Shock and annealing in aubrites: Implications for parent-body history

Abstract

Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more-pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact-induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near-surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4-level shock-damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.