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

• Published in: Earth and planetary science letters 2018-05, Vol.490, p.206-215
• Main Authors: Liu, Yang, Chen, Yang, Guan, Yunbin, Ma, Chi, Rossman, George R., Eiler, John M., Zhang, Youxue
• Format: Article
• Language: English
• Subjects: impact melts; magmatic water; Mars; Martian volatiles; subsurface water
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2018.03.019

•We observed H- and D-intake by maskelynite and pyroxenes in or near impact melts.•H contents and δD values of impact melts suggest dominance of subsurface source.•Subsurface fluids in EETA 79001 equilibrated with present-day atmosphere.•H contents and δD values of merrillite suggest alteration by post-crystallization process. We report volatile concentrations and hydrogen isotope compositions of impact melts and minerals in EETA 79001. We observed chemical changes in pyroxene, maskelynite (or feldspathic glass), and merrillite in contact with or inside impact melts. All pyroxene grains analyzed here are inside or close to impact melt pockets and contain 10–41 ppm H2O and enriched in D (δD=+1729 to +3707‰), with the highest values found in a grain enclosed in an impact melt pocket. Maskelynite or feldspathic glass contains 6.3 to 98 ppm H2O with δD values of +1604 to +3938‰. The high H2O and δD values were obtained in those enclosed inside or in contact with the impact melts, whereas low H2O content (4 ppm) and terrestrial-like D/H value (δD of −90±82‰) were found in one maskelynite grain away from impact melts contains. Rims of ∼5 μm thickness of merrillite grains next to impact melts display Na-depletion by ∼0.9 wt%, and the sides in contact with impact melts show Mg-enrichment by ∼0.5 wt%. However, the H2O and δD values of merrillite interiors (39–242 ppm H2O and δD of +1682 to +3884‰) do not show correlation with their proximity to the impact melts. Rather, δD and 1/H2O of merrillite form a negative trend different from that of impact melt pockets and maskelynite, suggesting post-crystallization or late-crystallization interactions with the crustal fluids. 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 H2O, F, Cl, P2O5, and δD values of impact melts and feldspathic glass 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 represents pre-impact alteration materials by subsurface water. Alteration from the subsurface water, equilibrated with the present-day-like Martian atmosphere, occurred after the crystallization of the rock (∼170 Ma) and before impact launch (∼0.7 Ma). Our conclusion is different from the previous