Stepped Acid Extractions of CO2 from Ancient Carbonates in Martian Nakhlites (MIL 03346, 090030, 090032, 090036) Show Distinct δ18O and δ13C Isotopic Values Compared to Secondary Terrestrial Carbonates Formed on Ordinary Chondrites (OC) Collected from Antarctica

This study finds that 1) Martian Nakhlite meteorites contain insitu carbonates with distinctive δ13C from terrestrial carbonates formed on Antarctic Ordinary Chondrites (OCs), and 2) Martian carbonate formation δ18O values for atmospheric CO2 and meteoric water can be predicted with a mixing model created from Antarctic OC carbonate data. Nakhlite and OC meteorites collected in Antarctica contain both calcites and non-calcite carbonates. Rock samples were crushed, dissolved in pure phosphoric acid, and allowed to react at the following conditions: 1 hr@30°C (Rx0, fine calcite), 18 hr@30°C (Rx1, course calcite), and 3 hr@150°C (Rx2, siderite and/or magnesite). The collected CO2 was purified with a Thermo Trace GC and analyzed on a Thermo MAT 253 IRMS in dual inlet mode. Ten OC meteorite samples collected from three different Antarctic regions (RBT, ALH, MIL) were analyzed. These samples had no pre-terrestrial aqueous alterations, yet evaporite minerals were visible on the fusion crust. It is deduced these OC carbonates were completely terrestrial. These calcites have δ13C=+6‰ and are consistent with equilibrium formation to Earth atmospheric CO2 δ13C=-7‰ at 0°C to 10°C. Siderite or magnesite fractionation may create slightly heavier δ13C as seen in the Rx2 results. The range of δ18O from +3‰ to +30‰ is heavier than expected if carbonate forms in equilibrium with only meteoric water. A δ18O mixing model is created with Earth atmospheric CO2 and meteoric water as end members. This model predicts the OC calcites form with 60%-90% contribution from atmospheric CO2 at 0°C, and the non-calcites form with 40-60% contribution from atmospheric CO2. Four martian Nakhlites collected from the Antarctic Miller Range were analyzed. These samples contain low carbonate concentrations (avg. 0.007% by weight) with distinctly heavier δ13C = +7‰ to +59‰. In general, these carbonates are lighter than expected if formed in equilibrium with the modern martian atmosphere (δ13Ccalcite ≈+60); however they may reflect formation values with an ancient (<1.3 Ga) martian atmosphere. If the martian carbonates formed with oxygen contribution ratios similar to the OC terrestrial carbonates, then the mixing model parametrically provides Mars δ18O for atmospheric CO2 and meteoric water that creates the measured values.