Evaluating the Effect of Dolomite Mineralogy on Oxygen Isotope Fractionation Using High-Temperature Dolomitization Experiments

Date of Award

5-2026

Degree Name

Doctor of Philosophy

Department

Geological and Environmental Sciences

First Advisor

Stephen E. Kaczmarek, Ph.D.

Second Advisor

Sierra V. Petersen, Ph.D.

Third Advisor

Ian Z. Winkelstern, Ph.D.

Fourth Advisor

David O. Zakharov, Ph.D.

Keywords

Carbonates, cation ordering, diagenesis, dolomite, elemental, isotopes

Abstract

Geoscientists routinely use the oxygen isotope composition of carbonate minerals to guide interpretations about the conditions of formation. Published oxygen isotope fractionation for dolomite, a Ca-Mg carbonate mineral, show wide discrepancies, limiting the use of δ¹⁸Odolomite in paleoenvironmental reconstructions. In this study we hypothesize that these discrepancies are caused by variations in magnesium-calcium cation ordering – a mineralogical parameter that varies widely in natural dolomite but is rarely considered in geochemical studies.

This hypothesis was tested using high-temperature dolomitization experiments conducted at six temperatures between 150 and 250 °C. Time-series experiments captured the step-wise replacement of calcite by very high-magnesium calcite (VHMC) to poorly- then well-ordered dolomite. Paired mineralogic and isotopic measurements document the evolution of oxygen isotope fractionation throughout the reaction.

Results show that oxygen isotope fractionation varies systematically with mineralogy during dolomitization. VHMC and ordered dolomite exhibit distinct mineral–water fractionation, with differences that decrease with increased temperature and converge near 250 °C. Experiments producing calcite under comparable conditions demonstrate that cation order, rather than Mg incorporation, controls the observed fractionation differences. Fluid–solid isotope measurements suggest that once ordered, dolomite largely preserves its isotopic signature.

Our results suggest that cation ordering provides a quantitative metric for evaluating mixed VHMC–dolomite assemblages. Using this relationship, we developed a model in which the effective oxygen isotope fractionation factor varies continuously with cation order. These results reconcile many discrepancies in the dolomite oxygen isotope literature and provide a practical approach for interpreting δ¹⁸O values in natural dolomites.

Access Setting

Dissertation-Abstract Only

Restricted to Campus until

5-1-2028

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