Rare earth element zonation in Pacific ferromanganese nodules

Rare earth element zonation in Pacific ferromanganese noduies H. ELDERFIELD,C. J. HAWKESWORTH* and M. J. GREAVES Department of Earth Sciences. University of Leeds, Leeds LS29JT. U.K and S. E. CALVERT Department of Oceanography, University of British Columbia, Vancouver, B.C., Canada V6T I W5 (Received

Abstract-The

11 November

1980; accepted

lower surfaces of ferromanganese

in rerisedjbrm

12 March 1981)

nodules from the north equatorial Pacific Ocean, which

are enriched in Mn, Cu and Ni, and the upper surfaces, which are enriched in Fe, P and Co, have been analyzed for La Ce. Nd. Sm. Eu. Gd. Dv. Er and Yb. The REE contents are lower and the Ce anomalv is smaller in the lower surfaces than in ihe upper surfaces. The magnitude of the Cc anomaly increase. with decreasing Mn/Fe ratio, indicative of a seawater origin. The zonal distribution of the other REE supports the conclusion derived previously from inter-nodule and nodule/sediment relationships that diagenetic fixation of rare earths in sediments affects their enrichment by nodular iron oxyhydroxides.

RECENTwork on the geochemistry of north equatorial

Pacific ferromanganese nodules and sediments has shown that diagenetic processes may provide an important mechanism for the redistribution of metals at the sea floor and may govern the mode of incorporation of such metals into the nodules (GREENSLATEet al., 1973; HQRN er al., 1973; CALWRT and PRICE, 1977). A significant piece of evidence supporting this suggestion is the observation by RAAB(1972) that the opposing surfaces of discoidal nodules have distinctly different compositions, the lower surfaces being relatively enriched in Mn. Cu, Ni, MO and i!n and the upper surfaces being enriched in Fe, Co and Pb. CALVERT and PRICE(1977) explained this difference by the diagenetic enrichment from the underlying sediment of metals associated with manganese phases on the undersides of the nodules and the direct removal of those metals from the overlying seawater, possibly by oxidation, onto the upper surfaces where they are associated with iron phases. Previous studies of the rare earth element (REE) geochemistry of nodules (GOLDBERG et al., 1963; GLASBY, 1973; PIPER, 1974) have concluded that the REE are added to nodules directly from seawater. However, from a comparison of nodules and associated sediments from the north equatorial Pacific, we have recently concluded that, for the trivalent REE (3 + REE), diagenetic remobilization reactions also play a key role in determining the REE composition of ferromanganese nodules (ELDERFIELDer al., 1981). On the other hand, Ce appears to be the only REE which is derived predominantly from the overlying

* Present address: Department of Earth Sciences, The Open University, Milton Keynes, MK76AA. U.K.

Hence, such nodules might be expected to show a zonation in REE contents analogous to that found for Mn. Fe, Cu. Co, Ni. MO. Pb and Zn. We describe here the REE distributions in the upper and lower surfaces of nodules from the equatorial Pacific and compare them with the transition metal contents in the same samples in order to see if inter-nodule variations are reflected in intra-nodule variations for this group of metals. seawater.

INTRODUCTION

MATERIALS

AND METHODS

The nodules were obtained from the collections of the Centre National pour I’Exploitation des Ocians, Brest. Samples 1 and 2 (Table 1) were collected in the northern equatorial Pacific between the Clarion and Clipperton Fracture Zones in 4730m water depth; sample 3 was recovered in the southern equatorial Pacific in 5231 m water depth. The nodules were distinctly discoidal, the major axes ranging from 6 to 12cm. the minor axes ranging from 1.5 to 3 cm. The upper and lower surfaces were identified by their surface morphologies, textures, colour and the presence or absence of adhering sediment (see RAAB, 1972). Surfaces designated as lower surfaces were intensely black, deeply crenulated and marked by the presence of sediment in cracks and irrregularities. Surfaces designated as upper surfaces were brown to black, relatively smooth and without adhering sediment; occasional encrusting micro-organic remains were present. Samples were removed from the opposing surfaces of the nodules by scraping the surfaces to a depth of a few millimetres with a clean spatula, avoiding wherever possible sediment and organic material. The samples were finely powdered in agate.

REE analys& were carried out by mass spectrometric isotope dilution, Mn, Fe, Cu, Ni and Co by atomic absorption &ectrophotometry and P by calorimetry, following methods described previously (ELDERFIELD er al., 1981). The mineralogy of the samples was determined before analyses by conventional X-ray diffraction methods, mineral assignments following those given in CALVERTand PRICE (1977).

1231

1232

H.

ELDERFIELD et al.

