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We developed a near-infrared focused heating system (IRrDAC) for deformation experiments using a rotational diamond anvil cell. This study reports the results of annealing tests on silver and antigorite conducted at SPring-8 (BL47XU) using the IRrDAC system. The experimental results demonstrated the melting of silver and the dehydration of antigorite, confirming the capability of this system. The reproducible relationships between temperature and input power were also confirmed. The IRrDAC system enables deformation experiments at pressures equivalent to those of the lower mantle under homogeneous and stable temperatures and is expected to contribute to the understanding of deep Earth rheology. [ABSTRACT FROM AUTHOR]

The seismic anisotropy observed in Earth's lowermost mantle (D″ layer) is thought to originate from the lattice‐preferred orientation (LPO) of mantle materials induced by plastic deformation. Ferropericlase is the second most abundant mineral in the lower mantle, and its LPO induced by deformation may significantly contribute to the seismic anisotropy in the D″ layer. The slip system(s) of ferropericlase controlling the LPO in the D″ layer is still controversial, owing to technical difficulties in conducting high‐pressure deformation experiments. We conducted shear deformation experiments on MgO under pressures of up to 120 GPa and reported the LPO after the deformation experiments. Our results indicate that {100}<011> is the dominant slip system in the D″ layer. The LPO developed during shear deformation would result in shear wave splitting with VSH > VSV, consistent with seismic observations in the regions expected to accommodate large shear strain by subducted slabs in the D″ layer. Plain Language Summary: Heat from Earth's core induces active thermal convection involving plastic flow in the Earth's solid mantle. Owing to plastic flow, minerals with elastic anisotropy in the mantle are thought to be aligned in certain directions (lattice‐preferred orientation, LPO) and generate the seismic anisotropy observed in the lowermost mantle. In particular, the LPO of ferropericlase, the second most abundant mineral in the lower mantle, has garnered attention as a likely candidate for attributing the seismic anisotropy of the lowermost mantle. To quantitatively interpret the dynamics of mantle convection in the lowermost mantle from seismic observations, it is essential to understand the LPO patterns of ferropericlase during deformation under the lowermost mantle conditions (for instance, extremely high‐pressure conditions at 1.2 million atmospheric pressure). However, owing to technical difficulties in conducting high‐pressure deformation experiments, there have been uncertainties in the deformation‐induced LPO of ferropericlase in the lowermost mantle. We conducted shear deformation experiments of MgO under the lowermost mantle pressure for the first time and demonstrated that the LPO of MgO produced by shear deformation is indeed consistent with the seismic anisotropy observed in parts of the lowermost mantle expected to accommodate large shear deformation by subducted slabs. Key Points: Strain rate controlled shear deformation experiments on MgO were conducted under lowermost mantle pressure (120 GPa) for the first timeLarge strain deformation experiments on MgO show that the {100}<011> slip is dominant in the D″ layerA {100}<011> slip of MgO by shear deformation results in VSH > VSV shear splitting, consistent with seismic observations in the D″ layer [ABSTRACT FROM AUTHOR]

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 speculation (1,2). 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 region (3). Recent theoretical calculations (4) combined with estimates of iron partitioning between (Mg,Fe)Si[O.sub.3] perovskite and melt at shallower mantle conditions (5-7) suggest that melt is more dense than solids at pressures in the Earth's deepest mantle, consistent with analysis of shockwave experiments (8). 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 (M[g.sub.0.95] [Fe.sub.0.05])Si[O.sub.3] 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)Si[O.sub.3] 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)Si[O.sub.3] 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., Our melting experiments were performed on samples with bulk composition ([Mg.sub.0.89] [[Fe.sub.011]).sub.2] Si[O.sub.4] at pressures from 20 to 159 GPa in a laser-heated diamond-anvil cell (DAC; Supplementary Table 1). The [...]

