Simon M. Clark

Primary carbonatite melt from deeply subducted oceanic crust

Partial melting in the Earth’s mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical ‘reservoirs’ and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here we provide experimental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

A BEAMLINE FOR HIGH PRESSURE STUDIES AT THE ADVANCED LIGHT SOURCE WITH A SUPERCONDUCTING BENDING MAGNET AS THE SOURCE

A new facility for high-pressure diffraction and spectroscopy using diamond anvil high-pressure cells has been built at the Advanced Light Source on beamline 12.2.2. This beamline benefits from the hard X-radiation generated by a 6 T superconducting bending magnet (superbend). Useful X-ray flux is available between 5 keV and 35 keV. The radiation is transferred from the superbend to the experimental enclosure by the brightness-preserving optics of the beamline. These optics are comprised of a plane parabola collimating mirror, followed by a Kohzu monochromator vessel with Si(111) crystals (E/DeltaE approximately equal 7000) and W/B4C multilayers (E/DeltaE approximately equal 100), and then a toroidal focusing mirror with variable focusing distance. The experimental enclosure contains an automated beam-positioning system, a set of slits, ion chambers, the sample positioning goniometry and area detector (CCD or image-plate detector). Future developments aim at the installation of a second endstation dedicated to in situ laser heating and a dedicated high-pressure single-crystal station, applying both monochromatic and polychromatic techniques.

High pressure single-crystal micro X-ray diffraction analysis with GSE_ADA/RSV software

GSE_ADA/RSV is a free software package for custom analysis of single-crystal micro X-ray diffraction (SCμXRD) data, developed with particular emphasis on data from samples enclosed in diamond anvil cells and subject to high pressure conditions. The package has been in extensive use at the high pressure beamlines of Advanced Photon Source (APS), Argonne National Laboratory and Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The software is optimized for processing of wide-rotation images and includes a variety of peak intensity corrections and peak filtering features, which are custom-designed to make processing of high pressure SCμXRD easier and more reliable.

Pressure-induced polyamorphism and formation of ’aragonitic’ amorphous calcium carbonate

Pressure-Induced Polyamorphism and Formation of ‘Aragonitic’ Amorphous Calcium Carbonate Pressed for time : Amorphous calcium carbonate (ACC) undergoes a reversible amorphous–amorphous phase transition at 10 GPa, adopting an aragonite-like local order. This result suggests a mechanism by which Mg—a cation with smaller ionic radius than Ca—modifies the local order of ACC to an aragonite-like order by helping to decrease the molar volume of the amorphous phase. Angewandte Chemie

Halite-sylvite thermoelasticity

Abstract Unit-cell volumes of four single-phase intermediate halite-sylvite solid solutions have been measured to pressures and temperatures of ~28 kbar and ~700 °C. Equation-of-state fitting of the data yields thermal expansion and compressibility as a function of composition across the chloride series. The variation of the product α0·K0 is linear (ideal) in composition between the accepted values for halite and sylvite. Taken separately, the individual values of α0 and K0 are not linear in composition. α0 shows a maximum near the consolute composition (XNaCl = 0.64) that exceeds the value for either end-member. There is a corresponding minimum in K0. The fact that the α0·K0 product is variable (and incidentally so well behaved as to be linear across the composition series) reinforces the significance of the complementary maxima and minima in α0 and K0 (significantly, near the consolute composition). These extrema in α0 and K0 provide an example of intermediate properties that do not follow simply from values for the end-members. Cell volumes across this series show small, well-behaved positive excesses, consistent with K-Na substitution causing defects through lattice mismatches. Barrett and Wallace (1954) showed maximum defect concentrations in the consolute region. Defect-riddled, weakened structures in the consolute region are more easily compressed or more easily thermally expanded, providing an explanation for our observed α0 and K0 variations. These compliant, loosened lattices should resist diffusive transfer less than non-defective crystals and, hence, might be expected to show higher diffusivities. Tracer diffusion rates are predicted to peak across the consolute region as exchange diffusion rates drop to zero.

Thermal equations of state for B1 and B2 KCl

Abstract Compressibility of solids generally drops as compression proceeds. Bonds shorten and stiffen with density increase in the same structure. However for the B1-B2 phase transition in many measured alkali halides, bond lengths increase in going to the denser phase. And IR mode frequencies decrease (Hofmeister 1997), leading to the counter-intuitive expectation that compressibility should increase in going to the denser B2 phase. Past volume measurements have been equivocal at best in recognizing any such compressibility increase. New thermal equations of state for B1 and B2 KCl from in-situ X-ray diffraction measurements of phase volumes on Station 16.4 of the CLRC Daresbury Laboratory are no less equivocal about the issue of whether compressibility increases or decreases across the transition. In contrast, the new volume measurements show an easily resolved thermal expansion increase in going from B1 to B2 KCl. The product a·K, which is better known than either α or K, increases from 0.0195 ± 0.0005 kbar/°C in B1 to 0.0275 ± 0.0009 kbar/°C in B2 KCl. Yagi (1978) demonstrated a similar increase for KF, also supported mainly by increases in a. This increase can also be seen in RbCl (Walker et al. 2001). Bond weakening indicated by the thermal expansion increase is consistent with the elusive compressibility increase that is expected across the B1-B2 transition but which is not resolved from volume measurements. The thermal effects are more visible than the compressional effects on α·K across the transition. Bond tightening upon decompression reduces α, increases solid viscosity, and hence decreases the Rayleigh number. An upwelling of material undergoing a decompression phase change with decrease of coordination number may have its convective friskiness damped at such a transition

