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The Earth's core is composed mainly of iron. Since the liquid core coexists with solid at the inner core boundary (ICB), the melting point of iron at 330 GPa offers a key constraint on core temperatures. However, previous results using a laser-heated diamond-anvil cell (DAC) have been largely inconsistent with each other, likely because of an intrinsic large temperature gradient and its temporal fluctuation. Here we employed an internal-resistance-heated DAC and determined the melting temperature of pure iron up to 290 GPa, for the first time above 200 GPa by static compression experiments. A small extrapolation of the present experimental results yields a melting point of 5500 ± 220 K at the ICB, higher than 4850 ± 200 K reported by previous laser-heated DAC by Boehler (1993) but is lower than 6230 ± 500 K by Anzellini et al. (2013) . Accounting for the melting temperature depression due to core-alloying elements, the upper bounds for the temperature at the ICB and the core–mantle boundary (CMB) are estimated to be 5120 ± 390 K and 3760 ± 290 K, respectively. Such low present-day CMB temperature suggests that the lowermost mantle has avoided global melting, at least since early Proterozoic Eon.

SiO2 may have been expelled from the core directly following core formation in the early stages of Earth's accretion and onward through the present day. On account of SiO2's low density with respect to both the core and the lowermost mantle, we examine the process of SiO2 accumulation at the core‐mantle boundary (CMB) and its incorporation into the mantle by buoyant rise. Today, if SiO2 is 100–10,000 times more viscous than lower mantle material, the dimensions of SiO2 diapirs formed by the viscous Rayleigh‐Taylor instability at the CMB would cause them to be swept into the mantle as inclusions of 100 m–10 km diameter. Under early Earth conditions of rapid heat loss after core formation, SiO2 diapirs of ∼1 km diameter could have risen independently of mantle flow to their level of neutral buoyancy in the mantle, trapping them there due to a combination of intrinsically high viscosity and neutral buoyancy. We examine the SiO2 yield by assuming Si + O saturation at the conditions found at the base of a magma ocean and find that for a range of conditions, dispersed bodies could reach as high as 8.5 vol % in parts of the lower mantle. At such low concentration, their effect on aggregate seismic wave speeds is within observational seismology uncertainty. However, their presence can account for small‐scale scattering in the lower mantle due to the bodies' large‐velocity contrast. We conclude that the shallow lower mantle (700–1,500 km depth) could harbor SiO2 released in early Earth times.

Silicate perovskite-type phases are the most abundant constituent inside our planet and are the predominant minerals in Earth’s lower mantle more than 660 kilometers below the surface. Magnesium-rich perovskite is a major lower mantle phase and undergoes a phase transition to post-perovskite near the bottom of the mantle. Calcium-rich perovskite is proportionally minor but may host numerous trace elements that record chemical differentiation events. The properties of mantle perovskites are the key to understanding the dynamic evolution of Earth, as they strongly influence the transport properties of lower mantle rocks. Perovskites are expected to be an important constituent of rocky planets larger than Mars and thus play a major role in modulating the evolution of terrestrial planets throughout the universe.

Terrestrial planets are thought to experience episode(s) of large-scale melting early in their history. Fractionation during magma-ocean freezing leads to unstable stratification within the related cumulate layers due to progressive iron enrichment upward, but the effects of incremental cumulate overturns during MO crystallization remain to be explored. Here, we use geodynamic models with a moving-boundary approach to study convection and mixing within the growing cumulate layer, and thereafter within the fully crystallized mantle. For fractional crystallization, cumulates are efficiently stirred due to subsequent incremental overturns, except for strongly iron-enriched late-stage cumulates, which persist as a stably stratified layer at the base of the mantle for billions of years. Less extreme crystallization scenarios can lead to somewhat more subtle stratification. In any case, the long-term preservation of at least a thin layer of extremely enriched cumulates with Fe# > 0.4, as predicted by all our models, is inconsistent with seismic constraints. Based on scaling relationships, however, we infer that final-stage Fe-rich magma-ocean cumulates originally formed near the surface should have overturned as small diapirs, and hence undergone melting and reaction with the host rock during sinking. The resulting moderately iron-enriched metasomatized/hybrid rock assemblages should have accumulated at the base of the mantle, potentially fed an intermittent basal magma ocean, and be preserved through the present-day. Such moderately iron-enriched rock assemblages can reconcile the physical properties of the large low shear-wave velocity provinces in the present-day lower mantle. Thus, we reveal Hadean melting and rock-reaction processes by integrating magma-ocean crystallization models with the seismic-tomography snapshot.

International audience; The crystallization of a magma ocean (MO) early in Earth's history shaped the entire evolution of our planet. The buoyancy relations between the forming crystals and the residual melt is the most important but also the most unknown parameter affecting the large-scale structure and evolution of the MO. The accumulation of crystals, near the depth of neutral buoyancy between crystals and the coexisting melt, if happening at mid-depths, can separate convecting regions within the MO. Here we use jointly first-principles molecular-dynamics calculations and diamond-anvil cell experiments to obtain the density relations between the molten bulk silicate Earth and the bridgmanite crystals during the crystallization of the MO. The chemical evolutions of the liquid and the coexisting solid during progressive crystallization were constrained by experiments, and the relevant densities were calculated by molecular dynamics. We find that the first crystal of bridgmanite that is formed in a fully molten mantle is Fe-poor, and becomes neutrally buoyant at 110-120 GPa. Since the cooling of the deep MO is fast, and related convection is vigorous, however, first crystals remain entrained. As crystallization advances, the relative Fe content increases in the melt, and the pressure of neutral buoyancy decreases. At 50% solidification, close to the rheological transition, the pressure of the density crossover moves to ∼50 GPa. At this pressure, crystals form an interconnected network and block global convection currents, which in turn leads to the separation of the partly crystallized MO into a surficial MO and a basal MO through melt-solid segregation. Such a shallow segregation of a crystal mush at mid-mantle depth has important implications for the dynamics and timescales of early mantle differentiation. Moreover, the shallow segregation should have promoted the formation of a voluminous basal MO that evolves into a large geochemically enriched reservoir. Accordingly, the seismically observed residues of basal MO crystallization in the present-day mantle may host an unmixed reservoir for the missing budget of highly incompatible elements.

