PHYSICOCHEMICAL CONDITIONS FOR MELTING IN THE EARTH'S MANTLE CONTAINING A C-O-H FLUID (<i>from experimental data</i>)
Experimental data on phase transformations and melting in peridotite and eclogite systems with a C-O-H fluid at 6-30 GPa have been analyzed with special attention to the influence of redox conditions. It has been found that melting in systems with H<sub>2</sub>O depends heavily on its total content and considerably on its solubility in nominally anhydrous rock-forming minerals. Partial melting occurs when the total H<sub>2</sub>O content of the system exceeds the H<sub>2</sub>O storage capacity in the rock under given physicochemical conditions. Melting in CO<sub>2</sub>-containing systems is determined by carbonate stability and the chemical composition of the system, mainly its Na<sub>2</sub>O and K<sub>2</sub>O contents, and, to a smaller extent, the content of CO<sub>2</sub> itself. Studies of peridotite and eclogite systems containing H<sub>2</sub>O, CO<sub>2</sub>, H<sub>2</sub>O<sup> </sup>+<sup> </sup>CO<sub>2</sub>, and a reduced C-O-H fluid show that most solidi flatten out at pressures above 6-8 GPa when intersecting the geotherms of subduction and average mantle. Mantle melting at constant pressure in the presence of a C-O-H fluid depends not only on temperature but also on redox conditions. The oxidation of the system causes redox melting. The stability boundary of a Fe-Ni alloy (it may coincide with the lithosphere-asthenosphere boundary under cratons, 200-250 km) and the 410 km discontinuity are paramount to redox and decarbonatization-dehydration melting. Also, the paper provides evidence that subducted carbonates play the leading role in the big mantle wedge model for stagnant slabs. Volatile-containing eclogite systems melt at lower temperatures than peridotite ones (the difference is up to 100-200 °C). This suggests that eclogites are of global importance in mantle melting, which agrees with modern geochemical models.