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The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet's internal structure suggest the presence of a FeS layer overlying a silicon-bearing core. We performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si(-S) core is at least 1 log unit at pressure and temperature conditions relevant to Mercury's interior. Estimates of the lower bound of thermal conductivity for FeS and Fe-Si(-S) materials are calculated using the Wiedemann-Franz law. A thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field.

The solid inner core is at the center of the Earth, gravitationally held within the liquid outer core. It is one of the most dynamic parts of Earth's interior. Since the initial claim of inner core differential rotation relative to the mantle, its existence and rate have been challenged for over two decades. Here, we re-examine the seismic records of two megaton nuclear tests in Novaya Zemlya, Russia, three years apart, from the Large Aperture Seismic Array in Montana, USA. Using an improved static time correction from an antipodal earthquake, we refine the resolution of the beamforming of PKiKP and its inner-core scattered coda. Then, we measure the slight time shifts (tenths of seconds) between the inner-core-scattered waves from the two events with moving-time-window cross-correlation. Applying a novel back-projection method, we locate the inner-core regions that scatter the energy within the PKiKP coda based on its slowness and the lapse time. We then measure the inner core rotation, first assuming alignment with Earth's rotation axis, then finding the best-fitting differential rotation axis with a grid search. The rotation rates here are robust and consistent across the many scattered arrivals throughout the inner core scattering wavetrain. Our results indicate 0.10°/year inner core super-rotation rate from 1971 to 1974 aligned with Earth's rotation axis, or 0.125°/year with the rotation axis tilting about 8° from the Earth's rotation axis, which yields a marginally better fit to the observed time shifts.

Seeking geophysical explanations for the periodic ∼six-year oscillation (SYO) previously found in the Earth's length-of-day variation (ΔLOD), we analyze the global GPS displacement and geomagnetic data using the array processing technique of OSE (Optimal Sequence Estimation), and find clear evidences of the 6-yr signals which manifest as a westward rotary propagating wave of the sectoral spherical-harmonic pattern of degree-2 order-2 ( Y 22 ). Based on the period, the spatial pattern, and the amplitudes and the estimated phases that exhibit consistent synchronicity among the three datasets, we propose the following scenario: The mantle-inner core gravitational (MICG) coupling gives rise to a 6-yr axial torsional libration of the inner core controlled by the sectoral Y 22 density anomalies, or the equatorial ellipticities, in the inner core and the (lower) mantle, the angular momentum exchange between which manifests as the SYO in ΔLOD. It forces into action a pressure wave-2 cyclic in 6 yr through the fluid outer core, which in turn produces the corresponding GPS and geomagnetic variations. Our findings provide insight into the dynamics of the deep Earth, and corroborate a positive density anomaly for the lower-mantle Large Low-Shear-Velocity Provinces.

In this study, we present new experimental constraints on the phase stability and thermal equation of state of an important hydrous phase, δ-AlOOH, using synchrotron X-ray diffraction up to 142 GPa and 2500 K. Our experimental results have shown that δ-AlOOH remains stable at the whole mantle pressure–temperature conditions above the D″ layer yet will decompose at the core–mantle boundary because of a dramatic increase in temperature from the silicate mantle to the metallic outer core. At the bottom transition zone and top lower mantle, the formation of δ-AlOOH by the decomposition of phase Egg is associated with a ∼2.1–2.5% increase in density (ρ) and a ∼19.7–20.4% increase in bulk sound velocity ( V Φ ). The increase in ρ across the phase Egg to δ-AlOOH phase transition can facilitate the subduction of δ-AlOOH to the lower mantle. Compared to major lower-mantle phases, δ-AlOOH has the lowest ρ but greatest V Φ , leading to an anomalous low ρ / V Φ ratio which can help to identify the potential presence of δ-AlOOH in the region. More importantly, water released from the breakdown of δ-AlOOH at the core–mantle boundary could lower the solidus of the pyrolitic mantle to cause partial melting and/or react with Fe in the region to form the low-velocity FeO2Hx phase. The presence of partial melting and/or the accumulation of FeO2Hx phase at the CMB could be the cause for the ultra-low velocity zone. δ-AlOOH is thus an important phase to transport water to the lowermost mantle and helps to understand the origin of the ultra-low velocity zone.

The conventional view of Earth's inner core is that it began to crystallize at Earth's center when the temperature dropped below the melting point of the iron alloy and has grown steadily since that time as the core continued to cool. However, this model neglects the energy barrier to the formation of the first stable crystal nucleus, which is commonly represented in terms of the critical supercooling required to overcome the barrier. Using constraints from experiments, simulations, and theory, we show that spontaneous crystallization in a homogeneous liquid iron alloy at Earth's core pressures requires a critical supercooling of order 1000 K, which is too large to be a plausible mechanism for the origin of Earth's inner core. We consider mechanisms that can lower the nucleation barrier substantially. Each has caveats, yet the inner core exists: this is the nucleation paradox. Heterogeneous nucleation on a solid metallic substrate tends to have a low energy barrier and offers the most straightforward solution to the paradox, but solid metal would probably have to be delivered from the mantle and such events are unlikely to have been common. A delay in nucleation, whether due to a substantial nucleation energy barrier, or late introduction of a low energy substrate, would lead to an initial phase of rapid inner core growth from a supercooled state. Such rapid growth may lead to distinctive crystallization texturing that might be observable seismically. It would also generate a spike in chemical and thermal buoyancy that could affect the geomagnetic field significantly. Solid metal introduced to Earth's center before it reached saturation could also provide a nucleation substrate, if large enough to escape complete dissolution. Inner core growth, in this case, could begin earlier and start more slowly than standard thermal models predict.

A significant 6 yr oscillation exists in the length of day (LOD) on the interannual scales. There are mainly two models currently to explain this oscillation, i.e., mantle–inner core gravitational (MICG) coupling mode and the fast torsional waves within the fluid outer core. The former has been doubted, while the source of the excitation of the latter is not yet understood. Therefore, the mechanism of the 6 yr oscillation is still not clear. Here, by considering the mantle and inner core angular momentum, we investigate the MICG coupling mode and its natural period ( T 0 ). Given that the strength of gravitational core–mantle coupling ( Γ ¯ ) within a recently constrained range is quite weaker than that estimated previously, the mechanism of the 6 yr oscillation still can be attributed to MICG coupling mode (i.e., T 0 equals to 6 yrs), but, we require the inertia moment of fluid within the tangent cylinder involved in the 6 yr oscillation to be smaller than 1.23 × 10 35 kg m 2 . This interpretation can be used to constrain the electromagnetic (EM) coupling at the inner core boundary (ICB). In order to study quantitatively the constraints on the EM coupling at the core–mantle boundary (CMB) from the observed 6 yr oscillation with a quality factor Q (∼51.6), we further develop the mathematical expression between the Q value based on the observations and EM coupling at the CMB. According to the Γ ¯ value from recent estimates and assuming that the 6 yr oscillation is in a free decay, we can obtain the radial magnetic field at the CMB is 0.52 mT∼0.62 mT when conductance at the CMB is 108 S.

