Controls on the petrologic type of CM carbonaceous chondrites evaluated by geochemical equilibrium modellingOPEN ACCESS
Robin L. Haller, Martin R. Lee
MAPS, In Press, Journal Pre-proof, Available online 30 October 2025
“The most abundant group of carbonaceous chondrites are the Mighei-like (CM) meteorites, and they span petrologic types ranging from almost unaltered (CM3) to heavily aqueously processed (CM1). The factors that controlled the extent of aqueous alteration that CM chondrites experienced on their parent body/bodies are debated and remain poorly constrained. Geochemical models, and equilibrium models in particular, are powerful tools for emulating water–rock (W/R) interactions as a function of different parameters and conditions. In order to investigate possible CM chondrite alteration conditions and evaluate controlling factor(s) on petrologic type we modelled the interaction of a CM3 proxy, the CO3.0 chondrite Dominion Range 08006, with a fluid under different temperatures (1–150 °C), W/R ratios (by mass) (0.2–5) and solute concentrations (0.2–2 mol/kg CO2, 0.02–0.2 m NH3, 0.01–0.1 m H2S and 0.001–0.01 m HCl). Five additional scenarios that use the same parameter space but with differences in properties including pressure and redox conditions were also created to further investigate the controls on petrologic type. Systems that are CM chondrite-like from their close similarity to the mineralogy of CM meteorites as determined by sample analysis can form under a wide range of temperatures (1–140 °C), W/R ratios (by mass) (0.3 – 5), solute concentrations (0.2 – 2 m CO2), pH (8.5 – 12.6) and pe (−10.8 – −6.6). Across the different scenarios CM2-like systems are most abundant followed by CM1-like, whereas CM1/2-like systems are rare. Differences in petrologic type can be mainly attributed to variations in temperature, with CM1s overall being formed by alteration at higher temperatures (80–140 °C) than CM2s (1–105 °C). CM1/2 chondrites might be produced by elevated W/R ratios (by mass) and/or solute concentrations. From a mineralogical perspective, CM chondrites of different petrologic type might have originated from contrasting regions of a singular, thermally stratified parent body. Some differences between model results and CM chondrite samples could be addressed by more sophisticated tools like kinetic modelling.”
