Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-GL Geology

[S-GL39] Geochronology and Isotope Geology

Sun. May 24, 2015 9:00 AM - 10:45 AM A03 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Takahiro Tagami(Graduate School of Science, Kyoto University), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo), Chair:Takahiro Tagami(Graduate School of Science, Kyoto University), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

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

*Mizuho KOIKE1, Yuji SANO1, Naoto TAKAHATA1, Akizumi ISHIDA1, Naoji SUGIURA2, Mahesh Anand3 (1.Atmosphere and Ocean Research Institute, University of Tokyo, 2.Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 3.Department of Physical Sciences, The Open University, UK)

Keywords:martian meteorites, NanoSIMS, D/H ratios, U-Pb dating, phosphates, melt-inclusions

Introduction: Water is an important volatile for environments of terrestrial planets as well as their habitability. A number of recent studies have identified strong evidence for liquid water on past Mars, such as clay minerals and fluvial geomorphological features (e.g. [1][2]), whereas a comprehensive history of the martian environment remains complicated. The isotopic compositions of hydrogen (D/H) of present martian atmosphere is highly elevated (~5 times that of terrestrial water; [3]), which results from the extensive atmospheric escape. Martian meteorites are useful as they potentially provide valuable records including D/H of the past surface water and the mantle primitive water (e.g. [4][5][6][7][8]). However, due to their complicated history on Mars, it is challenging to understand their isotopic records accurately. Phosphates are helpful, for they can preserve both U-Pb chronology and D/H information. Here, we report D/H ratios and U-Pb ages of phosphates in two martian meteorites; an ancient orthopyroxenite, ALH 84001 (ALH), and a young enriched shergottite, LAR 06319 (LAR). In addition, we have also measured D/H ratio of melt-inclusion glass (MIs) in LAR.
Analytical methods: For ALH, 3 merrillite grains with known U-Pb ages [9] 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 [9]. 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 [7] 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 [4]. The U-Pb age of the same grains, 3990Ma, can be interpreted as an impact-induced reset age [9]. 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 [5]. 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 [7]. 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.
References: [1] Bibring et al. (2006) Science 312, 400-404. [2] Ehlmann et al. (2011) Nature 479, 53-60. [3] Owen et al. (1988) Science 240, 1767-1770. [4] Sugiura and Hoshino (2000) Meteorit. Planet. Sci. 35, 373-380. [5] Greenwood et al. (2008) Geophys. Res. Lett. L05203, 1-5. [6] Usui et al. (2012) Earth Planet. Sci. Lett. 357, 119-129. [7] Hu et al. (2014) Geochim. Cosmochim. Acta 140, 321-333. [8] Usui et al. (2015) Earth Planet. Sci. Lett. 410, 140-151. [9] Koike et al. (2014) Geochem. J. 48, 423-431.