Diverse Alteration Minerals Around Martian Impact Craters Revealed by MRO-CRISM: Indicators of Hydrothermal Activity or Subsurface Aqueous Alteration?

Details Diverse Alteration Minerals Around Martian Impact Craters Revealed by MRO-CRISM: Indicators of Hydrothermal Activity or Subsurface Aqueous Alteration? Diverse Alteration Minerals Around Martian Impact Craters Revealed by MRO-CRISM: Indicators of Hydrothermal Activity or Subsurface Aqueous Alteration?

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p34a..04e&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P34A-04

Other

1034 Hydrothermal Systems (0450, 3017, 3616, 4832, 8135, 8424), 5220 Hydrothermal Systems And Weathering On Other Planets, 5410 Composition (1060, 3672), 5464 Remote Sensing, 6225 Mars

Scientific paper

Hydrated silicates indicating aqueous alteration of mafic rocks have been identified in central peak, wall, and ejecta rocks of many impact craters. CRISM has revealed 1000s of exposures in the Southern Highlands, often associated with craters, with Fe/Mg smectite-bearing rocks, and a small percentage with more diverse alteration minerals. Chlorite is not uncommon and kaolinite, illite or muscovite, hydrated silica, and zeolites are sometimes present, albeit infrequently. Understanding the geologic setting and formation processes of aqueous mineral deposits is important for determining whether alteration pre-dated the impact crater or whether mineralogic products are more recent and represent evidence for post-impact alteration, perhaps in hydrothermal systems. Using combined CRISM-CTX-HiRISE observations, we assess this in detail for craters of unusually diverse alteration mineralogy in the region west of the Isidis basin. Three types of information are considered: (1) Stratigraphy and geomorphic setting of crater hydrated silicates, (2) constraints on the temperature, pressure, and geochemical conditions required for the formation and stability of each alteration mineral, and (3) models of the temperatures experienced in impact processes. Two main classes of craters with diverse alteration minerals exist between the Nili Fossae and Antoniadi basin: (1) craters dominated by chlorite with illite and smectite also present and (2) craters with Fe/Mg smectite, chlorite, and the zeolite analcime in the central peaks. In (2), materials in sands ringing the peak are hydrated and Si-OH bearing, which may indicate either hydrated silica (e.g. chalcedony) or an aqueously altered basaltic glass. These sands also correspond to the unique units identified by TES with elevated quartz and alkali feldspars and interpreted to be quartzofeldspathic (QF)/granitoid material. Fe/Mg-rich smectites or chlorites with accessory zeolite, silica, quartz, and K-feldspar result from hydrothermal alteration in terrestrial craters (e.g. Allen et al., 1982; Naumov, 2005) and suggest a possible reinterpretation of the QF material in (2) as hydrothermal in origin rather than igneous. Experimental data show analcime-smectite assemblages result from aqueously altered glasses and basalt powders at temperatures T < 200 C (Robert and Goffe, 1993). Illite-chlorite assemblages are stable at T~200- 260 C. Variation in the temperature, water-rock ratio, and fluid chemistry of a hydrothermal system may result in these distinctive mineral assemblages observed in Nili Fossae craters. An alternative hypothesis is that alteration materials associated with craters reflect changes in underlying crustal materials west of Isidis and that aqueous alteration pre-dated formation of the impact structures in which these minerals are mapped. Smectite transforms to illite upon burial at T>50-80 C, and illite and chlorite altered at depth may have been excavated by impact. Both hypotheses--subsurface pre-impact alteration and post-impact hydrothermal alteration--will be examined with new MRO data.

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