Shock-melted material in the Krymka LL3.1 chondrite: Behavior of the opaque minerals

Abstract

Six large millimeter- to centimeter-size regions of one specimen of the Krymka LL3.1 ordinary chondrite show evidence of having been completely or nearly completely shock-melted in situ, a phenomenon rarely observed in primitive chondrites. The shock pressure, nominally in the range of 75-90 GPa, could only have been 30-35 GPa in a porous material like fine-grained matrix. The melted regions have an igneous texture and their silicates are zoned and unequilibrated. Large metal-troilite intergrowths formed in these regions. The metal has a nickel content corresponding to martensite and the troilite contains up to 4.2 wt% nickel. Melting must have been very short and cooling very fast (>100 °C/h at high temperature). The metal contains up to 0.7 wt% phosphorus. Abundant chromite crystals and sodium-iron phosphate glass globules are found in troilite. The differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions. Surprisingly high Co concentrations (up to 13 wt%) were found in some metal grains in or at the periphery of melted regions. They likely resulted from sulfurization of metal by sulfur vapor produced during the shock. After solidification, at least one other shock led to mechanical effects in the melted regions.