Ion Microprobe U-Pb Dating of Calcium Phosphates in Martian Meteorites

Studies of the SNC (Shergottite, Nakllites, Chassignite) meteorites, which are thought to be impact ejecta from the planet Mars, have contributed significantly to the understanding of the igneous history of the planet. So far, an intensive work on the radiometric dating of Martian meteorites has been carried out, and it is well documented that some isotopic systems had been disturbed by the impact shock, which could be related to the ejection from Mars. Since the U-Pb isotopic system in phosphate is more resistant to secondary process, U-Th-Pb dating could be a reference for the comparison of the date of the pre-shock igneous event via other dating techniques. We have attempted to investigate the U-Th-Pb systematics of phosphates in various Martian meteorites (basaltic Shergottite, Nakhlite, Chassignite) using the Sensitive High Resolution Ion MicroProbe (SHRIMP) installed at Hiroshima University, Japan. Our techniques take advantage of the high sensitivity at high mass resolution, and are superior to the conventional TIMS analyses in the following ways: (1) a much smaller amount of sample is required, (2) because it is a quasi-destructive technique, after U-Pb analysis, the sample can be used for other studies such as REE analysis by SIMS, observation of texture and chemical composition by EPMA. These techniques are suitable for especially precious sample such as Martian/Lunar samples. In this study, we used polished thin sections of several Martian meteorites, which were provided by the Smithsonian Institution and NASA. The observed "Total Pb/U isochron ages" in the ²³⁸U/²⁰⁶Pb - ²⁰⁷Pb/²⁰⁶Pb - ²⁰⁴Pb/²⁰⁶Pb diagram of basaltic Shergotty show relatively young ages (217 +/- 110 Ma for Shergotty, 359 +/- 120 Ma for Zagami). On the other hand, those of Nakhlite (Nakhla & Lafayette) indicated the older ages of \sim1.3 Ga (although these are only preliminary result). At the conference, we will present a series of results from other Martian meteorites, and will discuss about the early/recent thermal history of Mars.