16
Geological Society, London, Special Publications doi: 10.1144/GSL.SP.1987.030.01.11 p253-267. 1987, v.30; Geological Society, London, Special Publications Joron Barry L. Weaver, David A. Wood, John Tarney and Jean Louis Tristan da Cunha Atlantic: Ascension, Bouvet, St. Helena, Gough and Geochemistry of ocean island basalts from the South service Email alerting new articles cite this article to receive free e-mail alerts when here click request Permission part of this article to seek permission to re-use all or here click Subscribe Collection London, Special Publications or the Lyell to subscribe to Geological Society, here click Notes © The Geological Society of London 2014 at University of Southern California on April 2, 2014 http://sp.lyellcollection.org/ Downloaded from at University of Southern California on April 2, 2014 http://sp.lyellcollection.org/ Downloaded from

Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

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Page 1: Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1987.030.01.11p253-267.

1987, v.30;Geological Society, London, Special Publications JoronBarry L. Weaver, David A. Wood, John Tarney and Jean Louis Tristan da CunhaAtlantic: Ascension, Bouvet, St. Helena, Gough and Geochemistry of ocean island basalts from the South

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new articles cite this article to receive free e-mail alerts whenhereclick

requestPermission

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Notes

© The Geological Society of London 2014

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Page 2: Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

Barry L. Weaver, David A. Wood, John Tarney and Jean Louis Joron

S U M M A R Y: Basaltic and hawaiitic lavas from the S Atlantic Ocean islands of Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha have been analysed for major elements and a wide range of trace elements. There is marked chemical (particularly trace-element) diversity between these islands which parallels observed Pb, Sr and Nd isotopic variations, and implies considerable large-scale heterogeneity in the source regions for ocean island volcanism. Trace-element and isotopic variation within individual islands suggests significant small-scale source heterogeneity.

Abundance ratios between highly-incompatible trace elements (Rb, Ba, Th, U, K, Ta, Nb and La) appear not to be fractionated during partial melting (except at low degrees of melting in the production of Tristan da Cunha lavas) and can be used to infer source characteristics. Lavas from the islands of Ascension, Bouvet and St. Helena have comparable highly- incompatible trace-element ratios (e.g. La/Nb, Th/Ta, La/Th, Th/U, Ba/La and Ba/Nb), with the exception of the apparent depletion of Rb and K relative to other highly- incompatible elements in St. Helena lavas. Tristan da Cunha and Gough lavas have similar La/Th ratios but much higher La/Nb, Th/Ta, Th/U, Ba/La and Ba/Nb ratios than do lavas from the other islands. These differences reflect depletion in Nb and Ta and enrichment in Ba relative to other highly-incompatible elements in Gough lavas.

The anomalous behaviour (enrichment) of Nb and Ta compared with other highly- incompatible trace elements, and specific consideration of relations between Ba, La and Nb, suggest that ancient subducted ocean crust, rather than primitive or depleted mantle, is the main component of the mantle source for ocean island basalts. Gough, Tristan da Cunha and Walvis Ridge lavas seem to be derived from such a source which has been contaminated by a component with high Ba/Nb, La/Nb and Ba/La ratios. Correlation between Ba/Nb and La/Nb in Gough basalts and hawaiites implies that these lavas contain variable proportions of this component. Pelagic sediment has the high Ba/Nb, La/Nb and Ba/La ratios required of this contaminant, and a few per cent of such material in the mantle source for Gough, Tristan da Cunha and Walvis Ridge lavas would account for their trace-element chemistry.

Introduction

Ocean island basalts (OIBs) are generally re- garded as being derived from chemically-anom- alous mantle sources, and are associated with hot-spot activity (Morgan 1971, 1972). The strong enrichment in incompatible trace elements evi- dent in these volcanics (e.g. Wood et al. 1981) represents a relatively recent event in the source region, as isotopic constraints demand that the magmas are derived from mantle sources with a time-integrated depletion in, for instance, Rb relative to Sr and Nd relative to Sm (O'Nions et al. 1977). The relationship of this source to the continental crust and the mantle source for 'normal' (depleted) mid-ocean ridge basalts (N- MORB) is of considerable interest, and has been extensively studied using isotope systematics (O'Nions et al. 1977; Sun 1980; White & Hofmann 1982; Zindler et al. 1982; White 1985). Four main hypotheses have been advanced to explain the origin of chemically-anomalous OIB. These hypotheses suggest that the origins of these volcanics are as follows:

1 From primordial lower mantle which has undergone little or no chemical fractionation since shortly after the formation of the Earth (Schilling 1973; Sun & Hanson 1975; Dupr6 & All6gre 1980; All6gre 1982). 2 From subducted ocean crust which has had a considerable residence time at the boundary between the upper and lower mantle (Hofmann & White 1982; Ringwood 1982; Vollmer 1983; White 1985). 3 From material which was originally part of the subcontinental mantle, but which is now resident in the deep asthenosphere (McKenzie & O'Nions 1983; Cohen et al. 1984). 4 From a mantle source which has been contam- inated by recycled continental crustal material (Hawkesworth et al. 1979; Cohen & O'Nions 1982; White 1985).

However, relatively few comprehensive trace- element data are available for ocean island lavas which might provide additional important con- straints on these possibilities. Here we report major and detailed trace-element data for OIBs

From: FITTON, J. G. & UPTON, B. G. J. (eds), 1987, Alkaline Igneous Rocks, Geological Society Special Publication No. 30, pp. 253-267.

253

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254 B . L . W e a v e r e t al .

from five islands in the S Atlantic Ocean: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha (Fig. 1).

These ocean islands are well characterized isotopically (O'Nions & Pankhurst 1974; O'Nions et al. 1977; Sun 1980; Cohen & O'Nions 1982; White & Hofmann 1982), and display considerable inter-island (and often intra-island) isotopic heterogeneity (Sun 1980; Cohen & O'Nions 1982). They are individually represent- ative of islands which are both typical and extreme in terms of the isotopic variation ob- served in ocean island volcanics.

