Elastic wave velocity measurements of Na₂Si₂O₅ glass at high pressure

Abstract

Elastic wave velocities of sodium disilicate (Na₂Si₂O₅; NS2) glass were determined up to 7 GPa using an ultrasonic pulse-echo overlap technique combined with synchrotron X-ray imaging in a Kawai-type multi-anvil apparatus. NS2 glass represents an intermediate degree of polymerization (NBO/T = 1), bridging the compositional gap between fully polymerized and depolymerized silicate glasses. The compressional wave velocity (V_P) increases monotonically with pressure, whereas the shear wave velocity (V_S) remains nearly constant up to ∼ 4 GPa before rising at higher pressures. In comparison with fully polymerized (silica, jadeite, albite) and depolymerized (diopside, enstatite) glasses, the pressure dependence of NS2 glass resembles that of depolymerized compositions. These results indicate that the transition between polymerized and depolymerized elastic behavior occurs near NBO/T = 1. Consequently, mafic melts (NBO/T < 1) may exhibit anomalous elastic properties under upper-mantle conditions, whereas ultramafic melts (NBO/T > 1) are expected to stiffen monotonically with increasing pressure.