Volatiles in silicate melts: From geophysical detection to primordial reservoirs

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Seismology is a major tool when investigating Earth’s deep interior. By analyzing the velocity and form of seismic waves, scientists can visualize and interpret structures and compositions at depth; a technique which is comparable to medical sonograms. The presence of partial melts in the Earth’s mantle is characterized by anomalously low seismic velocities. Low velocities have been observed under tectonic plates, at the lithosphere-asthenosphere boundary. They are also observed in subduction zones where one plate dives underneath an other. Melting is suspected to occur in several other regions of the mantle, notably above and below the transition zone which extends from 410 km to 670 km depth. However, in many of these regions the estimated temperature is below the melting temperature of rocks; unless rocks contain volatiles (water or CO2) which lower their melting temperatures. Here, the scientists are carrying out experiments at the extreme pressures and temperatures of the deep Earth. Their goal is to determine if the observed seismic anomalies can be attributed to the presence of volatile-rich silicate melts. They use state-of-the-art techniques to compress and heat up glass specimens while measuring their density and elastic properties. These properties are directly linked to seismic velocities. The high pressures are produced at the tip of two opposing diamonds or generated by huge electric and magnetic power pulses in the Thor and Z machines at Sandia National Laboratories (SNL, New Mexico). The results address fundamental questions in Earth Sciences. Can hydrous silicate melts be detected in the mantle? What are their physically properties and possible impact on mantle evolution? Could silicate melts generated during large planetary impacts retain primordial volatiles? This project supports an early-career female scientist. It provides support and training to graduate and undergraduate students at University of Colorado Boulder, and outreach to underrepresented groups in science. It also fosters an interdisciplinary collaboration between academia, SNL and industry. In silicate minerals water is mostly stored as hydroxyl (OH) and charge balanced by defects. At one atmosphere, this leads to lower elastic wave velocities and softer elastic moduli with increasing water content. However, on compression, volatile-rich silicate glasses are stiffer than their anhydrous counterparts. This unexpected effect has been attributed to the ability of volatile species to fill interstitial voids and/or bond as silanol (Si-O-H) groups. The dependences on pressure, temperature and composition of this effect is currently unknown. Yet, quantifying these dependences is critical to constrain equation-of-state models for amorphous silicates in the Earth‘s mantle. Here, the working hypothesis is that hydrous silicate glasses and melts are elastically stiffer at mantle pressures than their anhydrous counterparts. To test this hypothesis, the team uses a combination of static and dynamic compression methods. GHz-ultrasonic interferometry measurement are carried out in situ (at high pressure and temperature) in the laser-heated diamond-anvil cell. SNL Thor and Z machines are used for ramp and shock-ramp compression experiments up to pressures of one million atm (100+ GPa). Ramp-compression is a new method of dynamic compression that follows isentropic paths rather than the Hugoniot, which is usually probed in shock experiments. During dynamic compression in SNL machines, powerful pressure waves generated by the machines allow probing in situ (at high pressure and temperature) the elastic properties of the compressed amorphous silicates by laser interferometry (e.g. VISAR). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.