Carbonation and decarbonation of eclogites: the role of garnet

Carbonates are potentially significant hosts for primordial and subducted c arbon in the Earth's mantle. In addition, the coexistence of carbonate with silicates and reduced carbon (diamond or graphite), allows constraints to be placed on the oxidation state of the mantle. Carbonate-silicate-vapor re actions control how carbonate + silicate assemblages may form from carbon-b earing vapor + silicate assemblages with increasing pressure. In olivine-be aring rocks such as peridotite, considered the dominant rock type in the up per mantle, the lowest-pressure carbonate-forming reactions involve olivine ( + clinopyroxene) reacting with CO2 (e.g., Wyllie et al. 1983). In eclogi tic rocks, the essential mineral assemblage is omphacitic clinopyroxene + g arnet, without olivine. Therefore, alternative carbonate-forming reactions must be sought. The carbonation of clinopyroxene via the reaction dolomite + 2 coesite = diopside + 2 CO2 was studied experimentally by Luth (1995). T he alternative possibility that garnet reacts with CO, is explored here by determining the location of the reaction 3 magnesite + kyanite + 2 coesite = pyrope; 3 CO2 between 5 and 11 GPa in multi-anvil apparatus. At the tempe ratures greater than or equal to 1200 degrees C carbonation of eclogitic ro cks with increasing pressure will proceed initially by reaction with clinop yroxene, because the pyrope-carbonation reaction lies at higher pressures f or a given temperature than does the diopside-carbonation reaction. Dilutin g the pyrope component of garnet and the diopside component of clinopyroxen e to levels appropriate for mantle eclogites does not change this conclusio n. At lower temperatures, appropriate for "cold" slabs, it is possible that the converse situation will hold, with initial carbonation proceeding via reaction with garnet, but this possibility awaits experimental confirmation . Decarbonation of an eclogite under "normal mantle" geothermal conditions by a decrease in pressure, as in an ascending limb of a mantle convection c ell, would be governed by the formation of clinopyroxene + CO2, At higher p ressure than this reaction, any CO2 produced by the breakdown of magnesite reacting with kyanite and coesite would react with clinopyroxene to produce dolomite + coesite. Release of CO? from eclogite into mantle peridotite wo uld form carbonate at sub-solidus conditions and produce a dolomitic carbon ate melt if temperatures are above the peridotite-CO2 solidus.