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

Liu, Yang and Chen, Yang and Guan, Yunbin and Ma, Chi and Rossman, George R. and Eiler, John M. and Zhang, Youxue (2018) Impact-melt hygrometer for Mars: The case of shergottite Elephant Moraine (EETA) 79001. Earth and Planetary Science Letters, 490 . pp. 206-215. ISSN 0012-821X.

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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.

Item Type:Article
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URLURL TypeDescription
Guan, Yunbin0000-0002-7636-3735
Ma, Chi0000-0002-1828-7033
Rossman, George R.0000-0002-4571-6884
Zhang, Youxue0000-0002-7439-0086
Additional Information:© 2018 Elsevier B.V. Received 14 November 2017, Revised 8 March 2018, Accepted 12 March 2018, Available online 26 March 2018. This work was largely conducted at the Jet Propulsion Laboratory, which is managed by the California Institute of Technology (Caltech) under a contract with NASA. We thank constructive reviews from T. Usui and A. Udry. YL thanks the support from JPL-Caltech President's and Director's Fund and partial support by NASA Cosmochemistry grant NNN13D465T. We thank the Johnson Space Center Meteorite Working Group for allocating the samples. The EPMA and SEM were performed at the Caltech Geological and Planetary Science Division Analytical Facility, which is supported in part by NSF grants EAR-0318518 and DMR-00800065. SIMS analysis was done at the Caltech Microanalysis Center that is partially supported by the Gordon and Betty Moore Foundation.
Funding AgencyGrant Number
JPL President and Director's FundUNSPECIFIED
Gordon and Betty Moore FoundationUNSPECIFIED
Subject Keywords:Martian volatiles; impact melts; subsurface water; magmatic water; Mars
Record Number:CaltechAUTHORS:20180327-081337793
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Official Citation:Yang Liu, Yang Chen, Yunbin Guan, Chi Ma, George R. Rossman, John M. Eiler, Youxue Zhang, Impact-melt hygrometer for Mars: The case of shergottite Elephant Moraine (EETA) 79001, Earth and Planetary Science Letters, Volume 490, 15 May 2018, Pages 206-215, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85442
Deposited By: Tony Diaz
Deposited On:27 Mar 2018 18:27
Last Modified:27 Nov 2019 00:07

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