Geomagnetic intensity fluctuations provide important constraints on time-scales associated with dynamical processes in the outer core. PADM2M is a reconstructed time series of the 0–2 Ma axial dipole moment (ADM). After smoothing to reject high frequency variations PADM2M's average growth rate is larger than its decay rate. The observed asymmetry in rates of change is compatible with longer term diffusive decay of the ADM balanced by advective growth on shorter time scales, and provides a potentially useful diagnostic for evaluating numerical geodynamo simulations. We re-analyze the PADM2M record using improved low-pass filtering to identify asymmetry and quantify its uncertainty via bootstrap methods before applying the new methodology to other kinds of records. Asymmetry in distribution of axial dipole moment derivatives is quantified using the geomagnetic skewness coefficient, s g . A positive value indicates the distribution has a longer positive tail and the average growth rate is greater than the average decay rate. The original asymmetry noted by Ziegler and Constable (2011) is significant and does not depend on the specifics of the analysis. A long-term record of geomagnetic intensity should also be preserved in the thermoremanent magnetization of oceanic crust recovered by inversion of stacked profiles of marine magnetic anomalies. These provide an independent means of verifying the asymmetry seen in PADM2M. We examine three near-bottom surveys: a 0 to 780 ka record from the East Pacific Rise at 19°S, a 0 to 5.2 Ma record from the Pacific Antarctic Ridge at 51°S, and a chron C4Ar–C5r (9.3–11.2 Ma) record from the NE Pacific. All three records show an asymmetry similar in sense to PADM2M with geomagnetic skewness coefficients, s g > 0 . Results from PADM2M and C4Ar–C5r are most robust, reflecting the higher quality of these geomagnetic records. Our results confirm that marine magnetic anomalies can carry a record of the asymmetric geomagnetic field behavior first found for 0–2 Ma in PADM2M, and show that it was also present during the earlier time interval from 9.3–11.2 Ma.

Oxygen diffusion experiments in liquid iron have been performed at 3–18 GPa and 1975–2643 K using a multi-anvil apparatus. Diffusion couples consisted of a pure iron rod and a sintered disk of Fe 0.85 O 0.15 placed end-to-end in a vertical orientation. Images and chemical spot analyses were acquired along the full length of the quenched sample on lines perpendicular to the diffusion interface. Exsolution features that formed during quenching consist mostly of spherical oxide blobs of at least two size populations, as well as feathery dendritic textures in more oxygen-rich regions near the top of the samples. Diffusion during heating (i.e. prior to reaching the peak annealing temperature, T f ) is treated numerically to refine Arrhenian parameters from simultaneous least-squares fits to several concentration profiles obtained from experiments at constant pressure and variable T f . Diffusion coefficients range from ∼ 6 × 10 − 9 to ∼ 2 × 10 − 8 m 2 s − 1 over the P–T range of the study, with activation enthalpies of less than 100 kJ mol−1. We find a very weak effect of pressure on oxygen diffusion with an activation volume of 0.1 ± 0.1 cm 3 mol − 1 , in agreement with computational studies performed above 100 GPa. Arrhenian extrapolation of diffusion coefficients for oxygen to P–T conditions of the Earth's outer core yields faster average diffusion rates ( ∼ 3 × 10 − 8 m 2 s − 1 ) than for Si or Fe in silicon-rich liquid iron alloys or pure liquid iron ( ∼ 5 × 10 − 9 m 2 s − 1 ) reported previously. Oxygen diffusion data are used to constrain the maximum size of descending liquid metal droplets in a magma ocean that is required for chemical equilibration to be achieved. Our results indicate that if the Earth's core composition is representative of equilibrium chemical exchange with a silicate magma ocean, then it could only have been accomplished by large-scale break-up of impactor cores to liquid iron droplet sizes no larger than a few tens of centimeters.

Melting experiments were performed on the Fe–Fe 3 S system at high pressures between 34 and 254 GPa in a laser-heated diamond-anvil cell (DAC), using starting materials of fine-grained homogeneous mixtures of Fe and FeS ( 5.7 ( ± 0.3 ) wt.% S coexisted with both solid Fe 3 S and Fe containing 3.9 ( ± 0.4 ) wt.% S at 254 GPa and 3550 K. The eutectic liquid at inner core boundary (ICB) pressure includes less sulfur than is required to account for the density deficit of the outer core (≥10 wt.% S). Furthermore, the difference in sulfur concentration between coexisting liquid and solid is not sufficient to account for the observed density jump across the ICB. These indicate that sulfur cannot be a predominant light element in the core.

Multiple lines of geochemical and geophysical evidence suggest the Moon has a small metallic core, yet the composition of the core is poorly constrained. The physical state of the core (now or in the past) depends on detailed knowledge of its composition, and unfortunately, there is little available data on relevant multicomponent systems (i.e., Fe–Ni–S–C) at lunar interior conditions. In particular, there is a dearth of phase equilibrium data to elucidate whether a specific core composition could help to explain an early lunar geodynamo and magnetic field intensities, or current solid inner core/liquid outer core states. We utilize geochemical information to estimate the Ni, S and C contents of the lunar core, and then carry out phase equilibria experiments on several possible core compositions at the pressure and temperature conditions relevant to the lunar interior. The first composition is 0.5 wt% S and 0.375 wt% C, based on S and C contents of Apollo glasses. A second composition contains 1 wt% each of S and C, and assumes that the lunar mantle experienced degassing of up to 50% of its S and C. Finally a third composition contains C as the dominant light element. Phase equilibrium experiments were completed at 1, 3 and 5 GPa, using piston cylinder and multi-anvil techniques. The first composition has a liquidus near 1550 °C and solidus near 1250 °C. The second composition has a narrower liquidus and solidus temperatures of 1400 and 1270 °C, respectively, while the third composition is molten down to 1150 °C. As the composition crystallizes, the residual liquid becomes enriched in S and C, but S enrichment is greater due to the incorporation of C (but not S) into solid metallic FeNi. Comparison of these results to thermal models for the Moon allow an evaluation of which composition is consistent with the geophysical data of an early dynamo and a currently solid inner and liquid outer core. Composition 1 has a high enough liquidus to start crystallizing early in lunar history (4.3 Ga), consistent with the possible core dynamo initiated by crystallization of a solid inner core. Composition 1 also stays partially molten throughout lunar history, and could easily explain the seismic data. Composition 2, on the other hand, can satisfy one or the other set of geophysical data, but not both and thus seems like a poor candidate for a lunar core composition. Composition 3 remains molten to temperatures that are lower than current estimates for the lunar core, thus ruling out the possibility of a C-rich (and S-poor) lunar core. The S- and C-poor core composition studied here (composition 1) is consistent with all available geochemical and geophysical data and provides a simple heat source and mechanism for a lunar core dynamo (core crystallization) that would obviate the need for other primary mechanisms such as impacts, core–mantle coupling, or unusual thermal histories.