Gough and Tristan da Cunha lavas have Nd and Sr isotope compositions which fall on the mantle array and lie close to supposed bulk Earth 143Nd/144Nd and 87Sr/S%r ratios (O'Nions et al. 1977), although He isotopes are suggestive of a non-primitive source (Kurz et al. 1982). Com- pared with MORB and other OIBs from the Atlantic Ocean, Gough and Tristan da Cunha lavas display anomalous 2~176 2~ 2~ and 2~176 ratios, having non-radio- genic 2~176 ratios and somewhat radio- genic :~176 ratios. However, St. Helena lavas plot somewhat to the left of the Nd-Sr mantle array with 143Nd/144Nd ratios greater than and 87Sr/86Sr ratios less than bulk Earth values. Additionally, St. Helena lavas have unusually radiogenic Pb isotope compositions (Sun 1980). Ascension and Bouvet lavas might be termed more isotopically typical of ocean island volcanics; they plot on (or in the case of Ascension possibly slightly to the left of) the Nd-Sr mantle

C *o

F. A '"

~

FIG. 1. Map of the main ocean islands in the Atlantic Ocean to the S of latitude 40~ : M, Madeira; C, Canary Islands; CV, Cape Verde Islands; F, Fernando de Noronha; A, Ascension; SH, St. Helena; T, Trindade; TC, Tristan da Cunha; G, Gough; DT, Discovery Tablemount; B, Bouvet.

array, although Bouvet lavas have less radiogenic Nd and more radiogenic Sr than do Ascension lavas. Pb isotope ratios for Ascension and Bouvet lava,s are comparable with those of the majority of Atlantic Ocean islands (e.g. the Canary Islands, the Azores, the Cape Verde Islands and Fernando de Noronha (Sun 1980)).

Sampling and analytical techniques

Samples from Ascension (Daly 1925; Harris 1983), Bouvet (Verwoerd et al. 1976; Imsland et al. 1977; LeRoex & Erlank 1982), St. Helena (Daly 1927; Baker 1969) and Tristan da Cunha (Baker et al. 1964) were obtained from the collections of the British Museum (Natural History), London. The samples from Gough Island were obtained from the collections of LeMaitre (1962) at the Department of Earth Sciences, University of Cambridge. Relevant geological details for each island, some petro- graphic data and a limited amount of geochemical data are available in the above cited publications. In all instances samples were chosen to cover the full compositional range of lavas erupted on each island, although only data for basaltic and hawaiitic rocks are reported here.

Data for the major elements and the trace elements Cr, V, Ni, Zn, Rb, Sr, Ba, Zr, Nb, Y, Th, La, Ce and Nd were obtained by X-ray fluorescence (XRF) analysis at the University of Leicester using a Philips PWl400 automatic spectrometer. Major-element determinations were made using an Rh X-ray tube on glass fusion beads utilizing a lithium tetraborate-lithium metaborate flux and with calibrations produced against international standards. Major-element data have been corrected to take account of weight loss on ignition of the sample at 900~ Trace-element concentrations were determined on pressed powder pellets using calibrations produced against international standards and standard spikes. Mass absorption corrections were applied using the intensity of the Rh Kct Compton scatter peak for the elements Ni, Zn, Rb, Sr, Y, Zr, Nb and Th and by using the intensity of the W L0r Rayleigh scatter peak combined with Fe K~ counts to cross the Fe absorption edge for the elements Cr, V, Ba, La, Ce and Nd. A more detailed description of the analytical procedures is given by Weaver et al. (1985).

Data for the the trace elements Cr, Ni, Sc, Co, Cs, Rb, Ba, Zr, La, Ce, Eu, Tb, Hf, Ta, Th and U were obtained by epithermal neutron activa- tion analysis (ENAA) at C.N.R.S., Universit6 Pierre et Marie Curie, Paris. Full details of the

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Geochemistry of ocean island basalts 255

analytical procedure, the precision and the accuracy are given by Chayla et al. (1973) and Jaffrezic et al. (1980).

Where there is overlap in the tabulated data (Table 1) between the two analytical techniques we have reported the XRF data for the elements Cr, Ni, Rb, Ba, Zr and Ce and the NAA data for Th and La. A standard Fe203/FeO ratio of 0.22 was used in the calculation of CIPW normative compositions.

Geochemistry

Major elements

Representative analyses of lavas from the islands of Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha are presented in Table 1. In this paper we have attempted to select only those lavas which by chemical criteria appear to have undergone limited low-pressure crystal fraction- ation. Our main criteria in selecting samples have been based on uniformity of trace-element ratios rather than major-element composition. Essen- tially, only those lavas which have a basaltic or hawaiitic major-element chemistry are consid- ered.

In Fig. 2 normative compositional data for basalts and hawaiites from the islands are plotted in terms of the components ne-di-ol-hy-qz. Considerable variation in normative composition exists between islands, and often within individ- ual islands. Bouvet basalts and hawaiites are uniformly ol-di-hy normative and have relatively high di/ol and di/hy ratios (Fig. 2). St. Helena basalts and hawaiites (with the exception of one sample) are ne normative, while Ascension basalts and hawaiites are somewhat variable in compo- sition, ranging from ne normative to moderately

Di

/ k

Ol Hy

FIG. 2. Plot of normative components ne-ol-di-hy-qz in basalts and hawaiites from Bouvet (I~), Ascension (-k), Gough (�9 St. Helena (D) and Tristan da Cunha (0). All normative compositions were calculated using an Fe203/FeO ratio of 0.22.

hy normative (Fig. 2). There is an interesting contrast between the chemistry of Gough and Tristan da Cunha basalts and hawaiites. The Tristan da Cunha samples range from moderately to mildly ne normative, with some samples straddling the di-ol join with approximately constant ol/di ratios. Gough samples, however, are extremely variable in normative mineralogy, ranging from hawaiites with a negligible amount of normative ne (essentially lying on the di-ol join) through basalts and hawaiites with quite variable proportions of ol, di and hy to a hawaiite with a small amount of normative qz (Fig. 2).

