Martian low-temperature alteration materials in shock-melt pockets in Tissint: Constraints on their preservation in shergottite meteorites

Abstract

We apply an array of in situ analytical techniques, including electron and Raman microscopy, electron and ion probe microanalysis, and laser ablation mass spectrometry to the Tissint martian meteorite in order to find and elucidate a geochemical signature characteristic of low-temperature alteration at or near the martian surface. Tissint contains abundant shock-produced quench-crystallized melt pockets containing water in concentrations ranging from 73 to 1730 ppm; water content is positively correlated with Cl content. The isotopic composition of hydrogen in the shock-produced glass ranges from δD = 2559 to 4422 ‰. Water is derived from two distinct hydrogen reservoirs: the martian nearsurface (>500 ‰) and the martian mantle (-100 ‰). In one shock melt pocket comprising texturally homogeneous vesiculated glass, the concentration of H in the shock melt decreases while simultaneously becoming enriched in D, attributable to the preferential loss of H over D to the vesicle while the pocket was still molten. While igneous sulfides are pyrrhotite in composition (Fe 0.88-0.90 S), the iron to sulfur ratios of spherules in shock melt pockets are elevated, up to Fe 1.70 S, which we attribute to shock-oxidation of igneous pyrrhotite and the formation of hematite at high temperature. The D-and Cl-enrichment, and higher oxidation of the pockets (as indicated by hematite) support a scenario in which alteration products formed within fractures or void spaces within the rock; the signature of these alteration products is preserved within shock melt (now glass) which formed upon collapse of these fractures and voids during impact shock. Thermal modeling of Tissint shock melt pockets using the HEAT program demonstrates that the shock melt pockets with the greatest potential to preserve a signature of aqueous alteration are small, isolated from other regions of shock melt, vesicle-free, and glassy.3

Hydrogen Isotopic Variations in the Shergottites

“…Almost all phases in the shergottites, e.g., apatite, merrillite, melt inclusions, maskelynite, and impact melt glasses, have δD values up to~4000-6830% [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. A summary of water content and hydrogen isotopic compositions of the shergottites is listed in Table 1.…”

“…The water content of maskelynite in the literature is reported to vary from ~1 ppm to 7.6 wt. % [52,59,60,68,70,71,124,125] with δD varying from ~−400 to ~4500 ‰ (Figure 4). However, most analytical spots on maskelynite have water content lower than 200 ppm, with a large variation in δD values (inset diagram in Figure 4).…”

“…Two models have been proposed to interpret the variation in the REE patterns, Sr, Nd, and Hf isotopic compositions, and redox conditions: (1) distinct mantle sources and (2) variable to enriched, oxidized crustal material assimilation of an original depleted parental magma [1, 13,25,[30][31][32][33][34]. in apatite, amphibole, melt inclusion glasses, impact melt glasses, and anhydrous silicates display two distinct water reservoirs on Mars [50][51][52][53]58,60,[67][68][69][70][71][72]91,92]: The Martian mantle has a low δD value of 0% , whilst the Martian surficial/crustal water reservoir is higher at~5000-6000% , which probably isotopically equilibrated with the Martian atmosphere (δD = 4950 ± 1080% ) [49,56,57]. Usui et al [72] proposed an intermediate water reservoir (δD~3000% ) according to the relationship between H 2 O and δD of groundmass glasses from Y 980459.…”

“…The current hydrogen isotopic composition of the Martian atmosphere is thought to have equilibrated with surficial/underground water via exchange between the Martian hydrosphere and lithosphere [55][56][57].A meteorite should equilibrate with its parent body in hydrogen isotopic compositions during crystallization (~0% , [50][51][52][53]). However, most of the shergottites display δD values higher than that presumed upon basaltic crystallization (up to~4000-6830% ) [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75]. Such high values are inconsistent with parent magma degassing on Mars [76].…”

“…have been observed in the shergottites, although they are probably terrestrial in origin [83-88] (see Section 3). Although the shergottites lack any unambiguous signs of Martian mineral alteration, their highest δD values among the SNCs in both bulk (e.g.,~2100% for Shergotty vs~800% for ALH 84001) [89] and in situ (up to~4000-6830% for shergottites vs <3000% for non-shergottites) [35,[51][52][53][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75], is consistent with the progressive loss of Martian water over time [5,90].In this paper, we mainly focus on the progresses made on the hydroxyl, water, and hydrogen isotopic content of different phases in the shergottities, namely: apatite, merrillite, melt inclusions, maskelynite, impact melt glass, groundmass glass, and nominal anhydrous minerals. We aim to collate various measurements and studies in order to explore water-rock verses magmatic H isotopic processes.…”