Christopher M. Holl

The COMPRES/GSECARS gas-loading system for diamond anvil cells at the Advanced Photon Source

We have designed and constructed a new system for loading gases at high pressure into diamond anvil cells at pressures up to 200 MPa. The gases are used either as quasi-hydrostatic pressure media surrounding the sample or as the sample itself. The diamond cell is sealed using a clamping mechanism, which permits nearly any type of diamond anvil cell to be used. Online ruby fluorescence and video imaging systems allow in situ monitoring of the pressure and gasket deformation as the cell is sealed, resulting in a very high success rate in loading cells. The system includes interlocks and computer control that allow it to be safely and easily operated by visiting users at the Advanced Photon Source. We present preliminary X-ray diffraction data on volume compression of single-crystal magnesium oxide (MgO) in helium up to 110 GPa.

Structural systematics of hydrous ringwoodite and water in Earth’s interior

Abstract Seven separate samples of hydrous ringwoodite with compositions ranging from Fo100 to Fo89 and hydrogen contents from 0.2 to 1.1 wt% were synthesized in the 5000 ton multi-anvil press at the Bayerisches Geoinstitut. Synthesis conditions ranged from 18 to 22 GPa and 1400 to 1500 °C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mössbauer spectroscopy, and by analytical and high-resolution transmission electron microscopy. The crystals are optically isotropic, and the Fe-bearing samples are deep blue in color. Mössbauer spectroscopy and ELNE spectroscopy applied to the Fe-bearing samples indicates about 10% of the iron is in the ferric state. High-resolution TEM examination of one of the Fe-bearing samples indicates that the crystals are homogeneous and free of significant inclusions or exsolution features. Infrared spectra show a broad absorption band extending from about 2500 to 3600 cm-1 with maxima ranging from 3105 for the pure magnesian samples to 3150 cm-1 for the Fo89 samples. The crystal structures of the seven ringwoodite samples were refined by X-ray single-crystal diffraction. Refinement of cation site occupancies indicates full occupancy of the tetrahedral site for all samples, whereas the occupancy of the octahedral site appears to decrease systematically with H content. The principal hydration mechanism involves octahedral cation vacancies. The IR spectra are consistent with protonation of the short O-O approach on the tetrahedral edge, which would imply partial Mg-Si disorder.

Compression of single-crystal magnesium oxide to 118 GPa and a ruby pressure gauge for helium pressure media

Abstract The pressure-volume equation of state (EoS) of single-crystal MgO has been studied in diamondanvil cells loaded with helium to 118 GPa and in a non-hydrostatic KCl pressure medium to 87 GPa using monochromatic synchrotron X-ray diffraction. A third-order Birch-Murnaghan fit to the nonhydrostatic P-V data (KCl medium) yields typical results for the initial volume, V0 = 74.698(7) Å3, bulk modulus, KT0 = 164(1) GPa, and pressure derivative, K′ = 4.05(4), using the non-hydrostatic ruby pressure gauge of Mao et al. (1978). However, compression of MgO in helium yields V0 = 74.697(6) Å3, KT0 = 159.6(6) GPa, and K′ = 3.74(3) using the quasi-hydrostatic ruby gauge of Mao et al. (1986). In helium, the fitted equation of state of MgO underdetermines the pressure by 8% at 100 GPa when compared with the primary MgO pressure scale of Zha et al. (2000), with KT0 = 160.2 GPa and K′ = 4.03. The results suggest that either the compression mechanism of MgO changes above 40 GPa (in helium), or the ruby pressure gauge requires adjustment for the softer helium pressure medium. We propose a ruby pressure gauge for helium based on shift of the ruby-R1 fluorescence line (Δλ/λ0) and the primary MgO pressure scale, with P (GPa) = A/B{[1 + (Δλ/λ0)]B - 1}, where A is fixed to 1904 GPa and B = 10.32(7).

Elasticity of hydrous wadsleyite to 12 GPa: Implications for Earth's transition zone

Knowledge of the pressure effect on elasticity of hydrous olivine polymorphs is necessary to model seismic wave speeds for potential hydrous regions of the mantle. Here we report single-crystal elastic properties of wadsleyite, {beta}-Mg2SiO4, with 0.84 wt.% H2O measured to 12 GPa by Brillouin scattering. Pressure derivatives of the aggregate bulk modulus, K{prime} S0, and shear modulus, G{prime}0, of hydrous wadsleyite are 4.1(1) and 1.4(1) respectively. These values are indistinguishable within uncertainty from those of anhydrous wadsleyite. We estimate that {approx}1 wt.% H2O in wadsleyite at 410-km depth can reconcile seismic bulk sound velocities with a pyrolite-composition mantle by using our measured high-pressure elastic constants. If the H2O content of the mantle is much less than 1 wt.%, then other factors need to be considered to explain the velocity contrast of the 410-km discontinuity. Variations in water content with depth may also contribute to the anomalously steep seismic velocity gradient in the mantle transition zone.

