Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p44a..10s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P44A-10
Mathematics
Logic
1039 Alteration And Weathering Processes (3617), 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5415 Erosion And Weathering, 6225 Mars
Scientific paper
Mineralogical abundance of primary minerals versus secondary minerals, chemical mixing relationships, and elemental ratios have been used assess aqueous alteration at Gusev Crater and Meridiani Planum. However, limited work has used chemical data to quantify the duration of aqueous alteration on Mars. The objectives of this work are to combine laboratory dissolution rates with Ti-normalized mass-balance analysis of APXS and Mossbauer data to estimate aqueous alteration times on Mars. Wishstone rocks are candidate parent materials for the Watchtower materials. Mass-balance analysis of the Wishstone-Watchtower sequence indicated that chemical alteration caused 37% loss of Na from Wishstone in forming the Watchtower materials. Mineralogy assumed from the APXS indicated Na loss was attributed to oligoclase dissolution. Laboratory dissolution rates of oligoclase under arguably Mars relevant conditions (25 C pH 4) and assuming a particle size of 1mm were used to calculate an aqueous alteration time of 2200 years. The Mossbauer and APXS data were combined to calculate Fe losses from olivine and pyroxene dissolution in the same Wishstone-Watchtower sequence. Lower aqueous alteration times of 150 and 800 years were calculated for olivine and pyroxene, respectively. If all three minerals were exposed to similar aqueous conditions, then similar dissolution times would be expected for all minerals. A possible explanation for the variation of dissolution times between the three minerals will be provided below. Calculated aqueous alteration times are minimum times because laboratory rates are measured under high water to rock ratios, low ionic strength, and do not consider the formation of surface precipitates. Field conditions can have low water:rock ratios, higher ionic strengths, and surface precipitates that inhibit mineral dissolution. The high concentration of nano-phase iron oxides (npFeOx) in Watchtower could be derived from Fe release from olivine and pyroxene. The formation of surface npFeOx could inhibit dissolution and extend the aqueous alteration times of the olivine and pyroxene particles. Secondary precipitates derived from oligoclase dissolution at pH 4 are not likely for oligoclase surfaces. This hypothesis suggests that the calculated olivine and pyroxene dissolution times would then increase and possibly approach the calculated oligoclase time. Despite the disparities in the dissolution times, this work demonstrates the value of combining Ti-normalized mass-balance with laboratory dissolution rates in assessing duration of aqueous activity on Mars. Results from this work will serve as the foundation for developing more sophisticated kinetic dissolution calculations that will provide improved estimates of aqueous alteration times on Mars.
Golden D.
Hausrath E.
Ming Dengming
Sutter Brad
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