MATHEMATICAL MODELING OF THE ISOTOPIC EVOLUTION OF A MANTLE WITH CONVECTION

Abstract

Several published methods for calculating the variation in isotopic ratios in open systems involve a description of the partitioning among phases of simple ratios of the concentrations of components in coexisting phases. As it was shown elsewhere, the use of co-crystallization coefficients is preferable in many cases, because these coefficients are less sensitive to external conditions. In addition, the conversion to ratios of elements decreases the error of the results, since it is no longer necessary to calculate the phase quantities lost from the system. The authors analyze radioactive decay, associated with selective loss of the most incompatible microelements from the sublithospheric mantle as a result of the loss of partial melts or fluids to the lithosphere using the case of the SM-Nd isotope system as an illustration. Data on the isotopic composition of neodymium in products of igneous activity of the mantle indicate that the geochemical and isotopic differentiation of global magma chambers probably began 4 to 3.5 billion years ago.