MICROANALYSIS OF IDDINGSITE ALTERATION IN MARTIAN METEORITE NORTHWEST AFRICA 10416
NWA 10416 is an olivine-phyric basalt which has been confirmed to be of Martian origin via oxygen isotope analysis. It is composed primarily of augite, orthopyroxene, plagioclase + maskelynite, olivine, and various oxides. It also contains large, 1-2 mm phenocrysts of olivine whose composition suggests that they are xenocrysts in disequilibrium with their host basalt. Optical microscopy, electron microprobe analysis, and transmission electron microscopy have confirmed that the cores of these xenocrysts have altered to iddingsite, an hydrous assemblage composed of multiple phases including ferric laihunite, various iron oxides and ferrihydrite, and amorphous 'silica gel.' The iddingsite grains are rimmed by pristine olivine in equilibrium with the groundmass, indicating that the xenocrysts must have altered to iddingsite before the basalt had fully crystallized. Terrestrial weathering is also present in the form of calcite veins deposited in fractures which cut across the entire rock, including altered iddingsite areas. This type of weathering is typical of the Sahara desert but cannot account for the presence of iddingsite, as conditions necessary for hydrothermal alteration do not exist in any terrestrial desert.