High-Pressure Compressibility of Silicate Liquid and Melt using Brillouin Scattering

Sergey N. Tkachev¹, Murli H. Manghnani¹, Quentin Williams², Li Chung Ming¹

¹Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822

²Department of Earth Sciences, University of California, Santa Cruz, CA

murli@hawaii.edu

Brillouin scattering measurements have been carried out on Na₂Si₂O₅ glasses and melts under in situ pressures to 8 GPa at temperatures from 300 K to 850 K in an externally resistively-heated DAC. For the first time, the 'modified' platelet scattering geometry has enabled us to determine V_P, independently from refractive index, and hence adiabatic compressibility and density of silicate melts as a function of pressure and temperature. In accordance with existing models on densification of silicate glasses and melts, the more compact arrangement of atoms has been assumed to account for the observed increase in density of the produced melts at high P-T conditions. A marked change in pressure derivative of adiabatic bulk modulus (dK/dP=K´) exists between liquid and glass forms of hydrous Na₂Si₂O₅ : the notably higher K´ of the liquid is consistent with the liquid being able to access a wider range of compressional mechanisms. The large K´ of the liquid thus illustrates that the means of compaction of the liquid may differ substantially from that of the glass.