Shock-metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB-07A, Bosumtwi impact crater, Ghana

Abstract

Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4-415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by ω{1013 }-equivalent crystallographic planes, although significant percentages of π{1012} and π other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40-45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water- or volatile-rich target for the Bosumtwi impact event.