Banded iron formations (BIFs), consisting of layers of iron oxide and silica, are far denser than normal mantle material and should have been subducted and sunk into the deep lower mantle. We performed melting experiments on Fe2SiO4 from 26 to 131 GPa in a laser-heated diamond-anvil cell (DAC). The textural and chemical characterization of a sample recovered from the DAC revealed that SiO2 is the liquidus phase for the whole pressure range examined in this study. The chemical compositions of partial melts are very rich in FeO, indicating that the eutectic melt compositions in the FeO SiO2 binary system are very close to the FeO end-member. The eutectic temperature is estimated to be 3540 ± 150 K at the core–mantle boundary (CMB), which is likely to be lower than the temperature at the top of the core at least in the Archean and Paleoproterozoic eons, suggesting that subducted BIFs underwent partial melting in a thermal boundary layer above the CMB. The FeO-rich melts formed by partial melting of the BIFs were exceedingly dense and therefore migrated downward. We infer that such partial melts have caused iron enrichment in the bottom part of the mantle, which may have contributed to the formation of ultralow velocity zones (ULVZs) observed today. On the other hand, solid residues left after the segregation of the FeO-rich partial melts have been almost pure SiO2, and therefore buoyant in the deep lower mantle to be entrained in mantle upwellings. They have likely been stretched and folded repeatedly by mantle flow, forming SiO2 streaks within the mantle “marble cake”. Mantle packages enhanced by SiO2 streaks may be the origin of seismic scatterers in the mid-lower mantle.

The composition of the lower mantle $-$ comprising 56% of Earth's volume $-$ remains poorly constrained. Among the major elements, Mg/Si ratios ranging from $\sim$0.9$-$1.1, such as in rocky solar-system building blocks (or chondrites), to $\sim$1.2$-$1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing and thus homogeneous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity with viscosity variations of $\sim$20$\times$, such as due to the dominance of intrinsically strong (Mg,Fe)SiO$_3-$bridgmanite in low-Mg/Si domains, are sufficient to prevent efficient mantle mixing, even on large scales. Models predict that intrinsically strong domains stabilize degree-two mantle-convection patterns, and coherently persist at depths of $\sim$1,000$-$2,200 km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such "bridgmanite-enriched ancient mantle structures" (BEAMS) may reconcile the geographical fixity of deep-rooted mantle-upwelling centers, and fundamental geophysical changes near 1,000 km depth (e.g. in terms of seismic-tomography patterns, radial viscosity increase, lateral deflections of rising plumes and sinking slabs). Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.

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.

Melting experiments on a pyrolitic mantle material were performed in a pressure range from 34 to 179 GPa based on laser-heated diamond-anvil cell (DAC) techniques. The textural and chemical characterizations of quenched samples were made by using field-emission-type electron microprobe (FE-EPMA). Melts formed by 46 to 77 wt.% partial melting in this study were ultrabasic in composition and became more depleted in SiO2 and more enriched in FeO with increasing pressure. Melting textures indicate that the liquidus phase changed from ferropericlase to MgSiO3-rich perovskite at least above 34 GPa and further to post-perovskite. The first phase to melt (disappear) changed from CaSiO3 perovskite to (Mg,Fe)O ferropericlase between 68 and 82 GPa. The stability of ferropericlase above solidus temperature shrinks with increasing pressure (melting last below 34 GPa and first 82 GPa), resulting in higher (MgO + FeO)/SiO2 ratio in partial melt at higher pressure. Additionally, the Fe-Mg distribution coefficients (KD) between perovskite/post-perovskite and melt decreased considerably with increasing pressure, leading to strong Fe-enrichment in partial melts. It supports dense partial melts in a deep lower mantle, which migrate downward to the core mantle boundary (CMB).

Ultrahigh-pressure and -temperature (P-T) experimental techniques have progressed rapidly in recent years. By combining them with X-ray diffraction measurements at synchrotron radiation facilities, it is now possible to examine deep Earth mineralogyin situat relevant highP-Tconditions in a laser-heated diamond anvil cell (DAC). The lowermost part of the mantle, known as the D″ layer, has long been enigmatic because of a number of unexplained seismological features. Nevertheless, the discovery of a phase transition from MgSiO3perovskite to ‘post-perovskite’ above 120 GPa and 2400 K indicates that post-perovskite is a principal constituent in the lowermost mantle, which is compatible with seismic observations. The ultrahighP-Tconditions of the Earth’s core have not been accessible by static experiments, but the structure and phase transition of Fe and Fe-alloys are now being examined up to 400 GPa and 6000 K by laser-heated DAC studies.

Understanding effects of non-hydrostatic pressure on phase transitions in minerals relevant to the Earth’s mantle is important to translate the observable seismic signals to not directly observable mineralogical models for the deep Earth. SiO 2 can occur as a free phase in subducting slabs, which contain sedimentary layers and/or mid-ocean-ridge basalts. In this study, we report on the effect of deviatoric strain on the pressure-induced phase transition in SiO 2 and its consequences on the seismic signal. The acoustic velocity in polycrystalline stishovite across the post-stishovite phase transition was measured by Brillouin scattering in the pure SiO 2 system at room temperature under deviatoric stress. High-pressure synchrotron X-ray diffraction data were also collected at SPring-8. A linear fit to the symmetry-breaking strain values and the pressure of the transverse velocity minimum indicate a transition pressure between 25 and 35 GPa, which is about 20 GPa lower than that under hydrostatic conditions. The transverse velocity dropped by about 3% at around 25 GPa in this study. This is much smaller than the prediction from ab initio calculations that a transverse velocity reduces by ~60% at around 50 GPa under hydrostatic conditions. The results of the present study indicate that the deviatoric stress lowers the transition pressure and reduces the acoustic velocity change associated with the post stishovite phase transition. Sedimentary and mid ocean ridge basalt (MORB) layers in a subducting slab are likely sites for finding stishovite and its high-pressure polymorphs in the deep earth. Seismic observations of deep earthquakes occurring in subducting slabs indicate the existence of considerable stress in down-going slabs. This study suggests that nonhydrostatic deviatoric stress is one of the possible reasons for the absence of general seismic features that can be directly related to the post-stishovite phase transition in subducting slabs at 1500 km depth. The phase transition of stishovite under deviatoric stress, which occurs at shallower depths, can affect the local seismic scattering structures and the rheological behavior of a subducting slab in the mid-lower mantle region.