Equation of state of hydrous Fo90 ringwoodite to 45 GPa by synchrotron powder diffraction

Abstract The equation of state of Fo90 hydrous ringwoodite has been measured using X-ray powder diffraction to 45 GPa at the GSECARS beam line at the Advanced Photon Source synchrotron at Argonne National Laboratory. The sample was synthesized at 1400°C and 20 GPa in the 5000 ton multi anvil press at Bayerisches Geoinstitut in Bayreuth. The sample has the formula Mg1.70Fe0.192+Fe0.023+H0.13-Si1.00O4 as determined by electron microprobe, Fourier transform infrared and Mössbauer spectro-scopies, and contains ~0.79% H2O by weight. Compression of the sample had been been measured previously to 11 GPa by single crystal X-ray diffraction. A third-order Birch-Murnaghan equation of state fit to all of the data gives V0 = 530.49±0.07 Å3, K0 = 174.6±2.7 GPa and Kʹ = 6.2±0.6. The effect of 1% H incorporation in the structure on the bulk modulus is large and roughly equivalent to an increase in the temperature of ~600°C at low pressure. The large value of Kʹ indicates significant stiffening of the sample with pressure so that the effect of hydration decreases with pressure.

Correction to “Effects of hydration on the elastic properties of olivine”

[1] In the paper ‘‘Effects of hydration on the elastic properties of olivine’’ by S. D. Jacobsen et al. (Geophysical Research Letters, 35, L14303, doi:10.1029/2008GL034398, 2008), the sample of hydrous olivine labeled hy-Fo97 with (001) orientation in the bottom plot of original Figure 1b has been subsequently identified by Raman spectroscopy as OH-chondrodite, (Mg,Fe)5Si2O8(OH)2 [e.g., Lin et al., 1999]. The OH-chondrodite co-existed with hydrous forsterite in the synthesis run, and all other samples in the study have been confirmed to be hydrous forsterite. Upon removing the OH-chondrodite platelet from the fit, we obtain a corrected set of elastic constants (Cij) and crystallographic orientations for hy-Fo97 using a two-plane fit, displayed in corrected Figure 1 and presented in corrected Table 1. The original Table 2 of anisotropy factors has been updated and presented here in corrected Table 2. Brillouin spectra from the two remaining orientations of hy-Fo97 determine eight of the nine Cij, leaving C12 unconstrained. As a result, C12 was fixed to the value obtained for hy-Fo100 and a large uncertainty of ±5 GPa in this parameter was assumed in calculating the aggregate bulk (KS0) and shear (G) moduli. [2] In addition, a minor correction to the elastic constants of hydrous forsterite (hy-Fo100) is presented in revised Table 1 because the original calculation used an earlier estimated density of 3.19 g/cm, instead of the actual measured X-ray density of 3.180(3) g/cm. The measured X-ray density of 3.180(3) g/cm was correctly reported in the original text, but not used in the calculation of Cij. The revised Cij of hydrous forsterite are affected by only 0.2–0.3% from the original calculation as a result of the error. [3] The revised values of elastic properties for hy-Fo100 and hy-Fo97 presented in the corrected Table 1 apply to the following statements in the text: [4] The last four sentences of paragraph [1] should read: The adiabatic bulk (KS0) and shear (G0) moduli of hy-Fo100 are 125.4(±0.2) GPa and 79.6(±0.1) GPa, respectively. For hy-Fo97, we obtain KS0 = 125.2(±0.8) GPa and G0 = 77.7(±0.3) GPa. Compared with anhydrous forsterite, the combined effects of 3 mol% Fe and 0.8 wt% H2O reduce bulk and shear moduli by 2.9(±0.6)% and 4.5(±0.4)% respectively, with greater reductions expected for more iron-rich Fo90 mantle compositions. Although lattice preferred orientation (LPO) studies have not been carried out under relevant conditions of water or pressure, analysis of idealized single-crystal anisotropy for various known LPO types predicts no more than 2% effect of hydration on Swave splitting anisotropy in olivine. [5] The last sentence of paragraph [9] should read: We measured two platelets of hy-Fo97 with fitted orientations of (100) and (010), shown in the corrected Figure 1b. [6] The last two sentences of paragraph [10] should read: The addition of 0.89 wt% H2O to forsterite in our hy-Fo100 samples shows a reduction of all Cij by 1.8–4.3%, except C33, which is reduced by only 0.8%. For hy-Fo100, we obtain KS0 = 125.4(±0.2) GPa and G0 = 79.6(±0.1) GPa, which are about 2.7% and 2.2% lower than anhydrous forsterite, respectively. [7] The first two sentences of paragraph [11] should read: Comparing the Cij of hy-Fo97 with anhydrous Fo100 to ascertain the net effect of iron and hydration shows that there is a large reduction in Cij by 2.4–6.4%, except for C23, which increased by 2.1%. For hy-Fo97, we obtain KS0 = 125.2(±0.8) GPa and G0 = 77.7(±0.3) GPa, which are 2.9% and 4.5% lower than anhydrous forsterite. [8] The last sentence of paragraph [11] should read: The aggregate hy-Fo97 velocities Vp and Vs (with only 3 mol% Fe) are 2.1% and 2.4% lower, respectively, than anhydrous forsterite, suggesting that hydrous Fo90 olivine, closer to mantle composition, would exhibit even further reduced velocities.