Timescales of CM carbonaceous chondrite aqueous alteration determined by kinetic modelling

Mighei-like (CM) meteorites are the most abundant group of hydrated carbonaceous chondrites. They display a wide range of degrees of aqueous alteration, from very mild to near complete, as demonstrated by the relative abundances of phyllosilicates and surviving anhydrous silicates. The reasons for differences in the extent of alteration are debated, and probably reflect variations within the CM parent body/bodies of one or more of water/rock (W/R) ratio, temperature, and timescale. The least understood is timescale, and so to constrain the duration of aqueous alteration and to investigate precipitation conditions of key CM minerals, we created kinetic models with the software PHREEQC. These models simulated a mineralogically detailed alteration of the CM3 proxy Dominion Range 08006 (CO3.00) by an aqueous fluid for different values of W/R ratio (by mass) and temperature over the timespan of 10 5–105 years with incremental time steps. Additional laboratory experiments in which chips of Dominion Range 08006 were reacted with either pure water or a carbonated aqueous solution under reducing conditions were undertaken to validate parts of the models and to evaluate formation conditions of calcite, as the study of calcite in CM chondrites provides invaluable information on aqueous alteration timings and conditions. The models suggest that regardless of their petrologic type, CM chondrites could have been altered from a CM3 over timescales ranging from months to ~10,000 years depending on temperature. The more highly altered CM1 chondrites that contain magnetite instead of cronstedtite would have needed to react at temperatures of at least 100◦C but not necessarily different W/R ratios compared to less altered CM2s. Some of the modelled systems suggest that minimally altered CM chondrites like Asuka 12169 could have been exposed to aqueous solutions at ~1◦C and W/R ratios (by mass) of ~3 or higher. Significant quantities of calcite only form in laboratory experiments with a carbonated solution, suggesting that at least for early formed calcite, aqueous carbonate was sourced from accreted C-bearing ice instead of indigenous organic carbon. According to the models, a typical CM2 chondrite could have been altered from a CM3 lithology within ~1,000 years at 50◦C and 0.8 W/R ratio (by mass).