In order to examine pressure–volume–temperature (P–V–T) relations for CaSiO3 perovskite (Ca-perovskite), high-temperature compression experiments with in situ X-ray diffraction were performed in a laser-heated diamond anvil cell (DAC) to 127 GPa and 2,300 K. We also employed an external heating system in the DAC in order to obtain P–V data at a moderate temperature of 700 K up to 113 GPa, which is the reference temperature for constructing an equation of state. The P–V data at 700 K were fitted to the second-order Birch–Murnaghan equation of state, yielding K 700,1bar = 207 ± 4 GPa and V 700,1bar = 46.5 ± 0.1 A3. Thermal pressure terms were evaluated in the framework of the Mie–Gruneisen–Debye model, yielding γ 700,1bar = 2.7 ± 0.3, q 700,1bar = 1.2 ± 0.8, and θ 700,1bar = 1,300 ± 500 K. A thermodynamic thermal pressure model was also employed, yielding α700,1bar = 5.7 ± 0.5 × 10−5/K and (∂K/∂T) V = −0.010 ± 0.004 GPa/K. Computed densities along a lower mantle geotherm demonstrate that Ca-perovskite is denser than the surrounding lower mantle, suggesting that Ca-perovskite-rich rocks do not rise up through the lower mantle. One of such rocks might be a residue of partial melting of subducted mid-oceanic ridge basalt (MORB) at the base of the mantle. Since the partial melt is FeO-rich and therefore denser than the mantle, all the components of subducted MORB may not return to shallow levels.

Thermal conductivity is essential for controlling the rate of core heat loss and long-term thermal evolution of the Earth, but it has been poorly constrained at the high pressures of Earth's lowermost mantle. We measured the lattice component of thermal diffusivity, heat transport by scattering of phonons, of both MgSiO 3 perovskite (Pv) and post-perovskite (PPv) at high pressures of up to 144 GPa and at room temperature. Lattice thermal conductivity of Pv-dominant lowermost mantle assemblage obtained in this study is about 11 W/m/K, while PPv-bearing rocks exhibit ∼60% higher conductivity. Since such Pv value is comparable to the conventionally assumed lowermost mantle conductivity, our findings do not significantly alter but support the recent notion of high core–mantle boundary heat flow along with a young inner core and high temperatures in the early deep Earth.

We report the measurements of aggregate shear velocity ( V S ) of CaSiO 3 perovskite (CaPv) at high pressure ( P ) between 32 and 133 GPa and room temperature ( T ) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured P – V S data show the shear modulus and its pressure derivative at ambient condition to be G 0 =115.8 GPa and G '=1.20, respectively. The zero-pressure shear velocity is determined to be V S0 =5.23 km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O 3 perovskite at 1 bar. Our experimental results are broadly consistent with the earlier calculations on tetragonal CaPv but exhibit lower velocity at equivalent pressure. Such tetragonal CaPv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle. While primitive mantle includes certain amount of CaPv, a depleted peridotite (former harzburgite) layer in subducted oceanic lithosphere is deficient in CaPv and enriched in ferropericlase in the lower mantle. Such harzburgite exhibits 0.9% faster V S and 0.7% slower bulk sound velocity ( V Φ ) at the lowermost mantle P – T conditions if CaPv is present in the tetragonal form in the surrounding mantle. The observed fast V S and slow V Φ anomalies in the D” layer underneath the circum-Pacific region might be attributed in large part in the presence of subducted harzburgitic materials.

The relative densities of melt and solid in the deep mantle have important implications for the chemical evolution of our planet. Recent theoretical calculations suggested that a density crossover may occur in the deepest mantle, but the important effect of the melt–solid partitioning of iron has been examined only at relatively low pressures (less than 25 gigapascals). Nomura et al. extend measurements of iron partitioning between (Mg,Fe)SiO3 perovskite and melt over the entire mantle pressure range, and find that a precipitous change occurs at about 76 GPa, resulting in stronger iron enrichment in melts at higher pressures. These results imply that liquid (Mg,Fe)SiO3 becomes more dense than coexisting solid at a depth of about 1,800 kilometres in the mantle. A melt has greater volume than a silicate solid of the same composition. But this difference diminishes at high pressure, and the possibility that a melt sufficiently enriched in the heavy element iron might then become more dense than solids at the pressures in the interior of the Earth (and other terrestrial bodies) has long been a source of considerable speculation1,2. The occurrence of such dense silicate melts in the Earth's lowermost mantle would carry important consequences for its physical and chemical evolution and could provide a unifying model for explaining a variety of observed features in the core–mantle boundary region3. Recent theoretical calculations4 combined with estimates of iron partitioning between (Mg,Fe)SiO3 perovskite and melt at shallower mantle conditions5,6,7 suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments8. Here we extend measurements of iron partitioning over the entire mantle pressure range, and find a precipitous change at pressures greater than ∼76 GPa, resulting in strong iron enrichment in melts. Additional X-ray emission spectroscopy measurements on (Mg0.95Fe0.05)SiO3 glass indicate a spin collapse around 70 GPa, suggesting that the observed change in iron partitioning could be explained by a spin crossover of iron (from high-spin to low-spin) in silicate melt. These results imply that (Mg,Fe)SiO3 liquid becomes more dense than coexisting solid at ∼1,800 km depth in the lower mantle. Soon after the Earth's formation, the heat dissipated by accretion and internal differentiation could have produced a dense melt layer up to ∼1,000 km in thickness underneath the solid mantle. We also infer that (Mg,Fe)SiO3 perovskite is on the liquidus at deep mantle conditions, and predict that fractional crystallization of dense magma would have evolved towards an iron-rich and silicon-poor composition, consistent with seismic inferences of structures in the core–mantle boundary region.

The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75 GPa and 300–2490 K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive dP/dT slope of 11.1 MPa/K and the transition occurs about 70 GPa at 2200 K along the typical mantle geotherm. This corresponds to a depth of 1700 km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850 km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO2 phase containing Al2O3 and H2O or by the phase transition in Al-bearing CaSiO3 perovskite from cubic to tetragonal structure.

