Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth's crust and upper mantle

Clark, A. N., Lesher, C. E., Jacobsen, S. D., Wang, Y. "Anomalous density and elastic properties of basalt at high pressure: Reevaluating of the effect of melt fraction on seismic velocity in the Earth's crust and upper mantle" (2016) J. Geophys. Res., 121, 6, 4232-4248

1Department of Earth and Planetary Sciences, University of California, Davis, California, USA,

2Now at CNRS UMR 7590, Institut de minéralogie et de physique des milieux condensés, Université Pierre-et-Marie-Curie, Paris, France,

3Department of Geoscience, Aarhus University, Aarhus, Denmark,

4Department of Earth and Planetary Sciences, Northwestern University, Evanston, Illinois, USA,

5Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA

Independent measurements of the volumetric and elastic properties of Columbia River basalt glass were made up to 5.5 GPa by high-pressure X-ray microtomography and GHz-ultrasonic interferometry, respectively. The Columbia River basalt displays P and S wave velocity minima at 4.5 and 5 GPa, respectively, violating Birch’s law. These data constrain the pressure dependence of the density and elastic moduli at high pressure, which cannot be modeled through usual equations of state nor determined by stepwise integrating the bulk sound velocity as is common practice. We propose a systematic variation in compression behavior of silicate glasses that is dependent on the degree of polymerization and arises from the flexibility of the aluminosilicate network. This behavior likely persists
into the liquid state for basaltic melts resulting in weak pressure dependence for P wave velocities perhaps to depths of the transition zone. Modeling the effect of partial melt on P wave velocity reductions suggests that melt fraction determined by seismic velocity variations may be significantly overestimated in the crust and upper mantle.

http://onlinelibrary.wiley.com/doi/10.1002/2016JB012973/abstract