Search

A new diamond‐anvil cell apparatus for in situ synchrotron X‐ray diffraction measurements of liquids and glasses, at pressures from ambient to 5 GPa and temperatures from ambient to 1300 K, is reported. This portable setup enables in situ monitoring of the melting of complex compounds and the determination of the structure and properties of melts under moderately high pressure and high temperature conditions relevant to industrial processes and magmatic processes in the Earth's crust and shallow mantle. The device was constructed according to a modified Bassett‐type hydrothermal diamond‐anvil cell design with a large angular opening (gθ = 95°). This paper reports the successful application of this device to record in situ synchrotron X‐ray diffraction of liquid Ga and synthetic PbSiO3 glass to 1100 K and 3 GPa. [ABSTRACT FROM AUTHOR]

Abstract We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO–MgO–SiO 2 –CO 2 (FMSC) and CaO–MgO–SiO 2 –CO 2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of ∼35 to 90 GPa at temperatures of ∼1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO 2 –V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite ±Ca-perovskite + CO 2 at pressures in the range of ∼40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than ∼1500 km. We also identify carbonate and CO 2 –V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes. Highlights • Experiments were made in the systems FMS-CO2 and CMS-CO2 at lower mantle pressures. • Carbonate in these systems react with silica in the lower mantle. • Decarbonation reactions with negative P–T slopes form CO 2 or diamond plus oxygen. • Carbonate in oceanic crust is limited to ∼1500 km depth along subduction geotherms. • Diamond is the stable form of carbon in oceanic crust at ambient mantle temperature. [ABSTRACT FROM AUTHOR]

In this article, the specification and application of the new double‐sided YAG laser‐heating system built on beamline I15 at Diamond Light Source are presented. This system, combined with diamond anvil cell and X‐ray diffraction techniques, allows in situ and ex situ characterization of material properties at extremes of pressure and temperature. In order to demonstrate the reliability and stability of this experimental setup over a wide range of pressure and temperature, a case study was performed and the phase diagram of lead was investigated up to 80 GPa and 3300 K. The obtained results agree with previously published experimental and theoretical data, underlining the quality and reliability of the installed setup. A double‐sided, off‐axis laser‐heating system for monochromatic X‐ray diffraction experiments using a diamond anvil cell has been installed at beamline I15 at Diamond Light Source. Measurements of the pressure–temperature phase diagram of Pb demonstrate the reliability of the setup and the high quality of the data. [ABSTRACT FROM AUTHOR]

The high-pressure structures of nickel monosilicide (NiSi) have been investigated to 124 GPa by synchrotron-based X-ray powder diffraction studies of quenched samples from laser-heated diamond anvil cell experiments, and the equations of state of three of these phases have been determined at room temperature. NiSi transforms from the MnP (B31) structure (space group Pnma) to the ɛ-FeSi (B20) structure (space group P213) at 12.5 ± 4.5 GPa and 1550 ± 150 K. Upon further compression, the CsCl (B2) structure (space group Pm3 m) becomes stable at 46 ± 3 GPa and 1900 ± 150 K. Thus, NiSi will be in the B2 structure throughout the majority of the Earth's mantle and its entire core, and will likely form a solid solution with FeSi, which is already known to undergo a B20 → B2 transition at high pressure. Data from the quenched (room-temperature) samples of all three phases have been fitted to the third-order Birch-Murnaghan equation of state. For the MnP (B31) structure this yields K0 = 165 ± 3 GPa with K0′ fixed at 4 and V0 fixed at 12.1499 Å3 atom−1 [ V0 from unpublished neutron diffraction measurements on the same batch of starting material; Wood (2011), personal communication]. For the ɛ-FeSi (B20) structure, K0 =161± 3 GPa and K0′ = 5.6 ± 0.2 with V0 fixed at 11.4289 Å3 atom−1. For the CsCl (B2) structure, K0 = 200 ± 9 GPa, K0′ = 4.6 ± 0.1 and V0 = 11.09 ± 0.05 Å3 atom−1. The ambient volume of NiSi, therefore, decreases by 6% at the first phase transition and then by a further 3% at the transition to the CsCl structure. Traces of additional NiSi structures predicted by Vočadlo, Wood & Dobson [ J. Appl. Cryst. (2012), 45, 186-196; part I], and labelled therein as Pbma-I, Pnma-II, and possibly also Pnma-III and P4/ nmm, have been detected. [ABSTRACT FROM AUTHOR]

