Oxygen isotopic record of silicate alteration in the SNC meteorite Lafayette

Details Oxygen isotopic record of silicate alteration in the SNC meteorite Lafayette Oxygen isotopic record of silicate alteration in the SNC meteorite Lafayette

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..775r&link_type=abstract

Meteoritics & Planetary Science, vol. 33, no. 4, pp. 775-784.

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Scientific paper

Samples from a suite of SNC meteorites were analyzed for their oxygen isotope ratios by a modified version of the laser fluorination technique. Measured isotopic ratios (17O/16O and 18O/16O) from bulk samples of the shergottites, EETA79001, Shergotty and Zagami, the nakhlite Lafayette, and Chassigny are similar to those reported in the literature, as are those from olivine and pyroxene mineral separates from Lafayette. Iddingsite, a preterrestrial alteration product of Lafayette, was measured for the first time as a separate phase. Oxygen isotope ratios increase with the percentage of iddingsite in a sample to a maximum d18O of 14.4e for a ~90% separate. Based on these measurements, end member iddingsite has a d18O of 15.6e, which places it among other 18O-enriched secondary phases (carbonate and silica) observed in SNC meteorites. The relatively large difference in d18O between iddingsite and the olivine and pyroxene it replaces (~11 ) is typical of low temperature alteration products. A range of crustal fluid d18O values can be interpreted from the d18O for end member iddingsite, assuming isotopic equilibrium was achieved during low temperature hydrous alteration (<100C Treiman et al., 1993). The calculated range of values, -15 to 5, depends on many factors including: 1) the modal mineralogy of iddingsite, 2) potential isotopic exchange among other oxygen-bearing phases such as host silicate and carbonate, and 3) exchange with evolved or exotic oxygen reservoirs on Mars. Despite the lack of constraints, the calculated range is consistent with isotopic exchange, possibly equilibria among components of the CO2-carbonate-iddingsite-H2O system at low temperature. SNC meteorite samples of this study have D17O values that are indistinguishable from bulk Mars (0.30), except for a single small sample of iddingsite which has an anomalous D17O of ~1.4. While analytical difficulties make isotopic measurements for this sample problematic, the D17O is similar in direction to D17O reported for waters extracted from bulk samples of Lafayette (Karlsson et al., 1992). If the D17O for iddingsite is confirmed, it can be concluded that evolved or exotic fluids on Mars have contributed volatiles to the oxygen reservoir from which iddingsite formed 130 to 700 Ma ago.

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