Carbonation of peridotites and decarbonation of siliceous dolomites represented in the system CaO-MgO-SiO_2-CO_2 to 30 kbar

The decarbonation of siliceous dolomite produces rocks including the minerals forsterite, orthopyroxene and clinopyroxene, characteristic of peridotites. Selected reactions in the system CaO-MgO-SiO_2 –CO_2 are reviewed and published data at crustal and mantle pressures are compared with previously unpublished experimental results between 15 and 30 kbar for five reactions: (0) magnesite + clinopyroxene = dolomite + orthopyroxene; (1) magnesite + quartz = enstatite + CO_2; (5) dolomite + orthopyroxene + quartz = clinopyroxene + CO_2; (3) magnesite + enstatite = forsterite + CO_2; (6) dolomite + orthopyroxene = clinopyroxene + forsterite+ CO_2. Reactions (0), (1) and (5) meet at an invariant point near 1090°C and 34 kbar. Reactions (3) and (6) represent the carbonation of model harzburgite and Iherzolite, respectively. Dolomites in reaction (6) contain more than 70 wt.% CaCO_3, at temperatures below the crest of the calcite-dolomite solvus, they are magnesian calcites. Phase relationships for carbonated model peridotites in the presence of H_2O, compared with estimated depths and temperatures of equilibration of xenoliths from the Premier Mine kimberlite, indicate that within limited depth intervals solid magnesite-harzburgite can coexist with partially melted lherzolite. Eruption of kimberlite could transport xenoliths of Iherzolite and magnesite-harzburgite. Experiments indicate that the magnesite dissociates within minutes during uprise. This observation is consistent with the proposal of Boyd and Gurney that low-calcium garnets in kimberlites of the Kaapvaal-Rhodesian craton are produced by disruption of magnesite disseminated through depleted harzburgites in the roots of the craton, within the diamond stability field