Table 1. Compositions of ferromanganese nodules sample

Mn

Fe

F

co

cu

Ni(%) Mn/Fe La

20.6 9.20 0.176 0.300 0.537 0.882 2.2 Bottom 22.3 6.82 0.125 0.255 0.868 0.992 3.3

liTo~

Ce

h'd Sm

Eu

Gd

Dy

Er

yb ‘ppm)

117 624 170 42.5 9.25 Al.4 33.; 16.3 89.3 h41 125 31.1 7.10 27.9 23.7 il.0 !O.i

2:ToP 23.2 6.92 0.141 0.296 0.833 1.16 Bottom 22.7 6.68 0.137 0.269 0.823 1.15

3.4 3.4

97.2 403 136 32.3 7.89 - 26.2 13.4 l:.i 94.1 394 133 32.9 7.78 29.7 25.5 12.7 11.1

3rTop 23.2 5.92 0.159 0.235 0.768 1.03 .Bottom 24.6 5.16 0.112 0.153 1.05 1.22

3.9 4.8

83.8 258 110 28.1 6.80 26.3 24.0 12.6 I1.h 63.3 215 109 27.4 6.76 25.6 22.4 11.5 10.3

RESULTS AND DPXUSSION The analytical results are given in Table 1. The Mn/Fe ratios range from approximately 2-5; in two cases the ratio is higher in the bottom sub-sample, whereas in the remaining sample the ratio is identical in top and bottom sub-samples. However, the Fe content is distinctly higher in the top sub-sample in all three cases. This is similar to the compositional variations noted in other collections of nodules (see CALVERT et al., 1978). Likewise, the minor transition metals are also present in different concentrations in the top and bottom sub-samples, except for Cu and Ni in sample No. 2, consistent with the observations of RAAB(1972), and P is also higher in the top subsamples. The mineralogy of the Mn-bearing phases shows no significant differences in the sub-samples from the opposing surfaces of the nodules. Todorokite. indicated by the presence. of reflections at 960 and 480 pm and 6-Mn02, indicated by reflections at 244 and 144pm of equal intensity to those characteristic of todorokite, are present. On the other hand the intensity of the principal quartz rdlection (at 334pm) is distinctly higher in the bottom sub-samples; this is consistent with the presence of a larger amount of aluminosilicate debris in the lower surfaces of the nodules studied by Raab (1972).

In the sample showing little difference in the Mn/Fe between designated top and bottom sub-samples. the REE concentrations are around 29’; higher in the top sub-samples; this difference is not analytically significant. For the other two samples, however, significant differences in the REE contents are found. Table 1 shows that the lower surface of each nodule is depleted in REE relative to the upper surface, by _ 407, for nodule No. 1 and by _ loo/, for nodule No. 3. For the latter sample, the light and heavy REE show a marked depletion, whereas the differences in Nd-Gd are very small. The pattern of REE distribution is retained when all samples are considered without reference to their origin, highest REE contents being found in samples with the lowest Mn/Fe ratios and vice versa. These results are consistent with the genetic model for the mode of incorporation of REE in nodules described previously (ELDERFIELD et al.. 19811. Thus, REE are conveyed from seawater to sediments in association with iron oxyhydroxides and are incorporated into nodules in both Fe-rich and P-rich phases depending upon the extent of diagenetic reaction in the surface sediments. In the present case, a P-phase does not appear to be important; Fe/P ratios in the sub-samples are relatively high and more typical of nodules from areas other than the equatorial Pacific

8

6

wt

10

% iron

Fig. 1. Relationships

between

REE and Fe.

REE zonation

in Pacific ferromanganese nodules M4Ub

100

-

1233 m4ub

botlwn

a.a.nN.E~~~rorlalPa&c intu- nadute ..a.=compoS*mnnlMrloril OLO oCooN4 nG4

..

co .:‘;.r: :: I.‘,. ::. ..I.’ ,[ . .:: :: ::

.

..:y ..I.’ :: . . .,

6

:: ::

V4

1.2

08

0

02

v4

wt. % cobalt

wt. 4, copper

Fig. 2. Relationships between: (a) REE (other than Ce) and Cu; Sm. Eu, Dy, Er and Yb (not shown) show similar sensitivities to Cu; (b) Ce and Co. The shaded areas show the relationships found from northern Equatorial Pacific inter-nodule compositional variations (EDERFIELD et al.,1981).

(see CALVERT and PRICE, 1977; ELDERFIELD et al., 1981). Figure 1 shows that an Fe-phase is the principal carrier for the REE. The zonation in the concentrations of the REE in discoidal equatorial Pacific nodules can be ascribed to two mechanisms. Firstly, the upper surfaces of the nodules have higher Fe contents than the lower surfaces. Such a difference reflects the existence of diagenetic reactions in the associated sediments. In sediments associated with nodules having high Mn/Fe ratios, a larger proportion of the deposited iron oxyhydroxides have reacted with. opaline silica to form smectite, thereby leaving less available for incorporation into the nodules (LYLEet al., 1977; CALVFXT et al., 1978). During this reaction, the REE are presumably released to other sediment phases or to the pore waters but, in contrast to Cu, Ni, MO and Zn, which are incorporated into the surfaces of nodules growing within the sediment, they are scavenged by phos-

