Oxygen and carbon stable isotope composition of the weathering Mg‐carbonates formed on the surface of the LEW 85320 ordinary chondrite: Revisited

Abstract

New δ 13 C and δ 18 O values were measured to constrain the formation mechanisms of two previously identified generations (Antarctica and Houston) of terrestrial weathering carbonates, nesquehonite, which formed on the surface of the Lewis Cliff 85320 ordinary chondrite. These two nesquehonite generations exhibit a characteristic carbon and oxygen isotopic trend which starts with a linear δ 13 C-δ 18 O covariation followed by a continuous increase in δ 18 O with little to no change in δ 13 C. Based on the newly developed nesquehonite-water oxygen isotope thermometry and the measured δ 18 O value of melted ice from Antarctica, the formation temperature of the Antarctic nesquehonite generation was estimated to be −1.9 AE 3°C. Houston nesquehonite generation was most likely formed from an evaporative residue of the melted Antarctic ice at 30 AE 4°C. Modeling the isotopic trend of the two generations suggests that evaporation created large δ 18 O variations in the nesquehonite while carbon isotopes were stabilized by exchange between the parent liquid and atmospheric CO 2. Thus, we suggest that carbonates formed by terrestrial weathering in Antarctica should possess a wide spread in δ 18 O values and a narrow range in δ 13 C values under dry and cold conditions. Finally, the observed wide spread of δ 18 O in Martian carbonates (e.g., Allan Hills 84001 and Nakhla) can be explained by evaporation near 0°C and may have formed by a similar mechanism to that of the nesquehonite in the LEW 85320. Meanwhile, the wide spread of δ 13 C suggests that two carbon reservoirs with distinct isotope compositions on Mars (e.g., the atmospheric carbon and magmatic carbon) may be actively participating in the formation process.

Planning Implications Related to Sterilization-Sensitive Science Investigations Associated with Mars Sample Return (MSR)

“…Furthermore, as noted above, all indications to date of carbonate minerals on Mars from orbital spectroscopy correspond to Mg-carbonate; the orbital spectra are consistent with the presence of the hydrous Mg-carbonate hydromagnesite (Calvin et al, 1994;Ehlmann et al, 2008b;Horgan et al, 2020). Hydrous carbonates contain H and O as molecular water, and some also contain hydroxyl (Jull et al, 1988;Velbel et al, 1991;El-Shenawy et al, 2020;Tosca et al, 2021). Water from these minerals is evolved (these minerals are "dehydrated") at temperatures (T D ) less than proposed sterilization temperatures (T S ).…”

“…Hydromagnesite is one of many hydrous Mg-carbonate minerals. Hydrous Mg-carbonates are sparsely distributed on Earth and occur naturally in cold climates in which dehydration kinetics are slow (Jull et al, 1988;Velbel et al, 1991;El-Shenawy et al, 2020) and in evaporitic lakes (Coshell et al, 1998;Russell et al, 1999).…”

Characteristics, Origins, and Biosignature Preservation Potential of Carbonate‐Bearing Rocks Within and Outside of Jezero Crater

“…Given that such low water:rock ratio evaporative/alteration processes form <1 vol. % carbonate in martian meteorites (McSween, 1994; Niles et al., 2013) and meteorites with carbonate rinds (El‐Shenawy et al., 2020; Jull et al., 1988), they may not generate sufficient quantities of carbonate to be detected in CRISM VNIR data. Surface water infiltration and evaporation generates carbonate layers in basaltic rocks on Earth (e.g., Knauth et al., 2003), but this process may not generate the 20%–25% carbonate modal abundance estimated for the entire ROB unit using CRISM (Edwards & Ehlmann, 2015).…”

Time-Sensitive Aspects of Mars Sample Return (MSR) Science

“…Although comprehensive studies are limited, highly hydrated Mg-carbonate phases are generally more stable at low RH, whereas higher RH and T values promote conversion to hydromagnesite (Wilson et al, 2013;Morgan et al, 2015). In addition to these studies, geochemical and mineralogical investigations have documented the formation of hydrous Mg-carbonate phases through interaction between thin films of water and Mg-silicate minerals over a period of several years at low temperatures characteristic of the Antarctic (Jull et al, 1988;Velbel et al, 1991;El-Shenawy et al, 2020). In general, this work supports the conclusion that hydrated Mg-carbonates are susceptible to volatile loss and recrystallization and transformation over timespans of months and perhaps longer.…”