Wendy R. Panero

Compressibility and thermal expansion of hydrous ringwoodite with 2.5(3) wt% H2O

Abstract Ringwoodite (γ-Mg2SiO4) is the stable polymorph of olivine in the transition zone between 525-660 km depth, and can incorporate weight percent amounts of H2O as hydroxyl, with charge compensated mainly by Mg vacancies (Mg2+ = 2H+), but also possibly as (Si4+ = 4H+ and Mg2+ + 2H+ = Si4+). We synthesized pure Mg ringwoodite containing 2.5(3) wt% H2O, measured by secondary ion mass spectrometry (SIMS), and determined its compressibility at 300 K by single-crystal and powder X-ray diffraction (XRD), as well as its thermal expansion behavior between 140 and 740 K at room pressure. A third-order Birch-Murnaghan equation of state (BM3 EOS) fits values of the isothermal bulk modulus KT0 = 159(7) GPa and (dKT/dP)P=0 = K′ = 6.7(7) for single-crystal XRD; KT0 = 161(4) GPa and K′ = 5.4(6) for powder XRD, with KT0 = 160(2) GPa and K′ = 6.2(3) for the combined data sets. At room pressure, hydrous ringwoodite breaks down by an irreversible unit-cell expansion above 586 K, which may be related to dehydration and changes in the disorder mechanisms. Single-crystal intensity data were collected at various temperatures up to 736 K, and show that the cell volume V(cell) has a mean thermal expansion coefficient αV0 of 40(4) ×10−6/K (143-736 K), and 29(2) ×10−6/K (143-586 K before irreversible expansion). V(Mg) have α0 values of 41(3) ×10−6/K (143-736 K), and V(Si) has α0 values of 20(3) ×10−6/K (143-586 K) and 132(4) ×10−6K (586-736 K). Based on the experimental data and previous work from 29Si NMR, we propose that during the irreversible expansion, a small amount of H+ cations in Mg sites transfer to Si sites without changing the cubic spinel structure of ringwoodite, and the substituted Si4+ cations move to the normally vacant octahedral site at (½, ½, 0). Including new SIMS data on this and several Mg-ringwoodite samples from previous studies, we summarize volume-hydration data and show that the Mg2+ = 2H+ dominates up to about 2 wt% H2O, where a discontinuity in the volume vs. H2O content trend suggests that other hydration mechanisms become important at very high H2O contents.

Temperature gradients in the laser‐heated diamond anvil cell

A semianalytic heat transfer model reproduces the temperature distributions measured inside the laser-heated diamond cell and constrains the temperature dependence of the thermal conductivity of dielectric mineral samples at high pressures and temperatures. Both axial and radial heat conduction determine the observed temperature distributions obtained from the TEM00 (Gaussian) and TEM01* laser modes. Measurements on the high-pressure perovskite-dominated assemblage of peridotite are consistent with the expected T−1 dependence of thermal conductivity at 45 GPa and temperatures between 1500 and 3800 K.

Forsterite, hydrous and anhydrous wadsleyite and ringwoodite (Mg2SiO4): 29Si NMR results for chemical shift anisotropy, spin-lattice relaxation, and mechanism of hydration

Abstract We present a detailed 29Si NMR spectroscopic study of isotopically enriched samples of forsterite and of anhydrous and hydrous wadsleyite and ringwoodite (α, β, and γ phases of Mg2SiO4), which complement previous extensive studies of these minerals by XRD and vibrational spectroscopy. VISi is not detected in any of the phases at levels of about 0.1 to 0.5%. When coupled with recent theoretical calculations on ringwoodite, this suggests the possibility of re-ordering of high-temperature octahedral-tetrahedral disorder during cooling. Cross-polarization (29Si{1H} CPMAS) NMR supports the protonation of O1 oxygen atoms in hydrous wadsleyite without formation of significant amounts of Si-OH groups. In contrast, new NMR peaks appear in hydrous ringwoodite that cross-polarize very rapidly, indicating very short Si-H distances and the presence of Si-OH, as expected from models in which much of the H+ substitutes into Mg2+ vacancies. Static NMR spectra provide new constraints on chemical shift anisotropies in wadsleyite and are fully consistent with the cubic structure of ringwoodite. Spin-lattice relaxation in all phases is much better fitted by a stretched exponential function than with a more conventional “T1” exponential, as expected when relaxation is dominated by paramagnetic impurities. However, the effects of paramagnetic impurity on ion contents on relaxation, and on the formation of newly observed minor peaks that may result from “pseudo-contact shifts,” appear to depend on mineral structure, and will require considerable future study to understand in detail.

Influence of anelastic surface layers on postseismic thrust fault deformation

We present the results of a systematic modeling study of postseismic deformation following blind thrust earthquakes. The results include qualitative and quantitative predictions of the surface movement caused by relaxation in viscoelastic near-surface layers. Finite element forward models are used in conjunction with elastic dislocation inversions to characterize the post-seismic deformation. A viscoelastic surface layer overlying a blind thrust fault in an elastic basement shows characteristic signatures of postseismic surface movement. Simple equivalent elastic dislocations located in the hanging wall wedge are found to provide an effective proxy for near-surface postseismic relaxation in two-dimensional numerical simulations. A model survey of a range of fault dip angles and layer geometries shows the time evolution and geometry of the proxy fault to be simply related to fault dip and sediment thickness. The results are of significance in the interpretation of postseismic Global Positioning System (GPS) strain data from the 1994 Northridge, California, earthquake and other similar events in regions characterized by poorly consolidated or otherwise anelastic layers overlying the brittle seismogenic zone.