An infrared spectroscopic study of the OH stretching frequencies of talc and 10-Å phase to 10 GPa

Abstract The effects of pressure on the OH stretching frequencies of natural talc and two samples of synthetic 10-Å phase have been measured using a diamond-anvil cell and a synchrotron infrared source. The 10-Å phase was synthesized at 6.0-6.5 GPa, 600 °C for 46 hours (sample 10Å-46) and 160 hours (10Å-160). Spectra were collected up to 9.0 GPa (talc), 9.9 GPa (10Å-46), and 9.6 GPa (10Å-160). The OH stretching vibration of Mg3OH groups in talc occurs at 3677 cm-1 at ambient pressure, and increases linearly with pressure at 0.97(2) cm-1 GPa-1. The same vibration occurs in 10-Å phase, but shows negligible pressure shift up to 2 GPa, above which the frequency increases linearly to the maximum pressure studied, at a rate of 0.96(3) cm-1 GPa-1 (10Å-46) and 0.87(3) cm-1 GPa-1 (10Å- 160). Two other prominent bands in the 10-Å phase spectrum are suggested to be due to stretching of interlayer H2O, hydrogen-bonded to the nearest tetrahedral sheet. These bands also show little change over the first 2 GPa of compression, as most of the compression of the structure is taken up by closing non-hydrogen bonded gaps between interlayer H2O and tetrahedral sheets. Between 2 and 4 GPa, changes in band intensities suggest a rearrangement of the interlayer H2O.

Determination of the variation of the fluorescence line positions of ruby, strontium tetraborate, alexandrite, and samarium-doped yttrium aluminum garnet with pressure and temperature

The pressure and temperature dependent fluorescence line-shift of strontium tetraborate has been measured concurrently with x-ray diffraction from the pressure standards sodium chloride or gold. Temperature was found to have a small effect on the fluorescence line-shift under pressure. We found a maximum pressure uncertainty of ±1.8 GPa at 25 GPa (7.2%) and 857 K when making no temperature correction. The fluorescence line-shifts for ruby, Alexandrite, and samarium-doped yttrium aluminum garnet were also determined, using our strontium tetraborate calibration to determine pressure and a thermocouple to measure temperature. Fluorescence measurements were extended up to 800 K for ruby and Alexandrite. Temperature was found to have a small effect on the fluorescence line-shift of samarium-doped yittrium aluminum garnet. We found a maximum uncertainty of ±2.7 GPa at 25 GPa (11.1%) and 857 K when no temperature correction was applied. We determined equations relating to the fluorescence line position from thes...

Thermo-compression of pyrope-grossular garnet solid solutions: Non-linear compositional dependence

Abstract Unit-cell parameters of a series of synthetic garnets with the pyrope, grossular, and four intermediate compositions were measured up to about 900 K and to 10 GPa using synchrotron X-ray powder diffraction. Coefficients of thermal expansion of pyrope-grossular garnets are in the range 2.10-2.74 × 10-5 K-1 and uniformly increase with temperature. Values for the two end-members pyrope and grossular are identical within experimental error 2.74 ± 0.05 × 10-5 K-1 and 2.73 ± 0.01 × 10-5 K-1, respectively. Coefficients of thermal expansion for intermediate compositions are smaller than those of end-members and are not linearly dependent on composition. Bulk modulus of grossular is Κ0 = 164.3(1) GPa (with Κ0′ the pressure derivative of the bulk modulus fixed to 5.92) and bulk modulus of pyrope is Κ0 = 169.2(2) GPa (with Κ0′ fixed to 4.4) using a third-order Birch-Murnaghan equation of state, which are consistent with previously reported values. The bulk moduli of garnets of intermediate composition are between ~155 and ~160 GPa, smaller than those of the end-members no matter which Κ0′ is chosen. The compositional dependence of bulk modulus resembles the compositional dependence of thermal expansion. Intermediate garnets on this binary have large positive excess volume, which makes them more compressible. We find that excess volumes in the pyrope-grossular series remain relatively large even at high pressure (~6 GPa) and temperature (~800 K), supporting the observation of crystal exsolution on this garnet join. The curiously “W”-shaped compositional variation of thermal expansion and bulk modulus is anti-correlated with the compositional dependence of microstrain documented in our companion paper (Du et al. in preparation) on the excess volumes in this series of garnets. Minimum thermal expansions and bulk moduli go with maximum microstrains.

Observation of polyamorphism in the phase change alloy Ge1Sb2Te4

A high-pressure synchrotron x-ray diffraction study of the phase change alloy Ge1Sb2Te4 demonstrates the existence of a polyamorphic phase transition between the “as deposited” low density amorphous (LDA) phase and a high density amorphous (HDA) phase at ∼10 GPa. The entropy of the HDA phase is expected to be higher than that of the LDA phase resulting in a negative Clapeyron slope for this transition. These phase relations may enable the polyamorphic transition to play a role in the memory and data storage applications.