HR: 16:00h
AN: P14A-01    [Abstracts]
TI: Ancient Crust, Hydrolytic Weathering, and Impact Melt Preserved in the Isidis Basin, Mars
AU: * Mustard, J F
EM: John_mustard@brown.edu
AF: Brown University Geological Sciences, Box 1846, Providence, RI 02912
AU: Poulet, F
EM: francois.poulet@ias.u-psud.fr
AF: Institud d'Astrophysique Spatiale, Orsay Campus, Orsay, 91405 France
AU: Head, J W
EM: james_head_III@brown.edu
AF: Brown University Geological Sciences, Box 1846, Providence, RI 02912
AU: Mangold, N
EM: mangold@geol.u-psud.fr
AF: Université Paris-Sud, ORSAY Cedex, Orsay, 91405 France
AU: Bibring, J P
EM: bibring@ias.u-psud.fr
AF: Institud d'Astrophysique Spatiale, Orsay Campus, Orsay, 91405 France
AB: The Isidis impact basin was formed in ancient cratered uplands but has since experienced volcanic modification of the western basin rim (Syrtis Major Planitia), tectonic deformation of the impact basin rim (Nili Fossae), and filling of the Isidis Basin (Hesperian ridged plains and the Vastitas Borealis Formation). This wide array of post-basin geomorphic units led to the general impression that primary Isidis Basin impact deposits had been highly modified and largely buried by eolian, volcanic, and fluvial deposits, and general weathering processes. Mineralogy derived from OMEGA data (olivine, pyroxene, phyllosilicates) coupled with high resolution imaging (MOC, THEMIS, HRSC) and topography (MOLA) show that the composition of early Mars is preserved in this region in the form of crustal composition, altered crust, and melt from the Isidis event. Noachian crust in the northwest quadrant of the Isidis basin exhibits enrichment in low-Ca pyroxene interpreted to result from early crust formation. This region also exhibits enrichment in olivine that was first reported from TES observations and interpreted to be exposures of ancient crust or post-Isidis ultramafic volcanism. Olivine occurrences mapped with OMEGA and merged with imaging data that show that in some regions of Nili Fossae the olivine occurs in distinct outcrops several 10s of meters thick draped over the underlying bedrock. The morphology of these outcrops resembles impact melts observed on Mars and the Moon. With analogy to the Orientale basin on the Moon, these olivine-rich outcrops occur between the inner and outer rings of the Isidis basin. In Orientale, this location hosts the largest concentration of impact melt. Olivine observed in Libya Montes also lies between these rings. In Nili Fossae, the olivine deposits sit on outcrops enriched in phyllosilicate minerals, likely Fe-rich smectite clays. This altered bedrock occurs over a large region of the Noachian crust. Thus this region preserves a Noachian-aged, large-scale alteration formed due to low-grade hydrothermal alteration or weathering that are draped by impact melt from the formation of the Isidis Basin.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 5464 Remote sensing
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005