FELDSPAR DEFORMATION AS AN INDICATOR OF LOW-BAROMETRY SHOCK: PETROGRAPHIC INVESTIGATION OF EJECTA FROM THE TENOUMER IMPACT CRATER, MAURITANIA

Cratering theory and modelling has shown that shock pressures rapidly decrease with depth in an impact event. As a result, material from greatest depth receives the least impact energy and is deposited closest to the crater rim upon excavation. Here we present details of grain-scale deformation within the ejecta blanket of a simple crater (1.9 km diameter) to trace differential shock pressures.

The Tenoumer impact crater (22°55’N, 10°24’W) lies within Archean and Paleoproterozoic crystalline rock of the Reguibat Shield, Mauritania. Barometric estimations from quartz planar deformation features (PDFs) observed in distal melt ejecta suggest peak impact pressures of approximately 20 GPa. Within lithic breccias of the ejecta blanket, only the most distally deposited material shows any obvious quartz deformation, indicating that the majority of attenuated shock pressures were below that necessary to produce quartz PDFs. Samples from the outer ejecta blanket (1-2.5 radii from the crater), however, show a broad variety of feldspar deformation features, including: offset twin planes, alternate-twin alteration, deformation lamellae in alternate albite twins, multiple orientations of possible PDFs – some of which are through-going and some of which are constrained to alternating twins, and microcline inclusions (recrystallization from diaplectic glass?) within plagioclase.

Micro-Raman analysis of inclusion-bearing feldspars show broadened peaks at 1110 and 514 cm^-1, and a loss of the peak at 576 and 454 cm^-1, which is consistent with features in experimentally shocked feldspars (Velde and Boyer 1985), and microprobe analysis of differentially deformed albite twins show no geochemical variation that might suggest a non-shock origin. Combined, these observations suggest that shock features in feldspars may be useful in defining a range of shock pressures that may overlap, but which are predominantly lower than, that defined by quartz PDF barometry.