2015 Volume 124 Issue 3 Pages 355-370
Chemical differentiation from pyrolitic lherzolite to harzburgite due to partial melting and melt extraction process causes the chemical heterogeneity in the Earth's upper mantle. Phase relation and chemical variations of residues obtained with melting experiments of dry and hydrous pyrolitic lherzolite are compared to understand the effects of water on chemical differentiation. In dry conditions, orthopyroxene/olivine ratio decreases with increasing degree of melting. In hydrous conditions, the stability field of residual orthopyroxene, however, expands relative to olivine above solidus, and the harzburgitic residue contains a large amount of Mg-rich (Mg# > 0.93) orthopyroxene above 4 GPa in pressure. The chemistry of residues obtained from hydrous experiments agrees well with chemical variations of continental cratonic garnet harzburgite. This observation indicates that cratonic harzburgite with a high orthopyroxene content possibly reflects formation by melt depletion under various water contents from almost anhydrous to 2 wt% in the upper mantle at depths of about 100 to 200 km. The orthopyroxene-rich harzburgite, similar to continental cratonic harzburgite, may be formed at deep mantle wedges in the present Earth because water is being dragged into the deep mantle wedge by subducting slabs. Orthopyroxene-rich harzburgite may be detected in seismological observations because a jump in elastic wave velocities occur at 9-10 GPa (270-300 km in depth) in the harzburgite due to the orthorhombic to high-pressure monoclinic phase transition in (Mg, Fe) SiO3 pyroxene. A small jump in seismic velocities at about 250-300 km in depth, the X discontinuity, has occasionally been observed in seismic profiles at some subduction zones in southern Africa and the southern Pacific. The phase transition of (Mg, Fe) SiO3 pyroxene in orthopyroxene-rich harzburgite correspond to the X discontinuity.
