Shock experiments on maskelynite-bearing anorthosite

Abstract

Shock experiments, over the pressure range 10–60 GPa, have been carried out on naturally shocked maskelynite-bearing anorthosite from the Mistastin impact structure, Labrador, Canada. With the exception of fracturing at ∼ 10 GPa, the petrographic and textural characteristics of the maskelynite are not changed by reshocking up to 60 GPa. Reshocking has no apparent effects on pyroxene and amphibole, which are present in minor amounts in the rock. The overall texture of the rock is not affected by reshocking, with the implication that multiple shock events in non-brecciated samples are not readily detectable by simple petrographical observation. Abnormal refractive index values for maskelynite, however, may provide a means of detecting a second shock event. Reshocking at pressures lower than the estimated initial shock pressure (33–35 GPa) tend to produce an increase in the refractive index of the original maskelynite, with a large effect between about 10 and 20 GPa. Refractive indices of maskelynite reshocked to 19.7 GPa are close to that of unshocked labradorite crystals, though no changes in the petrographic and X-ray characteristics of maskelynite are detectable. Reshocking at pressures higher than the initial shock tends to lower the refractive index of maskelynite. The increase in refractive index of maskelynite reshocked to pressures lower than the initial pressure is interpreted as due to shock densification of the diaplectic glass above the Hugoniot elastic limit and below the mixed phase regime. The refractive index data suggest that the low-high-low density transition of maskelynite occurs about 8 GPa below that of the crystal of corresponding composition. Based on these results, the abnormally high refractive indices of Shergotty maskelynite is interpreted as indicating a second shock event in the range 10–20 GPa, following an earlier shock event at 30–50 GPa which transformed the original plagioclase to maskelynite.