Crystallization differentiation of intrusive magmatic melt: Development of a convection-accumulation model

Abstract

The paper summarizes the principal results obtained over the past three decades at the Vernadsky Institute and the Department of Geochemistry of the Moscow State University by the computer simulation of basaltic magma differentiation in magma chambers. The processes of diffusion-controlled mass transfer in a chamber are demonstrated to be principally limited by the heat resources of the cooling magma and cannot play any significant role during the large-scale partitioning of melt components. The leading mass-transfer mechanism is the settling of crystals from convecting magma in the form of suspension flows that are enriched and depleted in the solid phase. The physical prerequisite for the onset of this concentration convection is the existence of boundary layers, which are characterized by volume crystallization and a gradient distribution of the suspended phases. Considered in detail are the principles used in the development of algorithms with regard for the occurrence of a boundary layer and the “instantaneous” stirring of the crystallizing magma that does not hamper the settling of mineral grains forming the cumulus. The plausibility of the convection-accumulation model is illustrated by the example of the reconstructed inner structure of differentiated Siberian traps. In application to these bodies, it is demonstrated that the solutions of the forward and inverse simulation problems with the use of geochemical thermometry techniques are identical. This is a convincing argument for the predominance of convection-accumulation processes during the formation of thin tabular magmatic bodies. The further development of the computer model for the differentiation dynamics should involve the processes of compositional convection related to the migration and reactivity of the intercumulus melt.