ASCENDING SUBDUCTED OCEANIC-CRUST ENTRAINED WITHIN MANTLE PLUMES

Upward migration of subducted oceanic crust from deep in the upper man tle is discussed based on high-pressure experimental data. Numerical c alculation reveals that a subducted oceanic crust, which is negatively buoyant in the present upper mantle, can ascend from the transition z one and be observed as a discrete magma source at the surface only whe n it has been broken into fragments comparable in dimension with its t hickness and involved in a solid diapir of heated peridotite. For exam ple, a solid diapir of heated peridotite with a temperature excess of 200 degrees C can contain up to 15 similar to 20 vol% of fragments of oceanic crust when it passes through the upper mantle, and it may caus e extensive basaltic volcanism at the surface.