Evidence for stable grain boundary melt films in experimentally deformed olivine-orthopyroxene rocks

The microstructure of olivine-olivine grain boundaries has been studied in experimentally deformed (1200-1227 degrees C, 300 MPa) partially molten oli vine and olivine-orthopyroxene rocks. In-situ melting produced similar to 1 vol% melt in all samples studied. Grain boundary analyses were carried out using a number of trammission electron microscopy techniques. The grain bo undary chemistry in undeformed olivine-orthopyroxene starting material show ed evidence for the presence of an intergranular phase along some, but not all, of the olivine-olivine boundaries. In the deformed samples, ultrathin Si-rich, Al- and Ca-bearing amorphous films have been observed along all in vestigated olivine-olivine grain boundaries. The chemistry of the grain bou ndaries, which is considered to be indicative for the presence of a thin fi lm, was measured with energy-dispersive X-ray spectroscopy (EDX) and energy -filtering imaging. The amorphous nature of the films was confirmed with di ffuse dark field imaging, Fresnel fringe imaging, and high-resolution elect ron microscopy. The films range in thickness from 0.6 to 3.0 nm, and EDX an alyses show that the presence of Al and Ca is restricted to this ultrathin film along the grain boundaries. Because thin melt films have been observed in all the samples, they are thought to be stable features of the melt mic rostructure in deformed partially molten rocks. The transition from the occ asional presence of films in the undeformed starting material to the genera l occurrence of the films in deformed materials suggests that deformation p romotes the formation and distribution of the films. Alternatively, hot-pre ssing may be too short for films to develop along all grain boundaries. A d ifference in creep strength between the studied samples could not be attrib uted to grain boundary melt films, as these have been found in all deformed samples. However, a weakening effect of grain boundary melt films on olivi ne rheology could not be ruled out due to the lack of confirmed melt-film f ree experiments.