We developed a thermodynamic model to explore the joint solubility of Mg, Si, and O in liquid Fe on the basis of high‐pressure metal‐silicate partitioning data in the literature, with more Mg kept in the metal when Si and O are present. With <1.7 ± 0.5 wt% Mg, the metal in the young Earth's core retains Mg as the core evolves and crystallizes SiO2. Higher Mg concentrations require either the late addition of metal that equilibrated with silicate in a super‐liquidus magma ocean or the assimilation of silicate into the core at the time of a giant impact. Above 1.7 wt% Mg, (Mg,Fe)‐silicate melts also exsolve from the core and transfer core‐hosted elements to the mantle. Fractional crystallization of the core‐derived silicate melts in the core or at the core‐mantle boundary could, additionally, yield a persistent molten silicate layer that could also contribute to ultralow velocity zone formation in the lowermost mantle. Plain Language Summary: Some think that crystallization of oxide in the liquid core, either MgO or SiO2, could have been responsible for driving core convection and geodynamo since early Earth times, at least until iron started crystallizing the inner core. However, the joint solubility of Mg, Si, and O in liquid Fe has not been explored yet. Here we developed a thermodynamic model to examine the exsolution of MgO and SiO2 from liquid Fe at core‐mantle boundary (CMB) pressure based on earlier experimental data on metal‐silicate partitioning. Our main findings are that (1) more Mg is retained in the presence of Si and O in metal; (2) with <1.7 wt% Mg, the core expels solid SiO2 but not MgO; and (3) with higher Mg, magnesium silicate melt also exolves and transfers core‐hosted elements to the mantle. The seismologically observed molten silicate layer above the CMB may have been originally derived from the core. Key Points: Mg uptake in liquid iron is controlled by the liquid's Si content; silicate Mg is reduced by Si in the metalIf the core later expelled Mg ingested while accreting as (Mg,Fe)‐silicate, it could transfer siderophile trace elements back to the mantleULVZs at the base of the mantle could be related to fractional crystallization of (Mg,Fe) silicate expelled by the core [ABSTRACT FROM AUTHOR]

Highlights • The solidus of KLB-1 peridotite and N-MORB were simultaneously determined at 25–27 GPa. • The solidus of N-MORB is identical to the KLB-1 peridotite at 25 GPa but lower at 27 GPa. • The crossover of melt fraction between KLB-1 peridotite and N-MORB occurs at 25–27 GPa. • Partial melts of N-MORB are denser than those of KLB-1 peridotite at a given mantle depth. • Melt distribution in the Earth may be different between KLB-1 peridotite and N-MORB. Abstract Interpretation of melting phase relationships of mantle peridotite and subducted basaltic crust is important for understanding chemical heterogeneity in the Earth's interior. Although numerous studies have conducted melting experiments on peridotite and mid-ocean ridge basalt (MORB), and suggested that the solidus temperature of MORB is lower than that of peridotite at whole mantle pressure conditions, both solidus temperatures overlap within their uncertainties. In this study, we conducted simultaneous experiments on KLB-1 peridotite and normal MORB (N-MORB) at pressures from 25 GPa to 27 GPa and temperatures from 2398 K to 2673 K, to compare the solidus temperatures and their melting phase relations. The experimental results show that the solidus temperature of the N-MORB is nearly identical to the KLB-1 peridotite at 25 GPa but lower at 27 GPa. In addition, we found that the crossover of melt fractions between KLB-1 peridotite and N-MORB occurs at 25–27 GPa. These changes are likely to be attributed to the majorite-bridgmanite transition of MORB. This indicates that the dominant melting component may change depending on the location of the uppermost lower mantle. Our calculation result on the density of partial melts along the mantle geotherm suggests that partial melts of KLB-1 peridotite are gravitationally stable around the top of the transition zone, whereas partial melts of N-MORB are gravitationally stable even at the top of lower mantle. These results suggest that the distribution of partial melts may be different between KLB-1 peridotite and N-MORB in the deep Earth. Our results may be useful for understanding the fate of partial melts of peridotitic mantle and recycled basaltic crust. [ABSTRACT FROM AUTHOR]

A rotational diamond anvil cell is the most suitable deformation apparatus with which to investigate the rheological properties of deep-Earth materials at pressures similar to those found in the lower mantle and core. However, slip between the sample and piston is still a problem, since the slip prevents the attainment of a constant strain rate and interferes with the uniform deformation of a sample. In this paper, we report that using a diamond anvil with deep grooves results in a marked improvement in the coupling between the sample and the diamond anvils. [ABSTRACT FROM PUBLISHER]

