ABSTRACT
RATIONALE: Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO2 for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of 13 C-18 O bonding preference in the mineral lattice, which is captured in the product CO2 , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO3 2- to mass 47 in CO2 ) during the acid digestion. METHODS: We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ47 acid fractionation. Experimental Δ47 data were acquired by processing the sample CO2 gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer. RESULTS: We showed that the acid digestion reaction, which results in the formation of CO2 enriched with 13 C-18 O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ47 acid fractionation and reaction temperature as Δ47 = -0.30175 + 0.57700 × (105 /T2 ) - 0.10791 × (105 /T2 )2 , with T varying between 298.15 and 383.15 K. CONCLUSIONS: We propose a reaction mechanism that shows a higher slope (Δ47 acid fractionation vs. 1/T2 curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.
ABSTRACT
RATIONALE: Acid digestion of carbonates to release CO2 is a crucial and sensitive step in sample preparation for clumped isotope analysis. In addition to data reduction and instrumental artefacts, many other uncertainties in the clumped isotope analysis of carbonates arise from the method used for the preparation of CO2 . We describe here an in-house-designed reaction vessel that circumvents degassing and contamination problems commonly associated with the McCrea-type digestion protocols. METHODS: We designed a leak-free break seal reaction vessel (made of Pyrex™) suitable for prolonged acid digestion at 25°C. Using this new vessel, several carbonate reference materials widely used in the clumped isotope community and other in-house laboratory standards were acid-digested and analysed for their δ13 C, δ18 O and Δ47 values with a dual inlet MAT 253 isotope ratio mass spectrometer following standard gas chromatography purification and data evaluation protocols. RESULTS: Long-term reproducibility in Δ47 determination was established using international references and in-house working standards as follows (mean and SE): Carrara-1 (0.395 ± 0.002, n = 43), Carrara-2 (0.441 ± 0.003, n = 22), OMC (0.587 ± 0.004, n = 16), NBS 19 (0.393 ± 0.005, n = 10), NBS 18 (0.473 ± 0.003, n = 5), ETH 1 (0.271 ± 0.005, n = 7), ETH 3 (0.698 ± 0.005, n = 3), MZ (0.715 ± 0.002, n = 3) and several others. CONCLUSIONS: A new method using a break seal tube was found to be efficient for the clumped isotope analysis of carbonates that require longer reaction time at 25°C. This method yields good precision in Δ47 analysis and was found to be suitable for acid digestions at any desired temperature.
