EFFECTS OF SHOCK ON FERRIC IRON AND MAJOR ELEMENTS IN PLAGIOCLASE, PYROXENE, AND OLIVINE

The redox state of Fe-bearing minerals can serve as a sensitive proxy of the redox state of planetary surfaces and atmospheres. Although Fe-bearing minerals in lunar samples and Martian meteorites have been well studied, those petrologic results have been interpreted in light of poorly-constrained shock effects. The goal of this study is to evaluate the changes in individual mineral chemistries and Fe oxidation state as a result of shock. This study examines the run products of shock experiments originally chosen from the mafic layered Stillwater Complex in Montana by Johnson and Horz. These experiments shocked aliquots of anorthosite (>90% An75 plag), and orthopyroxenite (>90% En85 opx) under an oxygen atmosphere of 10-3 torr at pressures ranging from 0 to 60 GPa. This study attempts to quantify the effects of shock on Fe content and the Fe3+/FeTotal ratio in plagioclase (initially 52%), orthopyroxene (initially ~16%), clinopyroxene (initially ~16%), and olivine (initially 0%) from these Stillwater rocks. Major element chemistries of all minerals did not change under shock pressures up to 60 GPa. Redox state of iron was measured in situ on 10 x 15 micron spots by synchrotron micro-XANES spectroscopy at the NSLS, at Brookhaven National Laboratory. The Fe3+/Fetotal ratio in plagioclase decreases from highly oxidized (~55%) under low shock pressures (up to 20 GPa) to more reduced (~15%) at pressures over 38 GPa. This change in plagioclase is related to the structural conversion of plagioclase to maskelynite. Higher shock pressures yield a small increase in olivine Fe3+/Fetotal of 8 – 10%; such oxidation has also been observed in Martian peridotite ALH77005, where localized shock oxidation of olivine has been quantified. Fe3+/Fetotal remains unchanged in the shocked pyroxenes, suggesting a steric control on reduction and/or oxidation. These data suggest that shock pressures experienced by meteorites and rocks on planetary surfaces may be sufficient to either change Fe oxidation state or render the samples susceptible to rapid changes without otherwise altering chemical composition.