THE LINK BETWEEN CATION ORDERING AND OXYGEN ISOTOPE FRACTIONATION IN EXPERIMENTAL DOLOMITES

Early diagenetic dolomites are abundant in the rock record and potentially offer an untapped resource to study Phanerozoic paleoclimate. The use of these dolomites as a paleoclimate proxy record is presently inhibited, however, by large uncertainties in the published oxygen water-mineral fractionation values. Laboratory experiments show that dolomitization takes place via a series of mineral transformations from very high-Mg calcite to poorly ordered dolomite to well-ordered dolomite. Field data also indicate that ancient dolomite also vary widely in their mineralogical attributes, such as stoichiometry (mole% MgCO₃) and cation ordering. To date, published isotope fractionation factors have not discriminated between the various Ca-Mg-carbonate minerals, all of which are referred to as dolomite. We hypothesize here that much of the uncertainty in oxygen isotope fractionation factors can be attributed to quantifiable differences in isotopic fractionation between the various Ca-Mg-carbonate phases.

To test this hypothesis, well-controlled, high temperature dolomitization experiments are performed to track δ¹⁸O_carb, δ¹⁸O_fluid, the Δ₄₇ clumped isotope composition through dolomitization stages. Preliminary data indicate that δ¹⁸O_carb correlates with cation ordering. Results from high-temperature dolomites exhibit an initial decrease in δ¹⁸O_carb with increasing cation ordering followed by an increase in δ¹⁸O_carb with progressive cation ordering. Given that these laboratory experiments are controlled for all other variables (i.e. temperature, fluid chemistry, reactant mineralogy and reactant size), observed differences in δ¹⁸O_carb are interpreted to reflect isotope fractionation variations driven by Ca-Mg cation ordering. These results imply that mineralogy must be considered when interpreting δ¹⁸O_carb in dolomites.