Rationale: Acid digestion of carbonates to release CO 2 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 CO 2 .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 inhouse laboratory standards were acid-digested and analysed for their δ 13 C, δ 18 O and Δ 47 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 Δ 47 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 Δ 47 analysis and was found to be suitable for acid digestions at any desired temperature.

Triple oxygen isotope systematics has evolved as a powerful tool in understanding various earth system processes. It has proven reliable in assessing paleoenvironmental conditions from natural archives (e.g., waters, ice cores, biota, sediments, etc.) owing to recent advances in high-precision mass spectrometric analysis. Toward the standardization of triple oxygen isotope analyses in carbonates, we describe a procedure of high-precision Δ′17O analysis of carbonates by a two-step protocol: acid digestion of carbonates to evolve CO2 followed by the catalytic CO2–O2 exchange method. The Δ′17O values of a suite of carbonate reference materials and several carbonates of different origins have been determined with good precision (∼0.007‰). The accuracy of sample Δ′17O values is dependent on the accuracy of Δ′17O composition of the reference CO2 used in determining the effective fractionation (θs) in the experimental setup. The obtained Δ′17O values (λ = 0.528, versus VSMOW) for NBS18-CO2 (−0.119‰) and NBS19-CO2 (−0.169‰) show a difference of 0.050‰, similar to that obtained elsewhere via complete fluorination. The analyzed carbonates mostly conform to equilibrium mass-dependent fractionation laws, but we encountered a suite of samples from cold seeps, caves, and metasomatic environments that have Δ′17O values indicative of disequilibrium fractionation. We show that a combination of clumped isotope composition (Δ47) that provides estimates of formation temperature and triple oxygen isotope ratios in carbonates can help in reconstructing past environments, where paired carbonate data (δ13C−δ18O−Δ47–Δ′17O) and parent water data (δ17O−δ18O−Δ′17O) are particularly useful.

Rationale:Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO 2 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 CO 2 , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO 3 2− to mass 47 in CO 2 ) 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 Δ 47 acid fractionation. Experimental Δ 47 data were acquired by processing the sample CO 2 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 CO 2 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 Δ 47 acid fractionation and reaction temperature as Δ 47 = −0.30175 + 0.57700 × (10 5 /T 2 ) -0.10791 × (10 5 /T 2 ) 2 , with T varying between 298.15 and 383.15 K. Conclusions:We propose a reaction mechanism that shows a higher slope (Δ 47 acid fractionation vs. 1/T 2 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.

The Amba Dongar carbonatite‐alkaline complex is regarded as a late phase intrusion (ca. 65 Ma) associated with Deccan volcanism. Existing age constraints on the Amba Dongar carbonatite are based on phlogopite 40Ar–39Ar plateau ages derived from carbonatite and associated alkaline rocks within the complex. We explore the utility of apatite, a common accessory mineral in Amba Dongar carbonatites, as a petrogenetic and geochronological tool to constrain further the evolution of the carbonatites. Apatite exhibits different crystal forms—from hexagonal stubby prisms to ovoid and elongate grains. Apatite occurs as cumulate and disseminated grains that are either primary, fractured, or recrystallized. SrO content is high in primary apatite (3.17 wt%) but less in recrystallized (1.90 wt%) and disseminated grains (0.47 wt%). MnO and Cl are negligible while FeO is typically low in all samples. SiO2 content is low except for a few primary grains which measure up to 1.35 wt%, correlating with high REE content. LREEs dominate the total REE budget of Amba Dongar apatite, with HREEs below EPMA detection limit in most samples. Total REE content is generally low, though some primary apatite contains up to 3.19 wt% REEs. We also report UPb apatite ages from the carbonatites analysed in situ by LA‐ICP‐MS. The data from all samples combined on a UPb Tera–Wasserburg concordia yield a lower intercept age of 65.4 ± 2.5 Ma (MSWD = 2.8, 207Pb/206Pbi = 0.8272 ± 0.0028), and the Amba Dongar carbonatites are thus interpreted to represent a phase of coeval magmatic activity with Deccan volcanism.

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