We carried out melting experiments on Fe–Si alloys to 127 GPa in a laser-heated diamond-anvil cell (DAC). On the basis of textural and chemical characterizations of samples recovered from a DAC, a change in eutectic liquid composition in the Fe–FeSi binary system was examined with increasing pressure. The chemical compositions of coexisting liquid and solid phases were quantitatively determined with field-emission-type electron microprobes. The results demonstrate that silicon content in the eutectic liquid decreases with increasing pressure to less than 1.5 ± 0.1 wt.% Si at 127 GPa. If silicon is a single light element in the core, 4.5 to 12 wt.% Si is required in the outer core in order to account for its density deficit from pure iron. However, such a liquid core, whose composition is on the Si-rich side of the eutectic point, crystallizes less dense solid, CsCl (B2)-type phase at the inner core boundary (ICB). Our data also show that the difference in silicon concentration between coexisting solid and liquid is too small to account for the observed density contrast across the ICB. These indicate that silicon cannot be the sole light element in the core. Previous geochemical and cosmochemical arguments, however, strongly require ∼6 wt.% Si in the core. It is possible that the Earth's core originally included ∼6 wt.% Si but then became depleted in silicon by crystallizing SiO2 or MgSiO3.

Earth's magnetic field is generated by the liquid outer core and sensitively depends on the thermal conductivity of the core. The dominant component of the Earth's core is Fe (∼85%) and Ni (∼10%). However, current estimates on FeNi liquids have not been previously tested at high pressures. In this paper, ab initio simulations were first applied to calculations of the thermal and electrical conductivities of FeNi liquids at Earth's outer core conditions. The thermal conductivity along the adiabatic curve for FeNi fluid ranges from 120.52 to 202.80 W/m/K, but pure Fe ranges from 125.07 to 216.18 W/m/K. The age of the inner core calculated with thermal conductivity of FeNi fluid is 0.019 Ga longer than pure Fe. Nickel effect on the age of the inner core is of the same order with the uncertainty of density jump and latent heat at the inner-core boundary. Furthermore, by analyzing the effective temperature gradient, the present thickness of thermal stratification calculated with thermal conductivity of FeNi liquid is 64.5 km thinner than that of pure Fe.

The partitioning of light elements between liquid and solid at the inner core boundary (ICB) governs compositional difference and density deficit between the outer and inner core. Observations of high S and low Fe concentration on the surface of Mercury from MESSENGER mission indicate that Mercury is formed under much more reduced conditions than other terrestrial planets, which may result in a Si and S-bearing metallic Fe core. In this study, we conducted high-pressure experiments to investigate the partitioning behavior of Si and S between liquid and solid in the Fe-Si-S system at 15 and 21 GPa, relevant to Mercury's ICB conditions. Experimental results show that almost all S partitions into liquid. The partitioning coefficient of Si (DSi) between liquid and solid is strongly correlated with the S content in liquid (X S liquid ) as: log 10 ⁡ ( D Si ) = 0.0445 + 5.9895 ⁎ log 10 ⁡ ( 1 − X S liquid ) . Within our experimental range, pressure has limited effect on the partitioning behavior of Si and S between liquid and solid. For Mercury with an Fe-Si-S core, compositional difference between the inner and outer core is strongly dependent on the S content of the core. The lower S content is in the core, the smaller compositional difference and density deficit between the liquid outer core and solid inner core should be observed. For a core with 1.5 wt% bulk S, a model ICB temperature would intersect with the melting curve at ∼17 GPa, corresponding to an inner core with a radius of ∼1600 km.

The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.

Thermal and compositional convection in Earth's core are thought to be the main power sources driving geodynamo. The viability and strength of thermally and compositionally-driven convection over Earth's history depend on the adiabatic heat flow across the core-mantle boundary (CMB) which is governed by the thermal conductivity of a constituent Fe-Ni-light element alloy at the pressure-temperature (P-T) conditions relevant to the core. Silicon is often proposed to be an abundant light element alloyed with Fe along with ∼5 wt% Ni, but the thermal transport properties of Fe-Ni-Si alloys at high P-T remain largely uncertain. Here we measured the electrical resistivities of Fe-10wt%Ni and Fe-1.8wt%Si alloys up to ∼142 GPa and ∼3400 K using four-probe van der Pauw method in laser-heated diamond anvil cell experiments. Our results show that the resistivities of hcp-Fe-1.8Si and Fe-10Ni display quasi-linear temperature dependence from ∼1500 to 3400 K at each given high pressure. Addition of ∼2 wt% Si in hcp-Fe significantly increases its resistivity by ∼25% at ∼138 GPa and 4000 K, but Fe-10wt%Ni has similar resistivity to pure hcp-Fe at near CMB P-T conditions. Using our measured values of electrical resistivities, we model thermal conductivities via the Wiedemann-Franz law, giving a nominal thermal conductivity of ∼50 W m−1 K−1 for liquid Fe-5Ni-8Si alloy at the topmost outer core, implying an adiabatic (conductive) core heat flow of ∼8.0 TW. The outer core has a much lower thermal conductivity than the inner core due to light-element differentiation across the solidifying inner-core boundary. Our studies imply that the adiabatic core heat flow is low enough to enable thermal convection to drive the geodynamo over most and possibly all of Earth's history, while the strength of compositional convection increases with the inner-core growth and accounts for ∼83% of the buoyancy flux to the present-day geodynamo.

The melting points of fcc- and hcp-structured Fe_(0.9)Ni_(0.1) and Fe are measured up to 125 GPa using laser heated diamond anvil cells, synchrotron Mossbauer spectroscopy, and a recently developed fast temperature readout spectrometer. The onset of melting is detected by a characteristic drop in the time-integrated synchrotron Mossbauer signal which is sensitive to atomic motion. The thermal pressure experienced by the samples is constrained by X-ray diffraction measurements under high pressures and temperatures. The obtained best-fit melting curves of fcc-structured Fe and Fe_(0.9)Ni_(0.1) fall within the wide region bounded by previous studies. We are able to derive the γ–ϵ–l triple point of Fe and the quasi triple point of Fe_(0.9)Ni_(0.1) to be 110 ± 5GPa, 3345 ± 120K and 116 ± 5GPa, 3260 ± 120K, respectively. The measured melting temperatures of Fe at similar pressure are slightly higher than those of Fe_(0.9)Ni_(0.1) while their one sigma uncertainties overlap. Using previously measured phonon density of states of hcp-Fe, we calculate melting curves of hcp-structured Fe and Fe_(0.9)Ni_(0.1) using our (quasi) triple points as anchors. The extrapolated Fe_(0.9)Ni_(0.1) melting curve provides an estimate for the upper bound of Earth's inner core–outer core boundary temperature of 5500 ± 200K. The temperature within the liquid outer core is then approximated with an adiabatic model, which constrains the upper bound of the temperature at the core side of the core–mantle boundary to be 4000 ± 200K. We discuss a potential melting point depression caused by light elements and the implications of the presented core–mantle boundary temperature bounds on phase relations in the lowermost part of the mantle.

