seawater ria higher valency states (i.e. Ce”. Co3’, Pb”, Ti”) has been proposed (GOLDBERG, 1961; SILLEN,19611 BURNS.

.Nd:--: . . .. ! 1200

a

b

.

.

.

La

I

Sm

01

.

0.4

0.8

Wf % CoPPer

Fig. Il.

Relationships

1.2

01

03 0.2 wf 9'0 Cobalt

between (a) 3 c REE and Cu. and rbl Cc and Co. in r‘uuatcrr~al Pmtic

nodules

J

0.4

527

Rare earth element geochemistry

196.5). In particular. oxidation of Co’+ in seawater followed by uptake of Co 3+ in nodules has been commonly assumed (e.g. PRICE and CALVERT, 1970; BURNS and BURNS, 1977). Therefore the processes which are thought to cause enrichment of Co in nodules also favour enrichment of Ce. This is iilustrated by the significant positive correlation between Co and Ce contents in the nodules studied here (Fig. 1lb). The correlation between Co and Ce was not found for the 3 + REE and that between Cu and the 3 + REE was not duplicated with Ce. Note afso from Fig. 10 that the pattern of decreasing 3 + REE contents in nodules with increasing Mn/Fe ratios appears to be a general one for all the samples we have studied. including the B-Mn02-bearing nodules, whereas the 6.Mn02 samples have widely divergent Ce anomalies. Further studies of the REE geochemistry of nodules from oceanic seamounts where Co enrichments are marked (see PRICE and CALVERT, 1970, Fig. 6) and of nodules from shallow depths, where, apparently, REE scavenging by phosphatic material does not, occur and where distinctly different REE patterns are found (PIPER. 1974a). would be worth-

BURNS R. G. and ine Manganese

BURNSV. M. (1977) Mineralogy. In MarDeposits (ed. G. P. Glasby), pp. 185248.

Elsevier. BURNSV. M. and BURNS R. G. (1978) Post-depositional metal enrichment processes inside manganese nodules from the North Equatorial Pacific. Earfh Planer. Sci. Left.

39,

341-348.

CALVERTS. E. (1978) Geochemistry of oceanic ferromanganese deposits. Phil. Trans. R. Sot. London Ser. A 290, 43-73.

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