Experimental Deformation of Dolomite and Mechanisms of Flow in the Calcium-Magnesium Carbonate System

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0107078NewmanCarbonates are common lithologic units in mountain belts, and their deformation plays an important role in the development of large-scale structures. Field and experimental studies show that fracture strengths of carbonates at near-surface conditions are comparable to those of siliciclastic rocks; however, flow strengths of carbonates deformed at depth are relatively low and penetrative strains are localized within them. Deformed limestones and marbles made up primarily of calcite exhibit abundant evidence of internal strain by mechanical twinning, microcracking, solution transfer, dislocation glide, dislocation creep, and diffusional creep. Evidence of recovery and dynamic recrystallization at elevated temperatures is ubiquitous. These processes also occur in experimentally deformed calcite, carried out at known temperatures, pressures and strain rates. The measured mechanical properties from these experiments provide constraints on the environmental conditions required for deformation, and the rheologies needed to model the tectonics and structural development of continental collisions.Carbonate units made up primarily of dolomite (CaMg(CO3)2) have significantly different mechanical properties from those made up of calcite (CaCO3). Field and experimental observations of dolomite and calcite indicate that the strength of dolomite exceeds that of calcite significantly, yet dolomites deformed at high temperatures also exhibit evidence of twinning, dislocation creep, and dynamic recrystallization. Most experimental studies of dolomite deformation bear on fracture properties and only a few, exceptional studies, have been done that provide flow strengths and mechanisms of penetrative deformation. Early high temperature experiments on dolomites provide information on crystal plastic deformation mechanisms, but systematic measurements of flow strengths that can be used to determine the high temperature rheology of this carbonate are lacking. This research addresses the high temperature deformation of dolomite through controlled deformation experiments; the objectives include determining the flow law for dolomite, determining the deformation processes associated with this rheology, and evaluating the conditions required for its deformation in orogenic belts. Experiments are being conducted on natural fine-grained dolomite and hot pressed synthetic dolomite in triaxial compression at confining pressure of 400 MPa, temperatures of 400 degree to 900 degree, strain rates of 10-4 to 10-7 s-1. Optical microscopy, SEM, and TEM are being used to characterize microstructures and document deformation mechanisms.In addition to the tectonic and structural problems this research addresses, the comparison of the mechanical properties of rhombohedral carbonates of differing composition addresses fundamental questions involving the mineral physics and chemistry of deformation. The high strength of dolomite relative to that of calcite raises a number of interesting questions, including: (1) How does substitution of one cation, Mg, for another, Ca, affect the deformation process?; (2) How do bond strengths affect the strengths and thermal activation barriers to deformation processes?; (3) How does cation ordering affect deformation?; (4) How do microstructures associated with order/disorder and lamellar compositional variations affect deformation and strength?Comparison of results for specimens made up of the magnesium end-member carbonate, magnesite (MgCO3), as well as a few samples of end-member calcite, deformed at comparable conditions to those of the dolomite experiments, may elucidate how deformation depends on crystal chemistry and structure. This research compares the strengths of these three carbonates at a common set of conditions, the deformation mechanisms that are activated in each, and the activation energies that presumably tell us about the rate-controlling processes.