Clearly, few (if any) of the samples considered in this study are representative of primary mantle melts; in general Mg numbers (100 Mg/(Mg+ FEZ+)) and Ni and Cr contents (e.g. Table 1) are low, and those lavas with Mg numbers which might be appropriate for primary mantle melts are generally olivine- and/or clinopyroxene- phyric. Indeed, in these islands the major-element compositions are largely controlled by removal or accumulation of the assemblage olivine+ clinopyroxene + plagioclase + Fe-Ti oxide in bas- altic and hawaiitic lavas (Zielinski & Frey 1970; LeRoex & Erlank 1982; Harris 1983; LeRoex 1985).

Trace elements

In this section we are primarily concerned with assessing the compositional variation in the mantle sources for OIBs. The most useful ele- ments for this purpose are those trace elements which have very low bulk crystal-liquid partition coefficients D during both partial melting and fractional crystallization processes. In particular, ratios between such elements should be repre- sentative of the ratios in the mantle source regions. Cs, Rb, Ba, Th, U, K, Ta, Nb and La are highly-incompatible trace elements for which D is very low (less than or equal to 0.01), and the behaviour of these elements will be emphasized. In evolved compositions (mugearites and trach- ytes) from each island ratios between some of these trace elements become influenced by low- pressure crystal-liquid processes. In the less evolved basaltic and hawaiitic compositions considered here there is no evidence for highly- compatible trace-element fractionation attribut- able to low-pressure crystallization. However, the degree to which, in particular, alteration and partial melting are capable of affecting abun- dance ratios needs to be assessed.

Alteration

In general the abundances of the large-ion lithophile (LIL) elements Cs, Rb, Ba, Th, U, K

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Page 5: Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

256 B. L. W e a v e r et al.

~ .,~

ial

. <

P"~.CN r

, :~r"q

r ~ . . , r

,#

P,,

. . . . ~ ~ ~ ~ o I ~ . . . .

~ ~ ~�9 ~ I ~ ~ ~ ~ ~ ~ ~ ~

�9 ~ ~ o o o O o ~ ~ ~

g r a

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Page 6: Geochemistry of ocean island basalts from the South Atlantic: Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha

Geochemistry o f ocean &land basalts 257

and Sr are susceptible to variation owing to low- temperature alteration processes (Hart et al. 1974; Clague & Frey 1982). Element mobility accompanying sub-aerial alteration and hydra- tion is evident in a number of the ocean island lavas studied, although the effects are not always consistent. Cs abundances are particularly unre- liable, and can be either enriched or depleted relative to immobile trace elements (Nb, Ta, La) in both altered and otherwise fresh lavas. U is moderately mobile upon alteration, usually with a loss of U leading to an increase in Th/U ratio in altered lavas (Fig. 3). K and Rb appear to be somewhat less mobile than Cs and U, the mobility

of K and Rb generally resulting in an increase in the K/Rb ratio. There is also some evidence that Ba and Th may display more limited mobility. These effects clearly contribute to the scatter evident in the LIL element plots of Fig. 3, and it is necessary to use care in the interpretation of the behaviour of some of these elements.

Partial melting

For trace elements to be good indicators of mantle source composition they must not be strongly fractionated during partial melting (e.g. by low degrees of melting or owing to the effect of

L 8

ell,..- /

T h

I I I

T h

Rb

I I I

, s J []4 Z41~*/S " ~ _ L J ~

Th L I I L 2 4 6 8 1 0

K2o

y / /

f S TI1

I I I I I

Ba

.

[] .

~ ' * [] ~ �9 T h

i i i [ 1

:/y / /

/ . ~ w /

/ / / : / g4v" I I i i 1,, 2 4 6 8 10

l O O O

8 0 0

6 0 0

4 0 0

2 o o T h

FiG. 3. Plots of Th against other highly-incompatible trace elements for the islands of Ascension ( , ) , Bouvet (1), St. Helena (F1), Gough (O) and Tristan da Cunha (0).

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258 B. L. Weaver et al.

residual mineralogy). This is clearly a problem with alkali basalts, as the degree of melting may be less than 5~o-10~ (Clague & Frey 1982). At these low degrees of melting, phases other than olivine, orthopyroxene, clinopyroxene and garnet (such as amphibole and/or phlogopite), which are capable of fractionating Ba, K, Rb and Th from other highly-incompatible trace elements (Phil- potts & Schnetzler 1970), may be residual.

From the systematic behaviour of highly- incompatible trace elements outlined below we believe that for Ascension, Bouvet, St. Helena and Gough there is little chemical fractionation attributable to partial melting. However, anom- alous values for some trace-element ratios in Tristan da Cunha lavas suggest that these ratios may not be representative of the source. The geographical proximity of Gough and Tristan da Cunha implies that these islands are related to the activity of the same hot spot, and indeed Nd, Sr, Pb and He isotopes indicate a chemically similar source for their lavas. However, the strongly ne-normative nature of Tristan da Cunha lavas relative to Gough lavas (Fig. 2) may be the result of a smaller degree of partial melting. Additionally, significant differences in rare-earth- element (REE) fractionation between the two islands (as expressed by Cen/Yn ratios which have values of 8.0 and 10.8 for Gough and Tristan da Cunha respectively (Table 2)) could be inter- preted in terms of a lower degree of partial melting in the production of the Tristan da Cunha lavas. Similar 143Nd/144Nd ratios (Cohen & O'Nions 1982; White & Hofmann 1982) indicate that the source for Tristan da Cunha lavas has had a similar time-integrated Sm/Nd ratio to that of the source for Gough lavas.

Relationships amongst the trace elements Rb, Ba, K and Th are somewhat ambiguous regarding the possible role of amphibole and/or phlogopite

as residualphases in the mantle source for Tristan da Cunha lavas. Partition-coefficient data suggest that DR > ORb ~ DBa for amphibole, whereas DK ,~ DRb > DBa for phlogopite (Philpotts & Schnetzler 1970). Th/Nb and Th/Ta ratios (Fig. 3; Table 2) are not fractionated signficantly during different degrees of melting in the production of Gough and Tristan da Cunha lavas. Variations in Ba/ Nb, K/Nb and Rb/Nb ratios (Table 2) demon- strate that Ba and K are equally partitioned into the residue in the production of Tristan da Cunha lavas, but that Rb behaves in a more incompatible fashion. However, for residual amphibole and/or phlogopite Ba should behave more incompatibly than K. Clearly it is not possible to constrain the residual mantle mineralogy tightly during the partial-melting event which generated the Tristan da Cunha lavas, except that phases retaining K, Ba and (to a lesser extent) Rb appear to be required.

