10:30 AM - 10:45 AM
[SGL39-07] Investigation of martian surface history: NanoSIMS analyses of D/H ratios and U-Pb chronology of martian meteorites
Keywords:martian meteorites, NanoSIMS, D/H ratios, U-Pb dating, phosphates, melt-inclusions
Analytical methods: For ALH, 3 merrillite grains with known U-Pb ages  were selected for D/H analyses. For LAR, several phosphate grains and MIs were found in a thin section, using a SEM-EDS. Both U-Pb and D/H analyses were carried out using a NanoSIMS 50 installed at AORI, Univ. of Tokyo. Before SIMS analyses, the samples were baked at ~100℃ in a SIMS air-lock overnight before/after gold coating to remove adsorbed water. The analytical methods of U-Pb dating were the same as the previous study . The D/H analyses were conducted on the phosphates in ALH and LAR and MIs in LAR. A Cs+ primary ion beams with 200pA/1nA was used for phosphates and MIs, respectively. An electron gun was used for charge compensation. Negative secondary ions of 1H-, 2D-, 12C- and 18O- were collected. A natural terrestrial apatite from Morocco and NIST SRM 610 were used as standards. To avoid terrestrial H contamination, most of which are background H in the analysis chamber and hydrocarbon in the sample cracks, careful analytical protocols following a previous study  were conducted.
Results & Discussion: The δD values of ALH merrillite varied from -300 to 1970‰ (Fig). The obtained highest value is similar to those of ALH carbonates and maskelynite . The U-Pb age of the same grains, 3990Ma, can be interpreted as an impact-induced reset age . It is likely that their D/H ratios reflect 3990Ma surface water, incorporated during the impact and/or a later hydrous metamorphism. The high D/H mainly supports a previously proposed two-stage evolution . On the other hand, phosphates in LAR yielded a total Pb/U age as 167+/-57 Ma. This is consistent with other radiometric ages within uncertainty, suggesting the U-Pb system in the phosphates has been preserved since crystallization of the host rock. δD values of LAR apatite, merrillite and MIs were 3340-4380‰, 1070-5260‰ and 1150-6830‰, respectively (Fig). The D/H may have mineral trends: MIs > merrillite > apatite. A similar trend was reported previously . While apatite possibly recorded the magmatic water at the timing of crystallization, MIs might have incorporated water from another reservoir with extremely high D/H ratios.
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