We found a match
Your institution may have access to this item. Find your institution then sign in to continue.
- Title
WATER ON MARS: CORRELATED MICROSCALE ANALYSES OF HYDROGEN ISOTOPES, WATER CONTENTS, AND REDOX STATE IN MARTIAN PYROXENES.
- Authors
Davidson, J.; Wadhwa, M.; Sutton, S. R.; Hervig, R. L.
- Abstract
Introduction: Determining the nature of water in planetary materials aids our understanding of the source(s), abundance, and evolution of planetary water [1]. As Mars has no plate tectonics, the water reservoirs in the mantle and atmosphere are mostly isolated, and likely buffered by a third reservoir in the crust [2]. The nakhlites Nakhla and Lafayette potentially originate from the same lava flow [3] and may contain magmatic water in their igneous minerals (e.g., [4]). The regolith breccia Northwest Africa (NWA) 7034 and its pairing group are the only known samples with compositions representative of the average martian crust [5], and contain lithologies not identified in other martian meteorites [6]. Nakhla, Lafayette, and NWA 7034 are some of the least-shocked martian samples available for study (all <20 GPa [7,8]), and so may have preserved primary magmatic signatures to a greater extent than more shocked martian meteorites. Collectively, they provide the opportunity to investigate the hydrogen (isotopic composition, δD in per mil, and water concentration, H2O in ppm) and Fe redox (Fe3+/ΣFe) systematics of a variety of geologic settings at different times in martian history. Here we present correlated δD-H2O-Fe3+/ΣFe systematics for the early-formed primary igneous mineral pyroxene, which may be one of the most reliable phases for determining the nature and abundance of primary water (e.g., [10-12]), in the martian meteorites Nakhla, Lafayette, and NWA 7034. Analytical Methods: Interior, fusion-crust free meteorite chips (one Nakhla, one Lafayette, two NWA 7034) were co-mounted with terrestrial standards in indium metal in four Al discs; no water was used in sample preparation. Quantitative compositional analyses of pyroxene were obtained with a Cameca SX-100 electron probe microanalyzer (EPMA) at the University of Arizona (20 kV, 20 nA) while high-resolution secondary and backscattered electron (BSE) imaging was undertaken on ASU's JEOL JXA-8530F EPMA (15 kV, 15 nA). Measurements of H isotope compositions and H2O contents of pyroxenes were performed on the Cameca IMS-6f SIMS at ASU [11,12]. The Fe K X-ray Absorption Near-Edge Structure (XANES) spectra were collected on spots adjacent to the SIMS pits using station 13-ID-E at the GSECARS X-ray microprobe at the Advanced Photon Source at Argonne National Lab, following the method of [13] and [14]. These analytical methods are described in more detail in [11,12]. Results: Nakhlites. Pyroxenes in Nakhla have heavy H isotope compositions (δD = 310 ± 170 ‰ to 1440 ± 120 ‰), low H2O contents (<10 ± 2 ppm to 90 ± 17 ppm) [15], and average Fe valence values of 2.139 to 2.171 [11]. Pyroxenes in Lafayette have similarly heavy H isotope compositions (δD = 840 ± 60 ‰ to 1630 ± 120 ‰), low H2O contents (~10 ± 2 ppm to 30 ± 6 ppm) [15], and Fe valence values from 2.115 to 2.175 [10]. Regolith breccia. Phenocrystic groundmass-hosted pyroxenes in NWA 7034 have lighter H isotope compositions (δD = -160 ± 12 ‰ to 275 ± 220 ‰) and higher H2O concentrations (40 ± 8 ppm to ~2240 ± 500 ppm) than nakhlite pyroxenes. Pyroxenes from Fe-, Ti-, and P-rich (FTP) clasts have generally heavier H isotope compositions (δD = 130 ±110 ‰ to 330 ± 110 ‰) and lower H2O concentrations (~100 ± 20 ppm to 900 ± 180 ppm) than groundmass pyroxenes. Pyroxenes in FTP clasts have systematically higher Fe valence values (average = 2.144 ± 0.045) than the pyroxene phenocrysts (average = 2.079 ± 0.022), similar to nakhlite pyroxenes (average = 2.153 ± 0.016). Discussion: Pyroxenes from both nakhlites and the regolith breccia exhibit overall inverse relationships between δD and water concentrations. This may result from degassing via dehydrogenation (i.e., H2 loss), which typically leads to isotopically heavier δD and an inverse relationship between δD and water concentrations (e.g., [16]). Alternatively, this trend may result from mixing between two distinct reservoirs (one with high δD and low H2O content, and the other with low δD and high H2O content). Combined δD-H2O-Fe3+/ΣFe systematics suggest that pyroxenes in both the nakhlites and NWA 7034 experienced a mix of dehydration (i.e., H2O loss) and dehydrogenation (H2 loss). Overall, some NWA 7034 pyroxenes contain more water than nakhlite pyroxenes, perhaps due to post-eruption addition of water on Mars (as seen in apatites from NWA 7034 [9]). These differences likely reflect the different environments on Mars in which the igneous components of the martian crust were formed and later altered.
- Subjects
LAFAYETTE (La.); HYDROGEN isotopes; HYDROGEN analysis; PYROXENE; UNIVERSITY of Arizona; ELECTRON probe microanalysis; ISOTOPIC analysis
- Publication
Meteoritics & Planetary Science, 2022, Vol 57, p6109
- ISSN
1086-9379
- Publication type
Article