Source characteristics

Abundances of La, Ta, Rb, K, Ba and U are plotted against Th (used as a general index of fractionation) in Fig. 3. In addition, selected element ratios (with associated l a standard deviations) for each island are tabulated in Table 2. Mean La/Th ratios are constant for the islands with the exception of Tristan da Cunha, which has a lower La/Th ratio owing to slightly more compatible behaviour of La during the low degree of partial melting. A rather larger variation in Th/Ta ratios between the islands is evident, with Gough and Tristan da Cunha having higher Th/ Ta ratios (1.47 and 1.50 respectively) than those of lavas from Ascension, Bouvet and St. Helena (range 1.00-1.09). Plots of Rb, K, Ba and U against Th display somewhat more scatter than those of Ta and La, largely owing to alteration

TABLE 2. Means and l a standard deviations for selected element ratios in basaltic and hawaiitic lavas from Gough, Tristan da Cunha, St. Helena, Ascension and Bouvet Islands

Ce,/Y, Zr/Nb La/Th Ba/Th Rb/Th Th/U Th/Ta La/Nb Th/Nb Ba/Nb K/Nb Rb/Nb Ba/La

Gough 8.0 6.8 9.1 154 9.5 4.86 1.47 0 . 9 7 0.105 16.1 432 0.99 16.6 0.9 0.8 0.6 20 0.9 0.28 0.15 0.13 0.009 2.8 47 0.15 1.6

Tristanda 10.8 4.2 7.8 103 8.2 4.50 1.50 0.86 0.108 11.4 307 0.88 13.2 Cunha 1.2 0.4 0.8 12 1.0 0.08 0.12 0 . 0 6 0.007 0.8 16 0.09 0.7

St. Helena 7.3 4.5 8.9 77 4.9 3.78 1.09 0 . 6 9 0.078 5.9 179 0.38 8.7 0.9 0.2 0.5 8 0.5 0.10 0 . 0 7 0 . 0 3 0.005 0.2 9 0.04 0.7

Ascension 4.6 5.3 8.7 92 6.9 3.54 1.05 0.65 0.073 6.8 230 0.46 10.3 0.5 0.4 1.0 12 0.8 0.17 0 . 0 9 0.04 0.006 0.5 17 0.05 1.2

Bouvet 3.9 6.9 9.3 83 7.1 3.68 1.00 0 . 6 8 0.073 6.0 272 0.52 8.9 0.1 0.3 0.4 4 0.4 0.19 0 . 0 3 0.04 0.003 0.3 13 0.02 0.4

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Geochemistry of ocean island basalts 259

effects. However, it is apparent that St. Helena lavas have lower Rb/Th ratios (average 4.9) than those of Ascension and Bouvet lavas (average ratios 6.9 and 7.1), which are in turn lower than the Rb/Th ratios in Gough lavas (average 9.5). A very similar pattern is evident for K/Th ratios, with the exception that Bouvet lavas have a higher K/Th ratio than do Ascension lavas. Both Gough and Tristan da Cunha lavas (when altered samples are excluded) have considerably higher Th/U ratios (average 4.86 and 4.50 respectively) than those of Ascension, Bouvet and St. Helena lavas (range of average ratios, 3.54-3.78). There is high degree of variation in Ba/Th ratios, often within each island, but Ascension, Bouvet and St. Helena lavas have rather consistent ratios of 77-92, whilst the Ba/Th ratio of 154 in Gough lavas is high.

It is evident from the foregoing that, in general, the trace-element characteristics of the lavas from Gough and Tristan da Cunha (where fractiona- tion during melting affects some element ratios) are very different from the characteristics of the lavas from Ascension, Bouvet and St. Helena. Within the latter three islands the St. Helena lavas appear to be somewhat depleted in K and Rb relative to other highly-incompatible trace elements. It is also apparent that within each island there is often a degree of heterogeneity in element ratios (expressed by the standard devia- tion) which cannot be simply attributed to analytical uncertainty or alteration effects. This observation is in accord with the well-docu- mented isotopic heterogeneity within lavas from a single island (Sun 1980; Cohen & O'Nions 1982).

Figure 4 is a plot of Th against Zr, a less incompatible element that is likely to be fraction- ated from highly-incompatible trace elements (e.g. Th) at low to moderate degrees of partial

4 0 0

2 0 0

/ /

- T h

1 2 1 J 41 J 6L I 8J I 110 L

FIG. 4. Plot of Th against Zr for basalts and hawaiites from the islands of Ascension ( , ) , Bouvet (*), St. Helena (U]), Gough (C)) and Tristan da Cunha ($).

melting. The lavas of Bouvet and Ascension define good linear trends on this diagram which intersect the Zr axis near the origin, indicating slightly more compatible behaviour of Zr than Th during melting. A rather wide variation in Zr/Th ratios in Gough lavas results in an imprecise correlation between Zr and Th, with an extrapolated trend which intersects the Zr axis at a rather higher Zr value implying a significantly more compatible behaviour of Zr than Th during partial melting. Similarly, good correlation of Zr and Th abundances in lavas from St. Helena and Tristan da Cunha have high intercepts on the Zr axis (particularly in the case of Tristan da Cunha). Although difficult to quantify, owing to uncertain- ties in the degree of relative light REE enrichment in mantle source, high Ce,/Y, ratios for St. Helena, Gough and Tristan da Cunha lavas (7.3, 8.0 and 10.8 respectively) suggest a lower degree of partial melting in the generation of these lavas than for the Ascension and Bouvet lavas (with mean Cen/Y, ratios of 4.6 and 3.9 respectively). It is also clear that for the fomer three islands Zr does not behave as a highly-incompatible element during partial melting. Thus, depending upon the degree of melting, use of ratios employing Zr (e.g. Zr/Nb ratios) will not necessarily reflect source characteristics for OIBs. This becomes apparent from a plot of Zr against Nb (Fig. 5) where Tristan da Cunha lavas display a strong covaria- tion of Zr and Nb which has a considerable intercept on the Zr axis.

