Shock Experiments on Basalt - Ferric Sulfate Mixes at 21 GPa & 49 GPa and their Relevance to Martian Meteorite Impact Glasses

RELEVANCE TO MARTIAN METEORITE IMPACT GLASSES. M.N. Rao, D.K. Ross, T.H. See, L.E. Nyquist, S. Sutton, P. Asimow, SARD/ESCG, NASA Johnson Space Center, Houston, TX 77058 (thomas.h.see@nasa.gov), ARES, NASA Johnson Space Center, Houston, TX 77058, APS CARS & Dept. of Geophysical Sciences, Univ. of Chicago, Chicago, IL 60439, Div. of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125. Introduction: Large abundance of Martian atmospheric gases and neutron-induced isotopic excesses as well as Rb-Sr isotopic variations [1-3] determined in some impact glasses in basaltic shergottites (e.g., Shergotty #DBS, Zagami #H1 and EET79001 #27, #8 and #104) provide definitive evidence for the occurrence of a Martian reglotith component in their constituent mineral assemblages. Some of these glasses, known as gas-rich impact-melts (GRIM), contain numerous micron-sized iron sulfide blebs along with minor amounts of iron sulfate particulates. As these GRIM glasses contain a Martian regolith component and as iron sulfates (but not sulfides) are found to occur abundantly on the Mars surface, we suggested that the sulfide blebs in GRIMs were likely generated by shock-reduction of the parental iron sulfate bearing regolith material that had been incorporated into the cavities / crevices of basaltic host rock prior to the impact event on Mars [4]. To test whether the sulfates could be reduced to sulfides by impact shock, we carried out laboratory shock experiments on a basalt plus ferric sulfate mixture at 49 GPa at the Caltech Shock Wave Laboratory and at 21 GPa at Johnson Space Center (JSC) Experimental Imapact Laboratory. The experimental details and the preliminary results for the Caltech 49 GPa experiment were presented at LPSC last year [5]. Here, we report the results for the 21 GPa experiment at JSC and compare these results to obtain further insight into the mechanism of the bleb formation in the GRIM glasses. 21 GPa experiment: The 21 GPa experiment was carried out in the FlatPlate Accelerator, a 20 mm smoothbore, horizontal powder propellant gun located in the Experimental Impact Laboratory at JSC [6]. The target consists of moderately compressed disc of powder [Columbia River Basalt (CRB) and ferric sulfate Fe2(SO4)3, 9H2O, 65 μm grain size, ~45% porosity] that was placed in a 9 x 0.8 mm sample well in a stainless steel (SS) target assembly. The assembly was, in turn, pressed into a SS target holder and the face of the combined assembly was machined flat. The target was then impacted with a 304 SS flyer plate travelling at 1.01 km/s. The resulting impact generated a planar shock wave of 21 GPa in the sample mix [6]. The SS assembly was then machined open to recover the shocked sample material that was subsequently prepared as a thin section for petrographic and FE-SEM investigations. There are several regions in this polished thin section that show no evidence of melting which suggests that the temperature increase due to shock in these spots was relatively low. The lowest temperature minerals in the shocked sample are apatites in the CRB and the Fe2(SO4)3, 9H2O which we had added to the target mixture.