OXYGEN ISOTOPIC RECORD OF SILICATE ALTERATION IN THE SHERGOTTY-NAKHLA-CHASSIGNY METEORITE LAFAYETTE

Samples from a suite of Shergotty-Nakhla-Chassigny (SNC) meteorites we re analyzed for their O isotopic ratios by a modified version of the l aser fluorination technique. Measured isotopic ratios (O-17/O-16 and O -18/O-16) 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 mi neral separates from Lafayette. Iddingsite, a preterrestrial alteratio n product of Lafayette, was measured for the first time as a separate phase. Oxygen isotopic ratios increase with the percentage of iddingsi te in a sample to a maximum delta(18)O of 14.4 parts per thousand for a similar to 90% separate. Based on these measurements, end-member idd ingsite has a delta(18)O of 15.6 parts per thousand, which places it a mong other O-18-enriched secondary phases (carbonate and silica) obser ved in SNC meteorites. The relatively large difference in delta(18)O b etween iddingsite and the olivine and pyroxene it replaces (similar to 11 parts per thousand) is typical of low-temperature alteration produ cts. A range of crustal fluid delta(18)O values can be interpreted fro m the delta(18)O for end-member iddingsite, assuming isotopic equilibr ium was achieved during low-temperature hydrous alteration (<100 degre es C; Treiman et al., 1993). The calculated range of values, -15 to 5 parts per thousand, depends on many factors including: (1) the modal m ineralogy of iddingsite, (2) potential isotopic exchange among other O -bearing phases such as host silicate and carbonate, and (3) exchange with evolved or exotic O reservoirs on Mars. Despite the lack of const raints, the calculated range is consistent with isotopic exchange, and possibly equilibria, among components of the CO2-carbonate-iddingsite -H2O system at low temperature. The SNC meteorite samples in this stud y have Delta(17)O values that are indistinguishable from bulk Mars (0. 30 parts per thousand), except for a single, small sample of iddingsit e that has an anomalous Delta(17)O of similar to 1.4 parts per thousan d. While analytical difficulties make isotopic measurements for this s ample problematic, the Delta(17)O is similar in direction to Delta(17) O reported for waters extracted from bulk samples of Lafayette (Karlss on et al., 1992). If the Delta(17)O for iddingsite is confirmed, it ca n be concluded that evolved or exotic fluids on Mars have contributed volatiles to the O reservoir from which iddingsite formed 130 to 700 M a ago.