Pressures and temperatures of ultrahigh-pressure metamorphism:: Implications for UHP tectonics and H2O in subducting slabs

Kyanite-phengite eclogites can yield robust P-T equilibration conditions that do not depend on the ferrous/ferric ratios of the constituent minerals. Even rocks lacking kyanite or phengite-for which the ferrous/ferric ratios are calculated stoichiometrically-can yield equilibration conditions that differ by < 0.4 GPa. The range of robust P-T conditions for kyanite-phengile UHP eclogites is more restricted than that reported for eclogites generally, with most in the range 600-800 degrees C and < 4 GPa: none fall in the forbidden zone defined by geothermal gradients of <= 5 K/km. Most UHP eclogites equilibrated at temperatures below the H2O-saturated solidus for crustal rocks. This is probably because hotter UHP terranes are sufficiently low in viscosity that they mix with their surroundings and lose their identity or re-equilibrate continuously during cooling until the solidus is reached. Lawsonite and chloritoid are not known from UHP eclogites.