High-Pressure Iron-Sulfur Compound, Fe3S2, and Melting Relations in the Fe-FeS System

An iron-sulfur compound (Fe3S2) was synthesized at pressures greater than 14 gigapascals in the system Fe-FeS. The formation of Fe3S2 changed the melting relations from a simple binary eutectic system to a binary system with an intermediate compound that melted incongruently. The eutectic temperature in the system at 14 gigapascals was about 400°C lower than that extrapolated from Usselman's data, implying that previous thermal models of Fe-rich planetary cores could overestimate core temperature. If it is found in a meteorite, the Fe3S2 phase could also be used to infer the minimum size of a parent body.     

The Melting Curve of Iron to 250 Gigapascals: A Constraint on the Temperature at Earth's Center

The melting curve of iron, the primary constituent of Earth's core, has been measured to pressures of 250 gigapascals with a combination of static and dynamic techniques. The melting temperature of iron at the pressure of the core-mantle boundary (136 gigapascals) is 4800 +/- 200 K. whereas at the inner core-outer core boundary (330 gigapascals), it is 7600 +/- 500 K. Corrected for melting point depression resulting from the presence of impurities, a melting temperature for iron-rich alloy of 6600 K at the inner core-outer core boundary and a maximum temperature of 6900 K at Earth's center are inferred. This latter value is the first experimental upper bound on the temperature at Earth's center, and these results imply that the temperature of the lower mantle is significantly less than that of the outer core.     

Physics of iron at Earth's core conditions

The bulk properties of iron at the pressure and temperature conditions of Earth's core were determined by a method that combines first-principles and classical molecular dynamic simulations. The theory indicates that (i) the iron melting temperature at inner-core boundary (ICB) pressure (330 gigapascals) is 5400 (+/-400) kelvin; (ii) liquid iron at ICB conditions is about 6% denser than Earth's outer core; and (iii) the shear modulus of solid iron close to its melting line is 140 gigapascals, consistent with the seismic value for the inner core. These results reconcile melting temperature estimates based on sound velocity shock wave data with those based on diamond anvil cell experiments.     

Temperatures in Earth's Core Based on Melting and Phase Transformation Experiments on Iron

Experiments on melting and phase transformations on iron in a laser-heated, diamond-anvil cell to a pressure of 150 gigapascals (approximately 1.5 million atmospheres) show that iron melts at the central core pressure of 363.85 gigapascals at 6350 +/- 350 kelvin. The central core temperature corresponding to the upper temperature of iron melting is 6150 kelvin. The pressure dependence of iron melting temperature is such that a simple model can be used to explain the inner solid core and the outer liquid core. The inner core is nearly isothermal (6150 kelvin at the center to 6130 kelvin at the inner core-outer core boundary), is made of hexagonal closest-packed iron, and is about 1 percent solid (MgSiO(3) + MgO). By the inclusion of less than 2 percent of solid impurities with iron, the outer core densities along a thermal gradient (6130 kelvin at the base of the outer core and 4000 kelvin at the top) can be matched with the average seismic densities of the core.     

Lithium, compression and high-pressure structure

Lithium is found to transform from a body-centered cubic (bcc) to a face-centered cubic (fcc) structure at 6.9 gigapascals (69 kilobars) and 296 kelvin. The relative volume of the bcc structured lithium at 6.9 gigapascals is 0.718, and the fcc structure is 0.25 percent denser. The bulk modulus and its pressure derivative for the bcc structure are 11.57 gigapascals and 3.4, and for the fcc structure are 13.1 gigapascals and 2.8. Extrapolation of the bcc-fcc phase boundary and the melting curve indicate a triple point around 15 gigapascals and 500 kelvin.     

Serpentine stability to mantle depths and subduction-related magmatism

Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to approximately 720 degrees C at 2 gigapascals, to approximately 690 degrees C at 3 gigapascals, and to approximately 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H(2)O by weight to depths of 150 to 200 kilometers in subduction zones. This H(2)O is in an ideal position for ascent into the hotter, overlying mantle where it can cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of approximately 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H(2)O than does the dehydration of altered oceanic crust.     

Solidus of Earth's deep mantle

The solidus of a pyrolite-like composition, approximating that of the lower mantle, was measured up to 59 gigapascals by using CO2 laser heating in a diamond anvil cell. The solidus temperatures are at least 700 kelvin below the melting temperatures of magnesiowustite, which in the deep mantle has the lowest melting temperatures of the three major components-magnesiowustite, Mg-Si-perovskite, and Ca-Si-perovskite. The solidus in the deep mantle is more than 1500 kelvin above the average present-day geotherm, but at the core-mantle boundary it is near the core temperature. Thus, partial melting of the mantle is possible at the core-mantle boundary.     

