CHEMICAL-EXCHANGE IN MIGMATITES DURING COOLING

The chemistry of the leucosome components in' limited-size migmatitic associations is generally believed to reflect the composition of the partial melt that could have been extracted to yield an anatectic magma. Several well-documented studies of migmatites indicate that the distributions of some trace elements in leucosome/melanosome pairs are inconsistent with equilibrium partial melting but are rather in agreement with chemical reequilibration during cooling. In migmatites, the liquid fraction does not leave the system. This may induce an alteration of the chemical composition of the whole migmatite system by extensive chemical exchange between the liquid and the solid residue during cooling. If this is the case, the leucosome and melanosome components are not representative of the chemical composition of the liquid and solid residue respectively, at the time of partial melting. A rough evaluation of the residence time of the melt fractions in cooling in-situ migmatitic bodies yields minimum estimates of 1 to 10 Ma. For such time spans the characteristic distances of diffusion-induced chemical transfer exceed the sizes of individual mineral grains, which is consistent with a re-equilibration process. The release of latent heat of crystallization in cooling migmatitic systems and the presence of an interstitial liquid phase are critical in determining the extent and scale of chemical exchange. The mechanism involves (1) reducing the cooling rate and (2) promoting the efficiency of chemical diffusion through the liquid phase. Thus, some doubt is cast on the ability of small-scale migmatitic systems to provide relevant information about chemical mass balance during partial crustal melting.