Physicochemical conditions for melting in the Earth's mantle containing a C-O-H fluid (from experimental data)
Physicochemical conditions for melting in the Earth's mantle containing a C-O-H fluid (from experimental data)
Russian Geology and Geophysics (May 2011) 52 (5): 475-492
- aliphatic hydrocarbons
- alkanes
- apparatus
- buffers
- carbon
- carbon dioxide
- carbonates
- diamond
- eclogite
- experimental studies
- fluid phase
- fugacity
- graphite
- hydrocarbons
- hydrogen
- igneous rocks
- laboratory studies
- mantle
- mantle wedges
- melts
- metamorphic rocks
- methane
- mineral composition
- native elements
- organic compounds
- oxygen
- partial melting
- peridotites
- phase equilibria
- plutonic rocks
- pressure
- quenching
- temperature
- transition zones
- ultramafics
- volatiles
- water
- 410-km discontinuity
- 670-km discontinuity
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) 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) O content of the system exceeds the H (sub 2) O storage capacity in the rock under given physicochemical conditions. Melting in CO (sub 2) -containing systems is determined by carbonate stability and the chemical composition of the system, mainly its Na (sub 2) O and K (sub 2) O contents, and, to a smaller extent, the content of CO (sub 2) itself. Studies of peridotite and eclogite systems containing H (sub 2) O, CO (sub 2) , H (sub 2) O+CO (sub 2) , 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 decarbonation-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 degrees C). This suggests that eclogites are of global importance in mantle melting, which agrees with modern geochemical models.