Oxygen and carbon stable isotope composition of the weathering Mg‐carbonates formed on the surface of the LEW 85320 ordinary chondrite: Revisited. (26th September 2020)

• Record Type: Journal Article
• Authors: El‐Shenawy, Mohammed I.; Niles, Paul B.; Ming, Doug W.; Socki, Rick; Righter, Kevin
• Is Part Of: Meteoritics & planetary science. Volume 55:Number 8(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-26
• Subjects: Meteorites -- Periodicals; Planetology -- Periodicals; 523.4
• DOI: 10.1111/maps.13553
• Languages: English
• ISSNs: 1086-9379

Abstract

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 ± 3 °C. Houston nesquehonite generation was most likely formed from an evaporative residue of the melted Antarctic ice at 30 ± 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 CO2 . 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.