We have conducted a series of melting experiments in the Fe–C system at pressures up to 25 GPa in the temperature range of 1473–2073 K. The results define the phase relations at several pressures, ...including the eutectic temperature and composition as a function of pressure, carbon partitioning between solid iron and liquid, and change of melting relations involving iron carbides. In order to interpolate and extrapolate the phase relations over a wide pressure and temperature range, we have established a comprehensive thermodynamic model in the Fe–C binary system. The calculated phase diagrams at pressures of 5, 10, and 20 GPa reproduce the experimental data, including the solubility of carbon in solid iron and the effect of pressure on the eutectic temperature and composition. The formation of Fe7C3 at pressures above 5 GPa is correctly modeled and the change of phase relations in the Fe–C system between 5 and 10 GPa is captured in the model. The model provides predictions of the phase relations at 136 GPa and 330 GPa, based on existing knowledge of the thermochemistry of the system at lower pressure. The calculated phase relations can be used to understand the role of carbon during inner core crystallization, predicting carbon distribution between the inner and outer cores and mineralogy of the solid inner core.
•We present melting data in the Fe–C system up to 25 GPa.•We have established a comprehensive thermodynamic model in the Fe–C binary system.•The model reproduces the observations and guides new experiments.•The model predicts carbon distribution between the inner and outer cores.
The thermal conductivity of the Earth's core can be estimated from its electrical resistivity via the Wiedemann–Franz law. However, previously reported resistivity values are rather scattered, mainly ...due to the lack of knowledge with regard to resistivity saturation (violations of the Bloch–Grüneisen law and the Matthiessen's rule). Here we conducted high-pressure experiments and first-principles calculations in order to clarify the relationship between the resistivity saturation and the impurity resistivity of substitutional silicon in hexagonal-close-packed (hcp) iron. We measured the electrical resistivity of Fe–Si alloys (iron with 1, 2, 4, 6.5, and 9 wt.% silicon) using four-terminal method in a diamond-anvil cell up to 90 GPa at 300 K. We also computed the electronic band structure of substitutionally disordered hcp Fe–Si and Fe–Ni alloy systems by means of Korringa–Kohn–Rostoker method with coherent potential approximation (KKR-CPA). The electrical resistivity was then calculated from the Kubo–Greenwood formula. These experimental and theoretical results show excellent agreement with each other, and the first principles results show the saturation behavior at high silicon concentration. We further calculated the resistivity of Fe–Ni–Si ternary alloys and found the violation of the Matthiessen's rule as a consequence of the resistivity saturation. Such resistivity saturation has important implications for core dynamics. The saturation effect places the upper limit of the resistivity, resulting in that the total resistivity value has almost no temperature dependence. As a consequence, the core thermal conductivity has a lower bound and exhibits a linear temperature dependence. We predict the electrical resistivity at the top of the Earth's core to be 1.12×10−6Ωm, which corresponds to the thermal conductivity of 87.1 W/m/K. Such high thermal conductivity suggests high isentropic heat flow, leading to young inner core age (<0.85 Gyr old) and high initial core temperature. It also strongly suppresses thermal convection in the core, which results in no convective motion in inner core and possibly thermally stratified layer in the outer core.
•Resistivity of Fe–Si alloys has been measured up to 90 GPa in a DAC.•Resistivity of hcp Fe–Si and Fe–Ni alloys has been calculated by means of KKR-CPA.•Experimental and theoretical results show excellent agreement and indicate resistivity saturation.•The saturation effect leads to the high thermal conductivity of the Earth's core.•The high conductivity strongly suppresses thermal convection in both liquid and solid cores.
Subduction is a key process for linking the carbon cycle between the Earth’s surface and its interior. Knowing the carbonation and decarbonation processes in the subduction zone is essential for ...understanding the global deep carbon cycle. In particular, the potential role of hydrocarbon fluids in subduction zones is not well understood and has long been debated. Here we report graphite and light hydrocarbon-bearing inclusions in the carbonated eclogite from the Southwest (S.W.) Tianshan subduction zone, which is estimated to have originated at a depth of at least 80 kilometers. The formation of graphite and light hydrocarbon likely results from the reduction of carbonate under low oxygen fugacity (∼FMQ - 2.5 log units). To better understand the origin of light hydrocarbons, we also investigated the reaction between iron-bearing carbonate and water under conditions relevant to subduction zone environments using large-volume high-pressure apparatus. Our high-pressure experiments provide additional constraints on the formation of abiotic hydrocarbons and graphite/diamond from carbonate-water reduction. In the experimental products, the speciation and concentration of the light hydrocarbons including methane (CH4), ethane (C2H6), and propane (C3H8) were unambiguously determined using gas chromatograph techniques. The formation of these hydrocarbons is accompanied by the formation of graphite and oxidized iron in the form of magnetite (Fe3O4). We observed the identical mineral assemblage (iron-bearing dolomite, magnetite, and graphite) associated with the formation of the hydrocarbons in both naturally carbonated eclogite and the experimental run products, pointing toward the same formation mechanism. The reduction of the carbonates under low oxygen fugacity is, thus, an important mechanism in forming abiotic hydrocarbons and graphite/diamond in the subduction zone settings.