High-temperature compression experiments with in situ X-ray diffraction of ferropericlase (Fp) with a composition of (Mg 0.81 Fe 0.19 )O were made in a laser-heated diamond anvil cell to pressures ( P ) of 116 GPa at a constant temperature ( T ) of 1600–1900 K. Room-temperature experiments with a laser annealing technique were also carried out on the same material. Anomalous unit-cell volume reductions that can be explained by the spin transition of ferrous iron were observed at P = 63–96 GPa and 45–63 GPa at T = 1600–1900 K and 300 K, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The observed density changes across this spin transition at T = 1600–1900 K and 300 K are about 1.6% and 1.0%, respectively, indicating that the spin transition pressure interval expands with increasing temperature. The thermal expansivity of Fp is large in the mid-lower mantle due to the effect of the spin transition. In a peridotitic composition, the spin transition in Fp increases the rock density by 0.35% at the lowermost mantle conditions. Calculated densities show that both perovskitic and peridotitic mantle models may explain the PREM lower mantle density. However, the peridotitic lower mantle model requires less assumption to satisfy the PREM density and is more self-consistent.

The electrical conductivities of natural pyrolitic mantle and MORB materials were measured at high pressure and temperature covering the entire lower mantle conditions up to 133 GPa and 2650 K. In contrast to the previous laboratory-based models, our data demonstrate that the conductivity of pyrolite does not increase monotonically but varies dramatically with depth in the lower mantle; it drops due to high-spin to low-spin transition of iron in both perovskite and ferropericlase in the mid-lower mantle and increases sharply across the perovskite to post-perovskite phase transition at the D″ layer. We also found that the MORB exhibits much higher conductivity than pyrolite. The depth–conductivity profile measured for pyrolite does not match the geomagnetic field data below about 1500-km depth, possibly suggesting the existence of large quantities of subducted MORB crust in the deep lower mantle. The observations of geomagnetic jerks suggest that the electrical conductivity may be laterally heterogeneous in the lowermost mantle with high anomaly underneath Africa and the Pacific, the same regions as large low shear-wave velocity provinces. Such conductivity and shear-wave speed anomalies are also possibly caused by the deep subduction and accumulation of dense MORB crust above the core–mantle boundary.

We have determined the phase transition boundary between NaCl-type (B1) and NiAs-type (B8) structures of FeO up to 208 GPa and 3800 K on the basis of synchrotron X-ray diffraction (XRD) measurements in situ at high pressure and temperature using a laser-heated diamond-anvil cell (DAC). The boundary is located at 200 GPa and 3200 K with positive Clapeyron slope. These results show that B1 phase of FeO is stable along the whole mantle geotherm, whereas B8 phase is stabilized at the inner core condition. Additionally, FeO coexisted with metallic Fe in the present experiments. We found that hexagonal close-packed (hcp) iron is stable over the entire present experimental conditions. Moreover, the direct chemical analyses of the recovered sample demonstrated that solid iron did not contain any detectable oxygen. While extensive solid solution between Fe and FeO has been speculated above 60–80 GPa, the present results strongly suggest that the system Fe–FeO is simple eutectic at least to 200 GPa pressure range.

We report in-situ petrological evidence of deep mantle origin from podiform chromitites in the Luobusa ophiolite, southern Tibet. Analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the Luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite, which indicate an ultrahigh-pressure origin of over 3 GPa (> 100 km deep). The presence of these lamellae, coupled with abundant micro-inclusions of clinopyroxene, requires high solubility of SiO2 and CaO in the host chromite, and suggests a precursor of chromite, a CaFe2O4-structured high-pressure polymorph, stable at pressures over 12.5 GPa (> 380 km deep). These nano-scale observations and geological occurrence indicate that the mantle peridotite under the Tibetan mid-ocean ridge was transported from the deep mantle (at least 100 km, probably more than 380 km deep) by the mantle convection. It implies that the root of mantle upwelling has a much deeper origin than previously believed.

Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO3 perovskite was investigated by a combination of laser-heated diamond-anvil cell (LHDAC) and analytical transmission electron microscope (TEM) to 146 GPa and 3,500 K. The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS). The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature (P–T). The chemical equilibrium between perovskite, ferropericlase, and molten iron at the P–T conditions of the core–mantle boundary (CMB) was calculated in Mg–Fe–Si–O system from these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase. We found that molten iron should include oxygen and silicon more than required to account for the core density deficit (

A phase transition of MgSiO3 perovskite, the most abundant component of the lower mantle, to a higher-pressure form called post-perovskite was recently discovered for pressure and temperature conditions in the vicinity of the Earth's core-mantle boundary. This discovery has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle called the D” region. Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. Here we discuss the current understanding of the core-mantle boundary region.

Subduction of mid-oceanic ridge basalt (MORB) gives rise to strong chemical heterogeneities in the Earth's mantle, possibly extending down to the core–mantle boundary. Phase relations in both pyrolite and MORB compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions. The results demonstrate that the post-perovskite phase transition occurs in pyrolite between 116 and 121 GPa at 2500 K, while post-perovskite and SiO2 phase transitions occur in MORB at ∼ 4 GPa lower pressure at the same temperature. Theory predicts that these phase changes in pyrolite and MORB cause shear wave velocity increase and decrease, respectively. Near the northern margin of the large low shear velocity province in the lowermost mantle beneath the Pacific, reflections from a negative shear velocity jump near 2520-km depth are followed by reflections from a positive velocity jump 135 to 155-km deeper. These negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of MORB and pyrolitic material. This may be a direct demonstration of the presence of accumulations of subducted MORB crust in the deep mantle.

Simultaneous volume measurements of MgSiO 3 post-perovskite (PPv) and perovskite (Pv) were performed in a diamond anvil cell (DAC) combined with synchrotron X-rays. An externally-heated DAC was used in addition to a laser-heated DAC for the volume measurement experiment at high temperatures. The volume data were collected in the stability field of post-perovskite from 115 to 130 GPa. The temperature generated in the externally-heated and the laser-heated DACs for the volume measurement were up to 832 and 2330 K, respectively. Using two different but complementary heating techniques, we collected the data at a wide temperature range from 300 to 2330 K. The obtained P-V-T data for PPv and Pv were fitted to a third-ordered Birch-Murnaghan equation of state (EOS). For a precise comparison of the volume between the two phases, the EOSs were constructed based on the same pressure scale of MgO. The simultaneous volume measurements and the volumes calculated from the determined EOSs demonstrate that the volume difference between PPv and Pv of about 1.5% is almost constant with increasing temperature to 4000 K at the transition. At the base of the mantle, this density difference corresponds to a temperature anomaly of 1300 K without the phase transition due to the very small thermal expansivity of minerals, which has a significant effect on mantle dynamics. The thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3.6. From a mantle convection study, this value suggests that huge and hot plumes are formed at the core–mantle boundary.

