Redox-influenced trace element compositional differences among variably aqueously altered CM chondrites

Abstract

It is generally agreed that the CM chondrites are remarkably compositionally uniform. To date, this observation has largely been based on major and selected trace element bulk elemental abundances. Here, we examine the trace element compositional uniformity of the CM chondrites in detail. We quantified the abundances of 43 trace elements in 21 CM chondrites displaying variable degrees of petrographically identified aqueous alteration. With these data, we used graphic and standard statistical methods for examining evidence for compositional differences with respect to degree of aqueous alteration. The results show that suites of variably aqueously altered CM chondrites have readily apparent and statistically significant trace element compositional differences. Higher degrees of aqueous alteration are associated with depleted bulk trace elemental abundances; however, when the variable mineralogy and hydration (H2O or OH) are taken into account, the compositional differences between variably aqueously altered suites of CM chondrites largely disappear. Nevertheless, some trace elements still show statistically significant differences between suites of CM chondrites that experienced extensive and milder degrees of aqueous alteration. These elements are observed to be redox sensitive species (e.g. Mn, Zn, Mo, Re, U), whose mobilities between aqueous solutions and solids are mediated by complex ions whose solubilities are sensitive to a complicated combination of either the presence of Mn or Fe (oxy)hydroxide surfaces, specific redox environments, solution pH, presence of organic matter, and phyllosilicate surface binding. The studies suggest that the apparent compositional differences of these elements reflect an evolving redox environment during the history of the CM chondrites. It is unclear if the compositional differences between variably aqueously altered CM chondrites are the result of anisochemical (open system) aqueous alteration for some redox condition sensitive trace elements on the CM parent body or bodies or if the differences are the result of the meteorites’ terrestrial residence.