Alkali mobility in shocked basalt

Abstract

Plagioclase (An_(45)-An_(55)) in unshocked and experimentally shocked samples of Vacaville basalt was analyzed by electron microprobe. Sodium X-ray intensity measurements were found to be unstable for the diaplectic and shock-fused plagioclases, and for synthetic plagioclase glasses with An ≾ 80. Unstable microprobe analyses have also been reported for shocked chondritic meteorites. That is, sodium is mobilized and volatilized under the electron beam, probably by a thermal mechanism. Crystalline plagioclase, even when shocked to just below the transition pressure to diaplectic glass, gives stable analyses as does synthetic plagioclase glass with An > 80, and shocked calcic plagioclase in Apollo 16 sample 68415. In this study, we conclude that: (i) there is no evidence for selective sodium loss due to the shock process; (ii) sodium mobilization under the electron beam reflects increasingly severe lattice damage for increasing shock pressures; (iii) sodium mobilization is, in any case, not significant for calcic plagioclase. Thus we find no evidence supporting shock-related alkali loss as an important lunar process for impacts generating maximum pressures below about 1 Mbar, corresponding to projectile velocities ~4 km/sec for metallic objects and ⪝6 km/sec for silicate objects.