Résumé
Melt phase influence on seismic properties in partially molten systems, here at magmatic chambers of mid-oceanic-ridge and at hotspots, is poorly constrained. This study brings new seismological observations of seismic anisotropy beneath French Polynesia and Galápagos archipelago. Measurements of shear-wave splitting and P waves polarization beneath French Polynesia show the involvement of two contributions: asthenospheric and lithospheric. The presence of melt in the upper mantle beneath the center of the Galápagos archipelago may result in the absence of anisotropy. We measure Q-1 of multiple ScS waves and the results confirm the existence of a higher attenuation zone beneath French Polynesian region than in the remainder of the South Pacific. Moreover, torsion oscillatory deformation experiments have been realized at high temperatures (600-1170°C) and at seismic frequencies in partially molten gabbronorites. Above the melting temperature, attenuation increases significantly with a stronger dependence to frequency. It is shown that the melt-related attenuation mechanism could be melt squirt flow. The dissipation mechanism due to melt squirt flow between grain boundaries may be superimposed on an attenuation mechanism associated with diffusion controlled grain boundary sliding. In addition, new viscosity measurements of two basaltic silicates and one andesitic silicate were realized. The temperature dependence of the viscosity is determined through the configurational entropy theory of relaxation process.