The distinctive trace-element characteristics of Gough and Tristan da Cunha as opposed to Ascension, Bouvet and St. Helena basalts and hawaiites is well displayed on plots of La, K20 and (particularly) Ba against Nb (Fig. 5). Average La/Nb ratios in Ascension, Bouvet and St. Helena lavas are rather uniform at 0.65-0.69 (Fig. 5; Table 2), but significantly higher at 0.97 in Gough lavas (Fig. 5; Table 2). There is some variation of K/Nb ratios between Ascension, Bouvet and St. Helena lavas with, as already noted, St. Helena basalts and hawaiites (K/Nb = 179) being somewhat depleted in K (Fig. 5) compared with Ascension and Bouvet basalts and hawaiites (K/ Nb values of 230 and 272). The K/Nb ratio of Gough lavas is high at 432 (Fig. 5; Table 2). Extremely large differences in Ba/Nb ratios are evident between lavas from the different islands (Fig. 5); Ascension, Bouvet and St. Helena lavas have consistent Ba/Nb ratios (Fig. 5) of 5.9-6.8, whereas the average Ba/Nb ratio in Gough lavas (Fig. 5) is very high at 16.1 but with considerable dispersion of values about the average ratio (Table 2).

Representative plots of incompatible element abundances normalized to an estimated primor-

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260 B. L . W e a v e r et al.

4 0 0

3 0 0

2 0 0

1 0 0 0

8 0 0

6 0 0

4 0 0

2 0 0

Zr

~ / Nb

I 1 I I I

/ /

/ / / /

I 410 I 1 I 2 0 6 0 8 0 1 0 0

La

K20

}

Nb

I I I I

Nb

I I I I I 2 0 4 0 6 0 8 0 1 0 0

FIG. 5. Plots of Nb against other incompatible trace elements for basalts and hawaiites from the islands of Ascension (~-), Bouvet (#), St. Helena (R), Gough (O) and Tristan da Cunha (0).

8 0

6 0

4 0

dial mantle composition (Wood et al. 1981) for basaltic and hawaiitic lavas from Ascension, Bouvet, St. Helena, Gough and Tristan da Cunha appear in Fig. 6. The order in which the elements are plotted corresponds to decreasing D from right to left in the diagram for equilibration of basaltic melt with four-phase lherzolite (Wood 1979). These diagrams illustrate some interesting features of the trace-element chemistry in addi- tion to those described above. The general patterns are of increasing normalized abundance from right to left in the diagrams, although amongst the highly-incompatible elements Cs-K there is a general decrease in normalized abun- dance with increasing incompatiblity (Fig. 6). The anomalous behaviour of Sr relative to Ce and Nd is due to plagioclase fractionation. Hf abundances define consistent negative anomalies relative to Zr and P, and Zr/Hfratios are constant between all five islands (range of average ratios from 43 to 45). These values are considerably higher than the primordial Zr/Hf ratio of 31

(Wood 1979), but similar to Zr/Hf ratios observed in other alkali basalt suites (Frey et al. 1978; Clague & Frey 1982). Maximum normalized abundances occur for Nb and Ta (Fig. 6), and Nb/Ta ratios are very uniform (within analytical error) both within and between islands (range of average Nb/Ta ratios from 13.7 to 14.1). Gough lavas are notable for the similar normalized abundances of Nb, Ta, La, K and Ba (Fig. 6). From Nb to Cs there is a general decrease in normalized highly-incompatible element abun- dances, with the exception of the islands of Tristan da Cunha and (particularly) Gough where Ba is enriched (producing a positive Ba spike) relative to other highly-incompatible LIL ele- ments (e.g. Cs, Rb, Th, U and K). The two most striking features of Fig. 6 are the large positive anomaly present for Nb and Ta in Ascension, Bouvet and St. Helena lavas, and the disappear- ance of this anomaly in Gough (and to an extent Tristan da Cunha) lavas with the simultaneous development of a positive Ba anomaly. The

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Geochemistry of ocean island basalts 261

1 0 0

=,

o lo

I - - Z <

0 1 0 n -

/ • ST. HELENA ~ ~ BOUVET

i i L i i i i i

- ~ ~ . ~ S C E N S I O N

i i i I I I I L I I I I i L I I l I I

GOUGH

, , , , J : , , , L L J L H ' f ' ~ ' b " ~ J R " ' v L , , ~ , , ~ J . ' d ' ' ' J ' xJb ' C S R b Ba Th U K Ta Nb L a C e S r N d P Z r E u T I T Y C s b B a h U K T a Nb L a C e S r P H f Z r E u T i Y

F]6.6. Plots of incompatible element abundances normalized to primordial mantle (Wood et al. 1981) for selected basalts and hawaiites from the islands of Ascension, Bouvet (stippled field), St. Helena, Gough and Tristan da Cunha.

magnitude of the Nb-Ta anomaly (as expressed by La/Nb and Th/Ta ratios in Table 2) is constant in Ascension, Bouvet and St. Helena lavas, and typical of OIBs from other islands in the Atlantic Ocean (B. L. Weaver et al., unpublished data) as well as of continental alkali basalts (e.g. Thomp- son et al. 1984).

Discussion

Amongst the highly-incompatible trace elements it is evident that the behaviour of, in particular, Ba, Nb and Ta may provide strong constraints on potential source reservoirs for OIBs. For instance, partial melting (especially at low de- grees of melting) of primordial (or primitive) mantle should produce melts with highly-incom- patible trace-element (Cs-La) abundances which, when plotted on normalized diagrams such as Fig. 6, would be either unfractionated or, more likely, slightly enriched in more highly-incompat- ible elements (e.g. Cs) over less highly-incompat- ible elements (e.g. La). It would not be possible to produce significant Nb-Ta or Ba anomalies (Fig. 6) from such a primordial source. Further- more, the depletion of Cs and Rb relative to K and La evident from Fig. 6 argues for derivation of OIBs from a source which is itself depleted in more highly-incompatible trace elements. It

would therefore seem that chemically primitive mantle (such as the lower mantle (All6gre 1982)) is not a suitable source for the production of OIBs.

