2008 Joint Annual Meeting (5-9 Oct. 2008): Cathodoluminescence Study of Shocked K-Feldspar from Bosumtwi Meteorite Crater

The Bosumtwi impact structure (Ghana, West Africa) is arguably the youngest and best-preserved terrestrial impact structure [1]. The crater has a pronounced rim, with a rim to rim diameter of about 10.5 km. The structure forms a hydrologically closed basin [2]. The country rocks are, mainly meta-graywacke, shale, and phyllite of the Early Birimian Supergroup and some granites of similar age. Suevite, which occurs in restricted locations to the north and to the south-southwest of the crater rim, contains melt fragments, diaplectic quartz glass, ballen quartz, and clasts derived from the full variety of target rocks [3]. The Bosumtwi granites have tonalitic to quartz-dioritic compositions [3]. Here, we characterize shocked K-feldspar samples by means of cathodoluminescence (CL) microscopy and spectroscopy. The samples employed here are K-feldspar specimens from syenite from a sampling location of Bosumtwi impact crater. In the OM-image, well characterized shock features in K-feldspar grains. In general, the edges of the planar fractures (PFs) are ending in wedge shape. The surface of some K-feldspar exhibits slightly kink banding effect. The investigation exhibited shear faults in K-feldspar grains, and abundant planar deformation features (PDFs) lamellae. The characteristic cathodoluminescence spectral features of shocked K-feldspar samples are emission bands at 450 (Al-O--Al center), 500 (Si-O-…M2+), and 680 nm (Fe3+). In the SEM-CL images, PDFs and micro-twinning patterns are also discernible indicating that this sample was subjected under the high shock pressure regime (around 15 GPa).

Consequently, cathodoluminescence microscopy and spectroscopy should be a powerful technique to characterize shock-induced microdeformations such as PDFs and micro-twinning in minerals from the Lake Bosumtwi impact structure.

References: [1] Scholz et al. (2002) Geology 30, 939-942. [2] Koeberl et al. (2007) Meteoritics & Planetary Science 42, 483-511. [3] Karikari et al. (2007) Meteoritics & Planetary Science 42, 513-540.