Geophysical Journal | 2011 volume 33 4

Melting under the upper mantle conditions

© Usenko O.V.

Modern petrological models describe the melting process as thermodynamically balanced. It has been shown in the work that initial melting is possible at the depth not less than 220 km. Phase diagrams plotted according to the results of experimental studies on melting of mantle lerzolite, demonstrate the effects unallowable for the process, which progress under balanced conditions. For example, in different experiments difference of temperatures at the beginning of the process, composition of the melt and degree of melting (under the same pressure) for the samples with close resemblance of composition has been fixed. Reduction of T interval between solidus and liquidus with pressure increase is observed in all experiments. Suggestion has been substantiated in the article that in case of ~ 3 GPa dramatic change of behavior (the character of interactions, directions of the progress of reactions) of the system crystal-melt, and the melting process itself is accompanied by increase of oxidation potential as a result of emanation of free oxygen. The higher is the pressure, the more intense is the process. Non-silicate chemically active components are produced. They interact selectively with lherzolite crystals. Composition of new increments of the melt depends on the composition of the first portions of liquid. In the proposed description the melting under mantle PT-conditions is not only a phase transition but also the beginning of a chain of physical-chemical processes accompanied by emanation of active components (including non-silicate fluids), their impact on crystal lattices of minerals and combining into new compounds. In the process of their interaction with crystal lattices of minerals under PT-conditions of the mantle electrochemical system is produced behavior of which is far from balanced. Description of the melting, that is proceeding under the pressure 7 GPa, with the help of phase reactions is ineligible because it must include formation (transformation) of deep fluid and its selctive interaction with lherzolite crystals.

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