Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11a1583s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11A-1583
Physics
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5430] Planetary Sciences: Solid Surface Planets / Interiors
Scientific paper
It is predicted that the primary crust of Mars crystallized from a magma ocean and would be well preserved at depth on a single plate planet but poorly exposed as impacts, volcanism and alteration has reworked the upper crust. In a few select locations, extensive excavation by impact or erosion has exposed unaltered mafic minerals of the Martian crust. The majority of these exposures occur within the uplifted central peaks and peak rings of Southern Highland craters. We examine the mafic compositions of these deeply excavated crustal rocks in an attempt to constrain the composition of the Martian crust and test models of planetary formation. The search for deeply excavated bedrock from HiRISE images is ongoing and has so far resulted in nearly 200 potential locations. Over half of these currently have CRISM spectroscopic observations with ~50 locations having good exposures of crustal rocks showing little to no alteration. It is this combination of deeply excavated minerals that has potential to tap the preserved primary crust of Mars. We focus our analysis on olivine and pyroxene as crustal formation models predict that these two minerals would dominate the modal mineralogy of the crystallizing crust with a garnet layer potentially stable at depth. The high-resolution visible and near-infrared spectroscopic data provided by the CRISM instrument is ideally suited for examining these compositional characteristics. Initial in-depth analysis of the central peak of Alga Crater shows excellent exposures of lithologies characterized by both olivine and pyroxene. The olivine-bearing unit here has a fayalitic composition and a dunite lithology. This ancient Fe-rich olivine is in stark contrast to the Mg-enriched olivine of the primitive mantle of Earth. The primary pyroxene-bearing unit was determined to be a low-calcium, high-Fe enstatite orthopyroxenite, consistent with the mineralogy of the ancient Mars meteorite ALH84001. These observations suggest that the crust crystallized into compositionally homogeneous units in close proximity to allow single impact to sample multiple lithologies. The units are consistent with the late-stage crystallization of a hot magma ocean enriched in Fe. Here we expand the results to all suitable Southern Highland exposures to check regional compositional consistency and examine global trends. Initial results support similar compositions in excavated crust throughout the southern highlands, though the presence of both olivine and pyroxene lithologies are rarely well exposed in the same central peak. Additional analysis will continue to test the emerging hypothesis that the upper primary crust is the direct result of the late stage crystallization of a magma ocean, with no density driven overturn, that results in compositionally segregated fayalite and enstatite rich crust.
Murchie Scott L.
Mustard John F.
Skok John R.
Tornabene Livio L.
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