This PhD work focuses on the rotational dynamics of the coupled inner core - outercore - mantle system. The conservation of the angular momentum our coupled Earth model indeed involves two direct electromagnetic torques at the fluid core boundaries and a remote gravitational torque between the inner core and the mantle. The rotational dynamics is described by four typical shears and studied in convective numerical simulations of the geodynamo which are able to reproduce the main characteristics of the geomagnetic field and its secular variation. This secular variation is mainly embodied by the westward drift of magnetic flux patches at the CMB, concentrated on the equator of the Atlantic hemisphere, and is well documented for the last four centuries. We provide constrains on the inner core differential rotation by expressing its link to the geomagnetic westward drift. This is performed through the formulation and the validation of dynamical electromagnetic torque models, which are then introduced in the conservation of the angular momentum of the system. In the long-term state, the global shear in the fluid outer core is distributed between the westward drift and the differential rotation of the inner core, in proportions controlled by the state of couplings. As a present day estimate of this shear is close to the observed westward drift, we conclude there is no differential rotation of the inner core on time-average. In the time-dependent state, we observed that the strength of gravitational coupling is the dominant parameter. This places limit on the decadal fluctuations of the inner core differential rotation, which should not exceed a few hundredths of degree per year

Many dynamo studies extrapolate numerical model results to planetary conditions by empirically constructing scaling laws. The seminal work of Christensen and Aubert (2006) proposed a set of scaling laws that have been used throughout the geoscience community. These scalings make use of specially-constructed parameters that are independent of fluid diffusivities, anticipating that large-scale turbulent processes will dominate the physics in planetary dynamo settings. With these ‘diffusion-free’ parameterizations, the results of current numerical dynamo models extrapolate directly to fully-turbulent planetary core systems; the effects of realistic fluid properties merit no further investigation. In this study, we test the validity of diffusion-free heat transfer scaling arguments and their applicability to planetary conditions. We do so by constructing synthetic heat transfer datasets and examining their scaling properties alongside those proposed by Christensen and Aubert (2006) . We find that the diffusion-free parameters compress and stretch the heat transfer data, eliminating information and creating an artificial alignment of the data. Most significantly, diffusion-free heat transfer scalings are found to be unrelated to bulk turbulence and are instead controlled by the onset of non-magnetic rotating convection, itself determined by the viscous diffusivity of the working fluid. Ultimately, our results, in conjunction with those of Stelzer and Jackson (2013) and King and Buffett (2013) , show that diffusion-free scalings are not validated by current-day numerical dynamo datasets and cannot yet be extrapolated to planetary conditions.

We present the thermochemical evolution of a downward crystallizing basal magma ocean (BMO) overlying the liquid outer core and probe its capability to dissipate enough power to generate and sustain an early dynamo. A total of 61 out of 112 scenarios for a BMO with imposed, present-day heat flux, Q B M O , values of 15, 18, and 21 TW and radiogenic heat, Q r , values of 4, 8, and 12 TW fully crystallized during the age of the Earth. Most of these models are energetically capable of inducing magnetic activity for the first 1.5 Gyrs, at least, with durations extending to 2.5 Gyrs; with final core-mantle boundary (CMB) temperatures of 4400 ± 500 K -well within current best estimates for inferred temperatures. None of the models with Q B M O = 12 TW achieved a fully crystallized state, which may reflect a lower bound on the present-day heat flux across the CMB. BMO-powered dynamos exhibit strong dependence on the partition coefficient of iron into the liquid layer and its associated melting-point depression for a lower mantle composition at near-CMB conditions -parameters which are poorly constrained to date. Nonetheless, we show that a crystallizing BMO is a plausible mechanism to sustain an early magnetic field.

We have examined the density and bulk sound velocity of liquid iron alloys, (Fe, Ni)X(H, Si, O, S, C)1-X, at Earth's outer core pressure and temperature conditions based on first-principles molecular dynamics simulations. The nonideal mixing effects on volume and velocity were found to be negligible for all combinations of different liquid alloys examined. By comparing the results with seismological observations, we searched for possible chemical compositions for the outer core. Hydrogen is found to be a primary light element when the inner-core boundary temperature TICB is 4,800 K to 5,400 K. If this is the case, it is suggested that a large amount of water was delivered to the Earth during its accretionary stage and that the present-day core temperature is relatively low. On the other hand, oxygen is the most important light element if TICB = 6,000 K, consistent with the previous calculations by Badro et al. (2014) at TICB = 6,300 K. To further constrain the chemical composition of the outer core, it is necessary to take into account other constraints besides its density and bulk sound velocity; melting temperature, simultaneous solubilities of multiple of light elements, and so forth.

Bulk translation of the Earth's inner core has been proposed as an explanation of observed quasi-hemispheric seismic structure. An important consequence of inner core translation would be the generation of a spherical harmonic degree one heat flow anomaly at the inner core boundary (ICB) that would provide an inhomogeneous forcing for outer core convection. We use geodynamo simulations to investigate the geomagnetic signature of such heterogeneity. Strong hemispheric heterogeneity at the ICB is found to produce a hemispheric signature in both the morphology of the magnetic field and its secular variation; in particular, we note the formation of high-intensity flux patches at high-latitudes and American longitudes in our model with strong ICB heterogeneity. In our simulations, this model provides the best match to the Earth's field over the past 400 years according to previously proposed measures of field structure. However, these criteria do not include the hemispheric balance of the field. We propose new criteria to measure this balance and find that our model with strong ICB heterogeneity produces the poorest match to the hemispheric balance of the historical geomagnetic field. Resolution of the hemispheric balance of the magnetic field throughout the Holocene would provide a strong test of any proposal of rapid inner core translation.

Phase relations of iron–silicon alloy (Fe–6.5 wt.% Si and Fe–9 wt.% Si) were investigated up to 407 GPa and 5960 K in a laser-heated diamond–anvil cell, which likely covers the entire pressure and temperature conditions of the Earth's inner core. Synchrotron X-ray diffraction measurements show that Fe–9 wt.% Si with a hexagonal close-packed (hcp) structure is stable to 4800 K at 330 GPa, corresponding to the pressure at the inner/outer core boundary, and decomposes into a mixture of Si-poor hcp and Si-rich CsCl-type (B2) phases at higher temperatures. We also found that the solubility of silicon in solid iron is relatively insensitive to temperature, decreasing from 9 to >6.5 wt.% over a range of 1500 K at 70 GPa. These suggest that the inner core is composed solely of the hcp phase, when the silicon content is up to 7 wt.% that likely accounts for the inner core density deficit as well as for the Mg/Si ratio and the Si isotopic composition of the mantle. Additionally, the present experiments demonstrate that the incorporation of silicon in iron expands the stability of hcp with respect to that of fcc.

The Reunion event is one of the earliest recognized periods of normal polarity within the reversed Matuyama chron. Named for the site at which it was first discovered on Reunion Island, it has since purportedly been found globally in both volcanic rocks and sediments, and thus has become a key chronostratigraphic marker. However, geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration, but also the number of Reunion events. New 40Ar/39Ar ages from eight Reunion Island lavas in three distinct sections are indistinguishable from one another, thereby placing the event at 2.200±0.007/0.010 Ma2.200±0.007/0.010 Ma (±2σ±2σ analytical/total uncertainty, note this format is used throughout the paper). The paleomagnetic behavior recorded at two of the island sites shows that the opposite (normal) polarity was reached and sustained for a period during which several lava flows were erupted. Whether this can be classified as a very short subchron bounded by a rapid set of back-to-back reversals, or as a special case of a geomagnetic excursion, is unclear. Hence, we choose to continue labeling the dynamo activity recorded by these Reunion Island lavas as an “event”. This event preceded a ∼38 kyr∼38 kyr period of normal polarity that we name the Feni subchron after its locality of discovery at ODP site 981. The Feni subchron was succeeded by the Huckleberry Ridge excursion for which 40Ar/39Ar sanidine and U–Pb zircon ages of 2.077±0.001/0.003 Ma2.077±0.001/0.003 Ma and 2.084±0.012/0.013 Ma2.084±0.012/0.013 Ma, respectively, from member B of the Huckleberry Ridge tuff in Idaho, are in agreement. These findings suggest that the full normal polarity recorded on Reunion Island is a singular brief period of unstable field behavior at the onset of a ∼125 kyr∼125 kyr bundling of dynamo instabilities from 2.20 to 2.07 Ma. Disturbances to the axial dipole component of earth's magnetic field during this period, and by analogy similar periods of temporally-clustered excursions during the early and late portions of the Brunhes chron, may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner-core or core–mantle boundaries; a proposition that might be tested by future numerical dynamo simulations.