In models for the chemical evolution of the crust-mantle system the continental crust and the mantle source for N-MORB are considered to be complementary reservoirs (e.g. Jacobsen & Wasserburg 1979; O'Nions et al. 1979; All6gre et al. 1983; All6gre & Rousseau 1984). To a first approximation those incompatible trace elements which are depleted in N-MORB are enriched in the continental crust. However, this is clearly not the case when consideration is given to the chemical budgets for Nb and Ta. Subduction- zone-related magmas, and indeed the bulk conti- nental crust itself, are strongly depleted in Nb and Ta relative to other highly-incompatible trace elements (e.g. Saunders et al. 1980; Thomp- son et al. 1984). For example, continental margin basalts and andesites typically have La/Nb ratios in the range 2.2-4.7 (data from the compilations of Ewart (1982)), while estimates of the bulk composition of the continental crust (Taylor & McLennan 1981 ; Weaver & Tarney 1984) suggest that the crust itself is similarly depleted, with La/ Nb ratios of 1.7-2.2. Island-arc volcanics have equally high La/Nb ratios (e.g. Tarney et al. 1981). Estimates of the composition of typical N- MORB (Sun 1980; Tarney et al. 1981 ; Pearce

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262 B . L . W e a v e r e t al.

1983) yield a La/Nb ratio of close to 1.0 which, considering the very similar D values for La and Nb during MORB genesis (Briqueu et al. 1984), represents the La/Nb ratio in the MORB reser- voir. A La/Nb ratio of 0.83-0.94 is likely for the chondritic Earth or primordial mantle (Sun 1980; Taylor & McLennan 1981; Thompson et al. 1984). Thus the production of continental crust with La/Nb > 2 from primordial mantle with La/ Nb ~ 0.9 requires a volumetrically large comple- mentary reservoir with a low (considerably less than 1) La/Nb ratio; the N-MORB source does not represent an appropriate reservoir. However, this reservoir would appear to be the source for ocean island (and continental) alkali volcanics with low La/Nb ratios (about 0.7).

It has been suggested that oceanic and conti- nental alkali basalts could be the product of partial melting of subducted ocean crust which has been resident in the deep mantle for a considerable period of time (Hofmann & White 1982; Ringwood 1982). Such a model is consistent with Pb isotope data for OIBs which imply derivation from source regions that have main- tained high U/Pb ratios (typical of altered oceanic crust) for periods of time of the order of (1.5- 2.0) x 103 Ma (Tatsumoto 1978; Sun 1980; Chase 1981). With regard to the behaviour of Nb and Ta, a key question is: what chemical modification occurs in oceanic crust during subduction ? Trace- element and isotopic considerations of the chem- istry of subduction-related magmas invariably require a component derived from the subducted ocean crust (Kay 1984; O'Nions 1984; Wilson & Davidson 1984). This component either could be a fluid produced by dehydration of the slab which carries LIL elements (and La, Ce?) into the overlying mantle wedge, or may involve a degree of partial melting of the slab, again under hydrous conditions (Kay 1984; Wilson & Davidson 1984; Wyllie 1984). The effect of these processes on the trace-element chemistry of the subducted ocean crust would be that LIL elements would be strongly depleted in subducting N-MORB-type ocean crust. However, it is also likely that the conditions pertaining during dehydration (and melting ?) of the ocean crust would stabilize minor phases (especially sphene (Hellman & Green 1979)) in the ocean crust which would retain high-field-strength elements such as Nb and Ta (Saunders et al. 1980; Briqueu et al. 1984; Thompson et al. 1984). Thus Nb and Ta would become strongly decoupled in their geochemical behaviour from other highly-incompatible trace elements, and the subducted ocean crust would develop low La/Nb and Ba/Nb ratios whilst being severely depleted in LIL elements. Unfor- tunately, lack of knowledge of phase relations

and partition coefficients for the probable min- eralogy of the ocean crust in the deep mantle (Ringwood 1982) precludes modelling of subse- quent partial melting of this material. It is perhaps probable that moderate degrees of melting (10~-15~) would produce undersatur- ated liquids with strong light REE enrichment (Hofmann & White 1982), positive Nb-Ta anomalies and increasing relative depletion in the more highly-incompatible trace elements reflecting the composition of the old ocean crust source. Such melts would mix with the mantle to form an enriched hot-spot component, the com- position of which is strongly reflected in the chemistry of lavas from Ascension, Bouvet and St. Helena. This component might further inter- act and mix with the depleted mantle source of N-MORB (producing transitional or T-type MORB), or could invade and vein (metasomatize) depleted upper-mantle peridotite (Menzies 1983; Hawkesworth et al. 1984) which might itself be subsequently melted to produce alkali basalts.

These general features are demonstrated in a plot of Ba/Nb against La/Nb in Fig. 7. As Nb is used as a denominator in the ratios plotted on both axes, mixing trends will be straight lines on

19

B a / N b

15

�9 A

/ ~ 5 2 7

N-MORB I I I ]

0 .6 0 .8 1.0 1.2

L a / N b

FIG. 7. Plot of Ba/Nb against La/Nb ratios for basalts and hawaiites from the islands of Ascension (~), Bouvet (,), St. Helena ([-]), Gough (O) and Tristan da Cunha (0). Also plotted are average ratios for basalts from DSDP Holes 525A, 527, 528 and 530A from the Walvis Ridge (data from Humphris and Thompson (1983)), and fields showing the average Ba/Nb and La/ Nb ratios in N-MORB and primordial mantle.

