Grain boundary melt films in an experimentally deformed olivine-orthopyroxene rock: Implications for melt distribution in upper mantle rocks.

An unexpected microstructure, with important implications for melt distribution in the upper mantle, has been found in a 70% olivine + 30% orthopyroxene rock experimentally deformed at 1500K, 300 MPa, where incipient partial melting occurred. As found in previous studies of comparable systems, most melt in the olivine-orthopyroxene sample resided in 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, along all grain boundaries, with total film fraction (F) of 0.0002. All of these melts are unusually SiO2-rich. Further work is needed to confirm that the thin films are not transient, although their coexistence with smoothly curved solid-melt interfaces and flat crystal faces suggests they are stable. If thin high-silica melt films are stable they might influence physical properties and melt extraction processes in regions of incipient melting and metasomatism in the upper mantle.