Implementation of experimental routines for analysis of Δ₄₇ clumped isotopes in carbonates at LAMIR Institute

Clumped isotope analysis on carbonates is a field of study concerned with the measurement of preference of bonding of heavy isotopes of carbon and oxygen in the carbonate mineral lattice, which is temperature-dependent. This level of clumping of heavy isotopes allows us to deduce mineral growth temperature, apart from the isotopic composition of the water related, like older methods require, becoming an important new approach for paleoclimate reconstruction. With recent infrastructure investments in LAMIR Institute (Laboratório de Análise de Minerais e Rochas) was acquired a 253 Plus IRMS (Thermo Fisher Scientific) equipped with Faraday cups with 10¹³ ohms amplification, suited for carbonate clumped isotope analysis of Δ₄₇ and Δ₄₈., became the first laboratory in Latin America for clumped, using the methodology developed by Caltech. The gas system denoted Autoline was also acquired and coupled on the dual inlet system of 253 Plus IRMS. The first goal has been to set an analysis routine for Δ₄₇ which is a measure of the deviation of the isotopologue ¹⁶O¹³C¹⁸O ratio to isotopologue ¹⁶O¹²C¹⁶O, in relation to the ratio of a random distribution of heavy isotopes. This stochastic prevision is the basis of the standardization. For that, it is necessary the preparation of CO₂ equilibrated with enriched water and heated for Δ₄₇ standardization to an absolute reference frame (ARF)¹, also known as “carbon dioxide equilibrium scale” (CDES)². For this purpose, we use a glass vacuum line with cryotraps of dry ice/ ethanol (-80°C) and liquid nitrogen (-196°C) like the vacuum line traditionally used for carbonate acid preparation. After purification, the gas is kept in a quartz ampoule and baked at 1000°C for 2 hours. This process rearranges the clumping of heavy isotopes in CO₂ molecules randomly. Autoline is an automated, online, common acid bath digestion and CO₂ purification system, based on multiple cryotraps of slush of dry ice/ethanol and liquid nitrogen and a GC column of Porapaq Q. The system is controlled by a LabView script (National Instruments) set to be triggered by IRMS software Isodat. The script controls a Zero Blank autosampler (Costech Analytical), pneumatic actuated valves, lifters for dewars of cryotraps, gas chromatograph, and thermometers. An amount of 10 mg of dry and powdered pure carbonate is separated into silver capsules per sample. Autoline also has a manual tube cracker intended for breaking heated gas ampoules. This way, the standardization gases undergo the same path of preparation as the sample gases. First results for 1000°C CO₂ have shown an Δ₄₇=-0.90‰ versus working gas, with a standard deviation of 0.01‰ (n=10). These gases have been prepared with a calibrated CO₂ with δ¹³C=-3.61‰ VPDB and δ¹⁸O=24.99‰ VPDB. During this period, we encountered difficulties with gas handling and purification, along with challenges related to the prototype characteristics of Autoline, necessitating expertise in electronics and computer programming. To address these issues, we made changes to the devices and rebuilt the vacuum line with improved valves. 1. DENNIS, K.J., AFFEK, H.P., PASSEY, B.H., SCHRAG, D.P., EILER, J.M. Defining an absolute reference frame for ‘clumped’ isotope studies of CO₂. Geochimica et Cosmochimica Acta 75(22):7117-7131. November 2011 2. HUNTINGTON, K.W., PETERSEN, S.V. Frontiers of Carbonate Clumped Isotope Thermometry. Annual Review of Earth and Planetary Sciences. Vol. 51, 2023. pp 611-641. May 2023