Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p52b..06g&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P52B-06
Physics
0400 Biogeosciences, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 3662 Meteorites, 4870 Stable Isotopes, 6240 Meteorites And Tektites
Scientific paper
The oxygen isotopic compositions of silica and SiO2-rich glass have been measured in situ by ion microprobe in the Martian meteorite ALH84001. Two different textural occurrences, a large anhedral (40x60æm) partially crystalline silica grain and a SiO2-rich glass-rich region of a granular band, have been studied. The large silica grain has been studied by electron microprobe [1] and micro-Raman spectroscopy [2] and is found to be compositionally similar to the fine-grained SiO2-rich regions in the granular band [1]. The SiO2-rich glass is finely intergrown with carbonate (of limited compositional range), chromite, feldspar and orthopyroxene. 18O/16O ratios were measured using the UCLA ims 1270 ion microprobe in multicollector mode using quartz mineral standards. Three analyses of the large silica grain in 2 analytical sessions gave a small range in oxygen isotopes (d18O=+24.41+/-0.80per mil to +25.3+/-2.6per mil; 2s); a similar range was measured for four regions of SiO2-rich glass in the granular band (d18O=+22.37+/-0.38per mil to +24.08+/-0.58 per mil; 2s). Our data show a limited range in d18O of silica which is in contrast to the large range found for ALH84001 carbonates (d18O: -10 to +28per mil [3-7]) and no evidence of low d18O values that would be indicative of magmatic or high-temperature silica [8]. Our results are heavier (by 2-4per mil) than a value published by [3] probably reflecting our more accurate correction for instrumental mass fractionation. The similar diffusivities for oxygen in calcite and quartz suggest that carbonate was deposited over a wide temperature range but that silica precipitated over a limited temperature interval. In the granular band, silica and carbonate appear to have been co-precipitated, potentially providing an opportunity to constrain the temperature and oxygen isotopic composition of the formation water for these two phases. The carbonate is too fine-grained for ion probe isotopic analysis, however the well-defined correlation of major-element and oxygen isotope compositions of ALH84001 carbonate [3-6] could be used to constrain its oxygen isotope composition. Electron microprobe analyses are in progress and will be reported at the meeting. These data should provide our best proxy for the oxygen isotope composition of water on early Mars. Implications for the search for life using the stable isotopes of oxygen will be discussed.
Greenwood James Paul
McKeegan Kevin D.
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