Seismic properties of lower crustal xenoliths from El Hoyazo (SE Spain): Experimental evidence up to partial melting

Seismic techniques provide unique tools to investigate the structure and, in combination with petrological, geochemical and petrophysical study, the composition of the lower crust. Controversies can be solved with comparative study of metamorphic terrains or xenoliths that occur adjacent to areas where seismic refraction/reflection data are available. Xenoliths represent a direct sampling of the inaccessible lower crust at the time of the volcanism, whilst exposed crustal sections can only be used as analogue of present day lower crust. The present study is focused on the measurements of compressional wave velocities up to conditions exceeding the beginning of melting (950 degrees C at 500 MPa confining pressure) on three garnet-biotite-sillimanite metapelitic xenoliths recovered from the Neogene dacites of El Hoyazo (SE Spain). They preserve widespread interstitial rhyolitic glass as evidence of primary melt extraction and represent the best example of partially molten lower crust in the Alboran Domain. The influence of glass on Vp is primarily reflected by anomalous positive dVp/dT while heating with velocity increasing at 500 MPa from 4.98 to 5.50 km s(-1) at room temperature to 5.85-6.79 km s(-1) at 650-700 degrees C. This corresponds to the glass transition where all the grain boundaries and most of the pores within the glass are closed. After this point, the velocity decreases to 6.2-6.5 km s(-1) at 950 degrees C where re-melting of the glass is achieved and additional partial melt produced. On cooling, the behavior is normal with negative dVp/dT. After the thermal treatment velocities are 30% higher (6.07-7.21 km s(-1)) and reveal that in the presence of intergranular melt velocity measurements at room temperature cannot be extrapolated to high temperatures. P-waves measured at melting conditions are in agreement with deep seismic refraction data and tomography in the area and corroborate the hypothesis that partial melts are actually present in Alboran lower crust. (c) 2006 Elsevier B.V. All rights reserved.