CE ANOMALIES IN THE LEW85300 EUCRITE - EVIDENCE FOR REE MOBILIZATION DURING ANTARCTIC WEATHERING

There is abundant evidence for widespread REE mobilization in Antarctic eucrites, resulting in Ce and Eu anomalies and lower whole-rock REE abundances relative to typical eucrites. Using secondary ion mass spectrometry (SIMS), we investigated the microdistribution of Ce anomalies in the Antarctic polymict eucrite LEW85300. Ce anomalies (positive and/or negative) are present in all three of the minerals (plagioclase, pyroxene and silica) analysed. The anomalies occur in grains from all four clasts as well as in mineral and lithic fragments from the surrounding matrix; thus there appears to be no lithologic association. Silica, which has been heavily shocked and altered, exhibits a LREE-enriched pattern with negative Ce and Eu anomalies, consistent with a derivation of the REE from Ca-phosphate dissolution: trivalent REE are leached from the phosphates, with preferential retention of tetravalent Ce, and redeposited on silica. Pyroxene is also widely affected by REE mobilization, due to an extensive, shock-induced microcrack network along cleavage planes. A tendency for the largest Ce anomalies to occur at low LREE concentrations suggests that pyroxene itself experienced REE leaching with concomitant Ce retention. Ce anomalies are rare in plagioclase because this mineral lacks an extensive network along which REE can be mobilized. The widespread REE mobilization and evidence for other forms of weathering requires that trace element data be used cautiously, especially for Antarctic eucrites.