Calibration of an Infrared Laser Spectroscopy System for Measurement of High Precision Clumped Isotope Ratios in Carbonates

Measurements of 13C-18O clumping in CO2 derived from carbonates have greatly expanded paleothermometry research. However, measurement is slow and expensive using isotope ratio mass spectrometry (IRMS) and requires large sample sizes, 17O corrections, and gas-chromatograph cleanup. In order to overcome these limitations, we have developed a method for clumped isotope ratio measurements of CO2 by tunable infrared laser differential absorption spectroscopy (TILDAS). Our previously published results using gas samples demonstrate that measurements using this method are as precise with the best IRMS systems and surpasses typical IRMS measurements in several key respects, such as small sample size (15 μmol of CO2, 1.5 mg equivalent calcite), rapid measurement (precision of 0.01‰ within 20 minutes, 1 S.E.), and no isobaric interferences. Here we present results from measurements of CO2 derived from phosphoric acid digestion of carbonates at 90 ˚C, using a new, automated bellows sample introduction system. Two-hundred and thirty-nine gas samples and 468 carbonates have been tested, including in-house laboratory standards (Carrera marble and a synthetic standard), interlaboratory standards (ETH-1, ETH-2, ETH-3, ETH-4), and two natural samples. Synthetic carbonates, precipitated at controlled temperatures from 7 to 70 ˚C, have also been measured. In total 77 measurements of 12 synthetic samples were used to produce a linear temperature calibration:Δ638 = (0.0366±0.0031*106/T2) +0.1972±0.0334, R2=0.95429 This relationship is specific to our laser system and manual extraction line at present. The slope (0.0022±0.0002‰/°C) of the ∆16O13C18O–temperature calibration is slightly shallower than that predicted by theoretical calculations but is in agreement with several previous calibrations and successfully predicts the formation temperature of two natural samples. Calibration of the clumped isotope composition of carbonates on a new laser system is a work in progress. The TILDAS system is set to play an important role in advancing clumped isotope thermometry and stable isotope geochemistry as a whole