Leading research of supercritical metamorphic fluids

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High-temperature and/or high-pressure regional metamorphic rocks have been regarded as the rocks most close to chemical equilibrium. During the research of this program, however, we have successfully discovered theoretically unexpected features not only in high-temperature (granulite-facies) regional metamorphic rocks from Sri Lanka and East Antarctica but also in high-pressure (elcogite-facies) regional metamorphic rocks from China. They are the formation of corundum (Al_2O_3) after pre-existing Al_2SiO_5 minerals (sillimanite in the high-temperature case and kyanite in the high-pressure case, respectively) in the vicinity of quartz (SiO_2) with intervening thin films of feldspar (less than c. 0.1 mm in thickness). The supercritical fluids are the most effective media of transportation of heat and material and, therefore, would prevent such reactions to form corundum after Al_2SiO_5 minerals in the presence of quartz, because the corundum+quartz assemblage is unstable compared with Al_2SiO_5s minerals in the crust and upper mantle. If, however, partial melting takes place in the rocks by the infiltration of H_2O-rich supercritical fluids along fructures, a chance to form corundum after Al_2SiO_5 minerals would happen, because both SiO_2 and Al_2O_3 do not diffuse rapidly in silicate melts formed around quartz. Thus rocks are separated into two portions, SiO_2-saturated and SiO_2-undersaturated.We performed experiments of partial melting of high-temperature coarse-grained pelitic rocks in the presence of H_2O-rich supercritical fluids to confirm the inferred formation of steep compositional gradients in the melts surrounding relict quartz grains. Thus, the role of H_2O-rich supercritical fluids during regional metamorphism has been revealed to be distinctly different under the subsolidus and "above-solidus" conditions.In addition, the duration of high-temperature conditions is estimated by the diffusion models of SiO_2 in melts.