Flotation of diamond in mantle melt at high pressure

Experiments show that diamond floats in a primitive mantle melt at around 20 gigapascals and 2360 degrees C and in a melt formed by partial melting of the transition zone at about 16 gigapascals and 2270 degrees C. These observations constrain magma densities at high pressure. Diamond precipitated or trapped in a silicate melt at the base of the transition zone or the lower mantle floats and has been accumulating in the transition zone since early in Earth's history. Thus, the transition zone could be a reservoir of diamond.     

Strength of diamond

The yield strength of diamond is measured under a pressure of 10 gigapascals at temperatures up to 1550 degrees C by the analysis of x-ray peak shapes on diamond diffraction lines in a powdered sample as a function of pressure and temperature. At room temperature, the diamond crystals exhibit elastic behavior with increasing pressure. Significant ductile deformation is observed only at temperatures above 1000 degrees C at this pressure. The differential yield strength of diamond decreases with temperature from 16 gigapascals at 1100 degrees C to 4 gigapascals at 1550 degrees C. Transmission electron microscopy observations on the recovered sample indicate that the dominant deformation mechanism under high pressure and temperature is crystal plasticity.     

Metallic CsI at pressures of up to 220 gigapascals

Direct electrical transport measurements in a diamond anvil cell provide evidence for the metallization of cesium iodide (CsI) at a pressure of 115 gigapascals. A drop in the temperature dependence of the resistance was found at pressures above 180 gigapascals, indicating that the CsI was superconductive. The superconductivity changed under the influence of a magnetic field to a lower critical temperature and disappeared above 0.3 tesla. The highest critical temperature at which superconductivity was observed was 2 kelvin, and the critical temperature decreased with increasing pressure.     

Melting of (Mg,Fe)2SiO4 at the Core-Mantle Boundary of the Earth

The lower mantle of the Earth is believed to be largely composed of (Mg,Fe)O (magnesiowustite) and (Mg,Fe)SiO3 (perovskite). Radiative temperatures of single-crystal olivine [(Mg0.9,Fe0.1)2SiO4] decreased abruptly from 7040 +/- 315 to 4300 +/- 270 kelvin upon shock compression above 80 gigapascals. The data indicate that an upper bound to the solidus of the magnesiowustite and perovskite assemblage at 4300 +/- 270 kelvin is 130 +/- 3 gigapascals. These conditions correspond to those for partial melting at the base of the mantle, as has been suggested occurs within the ultralow-velocity zone beneath the central Pacific.     

Protonic diffusion in high-pressure ice VII

Near ambient pressures, molecular diffusion dominates protonic diffusion in ice. Theoretical studies have predicted that protonic diffusion will dominate at high pressures in ice. We measured the protonic diffusion coefficient for the highest temperature molecular phase of ice VII at 400 kelvin over its entire stable pressure region. The values ranged from 10(-17) to 10(-15) square meters per second at pressures of 10 to 63 gigapascals. The diffusion coefficients extrapolated to high temperatures close to the ice VII melting curve were less by a factor of 10(2) to 10(3) than a superionic criterion of approximately 10(-8) square meters per second, at which protons would diffuse freely.     

Melting Temperature and Partial Melt Chemistry of H2O-Saturated Mantle Peridotite to 11 Gigapascals

The H2O-saturated solidus of a model mantle composition (Kilborne Hole peridotite nodule, KLB-1) was determined to be just above 1000°C from 5 to 11 gigapascals. Given reasonable H2O abundances in Earth's mantle, an H2O-rich fluid could exist only in a region defined by the wet solidus and thermal stability limits of hydrous minerals, at depths between 90 and 330 kilometers. The experimental partial melts monotonously became more mafic with increasing pressure from andesitic composition at 1 gigapascal to more mafic than the starting peridotite at 10 gigapascals. Because the chemistry of the experimental partial melts is similar to that of kimberlites, it is suggested that kimberlites may be derived by low-temperature melting of an H2O-rich mantle at depths of 150 to 300 kilometers.     

