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

Abstract

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 H_2O 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 H_2O with δ D values of +1604 to +3938‰. The high H_2O and δD values were obtained in those enclosed inside or in contact with the impact melts, whereas low H_2O content (4 ppm) and terrestrial-like D/H value (δ D 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 H_2O and δD values of merrillite interiors (39–242 ppm H_2O and δ D of +1682 to +3884‰) do not show correlation with their proximity to the impact melts. Rather, δD and 1/H_2O 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 H_2O, 4.3–13 ppm F, 13–50 ppm Cl, 707–2702 ppm S, and the δ D values of +3368 to +4639‰. The correlations between H_2O, F, Cl, P_2O_5, and δD values of impact melts and feldspathic glass are consistent with mixing between a volatile-rich and high δ D (+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 suggestion of an isotopically distinct subsurface water reservoir with a δ D value of +1000 to +2000‰ 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 δ D value of the subsurface source in EETA 79001 is ∼+4200‰, 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 in the last 600 Myrs.

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