Experimental high pressure investigation of partial melting in natural rocks and their influence on Vp and Vs

Plutonic and high-grade metamorphic rocks from the crystalline complexes of the Saxonian Erzgebirge and Granulitgebirge in Germany were used for experiments up to 0.5 GPa at laboratory temperature to measure the elastic wave velocities (v_p) and (v_s), under the minimized influence of pore space and cracks in an oil pressure chamber. The obtained high pressure data were also used to select samples for further experiments under the conditions of mineral dehydration and partial melting. We also applied a high performance gas pressure apparatus to investigate v_p and v_s on cylindrical samples up to 2 GPa and temperatures up to 1200°C. This paper shows the results of the elastic wave velocities v_p and v_s, measured simultaneously with encapsulated samples of granite, pyroxene granulite and pyroxenite under different partial melting conditions. Post-experimental microscopic analyses of the quenched samples including digital image processing were undertaken to determine the material and/or structural cause of a change in the physical properties, resulting in empirical relationships between elastic wave velocities and mineral content, dissipation and orientation of melt along grain boundaries. In a granite melt starts to form at 650°C with an amount of about 4 % at 900°C and reaching massive melting at temperatures above 1000°C. For basic to ultrabasic rocks at a pressure of 2 GPa a much smaller amount of melt has formed in the temperature interval above 900°C. At lower pressures the Poisson's ratio increases in the range of partial melting dramatically from 0.25 to about 0.4. For basic to ultrabasic rocks at 2 GPa pressure it keeps nearly unchanged up to 1000°C.