MICROANALYSIS OF IDDINGSITE ALTERATION IN MARTIAN METEORITE NORTHWEST AFRICA 10416

Recent data gathered by science missions on Mars overwhelmingly support the idea that Martian crust once held a substantial amount of water that exerted a prominent influence on its geological evolution. Geochemical analyses of Martian meteorites that have experienced aqueous alteration can supplement these data sets and provide a level of precision not possible with remote rover analyses. They can also serve as analogs for data gathered by future missions studying the Martian hydrologic cycle. Northwest Africa (NWA) 10416, a Martian meteorite found in Mali in 2015, shows signs of ‘iddingsite’ hydrous alteration. The altered zones have been subjected to various microanalytical techniques in order to understand the types and extent of hydrous alteration present at or near the Martian surface.

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.