The discovery of a post-perovskite phase transition in MgSiO 3 has significant implications for seismological observations in the D ″ region at the bottom of Earth's mantle. The D ″ discontinuity, which is manifested as a sharp positive seismic-wave velocity jump 200–300 km above the core–mantle boundary (at pressure of 119∼ 125 GPa), is one of the most enigmatic seismic features in this region. Whether this velocity increase may be due to the formation of a post-perovskite phase at the D ″ discontinuity has not, however, been directly addressed by experiments. Here we present the results of aggregate sound velocity measurements of the MgSiO 3 post-perovskite phase by Brillouin spectroscopy in the diamond anvil cell (DAC) up to a pressure of 172 GPa, in combination with infrared laser annealing of the sample. Based on these results and our recent high-pressure velocity measurements on perovskite, the aggregate shear wave velocity contrast across the perovskite to post-perovskite phase transition is at most 0.5%. This contrast is much smaller than typically observed across the D ″ discontinuity, indicating that the formation of an isotropic aggregate of the post-perovskite phase provides an insufficient velocity increase to explain the D ″ discontinuity. Lattice preferred orientation (LPO) of post-perovskite is likely to be crucial for explaining the D ″ discontinuity.

Processes that operated in the early Earth have largely been erased or overprinted by subsequent evolution. However, some traces may persist in the deep Earth as imaged by seismology. Large‐scale features with reduced seismic velocities are simply explained as variations of composition, and small‐scale ultra‐low velocity zones are explained by the presence of Fe‐rich material that may be partially molten. Both can originate from fractional crystallization of an originally thick basal magma ocean (BMO). Many questions are raised by this scenario regarding properties of melts with various compositions, in particular the partition coefficients between melt and crystals of various elements and their relative densities. After reviewing recent progress on both the structure of the lower mantle and the mineral physics associated with partial melting/freezing of silicates at high pressure, we discuss several ways in which a BMO can be produced. We argue that, in most cases, independently of whether a melt of composition similar to that of the bulk mantle is more or less dense than crystals in equilibrium, the compositional evolution of both the magma and the solid should lead to the formation of a dense, Fe‐rich BMO whose subsequent slow evolution would explain some features of the present lower mantle. 1 Laboratoire de geologie de Lyon, ENS de Lyon, Universite Lyon-1, CNRS, Lyon, France 2 Earth‐Life Science Institute, Tokyo Institute of Technology, Meguro, Tokyo, Japan c07.indd 123 8/24/2015 4:02:55 PM

The phase equilibria, structures, and properties of a high-pressure polymorph of silicate perovskite, called post-perovskite, are discussed. This phase was discovered in 2004 and provides a viable candidate for a deep mantle phase transition in the primary lower mantle mineral, silicate perovskite. Post-perovskite may exist in regions of the lowermost mantle at temperatures below about 3,000 K, and the phase transition from perovskite to post-perovskite may explain observations of a velocity discontinuity in seismic S waves and relatively strong anisotropy of the lower most mantle D″ region. The phase transition has important dynamical consequences for heat transport and boundary layer instabilities.

Here, we precisely determined the low-pressure stability limit of phase A in the Mg-end-member antigorite bulk composition defined as the reaction forsterite + water = phase A + enstatite (“water-line” or “water-storage line”) in a multi-anvil apparatus. Pressures were determined by in situ synchrotron X-ray diffraction measurements using NaCl as an internal pressure standard. Results demonstrate that the water-line is located at 800 °C and 8.5 GPa and at 550 °C and 5.1 GPa with a Clapeyron slope of 13.6 MPa/°C. We also examined the conditions for the formation of phase A by the decomposition of antigorite on the basis of the phase relations in the systems MgO–SiO 2 –H 2 O (MSH) and MgO–Al 2 O 3 –SiO 2 –H 2 O (MASH). A descending slab peridotite retains water in phase A beyond the stability of antigorite only when temperature in the slab is lower than 550 or 660 °C at 5.1 GPa (corresponding to 160 km depth), in the system MSH or MASH, respectively. The double seismic planes observed within the slabs may be caused by the dehydration reaction of antigorite or chlorite. Their merging depths, which could represent the high-pressure stability limit of antigorite or chlorite, are 160 km or deeper beneath northeast Japan, Aleutian, Kamchatka and Kuril. Water included in hydrous slab peridotite is transported into the transition zone and deeper mantle in these areas but recycled to the surface in other subduction zones.

Phase relations, mineral chemistry, and density of a natural mid-oceanic ridge basalt (MORB) composition were investigated up to 134 GPa and 2300 K by a combination of in-situ X-ray diffraction measurements and chemical analyses using transmission electron microscope (TEM). Results demonstrate that the MORB composition consists of MgSiO 3 -rich perovskite, stishovite, CaSiO 3 perovskite, and CaFe 2 O 4 -type Al-phase in the upper part of the lower mantle. The most abundant mineral of MgSiO 3 -rich perovskite undergoes phase transition to a CaIrO 3 -type post-perovskite phase above 110 GPa and 2500 K. Post-perovskite phase is similar in composition to perovskite except considerably high Na 2 O content. Stishovite transforms to CaCl 2 -type SiO 2 phase above 62 GPa and 2000 K and further to α-PbO 2 -type phase above 110 GPa. α-PbO 2 -type SiO 2 phase includes large amount of Al 2 O 3 , which significantly expands its stability relative to CaCl 2 -type phase. Phase transition of CaSiO 3 perovskite from tetragonal to cubic was also observed with increasing temperature. CaFe 2 O 4 -type Al-phase is stable to the bottom of the mantle. The density of MORB crust was calculated using volume data, combining with measured chemical compositions and calculated mineral proportions. The former MORB crust is denser than the average lower mantle at all depths greater than ∼ 720 km, contrary to earlier predictions. The subducted basaltic crust may have accumulated at the base of the mantle.

