CALIBRATING THE CARBONATE CLUMPED ISOTOPE THERMOMETER FROM 7 TO 70 ˚C BY INFRARED LASER SPECTROSCOPY

Measurements of ¹³C-¹⁸O clumping in CO₂ derived from carbonates have greatly expanded robust paleothermometry research. However, measurement is slow, requires large sample sizes, ¹⁷O corrections, GC cleanup, and is expensive. In order to overcome these limitations, we have developed an isotope ratio laser spectrometry method for clumped CO₂ measurements by tunable infrared laser differential absorption spectroscopy (TILDAS). Our previously published results using gas samples demonstrate that the method is competitive with the best isotope ratio mass spectrometry (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 interference problems. The reported TILDAS ∆_16O13C18O values, approximately equivalent to ∆₄₇, show a linear relationship with theoretical values. During the past 9 months, a manually operated gas extraction line was constructed for phosphoric acid digestion of carbonates at 90˚C, and the TILDAS instrument was upgraded with addition of an automated bellows sample introduction system. 239 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 2 natural samples. Synthetic carbonates, precipitated at controlled temperatures from 7 to 70 ˚C, have also been measured. In total 98 measurement of 14 synthetic samples were used to produce a linear temperature calibration. Although this relationship is specific to our laser system and manual extraction line at present, the slope of the ∆_16O13C18O–temperature calibration is in agreement with other published relationships. With further progress, the TILDAS system could play an important role in advancing clumped isotope thermometry and stable isotope geochemistry as a whole.