We review experimental phase equilibria associated with partial melting of mafic lithologies (pyroxenites) at high pressures to reveal systematic relationships between bulk compositions of pyroxenite and their melting relations. An important aspect of pyroxenite phase equilibria is the existence of the garnet-pyroxene thermal divide, defined by the enstatite - Ca- Tschermaks pyroxene diopside plane in CaO - MgO Al2O3 - SiO2 projections. This divide appears at pressures above ∼2 GPa in the natural system where garnet and pyroxenes are the principal residual phases in pyroxenites. Bulk compositions that reside on either side of the divide have distinct phase assemblages from subsolidus to liquidus and produce distinct types of partial melt ranging from strongly nepheline-normative to quartz-normative compositions. Solidus and liquidus locations are little affected by the location of natural pyroxenite compositions relative to the thermal divide and are instead controlled chiefly by bulk alkali contents and Mg-numbers. Changes in phase volumes of residual minerals also influence partial melt compositions. If olivine is absent during partial melting, expansion of the phase volume of garnet relative to clinopyroxene with increasing pressure produces liquids with high Ca/Al and low MgO compared with garnet peridotite-derived partial melts.
Bibliographical noteFunding Information:
We are grateful to Yaoling Niu for inviting us to contribute to this volume in honor of Mike O’Hara. In this paper and countless other experimental studies, the ample use of projections pioneered by O’Hara is but one expression of his remarkable legacy. We also acknowledge constructive reviews by Claude Herzberg, Michael Walter, and an anonymous reviewer. Significant support for this project came from a MEXT grant 15740318 to T.K. and NSF grants OCE 9706526 and OCE9876255 to M.M.H.
- Experimental petrology
- Mantle heterogeneity
- Partial melting
- Phase equilibrium