Maura S. Weathers

Shear‐promoted phase transitions in Fe2SiO4 and Mg2SiO4 and the mechanism of deep earthquakes

Phase transformations in end-member olivines have been investigated in the temperature range comparable to the interior of subducting slabs. This work constitutes the experimental evidence that the kinetics of transformation of silicate olivine (α phase) to modified spinel (β) and spinel (γ) phases is enhanced by shear deformation. Natural fayalite (α-Fe2SiO4) subjected to a pressure gradient from 0 to 25 GPa at 380°C in the diamond anvil cell (DAC) developed a ring of γ phase where the pressure was in the stability field of the γ phase and shear stress was large enough to promote the α→γ transition. The sample inside the ring, despite being at higher pressure, remained dominantly as α phase. The outermost, lower-pressure region of the sample also remained as α phase. In the Mg2SiO4 system, the transition from α to β was observed at 575°C in runs in which pressure covered the stability fields of β phase, γ phase and mixed oxides. These results show that the characteristic transformation temperature TCh can be lowered as much as ∼200°C by shear deformation. On the basis of these observations, we propose a nonhydrostatic kinetic boundary for the α→β and α→γ transitions in mantle olivine. For temperatures below this boundary, the transformations are kinetically inhibited, while above it, the transformations can be promoted by shear deformation. Therefore, olivine carried to a depth of several hundred kilometers in a subducting slab can remain as metastable α phase until shear deformation causes its transformation. We suggest that this region of shear-promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes.

Josephinite: Specimens from the earth's core?

Abstract Josephinite is a terrestrial iron-nickel alloy with an intergrown magnesium silicate, and arsenide and sulphide phases, and andradite garnet; several specimens have been found to contain elemental silicon and CaO · 2“FeO”. Josephinite is not awaruite, an iron-nickel mineral formed by serpentinization of ultramafic rocks. Because of its geologic setting and unique mineralogy we propose that josephinite might have originated in the region of the coremantle boundary, was transported via a deep-mantle “plume” and diatreme mechanism into lithosphere mantle that has been emplaced in the Klamaths by ophiolite obduction. Regardless of such a hypothesis, we report here the discovery of terrestrial silicon occurring with josephinite, which seems to preclude a lithosphere environment of origin for josephinite.

Compressibility of SiC up to 68.4 GPa

A diamond anvil cell was used with synchrotron radiation to measure the compressibility of the 6H polytype of SiC (moissanite) up to 68.4 GPa. NaCl was used as the pressure medium, and Au was used as the pressure calibrant. The effect of pressure gradient was minimized by using a very small x‐ray beam (0.015 mm diam), and uniaxial stress was minimized by using three times as much volume of NaCl as SiC. The 101 and 102 peaks of SiC were measured as a function of pressure, and the molar volumes based on them were in excellent agreement. Pressure was calculated from the 111, 200, and 220 peaks of Au. The data when fit to the Birch–Murnaghan equation yield a bulk modulus of K0=230.2±4.0 GPa if the K’0 is set at 4. These values are in good agreement with earlier unpublished measurements (K0=234.3±4.9 GPa; K’0=4) made by us in which NaCl was used as the pressure calibrant. Our compression measurements appear to indicate a stiffer behavior than those based on shock measurements.

Melting of carbon at 50 to 300 kbar

Diamond (∼1 μm) and graphite (1–10 μm) in NaCl were melted at 50 to 300 kbar in a diamond anvil cell using a pulsed YAG laser. The samples were removed from the cell and the structures of the quenched phases were studied by transmission electron microscopy.The melted regions of the samples were found to consist of nearly perfect spheres of carbon ranging in size from ∼1 μm down to less than a few nanometers. In the diamond sample melted at 300 kbar, the larger spherules (>0.2 μm) are polycrystalline diamond with either a granular or radial texture. The smaller spherules (<0.2 μm) give electron diffraction patterns with four diffuse rings that correspond to the 002, 100, 004 and 110 of graphite. This diffraction pattern is typical of disordered graphite randomly oriented about the c-axis. Dark field imaging, using a portion of the 002 ring, produces a “bow tie” figure in each of the smaller spherules. The orientation of the “bow tie” figure depends on the portion of the ring used to form the image, and indicates a radial orientation of the c-axis of the disordered graphite. The spacing between the 002 layers depends on the pressure at the time of melting. We interpret this to indicate that there is some sp3 bonding between layers in the disordered graphite in the smaller spherules. The smaller spherules may have the disordered graphite structure because of the effect of the size on the free energy relationship between the phases, or they may have been quenched more rapidly than the larger spherules thus preserving some of the character of the melt. If the latter explanation is correct, then our results may indicate that the diamond melt contains significant sp2 bonding.Lattice images (Fig. 12) of the internal structure of the smallest spherules observable (∼50 A) clearly show that the carbon layers are parallel to the surface of the spherules and that there is a great deal of disorder in the layers. These observations are entirely consistent with our conclusions based on the dark field images.

