Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p31g..06n&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P31G-06
Other
[1034] Geochemistry / Hydrothermal Systems, [3616] Mineralogy And Petrology / Hydrothermal Systems, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
We have analyzed data from MRO/CRISM, HiRISE, and CTX to study the massifs in the NW Hellas region. The Hellas basin is thought to have formed during the late heavy bombardment [Acuña et al. 1999] as a consequence of a massive impact. The impact not only excavated rocks from the deep stratigraphy, but it also deposited enough energy into the ejecta to support hydrothermal conditions [Newsom 1980]. Spectral observations of the mineralogy of the martian highlands north of Hellas suggest that the region was experiencing aqueous activity during that era [Pelkey et al. 2007]. Therefore, spectroscopic studies of the well-preserved massifs that form the rim and ejecta in northwest Hellas have the potential to reveal zones of hydrothermal alteration. Additionally, studies of the deep crustal rocks excavated as part of the ejecta are of particular relevance in light of recent discoveries of carbonate-bearing rocks exposed in complex craters on Mars [Michalski and Niles 2010; Wray et al. 2011]. Our analyses reveal outcrops in the massifs where evidence for products of hydrothermal alteration are observed. In particular, we find evidence for smectites, prehnite, chlorite, and illite exposed in these outcrops (Fig 1). The spectra of these altered units also exhibit a strong, broad concave-up absorption in the 1-1.5 μm region, consistent with the presence of Fe2+ in olivine, suggesting that only partial alteration has occurred. The mineralogy of hydrothermal alteration products is a function of the original composition of the host rock; the temperature, chemistry, and pH of the water; and the overburden pressure [DeRudder and. Beck 1963; Morris et al. 2001; 2003; Brown et al. 2010; Inoue et al. 2010]. On Earth, prehnite can form via low-grade metamorphism, where it occurs as part of the prehnite-pumpellyite metamorphic facies [Blatt and Tracy 1995], or as a product of the low-temperature (100-350°C) hydrothermal alteration of mafic rocks [Freedman et al. 2009; Marks et al. 2010]. Similarly, both chlorite and illite can occur as low-grade metamorphic or low-temperature hydrothermal products. In hydrothermal alteration, the pH of the water needed for the formation of these minerals is neutral to basic. Hence, the observations are consistent with low temperature hydrothermal alteration of mafic rocks in a neutral to basic aqueous environment.
Noe Dobrea Eldar Z.
Swayze Gregg A.
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