Ca-Mg inter-diffusion in synthetic polycrystalline dolomite-calcite aggregate at elevated temperatures and pressure

This study measures the reaction rate of dolomite and aragonite (calcite) into Mg-calcite at 800, 850, and 900 degrees C and 1.6 GPa. The dry synthetic dolomite-aragonite aggregate transformed very rapidly into dolomite-calcite polycrystalline aggregate while Mg-calcites formed at a relatively slow rate, becoming progressively richer in Mg with run time. We modeled the reaction progress semi-empirically by the first-order rate law. The temperature dependence of the overall transport rate of MgCO3 into calcite can be described by the kinetic parameters (E = 231.7 kJ/mol and A(o) = 22.69 h(-1)). Extrapolation using the Arrhenius equation to the conditions during exhumation of UHPM rocks indicates that the reaction of dolomite with aragonite into Mg-saturated calcite can be completed as the P-T path enters the Mg-calcite stability field in a geologically short time period (<1 Ky). On the other hand, the extrapolation of the rate to prograde metamorphic conditions reveals that the Mg-calcite formed from dolomitic marble in the absence of metamorphic fluid may not reach Mg-saturation until temperatures corresponding to high-grade metamorphism ( e. g., >340 degrees C and >10 My). SEM-EDS analysis of individual calcite grains shows compositional gradients of Mg in the calcite grains. The Mg-Ca inter-diffusion coefficient at 850 degrees C is around 1.68 x 10(-14) m(2)/sec if diffusion is the major control of the reaction. The calculated closure temperatures for Ca-Mg inter-diffusion as a function of cooling rate and grain size reveal that Ca/Mg resetting in calcite in a dry polycrystalline carbonate aggregate ( with grain size around 1 mm) may not occur at temperatures below 480 degrees C at a geological cooling rate around 10 degrees C/My, unless other processes, such as short-circuit interdiffusion along grain boundaries and dislocations, are involved.