+a4

phatic fish debris and are retained in the sediment. Hence, the diagenetic process that enriches nodules in Cu and Ni depletes them in REE (ELDERFIELD er al., 1981). Such an interpretation is consistent with the contrast in the enrichment of transition metals and the REE in the upper and lower surfaces of the modules examined here. In addition, the relationship between Cu, for exampk, and the REE in individual nodules fits the negative correlation between these metals found previously (Fig. 2a). The second mechanism causing the observed zonation in the REE involves the behaviour of Ce, which appears to be different from the 3 + REE. Nodules with the largest Mn/Fe ratio, and hence the largest diagenetic influence, have been found to have the smallest positive Ce anomalies and vice versa (ELDERFIELD et al., 1981). Here we find that the Ce content has a larger sensitivity to the Fe content (Fig. l), that the upper surfaces of the nodules have larger Ce

+;.*.+:-.. . ...::. .* . . . *.:::.$yl_

+o-2

..* ...+.. **. . ...*::... ..**.:q *+*: ..*::: * ..**...*:

-

no4ub_

w

..*_:.. .- ..*: * :.++&&

:.:.I

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N.EawtwmlPock

imn-nodub COWOS-nkmlvarlotlom CI onomaly*loglCa/fLo+~N41 *=

. ..*. *+:, I**I. *. 0

1

2

3

4

5

Mn/Fe Fig. 3. Relationships between Ce anomaly and Mn/Fe ratio; Ce anomaly = lo&e;{ La + f Nd). The shaded area shows the relationship found from northern Equatorial Pacific inter-nodule compositional variations (ELDERFIELD er al..1981).

H. ELDERFIELD et ul.

I234

anomalies and that the anomalies have a relationship to the Mn/Fe ratios similar to that found in the previous study (Fig. 3). This relationship lends support to the theory that Ce is added to nodules directly from seawater by means of its conversion to the quadrivalent state (GOLDBERG, 1954) and this is supported by the significant positive correlation between Ce and Co which is another element thought to be removed from seawater by oxidation (PRICE and CALVERT, 1970; CALVERT and PRKE. 1977). The nodule zones analyzed in this study show a similar relationship (Fig. Zb). In conclusion, the present study has demonstrated a zonation of the REE within ferromanganese nodules which is compatible with the variations in REE contents between different nodules identified previously. Such a zonation lends support to the role of diagenesis in the REE geochemistry of ferromanganese nodules.

composiuons and sedimentation in a small survey area of the equatorial Pacific. J. Mar. Res. 36. 161-183. ELDERF~ELD H., HAWKE~WORTH C. J., GREAVESM. J. and CALVERTS. E. (1981) Rare earth element geochemistry of

oceanic ferromanganese nodules and associated sediments. Geochim. Cosmochim. Acta 45, 513-528. GLA~BYG. P. (1973) Mechanisms of enrichment of the rarer elements in marine manganese nodules. iMar. Chem. 1. 105-125. GOLDBERGE. D. (1954) Marme geochemistry, 1. Chemical scavengers of the sea. J. Geol. 62, 249-265. GOLDBERG E. D.. KOIDE M.. SCHMITTR. A. and SMITH R. H. (1963) Rare earth distributions in the marine environment. .r. Groph~ls. Res. 68. 4209-4217. GREENSLATE J. L.. FRAZERJ. Z. and ARRHENIUSG. (1973) Origin and deposition of selected transition elements in the seabed. In Papers on the Origin und Disfributirm t,/ Munganese

Nodules

in

the

Pacific

trnd

Prospects

/iw

Exploration (ed. M. Morgenstein). Honolulu. Hawaii. pp. 45-69. HORN D. R., HORN B. M. and DELACHM. N. (1973) Copper and nickel content of ocean ferromanganese deposits and their relation to properties of the substrate. In Paoers on the Oriain and Disrriburion ot’ Manaanese Nidules in the Pa& und Prospects for E,$lora&n (ed. rlc~no~ledgemenrs-This work was supported by the M. Morgenstein), Honolulu. Hawaii, pp. 77-83. Natural Environment Research Council Grants GR3/3125 LYLE M., DYMONDJ. and HEATH G. R. (1977) Copperand GST/02/03. We thank M. GAUTHIERand J. LEMAIRE nickel enriched ferromanganese nodules and associated CNEXO. for supplying the nodules. crusts from the Bauer Deep, NW Nazca Plate. Earth Planet. Sci. Left. 35, 55-64. PRICEN. 8. and CALVERTS. E. (1970) Compositional vart-

REFERENCES CALVERTS. E. and PRICE N. B. (1977) Geochemical variation in ferromanganese nodules and associated sediments from the Pacific Ocean. Mar. Chem. 5, 43-74. CALVERTS. E.. PRICEN. B.. HEATHG. R. and MOORET. C. (1978) Relationship between ferromanganese nodule

ation in Pacific Ocean ferromanpnese nodules and its relationship to sediment accumulation rates. Mar. Geol. 9, 145-171. W. (1972) Physical and chemical features of Pacific deep sea manganese nodules and their implications to the genesis of nodules. In Fcrromungunusu Deposits on rhr Oceun Floor (ed. D. R. Horn), pp. 31-49. NSF.

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