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

1,2Mohammed I. El‐Shenawy,3Paul B. Niles,3Doug W. Ming,4Rick Socki,3Kevin Righter
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13553]
1Universities Space Research Association, NASA Johnson Space Center, Houston, Texas, 77058 USA
2Department of Geology, Beni‐Suef University, Beni‐Suef, 62511 Egypt
3NASA Johnson Space Center, Houston, Texas, 77058 USA
4Air Liquide, Delaware Innovation Campus, Newark, Delaware, 19702 USA
Published by Arrangement with John Wiley & Sons

New δ13C and δ18O 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 δ13C‐δ18O covariation followed by a continuous increase in δ18O with little to no change in δ13C. Based on the newly developed nesquehonite‐water oxygen isotope thermometry and the measured δ18O 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 δ18O 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 δ18O values and a narrow range in δ13C values under dry and cold conditions. Finally, the observed wide spread of δ18O 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 δ13C 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.

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