Martian Water in the Impact Melts in Eeta

Introduction: EETA 79001 is a particularly interesting martian meteorite because the impact melts in it contain noble gas signatures indicative of martian origin [1]. The noble gases were initially thought to be from martian atmosphere [1] and later revised to be from martian aqueous solution [2]. In either scenario, the impact melts acquired the noble gases could also have acquired martian H2O. If this H2O source exchanged hydrogen isotopes with recent martian atmosphere, it should have elevated deuterium content similar to the present martian surface values of 3900-7000 ‰ in δD [3] considering the young crystallization and cosmic-ray exposure ages of this meteorite [4]. However, previous studies on the impact melts in EETA 79001 yielded relatively low δD values of 1454-2900 ‰ [5,6]. Here we report new data on the impact melts in EETA 79001 and show that the H2O source acquired by EETA 79001 had high δD values within the range of recent martian atmosphere. Samples and Methods: We studied four impact melt pockets, one melt vein, and many maskelynites in three sections: EETA 79001,524, 715, and 717. Sample 524 is a polished section from Lithology A. Sample 715 and 717 are from Lithology C (“Bravo”, >1 cm impact pockets), and three pieces were prepared and mounted in indium. All impact melts and veins contain crystals and vesicles: the ones in 524 contain large olivines, pyroxenes and maskelynites surrounded by fine-crystalline devitrified glass, whereas those in 715 and 717 are mostly fine-crystalline (Fig 1). We focused our analyses on the fine-crystalline regions and avoided the large olivines and pyroxenes. For maskelynite, we analyzed some that are far away from impact melts and some fully enclosed in impact melts. The major-element compositions were analyzed with an electron microprobe and energy-dispersive spectroscopy on a scanning electron microscope. The volatiles (H2O analyzed as OH, F, Cl, P, and S) were analyzed with secondary ion mass spectrometry (SIMS). Natural and synthetic glasses were used as standards. The 2σ errors based on the reproducibility of the standards are 10% for H2O, P, and S, and 6 % for F and Cl. The detection limits are 5 ppm for H2O, 0.3 ppm for F, 0.2 ppm for Cl and P, and 0.1 ppm for S. Hydrogen isotopes were analyzed in separated SIMS sessions and reported as δD values relative to Vienna Standard Mean Ocean Water. The measured D/H values in the impact melts and maskelynites were corrected by the instrument mass fractionation factor of 1.016 (α) based on glass standards. The 2σ errors for δD values are 47-168 ‰, depending on the H2O concentration.