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

Abstract

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 -equivalent crystallographic planes, although significant percentages of and 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.

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

“…Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep ICDP borehole LB-07A in the Bosumtwi impact structure revealed two sequences of impactites below the post-impact crater sediments and above coherent basement rock (Coney et al 2007a;Morrow 2007). The upper impactites (333.38-415.67 m in depth) comprise an alternating sequence of suevite and lithic impact breccias.…”

“…Notably, no ballen quartz, which is abundant in fallout suevite (e.g., Boamah and Koeberl 2006), has been recorded in the withincrater impact breccias. Morrow (2007) describes quartz shock-metamorphic features in suevite samples from core LB-07A that were produced by: a) deformation, such as abundant planar microstructures and grain mosaicism; b) phase transformations, such as rare diaplectic quartz glass, and possible coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) postimpact transformations, such as abundant, variably decorated planar microstructures and patchy grain toasting. The quartz PDF orientations closely match those reported elsewhere from strongly shocked, crystalline-target impactites.…”

Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana

“…1. A limited petrographic description of these samples is given in Table 1 (for a more detailed description see Deutsch et al 2007;Ferrière et al 2007aFerrière et al , 2007bKarikari et al 2007;Morrow 2007). A single sample from core LB-05B was also analyzed.…”

“…1) of these two cores has been established by Koeberl et al (2007aKoeberl et al ( , 2007b. The petrography and shock characteristics of the impactites are described in detail in accompanying papers (Deutsch et al 2007;Ferrière et al 2007aFerrière et al , 2007bKarikari et al 2007;Morrow 2007).…”

The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB‐08A litho‐log, related ejecta, and shock recovery experiments

“…In addition, we discuss the shock metamorphic overprint of rocks in this core and provide results of the experimental validation of the shock assessment. Another lithological description for core LB-08A is provided by Ferrière et al (2007a); Coney et al (2007a) describe the lithoprofile and shock metamorphism in rocks (Morrow 2007) of core LB-07A.…”