δ 13 C and δ 18 O heterogeneities in carbonates: Nonlinear mixing in the application of dual-carbonate-clumped isotope thermometer.

Autor: White JH; School of the Environment, The University of Queensland, St Lucia, Queensland, Australia., Defliese WF; School of the Environment, The University of Queensland, St Lucia, Queensland, Australia.
Jazyk: angličtina
Zdroj: Rapid communications in mass spectrometry : RCM [Rapid Commun Mass Spectrom] 2023 Nov 15; Vol. 37 (21), pp. e9627.
DOI: 10.1002/rcm.9627
Abstrakt: Rationale: Dual-carbonate-clumped isotope thermometry assumes any departure from Δ 47 and Δ 48 co-equilibrium is due to disequilibrium processes; however, the effects of endmember mixing have not been evaluated for Δ 48 . We show that variations in δ 13 C and δ 18 O values within a sample can lead to offsets in Δ 47 and Δ 48 that can be mistaken for kinetic fractionation.
Methods: A numerical mixing model was developed to calculate the Δ 48 and Δ 47 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 Δ 48 and Δ 47 .
Results: Different mixing patterns arise from endmember mixing, with different patterns between Δ 48 and Δ 47 . Δ 47 can be both positively and negatively offset from equilibrium values by mixing; however, Δ 48 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 CO 2 degassing in dual-clumped isotope measurements.
Conclusions: Mixing between endmembers can result in patterns that resemble that of CO 2 degassing. However, these effects require a variation of greater than 5‰ in endmember δ 18 O values to have a significant effect relative to measurement errors on Δ 48 with the detection limits of modern mass spectrometers. Δ 47 remains more sensitive to endmember mixing effects and will display measurable mixing effects at 2‰ variation or less in endmember isotopic values.
(© 2023 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons Ltd.)
Databáze: MEDLINE
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