ScienceWeek

EARTH SCIENCE: ON THE CORE-MANTLE BOUNDARY

The following points are made by Thomas S. Duffy (Nature 2004 430:409):

1) The lowermost 250 km or so of Earth's mantle, known for historical reasons as D", is comparatively small in volume but potentially holds the key to understanding a host of geophysical phenomena -- among them the formation of plumes in the mantle, interactions between core and mantle, and the ultimate fate of subducting slabs of crust that are driven into the interior by tectonic forces. Investigations of this region largely depend on interpreting the behavior of seismic waves, which have shown that it is highly complex. Until recently, however, studies of the region's mineral properties at high pressures and temperatures had been unable to provide satisfying explanations for much of this complexity.

2) Part of the problem is that the extreme conditions in D" --pressures up to 135 gigapascals and temperatures probably ranging between 2000 K and 4000 K -- are difficult to reach in the laboratory. However, laboratory experiments and theory are finally coming together to bring this region into sharper focus. Iitaka et al(1) and Oganov and Ono(2) provide insights that link the calculated physical properties of a newly discovered(3) high-pressure crystal structure with seismic observations of the deep lower mantle.

3) Earth's mantle is composed mostly of dense silicate minerals containing magnesium, iron, calcium and aluminum. Experiments have shown that the lower mantle, extending from 660 km depth to the base of the mantle at about 2,900 km, is mainly composed of (Mg,Fe)SiO3 in a crystal structure known as "perovskite". Although the properties of this material are compatible with most observations for the lower mantle, the abrupt change in properties near the mantle's base has defied explanation in terms of perovskite behavior.

4) Thus, the experimental discovery by Murakami et al (3) of a "post-perovskite phase" in MgSiO3, at conditions comparable to the D" region, has stimulated considerable interest in the physical properties of the new phase. Given the difficulty of performing direct experiments under these conditions, first-principles quantum mechanical calculations of the type carried out by Iitaka et al(1) and Oganov and Ono(2) are especially useful for studying the deep Earth. In contrast to the perovskite structure that is widely adopted by many compounds, the post-perovskite phase seems to be rather uncommon. In this structure, each silicon cation remains surrounded by six oxygen anions --producing the octahedral coordination that is characteristic of the lower mantle. But rather than forming a corner-linked, three-dimensional network as in perovskite, in the post-perovskite phase the silicon octahedra share edges and corners to form a sheet-like structure with alternating magnesium and silicon layers.(4,5)

References (abridged):

1. Iitaka, T., Hirose, K., Kawamura, K. & Murakami, M. Nature 430, 442-445 (2004)

2. Oganov, A. R. & Ono, S. Nature 430, 445-448 (2004)

3. Murakami, M., Hirose, K., Kawamura, K., Sata, N. & Ohishi, Y. Science 304, 855-858 (2004)

4. Shim, S. -H., Duffy, T. S., Jeanloz, R. & Shen, G. Geophys. Res. Lett. 31, L10603 (2004)

5. Tsuchiya, T., Tsuchiya, J., Umemoto, K. & Wentzcovitch, R. M. Earth Planet. Sci. Lett. 224, 241-248 (2004)

Nature http://www.nature.com/nature

--------------------------------

Related Material:

GEOPHYSICS: ON THE CORE-MANTLE BOUNDARY

The following points are made by Edward J. Garnero (Science 2004 304:834):

1) At a depth of approximately 2900 km, the solid silicate rock of Earth's mantle meets the liquid iron alloy of the core. This region, called the core-mantle boundary (CMB), has long been pictured as a simple dividing zone. Recently, however, this neat model has been directly challenged by a broad range of discoveries. These include new evidence for a deep-mantle phase change (1), layering in the lowermost mantle (2), partial melting (3), mineral anisotropy (4), and small-scale convection with formation of whole-mantle plumes (5).

2) Because of these findings, researchers have created a new paradigm, in which deep-mantle layering and heterogeneity exist globally, with notable regional variations. This heterogeneity is intimately coupled to important processes of the interior, such as modulating heat flow out of the core, and hence fluid core convection currents, and the magnetic field (particularly during reversals). The CMB may also act as a repository for lighter elements that emerge from the core fluid. These phenomena are all consequences of the largest absolute temperature and density contrasts within the planet. Although many of the details of this new paradigm remain enigmatic, the growing body of evidence suggests that the deepest mantle and CMB contain the same degree of chemical, dynamical, structural, and thermal complexity as that of Earth's surface, and hence likely hold important clues for deciphering the evolution and present state of the entire interior.

