Elephant Moraine Polymict Eucrites: a Eucrite-Howardite Compositional Link

ELEPHANT MORAINE POLYMICT EUCRITES: A EUCRITE-HOWARBITE COMPOSITIONAL LINK H. Palme, B. Spettel, A. Burghele, G. Weckwerth, and H. Wanke, Max-Planck-Inst. fur Chemie, Mainz, F.R. Germany; J.S. Delaney and ha. Prinz, Dept. Mineral Sci., Amer. Mus. Natural Hist., New York, NY 10024. XRF and INAA chemical da ta for bulk samples and clasts were obtained for three polymict Antarctic eucrites: EETA 79004, 79005, and 79011. Two (79004 and 7901 1) have petrographic similarities (1) and all three have very similar compositions (Tables 1,2). They a r e compositionally intermediate between the rnonomict eucri tes and the most tteucritett-rich howardites, e.g. Malvern (Fig. 1, Table 3). These compositions f i t a chemical continuum between eucrites and diogenites, supporting the mixing hypothesis for howardites (2 and references therein). Elephant Moraine eucri tes have high Ti and Ta perhaps indicating an ilmenite enriched eucri t ic endmember. They a r e also depleted in CaO relative t o ALHA and Yamato polymict eucri tes (3) a s they have more low-Ca pyroxene modally than eucrites. (4, unpublished data) Minor and t race element contents of most clasts a re similar to the bulk, but with larger compositional spread, a situation common in howardites (5). I n e clasts from EETA 79004 and 79011 have nearly identical Fe/Mg ratios in the pyroxenes (1) despite textural and compositional variability. This may reflect homogenization during metamorphic annealing of these breccias (1). As a result, mobilisation of K and S is observed in bulk samples and clast. The high K contents of 79004 and of the some clasts a r e unique. The K/La ratios a r e in all cases above the typical eucri t ic value of 95. Mobility of K is not uncommon on meteorite parent bodies (e.g. LL-chondrites, 6). High S and Se contents a r e found in a basalt clast (79004,41, Table 1). Similar unusual enrichments of K and Se a r e also noticed for less metamorphosed 79002. Because of the similarity in bulk composition and the apparent, very unusual, K-enrichment, 79005 may be derived from the same source region on the EPB a s 79004 and 79011. A more likely reason for the K-enrichmen', however, may be terrestr ial weathering. All three meteorites contain terrestr ial weathering products including, perhaps, a K-bearing zeolite produced in the Antarctic environment (7). 79004 and 79005 have K-Ar ages of 3.3 and 3.8 by respectively (7). This would allow about 50% of terrestrial K in 79004. In addition t o the equilibrated mineral and lithic clasts in 79004 and 79011 there a r e a few unequilibrated mafic clasts (1). 79004,51 (clasa E:,36 of 1) has high FeO, low Ca and a V-shaped REE-pattern (Fig. 2). A mixture of a small fraction of t r ace element enriched plagioclase and a large amount of pyroxene could account for these features. The Se-content however is not correspondingly higher. This clast, in thin section (,36) i s a typical unequilibrated basalt. Feldspar chips ( ~ n ~ ~ 0 1 i ) from 79005 (Table 2) have the typical plagioclase REE pattern. However t h e absolute content of the REE is so high tha t the liquid from which the plagioclase crystallised must have been more 100 times enriched in REE