| 39397 |
abstract |
The Mars Exploration Rover Spirit investigated the igneous and alteration mineralogy
and chemistry of Home Plate and its surrounding deposits. Here, we focus on using
thermochemical modeling to understand the secondary alteration mineralogy at the Home
Plate outcrop and surrounding Columbia Hills region in Gusev crater. At high temperatures
(300 °C), magnetite occurs at very high W/R ratios, but the alteration assemblage
is dominated by chlorite and serpentine over most of the W/R range. Quartz, epidote,
and typical high-<i>T</i> phases such as feldspar, pyroxene, and garnet occur at low
W/R. At epithermal temperatures (150 °C), hematite occurs at very high W/R. A range
of phyllosilicates, including kaolinite, nontronite, chlorite, and serpentine are
precipitated at specific W/R. Amphibole, with garnet, feldspar, and pyroxene occur
at low W/R. If the CO<sub>2</sub> content of the system is high, the assemblage is
dominated by carbonate with increasing amounts of an SiO<sub>2</sub>-phase, kaolinite,
carpholite, and chlorite with lower W/R. At temperatures of hydrous weathering (13
°C), the oxide phase is goethite, silicates are chlorite, nontronite, and talc, plus
an SiO<sub>2</sub>-phase. In the presence of CO<sub>2</sub>, the mineral assemblage
at high W/R remains the same, and only at low W/R, i.e., with increasing salinity,
carbonate precipitates. The geochemical gradients observed at Home Plate are attributed
to short-lived, initially high (300 °C) temperature, but fast cooling events, which
are in agreement with our models and our interpretation of a multistage alteration
scenario of Home Plate and Gusev in general. Alteration at various temperatures and
during different geological processes within Gusev crater has two effects, both of
which increase the habitability of the local environment: precipitation of hydrous
sheet silicates, and formation of a brine, which might contain elements essential
for life in diluted, easily accessible form. |