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

Abstract

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.

Ultrasonic Velocity of Diopside Liquid at High Pressure and Temperature: Constraints on Velocity Reduction in the Upper Mantle Due to Partial Melts

“…Our data show no obvious abnormal pressure dependence (i.e., negative dependence or a change of slope in the velocity-pressure curve) of sound velocity for diopside liquid up to~4 GPa (Figure 4b), consistent with the prediction based on first principles molecular dynamics simulation results (Sun et al, 2011), but contradictory to previous studies on various silicate glasses where both V P and V S exhibit a change in slope at a pressure of a few gigapascals (Clark et al, 2016;Liu & Lin, 2014;Sakamaki et al, 2014;Sanchez-Valle & Bass, 2010;Zha et al, 1994). Figure 6a shows the density versus V P for diopside liquid, glass, and crystal as well as other depolymerized silicate glasses from experimental measurements.…”

“…First principles molecular dynamics simulations on MgSiO 3 liquid (Stixrude et al, ) also show that the acoustic velocities increase monotonically with pressures in the deep mantle with no visible anomaly, in contrast to the anomalous acoustic behavior reported in MgSiO 3 glass by Brillouin scattering (Liu & Lin, ; Sanchez‐Valle & Bass, ). Another important difference in acoustic properties between liquids and glasses is that shear waves can propagate through glasses (Clark et al, ; Liu & Lin, ; Sakamaki et al, ) but not through liquids (supporting information Figure S3).…”

“…Figure shows the calculated velocity reductions relative to PREM as a function of pressure and melt fraction using our measured sound velocities for diopside liquid (solid curve) and melt velocities modeled from results on cold‐compressed basalt glass (Clark et al, ; dashed curve). Our results show that the degree of velocity reduction increases significantly with the fraction of melt present in the mantle, while it decreases with pressure meaning that more melts are needed at higher pressures in order to reduce the velocity by a few percent.…”

“…Based on the observations that silicate glasses have a weak pressure or density dependence of acoustic velocities up to the upper mantle pressures, previous studies (e.g., Clark et al, 2016;Liu & Lin, 2014) suggest that the melt fraction needed for a given velocity reduction in the crust and mantle may be significantly , and solid (Li & Neuville, 2010). overestimated. But as demonstrated by our results, silicate liquids have much lower sound velocities than glasses do and do not exhibit any abnormal pressure dependence of velocity similar to that of glasses.…”

“…Until now, there are no reported high-pressure experimental data, to our best knowledge, on the sound velocity of any silicate liquid. Due to this limitation, many studies have used silicate glasses as analogs for liquids to infer the acoustic behavior of silicate liquids at high pressures (e.g., Clark et al, 2016;Liu & Lin, 2014;Malfait et al, 2011;Meister et al, 1980;Murakami & Bass, 2011;Sakamaki et al, 2014;Sanchez-Valle & Bass, 2010;Suito et al, 1992). These studies have shown that many silicate glasses exhibit an elastic anomaly with weak or even negative pressure dependence of sound velocity within the pressure range of a few gigapascals.…”