Forsterite, wadsleyite, and ringwoodite (Mg2SiO4): 29Si NMR constraints on structural disorder and effects of paramagnetic impurity ions

Abstract We present here high-resolution 29Si MAS NMR data for synthetic samples of forsterite (α-Mg2SiO4), wadsleyite (β), and ringwoodite (γ). Enrichment to >99% 29Si provides greatly enhanced signal-tonoise ratios and thus great sensitivity to small features in the spectra. At a detection limit of 0.1 to 0.5%, no six-coordinated Si (VISi) is observed in any of the polymorphs, although these results could be consistent with theoretical predications of 1 to 2% Mg-Si site disorder in ringwoodite if re-ordering occurs rapidly during cooling. Several small IVSi peaks in ringwoodite samples may be related to residual defects from this process. In forsterite and wadsleyite, several very small “extra” peaks are observed, many of which are at positions far outside the known range of chemical shifts for 29Si in silicates. These may be caused by “pseudo-contact” shifts from dipolar interactions with unpaired electron spins on trace impurities of paramagnetic transition metal cations.

Dry (Mg,Fe)SiO3 perovskite in the Earth's lower mantle

Combined synthesis experiments and first-principles calculations show that MgSiO3-perovskite with minor Al or Fe does not incorporate significant OH under lower mantle conditions. Perovskite, stishovite, and residual melt were synthesized from natural Bamble enstatite samples (Mg/(Fe + Mg) = 0.89 and 0.93; Al2O3 < 0.1 wt % with 35 and 2065 ppm weight H2O, respectively) in the laser-heated diamond anvil cell at 1600–2000 K and 25–65 GPa. Combined Fourier transform infrared spectroscopy, X-ray diffraction, and ex situ transmission electron microscopy analysis demonstrates little difference in the resulting perovskite as a function of initial water content. Four distinct OH vibrational stretching bands are evident upon cooling below 100 K (3576, 3378, 3274, and 3078 cm−1), suggesting four potential bonding sites for OH in perovskite with a maximum water content of 220 ppm weight H2O, and likely no more than 10 ppm weight H2O. Complementary, Fe-free, first-principles calculations predict multiple potential bonding sites for hydrogen in perovskite, each with significant solution enthalpy (0.2 eV/defect). We calculate that perovskite can dissolve less than 37 ppm weight H2O (400 ppm H/Si) at the top of the lower mantle, decreasing to 31 ppm weight H2O (340 ppm H/Si) at 125 GPa and 3000 K in the absence of a melt or fluid phase. We propose that these results resolve a long-standing debate of the perovskite melting curve and explain the order-of-magnitude increase in viscosity from upper to lower mantle.

The effect of sample thickness and insulation layers on the temperature distribution in the laser-heated diamond cell

The temperature gradients in the laser-heated diamond anvil cell can be modeled through the solution of the steady-state heat equation. For a given laser, the width of the hotspot is dependent on the thickness and thermal conductivity of the sample and of any thermal-insulation layers between the sample and the diamonds. For a given sample, insulation and peak temperature, thicker samples will have broader hotspots than thinner ones. Therefore, increasing the pressure on a sample will thin the gasket, sample and insulation, producing a narrower hotspot and increasing temperature gradients.

Structural Transitions and Electron Transfer in Coffinite, USiO4, at High Pressure

Abstract The compressibility, phase stability, and vibrational properties of coffinite (USiO4) were studied by in situ X-ray diffraction and infrared (IR) measurements at high pressures. An irreversible phase transition from the zircon-type to scheelite-type structure was found to occur at 14-17 GPa. Accompanying the structural transition, partial amorphization was also evident in the XRD analysis. The predicted transition pressure calculated by density functional theory is in good agreement with the experimental results. IR spectra also suggest that water is incorporated into the coffinite structure, and a pressure-induced electron transfer (U4+ → U5+) may also occur.

Hydrous ringwoodite to 5 K and 35 GPa: Multiple hydrogen bonding sites resolved with FTIR spectroscopy

Abstract Multiple substitution mechanisms for hydrogen in γ-(Mg,Fe)2SiO4, ringwoodite, lead to broad, overlapping, and difficult-to-interpret FTIR spectra. Through combined low-temperature, high-pressure synchrotron-based FTIR spectroscopy, the multiple bonding sites become evident, and can be traced as a function of temperature and compression. Multiple OH stretching bands can be resolved in iron-bearing and iron-free samples with 0.79-2.5(3) wt% H2O below 200 K at ambient pressure, with cooling to 5 K at 35 and 23 GPa resulting in the resolution of possibly as many as 5 OH stretching bands traceable at room temperature from 23 GPa down to 8 GPa. A distribution of defect mechanisms between ‴Mg″+2(H·) at 3100, 3270, and possibly 2654 cm-1, ‴Si″′+4(H·) at 3640 cm-1, and MgSi″+2(H·) at 2800 cm-1 can then be resolved. These multiple defect mechanisms can therefore explain the higher electrical and proton conductivity in ringwoodite when compared to wadsleyite, and therefore may be applied to resolve spatial variations in water storage in the Earth’s transition zone.

The role of carbon in extrasolar planetary geodynamics and habitability

The proportions of oxygen, carbon and major rock-forming elements (e.g. Mg, Fe, Si) determine a planets dominant mineralogy. Variation in a planets mineralogy subsequently affects planetary mantle dynamics as well as any deep water or carbon cycle. Through thermodynamic models and high pressure diamond anvil cell experiments, we demonstrate the oxidation potential of C is above that of Fe at all pressures and temperatures indicative of 0.1 - 2 Earth-mass planets. This means that for a planet with (Mg+2Si+Fe+2C)/O > 1, excess C in the mantle will be in the form of diamond. We model the general dynamic state of planets as a function of interior temperature, carbon composition, and size, showing that above a critical threshold of