252-8 Cathodoluminescence Study of Shocked K-Feldspar from Bosumtwi Meteorite Crater

Poster Number 222

See more from this Division: Topical Sessions
See more from this Session: Terrestrial Impact Structures: Origin, Structure, and Evolution (Posters)

Tuesday, 7 October 2008
George R. Brown Convention Center, Exhibit Hall E

Szabolcs Nagy, Petrology and Geochemistry, Eötvös Lorand University, Budapest, Hungary, Tasuku Okumura, Open Research Center, Okayama University of Science, Okayama, Japan and Arnold Gucsik, Geochemistry, Max Planck Institute for Chemistry, Mainz, Germany
Abstract:
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.

See more from this Division: Topical Sessions
See more from this Session: Terrestrial Impact Structures: Origin, Structure, and Evolution (Posters)