δ13 C and δ18 O heterogeneities in carbonates: Nonlinear mixing in the application of dual-carbonate-clumped isotope thermometer.

Abstract

Dual-carbonate-clumped isotope thermometry assumes any departure from Δ47 and Δ48 co-equilibrium is due to disequilibrium processes; however, the effects of endmember mixing have not been evaluated for Δ48 . We show that variations in δ13 C and δ18 O values within a sample can lead to offsets in Δ47 and Δ48 that can be mistaken for kinetic fractionation. A numerical mixing model was developed to calculate the Δ48 and Δ47 values of samples with heterogeneous isotope compositions. The model was used to test a variety of possible endmember mixing scenarios and produce a dataset of mixing offsets in both Δ48 and Δ47 . Different mixing patterns arise from endmember mixing, with different patterns between Δ48 and Δ47 . Δ47 can be both positively and negatively offset from equilibrium values by mixing; however, Δ48 can only be offset in a positive direction, producing an underestimate of formation temperature. The overall results suggest that endmember mixing can mimic kinetic fractionation caused by CO2 degassing in dual-clumped isotope measurements. Mixing between endmembers can result in patterns that resemble that of CO2 degassing. However, these effects require a variation of greater than 5‰ in endmember δ18 O values to have a significant effect relative to measurement errors on Δ48 with the detection limits of modern mass spectrometers. Δ47 remains more sensitive to endmember mixing effects and will display measurable mixing effects at 2‰ variation or less in endmember isotopic values.