HEMATITE, PYROXENE, AND PHYLLOSILICATES ON MARS - IMPLICATIONS FROM OXIDIZED IMPACT MELT ROCKS FROM MANICOUAGAN CRATER, QUEBEC, CANADA

Visible and near-IR reflectivity, Mossbauer, and X ray diffraction dat a were obtained on powders of impact melt rock from the Manicouagan Im pact Crater located in Quebec, Canada. The iron mineralogy is dominate d by pyroxene for the least oxidized samples and by hematite for the m ost oxidized samples. Phyllosilicate (smectite) contents up to similar to 15 wt % were found in some heavily oxidized samples. Nanophase hem atite and/or paramagnetic ferric iron is observed in all samples. No h ydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250 degree s C. Oxidative alteration is thought to have occurred predominantly du ring late-stage crystallization and subsolidus cooling of the impact m elt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near-IR band minimum correlated with th e extent of aleration (Fe3+/Fe-tot) and ranged from similar to 1000 nm (high-Ca pyroxene) to similar to 850 nm (bulk, well-crystalline hemat ite) for least and most oxidized samples, respectively. Intermediate b and positions (900-920 nm) are attributed to low-Ca pyroxene and/or a composite band from hematite-pyroxene assemblages. Manicouagan data ar e consistent with previous assignments of hematite and pyroxene to the similar to 850 and similar to 1000 nm bands observed in Martian refle ctivity spectra. Manicouagan data also shaw that possible assignments for intermediate band positions (900-920 nm) in Martian spectra are py roxene and/or hematite-pyroxene assemblages. By analogy with impact me lt sheets and in agreement with observables for Mars, oxidative altera tion of Martian impact melt sheets above 250 degrees C and subsequent erosion could produce rocks and soils with variable proportions of hem atite (both bulk and nanophase), pyroxene, and phyllosilicates as iron -bearing mineralogies. If this process is dominant, these phases on Ma rs were formed rapidly at relatively high temperatures on a sporadic b asis throughout the history of the planet. The Manicouagan samples als o show that this mineralogical diversity can be accomplished at consta nt chemical composition, which is also indicated for Mars from analyse s of soil at the two Viking landing sites.