The melting curve of Ni has been determined to 125 GPa using laser-heated diamond anvil cell (LH-DAC) experiments in which two melting criteria were used: firstly, the appearance of liquid diffuse scattering (LDS) during in situ X-ray diffraction (XRD) and secondly, plateaux in temperature vs. laser power functions in both in situ and off-line experiments. Our new melting curve, defined by a Simon–Glatzel fit to the data where T M ( K ) = [ ( P M 18.78 ± 10.20 + 1 ) ] 1 / 2.42 ± 0.66 × 1726 , is in good agreement with the majority of the theoretical studies on Ni melting and matches closely the available shock wave melting data. It is however dramatically steeper than the previous off-line LH-DAC studies in which determination of melting was based on the visual observation of motion aided by the laser speckle method. We estimate the melting point ( T M ) of Ni at the inner-core boundary (ICB) pressure of 330 GPa to be T M = 5800 ± 700 K ( 2 σ ) , within error of the value for Fe of T M = 6230 ± 500 K determined in a recent in situ LH-DAC study by similar methods to those employed here. This similarity suggests that the alloying of 5–10 wt.% Ni with the Fe-rich core alloy is unlikely to have any significant effect on the temperature of the ICB, though this is dependent on the details of the topology of the Fe–Ni binary phase diagram at core pressures. Our melting temperature for Ni at 330 GPa is ∼2500 K higher than that found in previous experimental studies employing the laser speckle method. We find that those earlier melting curves coincide with the onset of rapid sub-solidus recrystallization, suggesting that visual observations of motion may have misinterpreted dynamic recrystallization as convective motion of a melt. This finding has significant implications for our understanding of the high-pressure melting behaviour of a number of other transition metals. [ABSTRACT FROM AUTHOR]

The melting curve of NiSi has been determined to 70 GPa on the basis of laser-heated diamond anvil cell (LH-DAC) experiments in which changes in the gradient of temperature vs. laser power functions were used as the melting criterion. The melting curve was corroborated with in situ X-ray diffraction experiments in both the LH-DAC and multi-anvil press in which the appearance of liquid diffuse scattering in the diffraction patterns was used as the melting criterion. At all pressures, the NiSi melting curve is lower than that of FeSi, with the difference in melting temperature reaching a maximum of 900 K at 14 GPa. The location of the B31 + B20 + L triple point has been constrained to 12 ± 2 GPa and 1550 ± 100 K and the B20 + B2 + L triple point to 28.5 ± 1.5 GPa and 2165 ± 60 K. On the basis of the in situ LH-DAC experiments the Clapeyron slope of the B20 → B2 transition is estimated at -67 MPa K-1. Extrapolation of the B2-NiSi liquidus to core-mantle boundary (CMB) conditions (135 GPa) suggests the melting point of NiSi (3700 ± 400 K) will be only marginally lower than that of isostructural FeSi (4000 ± 200 K). Thus any (Fe,Ni)Si solid solution present within the D" layer is expected to remain solid, with the possible exception of the very hottest region adjacent to the CMB. [ABSTRACT FROM AUTHOR]

The phase boundary of the post-spinel transition in Mg2SiO4 was re-investigated by means of high P–T in situ X-ray diffractometry with a gold pressure marker in a Kawai-type apparatus. Rapid and continuous temperature changes were conducted to initiate dissociation of spinel, which tends to be inert after long annealing. Isothermal decompression at high temperature was conducted to form spinel from perovskite plus periclase. The phase boundary is located at ca. 22 GPa in the temperature range from 1550 to 2100 K, which is 1–1.5 GPa lower than the 660 km discontinuity. This discrepancy might be explained in terms of the pressure effect of thermocouple emf and inaccurate equation of state (EOS) for the pressure maker. The transition is found to be less sensitive to temperature than reported previously, with a Clapeyron slope ranging from −2 to −0.4 MPa/K. This small Clapeyron slope implies that the post-spinel transition would not be an effective barrier to mantle convection. [Copyright &y& Elsevier]