¹Robert W.Nicklas,¹James M.D.Day,²Zoltan Vaci,³Arya Udry,⁴Yang Liu,⁵Kimberly T.Tait
Earth and Planetary Science Letters 564, 116876 Link to Article [https://doi.org/10.1016/j.epsl.2021.116876]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093, USA
²Institute of Meteoritics, Department of Earth and Planetary Science, University of New Mexico, Albuquerque, NM, 87131, USA
³Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, USA
⁴Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵Department of Natural History, Royal Ontario Museum, Toronto, ON, M5S 2C6, Canada
Copyright Elsevier

Martian meteorites are the only available samples that can be directly measured to constrain the geological evolution of Mars. It has been suggested that the oxygen fugacity (fO2) of martian shergottite meteorites, which have low (∼7 wt.%) to high-MgO (∼30 wt.%) compositions, correlates with incompatible trace element enrichment (i.e., La/Yb), and 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and 176Hf/177Hf at the time of crystallization. These relationships have been interpreted to result from early magmatic processes segregating enriched and more oxidized from depleted and more reduced reservoirs in Mars. Here we use the V-in-olivine oxybarometer to constrain the fO2 of shergottites and the dunitic chassignites. These data, utilizing early crystallizing silicate phases, constrain the shergottite fO2 range to between −3.72 ± 0.07 and −0.21 ± 0.55 ΔFMQ (log units relative to the fayalite-magnetite-quartz buffer), with no correlation with trace element enrichment or Nd isotope systematics. Previously employed oxybarometers that use later-formed or multiple mineral phases, and that show such correlations, likely differ from the V-in-olivine oxybarometer in that they record effects from late-stage magmatic processes. In contrast to shergottites, chassignites are relatively oxidized, at +2.1 ± 0.4 to +2.2 ± 0.5 ΔFMQ. The chassignites, along with the nakhlites, have been proposed to be sourced from metasomatized lithospheric mantle, and their high fO2 strengthens this model. The new data implies that the martian mantle sources of shergottites have fO2 of −2.1 ± 1.8 ΔFMQ. This estimate indicates that the mantle and core of Mars are not in redox equilibrium and therefore that oxidation of the martian mantle following core formation is required.

¹Johan Vandenborre,³Laurent Truche,¹Amaury Costagliola,¹Emeline Craff,¹Guillaume Blain,¹Véronique Baty,²Ferid Haddad,²Massoud Fattahi
Earth and Planetary Science Letters 564, 116892 Link to Article [https://doi.org/10.1016/j.epsl.2021.116892]
¹Subatech, UMR 6457, Institut Mines-Télécom Atlantique, CNRS/IN2P3, Université de Nantes, 4, Rue Alfred Kastler, La chantrerie BP 20722, 44307 Nantes cedex 3, France
²GIP ARRONAX, 1 rue ARRONAX, CS 10112, 44817 Saint-Herblain Cedex, France
³University Grenoble Alpes, CNRS, ISTerre, CS 40700, 38058 Grenoble, France
Copyright Elsevier

Low molecular weight carboxylate anions such as formate (HCOO−), acetate (CH3COO−) and oxalate (C2O) have been shown to play an important role in supporting deep subsurface microbial ecosystems. Their origin whether biological or abiotic is currently highly debated, but surprisingly radiolytic production has rarely been considered, as it is the case for H2. Here, we address this question through dedicated irradiation experiments. Aqueous solutions containing carbonate, formate, acetate or oxalate have been irradiated using both the 60.7 MeV α-beam of the ARRONAX cyclotron (Nantes, France) and 661.7 keV γ-Ray in order to reveal the mechanism and chemical yield of radiation-induced dissolved carbonate degradation.

The yields (G-values) of carboxylate anions production/degradation in low-concentration carbonate solution (0.01 to 1 mmol L−1) are measured. Carbonate degradation occurs through three consecutive steps (Carbonate
Formate Acetate Oxalate) involving formate radical (CO2−•), dihydrogen (H2), and carbon dioxide (CO2) generation. Dissolved carbonate radiolysis provides a consistent pathway for both enhancing two-fold the radiolytic H2 production compared to pure water and generating carboxylic species, chiefly oxalate, readily available for microbes. Radiation-induced carbonate degradation may produce substantial amount (millimolar concentration) of carboxylate anions in ancient groundwaters from deep crystalline bedrocks. Subsurface lithoautotrophic microbial ecosystems may not only be supported by radiolytic H2 but also by carboxylate species from carbonate radiolysis. Carbonate radiolysis can be also an endogenous source of carboxylate species on Mars and other planetary bodies.