δ C and δ O heterogeneities in carbonates: Nonlinear mixing in the application of dual-carbonate-clumped isotope thermometer.

Rationale: Dual-carbonate-clumped isotope thermometry assumes any departure from Δ and Δ co-equilibrium is due to disequilibrium processes; however, the effects of endmember mixing have not been evaluated for Δ . We show that variations in δ C and δ O values within a sample can lead to offsets in Δ and Δ that can be mistaken for kinetic fractionation.

Methods: A numerical mixing model was developed to calculate the Δ and Δ values of samples with heterogeneous isotope compositions.