TEMPERATURE AND REDOX CRYSTALLIZATION CONDITIONS OF THE SOUTHAMPTON PALLASITE

The olivine and chromite compositions of the Southampton pallasite have been studied to determine the petrologic (T-fO₂) conditions of the planetary body from which the specimen originated. The meteorite contains olivine with a relatively homogeneous forsterite content (Fo88±1), and is therefore classified as pallasite main group (PMG). Chromite is an accessory mineral and occurs either as an inclusion (48) enclosed in olivine or as a massive crystal (4 mm and 1 mm in diameter). In contrast to olivine, the chromite show significant compositional variability in their Cr# (90-96) and Fe# (60-80) values, with the higher values being related to the inclusions and one massive chromite. The estimated equilibrium temperatures, determined by the olivine-spinel geothermometer, are 1400±65°C for the massive chromite and 1000-1400°C for the inclusions, the majority of results cluster around 1200±65°C. A minority of inclusions, hosted by two different olivine, exhibit higher temperatures (1500-1800°C). The fO₂ has been estimated using the olivine-spinel oxygen geobarometer, with the aforementioned temperatures (°C) and a pressure of 1 GPa. Experiments at 0.5 and 1.5 GPa have shown that pressure has a negligible effect on fO₂. Within the uncertainty range of the EPMA analyses, fO₂ estimates indicate reduced conditions for the massive chromite crystals (ΔFMQ=-3) and inclusions (ΔFMQ = -1 to -2), comparable to those of the primitive mantle of Earth and Venus (ΔFMQ = -1 to -2). The minimum estimates for the fO₂ at the time the meteorite formed may be preserved by the massive chromite crystals. It is possible that decrease in temperature and increase olivine crystallization caused a slight increase in the oxidation state of the system, as shown by the fO₂ of chromite inclusions.

Southampton pallasite planetary science oxygen fugacity chromite spinel