Mineralogy and composition of the upper mantle

Seismic velocities are calculated for two petrological models of the upper mantle, an olivine‐rich assemblage, pyrolite, and a garnet‐clinopyroxene rich, olivine bearing, assemblage, piclogite. These are compared with recent seismic profiles for various tectonic provinces. The shield data is most consistent with a cold olivine and orthopyroxene‐rich LID (the seismic lithosphere) extending to 150 km followed by a high temperature gradient and/or a change in mineralogy that serves to decrease the velocity. From 200 to 400 km the velocities follow a 1400°C adiabat. The rise‐tectonic mantle is much slower, presumably hotter and is likely to be above the solidus to depths of at least 300 km. The high V p /V s ratio of the lower oceanic lithosphere in the western Pacific is most consistent with eclogite.