OXYGEN ISOTOPE FRACTIONATION DURING PARTIAL MELTING IN MIGMATITES

ALLEN, Tim, Science Division–Geology, Keene State College, Keene NH 03431-4183, tallen@pisgah.keene.edu.

Detailed stable isotope, petrologic and geochemical studies of migmatites forming a "hot spot" in the Acadian metamorphic high of New England were carried out. Leucosome—melanosome relationships and the observed metamorphic reaction set suggests that the migmatization occurred as a result of partial melting. This partial melting model is supported by trace element partitioning between migmatite leucosome and melanosome components. Lowering of d18O values in the migmatites compared to adjacent parent schists, and the spatial association of migmatites with pegmatites suggest the melting was driven by magmatic water infiltration.

The d18O values of quartz mineral separates from migmatite leucosomes are up to 1 per mil higher than for quartz from migmatite melanosomes. A similar result has also been obtained from migmatites in Switzerland (Mazurek, 1992). This fractionation between the two components of migmatites cannot be obtained by the isotopic re- equilibration of the leucosome and melanosome mineral assemblages during cooling, nor by the influence of the infiltrating fluids either during partial melting or by selective post-migmatization alteration. These results suggest that there is a significant fractionation of oxygen isotopes between melts and residual solids during fluid-driven partial melting of pelitic rocks. This oxygen isotope fractionation can be modelled, coupled with the calculated partial melting reactions.

1993 Geological Society of America Abstracts with Program 25(6): A448.