Yingwei Fei

The fate of subducted basaltic crust in the Earth's lower mantle

The subduction of oceanic lithosphere into the Earths 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 mantles 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 layer, . 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.

Toward an internally consistent pressure scale.

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earths 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 earths 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 earths interior.

Mineralogy of the Martian interior up to core‐mantle boundary pressures

In order to determine the mineralogy of the Martian interior along a high-temperature areotherm, multianvil experiments have been performed with a model Martian mantle composition up to 23.5 GPa. The Dreibus and Wanke [19851 Martian mantle composition yields an upper mantle that consists of olivine + clinopyroxene + orthopyroxene + garnet at pressures up to 9 GPa. Above 9 GPa, orthopyroxene is no longer present. The transition zone is marked by the appearance of γ spinel at 13.5 GPa. Up to 15 GPa, clinopyroxene and majorite coexists with β phase and/or γ spinel. By 17 GPa, clinopyroxene is entirely replaced by majorite and the modal abundance of γ spinel increases at the expense of β phase. The dominant assemblage throughout most of the transition zone is γ spinel + majorite. Two experiments completed in the perovskite stability field indicate that the lower mantle consists of Mg-Fe silicate-perovskite, magnesiowustite, and majorite. CaSiO 3 -perovskite is not present in these experiments. Both the presence of a Martian lower mantle, i.e., an Mg-Fe silicate-perovskite bearing zone, and the phase assemblage stable in the Martian lower mantle are very sensitive to the temperature profile of the interior. A low-temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures. Regardless of the temperature profile assumed, the Martian upper mantle and transition zone will account for a larger proportion of the planets interior than is the case for the Earths interior because of the smaller size of Mars.

Effects of temperature and composition on the bulk modulus of (Mg,Fe)O

Abstract Isothermal static compression data for MgO at 300 and 1100 K were obtained by combining synchrotron X-ray diffraction techniques with an externally heated high-temperature diamond-anvil cell that is capable of achieving pressures greater than 125 GPa at temperatures up to 1100 K. The experiments at 300 K were conducted under both hydrostatic and nonhydrostatic conditions. The deviatoric stress in non-hydrostatic environment significantly effects the measured lattice parameters. Fits to the static compression data of MgO at 300 K yield a bulk modulus of 185(7) GPa and 160(2) GPa under nonhydrostatic and hydrostatic conditions, respectively. The deviatoric stress decreases with increasing temperature, and a nearly hydrostatic condition was achieved at temperatures above 900 K when NaCl was used as a pressure-medium. The bulk modulus of MgO was determined to be 135(3) GPa at a temperature of 1100 K, yielding its temperature derivative of 20.030(3) GPa/K. Comparing these results with previous studies in the system MgO-FeO, shows that the bulk modulus of (Mg,Fe)O decreases with increasing FeO content, from 160 GPa for MgO to 146 GPa for FeO.

In Situ Determination of the NiAs Phase of FeO at High Pressure and Temperature

In situ synchrotron x-ray diffraction measurements of FeO at high pressures and high temperatures revealed that the high-pressure phase of FeO has the NiAs structure (B8). The lattice parameters of this NiAs phase at 96 gigapascals and 800 kelvin are a = 2.574(2) angstroms and c = 5.172(4) angstroms (the number in parentheses is the error in the last digit). Metallic behavior of the high-pressure phase is consistent with a covalently and metallically bonded NiAs structure of FeO. Transition to the NiAs structure of FeO would enhance oxygen solubility in molten iron. This transition thus provides a physiochemical basis for the incorporation of oxygen into the Earths core.

Thermoelasticity of Silicate Perovskite and Magnesiowüstite and Stratification of the Earth's Mantle

Analyses of x-ray-diffraction measurements on (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiow�stite at simultaneous high temperature and pressure are used to determine pressure-volume-temperature equations of state and thermoelastic properties of these lower mantle minerals. Detailed comparison with the seismically observed density and bulk sound velocity profiles of the lower mantle does not support models of this region that assume compositions identical to that of the upper mantle. The data are consistent with lower mantle compositions consisting of nearly pure perovskite (>85 percent), which would indicate that the Earths mantle is compositionally, and by implication, dynamically stratified.

Experimental evidence that potassium is a substantial radioactive heat source in planetary cores

The hypothesis that 40K may be a significant radioactive heat source in the Earths core was proposed on theoretical grounds over three decades ago, but experiments have provided only ambiguous and contradictory evidence for the solubility of potassium in iron-rich alloys. The existence of such radioactive heat in the core would have important implications for our understanding of the thermal evolution of the Earth and global processes such as the generation of the geomagnetic field, the core–mantle boundary heat flux and the time of formation of the inner core. Here we provide experimental evidence to show that the ambiguous results obtained from earlier experiments are probably due to previously unrecognized experimental and analytical difficulties. The high-pressure, high-temperature data presented here show conclusively that potassium enters iron sulphide melts in a strongly temperature-dependent fashion and that 40K can serve as a substantial heat source in the cores of the Earth and Mars.

Subsolidus and melting phase relations of basaltic composition in the uppermostlower mantle

Abstract 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 synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3 perovskite up to 120 GPa

Abstract The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe0.05Mg0.95SiO3 (Pv05) and Fe0.1Mg0.9SiO3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mössbauer spectroscopy (SMS). Such investigations of extremely small and dilute 57Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe2+-like sites and one Fe3+-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe3+/ΣFe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe3+ remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a highspin to a low-spin state.

Experimental determination of element partitioning and calculation of phase relations in the MgO‐FeO‐SiO2 system at high pressure and high temperature

Mg-Fe partitioning between coexisting phases, magnesiowustite (Mw) and olivine (α), Mw and β-phase, Mw and spinel (γ), and Mw and perovskite (Pv), has been determined experimentally with piston-cylinder apparatus, the multianvil device and the diamond anvil cell technique at pressures between 2 and 28 GPa and temperatures between 1473 and 1773 K. The solution parameters of each solid solution were obtained by fitting the experimental data simultaneously using the Margules formulation. The optimized solution parameters (in J/mol) are WMwMg-Fe = 16100, WMwFe-Mg = 26300 − 5.56T; WαMg-Fe = 4500 + 130P, WαFe-Mg = 6500 + 130P; WβMg-Fe = 1000, WβFe-Mg = 2000; WγMg-Fe = 900 − 1.10T WγFe-Mg = 3900; and WPvMg-Fe = 4130 −1.37T + 110P, WPvFe-Mg = −4050 − 2.45T + 150 P where P is in GPa and T in K. These parameters are consistent with solution calorimetry and phase equilibrium data. Phase relations in the MgO-FeO-SiO2 system were calculated by using the Margules solution model combined with internally consistent thermodynamic data of the pure phases. The computation provides independent constraints for phase relations in the system, in addition to those directly determined in P-T-Xi space.