DECOUPLING OF CHEMICAL AND ISOTOPIC EXCHANGE DURING MAGMA MIXING

When silicate magmas of different chemical and isotopie compositions are brought into contact, their equilibration is governed by the kinetics of diffusion. Here I present experimental data showing that, between a basic and an acidic silicate melt, isotopic exchange proceeds much more quickly than chemical homogenization. Moreover, different isotopic systems (in this case, strontium and neodymium) exhibit different rates of equilibration. The transient compositional states reached in the exchange process depart markedly from the results expected for binary mixing of the two end-members1. This complexity must be acknowledged in efforts to unravel episodes of magma mixing, such as involving partial melting of (acidic) continental crust by basic magma. In such cases, the intimate mixing of chemically dissimilar magmas is hindered by their different viscosities, but they do selectively exchange mass by diffusion. ö © 1990 Nature Publishing Group.