Melting of Diamond

Radiation from a Q-switched YAG laser, focused on the (100) face of a single crystal diamond anvil in a high-pressure diamond cell, caused a portion of the diamond anvil face to melt. Potassium bromide mixed with graphite was under pressure between the anvils when melting occurred. The diamond surface melted at pressures greater than ∼120 kilobars and graphitized at lower pressures. Evidence for the melting and graphitization of the diamond was obtained by optical and scanning electron microscopy.

Native Iron Occurrences of Disko Island, Greenland

Field and petrographic relations indicate that the commonly accepted theory that the native iron of Disko formed by reduction of basalt or sulphides by inclusions of coal or carbonaceous sediments during eruption of basalt is not valid. Current models for basalt generation indicate that basalts are erupted through lithosphere. Xenoliths of iron in Disko Island basalt were, most likely, carried with the erupting basalt. Therefore, the iron xenoliths may be from the sub-lithosphere mantle, from regions deep enough for iron to have remained unoxidized.

Phase diagram and elastic properties of Fe 30% Ni alloy by synchrotron radiation

Diffusionless phase transitions of Fe-Ni alloy with 30 wt % Ni (Fe30Ni) were studied at high temperature and pressure by synchrotron radiation. Several isothermal runs ranging from 25°C to 600°C were carried out at up to 20–60 GPa to determine the phase relationships and elastic properties of the alloy. The ruby fluorescence method was used for pressure measurements at room temperature. The compressibility of the fcc phase of the alloy, determined up to 48 GPa, has a bulk modulus of 160 ± 15 GPa at room temperature assuming K′0 = 4. The average linear thermal expansion coefficient of the fcc phase of Fe30Ni was estimated to be 10.2 ± 1.5 × 10−6 K−1 between 25°C and 600°C. The pressure at high temperature was calculated using the equation of state of the fcc phase assuming that the compressibility of the alloy at high temperature remains the same as at 25°C. The Fe30Ni alloy has both fcc and bcc structures at ambient conditions. However, the bcc phase disappeared (bcc-out) after pressure was applied to the sample at various temperatures. The bcc-out transition boundary has a negative slope of −55° ± 5°C/GPa. The fcc phase transformed to a hexagonal close-packed phase at high pressure (hcp-in). The slope of the phase boundary of hcp-in was determined to be 25 ± 5°C/GPa. This result, combined with our previous investigation (Huang et al., 1988), indicates that the slope of the fcc/hcp phase boundary in Fe-Ni alloys remains constant with an increase of Ni content up to 30 wt %.

Anisotropic thermal expansion of calcite at high pressures; an in situ X-ray diffraction study in a hydrothermal diamond-anvil cell

Abstract Lattice parameters of calcite were measured at simultaneous high pressures and temperatures up to 10 kbar and 500°C. Samples of Solnhofen limestone and distilled, deionized water were loaded in a hydrothermal diamond-anvil cell. In situ energy-dispersive X-ray diffraction was used to determine lattice parameters along five H2O isochores with densities of 1.040, 0.964, 0.760, 0.670, and 0.595 g/cm3 from 30 to 500°C; these densities correspond to the ice-melting temperature of -7.7 °C and the liquid-vapor homogenization temperatures 92.4,274.5, 318.9, and 344.6 °C, respectively. In addition, data along the P axis were collected by hydrostatic compression at room temperature and along the T axis by heating at atmospheric pressure. Our results show that anisotropic thermal expansion of calcite continues up to 10 kbar, therefore making it a good double internal X-ray standard. Both a and c lattice parameters are fitted to second-order polynomials of pressure, temperature, and a cross term.

Liquid carbon in the lower mantle

Elemental carbon in the lower mantle might be a metallic liquid. If present in sufficient concentrations, this substance might promote the ascent of heat and matter from the deep mantle by locally increasing thermal conductivity and reducing viscosity. We propose that such carbon in the lower mantle induces mantle plumes.

Excess 3He and 21Ne in josephinite

Abstract Analyses of noble gases in josephinite, an Fe-Ni alloy-bearing rock from Josephine County, Oregon, indicate the presence of excess 3 He and 21 Ne. We infer a 3 He/ 4 He ratio in josephinite of ≈5 × 10 −4 . The origin of the 21 Ne excess is unknown; it could be the product of nuclear reactions within the earth, or possibly, from a previously unrecognized primordial noble gas component rich in 21 Ne and possibly also 3 He and 129 Xe.