Spectroscopic evidence for pressure-induced coordination changes in silicate glasses and melts

Infrared spectra demonstrate that at pressures above 20 gigapascals and room temperature the regular tetrahedral coordination of oxygen around both silicon and aluminum ions is severely disrupted in SiO(2), CaMgSi(2)O(6), and CaAlSi(2)O(8) composition glasses. The spectra are consistent with gradual, pressure-induced increases in the coordination numbers of silicon and aluminum. A variety of coordination environments, from sixfold to fourfold, appears to be present at pressures as high as about 40 gigapascals. This apparent change in coordination is not quenchable at room temperature: on decompression, the glasses return to tetrahedral coordination. This continuous and reversible coordination change in amorphous silicates explains the lack of observation of coordination changes in silicate glasses quenched from high pressure, the shallow melting slopes observed for mantle silicates at high pressures, and the possible presence of neutrally buoyant magmas deep within the terrestrial planets.     

Crystal structures at megabar pressures determined by use of the cornell synchrotron source

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10(-9) cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.     

Phase transition of FeO and stratification in Earth's outer core

Light elements such as oxygen in Earth's core influence the physical properties of the iron alloys that exist in this region. Describing the high-pressure behavior of these materials at core conditions constrains models of core structure and dynamics. From x-ray diffraction measurements of iron monoxide (FeO) at high pressure and temperature, we show that sodium chloride (NaCl)-type (B1) FeO transforms to a cesium chloride (CsCl)-type (B2) phase above 240 gigapascals at 4000 kelvin with 2% density increase. The oxygen-bearing liquid in the middle of the outer core therefore has a modified Fe-O bonding environment that, according to our numerical simulations, suppresses convection. The phase-induced stratification is seismologically invisible but strongly affects the geodynamo.     

Metasomatized lithosphere and the origin of alkaline lavas

Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their trace-element and isotopic characteristics. Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere. In melting experiments on natural amphibole-rich veins at 1.5 gigapascals, we found that partial melts of metasomatic veins can reproduce key major- and trace-element features of oceanic and continental alkaline magmas. Moreover, experiments with hornblendite plus lherzolite showed that reaction of melts of amphibole-rich veins with surrounding lherzolite can explain observed compositional trends from nephelinites to alkali olivine basalts. We conclude that melting of metasomatized lithosphere is a viable alternative to models of alkaline basalt formation by melting of recycled oceanic crust with or without sediment.     

B1-b2 transition in calcium oxide from shock-wave and diamond-cell experiments

Volume and structural data obtained by shock-wave and diamond-cell techniques demonstrate that calcium oxide transforms from the B1 (sodium chloride type) to the B2 (cesium chloride type) structure at 60 to 70 gigapascals (0.6 to 0.7 megabar) with a volume decrease of 11 percent. The agreement between the shockwave and diamond-cell results independently confirms the ruby-fluorescence pressure scale to about 65 gigapascals. The shock-wave data agree closely with ultrasonic measurements on the B1 phase and also agree satisfactorily with equations of state derived from ab initio calculations. The discovery of this B1-B2 transition is significant in that it allows considerable enrichment of calcium components in the earth's lower mantle, which is consistent with inhomogeneous accretion theories.     

Effect of a Coordination Change on the Strength of Amorphous SiO2

Measurements of the yield strength of SiO(2) glass to pressures as high as 81 gigapascals at room temperature show that the strength of amorphous silica decreases significantly as it is compressed to denser strctures with higher coordination. Above 27 gigapascals, as the silicon in amorphous SiO(2) is continuously transformed from fourfold to sixfold coordination, the strength of the glass decrases by more than an order of magnitude. These data confirm theoretical predictions that the mechanical properties of polymerized amorphous silicates are sensitive to pressure-induced structural transformations and suggest that the viscosity of silica-rich liquids decreases significantly at high pressures. Such a change in melt rheology could enhance the processes of chemical differentiation with depth in the Earth's mantle.     

Pressure-induced landau-type transition in stishovite

A Rietveld structural analysis of stishovite, with angle-dispersive x-ray diffraction synchrotron source at the European Synchrotron Radiation Facility, confirmed a CaCl2 form of stishovite distortion at 54 +/- 1 gigapascals but confirmed no further phase transformation up to 120 gigapascals. The deviatoric stress that is usually encountered at such pressures was relaxed after yttrium-aluminum-garnet-laser heating. A single Birch-Murnaghan equation of state fits volumes of stishovite and a CaCl2 form, showing that the tetragonal distortion occurs without a substantial change in volume. At the 54-gigapascal transition, the pressure-induced lattice modifications were similar to those found in a Landau-type temperature-induced transition. It is proposed that, above the transition pressure, the critical temperature increases above 300 kelvin, so that the lower entropy form becomes stable.