Impact-melt hygrometry for Mars: The case of shergottite Elephant Moraine (EETA) 79001

Yang Liua, Yang Chena, Yunbin Guanb, Chi Mab, George R. Rossmanb, John M. Eilerb, Youxue Zhangc

a: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

b: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

c: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA


    We report that volatile concentrations and hydrogen isotope compositions of impact melts and minerals in EETA 79001 Lithology A and Lithology C.  We observed chemical changes on maskelynite, pyroxene and merrillite relating to their proximity to impact melts.  Maskelynite in EETA 79001 contains 6.3 to 98 ppm H2O with δD values of +1604 to +3938 ‰. The high H2O and δD values were obtained in grains enclosed in or next to the impact melts, whereas one maskelynite grain away from the impact melts contains low H2O content (4 ppm) and terrestrial-like D/H value (δD of -90 82 ‰).  All pyroxene grains analyzed are either inside or next to the impact melts, and contain 10-41 ppm H2O and enriched D (+1729 to +3707 ‰), with the higher values found in a grain enclosed in the impact melts.  Merrillite grains next to impact melts contain Na depletion rims of ~5 μm thickness, and Mg-enrichment near the contact with impact melt.  However, the H2O and δD values of merrillite interiors (39-242 ppm H2O and δD of +1682 to +3884 ‰) do not show correlation with its proximity to the impact melts, suggesting nominally anhydrous silicate minerals are more prone to by affect by impact melts.  Rather the negative correlation between δD and 1/H2O of merrillite suggests possible post-crystallization  exchange with subsurface fluid.
    The impact melt pockets in EETA 79001 contain 121-646 ppm H2O, 4.3-13 ppm F, 13-50 ppm Cl, 707-2702 ppm S, and the δD values of +3368 to +4639 ‰.  The correlations between of H2O, F, Cl, P2O5, and δD values of impact melts are consistent with mixing between a volatile-rich and high δD (+3000 to +5000 ‰) endmember and a volatile-poor and low δD endmember.  The volatile-poor and low δD endmember is consistent with magmatic volatiles stored in silicates.  The volatile-rich and high δD endmember is alteration materials in the pre-impact vesicle-bearing rocks. The subsurface water had equilibrated with present-day-like Martian atmosphere and alteration occurred after the crystallization of the rock (~170 Ma) and before impact launch (~0.7 Ma).  The δD value of the subsurface source in EETA 79001 is similar to those in the Tissint meteorite (crystallization at ~600 Ma, impact launch at ~0.7 Ma) and LAR 06319 (crystallization at ~200 Ma, impact launch at ~3 Ma), suggesting stable water chemistry for the subsurface environment since 600 Ma.  Our conclusion is different from the previous suggestion of an isotopically distinct subsurface water reservoir with a δD value of +1000 to +2000 ‰.  Although heterogeneous subsurface water on Mars is possible, the previous study was likely biased by a limited number of analysis (n = 2) and possible terrestrial contamination.