Transmission electron microscopy of L-group chondrites, 2. Experimentally annealed Kyushu

Abstract

Kyushu (shock facies d) samples were annealed for approximately 90 hours at 800°C, 900°C and 1000°C, to provide a basis for interpreting natural effects in shock facies e. With increasing temperature, dislocation distributions in olivine progressively recover, giving curved shapes, loops and arrays. Cracks become healed and small voids are trapped on healed cracks and regular dislocation arrays (subgrain boundaries). Plagioclase crystallizes from maskelynite at 900°C and 1000°C, with small grain-sizes comparable to those in facies-e material. Orthopyroxene shows recovered substructures, while diopside retains shock-induced twinning. In conjunction with other experimental data, the results indicate annealing of facies-e material for ∼ 10 3 s at 1000–1050°C, consistent with published estimates of initial cooling rates from Ar-diffusion/radiometric studies. The fine structures are distinguishable from those due to slower cooling, and analogies can be drawn with primary cooling effects in ordinary chondrites.