On the siting of gases shock-emplaced from internal cavities in basalt

Abstract

Noble gases were extracted by stepped combustion and crushing from basalts which contained gas-filled cavities of controlled sizes prior to shock at 40 GPa. Analysis of fractions enriched and depleted in shock glass from a single sample gave a factor of 2 higher gas abundances in the glass-rich separate. Release patterns were nearly identical, suggesting similar siting (in glass) in both fractions. Crushing of a sample released ~45% of implanted noble gases, but only ~17% of N 2, indicating that most or all of the noble gas was trapped in vesicles. Analysis by SEM/EDS confirmed the presence of vesicles in glassy areas, with an average diameter of ~10 μm. Samples with relatively large pre-shock cavities were found to consist of up to 70–80% glass locally and generally exhibit greater local shock effects than solid and densely-packed paniculate targets at the same shock pressure, though the latter give higher gas emplacement efficiencies. The petrographic results indicate that in situ production of glassy pockets grossly similar to those in the shergottite EETA 79001 is possible from shock reverberations in the vicinity of a vug. However, the siting of the gases points to a more complex scenario, in which SPB gas and melt material were probably injected into EETA 79001.