Exsolution of Iron-Titanium Oxides in Magnetite in Miller Range (MIL) 03346 Nakhlite: Evidence for Post Crystallization Reduction in the Nakhlite Cumulate Pile

摘要

MIL 03346 is one of the most mesostasis-rich nakhlites (1) and thought to have equilibrated at oxygen fugacities near the fayalite-magnetite-quartz oxygen (FMQ) buffer ((2,3)). Studies of FeTi oxides in nakhlites have led to additional constraints on their equilibration temperatures and fO2s (4,5,6,7). Comparison of these results to fO2s calculated for shergottites indicates that nakhlites are among the most oxidized samples from the martian meteorite suite (2). The mesostasis of MIL 03346 contains skeletal titanomagnetite. Several scientists noticed several years ago (e.g. (8)) that this titanomagnetite contains very fine oxidation-driven exsolution lamellae (Figure 1). However, the lamellae are so small that they cannot be characterized by electron microprobe analysis (EMPA). Here we select several areas for focused ion beam (FIB) extraction, prepare transmission electron microscopy (TEM) foils, and identify and analyze the lamellae using TEM at the Johnson Space Center (JSC). The resulting analyses are combined with previous work on nakhlites to interpret the thermal and oxidation history of this meteorite group.