Date of Award

8-2021

Degree Name

Master of Science

Department

Geological and Environmental Sciences

First Advisor

Dr. Stephen E. Kaczmarek

Second Advisor

Dr. Michelle A. Kominz

Third Advisor

Dr. Robb H. Gillespie

Fourth Advisor

Dr. Andrew H. Caruthers

Keywords

Carbonates, diagenesis, dolomite, stoichiometry, proxy

Access Setting

Masters Thesis-Open Access

Abstract

Dolomite is a common, diagenetic, Mg-Ca carbonate mineral. Dolomitization occurs by a dissolution-precipitation reaction between a CaCO3 precursor (calcite or aragonite) and Mg bearing fluids. The amount of Mg in dolomite, referred to as dolomite stoichiometry (mol% MgCO3), varies in natural dolomites (38-50 mol% MgCO3). Laboratory experiments have shown that dolomite stoichiometry is dependent on physiochemical factors of the diagenetic fluids (i.e., temperature, salinity, and Mg/Ca). The impact of the CaCO3 precursor on stoichiometry, however, has not been studied directly. This study tests how the CaCO3 precursor size (sieve size) and mineralogy (calcite vs. aragonite) impact dolomite stoichiometry using high-temperature dolomitization experiments. The results show no difference in dolomite stoichiometry between the five different grain size cohorts, which range from 63 – 150 μm. This finding indicates that dolomite stoichiometry is independent of reactant surface area, and thus reaction rate. To the contrary reactant mineralogy did impact stoichiometry and calcite reactants consistently produced dolomite products that were approx. 5 mol% MgCO3 higher than aragonite reactants. The CaCO3 reactants also produced unique dolomite crystal textures. Aragonite reactants produced fine, non-planar to planar-s dolomite crystals, whereas calcite reactants produced larger, planar-e dolomite crystals.

The observation that calcite produces more stoichiometric (higher mol% MgCO3) dolomites indicates that the calcite crystal lattice may aid in the incorporation of Mg. The lack of change in stoichiometry for different reactant sizes supports this hypothesis by providing evidence that the difference in dolomite stoichiometry cannot be attributed to changes in solution chemistry driven by reactant solubility and dissolution rate. The results of this study indicate that the substrate on which dolomite precipitates can impact the incorporation of Mg and may explain some of the variability in dolomite stoichiometry observed in the rock record.

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