Petrographic Characteristics of Quartz in Suevitic Impact Breccia, Drillcore

Introduction: Individual grains and small lithic clasts in thin-sectioned suevitic breccia samples from the International Continental Scientific Drilling Program (ICDP) Bosumtwi impact crater drillcore LB07A were examined petrographically to evaluate the occurrence , abundance, and characteristics of unshocked and shock-metamorphosed quartz. The analyzed samples were taken across a ~42-m-long (138-ft-long) interval of the core, from a drilling depth of ~341 m (1120 ft) to ~383 m (1256 ft) (Table 1). To facilitate analyses, samples were chosen from an interval in the core containing a relatively high amount of fine-grained particles and small lithic clasts within po-lymict, matrix-rich suevitic breccia and micro-breccia (Fig. 1). A petrographic microscope with point-counting stage was used to obtain a semi-quantitative measure of (a) the overall quartz abundance and distribution , and (b) the relative proportion of unshocked to shocked-quartz grains within the interval (Table 1). Individual grains and lithic clasts recorded during the analysis ranged from 60 µm (coarse silt) to ~4 mm (granule) in diameter. Occurrence of quartz: Within the suevitic brec-cia, quartz occurs most commonly as isolated, relatively small (silt-to medium sand-sized) monocrystal-line grains; as isolated polycrystalline quartz and composite quartz-feldspar grains; and as inclusions within larger lithic clasts. Potential target-rock sources for the quartz, quartz-feldspar, and heterolithic quartz-bearing lithic grains within the impact breccia include (a) 2.2-2.1 Ga Birimian Supergroup mica schist, quartz-feldspathic banded schist, sedimentary and meta-sedimentary rock fragments (greywacke, meta-greywacke, quartzite), phyllite, and meta-volcanic grains; (b) 2.13-2.12 Ga Tarkwaian Supergroup clastic sedimentary and meta-sedimentary rock fragments, quartz-feldspar porphyry, and meta-volcanic grains; and (c) granitic fragments derived from post-Tarkwaian intrusions [1]. Quartz comprises an estimated 10-20 vol. % of the examined thin sections, with the remaining constituents dominated by feldspar grains, lithic clasts, very fine-to fine-grained clastic matrix, cryptocrystalline to very fine-grained altered and devitrified impact melt/glass, clay minerals, and other accessory minerals (mica, mafic, and opaque grains) within the matrix [cf. 1, text figs. 24-25]. Shock-metamorphic features: Common microscopic shock-metamorphic effects identified in the quartz grains include (a) irregular sub-planar and cur-vilinear fractures, associated with comminuted grains and possible grain-margin percussion features; (b) grain mosaicism and irregular petrographic extinction patterns; (c) " toasted " regions within grains, comprising dark, cloudy, nonpleochoic patches containing abundant micrometer-to submicrometer-scale fluid inclusions [2], often associated with denser concentrations of planar fractures (PFs) and planar deformation features (PDFs) (Fig. 2); (d) PFs, typically 5-10 µm wide and spaced >15 µm apart; (e) PDFs, discussed below; …