Experimental partitioning of rare earth elements and strontium: Constraints on petrogenesis and redox conditions during crystallization of Antarctic angrite Lewis Cliff 86010

Abstract

The partitioning of REE and Sr among anorthite, fassaitic pyroxene, and synthetic melts similar in bulk composition to angrite LEW 86010 was studied experimentally at 1 atm and 1175–1210°C using the percent level doping technique. Most experiments were at an oxygen fugacity 1 log unit above the iron-wüstite buffer, but Eu and Gd partitioning were studied from iron-wüstite to just above quartz-fayalite-magnetite. 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 log unit range fO 2 studied. Parent melts calculated by inverting natural pyroxene and anorthite cores from LEW 86010 using partition coefficients from this study are in excellent agreement with one another. This agreement is strong evidence for (1) equilibrium between the natural mineral cores at the time the meteorite crystallized, and (2) lack of subsequent subsolidus diffusive modification of REE abundances in the cores. The overall levels of the computed REE patterns agree well with REE abundances in bulk samples of LEW 86010, supporting the idea that this sample formed through a process approximating closed-system fractional crystallization. The observed variation of D Eu / D Gd with fO 2 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 about 1 log unit above iron-wüstite, considerably more oxidizing than conditions under which common basaltic achondrites such as eucrites are thought to have formed.