EXPERIMENTAL PARTITIONING OF RARE-EARTH ELEMENTS AND STRONTIUM - CONSTRAINTS ON PETROGENESIS AND REDOX CONDITIONS DURING CRYSTALLIZATION OFANTARCTIC ANGRITE LEWIS CLIFF-86010

The partitioning of REE and Sr among anorthite, fassaitic pyroxene, an d synthetic melts similar in bulk composition to angrite LEW 86010 was studied experimentally at 1 atm and 1175-1210-degrees-C using the per cent level doping technique. Most experiments were at an oxygen fugaci ty 1 log unit above the iron-wustite buffer, but Eu and Gd partitionin g were studied from iron-wustite to just above quartz-fayalite-magneti te. Pyroxene partition coefficients are correlated with Al content of the pyroxene. Despite the fassaitic nature of the synthetic pyroxenes, partition coefficients are not dramatically different from those for diopside. Pyroxene/melt REE partition coefficients range from .08 for La to .45 for Yb. Plagioclase/melt coefficients (except for Eu) range from .022 for La to .004 for Yb. D(Eu) varies by nearly a factor of 2 for pyroxene and nearly a factor of 5 for plagioclase over the four lo g unit range of fO2 studied. Parent melts calculated by inverting natu ral pyroxene and anorthite cores from LEW 86010 using partition coeffi cients from this study are in excellent agreement with one another. Th is agreement is strong evidence for (1) equilibrium between the natura l mineral cores at the time the meteorite crystallized, and (2) lack o f subsequent subsolidus diffusive modification of REE abundances in th e cores. The overall levels of the computed REE patterns agree well wi th REE abundances in bulk samples of LEW 86010, supporting the idea th at this sample formed through a process approximating closed-system fr actional crystallization. The observed variation of D(Eu)/D(Gd) with f O2 for pyroxene and plagioclase was combined with Eu and Gd abundances from mineral cores in LEW 86010 to estimate the oxygen fugacity under which this sample crystallized. Results indicate crystallization at a bout 1 log unit above iron-wustite, considerably more oxidizing than c onditions under which common basaltic achondrites such as eucrites are thought to have formed.