Eclogite formation and the rheology, buoyancy, seismicity, and H<subscr>2</subscr>O content of oceanic crust

A broad spectrum of variably altered igneous rocks with a wide range of grain sizes are compressed and heated over a wide range of pressure-temperature paths in subduction zones. Although experimental kinetic data cannot be extrapolated to predict the rates of blueschist and eclogite formation in nature, textural data from rocks indicate that transformation below temperatures of 150øC is minimal. Complete transformation of volcanic rocks occurs by •-250øC, but incomplete transformation of gabbroic rocks heated to 800øC has been observed. There are important consequences to the rapid transformation of volcanic rocks and the metastable persistence of gabbroic rocks into the blueschist and eclogite stability fields. Fast seismic velocities should be evident first in the upper oceanic crust and may be substantially retarded in the lower oceanic crust. The upper oceanic crust will be denser than asthenosphere before the lower oceanic crust. Early in the process of eclogite formation, volcanic rocks will be placed in deviatoric tension and the underlying coarser grained rocks in compression; with further reaction, the state of stress in gabbroic rocks will change from compressive to tensile. Earthquakes at shallow depths should be extensional in basalt and contractional in gabbro, changing at deeper levels to extensional throughout he crust.