Possible Role of Thermal Concentration Convection during the Fractionation of Components in Basic Intrusions

摘要

Concentration convection is among the factors responsible for the separation of components during the fractionation of basic melts in magma chambers. However, this factor has been disregarded in the known formulations of dynamic problems of this kind. It is likely that this situation is related to the fact that, in such formulations of the problem, attention has been focused on the volume effects of gravity phase separation. The local separation of components at the crystallization boundary is usually considered in approximation, which is used to estimate the effective coefficient of distribution k_(eff) between solid and liquid phases. The complexity of its implementation lies in the fact that it includes an unknown variable δ_C, i.e., the thickness of the diffuse boundary layer. We note that the experimental verification of theory in salt systems related to the dependence of k_(eff) on the rate of ingot consolidation, intensity of mixing, and angle of the container inclination was carried out without analyzing the factor mentioned above. We note that this factor must be taken into consideration in the analysis of the conditions of the separation of components in crystal cells in estimations of the ratio of the compositions of melted inclusions and volume liquid phase. Usually, δ_C is estimated from the solution of the problem of free convection at a stationary vertical wall. At present, we have a local automodel and a more complete solution of such a problem, in which the dependence of melt density on the local concentration of components and density jump and the phase interface are taken into account. For petrogenetic application, the use of the most complete approximation requires an additional analysis, which is given in this paper. Let us consider such a problem with a simple binary system Fe_2SiO_4 + Fe_3O_4 as an example.