Hosts of hydrogen in Allan Hills 84001; evidence for hydrous Martian salts in the oldest Martian Meteorite?

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Authors:Eiler, John M.; Kitchen, Nami; Leshin, Lauri; Strausberg, Melissa
Author Affiliations:Primary:
California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States
Arizona State University, United States
Volume Title:Meteoritics & Planetary Science
Source:Meteoritics & Planetary Science, 37(3), p.395-405. Publisher: Meteoritical Society, Fayetteville, AR, United States. ISSN: 1086-9379
Publication Date:2002
Note:In English. 32 refs.; illus.
Summary:The martian meteorite, Allan Hills (ALH) 84001, contains D-rich hydrogen of plausible martian origin (Leshin et al., 1996). The phase identity of the host(s) of this hydrogen are not well known and could include organic matter (McKay et al., 1996), phlogopite (Brearley, 2000), glass (Mittlefehldt, 1994) and/or other unidentified components of this rock. Previous ion microprobe studies indicate that much of the hydrogen in ALH 84001 as texturally associated with concretions of nominally anhydrous carbonates, glass and oxides (Boctor et al., 1998; Sugiura and Hoshino, 2000). We examined the physical and chemical properties of the hosts(s) of this hydrogen by stepped pyrolysis of variously pre-treated subsamples. A continuous-flow method of water reduction and mass spectrometry (Eiler and Kitchen, 2001) was used to permit detailed study of the small amounts of this hydrogen-poor sample available for study. We find that the host(s) of D-rich hydrogen released from ALH 84001 at relatively low temperatures (∼500°C) is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near-complete isotopic exchange with water within hours at temperatures of 200 to 300°C. These characteristics are most consistent with the carrier phase(s) being a hydrous salt (e.g., carbonate, sulfate or halide); the thermal stability of this material is inconsistent with many examples of such minerals (e.g., gypsum) and instead suggests one or more relatively refractory hydrous carbonates (e.g., hydromagnesite). Hydrous salts (particularly hydrous carbonates) are common on the Earth only in evaporite, sabkha, and hydrocryogenic-weathering environments; we suggest that much (if not all) of the "martian" hydrogen in ALH 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust.
Sections:Meteorites and tektites
Subjects:Achondrites; ALH 84001; Allan Hills Meteorites; Chemical composition; Chemical properties; Deuterium; Hydrogen; Isotopes; Mars; Martian meteorites; Meteorites; Mineral composition; Parent bodies; Physical properties; Planets; Stable isotopes; Stony meteorites; Terrestrial planets; Allan Hills; Antarctica; Victoria Land
Abstract Numbers:02M/4369
Record ID:2002035410
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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