Evaluating Dolomite Stoichiometry as a Proxy for the Chemistry of Dolomitizing Fluids

Abstract

Various proxies, such as stable isotopes and trace elements, are routinely used to constrain the conditions of dolomitization. Here, dolomite stoichiometry is evaluated as a proxy for the chemistry of the dolomitizing fluid. High-temperature experiments have shown that dolomite stoichiometry is controlled by Mg/Ca, temperature, and molarity (Na, K, Mg, Ca) of the dolomitizing fluid. Here, we evaluated systematic changes in dolomite stoichiometry as a means to provide geologically relevant information about the dolomitizing conditions. To do this, high-resolution stratigraphic measurements were acquired from an outcrop of the Cretaceous Upper Glen Rose Formation. In total, 292 vertical and 102 lateral samples were collected and examined at the centimeter scale using a suite of analytical tools, including powder X-ray diffraction (XRD), stable isotope geochemistry, thin section petrography, and scanning electron microscopy. The Upper Glen Rose is characterized by high-frequency depositional cycles that fluctuate between subtidal mud-dominated miliolid packstones to supratidal mud-cracked dolomitic mudstone caps. High-resolution data exhibit two geochemical and mineralogical patterns within individual depositional cycles. Regressive facies successions are associated with vertical increases in dolomite stoichiometry, percent dolomite, and δ¹⁸O values. In contrast, transgressive facies successions are associated with vertical decreases in dolomite stoichiometry, percent dolomite, and δ¹⁸O values. These patterns are consistent with a model of dolomitization whereby temporal changes in fluid chemistry (e.g., Mg/Ca, temperature, and molarity) reflect relative sea-level fluctuations during deposition and penecontemporaneous dolomitization. The high-resolution XRD dataset presented here is the first of its kind and suggests that dolomite stoichiometry may provide a valuable proxy for interpreting the chemistry of dolomitizing fluids.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018