Further Evidence for Geochemical Diversity, and Possible Bimodality, Among Cumulate Eucrites

Details Further Evidence for Geochemical Diversity, and Possible Bimodality, Among Cumulate Eucrites Further Evidence for Geochemical Diversity, and Possible Bimodality, Among Cumulate Eucrites

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.303w&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 303

Computer Science

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Scientific paper

We have used INAA, RNAA, and fused-bead analysis to determine the bulk compositions of numerous Antarctic eucrites (and also the LEW88516 SNC meteorite). Only a few of the most unusual eucrites can be discussed in the limited space here. Takeda et al. (1988) noted that Y791195 is a slowly cooled eucrite, with an equant, medium-grained texture, and pyroxene exsolution lamellae up to 10 micrometers across. In Y791195,81-3, we find lamellae up to 14 micrometers across. In this respect, Y791195 resembles RKPA80224, in which exsolution lamellae up to 12 micrometers across. We have previously discussed the evidence that RKPA80224 is a mildly accumulative rock that formed from an unusually low-mg parent melt. Our second analysis of RKPA80224 only partly confirms the unusually low incompatible trace element (ITE) content, but the Ce anomaly is consistently small (Ce/La = 0.90-1.02 x CI), and based on a weighted mean composition the implied parent melt is still unlike any noncumulate eucrite (see Fig. 1, which shows results from mass balance calculations modeling the sample as a mixture of cumulus px and plag, plus trapped melt). A parent melt similar to an extreme low-mg, variant of the "Nuevo Laredo Trend" would plausibly account for RKPA80224. The spectrum of possible parents for Y791195 is similar, even though its "true" Sm content is slightly obscured by weathering (Ce/La = 1.4 x CI). The [Sm] used in the figure is scaled to the highest CI-normalized REE concentration. Data of Mittlefehldt and Lindstrom (1991) indicate that except for exterior samples "showing extreme rustiness," Sm even in weathered eucrites is generally not altered beyond a few tens of pct. relative (sample size seems to account for more of the variation in [Sm] among interior, non-rusty samples). Even assuming a Sm content twice that assumed in the figure, the parent melt still must be well to the low-MgO/FeO, low-Sm side of all known eucrites. The LEW87002 eucrite is brecciated, but probably monomict. The pyroxene is uniformly Mg-rich (opx mg = 68), yet diogenitic px is not present. No Ce anomaly was detected. If lunar standards can be applied to eucrites, our RNAA siderophile result for Au gives a marginal indication of "pristinity": [Au] = 2.5 X 10^-4 times CI; also [Re] = 4.3 X 10^-4 times CI, but [Ir] = 7.4 X 10^-4 times CI (possibly linked to the unusually mafic nature of this rock). Like Binda and the mildly accumulative Pomozdino, LEW87002 appears to be the product of a melt along the moderate-mg, high- ITE "Stannem Trend." Collectively, these samples suggest that cumulate eucrites formed from parent melts more diverse than the known noncumulate eucrites. The data also hint at a geochemical bimodality for the parent melts, reminiscent of the bimodality among ancient lunar cumulates, which show a paradoxical tendency for high-mg cumulates to be more ITE-rich than low-mg cumulates. The same basic mechanism might be responsible: later melts are more directly linked to the high-mg mantle, but tend to be contaminated by mixing with residual melts left over from the slightly older Nuevo Laredo Trend linked cumulates. References: Delaney J. S. (1988) Lunar and Planet. Sci. XX, 236-237. Mittlefehldt D. W. and Lindstrom M. M. (1991) Geochim. Cosmochim. Acta 55, 77-87. Takeda H., Tagai T., and Graham A. (1988) Thirteenth Symp. Ant. Mets. (Tokyo), pp. 142-144. Figure 1, which in the hard copy appears here, shows cumulate and cumulate-like eucrites and possible parent melt compositions.

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