Studies of an artificially shock-loaded H group chondrite

Abstract

Five samples of the naturally unshocked Kernouve (H6) meteorite were artificially shock-loaded to pressures of 70, 165, 270, and 390 kbar and the silicates and metal examined optically, by scanning and transmission electron microscopy and by thermoluminescence (TL). Olivine deformation is closely comparable to that in naturally shocked meteorites, producing dislocations with Burgers vector [001]. At pressures of ⩽165 kbar, these are formed in well-defined slip planes. At 270 kbar, olivine develops optical mosaicism, has high dislocation densities throughout and is also highly fractured. Recovery, due to heating is minimal. In orthopyroxene, the deformation mechanism changes, from the clino-inversion to unit-dislocation slip, between 70 and 165 kbar. In diopside, (001) and (100) twinning was produced. Plagioclase is inferred to have been progressively converted to maskelynite, but some is still present in 270 kbar sample. The microhardness of the kamacite in the samples increases with shock pressure. The α → ϵ transformation pressure in the kamacite is 30–40 kbar higher than observed for iron meteorites. Annealed kamacite displays incipient polycrystallinity and α-martensite and taenite sometimes contains slip lines. Troilite acquired cracks, undulose extinction, twins, polycrystallinity and finally melted as the shock pressure increased. At pressures over 200 kbar there was a systematic decrease in the natural TL and the TL sensitivity. Detailed considerations of changes in the natural TL TL sensitivity ratio for various regions of the TL glow curve suggest that two processes were effective during shock; thermal drainage of electron traps and a reduction in the effective trap density. It is suggested that the latter process associated with the vitrification of feldspar, the TL phosphor. An additional sample was subjected to a shock pulse which was “spiked” instead of square. Very distinctive changes were apparent; thermal effects are conspicuous and with widespread annealing (~600–800°C) of metal and sulfide. Glassy, opaque veins were produced which are analogous to the black veins in shock-lithified gas-rich meteorites. Anomalous low-temperature TL was induced, suggesting that a new or modified phase or mineral has become the dominant TL phosphor.