Previous compilations of paleomagnetic studies have suggested that the Proterozoic geomagnetic field may have had as large as 12% quadrupole and 29% octupole components, relative to the dipole intensity. These values would have significant implications for geodynamo secular cooling and outer core dynamics, and for paleogeographic reconstructions. Herein we test the validity of one method, using paleomagnetic remanence of three Proterozoic large igneous provinces (LIPs), as well as younger paleomagnetic datasets, to determine zonal geomagnetic field models. Our results show that individual LIPs are not laterally widespread enough to support claims of significant global non-dipole field components in Proterozoic time. The geocentric axial dipole hypothesis remains viable for Proterozoic paleogeographic and geodynamic models.

Article history: Sound velocities of Fe and Fe-S liquids were determined by combining the ultrasonic measurements and synchrotron X-ray techniques under high pressure-temperature conditions from 1 to 8 GPa and 1573 K to 1973 K. Four different liquid compositions were studied including Fe, Fe-10 wt% S, Fe-20 wt% S, and Fe-27 wt% S. Our data show that the velocity of Fe-rich liquids increases upon compression and decreases with increasing sulfur content, whereas temperature has negligible effect on the velocity of Fe-S liquids. The sound velocity data were combined with ambient-pressure densities to fit the Murnaghan equation of state (EOS). Compared to the lunar seismic model, our velocity data constrain the sulfur content at 4 ± 3 wt%, indicating a significantly denser (6.5 ± 0. 5g /cm3) and hotter (1870 + 100 −70 K) outer core than previously estimated. A new lunar structure model incorporating available geophysical observations points to a smaller core radius. Our model suggests a top-down solidification scenario for the evolution of the lunar core. Such "iron snow" process may have been an important mechanism for the growth of

We performed compressional sound velocity and density measurements on liquid iron at pressures up to 800 GPa with a newly refined shock-compression method using a high-power laser. We found that sound velocity as a function of density can be fitted to a linear relation following Birch's law for hot dense liquid as well as for the solid phase of iron, with a slope ratio between the solid and liquid of approximately 1.5. A comparison of Birch's law for solid and liquid metals indicates that the sound velocity in the liquid phase is about 10% lower than that in the solid phase at melting point density, which is about 1.5 times larger than the initial density. We suggest that these relations between Birch's law coefficients for solid and liquid phases along the Hugoniot are universal for metals.

A variety of studies of the Earth's outer core report wave speeds near the top of the core slightly lower than reference models for core properties. One interpretation of the slower wavespeed profile is that it could represent a change in the core's light element concentration with depth in the core. I explore the consequences of this idea by interpreting the velocity profile as arising from diffusion gradients imposed in the outer core by various mechanisms. In order to estimate relative diffusion rates for light elements in liquid iron I also examine theories for transport properties of high pressure metallic liquids that are based on hard-sphere models. From the seismic wavespeed profile, an effective diffusivity may be obtained, which ranges from 0.1 to 10×10−7 m2s−1 depending on the particular boundary condition or initial condition chosen. The upper bound of the range is higher than expected from high pressure experiments and models of diffusivity in liquid metals for all elements except H. The lower bound is within the uncertainty of theoretical predictions and experimental determinations given the range of expected outer core temperatures if diffusion involves low Z elements. Plausible agreement arises from a class of models that represent diffusion out of a compositionally different layer existing from the time of the formation of the Earth. If the wavespeed profile in the core is diffusive in nature, the data suggest that it is an original feature of the core.

The MgO content of the highest MgO plume-related komatiites and picrites remained constant at 32 ± 2.5 % between 3.5 and 2.7 Ga, then fell to 21 ± 3 % by ca. 2.0 Ga, a value similar to the present day value. Because there is a linear relationship between the liquidus temperature of dry magmas and their MgO content this observation implies that the temperature of mantle plumes changed little between 3.5 and 2.7 Ga, and then fell by 200–250 °C between 2.7 and 2.0 Ga to a temperature similar to that of present plumes. We suggest that Archaean plumes originate from the core–mantle boundary and that their temperature remained constant because the temperature of the outer core was buffered by solidification of the Fe–Ni inner core. At about 2.7 Ga dense former basaltic crust began to accumulate at the core and eventually covered it to produce an insulating layer that reduced the heat flux out of the core and lowered the temperature of mantle plumes. The temperature of mantle plumes fell as the dense layer above the core thickened until it exceeded the critical thickness required for convection. Because heat is transferred rapidly across the convecting part of the insulating layer, any further increase in its thickness by the addition more basaltic material has no influence on the temperature at the top of the layer, which is the source of Post-Archaean mantle plumes. We equate the dense layer above the core with the seismically identified layer D″. Our analyses suggest the drop in plume temperatures produced by a dense insulating layer above the core will be about 40% once it starts to convect, which is consistent with the observed drop inferred from the decrease in the MgO content of komatiites and picrites at that time.

The structure of Earth's deep inner core has important implications for core evolution, since it is thought to be related to the early stages of core formation. Previous studies have suggested that there exists an innermost inner core with distinct anisotropy relative to the rest of the inner core. Using an extensive new data set of handpicked absolute travel time observations of the inner core phase PKIKP, we find that the data are best explained by variations in anisotropy between two hemispheres and do not require an innermost inner core. We demonstrate that observations of an innermost inner core are an artifact from averaging over lateral anisotropy variations. More significantly we show that hemispherical variations in anisotropy, previously only imaged in the upper inner core, continue to its centre. The eastern region has 0.5-1.5% anisotropy, whereas the western region has 3.5-8.8% anisotropy increasing with depth, with a slow direction at 57-61° to the Earth's rotation axis at all depths. Such anisotropy is consistent with models of aligned hcp or bcc iron aggregates.

Differential travel times and waveforms of PKIKP and PKiKP phases in the 129°–141° distance range, deconvolved for the effects of source time functions and average mantle attenuation operators, are inverted for velocity and attenuation in the uppermost 80 km of the inner core, and the velocity gradient and attenuation in the lowermost 200 km of the outer core (F region). Results confirm degree-one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1%. Although attenuation in the inner core also exhibits degree-one pattern, a high attenuation region ( Q p − 1 = [ 0.002 , 0.0067 ] ) extends to the central Pacific, which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation. The rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously. An observed mantle-like relationship between velocity and attenuation in the central Pacific can be interpreted as originating from higher than average homologous temperature, consistent with a model of inner core solidification driven by outer core convection coupled to core–mantle boundary thermal heterogeneity. Alternative mechanisms, including differences in grain size and impurity concentration, and lateral differences in solidification rate might be required to explain the structure of the inner core. The F region velocity and attenuation are best explained by the AK135-F velocity model and Q p − 1 = [ 0 , 0.001 ] respectively.

