The plastic deformation of iron at pressures of the Earth's inner core

Soon after the discovery of seismic anisotropy in the Earth's inner core, it was suggested that crystal alignment attained during deformation might be responsible. Since then, several other mechanisms have been proposed to account for the observed anisotropy, but the lack of deformation experiments performed at the extreme pressure conditions corresponding to the solid inner core has limited our ability to determine which deformation mechanism applies to this region of the Earth. Here we determine directly the elastic and plastic deformation mechanism of iron at pressures of the Earth's core, from synchrotron X-ray diffraction measurements of iron, under imposed axial stress, in diamond-anvil cells. The epsilon-iron (hexagonally close packed) crystals display strong preferred orientation, with c-axes parallel to the axis of the diamond-anvil cell. Polycrystal plasticity theory predicts an alignment of c-axes parallel to the compression direction as a result of basal slip, if basal slip is either the primary or a secondary slip system. The experiments provide direct observations of deformation mechanisms that occur in the Earth's inner core, and introduce a method for investigating, within the laboratory, the rheology of materials at extreme pressures.     

Stability of the body-centred-cubic phase of iron in the Earth's inner core

Iron is thought to be the main constituent of the Earth's core, and considerable efforts have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves. Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h.c.p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon, and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.     

Melting of the Earth's inner core

The Earth's magnetic field is generated by a dynamo in the liquid iron core, which convects in response to cooling of the overlying rocky mantle. The core freezes from the innermost surface outward, growing the solid inner core and releasing light elements that drive compositional convection. Mantle convection extracts heat from the core at a rate that has enormous lateral variations. Here we use geodynamo simulations to show that these variations are transferred to the inner-core boundary and can be large enough to cause heat to flow into the inner core. If this were to occur in the Earth, it would cause localized melting. Melting releases heavy liquid that could form the variable-composition layer suggested by an anomaly in seismic velocity in the 150 kilometres immediately above the inner-core boundary. This provides a very simple explanation of the existence of this layer, which otherwise requires additional assumptions such as locking of the inner core to the mantle, translation from its geopotential centre or convection with temperature equal to the solidus but with composition varying from the outer to the inner core. The predominantly narrow downwellings associated with freezing and broad upwellings associated with melting mean that the area of melting could be quite large despite the average dominance of freezing necessary to keep the dynamo going. Localized melting and freezing also provides a strong mechanism for creating seismic anomalies in the inner core itself, much stronger than the effects of variations in heat flow so far considered.     

Texturing of the Earth's inner core by Maxwell stresses.

Elastic anisotropy in the Earth's inner core has been attributed to a preferred lattice orientation, which may be acquired during solidification of the inner core or developed subsequent to solidification as a result of plastic deformation. But solidification texturing alone cannot explain the observed depth dependence of anisotropy, and previous suggestions for possible deformation processes have all relied on radial flow, which is inhibited by thermal and chemical stratification. Here we investigate the development of anisotropy as the inner core deforms plastically under the influence of electromagnetic (Maxwell) shear stresses. We estimate the flow caused by a representative magnetic field using polycrystal plasticity simulations for epsilon-iron, where the imposed deformation is accommodated by basal and prismatic slip. We find that individual grains in an initially random polycrystal become preferentially oriented with their c axes parallel to the equatorial plane. This pattern is accentuated if deformation is accompanied by recrystallization. Using the single-crystal elastic properties of epsilon-iron at core pressure and temperature, we average over the simulated orientation distribution to obtain a pattern of elastic anisotropy which is similar to that observed seismologically.     

Fine-scale heterogeneity in the Earth's inner core

The seismological properties of the Earth's inner core have become of particular interest as we understand more about its composition and thermal state. Observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects. The attenuation of seismic waves in the inner core is strong, and studies of seismic body waves have found that this high attenuation is consistent with either scattering or intrinsic attenuation. The outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths. Here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body-wave reflection from the inner-core boundary. The amplitude of these scattered waves can be explained by stiffness variations of 1.2% with a scale length of 2 kilometres across the outermost 300 km of the inner core. These variations might be caused by variations in composition, by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy.     

