Beschreibung: The content and transport of fluid phases such as water into the deep Earth is of great importance not only to correlate seismological models of the planet’s interior with mineralogical models, but also for the understanding of the evolution of the solid Earth as well as the Earth’s atmosphere. This study reports on the influence of water on the structural and physical properties of olivine, which is known to be the main constituent of the upper mantle. Two hydrous olivines of composition Fo 97 Fa 3 with water content of 4883 parts per million by weight (ppmw) (SZ0407A) and 8000 ppmw (SZ0407B) were synthesized at 1250 °C and 12 GPa. Single-crystal X-ray diffraction was used to determine unit-cell parameters of SZ0407A and SZ0407B at pressures up to 7.1 GPa at room temperature. Synchrotron powder X-ray diffraction and Raman scattering experiments were performed on sample SZ0407A in a diamond-anvil cell to 34 GPa at room temperature. For both samples, the compressibility is the largest along the b -axis and smallest along the a -axis. Using the compression ( V / V o ) vs. pressure data for sample SZ0407A to 29 GPa, in conjunction with the third-order Birch-Murnaghan equation of state, we calculate the isothermal bulk modulus and its pressure derivative as K o = 119.2(12) GPa and K ' o = 6.6(4). Single-crystal compression data for sample SZ0407A to 7 GPa give K o = 121.5(6) GPa and K ' o = 5.7(2); and for sample SZ0407B K o = 122.2(12) GPa and K ' o = 6.2(4). High-pressure Raman spectra for SZ0407A up to 34 GPa show a continuous shift of all the observed bands to higher frequency with increasing pressure; there is no indication of any first-order phase transition. However, the Raman spectra indicate subtle discontinuous changes around 22 GPa, unobserved in previously reported studies on anhydrous olivines.

Beschreibung: The mantle transition zone, at depths between 410 to 660 km, is characterized by two prominent discontinuities in seismic-wave velocity in addition to a relatively steep velocity gradient. Throughout this region garnet will be an abundant mineral, the composition of which will change depending on both depth and lithology. It is important, therefore, to be able to characterize the effects of these changes on seismic velocities, which means that models must incorporate reliable elasticity data on the dominant mineral end-members that can be accurately employed at mantle conditions. In this study elastic wave velocities of synthetic polycrystalline pyrope garnet (Mg 3 Al 2 Si 3 O 12 ) have been measured using ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction in a 1000-ton multi-anvil press. Measurements were performed at pressures up to 24 GPa, conditions compatible with the base of the transition zone, and at temperatures up to 1300 K. Least-squares refinement of the ambient-temperature data to a third-order finite strain equation yields values for the bulk and shear moduli and their pressure derivatives of K S0 = 172.0 ±1.6 GPa, G 0 = 89.1 ±0.5 GPa, K S / P = 4.38 ±0.08, and G/ P = 1.66 ±0.05. The determined temperature derivatives are K S / T = –17.8 ±2.0 MPa/K and G/ T = –7.9 ±1.0 MPa/K. High-temperature data were fitted to extract parameters for a thermodynamic model. As several high-pressure and -temperature studies have been performed on pyrope, fitting all of the available data provides a more robust assessment of the accuracy of velocity measurements and allows the uncertainties that are inherent in the various methodologies to be realized. When this model is used to determine pyrope velocities at transition zone conditions the propagated uncertainties are approximately 1.5 and 2.5% for v p and v s , respectively. To reduce these uncertainties it is important not only to measure velocities as close as possible to mantle temperatures but also to understand what causes the difference in velocities between studies. Pyrope v P and v S at mantle transition zone conditions are found to be approximately 2.4 and 3.7%, respectively, larger than recent determinations of majoritic garnet at the same conditions, implying a significant variation with chemistry that is mainly realized at high temperatures.

Beschreibung: The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H 2 O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K 1.5 Mg 2 Si 2 O 7 H 0.5 , which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by single-crystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P 6 3 / mcm , in contrast to other studies that have reported non-centrosymmetric space group P 6 3 cm . While it was possible to obtain good agreement indices for refinements in P 6 3 cm , there were strong correlations between atoms that are equivalent in P 6 3 / mcm , suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P 6 3 / mcm gave R 1 = 0.036, wR 2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4 c site lying on the sixfold axis on to three split 12 j sites 0.2 Å away, each having 1/4 occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8 a sub x 8 a sub x c sub , having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2 n , but are absent from l = 2 n + 1 layers. Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm –1 and three much weaker bands at 3690, 3560, and 3405 cm –1 . Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P 6 3 / mcm or 4 ≤ m ≤ 12 in P 6 3 cm . Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K 1.5 Mg 2 Si 2 O 7 H 0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P 6 3 / mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX 50 /Anhydrous-PhX 50 . Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions.

Beschreibung: Nominally hydrous high-pressure silicate phases such as the superhydrous phase B are of considerable importance for the understanding of the water-cycle between the surface and the interior of the Earth. This study tackles the controversial issue of hydrogen positions in superhydrous phase B, a phase believed to be potentially stable in cold subducting ultramafic slabs. To investigate the nature of hydrogen incorporation into the structure of superhydrous phase B, neutron powder diffraction experiments have been performed. A structural model based on Pnn 2 symmetry has been used for the analysis of the data, which is consistent with earlier spectroscopic studies. Application of Fourier synthesis with subsequent analyses of difference nuclear density maps and Rietveld fits reveal two distinct positions for deuterium, at 4 c (0.194, 0.052, 0.596) and at 4 c (0.186, 0.119, 0.388). This unambiguously shows that deuterium lies within large channels, which are formed between the edge-shared octahedra and vertex-linked tetrahedra along the b -axis of the structure. These results contrast with recent polarized single-crystal infrared spectroscopy studies where the position of one of two H atoms was estimated to lie close to the octahedral edge of an MgO 6 octahedron, thereby leaving the large structural channel empty.

