Orthopyroxenes upper mantle composition

The Earth s mantle is peridotitic in composition and is significantly depleted in silica relative to primitive chondrites. Seismological evidence shows that the mantle is layered and can be divided into an upper and lower mantle, separated by a transition zone at 400-660 km depth. Above the transition zone the mantle is dominated by olivine and orthopyroxene with minor garnet and clinopyroxene. The lower mantle is made up of phases Mg- and Ca-perovskite and magnesiowustite. Seismic velocity contrasts between the upper and lower mantle are thought to reflect the ph ase transformations between the two and are not related to differences in bulk chemical composition. The lower mantle is separated from the outer core by the D" layer, a hot thermal boundary layer of enigmatic composition. 

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