FIGURE 2.19 Photograph looking into a diamond cell at a 100-micron blue single crystal of hydrous ringwoodite (ideally Mg2SiO4 composition) held in situ at 30 GPa, corresponding to a depth of 800 km in Earth. The brown spots indicate where the sample has been heated with a laser to a few thousand degrees, causing a phase transformation to the assemblage MgSiO3 perovskite + MgO periclase that is thought to comprise most of Earth’s mantle below a depth of 660 km. SOURCE: Courtesy of Steven Jacobsen, Northwestern University. Used with permission.

we need to understand the processes that control the melting and freezing of rocks and minerals in the planetary interior. Melting of rocks involves complex chemistry, because rocks are typically composed of four or more mineral phases, none of which are pure. As rock melts, the composition and density of the liquid portion are different from those of the solid, and thus, with the help of gravity, one can segregate from the other. For example, the lava that erupts from volcanoes is both less dense and compositionally different from the parent mantle rock. Over Earth’s long history the repeated processes of melting, melt ascent due to buoyancy, and eruption onto the surface have completely rearranged many of its chemical elements. This process of planetary differentiation, making chemically distinct domains out of a homogeneous starting material, is one of the most fundamental features of planetary evolution (Questions 2, 4, and 5).

One of the more intriguing questions about melting is whether, under some conditions, magma may be denser than the surrounding solid mantle. Magma is highly compressible, so its density must increase rapidly with increasing pressure. The density of solids also increases with pressure but more slowly. Although there is so far only scant experimental and theoretical evidence, it suggests that magma can be denser than mantle rock deep inside Earth (Figure 2.20; Miller et al., 1991). The consequences of this for Earth’s evolution would be profound. If silicate melt sinks instead of rising toward the surface, it could be stored at depth for long periods, where it would be kept hot. The geochemical consequences of this inverted gravitational separation could also be important, but little is known about the distribution of trace elements between solids and liquids at high pressures. Iron-rich liquid would likely exist as a separate, denser phase than Earth’s silicate fraction and sink to the center, forming the core (Question 2). The timescale of this descent and the partitioning of elements between the iron-rich and silicate portions during core formation are still uncertain and have profound implications for the chemical composition of the core and the origin of the geomagnetic field (Question 4).

There is confirming evidence that liquid may be present in the deep mantle, especially near the coremantle boundary. Seismologists have identified thin layers of extremely low shear wave velocity at the base of the mantle, a characteristic of liquid. It has been suggested that this region could be made of dense, partially solidified magma and that it could even be a remnant of the Hadean planetary magma ocean (Williams and Garnero, 1996; see Question 2). If U, Th, and K are concentrated in this deep liquid, it could mean that the base of the mantle produces extra heat from radioactivity, which would affect how we think about the core dynamo and about the overall chemical composition of the mantle. If mantle liquid is in contact with the liquid outer core, it would also mean that chemical exchange across the boundary would be much more effective than if the mantle is solid; this would change the way we think about the origin of chemical heterogeneity in the mantle (Question 4). To resolve these issues we need to know much more about the properties of silicate liquids and solids at very high pressures and temperatures. Recent experimental advances, including measurements of liquid structure in situ at high pressure (Shen et al., 2004), will work hand in hand with theoretical and computer modeling. Modeling of high-pressure properties (Figure 2.20), using the principles of quantum mechanics, shows promise, although at present only a



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement