Signatures of the Martian regolith components entrained in some impact‐melt glasses in shergottites

Abstract

Martian regolith components are found in some impact melts (IM) containing Martian atmospheric gases in the shergottites Elephant Moraine (EET) 79001, Tissint, Zagami, and Shergotty. Excess sulfur abundances provide strong indicators for the presence of an exogenous component. High sulfur abundances and the SO3‐SiO2 correlation in polished thin section (PTS) EET 79001,507 (here #507) are comparable to those in Martian soils. Correlations of SO3 with FeO in #507 from Lithology B and of CaO and Al2O3 in EET 79001,506 (here #506) from Lithology A suggest the possible occurrence of two varieties of sulfate‐bearing phases in impact‐melt precursors. Fe/S (atomic) ratios of 1.02–1.34 determined in several sulfide blebs in #507 differ from those determined in igneous sulfides (Fe/S = 0.92), and suggest that most sulfide blebs in #507 are not related to igneous sulfides. Fe/S (atomic) ratios in a Tissint glass range from ~0.5 (pyrite) to >1.1 suggesting a mixture of sulfur‐bearing phases. S K‐XANES spectra of the blebs in EET 79001 and Tissint glasses show that sulfur occurs as mixed amorphous sulfide and sulfite. The δ34S values and the 87Sr/86Sr (I) ratios determined in EET 79001 impact melts are consistent with the proposition that the sulfide blebs result from decomposition of secondary sulfates into sulfites during shock heating followed by reduction to sulfides by isentropic cooling. These results suggest the presence in some shergottites of extraneous regolith components containing oxidized S‐bearing species resembling sulfur species present in Martian soils.

A new type of isotopic anomaly in shergottite sulfides

“…However, the small abundance of sulfur in shock melts (e.g., range of SO 3 in glass from 0.72 wt% in Tissint to 8.06 wt% for EET 79001 lithology C—Rao et al. ) suggests that the isotopic signatures we obtained are likely dominated by igneous sulfides. Possible correlations between sulfur isotopes and other geochemical indicators of assimilation may help distinguish between potential mechanisms for individual meteorites.…”

“…In addition, shock melt components in some shergottites carry chemical and isotopic signatures consistent with incorporation of regolith material, such as isotope ratios of Cl, Kr, and Sr (Rao et al. , ; Sharp et al. ).…”

“…Rao et al. () argued that sulfate from the regolith may have been incorporated into impact melts and then reduced to sulfide by isentropic cooling. Because our analyses utilized bulk rock samples that included any shock melt material that might have been present, it is possible that surficial sulfur in shock melt components contributed to the samples that we analyzed.…”

“…Isotopically anomalous sulfur may have been introduced to parent rocks through assimilation of solid surface material or through interaction with fluids carrying anomalous sulfur (Franz et al 2014). In addition, shock melt components in some shergottites carry chemical and isotopic signatures consistent with incorporation of regolith material, such as isotope ratios of Cl, Kr, and Sr (Rao et al 2011(Rao et al , 2018Sharp et al 2016). Rao et al (2018) argued that sulfate from the regolith may have been incorporated into impact melts and then reduced to sulfide by isentropic cooling.…”

“…In addition, shock melt components in some shergottites carry chemical and isotopic signatures consistent with incorporation of regolith material, such as isotope ratios of Cl, Kr, and Sr (Rao et al 2011(Rao et al , 2018Sharp et al 2016). Rao et al (2018) argued that sulfate from the regolith may have been incorporated into impact melts and then reduced to sulfide by isentropic cooling. Because our analyses utilized bulk rock samples that included any shock melt material that might have been present, it is possible that surficial sulfur in shock melt components contributed to the samples that we analyzed.…”