they could be reheated by heat flow from the core (and their own radioactive elements) and return to the near-surface environment as mantle plumes. There is geochemical support for this notion, which offers a direct mechanism for chemical exchange between the surface and the deep mantle (e.g., Hofmann, 1997; Bizimis et al., 2007). Some models suggest that subducted slabs do not sink that far before being thermally reassimilated by the mantle and that the basal layers of the mantle may be very old and relatively pristine. There is also geochemical evidence for this latter model in the form of high primordial 3He contents of large mantle plumes (Courtillot et al., 2003).

Both models are mute on whether there is chemical exchange between the mantle and core analogous to that between the mantle and the oceans. However, Os isotope data suggest that some mantle plumes contain components that may have come from the core (Brandon et al., 1999). This observation, if confirmed, would be consistent with a deep origin of mantle plumes, although uncertainty remains about whether this core signal could be transmitted through a basal mantle layer, which is both denser and more heterogeneous than the rest of the mantle (Figure 2.9; Garnero, 2000). Whether there is any chemical com

FIGURE 2.9 Inferred features at the core-mantle boundary (CMB). The notation D″ is the seismological designation of the heterogeneous zone at the base of the mantle. ULVZ is ultra-low-velocity zone. SOURCE: Garnero (2000). Reprinted with permission from Annual Review of Earth and Planetary Sciences. Copyright 2000 by Annual Reviews.

munication between the core and the lower mantle and what processes could allow this communication to be significant are topics of intense debate (e.g., Scherstén et al., 2004).

Summary

Earth’s internal evolution governs much of the planet’s evolution as a whole, but because the interior is mostly inaccessible to direct sampling, its study requires a combination of approaches. The seismic waves of earthquakes can be used to determine the elastic properties of Earth’s interior, and three-dimensional images of the mantle and core from these waves are being produced at systematically higher resolutions. The structures revealed by seismology are interpreted using new knowledge about Earth materials at high pressure, and great advances have been made in experimental and theoretical mineral physics. We now have sophisticated models for convection in the mantle and core and more precise geochemical and isotopic measurements of mantle rocks. But there are still first-order inconsistencies in the interpretations of available observations, especially for the style of convection and the number and origin of mantle plumes. Recent discoveries of structure and evidence for an unanticipated phase at the base of the mantle have added a new dimension to mantle studies, as knowledge from seismology, fluid dynamics, geochemistry, and cosmochemistry comes together.

Earth’s deep interior and surface are connected by volcanism and subduction. Volcanism modifies the internal chemical structure of planets, and great strides have been made in understanding the formation of magma and its transport from the mantle to the surface. But there is still no consensus on many aspects of Earth’s magmatic and geochemical history and their relation to surface conditions. For example, we do not know how much of Earth’s past volcanism was produced by mantle plumes and how much by plate tectonics, or why there were short periods of intense volcanic activity that could have changed the ocean basins, continents, and even global climate. In addition, we still have only hints about how subduction zones work and how the very existence of plates feeds back on the energetics of the mantle convection system. Finally, we are only now exploring the most fundamental connections between Earth’s core, magnetic field, mantle, and surface.



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