An internally consistent thermodynamic database for pure iron has been established to pressures (P) up to 360 GPa and temperatures (T) up to 7000 K from existing static experimental data and ...thermochemical measurements. The database includes body‐centered cubic (BCC) phases (α or δ phase), the face‐centered cubic (FCC) phase (γ phase), the hexagonal close‐packed (HCP) phase (ɛ phase), and the liquid phase. We describe fundamental thermodynamic relations as the Gibbs free energy divided into thermochemical and thermophysical terms. The thermochemical data were evaluated from existing metallurgy databases together with experimentally determined phase relations. The thermophysical term is obtained from the pressure‐volume‐temperature equations of state (EoS) for the phases. We constructed an EoS of the FCC phase from our recent internally‐heated diamond anvil cell (DAC) experimental data and assessed the EoS of the liquid phase from existing laser‐heated DAC experiments together with density data at P = 1 bar, 0.2 GPa, and along the Hugoniot. The HCP‐FCC‐liquid triple point is located at P = 90 GPa and T = 2800 K. The calculated melting temperature of HCP iron at the inner core boundary (P = 330 GPa) is 4900 K and the density change at melting is −1.2%. The core density deficits at the inner core boundary are 8.1 wt.% and 5.3 wt.% for the liquid outer core and solid inner core, respectively. The calculated melting temperature is much lower than that from dynamic shock wave experiments, suggesting that the HCP structure may not be stable in the inner core. We included a hypothetical high‐pressure BCC phase which could be stabilized above 220 GPa by a solid‐solid transition of high‐P BCC‐HCP phases. This hypothetical BCC phase should have a large entropy to give a high melting temperature in order to reconcile the existing discrepancies between the static and shock wave experimental studies.
The balance of carbon flux in subduction zones is critical to the deep carbon cycle. Carbonate-bearing lithologies are the major carbon carriers transported from Earth's surface into its interior at ...subduction zones. Recently, a number of studies have showed that carbon can be released from the subducting slab through metamorphic decarbonation and dissolution into C-H-O fluids. However, the evolution of the released C-H-O fluids during subduction-zone metamorphism is ambiguous and poorly explored. In this study, we found graphite-rich eclogite veins (VE) in the carbonated eclogites from the Southwestern (S.W.) Tianshan subduction zone. The observed graphite with high crystallinity and graphite-bearing fluid inclusions indicate the fluid-deposited origin. Phase equilibrium modelling for the host carbonated-eclogite (HE) in a closed system indicates that it has experienced a retrograde P-T path involving decompression with heating from 26.5 kbar at 487 °C to 20.6 kbar at 565 °C. The calculation showed that about 0.92–2.03 wt% of CO2 (0.25–0.55 g C per 100 g rock) could be released from the carbonated eclogite during its exhumation process, which is enough to provide the carbon source for graphite precipitation in the VE. Combined with petrological and isotopic results, we suggest that the graphites in the VE were precipitated from carbon-bearing fluids derived from the carbonated eclogites during exhumation metamorphism. The overall redox reaction is: FeO (in silicate, Grt, Omp or Gln) + FeS + (H2O + CO2) (released from Lws and Dol) → Fe2O3 (in Ep, Andradite or Hematite) + C (graphite) + SO42− + HCO3– + CO32−. Mass balance calculation indicates that carbonates could also be re-precipitated in the VE during fluid-rock interaction, in addition to the graphite precipitation. The finding of fluid-deposited graphite in the carbonated eclogites provides new insights into the fate of carbonic fluids formed in the subducted oceanic crust. We suggest that carbonic fluids formed in the carbonated eclogites by decarbonation or carbonate dissolution may also precipitate abiotic graphite or carbonates under favorable conditions during their migration in addition to the commonly recognized transportation to the mantle wedge.
