ScienceWeek

EARTH SCIENCE: ON THE MANTLE-CORE BOUNDARY

The following points are made by Cin-Ty Aeolus Lee (Science 2004 306:64):

1) Earth's interior is divided into a central core made of an iron-nickel alloy and a rocky mantle made of silicate and oxide minerals. Earth scientists often ignore the core in their treatments of the chemical and dynamic evolution of the mantle after core-mantle segregation. However, there has been growing debate on whether there may be significant chemical interaction between the core and the mantle. New work [1] presents data that suggest that the lower mantle may be enriched in iron compared to the upper mantle, and that this iron enrichment may indeed be due to core-mantle interaction.

2) If the lower mantle is enriched in iron through chemical reactions between the lower mantle and the liquid outer part of the Earth's core [2], then the dynamics of the core and mantle must be coupled. The iron enrichment will also influence the physical properties of the lower mantle (such as its density, elasticity, and electrical conductivity). However, direct evidence for an iron-rich lower mantle is lacking. Moreover, even if the lower mantle is enriched in iron, other explanations are plausible. For example, the mantle may retain a primordial compositional stratification. Alternatively, subduction of oceanic crust or deep-sea marine sediments, some of which may be enriched in iron and manganese, may cause the enrichment.

3) It is widely believed that volcanic hotspots are the surface manifestations of plumes rising up from the lower mantle or core-mantle boundary. Earth scientists therefore use hotspot volcanoes as windows into Earth's deep interior. Humayun et al [1] report the Fe/Mn ratios of basaltic lavas associated with a well-known hotspot, Hawaii. They demonstrate that the Hawaiian lavas have higher Fe/Mn ratios than basalts from mid-ocean ridges; the latter only tap the upper mantle.

4) Humayun et al [1] argue that the partitioning of iron and manganese between melts and the mantle is roughly equal, and that the Fe/Mn ratio of a melt should therefore closely reflect the Fe/Mn ratio of the melt source region. They thus suggest that the high Fe/Mn ratios of the Hawaiian lavas reflect a lower mantle enriched in iron, possibly due to long-term chemical interaction between the core and the mantle.

5) If this interpretation is correct, then the results provide the second observational evidence for a core component to the Hawaiian mantle source. The first evidence came from anomalously high concentrations of the isotope 186Os in Hawaiian lavas [3], which were attributed to a Hawaiian mantle source that has incorporated small amounts of outer-core material. The latter is hypothesized to have elevated 186Os due to its high Pt/Os ratio and radioactive decay of 190Pt to 186Os [3]. Others have argued that the 186Os anomalies are more likely to result from incorporation of subducted Fe-Mn-rich marine sediments [4,5], which also have high Pt/Os ratios. However, such sediments have low Fe/Mn ratios, which is inconsistent with the high Fe/Mn ratios of Hawaiian lavas reported by Humayun et al [1].

References (abridged):

1. M. Humayun, L. Qin, M. D. Norman, Science 306, 91 (2004)

2. E. Garnero, Science 304, 834 (2004)

3. A. D. Brandon et al., Science 280, 1570 (1998)

4. G. Ravizza, J. Blusztajn, H. M. Prichard, Earth Planet. Sci. Lett. 188, 369 (2001)

5. A. Schersten et al., Nature 427, 234 (2004)

Science http://www.sciencemag.org

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GEOPHYSICS: ON EARTH'S DEEP MANTLE

The following points are made by R.D. van der Hilst (Science 2004 306:817):

1) The present-day state and dynamics of Earth's interior hold the keys to understanding the early conditions of the solid Earth and its biosphere, hydrosphere, and atmosphere, and how these have evolved to the planet we now know. Earth's stable stratification into crust (between 5 and 70-km thick), mantle (from base of crust to ~2890-km depth), and core (2890- to 6371-km depth) has been known for half a century from seismic velocity measurements, but characterizing the heterogeneity within and the interaction between these concentric shells is a frontier of modern, cross-disciplinary research. New work(1) breaks new ground with compelling evidence for large-scale variations in composition in Earth's mantle.

2) Man-made probes into the Earth's interior barely reach a depth of ~10 km, and volcanism rarely brings up samples from deeper than ~150 km. These distances are dwarfed by Earth's dimensions, and our knowledge of the deeper realms is pieced together from a range of surface observables, meteorite and solar atmosphere analyses, experimental and theoretical mineral physics and rock mechanics, and computer simulations. A major unresolved issue concerns the scale and nature of mantle convection, the slow (1 to 5 cm/year) stirring that helps cool the planet by transporting radiogenic and primordial heat from Earth's interior to its surface. The mantle displays a velocity discontinuity at 660 km. Does convection occur within separate layers or over the whole mantle? Is the mantle effectively homogenized or has large-scale compositional heterogeneity survived long-term mixing? Classic models have focused either on convective layering (with the upper and lower layers having different, but constant, composition), or on isochemical whole-mantle overturn, but neither satisfies all multidisciplinary constraints (2-5).

3) Over the past decade, several discoveries have begun to reveal a lower mantle that is far more interesting -- and enigmatic --than the bland shell of near-constant properties considered in the classic models. Seismic tomography demonstrates that the 660-km discontinuity is, at least locally, permeable to convective flow, implying that any chemical stratification must be deeper. Moreover, slabs of subducted tectonic plates that sink into the lower mantle do not appear to all reach the core-mantle boundary, which may suggest poor vertical mixing of the mantle (5). Mantle plumes remain a topic of debate, in part because seismological constraints on their nature and size are still ambiguous. But the existence in the deep mantle, for instance beneath Africa, of seismically slow structures with velocity anomalies and gradients too large for a purely thermal origin is uncontroversial. Seismologists also discovered discrepant behavior of different types of seismic waves, another smoking gun for compositional heterogeneity. In particular, anomalous frequencies of Earth's free-oscillation modes yield a conspicuous anticorrelation between shear speed and mass density, which is inconsistent with a thermal cause.

4) Besides providing the data needed to calculate the effect on elastic parameters of changes in temperature, pressure (which is related to depth), and composition, mineral physicists are slowly unraveling the secrets of deep-mantle mineralogy. Recent findings include a modified perovskite structure near 88 GPa (~2000-km depth); a change from high- to low-spin state of iron between 70 GPa (~1700-km depth) and 120 GPa (~2600-km depth), which may affect rheology (and thus convective mixing) and iron partitioning among magnesiowoestite and perovskite; and a phase transformation to postperovskite near 120 GPa (between 2600- and 2700-km depth).

References (abridged):

1. J. Trampert et al., Science 306, 853 (2004)

2. A. W. Hofmann, Nature 385, 219 (1997)

3. P. J. Tackley, Science 288, 2002 (2000)

4. G. Schubert et al., Mantle Convection in the Earth and Planets (Cambridge Univ. Press, Cambridge, UK, 2001)

5. F. Albarede, R. D. van der Hilst, Philos. Trans. R. Soc. London A 360, 2569 (2002)

Science http://www.sciencemag.org

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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)

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