Tungsten isotopes provide important constraints on the ocean-island basalt (OIB) source regions. Recent analyses of μ182W in modern basalts with high 3He/4He originating from the core-mantle boundary region reveal two distinct features: positive μ182W in Phanerozoic flood basalts indicating the presence of primordial reservoir, and negative μ182W in modern OIBs. One possibility to produce large variations in μ182W is interaction between the mantle and outer core. Here, we report grain boundary diffusion of W in lower mantle phases. High pressure experimental results show that grain boundary diffusion of W is fast and strongly temperature dependent. Over Earth's history, diffusive transport of W from the core to the lowermost mantle may have led to significant modification of the W isotopic composition of the lower mantle at length scales exceeding one kilometer. Such grain boundary diffusion can lead to large variations in μ182W in modern basalts as a function of the distance of their source regions from the core mantle boundary. Modern oceanic island basalts from Hawaii, Samoa and Iceland exhibit negative μ182W and likely originated from the modified isotope region just above the core-mantle boundary, whereas those with positive μ182W could be derived from the thick Large Low Shear Velocity Provinces (LLSVPs) far from the core-mantle boundary (CMB). When highly-oxidized slabs accumulate at the CMB oxidizing the outer core at the interface, a large W flux with negative μ182W can be added to the silicate mantle. As a result, the source region of the OIB would be effectively modified to a negative μ182W.

The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9 wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200 GPa and ~145 GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42 GPa. The minimum temperature of an Fe–Si outer core is 4380 K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, onlymore » the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8 wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases.« less

The P-wave velocity ( V P ) of liquid Fe 57 S 43 was measured up to 5.4 GPa using an ultrasonic method combined with the synchrotron X-ray technique. The V P of liquid Fe 57 S 43 showed little change with temperature, but increased almost linearly from 3105±11 m/s to 3845±9 m/s with increasing pressure from 2.4 to 5.4 GPa. This can be approximated by V P [m/s]=2664+205.4× P , where P is the pressure in GPa. The V P of liquid Fe 57 S 43 at 2.4–5.4 GPa was significantly lower than that of pure liquid Fe. However, the pressure dependence of V P of the liquid Fe 57 S 43 was markedly higher than that of pure liquid Fe. The marked difference in the pressure dependences of V P between pure liquid Fe and liquid Fe 57 S 43 may cause V P crossover at around 7 GPa. As a result, the V P of liquid Fe 57 S 43 would become higher than that of pure liquid Fe at pressures higher than 7 GPa. Thus, S decreases V P at low pressures such as those of the lunar outer core, but would increase it at the high pressures of the Earth's outer core. Assuming the lunar core consists of a liquid Fe–FeS outer core and a solid Fe inner core, the expected V P in the lunar outer core ranges from 3756 to 4230 m/s.

The outer core of the Earth contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is critical to understand the high pressure–temperature properties and behavior of an iron–silicon alloy with a geophysically relevant composition (16 wt% silicon). We experimentally determined the melting curve, subsolidus phase diagram, and equations of state of all phases of Fe–16 wt%Si to 140 GPa, finding a conversion from the D03 crystal structure to a B2+hcp mixture at high pressures. The melting curve implies that 3520 K is a minimum temperature for the Earth's outer core, if it consists solely of Fe–Si alloy, and that the eutectic composition in the Fe–Si system is less than 16 wt% silicon at core–mantle boundary conditions. Comparing our new equation of state to that of iron and the density of the core, we find that for an Fe–Ni–Si outer core, 11.3±1.5 wt% silicon would be required to match the core's observed density at the core–mantle boundary. We have also performed first-principles calculations of the equations of state of Fe3Si with the D03 structure, hcp iron, and FeSi with the B2 structure using density-functional theory.

In situ X-ray diffraction experiments in the Fe–Fe 3 S system were performed up to 175 GPa and 3500 K using a laser-heated diamond anvil cell to investigate melting relationships in the system. Partial melting in the Fe–Fe 3 S system was observed based on the disappearance of X-ray diffraction peaks of solid Fe 3 S and texture observation of the recovered samples. The melting relationship of the Fe–Fe 3 S system as a function of pressure is evaluated based on Kraut–Kennedy law. Our results of melting relationships suggest that the temperature at the inner core boundary is between 4700(160) and 4930(330) K if sulfur is the only light element in the Earth's core. Assuming the adiabatic temperature gradient in the outer core, the temperature at the core–mantle boundary is estimated to be in the range of 3600–3770 K. The present temperature profile of the core is consistent with the core–mantle boundary temperature that can explain the core heat flux to maintain the core dynamo and the seismic structure at the base of the lower mantle.

The seismological exploration of the Earth's inner core has revealed unexpected and puzzling structural complexities. Its elastic anisotropy is now well established, and has been shown to vary spatially. There is a well defined hemispherical dichotomy in anisotropy and attenuation, and significant variations in the radial direction, with weaker anisotropy in the uppermost regions, and possibly different properties in an “innermost inner core” of radius 300–600 km. Perhaps even more puzzling is the observation of a stably stratified layer at the base of the outer core, which has recently been proposed to result from melting of inner core material. If confirmed, the presence of this layer has major implications for our understanding of convection in the outer core. Inner core differential rotation with respect to the mantle remains controversial. We will review here recent progress in understanding the dynamics of the inner core. A number of different models have been proposed for the development of the inner core's structure and formation of the stable layer at the base of the outer core, and particular care will be taken to clarify under which conditions the proposed mechanisms can be active.

Differential rotation is one of the key components needed to maintain a magnetic dynamo, therefore it is important to understand the processes that generate differential rotation in rotating bodies. In a rotating density-stratified fluid, local vorticity generation occurs as fluid parcels move radially, expanding or contracting with respect to the background density stratification. The convergence of this vorticity forms zonal flow structures as a function of the radius and the slope of the background density profile. While this effect is thought to be of importance in bodies that are quickly rotating and highly turbulent with large density stratifications such as Jupiter, it is generally neglected in bodies such as the Earth's outer core, where the density change is small. Simulations of thermal convection in the 2D rotating equatorial plane are conducted to determine the parameter regime where local vorticity generation plays a significant role in organizing the fluid flow. Three regimes are found: a dipolar flow regime, where the flow is not organized by the rotation, a transitional flow regime, and a differential flow regime, where the flow is strongly organized into differential rotation with multiple jets. A scaling law is determined based on the convective Rossby number and the density contrast across the equatorial plane, providing a simple way to determine in which regime a given body lies. While a giant planet such as Jupiter lies firmly in the differential flow regime as expected, the Earth's outer core is also found to lie in the differential flow regime indicating that, even in the Earth's outer core, where the density contrast is small, vorticity contributions via fluid movement through the density stratificationmay be non-negligible.

