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Project Progress

Geochronology on Martian meteorites is difficult because of their complicated history that includes crystallization from an igneous melt, Martian weathering processes, impact shock processes associated with ejection into space, space weathering processes and terrestrial alteration. Past work on ALHA84001 has applied very stringent mineral separation methods and geochemical techniques to see through all of the processes that have had an impact on this sample. Traditionally people have thought that the Sm-Nd system should be the most robust geochronometer and thus have used this system to determine the time of igneous crystallization of SNC meteorites. However, the bulk of Sm and Nd in ALHA84001 are in phosphate phases which are susceptible to some open system behavior during low pH Martian weathering processes. We have used the Lu-Hf system to avoid such possible open system behavior because the bulk of Lu and Hf are contained in orthopyroxene and chromite which are phases that would be resistant to open system behavior relative to phosphate in low pH environments. A mineral isochron which includes oxide, pyroxene and bulk rock defines an age of 4.091 ±0.030 Ga (Lapen et al., 2010). Sm-Nd analyses on the same minerals do not define an isochron and appear to be the result of some type of open system behavior likely associate with low pH weathering (Lapen et al., 2010).

Future work is devoted to conducting a Rb-Sr investigation on ultra high purity mineral separates using new analytical techniques that allow us to analyze ⁸⁷Sr/⁸⁶Sr ratios to a precision of 0.007% on as little as 500 picograms of Sr (10^-12 grams). We anticipate being able to analyze pyroxene, maskelynite (shocked plagioclase glass), and oxide phases that should allow us to see through possible alteration processes.