~30A

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G e o c h e m i s t r y o f ocean i s land basa l t s 263

this diagram. Since DBa ~< DNO ~< DLa, partial melt- ing of primitive mantle or N-MORB source mantle should produce liquids with somewhat lower La/Nb but somewhat higher Ba/Nb ratios than those in the source. Such a mechanism cannot account for the position of OIBs (as typified by Ascension and St. Helena) on this plot as melts from primordial or N-MORB source mantle, but does explain the relationship between primordial mantle and N-MORB source mantle (Fig. 7), with the latter being residual to the former. Continental crust, having high Ba/Nb (about 30-50) and La/Nb (greater than 2) ratios, plots well off the diagram to the top right. The primordial mantle composition can only be reconstituted by a three-component mixture of N-MORB mantle source, continental crust and OIB source (subducted ocean crust) in this diagram. It is interesting to note that the trend displayed by St. Helena lavas in Fig. 7 could reflect mixing between a hot-spot component and an N-MORB source. A similar, but less well- defined, trend could be present for Ascension lavas. In this context many of the transitional MORB from the SW Indian ridge (LeRoex et al. 1983) have variable La/Nb ratios but Ba/Nb ratios of less than 7 and may similarly represent mixing between the Bouvet hot spot and a depleted mantle source.

The anomalous chemistry of the Gough and Tristan da Cunha lavas is well displayed on Fig. 7. Also shown on this diagram are average Ba/Nb and La/Nb ratios for drilled basalts recovered from the Walvis Ridge (Humphris & Thompson 1983). Relative to Gough and Tristan da Cunha lavas, the Walvis Ridge basalts have lower abundances of highly-incompatible trace elements and lower Cen/Yn ratios (down to 1.5). The Walvis Ridge basalts therefore appear to be the products of higher degrees of partial melting than do the Gough and Tristan da Cunha lavas, but they still have the characteristically high Ba/ Nb and La/Nb ratios of the Gough and Tristan da Cunha basalts and hawaiites. This strengthens our argument that ratios involving the highly- incompatible trace elements are insensitive to degree of melting, except (as in the case of Tristan da Cunha) at very small degrees of partial melting. Interestingly, basalts from Site 530A (at the eastern end of the Walvis Ridge) have Ba/Nb and La/Nb ratios similar to those of Ascension, Bouvet and St. Helena lavas (Fig. 7). The Gough and Walvis Ridge lavas do not seem to be the products of partial melting of primordial mantle (as earlier argued for Ascension, Bouvet and St. Helena lavas); La/Nb ratios are higher (often substantially so) in these lavas than in primordial mantle (Fig. 7), which cannot occur under con-

ditions of partial melting where DNb,~DLa. Rather, the chemistry is best explained by mixing between a 'typical' hot-spot component (repre- sented by Site 530A basalt or Ascension, Bouvet and St. Helena lavas) and a component with high Ba/Nb and La/Nb ratios. It is clear from the Ba/ Nb-La/Nb relations in Fig. 7 that an N-MORB component or source is not a suitable end- member in any mixing model, as pointed out by Richardson et al. (1982) for Walvis Ridge lavas and LeRoex (1985) for Gough lavas. The wide range in, and good correlation between, Ba/Nb and La/Nb ratios in Gough lavas (Fig. 7) suggests variable proportions of the high Ba/Nb and La/ Nb ratio components in these lavas. This trend is not simply the result of dilution of Nb causing the Ba/Nb and La/Nb ratios to increase; with decreasing magnitude of the Nb-Ta anomaly (e.g. increasing La/Nb ratio) Ba abundances increase relative to other highly-incompatible trace elements such that there are reasonable correlations between La/Nb, Ba/Th and Ba/La ratios. Thus the end-member with the high Ba/ Nb and La/Nb ratios must be a component which itself has a negative Nb anomaly but a positive Ba anomaly. Such chemical characteristics are typical of pelagic sediments, which are strongly enriched in Ba relative to Th, Rb and Nb and in La relative to Nb (Hole et al. 1984; Kay 1984; Thompson et al. 1984). Terrigenous sediments are also a possible contaminant, but typically have rather low La/Nb, Ba/Nb and Th/Nb ratios which makes them unsuitable end-members in mixing models. Hence the trace-element data are qualitatively suggestive (as are the He isotope data (Kurz et al. 1982)) of a (pelagic) sedimentary component in the Gough, Tristan da Cunha and Walvis Ridge lavas.

We can make some attempt at quantifying the mixing process using Ba/Nb and La/Nb ratios, and approach the question of low-pressure mixing of the common parental ocean island magma type with pelagic sediment versus high-pressure mixing of components in the source region of some OIBs. A parental (or primary) OIB would have Ba/Nb and La/Nb ratios of 6.5 and 0.68 (typical of Ascension, Bouvet and St. Helena lavas (Table 2)), and might have a Nb content (corrected for the effects of low-pressure crystal fractionation) of 25 ppm. For an estimate of the composition of pelagic sediment we use the Pacific authigenic weighted mean sediment (PAWMS) of Hole et al. (1984), which has Ba/ Nb and La/Nb ratios of 1070 and 20.6 respectively and an Nb concentration of 1.25 ppm. A mixture of 20~ PAWMS + 80~ parental OIB is required to generate high Ba/Nb and La/Nb ratios (19.6 and 0.93 respectively) which approximate to the

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264 B . L . W e a v e r e t al.

values for these ratios in Gough lavas (Table 2). Varying the proportions of nannofossil ooze, diatom ooze, ferruginous clay and pelagic clay in the sediment (PAWMS comprises 95~ nannofos- sil ooze plus 5~ ferruginous clay) has little effect on element ratios in the sediment or on the amount of pelagic sediment required as a contam- inant. Addition of such large proportions of pelagic sediment would have a drastic effect on the major-element composition of the mixture for which there is little evidence. Also, why should such low-pressure mixing (presumably associated with a sub-volcano magma chamber) be appar- ently restricted geographically to the islands of Gough and Tristan da Cunha in the South Atlantic and to islands in the Indian Ocean (Dupr6 & All6gre 1983)? However, if mixing occurs in the source regions of some OIB the end- members would be pelagic sediment and sub- ducted ocean crust (the main mantle source component for OIB). The subducted ocean crust would have a Nb content of approximately 3 ppm and Ba/Nb and La/Nb ratios similar to those of the hot-spot material. In consideration of pelagic sediment older than Mesozoic, nannofossil and diatomaceous oozes would not be a component. For a ratio of ferruginous clay (Hole et al. 1984) to pelagic clay (Li 1982) of 2:8, the ancient pelagic sediment might have a Nb content of 12.3 ppm and La/Nb and Ba/Nb ratios of 5.7 and 341 respectively. In this case a mixture of 1~ pelagic sediment plus 99~ OIB source would have a La/Nb ratio of 0.92 and a Ba/Nb ratio of 22, similar to the values for these ratios in Gough lavas. Such a situation is compatible with the injection of a small proportion of sediment into the deep mantle along with subducted ocean crust.

