Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p34c..02m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P34C-02
Mathematics
Logic
[6055] Planetary Sciences: Comets And Small Bodies / Surfaces, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The abundance of clay mineral deposits in the >3.5 Ga Noachian crust of Mars suggests widespread water-rock interaction and aqueous alteration during the first billion years of the planet's history. In addition, the ancient rock record of Mars is remarkably well preserved compared with that of Earth due to the lack of crustal recycling. However, aqueous and eolian sedimentation, volcanism, and deposition of impact ejecta all likely contributed to the geologic record of Mars for several billion years, and all of these processes would contribute to the burial of near-surface sediments. On Earth, the burial of smectitic clay minerals results in the formation of mixed-layered clay minerals, commonly illite/smectite or chlorite/smectite, which are ultimately transformed to illite and chlorite, respectively. The rate and extent of these processes depend on factors such as initial smectite composition, fluid chemistry, time, and temperature. The oldest smectites on Earth may only be 600 Myr old, whereas smectites identified on Mars are inferred to be >3.5 Gyr old, a remarkable difference even in the absence of plate tectonics for the latter. If Martian clay mineral deposits are truly this ancient and still smectitic, then this would have profound implications for geothermal gradients and/or the duration of water-rock interaction in the ancient crust. Here we examine visible-near infrared CRISM spectra for a variety of Martian clay deposits in greater detail, exploring the possibility that many previously identified smectite deposits are more consistent with mixed-layered chlorite/smectite. Analysis of laboratory data for di- and trioctahedral smectites and mixed-layered chlorite/smectite reveals that subtle differences in the reflectance spectra between these clay minerals can be parameterized and thus allow them to be distinguished from each other. Alhough some ambiguity exists in distinguishing physical mixtures of chlorite and smectite from mixed-layered chlorite/smectite, these cases can often be identified by visual inspection of the reflectance spectra. Application of our derived parameters to CRISM spectra reveals that most clay mineral deposits on Mars are consistent with mixed-layered chlorite/smectite or chlorite, and that smectitic clays are relatively uncommon. This suggests that most clay minerals on Mars have experienced some degree of chloritization and diagenesis if they were originally formed as smectite. Future work that helps to constrain the exact degree of chloritization may also help to constrain the geothermal gradient on early Mars, longevity of crustal fluid circulation, and the potential for Martian clay deposits to preserve organic material.
Bish David L.
Bristow Thomas F.
Milliken Ralph
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