The Derivation and Interpretation of Geochemical Ratios Generated by Meteoritic Impact

Overview

Objectives/Goals Much has been done to determine the effects of shock metamorphism on the mineralogical structure of target material, but little has been done to document the effects of shock metamorphism on the chemical composition of the target material. This research quantifies the trace chemical changes induced by ultrahigh temperatures and pressure in granite samples due to meteoritic impact. Methods/Materials In this study, granite from the Silver Plume batholith was subjected to hypervelocity impact, which was induced by a two-stage light gas gun, using an aluminum projectile. X-Ray Fluorescence and Inductive Coupled Plasma Mass Spectrometry analyses were used to document the chemical composition of the target material before and after the exposure of target material to impact. Results Depletion and enrichment were noted as occurring on an elemental basis, as well as determining enrichment and depletion trends throughout entire affinities and groupings of elements as defined by V.M. Goldschmidt. Statistical analysis of the data shows relative homogeneity of the sample material and is useful in determining trends in enrichment and depletion. Data shows that enrichment is characteristic of the chalcophile elements (according to terrestrial affinity) and siderophile elements (as defined by meteoritic affinity). Ni, Cu, Ga, Pb, and Zn were each enriched in a consistent, significantly significant manner. Other elements were affected significantly, but in an inconsistent manner. Thin section analysis shows that a high silica polymorph formed, meaning that pressures of at least 170 GPa were reached, with temperatures near 1000 degrees C. Conclusions/Discussion Impact-induced shock metamorphism has a distinct and significant effect on the trace chemical composition of granite specimens. Trends of enrichment were seen in the chalcophile/siderophile affinity.