We measured the dihedral angle of molten iron in (Mg,Fe)SiO3-perovskite aggregate with increasing pressure and temperature using laser-heated diamond anvil cell. Results demonstrate that it decreases from 94° at ∼27 GPa and ∼2400 K to 51° at ∼47 GPa and ∼3000 K. This value is smaller than the critical angle of 60°, thus allowing iron melt to wet the grain boundaries of silicate perovskite and develop an interconnected melt network within the perovskite-dominant matrix, even at very small melt fractions. The quenched liquid iron contained substantial amounts of oxygen and silicon as pressure and temperature increase. Such a decrease in the dihedral angle is likely due to a reduction in the iron-perovskite interfacial energy by dissolving oxygen and silicon into the liquid iron from coexisting silicate perovskite. These suggest that a wetting behaviour of core melts in the solid silicate mantle changes above ∼40 GPa, corresponding to ∼1000 km depth in the present Earth, and efficient metal segregation may have proceeded by permeable flow in a late stage of accretion. Oxygen and silicon incorporated during core formation and later by core–mantle boundary process may be important light elements in the Earth's core.

Trace element partitioning data between CaSiO3-perovskite (CaPv), MgSiO3-perovskite (MgPv), calcium–aluminum silicate (CAS-phase), and coexisting melts in peridotite and mid-ocean ridge basalt (MORB) compositions were obtained at 25–27 GPa and 2400–2530 °C using multi-anvil apparatus and ion microprobe. Results clearly show that CaPv is the predominant host for large ion lithophile elements (LILE) in the lower mantle. Because of the overwhelmingly high CaPv/melt partition coefficients (>10 for many of the LILE), partial melting in the lower mantle causes strong enrichment of LILE in the CaPv-bearing solid phase residue. CaPv has the following partitioning characteristics: (1) uniformly high partition coefficients for heavy rare earth elements (HREE) (e.g. 15 for Yb), decreasing toward light REE (e.g. 7 for La), (2) systematically lower partition coefficients for high field strength elements (Nb, Zr, Ti) and Sr relative to neighboring REE, (3) high Th and U, and systematically low Pb partition coefficients. Previous high-pressure studies have shown that the stability field of CaPv above solidus temperature is much wider in basaltic composition than in peridotite, indicating that melting of subducted oceanic crust in the lower mantle could produce significant geochemical CaPv signatures. Strong enrichment in Th and U relative to Pb in CaPv would result in radiogenic Pb isotopic compositions of the CaPv-bearing solid residue. Some clinopyroxenes in plume mantle peridotite xenoliths possess trace element patterns closely resembling those of natural CaPv found in diamonds and CaPv from the present experiments, suggesting that they were inherited from the CaPv-bearing precursor. In contrast, CaPv is the first phase to disappear during partial melting of peridotite above 24 GPa, and its geochemical signature may not be observable in nature. (MgPv + CaPv) fractional crystallization from a magma ocean has previously been put forward as a mechanism for Si depletion of the upper mantle. However, our data show that the magma ocean Sm/Ba ratio would deviate significantly from observed chondritic values when fractionation exceeds only 6%, if the crystallizing mixture were 10% CaPv and 90% MgPv. A significant hidden perovskite reservoir in the lower mantle can therefore be excluded.

In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO 6 -octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

MgSiO3 perovskite is believed to be a dominant mineral at least in the upper part of the Earth's lower mantle, but its stability and possible phase transition in deeper levels were not known. Recently we discovered the phase transition from MgSiO3 perovskite to a new high-pressure form (space group: Cmcm) above 125 GPa and 2500 K on the basis of in-situ x-ray diffraction measurements [1]. This phase transition is most likely responsible for the origin of the D” seismic discontinuity observed around 2700 km depth, and the MgSiO3 post-perovskite phase is a main constituent mineral in the D” region. Here we introduce the details of high-pressure experiment and crystal structure determination, and discuss the seismic anomalies in the lowermost mantle.

The phase relations and the element partitioning in a mid-oceanic ridge basalt composition were determined for both above-solidus and subsolidus conditions at 22 to 27.5 GPa by means of a multianvil apparatus. The mineral assemblage at the solidus changes remarkably with pressure; majorite and stishovite at 22 GPa, joined by Ca-perovskite at 23 GPa, further joined by CaAl4Si2O11-rich CAS phase at 25.5 GPa, and Mg-perovskite, stishovite, Ca-perovskite, CF phase (approximately on the join NaAlSiO4-MgAl2O4), and NAL phase ([Na,K,Ca]1[Mg,Fe2+]2[Al,Fe3+,Si]5.5–6.0O12) above 27 GPa. The liquidus phase is Ca-perovskite, and stishovite, a CAS phase, a NAL phase, Mg-perovskite, and a CF phase appear with decreasing temperature at 27.5 GPa. Partial melt at 27 to 27.5 GPa is significantly depleted in SiO2 and CaO and enriched in FeO and MgO compared with those formed at lower pressures, reflecting the narrow stability of (Fe,Mg)-rich phases (majorite or Mg-perovskite) above solidus temperature. The basaltic composition has a lower melting temperature than the peridotitic composition at high pressures except at 13 to 18 GPa (Yasuda et al., 1994) and therefore can preferentially melt in the Earth’s interior. Recycled basaltic crusts were possibly included in hot Archean plumes, and they might have melted in the uppermost lower mantle. In this case, Ca-perovskite plays a dominant role in the trace element partitioning between melt and solid. This contrasts remarkably with the case of partial melting of a peridotitic composition in which magnesiowustite is the liquidus phase at this depth.

A laser heating diamond anvil cell experiment, with an angle-dispersive X-ray diffraction using synchrotron radiation source at the SPring-8, has been developed to observe the phase transition in silica (SiO2) between the P42/mnm (rutile-type) and Pnnm (CaCl2-type) up to pressures of 100 GPa and at temperatures up to 2200 K. The transition was observed in the vicinity of 55 GPa at room temperature, and showed a positive temperature dependence of the transition pressure. The phase boundary was determined to follow the equation P (GPa)=(51±2)+(0.012±0.005)×T (K). Our result gives a transition pressure of near 80 GPa and a depth of 1900 km at an expected lower mantle temperature of 2000–2500 K. Therefore, this SiO2 transition is not the cause of recent observations of seismic anomalies between 800 and 1600 km depth in the mid–lower mantle.

