The redox state of planetary basalts

Abstract

A well established data base exists for the oxidation‐reduction (redox) states of basalts on the Earth and on the Moon; the former equilibrate along the fayalite‐magnetite‐quartz buffer curve, and the latter crystallize in the field of metallic iron stability, below the iron wustite buffer. Preferred accumulation of volatiles and the disproportionation of water into hydrogen and oxygen in thick terrestrial lavas and in ponded lava lakes results in high states of oxidation equivalent to that of hematite stability. At the other extreme, lunar basalts exhibit the effects of subsolidus reduction and estimates yield T = 700‐1000°C and fO2 = 10−16 to 10−23 atms. Based on a number of cosmogenic properties, models for planetary interiors, and the sequence of condensation with heliocentric distance from the protosun, estimates for the redox states of inner solar system planetary basalts yield the following results: Basalts on Mercury and the Moon crystallize below the iron‐wustite buffer curve; Venusian basalts are more oxidized than those on Mercury, less oxidized than those on Earth, and crystallization within the field of wustite stability is suggested; basalts on Earth are dominantly in the field of magnetite stability in close proximity to the fayalite‐magnetite‐quartz buffer curve; Martian basalts are estimated to crystallize in the upper regions of magnetite stability and well into the hematite field of stability expressed in terms of temperature and oxygen fugacity.