An Experimental Study of the Breakdown of Dolomite in Mixed-Volatile Metamorphic Environments

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

Labotka EAR-0087553 The compositions and abundances of minerals in metamorphosed limestone and dolomite rock are commonly used to determine properties of the metamorphic fluid. The important properties are the composition of the fluid, the fluid flux, and terrain-scale patterns of fluid flow. In metamorphic systems, the development of mineral assemblages depends on the relative values of several rate constants, such as hydraulic conductivity, thermal conductivity, diffusion and dispersivity, and reaction progress. In this study, we plan to determine the kinetics of reactions that affect the stability of dolomite in mixed-volatile environments. A common sequence of reactions in dolomitic rocks metamorphosed under H2O-rich conditions is dolomite = calcite + periclase + CO2, followed by periclase + H2O = brucite. Similar reactions occur during metamorphism of ultramafic rocks in mixed-volatile fluids. The dolomite system is ideal for the study of reaction kinetics because the assemblages occur in a variety of silica-poor rocks in contact and regional metamorphic terrains. Our planned set of experiments consists of three parts. First, we will conduct time-series experiments on powdered starting materials in hydrothermal vessels to provide upper limits on mineral and isotope reaction rates. Second, we will observe fast and unquenchable reactions in situ in the hydrothermal diamond-anvil cell with the aid of laser Raman spectroscopy. In the third part of experimental investigation we plan to use cylinders of dolomite rock as starting material to mimic more closely the natural reactions. In all experiments, we plan to use isotopically labeled fluid to determine the rates of not only the mineralogic reactions, but also the isotope-exchange reactions in the same experiments. We will compare the results of the experimental study, particularly of the rock core experiments, with observations of the contact-metamorphosed dolomite from Marble Canyon, Texas, which experienced both the periclase-forming reaction and the brucite-forming retrograde reaction.