Additional constraints on mixing processes in Gough, Tristan da Cunha and Walvis Ridge lavas come from Pb, Sr and Nd isotope data (Sun 1980; Cohen & O'Nions 1982; Richardson et al. 1982). Isotopic trends within Walvis Ridge basalts suggest a limiting composition for the enriched end-member with radiogenic Sr (8~Sr/ 86Sr=0.7058) and non-radiogenic Nd (143Nd/ 144Nd = 0.5122) and Pb (e.g. 2~176 = 17.1) (Richardson et al. 1982). Amongst the Walvis Ridge and Gough and Tristan da Cunha lavas, Site 525A basalts have isotopic characteristics suggesting the greatest proportion of this enriched end-member, which from the Ba/Nb-La/Nb trend in Fig. 7 is equated with the high Ba/Nb and La/Nb ratio component. The isotopic com- position of the depleted end-member is not well constrained by the Pb, Sr and Nd data (Richard- son et al. 1982). However, Ba/Nb-La/Nb rela- tions (Fig. 7) imply a hot-spot (rather than

MORB) end-member represented by Site 530A basalts, for which isotope data are lacking. The trend defined by Walvis Ridge and Tristan da Cunha lavas on a 2~176176176 dia- gram (Richardson et al. 1982) suggests that this depleted end-member could have 2~176 and 2~176 ratios similar to those of Ascen- sion and Bouvet lavas. If such a hot-spot component has a 2~176 ratio of approxi- mately 19.5, then about 40~ of the Pb in Gough and Tristan da Cunha (and about 70~ of the Pb in Site 525A) lavas would be derived from the component with high Ba/Nb and La/Nb ratios. A 2~176 ratio for the hot-spot component in excess of those of Ascension and Bouvet lavas would be required, however, as the 2~176 2~176 trend for Gough, Tristan da Cunha and Walvis Ridge lavas (Richardson et al. 1982) is parallel to that for MORB and other OIBs from the Atlantic. Thus the hot-spot component may not have isotopic compositions directly compa- rable with those of other islands in the S Atlantic.

The isotopic composition of the limiting en- riched end-member is not apparently compatible with that of typical pelagic sediment. Although non-radiogenic Nd and radiogenic Sr are consist- ent with the characteristics of pelagic sediment, non-radiogenic Pb is not. Modern Atlantic pelagic sediments generally have rather radi- ogenic Pb (2~176 ~ 19, 2~176 ~ 15.7 and 2~176 ~ 39.1 (Sun 1980)). However, if the model for derivation of OIB dominantly from old subducted ocean crust with the possibility of contamination occurring in the source is ac- cepted, the sedimentary component of interest will be that subducted with the ocean crust at some considerable time (of the order of (1.5- 2.0) x 103 Ma) in the past. The initial isotopic composition of this sediment is obviously uncer- tain, but it is likely to have had, relative to upper mantle at that time, radiogenic Sr and Pb and non-radiogenic Nd. The pelagic sediment would have had a Sm/Nd ratio less than the chondritic value (Sm/Nd ~ 0.20), a low Rb/Sr ratio (Rb/Sr 0.05) and very low U/Pb and Th/Pb ratios (# ~ 2 owing to high Pb contents in pelagic sediments of up to 50 ppm (Cohens & O'Nions 1982)). These arguments assume that the sediment maintains its chemical integrity through the subduction process. This is unlikely, but the chemical effects associated with dehydration and/or partial melt- ing of sediment during subduction are very poorly constrained. Subsequent to subduction, growth of radiogenic Nd, Sr and Pb would be inhibited (strongly so in the case of Pb) in this material, and could result in the present isotopic composi- tion predicted for the end-member with high Ba/ Nb and La/Nb ratios observed in the Gough,

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Geochemistry of ocean island basalts 265

Tristan da Cunha and Walvis Ridge lavas. M u c h of the foregoing must remain speculative until the composi t ion of this componen t can be more rigorously character ized. The var ia t ion of trace- e lement ratios (Fig. 7) in Gough lavas suggests that a combined detai led t race-e lement and isotopic study of this island might be of par t icular impor tance in approach ing this problem.

In model l ing the petrogenesis of Walvis Ridge and Gough lavas, R ichardson et al. (1982) and LeRoex (1985) ascribed the unusual isotopic and t race-e lement characteris t ics of these lavas to a 'double-enr ichment ' event in the mant le source

region. We would equate the first t race-e lement ' en r i chment ' event to a mel t componen t der ived dominan t ly from old subducted ocean crust in the source region for OIB. This event produces OIB with the ra ther uni form t race-e lement composi t ion of Ascension, Bouvet and St. He lena lavas. The second ' en r i chment ' event observable in the geochemis t ry of Gough, Tris tan da Cunha and Walvis Ridge lavas can be equated with con tamina t ion of the OIB source region by a small amount (1%-2%) of ancient pelagic sedi- ment which was subducted into the deep mant le along with the ocean crust.

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BARRY L. WEAVER, School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A.

DAVID A. WOOD, Amoco Europe and West Africa Inc., Amoco House, 1 Stephen Street, Tottenham Court Road, London W 1P 2AU, UK

JOHN TARNEY, Department of Geology, University of Leicester, Leicester LE1 7RH, U.K. JEAN LOUIS JORON, Group des Sciences de la Terre, Laboratoire Pierre Sue, C.N.R.S., Centre

d'Etudes Nucl6aires de Saclay, BP 2, 91190 Gif-sur-Yvette, France.

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