Shock-induced deformation features in terrestrial peridot and lunar dunite

Abstract

Single crystals of terrestrial olivine (Fo90) were experimentally shock-loaded along [010] to peak pressures 280, 330, and 440 kbar and the resulting deformation features were compared to those present in olivine from lunar dunite 72415. Recovered fragments were examined on the Universal stage to determine the orientation of the planar fractures. With increasing pressure the percentage of pinacoids ({100}, {010}, and {001}) and prisms ({hkO}, {hOl}, and {Okl}), decreases (~40 to ~25%), whereas the percentage of bi pyramids {hkl} increases (~20-50% ). The complexity of the distribution of bipyramids also increases with increasing pressure. Other shock-induced deformation features, including varying degrees of recrystallization, are found to depend on pressure as observed by others. Lunar dunite 72415 was examined and found to contain olivine with well-developed shock deformation features. The relative proportion of pinacoid, prism, and bipyramid planar fractures measured for olivine from 72415 indicates that this rock appears to have undergone shock pressure in the range 330-440 kbar. It displays a preponderance of bipyramid fractures along (2 h = k) planes which have not previously been reported in laboratory or naturally shocked samples. If this dunite was brought to the surface of the moon as a result of excavation of an Imbrium event-sized impact crater, the shock-pressure range experienced by the sample and the results of cratering calculations suggest that it could have originated no deeper than 50-150 km.

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