Evolution of Magmatic and Diamond-Forming Systems of the Earth's Lower Mantle by Anna V. Spivak & Yuriy A. Litvin

Author:Anna V. Spivak & Yuriy A. Litvin
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

3.4 Experimental and Natural Evidence for Ultrabasic-Basic Evolution of Lower Mantle Magmatism: Peritectic Control and Fractional Mechanisms

The evolution of the ultrabasic-basic magma cannot occur in equilibrium regime. The peculiarity of the equilibrium melting and subsequent crystallization lies in the fact that any general composition of the equilibrium system is fixed from the beginning of the melting-crystallization processes to their end in the subsolidus conditions. In the case being under consideration, formation of stishovite as the subsolidus phase cannot be realized at the equilibrium melting-crystallization processes for compositions associated with the ferropericlase FPer + ferrobridgmanite FBrd + Ca-perovskite CaPrv subsolidus field (Fig. 3.8).

Thus, it is valid to say that the peritectic reaction of bridgmanite does no more than opens the door to ultrabasic-basic evolution of lower mantle magmatism. What actually happens under the equilibrium conditions is that the peritectic reaction of bridgmanite in the ultrabasic-basic system considered here is the physico-chemical mechanism that creates in principle a possibility for the magmatic ultrabasic-basic evolution of the primitive ultrabasic material of the lower mantle at decreasing temperature. The triangulation procedure subdivides the subsolidus complex diagram of the multicomponent lower mantle system MgO–FeO–SiO2–CaSiO3 (Fig. 3.6) into the set of tetrahedral symplexes. The procedure is attended with making the common conjugate surfaces between the simplexes. The equilibrium physico-chemical processes may be exclusively realizable at compositions limited by a separate simplex in spite of this symplex is bonded by temperature-lowering univariant-invariant-univariant liquidus elements with its neighbouring simplex. It can be said that the conjugate faces of the neighbouring simplexes are of sort of the physico-chemical barrier for the equilibrium composition evolution of the melted materials. The conjugate faces are of no concern for the eutectic liquiduses which in themselves are the low-temperature physico-chemical barriers for magmatic evolution.

Thus, the ultrabasic-basic magmatic evolution of the native substance of the lower mantle is impossible under conditions of equilibrium melting and crystallization of the corresponding petrological systems, when the starting and final compositions of the crystallizing system must coincide.

The effective ultrabasic-basic evolution of the lower mantle magmas has been made possible with the peritectic control only under the regime of their fractional crystallization. This is related to the fact that the ultrabasic-basic evolution of a magma generated in the ultrabasic field FPer + Brd is impossible under equilibrium conditions, because the bulk composition of the system remains constant and the same in all episodes of the equilibrium melting and crystallization of a particular starting composition. The process of fractional crystallization, when the formed solid phases are removed from the physico-chemical contact with crystallizing melt, continuously changes the composition of the crystallizing melt which always coincides in such a case with the changing bulk composition of the system. This ensures a continuous fractional removal of minerals recrystallized at lower temperatures from magmas melts, which is accompanied by a balanced change of the compositions of the residual melts. This kind of continuous updating of the residual melt compositions also modifies the general composition of the fractionating petrological system from beginning to end and determines its intermediate compositions (in the idealized approximation).

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