SOLID-STATE CARBONATE CLUMPED ISOTOPE THERMOMETRY: PROGRESS AND PROSPECTS (Invited Presentation)

The solid-state carbonate clumped isotope geothermometer is based on the temperature dependent propensity for heavy isotopes to 'clump' into the same molecules (e.g., ¹³C¹⁸O¹⁶O₂^2-in carbonate minerals), combined with the ability of O and C to diffuse in the solid state at elevated temperature. The latter allows concentrations of isotopic clumps to come into thermal equilibrium after a change in mineral temperature. Encouraged by John Ferry and following on his pioneering application of clumped isotopes to dolomite formation, we initiated a series of experiments designed to reveal the kinetics of this clumped isotope "reordering" process. Such kinetics are central to applications ranging from metamorphic processes to preservation of paleoclimate archives.

The kinetics inferred from heating optical calcite (T ~ 300 – 800 C) on laboratory timescales deviate slightly from first-order. Two explanations have been proposed, one invoking non-equilibrium defects that enable initial rapid bond reordering followed by slower, first-order reordering after the defects have been annealed, and another invoking varying concentrations of 'pairs'—adjacent carbonate ions, one with ¹³C and the other with ¹⁸O—the penultimate configuration prior to creation of a clumped ion (or, the initial configuration after destruction of a clump). The two models lead to broadly similar predictions (though with potentially important differences) about the geological temperatures and timescales over which the reordering process is active.

 Within each model, there is a remarkable agreement in kinetic parameters for the handful of different optical calcites studied in different laboratories. Additionally, it appears that the same kinetics operate over a vast range of timescales and temperatures, meaning that the thermometer is applicable to processes occurring over < 10^-6 y (e.g., impact heating) to >10⁸ y timescales (burial / exhumation in sedimentary basins). An important frontier is understanding how mineralogy, minor and trace element chemistry, and presence of water influence reordering. Dolomite has already been shown to be far more recalcitrant than calcite, and there is an indication that individual rock samples can contain multiple independent thermometers owing to variations in carbonate chemistry.