Melting Moments The boundary between Earth's core and mantle defines where the iron-rich liquid outer core meets the more chemically heterogeneous solid lower mantle and is marked by a sharp thermal gradient of nearly 1500 kelvin. The precise relationship between temperature and melting of the lowermost mantle constrains the structure and heat flow across the core-mantle boundary. In order to identify trace amounts of liquid as melting initiates, Nomura et al . (p. 522 , published online 16 January) performed x-ray microtomographic imaging of rocks of a primitive mantle composition that had been subjected to high pressures and temperatures in a diamond anvil cell. The experimentally determined maximum melting point of 3570 kelvin suggests that some phases typically thought to lose stability in the lowermost mantle, such as MgSiO 3 -rich post-perovskite, may be more widely distributed than expected.

Here we report the phase boundary of pyrope garnet (Mg3Al2Si3O12) to Al-bearing silicate perovskite plus corundum, with the highest transition pressure determined by in situ measurements in a multi-anvil apparatus at high temperature. The consistency of the pressure scales by different standards of Au, NaCl, Pt, W, and Mo at high temperature was also evaluated by in situ X-ray measurements. Our results, together with recent in situ measurements of the post-spinel transition in Mg2SiO4 [Irifune et al., Science 279 (1998) 1698–1700] and the ilmenite–perovskite transition in MgSiO3 [Ono et al., Geophys. Res. Lett. (2000) submitted], show that pressures determined in conventional quench experiments [Ito and Takahashi, J. Geophys. Res. 94 (1989) 10637–10646] could have been overestimated by more than 2 GPa at pressures corresponding to the bottom of the transition zone. On the basis of the in situ measurements, the post-spinel transition occurs at a depth (∼600 km) that is too shallow to match with the 660-km seismic discontinuity in the Earth’s mantle. Therefore, an olivine dominant mantle compositional model may be inconsistent with the seismic observations. Alternatively, we propose a pyroxene–garnet dominant transition zone with an appropriate Al2O3 content (ca. 6–8 mol%), in which majorite garnet transforms to perovskite at the depth of the 660-km discontinuity. Any alternative models would have to consider chemical stratification in the mantle.

The subduction of oceanic lithosphere into the Earth's deep interior is thought to drive convection and create chemical heterogeneity in the mantle. The oceanic lithosphere as a whole, however, might not subduct uniformly: the fate of basaltic crust may differ from that of the underlying peridotite layer because of differences in chemistry, density and melting temperature. It has been suggested that subducted basaltic crust may in fact become buoyant at the mantle's 660-km discontinuity, remaining buoyant to depths of at least 800 km, and therefore might be gravitationally trapped at this boundary to form a garnetite layer1, 2. Here we report the phase relations and melting temperatures of natural mid-ocean ridge basalt at pressures up to 64 GPa (corresponding to ∼1,500 km depth). We find that the former basaltic crust is no longer buoyant when it transforms to a perovskitite lithology at about 720 km depth, and that this transition boundary has a positive pressure–temperature slope, in contrast to the negative slope of the transition boundary in peridotite. We therefore predict that basaltic crust with perovskitite lithology would gravitationally sink into the deep mantle. Our melting data suggest that, at the base of the lower mantle, the former basaltic crust would be partially molten if temperatures there were to exceed 4,000 K.

For understanding mantle melting processes, incremental batch melting of peridotite was experimentally investigated. The experiments simulated partial melting of a mantle with 1344°C potential temperature upon adiabatic decompression, which segregates melt increments at 2, 1.5, 1 and 0.5 GPa. The compositions of incremental melts formed at respective pressures were obtained using `diamond aggregates method'. Mass balance calculations show that they are 4.9–6.5 wt.% partial melts in each experiment and that the mantle melts 21.0 wt.% by decompression to 0.5 GPa. Comparison with batch melting experiments on the same lherzolite provides an estimation of the effects of melt segregation on polybaric mantle melting. It was clearly demonstrated that melt removal from the melting system reduces the amount of melt produced. To form a certain thickness of oceanic crust at ridges by near fractional mantle melting, a potential temperature several tens of degrees higher than that for batch melting is required. The compositions of incremental melts are characterized by high FeO* and MgO and low TiO2, Al2O3 and Na2O contents compared to those of batch melts. On the other hand, the aggregates of melt increments are similar in composition to batch melts. Both incremental melting and batch melting can thus produce a melt with similar major element composition from the identical source lherzolite, but a higher potential temperature is required for batch melting.

New compositional data are presented for recent (

Partial melting of a natural lherzolite (KLB-1) under H2O-undersaturated conditions was experimentally investigated at 1 GPa over the temperature range from 1100 to 1350°C. The effect of H2O added to the dry lherzolite ranged from 0.1 to 0.9 wt% of the melt composition and the extent of melting was quantitatively estimated by comparison with the results of anhydrous melting experiments on the same lherzolite [1]. With the new capsule configuration presented here, partial melt layers of more than 30 μm in width formed along the inner capsule wall. The composition of quenched glass could successfully be obtained by direct microprobe analyses of these melt layers unmodified during quenching. The degree of partial melting was estimated from mass balance calculations using sodium concentrations. Au75Pd25 and Ag70Pd30 tubes were used as sample containers to minimize iron loss. SIMS analyses of hydrogen in quenched glass showed that loss of hydrogen through the Au75Pd25 capsule was minimal during the experiment. A small amount of H2O in the lherzolite lowers the solidus temperature from slightly below 1250°C to less than 1100°C and significantly increases the extent of melting. The experimental results quantitatively show the increase in extent of melting as a linear function of H2O content in the partial melt above 1200°C. Partial melts with less than 2.5 wt% H2O formed at temperatures above 1200°C are close in major element composition to the anhydrous melts formed by the same degree of partial melting. On the other hand, melts with more than 3 wt% H2O formed at 1100°C are clearly enriched in SiO2 and depleted in MgO compared to the anhydrous melts. The larger amounts of H2O in the mantle source in some hotspot regions such as the Azores platform along the Mid-Atlantic Ridge contribute to a higher extent of melting, resulting in relatively thick oceanic crust.

