Volumetric and elastic properties of basalt at high pressure by X-ray microtomography and GHz-ultrasonic interferometry

Independent measurements of volume and sound velocity of basaltic glass (BCR-2) were made up to 5 GPa at room temperature by X-ray microtomography (HPXMT) and GHz- ultrasonic interferometry, respectively. These results are used to constrain bulk and shear moduli, and demonstrate that basaltic glass violates Birch's law as found for vitreous silica and other network glasses. Specifically for basalt, there is a 12% increase in density from 0 - 4.5 GPa, while P- and S-wave velocities decrease by 2.8 and 7.2%, respectively over the same pressure range. At higher pressures a positive correlation exists between density, and P- and S-wave velocities. There is no detectable frequency dependence for P- and S-wave velocities between 1-2 GHz, and velocities determined on compression and decompress are identical within uncertainties demonstrating fully elastic behavior up to at least 5 GPa. We find the pressure of P- and S-wave velocity minima for basaltic glass are ~2 GPa higher than for vitreous silica. We further find that overall velocity reductions approaching the minima are more modest than for vitreous silica and posit that this behavior reflects network flexibility approaching the effective locking limit during compression. Expansion of the pressure range of anomalous behavior coupled with the more modest overall velocity reduction relative to silica is attributed to the more depolymerized (disordered) structure of basaltic glass. We postulate that the velocity decrease observed in basaltic glass extends into the liquid state consistent with the recent work of Sakamai et al. (2013) and Sanloup et al. (2013). If this is true, the effect of melt on decreasing seismic velocities will be greater than currently assumed, and thus estimates of melt fraction in the upper mantle from velocity anomalies may be overestimated. T. Sakamaki et al., Nat. Geosci. (2013). C. Sanloup et al., Nature (2013).