A robust methodology for triple (Δ47, Δ48, Δ49) clumped isotope analysis of carbonates

Analysis of triangle(48) and triangle(47) values of CO2 released from phosphoric acid digestion of carbonates enables the rate-limiting kinetic processes involved in carbonate mineralization to be identified and formation temperatures to be determined by accounting for these kinetic biases. This method has the benefit of only requiring analysis of a single carbonate phase. To resolve temperature from the kinetic information, mass spectrometric measurements of highest accuracy and precision are required.Here, we outline a robust methodology for high-precision triple (triangle(47,) triangle(48), triangle(49)) clumped isotope analysis using turbo-pumped gas and carbonate preparation lines, a common acid bath and the dual inlet of a Thermo Scientific 253plus gas source mass spectrometer, equipped with additional m/z(47.5) half mass cup. By analysis of >400 replicates each, we report long-term (50 months) triangle(47, CDES 90) and triangle(48, CDES 90) values for three internationally accepted (ETH-1, -2 and -3) and two internal carbonate standards (GU1, Carrara) unbiased by scale compression. In order to obtain highest accuracy and precisions closest to shot noise limit it is essential to account for a sample size effect, correct raw intensities for a negative background utilizing the half mass cup, avoid pressure imbalances between sample and reference gas and distribute samples evenly across a given analytical session.We confirm triangle(47, CDES 90) values that were recently assigned to ETH-1 and ETH-2. Our long-term average triangle(47, CDES 90) value of 0.6032 parts per thousand for ETH-3, on the contrary, occurs 10 ppm lower than its recently assigned value. As a consequence, triangle(47, CDES 90) values of samples having relatively low temperatures of formation are not consistent with triangle(47, I-CDES) values. We show that GU1, a synthetic carbonate deriving from the hydroxylation of CO2 and exhibiting a strong anti-clumped triangle(48, CDES 90) value, appears to be homogeneous in its isotopic composition. If used in combination with ETH-1, -2 and - 3, it may allow projection of background corrected raw data without any bias into triangle(47)-triangle(48) CDES 90 space, provided our long-term dual clumped isotope values for these carbonate anchors are considered.Stability of mass spectrometric measurements is related to filament age and/or room temperature fluctuations and can be monitored using reference gas m/z(49) intensity. Under the most stable analytical conditions, the triangle(49, CDES 90) values of equilibrated gases and carbonates reveal a trend with formation temperature. Using data from the most stable sessions, we present a temperature calibration for triangle(49, CDES 90) in CO2 evolved from phosphoric acid digestion of carbonates.