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

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 (delta D = +1729 to +3707 parts per thousand), 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 delta D values of +1604 to +3938 parts per thousand. The high H2O and delta D 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 (delta D of -90 +/- 82 parts per thousand) were found in one maskelynite grain away from impact melts contains. Rims of similar to 5 mu m thickness of merrillite grains next to impact melts display Na-depletion by similar to 0.9 wt%, and the sides in contact with impact melts show Mg-enrichment by similar to 0.5 wt%. However, the H2O and delta D values of merrillite interiors (39-242 ppm H2O and delta D of +1682 to +3884 parts per thousand) do not show correlation with their proximity to the impact melts. Rather, delta D 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.& para;& para;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 delta D values of +3368 to +4639 parts per thousand. The correlations between H2O, F. CI, P2O5, and delta D values of impact melts and feldspathic glass are consistent with mixing between a volatile-rich and high delta D (+3000 to +5000%0) endmember and a volatile-poor and low SD endmember. The volatilepoor and low delta D endmember is consistent with magmatic volatiles stored in silicates. The volatile-rich and high delta D 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 (similar to 170 Ma) and before impact launch (similar to 0.7 Ma). Our conclusion is different from the previous suggestion of an isotopically distinct subsurface water reservoir with a delta D value of +1000 to +2000 parts per thousand in EETA 79001. Although heterogeneous subsurface water on Mars is possible, the previous study was likely biased by a limited number of analyses (n = 2) and possible terrestrial contamination. The delta D value of the subsurface source in EETA 79001 is similar to+4200 parts per thousand, similar to those in the Tissint meteorite (crystallization at similar to 600 Ma, impact launch at similar to 0.7 Ma) and IAR 06319 (crystallization at similar to 200 Ma, impact launch at similar to 3 Ma), suggesting stable water chemistry for the subsurface environment in the last 600 Myrs. (C) 2018 Elsevier B.V. All rights reserved.