Melting of iron at the physical conditions of the Earth's core

Seismological data can yield physical properties of the Earth's core, such as its size and seismic anisotropy. A well-constrained iron phase diagram, however, is essential to determine the temperatures at core boundaries and the crystal structure of the solid inner core. To date, the iron phase diagram at high pressure has been investigated experimentally through both laser-heated diamond-anvil cell and shock-compression techniques, as well as through theoretical calculations. Despite these contributions, a consensus on the melt line or the high-pressure, high-temperature phase of iron is lacking. Here we report new and re-analysed sound velocity measurements of shock-compressed iron at Earth-core conditions. We show that melting starts at 225 +/- 3 GPa (5,100 +/- 500 K) and is complete at 260 +/- 3 GPa (6,100 +/- 500 K), both on the Hugoniot curve-the locus of shock-compressed states. This new melting pressure is lower than previously reported, and we find no evidence for a previously reported solid-solid phase transition on the Hugoniot curve near 200 GPa (ref. 16).     

Hemispherical variations in seismic velocity at the top of the Earth's inner core

Knowledge of the seismic velocity structure at the top of the Earth's inner core is important for deciphering the physical processes responsible for inner-core growth. Previous global seismic studies have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals-observed on both global seismograph stations and several regional seismic networks-are systematically larger, by about 0.8 s, for waves that sample the 'eastern hemisphere' of the inner core (40 degrees E to 180 degrees E) compared to those that sample the 'western hemisphere' (180 degrees W to 40 degrees E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.     

Thermal and electrical conductivity of iron at Earth's core conditions

The Earth acts as a gigantic heat engine driven by the decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing (as the solid inner core grows), and on chemical convection (due to light elements expelled from the liquid on freezing). The power supplied to the geodynamo, measured by the heat flux across the core-mantle boundary (CMB), places constraints on Earth's evolution. Estimates of CMB heat flux depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent experimental and theoretical difficulties. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles--unlike previous estimates, which relied on extrapolations. The mixtures of iron, oxygen, sulphur and silicon are taken from earlier work and fit the seismologically determined core density and inner-core boundary density jump. We find both conductivities to be two to three times higher than estimates in current use. The changes are so large that core thermal histories and power requirements need to be reassessed. New estimates indicate that the adiabatic heat flux is 15 to 16 terawatts at the CMB, higher than present estimates of CMB heat flux based on mantle convection; the top of the core must be thermally stratified and any convection in the upper core must be driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted, and future models of mantle evolution will need to incorporate a high CMB heat flux and explain the recent formation of the inner core.     

Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity

Seismic waves sampling the top 100 km of the Earth's inner core reveal that the eastern hemisphere (40 degrees E-180 degrees E) is seismically faster, more isotropic and more attenuating than the western hemisphere. The origin of this hemispherical dichotomy is a challenging problem for our understanding of the Earth as a system of dynamically coupled layers. Previously, laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core, which in turn can induce textural heterogeneity on the inner core solidification front. The resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo, specifically magnetic flux concentrations in the time-average palaeomagnetic field over the past 5 Myr, and preferred eddy locations in flows imaged below the core-mantle boundary by the analysis of historical geomagnetic secular variation. Here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large-scale, long-term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle. The main feature of this thermochemical 'wind' is a cyclonic circulation below Asia, which concentrates magnetic field on the core-mantle boundary at the observed location and locally agrees with core flow images. This wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary, suggesting that lateral seismic anomalies at the top of the inner core result from mantle-induced variations in its freezing rate.     

地核物质的状态方程和参数

 解释了3种地核物质状态方程的异同,验证了Vinet et al. EOS(equation of state)比Shanker et al.EOS更接近Stacy EOS的结论,后者与高压极限的热动力学一致、而且与来源于地震波的数据完全匹配;同时与地震波测量数据进行了对照;给出了内核边界压力下物质摩尔体积随温度的变化函数,预言了密度亏损的温度变化.这对于揭示地球深部物质特性、研究地核内的轻物质组分以及深部动力学原理具有重要意义.     

