Recent experimental data on phase transformations and melting in peridotite and eclogite systems with a C-O-H fluid at pressures up to about 30 GPa are reviewed with special attention to the effect of redox conditions. The fundamental differences for partial melting in systems with H2O, CO2 and reduced C-O-H fluid (CH4-H2O-H2) are outlined. Melting in systems with H2O depends mainly on the total H2O content and is controlled by H2O solubility in nominally anhydrous minerals. Partial melting occurs when the total H2O content of the system exceeds the H2O storage capacity in the minerals of the rock under given P-T-X-fO2 conditions. Melting in systems with CO2 is determined by carbonate stability and is strongly affected by the alkali (particularly, K2O) content in the system. The onset of melting is relatively insensitive to the total CO2 content. Studies of peridotite and eclogite systems saturated with H2O and CO2 show that H2O-bearing phases, such as dense hydrous silicates, superhydrous phase B and phase D, can control initial melting and low degree partial melts are silica-rich. Moreover, most solidi are flattening out at pressures above 6-8 GPa. The solidi of peridotite and eclogite with coexisting reduced C-O-H fluid (presumably CH4 + H2O, at the oxygen fugacity near the Fe-FeO buffer) are located 300-500 ?C above the solidi of the systems with H2O and CO2 at 15 GPa. At the same time they are still 300-400?C lower than volatile-free solidi. Thus, we provide a first calibration of the dependence of mantle melting at constant pressure on redox conditions. The stability boundary of Fe-Ni alloy, which may coincide with the lithosphere-asthenosphere boundary under cratons (200-250 km), the 410km discontinuity and the transition zone itself, may be paramount to redox and decarbonation-dehydration melting and freezing. Subducted carbonates rather than water may control melting in the ''big mantle wedge'' model for stagnant slabs. We proposed a phenomenological model for the segregation of a slab-derived carbonate melt in the transition zone, which can move as a diapir through the transition zone and upper mantle and initiate magmatism at the surface.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)