GRAIN-BOUNDARY MELT FILMS IN AN EXPERIMENTALLY DEFORMED OLIVINE-ORTHO-PYROXENE ROCK - IMPLICATIONS FOR MELT DISTRIBUTION IN UPPER-MANTLE ROCKS

An unexpected microstructure, with important implications for melt dis tribution in the upper mantle, has been found in a 70% olivine + 30% o rthopyroxene rock experimentally deformed at 1500K, 300 MPa, where inc ipient partial melting occurred. As found in previous studies of compa rable systems, most melt in the olivine-orthopyroxene sample resided i n a network of grain-edge tubes and occasional thick (50-500 nm)layers . We infer, using electron microscopy at its highest resolution, that melt also existed in another form as glass films 1.0-1.5 nn thick, alo ng all grain boundaries, with total film fraction (F) of 0.0002. All o f these melts are unusually SiO2-rich. Further work is needed to confi rm that the thin films are not transient, although their coexistence w ith smoothly curved solid-melt interfaces and flat crystal faces sugge sts they are stable. If thin high-silica melt films are stable they mi ght influence physical properties and melt extraction processes in reg ions of incipient melting and metasomatism in the upper mantle.