Deep Earth and Mineral Physics

Most large-scale geological processes, such as mantle convection and plate tectonics, involve plastic deformation of rocks. However, quantitative experimental studies of plastic properties under deepmantle conditions are challenging, and major progress in this area has often been associated with the development of new techniques. Until very recently, reliable studies have been conducted only at pressures less than ~0.5 GPa (~15 km depth in Earth). By combining novel techniques of synchrotron-based in situ stress–strain measurements with newly designed high-pressure apparatuses, a new generation of experimental studies of plastic deformation of minerals under deep-mantle conditions is emerging. These studies constrain the pressure dependence of deformation of minerals such as olivine and the slip systems in high-pressure minerals such as wadsleyite and perovskite. These results have important implications for the depth variation of mantle viscosity and the geodynamic interpretation of seismic anisotropy.

Aphase transition of MgSiO3 perovskite, the most abundant component of the lower mantle, to a higher-pressure form called post-perovskite was recently discovered for pressure and temperature conditions in the vicinity of the Earth’s core–mantle boundary. This discovery has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle called the D” region. Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. Here we discuss the current understanding of the core–mantle boundary region.

More than 90 percent of the Earth’s mass is composed of iron, oxygen, silicon and magnesium, distributed among a metal-rich core, a silicate-rich mantle and more highly fractionated crustal rocks (less than 1% of the total). Mantle and core compositions can be approximated quite easily provided the bulk-Earth composition is assumed to be the same as that of appropriate meteorites. Critical mineral-physics data, some of which are reviewed in this article, are then needed to develop viable compositional and thermal Earth models, thus leading to a better knowledge of the deepest rocks in the Earth.

The upper mantle is the source of almost all magmas. It contains major transitions in rheological and thermal behaviour that control the character of plate tectonics and the style of mantle dynamics. Essential parameters in any model to describe these phenomena are the mantle’s compositional and thermal structure. Most samples of the mantle come from the lithosphere. Although the composition of the underlying asthenospheric mantle can be estimated, this is made difficult by the fact that this part of the mantle partially melts and differentiates before samples ever reach the surface. The composition and conditions in the mantle at depths significantly below the lithosphere must be interpreted from geophysical observations combined with experimental data on mineral and rock properties. Fortunately, the transition zone, which extends from approximately 410 to 660 km, has a number of characteristic globally observed seismic properties that should ultimately place essential constraints on the compositional and thermal state of the mantle.

Seismological studies give us a high-definition 3-D picture of the Earth’s interior in terms of seismic velocity and density. Near the surface, observations of these properties can be compared with rock samples. As we go deeper into the Earth, interpretation of seismic data is more difficult. Laboratory measurements of velocities and other elastic properties of minerals are the key to understanding this seismic information, allowing us to translate it into quantities such as chemical composition, mineralogy, temperature, and preferred orientation of minerals. Here we present a description of modern techniques for measuring elastic properties at high pressures and temperatures, emphasizing those most relevant to understanding the interior of the Earth and other planets.

Very few rocks on the Earth’s surface come from below the crust. In fact, most of Earth’s interior is unsampled, at least in the sense that we do not have rock samples from it. So how do we know what is down there? Part of the answer comes from laboratory and computer experiments that try to recreate the enormous pressure–temperature conditions in the deep Earth and to measure the properties of minerals under these conditions. This is the realm of high-pressure mineral physics and chemistry. By comparing mineral properties at high pressures and temperatures with geophysical observations of seismic velocities and density at depth, we get insight into the mineralogy, composition, temperature, and deformation within Earth’s interior, from the top of the mantle to the center of the planet.