Characterization of Mineral Assemblages Associated with Shock Veins in L5 Chondrite Dhofar 1970

English Abstract:

The purpose of this research was to investigate a previously unstudied L5 chondrite, Dhofar 1970, available through loan from the University of Alberta Meteorite Collection. Dhofar 1970 was strongly affected by shock metamorphism, which caused small volumes of the rock to melt, forming a complex network of 10-1200 μm thick shock veins and isolated shock melt pockets. This study focuses on characterizing the composition and microtextures of minerals associated with Dhofar 1970 shock veins. Using a combination of optical microscopy, SEM, EMPA and Raman spectroscopy, the following minerals were identified: olivine, ringwoodite, jadeite, maskelynite, majorite and magnesiowüstite. Deformation in host rock minerals, including complete transformation of plagioclase to maskelynite, classify Dhofar 1970 as S6 or very strongly shocked. Majorite, observed as 1-3 μm size equant crystals has co-crystallized with magnesiowüstite from the shock melt. When compared to published experimental work on mineral stability and identical mineral assemblages in shock melts of other well-studied chondrites, this assemblage indicates Dhofar 1970 experienced a pressure of 18-25 GPa and temperature of 2000-2400 ^oC during impact collision. This estimate of shock conditions is significantly lower than estimates of ~55-75 GPa derived from the shock classification for ordinary chondrites. This discrepancy is attributed to the shorter duration of shock recovery experiments compared to those experienced by naturally shocked rocks and by reaction kinetics which are greatly enhanced close to hotter portions of the rock within and adjacent to the shock veins. Round clasts within the shock vein were also documented, with Fe-rich ringwoodite rims and Fe-poor olivine cores. Identical textures reported from Peace River and Y-74445 chondrites were interpreted to have formed by fractional crystallization. The textures documented in this study support solid-state transformation, which requires longer time for diffusion of Fe-Mg between ringwoodite and olivine.