Thin amorphous films (1-2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos, Arizona

Abstract

Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width (1–2 nm) were detected along olivine grain boundaries. The Al2O3, TiO2 and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole, and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source.