Seismological evidence for mosaic structure of the surface of the Earth's inner core

The transition from the Earth's solid inner core to liquid outer core is the location where the inner core grows and from which compositional convection in the outer core originates. Most seismological models of the Earth describe the inner-core boundary as sharp and simple, although experimental data requiring the presence of a thin transition layer at the bottom of the outer core have been reported. The density jump at the inner-core boundary--an important parameter determining gravitational energy release and constraining the compositional difference between the inner and outer core-is also not well known. Estimates of this density jump obtained using free-oscillation eigenfrequencies give low values of 0.25-1.0 g cm(-3), whereas a method using the amplitude ratio of core-reflected phases yielded values of 0.6-1.8 g cm(-3) (refs 14, 15, 16-17). Here we analyse properties of waves precritically reflected from the Earth's inner core (PKiKP phases) that show significant variability in amplitude, consistent high-frequency content and stable travel times with respect to a standard Earth model. We infer that the data are best explained by a mosaic structure of the inner core's surface. Such a mosaic may be composed of patches in which the transition from solid inner to liquid outer core includes a thin partially liquid layer interspersed with patches containing a sharp transition.     

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core

The nature of the stable phase of iron in the Earth's solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.     

地球内核物态热效应研究

 在考虑Grüneisen系数、Debye温度随体积变化的基础上,详细讨论了各部分热压的贡献,并且结合K-primed冷压方程,得到了地球内核较为全面的物态方程,分析了内核p-V-T的关系,给出了地球内核的温度分布.其结果详实有据,为以后深入研究地球内核甚至液态外核的物态性质提供了坚实的基础.     

Slow differential rotation of the Earth's inner core indicated by temporal changes in scattering

The finding that the Earth's inner core might be rotating faster than the mantle has important implications for our understanding of core processes, including the generation of the Earth's magnetic field. But the reported signal is subtle--a change of about 0.01 s per year in the separation of two seismic waves with differing paths through the core. Subsequent studies of such data have generally supported the conclusion that differential rotation exists, but the difficulty of accurately locating historic earthquakes and possible biases induced by strong lateral variations in structure near the core-mantle boundary have raised doubt regarding the proposed inner-core motion. Also, a study of free oscillations constrained the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography and mantle heterogeneity might lock the inner core to the mantle. The recent detection of seismic waves scattered in the inner core suggests a simple test of inner-core motion. Here we compare scattered waves recorded in Montana, USA, from two closely located nuclear tests at Novaya Zemlya, USSR, in 1971 and 1974. The data show small but coherent changes in scattering which point toward an inner-core differential rotation rate of 0.15 degrees per year--consistent with constraints imposed by the free-oscillation data.     

Silicon in the Earth's core

Small isotopic differences between the silicate minerals in planets may have developed as a result of processes associated with core formation, or from evaporative losses during accretion as the planets were built up. Basalts from the Earth and the Moon do indeed appear to have iron isotopic compositions that are slightly heavy relative to those from Mars, Vesta and primitive undifferentiated meteorites (chondrites). Explanations for these differences have included evaporation during the 'giant impact' that created the Moon (when a Mars-sized body collided with the young Earth). However, lithium and magnesium, lighter elements with comparable volatility, reveal no such differences, rendering evaporation unlikely as an explanation. Here we show that the silicon isotopic compositions of basaltic rocks from the Earth and the Moon are also distinctly heavy. A likely cause is that silicon is one of the light elements in the Earth's core. We show that both the direction and magnitude of the silicon isotopic effect are in accord with current theory based on the stiffness of bonding in metal and silicate. The similar isotopic composition of the bulk silicate Earth and the Moon is consistent with the recent proposal that there was large-scale isotopic equilibration during the giant impact. We conclude that Si was already incorporated as a light element in the Earth's core before the Moon formed.     

The Earth's 'missing' niobium may be in the core

As the Earth's metallic core segregated from the silicate mantle, some of the moderately siderophile ('iron-loving') elements such as vanadium and chromium are thought to have entered the metal phase, thus causing the observed depletions of these elements in the silicate part of the Earth. In contrast, refractory 'lithophile' elements such as calcium, scandium and the rare-earth elements are known to be present in the same proportions in the silicate portion of the Earth as in the chondritic meteorites-thought to represent primitive planetary material. Hence these lithophile elements apparently did not enter the core. Niobium has always been considered to be lithophile and refractory yet it has been observed to be depleted relative to other elements of the same type in the crust and upper mantle. This observation has been used to infer the existence of hidden niobium-rich reservoirs in the Earth's deep mantle. Here we show, however, that niobium and vanadium partition in virtually identical fashion between liquid metal and liquid silicate at high pressure. Thus, if a significant fraction of the Earth's vanadium entered the core (as is thought), then so has a similar fraction of its niobium, and no hidden reservoir need be sought in the Earth's deep mantle.