QUANTIFYING THE ∆63-∆47 OFFSET: DETERMINING MINERAL- AND TEMPERATURE-DEPENDENT ACID FRACTIONATION FACTORS THROUGH MASS SPECTROMETRIC ANALYSES OF STOCHASTICALLY-REORDERED CARBONATES

“Clumped” isotope thermometry—the relationship between the formation temperature of a carbonate mineral and the relative abundance of ¹³C—¹⁸O bonds in its crystal lattice—is a novel geochemical proxy with a wide range of applications in paleoclimatology, geobiology, and paleoceanography. It is based on the thermodynamic propensity for rare, heavy isotopes to bond at greater rates at lower temperatures, while at high temperatures, a stochastic distribution of heavy isotopologues is achieved.

Unfortunately, precision mass spectrometric determination of the abundance of isotopologues in solid materials has proven difficult; instead, the isotopic composition of carbonates has traditionally been measured through acid digestion and subsequent analysis of the product CO₂ gas. For example, clumped isotope thermometry typically relates formation temperature to ∆₄₇, the abundance of 47-amu isotopologues relative to the predicted stochastic distribution.

As a consequence, the degree of fractionation that occurs between solid (∆₆₃) and gaseous (∆₄₇) phases has largely gone unstudied. By melting calcite and witherite powder at high pressures and temperatures (~1650ºC), we have produced a suite of carbonates predicted to have stochastic distributions of CO₃^2- isotopologues (i.e., ∆₆₃ values of 0‰). Thus, the measured ∆₄₇ values of CO₂ produced from these samples through acid digestion should equal the degree of fractionation that occurs.

We perform these measurements at a range of acid temperatures on several digestion apparatuses in order to deduce and quantify controls on acid digestion fractionation factors. We also calculate acid digestion fractionation factors using different sets of constants and compare our results to previously published estimates.