The Earth's lower mantle is composed of bridgmanite, ferropericlase, and CaSiO-rich perovskite. The melting phase relations between each component are key to understanding the melting of the Earth's lower mantle and the crystallization of the deep magma ocean. In this study, melting phase relations in the MgSiO-CaSiO system were investigated at 24 GPa using a multi-anvil apparatus. The eutectic composition is (Mg,Ca)SiO with 81-86 mol% MgSiO. The solidus temperature is 2600-2620 K. The solubility of CaSiO component into bridgmanite increases with temperature, reaching a maximum of 3-6 mol% at the solidus, and then decreases with temperature. The same trend was observed for the solubility of MgSiO component into CaSiO-rich perovskite, with a maximum of 14-16 mol% at the solidus. The asymmetric regular solutions between bridgmanite and CaSiO-rich perovskite and between MgSiO and CaSiO liquid components well reproduce the melting phase relations constrained experimentally. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The phase transition boundary between stishovite and CaCl2-type structure in pure SiO2 was investigated at 45–75GPa and 300–2490K 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 11.1MPa/K and the transition occurs about 70GPa at 2200K along the typical mantle geotherm. This corresponds to a depth of 1700km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850km 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. [ABSTRACT FROM AUTHOR]

A high-pressure in situ X-ray laminography technique was developed using a newly designed, laterally open diamond anvil cell. A low X-ray beam of 8 keV energy was used, aiming at future application to dual energy X-ray chemical imaging techniques. The effects of the inclination angle and the imaging angle range were evaluated at ambient pressure using the apparatus. Sectional images of ruby ball samples were successfully reconstructed at high pressures, up to approximately 50 GPa. The high-pressure in situ X-ray laminography technique is expected to provide new insights into the deep Earth sciences. [ABSTRACT FROM AUTHOR]

Magnesium partitioning between metal and silicate was experimentally investigated between 34 and 138 GPa, 3,500 and 5,450 K using laser‐heated diamond anvil cells. The 22 measurements are combined with previously published data (total of 49 measurements) to model magnesium metal‐silicate partitioning using a thermodynamically consistent framework based on the interaction parameter formalism. The observations support the mechanism of MgO dissolution in the metal, ruling out other mechanisms. The magnesium partition coefficient depends on temperature and metal composition, but not on pressure or silicate composition. The equilibrium concentration and the exsolution rate of MgO in Earth's core can therefore be calculated for any P, T, and composition. Using a core thermal evolution model, the buoyancy flux converts to a magnetic field at Earth's surface, with dipole intensities between 40 and 70 μT prior to inner core growth, consistent with the paleomagnetic record going back to the Archean. Plain Language Summary: We measure the incorporation of magnesium oxide (one of the main components of Earth's mantle) into iron (the main constituent Earth's core), using extremely high pressure and temperature experiments that mimic the conditions of Earth's mantle and core. We find that magnesium oxide dissolution depends on temperature but not on pressure, and on metal (i.e., core) composition but not silicate (i.e., mantle) composition. Our findings support the idea that magnesium oxide dissolved in the core during its formation will precipitate out during subsequent core cooling. The precipitation should stir the entire core to produce a magnetic field in Earth's distant past, at least as intense as the present‐day field. Key Points: A new thermodynamic model of magnesium metal‐silicate partitioning is proposed, fitting all previously published data along with 22 new experimentsMgO, not Mg, dissolves into iron; partitioning depends on T and metal composition, and is independent of P and silicate compositionMgO exsolution during Earth's core cooling (~10−6 K−1) produces a buoyancy flux on the order of 105 kg/s, able to generate a dipolar magnetic field of 40–70 μT [ABSTRACT FROM AUTHOR]

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. • Density crossover between melt and bridgmanite starts at the base of the magma ocean. • Density crossover moves upwards in the magma ocean. • Magma ocean evolution followed batch crystallization up to 50% by volume. • Rheological transition around the depth corresponding to 50–60 GPa pressure. • Evolution of magma ocean deeper than 1300 km yields basal magma ocean. [ABSTRACT FROM AUTHOR]

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 Fe 2 SiO 4 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 SiO 2 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 SiO 2 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 SiO 2 , 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 SiO 2 streaks within the mantle “marble cake”. Mantle packages enhanced by SiO 2 streaks may be the origin of seismic scatterers in the mid-lower mantle. [ABSTRACT FROM AUTHOR]