The gravitational torque applied on the Earth by the other celestial bodies generates periodic variations in the orientation of the Earth's rotation axis in space which are called nutations. Observations of Earth's nutations allow for insights into the physical properties of the inner core because of the presence of a normal mode, the Free Inner Core Nutation (FICN), which is characterized by a tilt of the inner core figure and rotation axes with respect to the mantle and outer core. The frequency of the FICN is controlled by the strength of the mechanical coupling acting at the inner core boundary (ICB) and by the ability of the inner core to deform under the action of centrifugal and gravitational forces. Attenuation of the FICN reflects energy dissipated by electromagnetic (EM) and viscous friction at the ICB and through viscous relaxation of the inner core. Here, we show that it is possible to explain the observed frequency and damping of the FICN by a combination of EM coupling at the ICB and viscoelastic deformation of the inner core. This imposes a strong constraint on the viscosity of the inner core which has to be in the range ~ 2–7 × 1014 Pa s. We also obtain an estimate of the RMS strength of the radial magnetic field at the ICB, which has to be between 4.5 and 6.7 mT. Interestingly, if a viscoelastic Maxwell rheology is assumed for the inner core, our estimated inner core viscosity is in very good agreement with the shear quality factor inferred from seismic normal modes observations. This suggests that the viscous deformation of the inner core at the nutation (diurnal) time scale and at the seismic normal modes time scale may be due to the same physical mechanisms.

The phase fcc FeHx (x ~ 1) was synthesized at pressures over 30 GPa and temperatures over 1600(50) K. At room temperature this iron hydride is stable at pressures from 19(1) GPa up to at least 68(2) GPa (the highest pressure achieved in this study). A fit of the PV data collected for fcc FeHx at room temperature gives the following parameters for the equation of state: V0 = 53.8(3) A3, K0 = 99(5) GPa, K′ = 11.7(5). Using this data the amount of H required to match the density of the Earth's core was estimated to be 0.5–1 wt.% hydrogen in the outer core and 0.08–0.16 wt.% hydrogen in the inner core. Our results also suggest that hydrogen and carbon do not occur together in the Earth's core.

Wustite, Fe(1-x)O. is an important component in the mineralogy of Earth's lower mantle and may also be a component in the core. Therefore the high pressure, high temperature behavior of FeO, including its phase diagram and equation of state, is essential knowledge for understanding the properties and evolution of Earth's deep interior. We performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve simultaneous high pressures and temperatures. Wustite was mixed with iron metal, which served as our pressure standard, under the assumption that negligible oxygen dissolved into the iron. Our data show a positive slope for the subsolidus phase boundary between the B1 and B8 structures, indicating that the B1 phase is stable at the P-T conditions of the lower mantle and core. We have determined the thermal equation of state of B1 FeO to 156 GPa and 3100 K. finding an isothermal bulk modulus K(o) = 149.4 +/- 1.0 GPa and its pressure derivative K(o)' = 3.60 +/- 0.4. This implies that 7.7 +/- 1.1 wt.% oxygen is required in the outer core to match the seismologically-determined density, under the simplifying assumption of a purely Fe-O outer core. (C) 2011 Elsevier B.V. All rights reserved.

The solidus and liquidus temperatures of the Fe75O5S20 alloy are determined up to 157 GPa using a laser-heated diamond anvil cell combined with in situ X-ray diffraction technique. Fe (fcc/hcp), Fe3S2/Fe3S, and FeO B1/B8/rhombohedral phases are stable under subsolidus conditions. First, Fe3S2 or Fe3S phase melts at a temperature close to the eutectic point of the Fe–Fe3S system, suggesting that the alloying effect of 5 at.% oxygen on the eutectic temperature in the Fe–Fe3S system is minor. Then FeO melts at several hundreds of degrees Kelvin higher than the solidus, and Fe is a liquidus phase in this system. The liquidus temperature is 260–670 K lower than the melting temperature of pure Fe because of the alloying effect of S and O on the melting temperature of Fe. Based on our results, the temperatures at the core/mantle boundary (TCMB) and at the boundary of the inner/outer core (TICB) are estimated to be 3600 ± 200

We have carried out in situ X-ray diffraction experiments on the FeS–H system up to 16.5 GPa and 1723 K using a Kawai-type multianvil high-pressure apparatus employing synchrotron X-ray radiation. Hydrogen was supplied to FeS from the thermal decomposition of LiAlH 4 , and FeSH x was formed at high pressures and temperatures. The melting temperature and phase relationships of FeSH x were determined based on in situ powder X-ray diffraction data. The melting temperature of FeSH x was reduced by 150–250 K comparing with that of pure FeS. The hydrogen concentration in FeSH x was determined to be x = 0.2–0.4 just before melting occurred between 3.0 and 16.5 GPa. It is considered that sulfur is the major light element in the core of Ganymede, one of the Galilean satellites of Jupiter. Although the interior of Ganymede is differentiated today, the silicate rock and the iron alloy mixed with H 2 O, and the iron alloy could react with H 2 O (as ice or water) or the hydrous silicate before the differentiation occurred in an early period, resulting in a formation of iron hydride. Therefore, Ganymede's core may be composed of an Fe–S–H system. According to our results, hydrogen dissolved in Ganymede's core lowers the melting temperature of the core composition, and so today, the core could have solid FeSH x inner core and liquid FeH x –FeSH x outer core and the present core temperature is considered to be relatively low.

A heterogeneous heat flux at the core-mantle boundary can maintain time-averaged convective structures in the fluid core. This includes a steady pattern of heterogeneous heat flux at the inner core boundary which leads to aspherical inner core growth. If this growth pattern is longitudinally misaligned with the mantle-induced geoid, the latter would impart a gravitational torque on the newly created topography of the inner core. Allowing for continuous melting/solidification and viscous deformation of the inner core, a steady gravitationally driven differential rotation of the inner core with respect to the mantle can be sustained. In this work, we present calculations of inner core rotation driven by such a mechanism using recently published models of the heat flux at the inner core boundary and of the geoid at the base of the mantle. We show that, for a fast mean inner core growth rate of 1 mm/yr and a bulk viscous relaxation time longer than 100 yr, the inner core differential rotation can be as high as 100 deg/Myr, in the westward direction. Although this is much too slow (and in the wrong direction) to explain the seismically inferred inner core rotation (eastward, of the order of 0.2 deg/yr), this mechanism by itself would rotate the inner core by one full rotation in only a few million years. This gravitational torque would then partly offset the viscomagnetic torque from the steady eastward zonal flow near the inner core boundary, the driving mechanism typically invoked to explain a steady inner core super-rotation. Under the combined action of these two torques, the overall steady differential rotation of the inner core may then be small. This would allow the development of an inner core texture with a distinct longitudinal pattern connected to its aspherical freezing rate, as has been suggested to explain seismic observations.

