Mineralogy, composition, and alteration of Mars Pathfinder rocks and soils: Evidence from multispectral, elemental, and magnetic data on terrestrial analogue, SNC meteorite, and Pathfinder samples

Major element, multispectral, and magnetic progenies data were obtained at Ares Vallis during the Mars Pathfinder mission. To understand the compositi onal, mineralogical, and process implications of these data, we obtained ma jor element, mineralogical, and magnetic data for well-crystalline and nano phase ferric minerals, terrestrial analogue samples with known geologic con text, and SNC meteorites. Analogue samples include unaltered, palagonitic, and sulfatetic tephra from Mauna Kea Volcano (hydrolytic and acid-sulfate a lteration), steam vent material from Kilauea Volcano (hydrolytic alteration ), and impactites from Meteor Crater (relithification). Salient results for Mars Pathfinder include: (1) Band depths BD530b and BD600 and the reflecti vity ratio R800/R750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of H2O and occurri ng in composite particles along with subordinate amounts of other ferric mi nerals. Hematite and hematite plus nanophase goethite are most consistent w ith the data, but maghemite, akaganeite, schwertmannite, and nanophase lepi docrocite are also possible interpretations. Ferric oxides that are consist ently not favored by the data as sole alteration products are jarosites and well-crystalline goethite and lepidocrocite. (2) The strength of the ferri c adsorption edge (R750/R445) implies the Fe3+/Fe2+ values for Pathfinder r ock and soil are within the ranges 0.7-3 and 3-20, respectively. (3) Ferrou s silicates are indicated for subsets of Pathfinder rocks and soils. One su bset has a band minimum near 930 nm that can attributed to low-Ca pyroxene. Alternatively, the band could be a second manifestation of certain ferric oxides, including nanophase goethite, maghemite, akaganeite, and schwertman nite. Another subset has a negative spectral slope from similar to 800 to 1 005 nm which could result from the high-energy wing of a high-Ca pyroxene a nd/or olivine band, a mixture of bright and dark materials, and, for rocks, thin coatings of bright dust on dark rocks. (4) Chemical data on Pathfinde r rocks and soils are consistent with two-component mixtures between an "an desitic" rock with low MgO and SO3 concentrations (soil-free rock) and a gl obal, basaltic soil with high MgO and SO, concentrations (rock-free soil). Pathfinder rock-free soil can be modeled as a chemical mixture of SNC meteo rites and the Pathfinder soil-free rock. (5) Pathfinder soil cannot be obta ined by chemical alteration of Pathfinder rocks by any of the hydrolytic an d acid-sulfate alteration processes we studied. Presumably, global mixing h as obscured and possibly erased the elemental signatures of chemical altera tion. (6) The strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite (Fe-T i spinels). The saturation magnetization of the tephra samples (0.5-2.0 Am- 2/kg) is at or below the low end of the range inferred for Martian dust (4/-2 Am-2/kg), implying that lithogenic Fe-Ti spinels are a possible candida te for the Martian strongly magnetic phase. (7) The predominantly palagonit ic spectral signature and magnetic nature of Martian soil and dust are cons istent with glassy precursors with imbedded Fe-Ti spinel particles. Compari son with lunar glass production rates suggests that production of sufficien t quantities of glassy materials on Mars by volcanic and impact processes i s sufficient to account for these observations.