Disequilibrium features in experimentally shocked mixtures of olivine plus silica glass powders |
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| Authors: | Rand B Schaal |
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| Institution: | (1) 1335 First Street, 95020 Gilroy, CA, USA |
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| Abstract: | Seventeen shock-recovery experiments were performed on powder mixtures of one part (by weight) olivine (St. John's forsterite) plus two parts silica glass (pure vitreous silica) in order to characterize the physical and chemical interaction of two chemically incompatible components during shock. Powders of <45  m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6.2 to 64.2 GPa (1 GPa= 10kbar).Petrographie features observed in thin section attest to mechanical and thermal metamorphism. Samples shocked to pressures from 6.2 to 39.3 GPa form compacted, mosaic, granular aggregates with fractured and strained grains. Samples shocked to pressures from 42.9 to 64.2 GPa form vesicular, mixed melts containing flow schlieren and relict olivine fragments. Petrographic disequilibrium is manifested in cataclastic textures showing deformational anisotropy and in thermal effects showing non-uniform intergranular melting. This disequilibrium is caused by an irregular pressure distribution resulting from the rapid collapse of pore spaces.The chemical composition of the shock melts are similar in each of six samples shocked to pressures of 42.9 to 64.2 GPa. Melt chemistry is bimodal in each sample. Colorless melts are 99.9% SiO2 and represent pure silica glass melts; pale to dark green melts range in composition from 47% to 64% SiO2 and represent a progressive mixture of olivine melt (41% SiO2) with silica glass melt. Surprisingly, the compositions of the colored glasses are intermediate between the composition of pure olivine and the bulk composition of the original starting material (79% SiO2) and are similar to enstatitic pyroxene compositions (50% to 57% SiO2; 33% to 37% MgO). Although bulk compositions of shocked samples are unchanged, the creation of melts with pyroxene compositions instead of bulk sample compositions may indicate that an incipient eutectic-type fusion may have occurred in small olivine-normative domains surrounding individual olivine grains. Chemical disequilibrium is evidenced by the creation of these olivine-normative melts from a quartz-normative starting compositions and by the chemical heterogeneity in the melts. |
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