Paleomagnetic results obtained from rocks of Ediacaran age in several localities in Laurentia and Baltica persistently display co-existence of two magnetization components, one shallowly and the other steeply inclined. Both components pass criteria for a primary magnetization while geological considerations and radiometric age dating indicate that these magnetizations are surprisingly close in age. The conventional interpretation of these results, translating the inclination into paleolatitudes using the geocentric axial dipole hypothesis, would imply that rocks acquired magnetizations in positions switching back and forth between equatorial and near-polar latitudes. In a geographic reference frame, such large-scale and fast (>45 cm/yr) migrations of a continent have been rejected as dynamically implausible; neither plate tectonics nor True Polar Wander are thought to be able to attain the required velocities. A highly irregular behavior of the geomagnetic field during the Ediacaran, possibly an alternation of the geomagnetic dipole axis between a co-axial and an equatorial alignment, remains the only viable explanation for the paleomagnetic data. Such a behavior entails specific outer core conditions, which in turn impose strong constraints on the possible models for the thermal evolution of the Earth's interior.

article Coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some ocean island basalts have been interpreted to reflect material transfer from Earth's outer core to the base of the mantle. The outer core is a viable source for these coupled enrichments if it is able to maintain sufficiently high Pt/Os and Re/Os ratios for sufficient time. Iron meteorite studies and some experimental data indicate that progressive crystallization of the inner core is a plausible mechanism for generating the required high Pt/Os and Re/Os ratios in the outer core. However, the conditions of the experiments and meteorite parent bodies are far different from those in Earth's core, particularly in terms of pressure. Here we present experimental results on the partitioning of Os, Re and Pt betweensolidironandliquidironsulfidesolutionsoverawiderangeofpressures,from3.3to22GPa.Thesolid/ liquid partition coefficients for these elements decrease, and become more similar to each other, as pressure increases. This behavior is consistent with an increasing misfit of each element in the iron lattice with increasing pressure, due to the small compressibility of Os, Re and Pt relative to iron. The partition coefficients, anddifferencesamongthem,aretoosmallattheconditionsof theseexperimentstogeneratetheradiogenicOs isotope anomalies observed in some plume-derived lavas. At 22 GPa the partition coefficients are such that inner core crystallization elevates the Pt/Os ratio of the outer core by only 11%, and the Re/Os ratio by only 6%; after 4.5 Gyr, the calculated 186 Os/ 188 Os ratio of the outer core is only 0.0012% higher than it would be in the

We present a three-dimensional model of the seismic anisotropy and texturing of iron crystals in the inner core. The form of the anisotropy changes suddenly at slightly less than half of the inner core radius. The outer part is composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the Earth. The inner part may be composed of a different phase of crystalline iron or have a different pattern of alignment.

A series of melting experiments in the Fe-rich portion of the Fe–Ni–S system have been conducted at 19–23 GPa and 800–1100 °C. The solubility of S in the Fe–Ni solid alloy and the eutectic melting in the Fe–Ni–S system were determined as a function of Ni content. The maximum S solubility in the Fe–Ni alloy is 2.7 wt.% at 20 GPa and the eutectic temperature. The eutectic melting temperature in the Fe–Ni(5wt.%)–S system is ~ 1000 °C lower than the melting point of pure Fe at 20 GPa. We also found that Ni can substitute Fe in the Fe 3 S structure to form (Fe,Ni) 3 S solid solutions up to at least a Fe/Ni atomic ratio of 0.5. Similar to melting behavior in the Fe–FeS system, the eutectic melting relations in the Fe–Ni–S system could produce inner and outer cores with the right light element balance to account for the density difference between the solid inner core and the liquid outer core.

We present a high-resolution paleomagnetic record from 682 discrete samples from Eltanin 27–21 (69.03°S 179.83°E), a 16-meter long piston core recovered in 1968 at a water depth of 3456 m by the USNS Eltanin as part of Operation Deep Freeze. After removal of a low-coercivity overprint, most samples yield stable characteristic remanent magnetization directions. The downhole variation in the magnetic inclination provides a well-resolved magnetostratigraphy from the Brunhes Chron (0–0.781 Ma), through the Reunion Subchron (2.128–2.148 Ma), and into Chron C2r.2r. The sedimentation rates are sufficiently high that even short-term geomagnetic features, like the Cobb Mountain excursion, are resolved. The record from Eltanin 27–21 provides new insights into the behavior of the geomagnetic field at high latitudes, about which very little is currently known. Using the variability in the inclinations during stable polarity intervals, we estimate that the dispersion in the paleomagnetic pole position over the past ~ 2 Myr is 30.3° ± 4.3°, which is significantly greater than observed at low to mid latitude sites. The higher dispersion observed at Eltanin 27–21 is consistent with numerical modeling of the geodynamo. That modeling has shown that polar vortices can develop in the Earth's core within the tangent cylinder, defined as the cylinder coaxial with the Earth's rotation axis and tangent to the inner core/outer core boundary. The polar vortices produce vigorous fluid motion in the core, which creates greater geomagnetic field variability above the tangent cylinder at the surface of the Earth. The tangent cylinder intersects the Earth's surface in the polar regions at 79.1° latitude, which is relatively close to the latitude of Eltanin 27–21.

Body waves interacting with the boundary of the solid inner core at narrow and wide angles of incidence provide independent constraints on a heterogeneous texture that may originate from the process of solidification. The equatorial, quasi-eastern hemisphere, of the uppermost 50–100 km of the inner core is characterized by a higher isotropic P wave velocity, lower Qα's inferred from PKIKP, and simpler PKiKP pulses compared to adjacent regions in the western hemisphere and polar latitudes. Compared to this region, the adjacent western (primarily Pacific) equatorial region is characterized by higher Qα's and a higher level of coda excitation following PKiKP. Lateral variations in both inner core Qα inferred from transmitted PKIKP and inner core heterogeneity inferred from the coda of reflected PKiKP can be modeled by lateral variations in a solidification fabric. In an actively crystallizing eastern equatorial region, characterized by upwelling flow in the outer core, fabrics that explain strong attenuation and the absence of attenuation and velocity anisotropy in short range (120°–140°) PKIKP and weak PKiKP codas have an anisotropy of scale lengths with longer scale lengths in the vertical direction, perpendicular to the inner core boundary. In less actively solidifying regions in the equatorial western hemisphere, longer scale lengths tend to be more parallel to the inner core boundary, consistent with outer core flow tangent to the inner core boundary or viscous shearing and recrystallization in the horizontal direction away from more actively crystallizing regions in the eastern hemisphere. This texture is less effective in attenuating PKIKP by forward scattering. Lateral variation in the equatorial western hemisphere between vertical versus horizontal oriented plate-like textures may explain lateral variations from weak to strongly back-scattered PKiKP coda and from strong to weak velocity and attenuation anisotropy in short range PKIKP. © 2007 Elsevier B.V. All rights reserved.

Although oxygen is a possible light element in the Earth's core, the effect of pressure on the concentration of this element in metallic iron has been a controversial issue for the last 20 yr. Completely opposite pressure effects have been advocated based on studies of phase relations and element partitioning, respectively. Here we report new data on the partitioning of FeO between magnesiowustite and liquid iron over a wide pressure–temperature range (3–25 GPa and 2273–3200 K). The proportion of FeO partitioning into liquid iron decreases with increasing pressure below 10 GPa but increases with increasing pressure above 10–15 GPa. The change in the pressure effect is caused by the Fe + O component having different compressibilities in magnesiowustite and liquid Fe respectively. The new experimental data, together with results from previous studies obtained over a wide P–T range (2–139 GPa, 2273–3150 K), have been fitted by a thermodynamic model that enables the results to be extrapolated to conditions of the outer core. Assuming core–mantle equilibrium, the results show that the outer core is undersaturated in oxygen, which causes a thin layer at the very base of the mantle to be strongly depleted in FeO. The results of core formation modeling indicate that oxygen is likely to be the main light element in the Earth's core (e.g. 7–8 wt.%) and that the FeO content of the proto-earth may have been similar to that of present day Mars (e.g. 18 wt.% FeO in the mantle).