Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13e..01b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13E-01
Mathematics
Logic
[1060] Geochemistry / Planetary Geochemistry, [5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The nakhlite olivine clinopyroxenite meteorites preserve a detailed record of water-rock interaction on Mars. Detailed knowledge of the mineralogy of the secondary assemblages [1,2,3] now allow us to accurately model the fluid from which it was deposited [4]. This combined mineralogical and fluid characterisation provides constraints on the near surface conditions associated with the similar assemblages identified e.g. with CRISM on the surface of Mars [5]. Brittle fractures in the olivine grains of the Lafayette nakhlite are filled with a zoned assemblage of siderite at the margins of veins, then phyllosilicate, and finally silicate gel in the centre of the veins [2,3]. The mesostasis also contains parts of this alteration assemblage. The silicate gel is the most abundant secondary phase within the nakhlites. The phyllosilicate in Lafayette is a complex mixture of 2:1 and 1:1 phyllosilicate: Fe smectite and Fe serpentine. Soluble salts are also present although some of these e.g. in Y000749 are terrestrial contamination. The gel has a similar composition to the Lafayette phyllosilicate, with its amorphous nature shown by HRTEM [3]. The carbonate and gel compositions vary between in respect to their position in the nakhlite cumulate pile [2,3]. For instance, Lafayette and NWA998, which sample the bottom of the nakhlite cumulate pile [6] near the fluid source have a high gel Mg#, and the nakhlites which sample levels closer to the martian surface, a lower Mg#. We have used CHILLER [7] to model the hydrothermal brine from which the secondary minerals were deposited. The starting materials are derived from a mixture of Lafayette bulk, Lafayette olivine and mesostasis. The early parts of the fluid's history with formation of the siderite are associated with dissolution of the olivine. The carbonate compositions in the nakhlites suggest water/rock (W/R) ratios ~10, 100oC, pH 5.5. As the fluid evolved the bulk and mesostasis become components of the dissolved rock composition. Once the CO2 is nearly exhausted, the carbonate is partially replaced by phyllosilicate [3]. This part of the fluid's history has a lower W/R 5-10, 50-100oC and a higher pH 7-8. The W/R ratio continues to drop with precipitation of the gel and soluble salts in some parts of the nakhlite pile. The relatively rapid cooling and metastable nature of much of the secondary assemblage together with the fluid pathway from deep to shallow is consistent with an origin of the fluid through impact melting of buried ice. The nakhlite secondary assemblage and fluid conditions offer an insight into the formation of similar assemblages identified near impact craters and on other parts of the martian surface where smectite (saponite and nontronite) like those seen in Lafayette are the most common phyllosilicates [1].
Bridges John
Schwenzer Susanne P.
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