ELECTRON PROBE MICROANALYSIS, MICRO X-RAY DIFFRACTION, AND DEUTERIUM-HYDROGEN ANALYSIS OF HYDROUS ALTERATION IN MARTIAN METEORITES NORTHWEST AFRICA 10416 AND 8159

NWA 8159 is an augite basalt, while NWA 10416 is an olivine-phyric basalt, both of which have been confirmed to be of Martian origin via oxygen isotope analysis.

Electron Probe Microanalysis: NWA 10416 contains large (1-2 mm) olivine phenocrysts which are visible in hand specimen and clearly show signs of iddingsite alteration. Electron backscatter and qualitative elemental maps of these regions were created using the JEOL-8200 Superprobe at the University of New Mexico. These show distinct zones of high aluminium and iron and low silica and magnesium (Fig. 2). The alteration does not appear to be connected with the terrestrial weathering of the meteorite, visible as high Ca calcite zones in the fractures. Because of the similarities of the meteorites, electron microanalysis was performed on the olivines in NWA 8159, which are microcrystalline, to see whether similar alteration could be found. The elemental maps created show analogous alteration patterns, though they approach the resolution limits of the electron microprobe. Quantitative elemental analysis was also performed on altered points within the olivine grains which returned low totals for oxide percentages on the order of 88-92 weight percent, suggesting the presence of hydrated mineral phases.

Discussion: While laihunite has been identified as one of the hydrothermal phases, the alteration in NWA 8159 and 10416 displays microcrystalline texture that requires further analysis by instruments capable of higher resolution analysis. Therefore sections will be cut from both meteorites using focused ion beam milling. These sections will be viewed under a transmission electron microscope so that additional phases may be identified. To constrain the Martian origin of the alteration in NWA 8159, D/H analysis via secondary ion mass spectroscopy (nanoSIMS) will be necessary, since the affected olivine grains in the sample are microcrystalline.