EXPERIMENTAL DETERMINATION OF THE REACTION ARAGONITE + MAGNESITE = DOLOMITE AT 5 TO 9 GPA

Abstract

Martinez et al. bracketed the decomposition reaction of dolomite to aragonite + magnesite between 5.0 and 5.5 GPa at 600 C using in-situ XRD in a multi-anvil apparatus. They then extrapolated their results using the available thermodynamic data such that the breakdown reaction occurs at ~1,000 C at 6 GPa. If correct, this extrapolation has implications for the type(s) of stable carbonate(s) in the mantle. For example, the reaction magnesite + diopside = dolomite + enstatite would be metastable at P >7 GPa relative to the aragonite-containing analog, and thus aragonite, rather than dolomite, might be stable near the solidus of lherzolite. This extrapolation would also imply that aragonite and magnesite would be stable in garnet + clinopyroxene assemblages at pressures in the stability field of diamond. To test this extrapolation, experiments were conducted at higher temperatures in a multiple-anvil apparatus. Results of initial experiments were consistent with the results of Martinez et al.. At higher temperatures, however, the reaction occurs at higher pressures than predicted by Martinez et al., requiring significant curvature to the reaction boundary. This curvature (increasing dP/dT with increasing T) is consistent with increasing disorder in the dolomite with increasing temperature, increasing the stability of dolomite relative to aragonite plus magnesite. The experimental results restrict aragonite + magnesite-bearing assemblages to low temperature. Aragonite will not be present in near-solidus lherzolite, and can be present in orthopyroxene-free eclogites and pyroxenites only at low temperature.