ASSESSING MO ISOTOPE VARIATIONS IN SILICICLASTIC SEDIMENTARY ROCKS DEPOSITED BEFORE THE GREAT OXIDATION EVENT

Molybdenum isotopic compositions (δ^98/95Mo) in terrigenous sedimentary rocks are emerging as a valuable tool for characterizing chemical weathering conditions. A previous study demonstrated that glacial diamictites deposited before the Great Oxidation Event (GOE) possess unfractionated δ^98/95Mo values (+0.03 ± 0.18‰) whereas post-GOE glacial diamictites are enriched in isotopically light Mo (Greaney et al., 2020 EPSL 534, 116083). The negative shift in δ^98/95Mo was interpreted to reflect the onset of oxidative weathering because isotopically light Mo is preferentially retained in Fe oxides during modern chemical weathering (e.g., Greaney et al., 2021 Chem. Geol. 566, 120103). However, isotope fractionation during anoxic weathering is poorly discussed because modern analogues for pre-GOE weathering are lacking.

To further understand δ^98/95Mo variations among terrigenous sedimentary rocks, we focus on chemical compositions of pre-GOE sandstones from the 2.45-Ga Matinenda Formation in the Huronian Supergroup, Canada. The Matinenda Formation form in shallow braided channels and is known to contain detrital U-bearing minerals and pyrite (Fralick & Miall, 1989 Sediment. Geol. 63 127-153). Detrital uranothorite and pyrite were also observed in the analyzed samples. The δ^98/95Mo of the analyzed samples range from -0.32 to 0.50‰ with an average value of +0.03 ± 0.39‰. The average value matches well with that of pre-GOE diamictites. However, the analyzed samples show more scattered values. Because Mo concentrations roughly covary with total sulfur contents in the samples, the scatter likely reflects heterogeneity in Mo isotopic compositions of detrital pyrites. Anoxic weathering probably did not impart analytically resolvable Mo isotope fractionation to the sandstones of the Matinenda Formation.