First Principles Theory of Mantle and Core Phases

Abstract

The Earth’s interior is an extreme environment where the pressure and temperature (up to 3.6 Mbar or 360 GPa, and approximately 6000 K, respectively at Earth’s center) are sufficiently high to affect the structure, physics, and chemistry of minerals in often surprising ways that may confound our intuition based on studies at near-ambient conditions. Though the Earth’s interior deeper than 12 km has not been subject to in situ observation, observational evidence from seismology, combined with experimental and theoretical studies of Earth materials under extreme conditions, has allowed us to construct a picture of this remote region of the planet (Fig. 1⇓). Although the mineralogy of the uppermost mantle is familiar from studies of xenoliths, the deepest portion of the silicate mantle (2890 km depth, or 136 GPa) is more than twice as dense as average continental crust (Dziewonski and Anderson 1981), and is thought to be composed primarily of a phase not yet seen at the Earth’s surface: an Mg-rich meta-silicate with the perovskite structure. The Earth’s core is thought to be composed primarily of iron and is subdivided into a liquid outer part and a solid inner part. The inner core is 65% denser than iron at ambient conditions, partly due to the effects of compression, and partly to the stabilization, at high pressure, of a close-packed, paramagnetic phase of iron. Figure 1. Properties of the Earth’s interior: the density (bold line) as determined seismologically and the pressure (light line) calculated from the density distribution (Dziewonski and Anderson 1981). The density profile reflects the divisions of the Earth’s interior into mantle and core and the further subdivision into upper mantle, transition zone, and lower mantle, liquid outer core, and solid inner core. The temperature distribution cannot be directly observed below the uppermost crust and must be inferred. The bold …