Generation and crystallization of an amphibolite shear melt: an investigation using radial friction welding apparatus

A cylinder of amphibolite comprising the assemblage amphibole+clinozoisite+albite has been frictionally melted using radial friction welding apparatus. This was achieved by rotating a steel ring at 750 rpm and a force of 98 kN for 10 s against a stationary steel casing which housed the rock sample. The ring penetrated the casing then proceeded to rotate and compress the sample to 95% of its original volume until it cracked at right angles to its length. This generated a whole rock silicate melt which injected the crack and on cooling produced glass, crystallites and vesicles. Melting occurred in two stages: an initial low pressure melting event with crystallization to augite+Fe-rich anorthite, followed by a high pressure melting event with crystallization to fassaitic clinopyroxene. It is estimated that pressures of 0.5 GPa rising to 1 GPa were realized at the ring-rock interface. Under these conditions fassaite superseded augite+anorthite crystallization due to the increased solubility of Ca-Tschermak's component in clinopyroxene. The high pressure event provides a crude analogue for the frictional melting of basic rock at depths of 15–30 km in a seismogenic fault: a situation realized along the slip zone between cold descending lithosphere and overlying mantle during subduction.