Non-linear mixing effects on mass-47 CO2 clumped isotope thermometry: Patterns and implications

RATIONALE: Mass-47 CO2 clumped isotope thermometry requires relatively large (similar to 20 mg) samples of carbonate minerals due to detection limits and shot noise in gas source isotope ratio mass spectrometry (IRMS). However, it is unreasonable to assume that natural geologic materials are homogenous on the scale required for sampling. We show that sample heterogeneities can cause offsets from equilibrium Delta(47) values that are controlled solely by end member mixing and are independent of equilibrium temperatures. METHODS: A numerical model was built to simulate and quantify the effects of end member mixing on Delta(47). The model was run in multiple possible configurations to produce a dataset of mixing effects. We verified that the model accurately simulated real phenomena by comparing two artificial laboratory mixtures measured using IRMS to model output. RESULTS: Mixing effects were found to be dependent on end member isotopic composition in delta C-13 and delta O-18 values, and independent of end member Delta(47) values. Both positive and negative offsets from equilibrium Delta(47) can occur, and the sign is dependent on the interaction between end member isotopic compositions. The overall magnitude of mixing offsets is controlled by the amount of variability within a sample; the larger the disparity between end member compositions, the larger the mixing offset. CONCLUSIONS: Samples varying by less than 2 % in both delta C-13 and delta O-18 values have mixing offsets below current IRMS detection limits. We recommend the use of isotopic subsampling for delta C-13 and delta O-18 values to determine sample heterogeneity, and to evaluate any potential mixing effects in samples suspected of being heterogonous. Copyright (C) 2015 John Wiley & Sons, Ltd.