Experimental determination of the reaction aragonite plus magnesite = dolomite at 5 to 9 GPa

Martinet et al, bracketed the decomposition reaction of dolomite to aragoni te + magnesite between 5.0 and 5.5 GPa at 600 OC using in-situ XRD in a mul tianvil apparatus. They then extrapolated their results using the available thermodynamic data such that the breakdown reaction occurs at similar to1, 000 degreesC 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 G Pa relative to the aragonite-containing analog, and thus aragonite, rather than dolomite, might be stable near the solidus of Iherzolite. This extrapo lation would also imply that aragonite and magnesite would be stable in gar net + clinopyroxene assemblages at pressures in the stability field of diam ond. To test this extrapolation, experiments were conducted at higher tempe ratures in a multiple-anvil apparatus. Results of initial experiments were consistent with the results of Martinet et al. At higher temperatures, howe ver, the reaction occurs at higher pressures than predicted by Martinet et al., requiring significant curvature to the reaction boundary. This curvatu re (increasing dP/dT with increasing T) is consistent with increasing disor der in the dolomite with increasing temperature, increasing the stability o f dolomite relative to aragonite plus magnesite. The experimental results r estrict aragonite + magnesite-bearing assemblages to low temperature. Arago nite will not be present in near-solidus Iherzolite, and can be present in orthopyroxene-free eclogites and pyroxenites only at low temperature.