ULTRAHIGH-PRESSURE METAMORPHISM—PAST RESULTS AND FUTURE PROSPECTS

Fifty years ago P-T conditions and geologic environments required for the formation of blueschists, eclogites, and garnet lherzolites were unknown. Diamond was regarded as a high-pressure mineral, but its physical conditions of crystallization were still uncertain. With advent of high-pressure synthesis equipment and more precise calorimetry, minerals such as jadeite, aragonite, pyrope, and the dense polymorphs of silica and carbon were shown to be stable at elevated pressures and moderate temperatures. Low geothermal gradients implied by these P-T stability fields are understandable on a dynamic Earth characterized by mantle circulation and lithosphere subduction. Integration of experimental studies with plate tectonics elucidates the generation of both oceanic and continental crusts; combined with geochemical, geophysical, and isotopic data, phase equilibria provide important constraints regarding the constitution and differentiation of the upper mantle.

Circumpacific-type blueschists and eclogites worldwide form in penetratively deformed allochthonous slabs and nappes verging seaward, requiring underflow of oceanic basement (30-50 km) during metamorphism. Neoblastic coesite and microdiamond inclusions found in tough, rigid container minerals demonstrate that Alpine-type continental collision involves partial recovery of rocks far more deeply subducted (100-130 km) than previously imagined. Even more surprising, garnet lherzolites from the central Alps, east-central China, and western Norway display mineral intergrowths and exsolution lamellae reflecting the initial presence of majoritic garnet, requiring even greater depths of origin of host peridotites (>300 km). Nano-minerals hold yet another key to deciphering the actual depths of subduction and mantle return flow. The duration of storage at great depth and exhumation rates are current problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through both hydrous and nominally anhydrous phases. Mantle petrochemistry and dynamics (plumes + plates) control the evolving architecture of the crust and the biosphere dependent on it, hence synergies between advanced technologies and condensed ultrahigh-pressure materials will lead to a fuller understanding of the Earth in time and space.