Shock-induced fine-grained recrystallization of olivine: Evidence against subsolidus reduction of Fe^(2+)

Abstract

Electron spin resonance (ESR) studies have been carried out on three single grains of terrestrial olivine (F0_(90)) shock loaded along [010] to peak pressures of 280, 330, and 440 kbar. The results indicate that neither metallic Fe similar to that observed in returned lunar soils nor paramagnetic Fe^(3+) caused by oxidation of Fe^(2+) has been produced in these shock experiments. Trace amounts of Mn^(2+) have been detected in both shocked and unshocked olivine. The ESR signals of Mn^(2+) show spectral features which are found to correlate with the degree of shock-induced recrystallization observed petrographically. The increasing mass fraction of recrystallized olivine correlates with increasing shock pressures. This phenomenon is modelled assuming it results from the progressive effect of the shock-induced transformation of the olivine to, a yet unknown high-pressure phase, and its subsequent reversion to the low-pressure, olivine phase. The mass fraction of recrystallized material is predicted to be nearly linear with shock pressure.