Melt generation by incremental batch melting of peridotite with a complete melt withdrawal was experimentally investigated. The experiments were made using the “diamond aggregate method” at 1.5 GPa with increasing temperature from 1275° to 1425°C and total degree of partial melting to 22.6 wt.%. The incremental batch melting significantly differs from the batch melting in amount and composition of melt formed. The total degrees of partial melting formed by incremental batch melting are considerably lower than those formed by batch melting at the same temperatures. The compositions of incremental melts change more rapidly than do those of batch melts with increasing temperature. The accumulated compositions of incremental melts are, however, similar to those of batch melts formed by the same total degrees of partial melting. If melts are generated isobarically, both the melting processes can form melts with similar major oxide compositions, but the temperatures needed are significantly different.

A systematic compilation of the geochemical data collected during the starmer programme is presented, in addition to all published data gathered along the North Fiji Basin (NFB) spreading system. From the 194 samples selected, the geochemical variations observed along the different spreading segments can be related to uni- or multi-modal origin of distinctive magma sources. These variations are interpreted as directly linked to the geodynamical features which surround the NFB. In the southern NFB, on the N174°E segment, N-MORB are present, but the low rate (or dead) subduction along the Hunter ridge still marks its influence as several basalts show a significant subduction-related contamination with negative Nb anomaly and high LOI. Several others are marked by a weak E-MORB source contribution, that may be related to subducted-OIB seamounts from the South Fiji Basin. In the central NFB, along the N-S segment which represents the most active and morphologically regular spreading ridge of the NFB, the magma source produces only N-MORB, with depleted LILE, HFSE, and LREE patterns, except in its northernmost part where magmatic signatures similar to that of the N15° segment appear. Along the N15° segment, three distinctive sources coexist and produce a wide range of geochemical signatures on the basalts collected; (1) a N-MORB source signature; (2) a transitional towards E-MORB source signature with a negative Nb anomaly indicating that some subduction related contamination still exists beneath the NFB (the basalts derived from this source might also have been called BABB previously); and (3) a source signature transitional towards E-MORB or OIB marking the start of an influence that increases towards the northern NFB as the Rotuma-Samoan hot spot lineament is approched. Around 17°S, on the Kaiyo station 4 site explored and sampled by the Nautile deep-sea submersible, as well as in the triple junction area, the same variability derived from three distinctive mantle sources is observed. Along the N160° segment, the three sources still coexist, but the influence of the E-MORB or OIB source (hot spot-related) increases, whereas the influence of the subduction-related source decreases. Along the Pandora-Rotuma ridge, the OIB-derived lava type is the only one present, the eventual contribution from other sources being completely diluted. Thus, the geochemistry of the NFB basalts is directly influenced by (1) the regional geodynamic environment, such as subduction zones and/or hot spot trails, (2) the geodynamical regime and stability of this type of back-arc system. The influence of the New Hebrides subduction, located some 500 km west of the active spreading ridge, is still perceptible, although weak, in the whole northern half of the NFB. However, this infuence is not directly linked to the presently active subduction, but originates rather from the partial melting of an upper mantle source that suffered subduction contamination during the clockwise rotation of the New Hebrides arc leading to the opening of the NFB during the past 12 Myr. In the northern NFB, many basalts result from the mixing of an N-MORB and a OIB source, similar to transitional alkalic lavas from oceanic intra-plate magmatism. This sources mixing increases northwards from 18°20′S to 12°S.

[1] Strong seismic anisotropy observed in the Earth's lowermost mantle, the D″ layer, is likely attributed to the lattice-preferred orientation (LPO) of its predominant mineral, magnesium-silicate post-perovskite (PPv). Here we report simultaneous high-pressure and -temperature plastic deformation experiments on MnGeO3 PPv from 77 to 111 GPa and from 83 to 105 GPa at 2000 K using the membrane-type diamond-anvil cell (DAC). Radial X-ray diffraction measurements demonstrate that the (001) plane aligned perpendicular to the compression direction, indicating deformation dominated by slip on (001). In strong contrast to other slip planes proposed previously, the (001) slip plane can cause significant shear-wave splitting with polarization in line with seismic observations. D″ anisotropy can therefore be primarily reconciled with LPO of the PPv phase.

In this article the author examines aspects of the composition of the Earth. At issue is the discovery of a mineral deep in the Earth's mantle called postperovskite. At high pressure the common mineral in the Earth's mantle experiences a structural change and becomes more dense. It is suggested that heat is more efficiently carried in this layer than had been previously thought. The effects of the transportation of heat on the development of continents and the magnetic field of the Earth is discussed. INSETS: DEEP EARTH IN THE LAB;CONVECTION ON STEROIDS;MANTLE AND CORE EVOLUTION.

[1] In situ crystallography based on diamond anvil cells have been extended to the multimegabar regime. Temperatures in these experiments have crossed the 2500 K mark. Yet, current high pressure-temperature (PT) standards of calibration produce uncertainties that inhibit clear conclusions about phenomena of importance to planetary processes, e.g., the postperovskite transition in Earth’s mantle. We introduce a new thermal equation of state (EOS) of MgO which appears to be predictive up to the multimegabar and thousands of kelvin range. It is obtained by combining first principles local density approximation quasi-harmonic (QHA) calculations with experimental lowpressure data. This EOS agrees exceptionally well with shock compression data. The postspinel and postperovskite phase boundaries recalculated using our EOS match seismic observations. The latter, in particular, supports the idea that postperovskite transforms back to perovskite before the core-mantle boundary. The recalculated experimental Clapeyron slope of the postperovskite transition is also more consistent with those obtained by first principles calculations. Citation: Wu, Z., R. M. Wentzcovitch, K. Umemoto, B. Li, K. Hirose, and J.-C. Zheng (2008), Pressure-volume-temperature relations in MgO: An ultrahigh pressure-temperature scale for planetary sciences applications, J. Geophys. Res., 113, B06204

Recent discovery of a phase transition from perovskite to post-perovskite suggests that the physical properties of Earth's lowermost mantle, called the D″ layer, may be different from those of the overlying mantle. We report that the electrical conductivity of (Mg 0.9 Fe 0.1 )SiO 3 post-perovskite is >10 2 siemens per meter and does not vary greatly with temperature at the conditions of the D″ layer. A post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and the solid mantle, which can explain the observed changes in the length of a day on decadal time scales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.