Evidence for High and Low Temperature Alteration across Home Plate, Gusev Crater

Over the last ~2 years in Gusev Crater, the Mars Exploration Rover Spirit has observed coherent variations in mineralogy and geochemistry along an almost circular traverse of Home Plate, an 80 m-diameter outcrop of layered, basaltic tephra. Observations of Home Plate from orbit by the High Resolution Imaging Science Experiment (HiRISE) camera (0.3 m per pixel) and from the summit of Husband Hill (0.7 km to the north) by the Panoramic Camera (Pancam) onboard Spirit show clear longitudinal differences in visible/near- infrared (VNIR) colors, where its eastern region is more blue and western region is more red. Up close, Pancam spectra of rock targets brushed by the Rock Abrasion Tool (RAT) revealed similar variations and confirm that color contrasts observed at greater distances reflect meaningful differences in outcrop mineralogy. Mineralogical observations by the Spirit Mössbauer Spectrometer and Miniature Thermal Emission Spectrometer (Mini-TES) are consistent with the VNIR data, indicating that pyroxene and magnetite dominate the Fe-bearing assemblage at the east side, while olivine, nanophase ferric oxide (npOx), and glass are more abundant at the west. Alpha Particle X-Ray Spectrometer (APXS) observations indicate that eastern Home Plate has higher concentrations of Si, Al, Zn, Ni, and K, while Cl and Br are higher in the west. Compositional similarities in major elements between the two sides of Home Plate, as well as geologic observations indicate that upper, cross-bedded materials that span Home Plate belong to the same stratigraphic unit. However the compositions of more fluid-soluble elements and Fe-bearing minerals in the upper unit vary independently of stratigraphy. We propose that these variations are the result of two distinct alteration regimes: one that produced npOx at the west and another that recrystallized olivine to form pyroxene by Si addition at the east. Abundant npOx at the west is the likely product of breakdown and oxidation of glass or other igneous phases by either low temperature hydrothermal alteration or chemical weathering. Some mass transport during the recrystallization event is implied by small but systematic changes in composition across Home Plate (e.g., decreasing SiO2 and Zn from east to west). Under hydrothermal conditions, SiO2 solubility is increased and Zn and Ni can form temperature-dependent complexes with Cl. The higher concentrations of SiO2, Zn, and Ni in eastern Home Plate rocks indicate that higher temperatures were likely attained there (likely ~300° C to subsolidus temperatures). The localized nature of the high temperature alteration indicates perhaps that the event was relatively short-lived, temperature gradients were steep, and lateral advection was minor across Home Plate.