Generation of abnormal trace element abundances in Antarctic eucrites by weathering processes

Abstract

Based on REEs, Antarctic eucrites can be divided into two groups: those showing normal trace element characteristics (e.g., similar to Juvinas) and those showing abnormal trace element abundances. Many Antarctic eucrite, polymict eucrite, and basaltic clast samples show the abnormal trace element abundances with REE patterns exhibiting positive Ce anomalies (sometimes negative Ce anomalies), positive Eu anomalies, and low abundances of the remainder of the REEs, with the LREEs generally being at lower relative abundances than the HREEs. Most samples of crystalline clasts from the polymict eucrites LEW85300, LEW85302, and LEW85303 show the abnormal patterns, while the glassy matrixes of these meteorites show normal patterns. Exterior samples generally show more abnormal patterns (larger anomalies, greater depletions) than interior samples from the same meteorites. Comparison of all basaltic eucrite literature data combined with our data shows that positive and negative Ce anomalies and positive Eu anomalies are found in about 61% of Antarctic eucrite analyses and are virtually unknown in non-Antarctic eucrite analyses. Further, positive Ce anomalies and positive Eu anomalies are commonly associated with Antarctic eucrites having low REE concentrations. Consideration of mineral/melt partition coefficients shows that it is unlikely that Ce anomalies are magmatic features from the HED parent body. Cerium anomalies on earth are generally restricted to the weathering zone where the relative ease of oxidizing Ce to the +4 state allows for fractionation of Ce from the +3 REEs. We believe the unusual REE patterns of abnormal Antarctic eucrites arise from weathering effects generated in or on the Antarctic ice. Our suggested scenario involves formation of melt water and its equilibration with the atmosphere which promotes dissolution of REE-rich phosphates and oxidation of Ce. Tetravalent Ce can then be fractionated from the trivalent REE in solution. The details of the weathering process are unclear and will require detailed chemical and SEM investigations of eucrites for their elucidation. We predict that rapidly chilled eucrites with glassy, rather than crystalline, mesostases will be more likely to survive the Antarctic environment without alteration of their REE patterns. Occasional S, Se, and K enrichments are likely due to weathering in the Antarctic environment as well, but these enrichments are not well correlated with Ce anomalies.