The Pressure Is On: High-Pressure Mineral Physics

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It's hard to imagine what it would be like deep inside the earth's surface, where the intense heat and the crushing pressure of the incredible mass above is enough to turn familiar substances like water and rock into almost unrecognizable materials. Under such high pressures, water becomes more acidic than the terribly corrosive, concentrated hydrofluoric acid. Crystals nearly double in density. Some gases--xenon, for example--not only solidify under the weight of the earth's heavy mantle, they actually become metals. A strange and wonderfully interesting place to contemplate.

It's more than just a curiosity, though. Scientists need to understand the behavior of materials deep inside the earth in order to understand the geologic processes that shape our planet. The UW High Pressure Mineral Physics Laboratory is one of the leaders in successfully simulating deep earth conditions with that goal in mind.

"The earth sciences are in the midst of a revolution," says J. Michael Brown, UW geophysics professor and researcher in the Lab. "The era of identifying and classifying geologic phenomena is being replaced by efforts to understand the underlying processes." Now, traditional field work is being supplemented by laboratory experiments and theoretical work. For example, the drifting of continents, called plate tectonics, is linked to the transport of heat from the earth's deep interior to the surface. Currently, scientists don't know whether convective circulation of the solid mantle extends from the earth's core to the surface, or if these movements are restricted to the upper part of the mantle only. To answer that question will require knowing the thermodynamic properties of minerals under these truly extreme circumstances.

Research at the UW Mineral Physics Lab has provided the first experimental estimates of deep earth conditions: At the core-mantle boundary, the pressure is estimated to be 1.4 million times the atmospheric pressure at sea level, and the temperature may be as high as 4,000 Kelvin (over 7,000° F).

Recently, Brown and chemistry professor Leon Slutsky have applied a new ultra-short-pulsed laser technique to the problem of measuring chemical and physical properties of materials at high pressure and temperature. The technique, called laser-induced phonon spectroscopy, or LIPS, involves generating intense laser pulses (peak power equivalent to the total output of a commercial power plant, e.g. 1,000 megawatts) for time intervals on the order of 100 trillionths of a second. Using LIPS, Brown and Slutsky can make a variety of measurements on mineral samples, including thermal conductivity, viscosity, elasticity, and chemical reaction rates. "We are the first, and currently only, group to apply these techniques in the earth sciences," says Brown.

Research at the UW High Pressure Mineral Physics Lab has shed new light on the melting of iron under earth core conditions. "The principal constraint on the thermal structure of the deep earth is the high pressure melting characteristics of iron, the main constituent of the earth's core," Brown explains. Using a new shock wave technique developed at the Lab, Brown reported in 1986 the first direct measurement of the melting point of iron at a pressure of 2.4 million atmospheres. This pioneering work has been widely cited and used by other laboratories.

Work reported in 1991 by Brown and colleagues was the first to show that hydrogen can be dissolved in iron at high pressure and may be an important volatile component of the earth's mantle and core. "Even in small quantities," says Brown, "hydrogen causes large changes in some physical properties of iron. For example, it is a major cause of metal weakening in chemical reaction vessels."

The first measurements of sound velocities in minerals and simple fluids at ultra-high pressure were performed at the Lab in a diamond-anvil high pressure cell. First reported in 1989, such measurements are critically important in understanding the thermal and compositional state of the earth's interior. The diamond anvil cell holds a tiny sample of a mineral between two diamonds. Surrounding the sample inside the cell is argon, an inert gas. Tightening the screws on the frame of the cell brings the two diamonds closer together, increasing the pressure on the argon, and therefore indirectly increasing the pressure on the sample. Next to each sample is a tiny ruby chip that is used to calibrate the pressure measurement. When illuminated with blue or green light, the ruby gives off red fluorescence. As the pressure on the ruby increases, the wavelength of the fluroescence also increases. Pressure can be measured in this way to over 800 kilobar (even as high as 5,000 kilobar, exceeding the pressure at the center of the earth) within 0.2 kilobars.

Research at the Lab also is shedding new light on the mechanisms that lead to earthquakes. Scientists generally believe that deep earthquakes, occurring in subducting plates at depths of more than 400 km, are not caused by normal frictional processes. The recent discovery by Brown and colleagues of a room temperature transition of the mineral olivine to the mineral spinel at high pressure suggests a new mechanism that may explain how deep faults rupture.

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