•Graphite-rich eclogite veins were firstly found in carbonated eclogite from S.W. Tianshan, China.•The graphites were precipitated from C-H-O fluids released from the carbonated eclogites during exhumation.•C-H-O fluids released from the carbonated eclogites may not be directly and fully transported to the mantle wedge.•C-H-O fluids may precipitate abiotic graphite or carbonates under favorable conditions during their migration.
We conducted high‐pressure experiments on hexagonal close packed iron (hcp‐Fe) in MgO, NaCl, and Ne pressure‐transmitting media and found general agreement among the experimental data at 300 K that ...yield the best fitted values of the bulk modulus K0 = 172.7(±1.4) GPa and its pressure derivative K0′ = 4.79(±0.05) for hcp‐Fe, using the third‐order Birch‐Murnaghan equation of state. Using the derived thermal pressures for hcp‐Fe up to 100 GPa and 1800 K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp‐Fe. The thermal equation of state of hcp‐Fe is further used to calculate the densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earth's inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000 K.
Key Points
Intercalibrated pressure scales lead to a consistent compression curve of hcp‐iron up to 300 GPa
Thermal equation of state of hcp‐iron is refined by combining static and dynamic data
The inner core density deficit is 3.6% at an inner core boundary temperature of 6000 K
Abstract
The essential data for interior and thermal evolution models of the Earth and super-Earths are the density and melting of mantle silicate under extreme conditions. Here, we report ...an unprecedently high melting temperature of MgSiO
3
at 500 GPa by direct shockwave loading of pre-synthesized dense MgSiO
3
(bridgmanite) using the Z Pulsed Power Facility. We also present the first high-precision density data of crystalline MgSiO
3
to 422 GPa and 7200 K and of silicate melt to 1254 GPa. The experimental density measurements support our density functional theory based molecular dynamics calculations, providing benchmarks for theoretical calculations under extreme conditions. The excellent agreement between experiment and theory provides a reliable reference density profile for super-Earth mantles. Furthermore, the observed upper bound of melting temperature, 9430 K at 500 GPa, provides a critical constraint on the accretion energy required to melt the mantle and the prospect of driving a dynamo in massive rocky planets.
Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (V
) and high Poisson's ratio (σ) in the solid inner core. To date, neither ...V
nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present V
and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low V
and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease V
and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.
The heat extracted from the core by the overlying mantle across the core‐mantle boundary controls the thermal evolution of the core. This in turn leads to the solidification of the inner core in ...association with the exsolution of light alloying elements into the liquid outer core. Although the temperature (T) at the inner core boundary (ICB) would be adjusted to account for the effects of the light elements, the melting T of Fe places an upper bound at the ICB and it is a vital point in the thermal profile of the core. Here, we determine the melting T of Fe in the multi‐anvil press by characterizing the interface of Fe‐W interaction. Our data place a tighter constraint on the melting curve of Fe between 8 and 21 GPa, that is directly applicable to small planetary bodies and serves as an anchor for melting curve of Fe at higher pressure.
Plain Language Summary
The melting temperature (T) of Fe is a fundamental parameter in constraining the thermal structure and evolution of the cores of the rocky planets and their satellites. Here, we precisely determine the melting T of Fe by characterizing the inter‐metallic diffusive interaction between Fe‐W at the melting transition using a new method “inter‐metallic fast diffusion” in multi‐anvil press. We measured the melting T of Fe at various fixed pressures between 8 and 21 GPa. We determined the melting T within an uncertainty of about 30 K, which is higher in precision than the reported errors in previous studies. Our data set provides a tighter constraint on the melting curve of Fe measured in the large‐volume press. In addition, we used the data set to critically evaluate the melting curve of Fe up to 80 GPa which has a large discrepancy in the existing melting data produced in laser‐heated diamond anvil cell.
Key Points
The melting curve of Fe was precisely measured up to 21 GPa in multi‐anvil press by Characterizing the interface of Fe‐W interaction
We used our data set to critically evaluate the melting curve of Fe up to about 80 GPa which provided an independent check on diamond anvil cell datasets
Toward an internally consistent pressure scale Fei, Yingwei; Ricolleau, Angele; Frank, Mark ...
Proceedings of the National Academy of Sciences - PNAS,
05/2007, Letnik:
104, Številka:
22
Journal Article
Recenzirano
Odprti dostop
Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earth's interior is critically dependent on the accuracy of pressure ...measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earth's interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure-temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earth's interior.
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