Chondrule Flattening by Shock Recovery Experiments on Unequilibrated Chondrites

摘要

Shock recovery experiments were conducted using ALH-78084 H3 and Y-793375 L3 chondrites in the shock pressure range of 11-43 GPa to reproduce shock-induced melting and chondrule flattening. Shock-induced melt and chondrule flattening in 15 H3, 23 L3, and 23 LL3 ordinary chondrites were also investigated for comparisons. The shock experiments proved that, at least in unequilibrated ordinary chondrites, shock-induced melting occurred beyond 11 GPa. The melting occurs at the boundary between chondrule and matrix. The melts included fine-grained silicate minerals, glasses, and ameba or spherical metallic Fe-Ni or metallic Fe-Ni-iron sulfide with a eutectic texture that coincided with shock-induced melts in the investigated H/L/LL3 ordinary chondrites. Shock experiments also proved that shock-induced flattening of chondrules occurs and the flattening degree increases with increasing shock pressure. Considering the shock experiments not only of ordinary chondrites but also of carbonaceous chondrites, the flattening degree is not significantly affected by the density, porosity, and chondrule/ matrix ratio of chondrites. The long axes of chondrules in shocked ALH-78084 H3 and Y-793375 L3 chondrites have preferred orientations and the degree increases with increasing shock pressure. It is difficult to estimate quantitatively the shock pressure recorded in unequilibrated ordinary chondrites using the empirical formula between the aspect ratio of chondrules and shock pressure. Nevertheless, the investigated L/LL3 ordinary chondrites with shock-induced melts had higher aspect ratios (median, 1.36) and more strongly preferred orientations than those without shock-induced melts (median, 1.25).