3) For more than two decades, it has been recognized that the top and bottom few hundred kilometers of Earth's rocky mantle contain the planet's strongest seismic heterogeneity over long lateral distance scales ( greater than 2000 km). Several recent studies highlight seismic analyses that image the deep mantle and CMB region at much shorter scales, around hundreds of kilometers laterally and sometimes less than tens of kilometers vertically. Partly owing to Earth's limited geographical distribution of earthquakes and recorders, only isolated patches of the deep mantle can be investigated at this level of detail.

4) Three of the most thoroughly studied regions are (i) beneath Central America and the Caribbean Ocean, an area underlying past and present subduction that is grossly characterized with higher than average seismic velocities; and two regions where several hotspot volcanoes litter the surface (ii) beneath the central Pacific Ocean and (iii) beneath the southern Atlantic Ocean. The distribution and magnitude of inferred seismic heterogeneity, anisotropy, and layering differ significantly between these regions.

References (abridged):

1. M. Murakami et al., Science 304, 855 (2004)

2. T. Lay, E. J. Garnero, Q. Williams, Phys. Earth Planet. Int., in press.

3. S. Rondenay, K. Fischer, J. Geophys. Res. 108, 2537, 10.1029/2003JB002518 (2003)

4. A. K. McNamara, P. E. van Keken, S. Karato, J. Geophys. Res. 108, 2230, 10.1029/2002JB001970 (2003)

5. P. J. Tackley, Geochem. Geophys. Geosyst. 3, 10.1029/2001GC000167 (2002)

Science http://www.sciencemag.org

--------------------------------

Related Material:

EARTH SCIENCE: ON THE EVOLUTION OF EARTH'S INTERIOR

The following points are made by David Stevenson (Nature 2004 428:476):

1) The basic divisions of Earth's internal structure (crust, mantle and core) have been known for a long time. But the evolutionary path that gave us this structure, and that provides the dynamics of plate tectonics, volcanism, and magnetic-field generation, remains poorly understood. Why do we have plate tectonics? What is the nature and extent of melting deep within Earth? How does the core manage to keep generating such a richly complex magnetic field?

2) A simple view of Earth's evolution invokes the first law of thermodynamics, estimates of radiogenic heat production -- that produced by radioactivity -- and some straightforward scaling arguments that emerge from our theoretical and experimental understanding of thermal convection. In this view, Earth started hot and cooled through geological time at a rate that closely mimics the decreasing rate at which the radiogenic heat production declines. This decreasing rate arises because the radiogenic heat sources have a variety of half-lives, and the more long-lived sources become increasingly important with time. Earth is now presumably shifting to the predominance of Th-232, the longest-lived of the sources. The other parameter of great importance in simple models is the temperature dependence of mantle viscosity, because this dictates the vigor of mantle convection and plate tectonics as the heat flow declines. In this approach, Earth's core has a heat flow that is strongly coupled to mantle evolution through the mantle's ability to cool and accept heat from the core.

3) Models of this kind are easy to construct and boringly monotonic. Furthermore, they cannot explain the widely accepted factor-of-two ratio for current Earth heat output to current radiogenic heat production. Our planet was more eventful than these simple models allow. Whereas Earth scientists have no desire to repeal the first law of thermodynamics, they are willing to challenge almost everything else. Recently, major disagreements have emerged in attempts to understand the energy budget of Earth's core, and there are still many uncertainties over how to incorporate the effects of plates, water, melting and layering into our picture of mantle circulation.

4) In the current debate about Earth's core, there is a contrast between low- and high-dissipation pictures of the dynamo responsible for the planet's magnetic field. These respectively make different assumptions about the geometry of the magnetic fields and associated electrical currents in the core. There is much publication activity on this topic(1-4), but no consensus, because we do not know the full complexity of the field and currents involved. A new study(5) invokes experimental dynamos as well as theoretical ideas, and suggests that dynamos may be less dissipative than some suppose, thereby making a dynamo easier to sustain. Uncertainties about the electrical and thermal conductivity of the core material, and its phase diagram, are also large enough to have a major effect on estimates of dissipation and convective vigor.

References (abridged):

1. Buffett, B. A. Geophys. Res. Lett. 29, 1566 (2002)

2. Gubbins, D. et al. Geophys. J. Inter. 155, 609-622 (2003)

3. Labrosse, S. Phys. Earth Planet. Inter. 140, 127-143 (2003)

4. Nimmo, F. et al. Geophys. J. Inter. 156, 363-376 (2004)

5. Christensen, U. & Tilgner, A. Nature (in the press).

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com