Beschreibung: At lower mantle conditions, subducted mid oceanic ridge basalts (MORB) will crystallize more than 20 vol% of an aluminum-rich phase, which is referred to generally as the new aluminum (NAL) phase. Given that a significant proportion of the lower mantle may be comprised of subducted crust, the NAL phase may contribute to the bulk elastic properties of the lower mantle. In this study we report for the first time the structure, Raman spectrum and elasticity of single crystals of Na 0.41 [Na 0.125 Mg 0.79 Al 0.085 ] 2 [Al 0.79 Si 0.21 ] 6 O 12 NAL phase, synthesized at 2260 °C and 20 GPa. The single-crystal structure refinement of NAL, which is consistent with the space group P 6 3 / m , reveals dynamic disorder of Na atoms along channels within the structure. The elastic tensor was experimentally determined at ambient conditions by Brillouin scattering spectroscopy. The elastic modulii obtained from the Voigt-Reuss-Hill approximation using the elastic constants determined in this study are K S = 206 GPa and μ = 129 GPa, whereas the isotropic compressional and shear sound velocities are v P = 9.9 km/s and v S = 5.8 km/s. The NAL phase is elastically anisotropic, displaying 13.9% compressional and shear wave anisotropy. Elastic constants as well as Raman active modes of NAL have also been calculated using density-functional theory and density-functional perturbation theory.

Beschreibung: Phase relations of magnesioferrite-magnetite solid solutions (Mg,Fe 2+ )Fe 2 3+ O 4 were investigated at pressures of 9–23 GPa and temperatures of 1200–1600 °C. Our new results indicate that the assemblage Mg 2 Fe 2 O 5 + Fe 2 O 3 reconstitutes to a hp-MgFe 2 O 4 phase at 20 GPa and 1300–1500 °C. The stability field of hp-MgFe 2 O 4 begins at ~1300 °C and widens to higher temperature. At lower temperature (1200–1300 °C) Mg 2 Fe 2 O 5 + Fe 2 O 3 breaks down to the new phase assemblage Mg 3 Fe 4 O 9 + Fe 2 O 3 with its stability field expanding to higher pressures and temperatures at the expense of hp-MgFe 2 O 4 . The Mg 3 Fe 4 O 9 phase has the same crystal structure that recently reported for Fe 7 O 9 , and thus represents the Mg-end-member. From powder X-ray diffraction, we find that hp-MgFe 2 O 4 has a structure consistent with an orthorhombic unit cell belonging to the Pmcn space group (no. 62). However, it could have undergone a transformation from a different structure during decompression. Experiments conducted with a Mg 0.5 Fe 2+ 0.5 Fe 2 3+ O 4 composition demonstrate that the addition of Fe 2+ significantly changes the topology of the phase relations compared to the MgFe 2 O 4 end-member system. At 10–11 GPa and 1000–1600 °C, Mg 0.5 Fe 0.5 Fe 2 O 4 breaks down to the assemblage MgFeFe 2 O 5 + Fe 2 O 3 , with the phase boundary described by: P (GPa) = 2.0 x 10 – 3 x T (°C) + 8.2. No stability field for the constituent oxides [i.e., (Mg,Fe)O + Fe 2 O 3 ] exists, in contrast to that observed for the MgFe 2 O 4 end-member. The stability of the assemblage MgFe 2+ Fe 2 3+ O 5 + Fe 2 O 3 is limited at higher pressures and appears to pinch out to higher temperatures. At 15–16 GPa and temperatures up to 1350 °C, this assemblage reconstitutes to form a hp-Mg 0.5 Fe 0.5 Fe 2 O 4 phase. However, at higher temperatures a new assemblage of (Mg,Fe) 3 Fe 4 O 9 + Fe 2 O 3 appears. The occurrence of such compositions suggests that solid solution may be complete across the Mg 3 Fe 4 O 9 –Fe 7 O 9 binary. Our results further demonstrate that phase relations even in simple Fe-Mg oxides can become complex at high pressures and temperatures and that phases with various novel stoichiometries (i.e., Mg 3 Fe 4 O 9 ) may become stable. In addition, this study has implications for natural samples by helping to place constraints on the range in pressure and temperature at which a given sample formed. For instance, magnetite or magnesioferrite entrapped as inclusions in diamond could have either have crystallized directly, or formed from precursor phases at depths that exceed the stability of the spinel-structured phases. Evidence for such high-pressure transformations can potentially be found by investigating micro-textures.

Beschreibung: In situ synchrotron X-ray powder diffraction measurements using a Paris-Edinburgh pressure cell were performed to investigate the nature of the high-pressure breakdown reaction of magnetite (Fe 3 O 4 ). Refinement of diffraction patterns reveals that magnetite breaks down via a disproportionation reaction to Fe 4 O 5 and hematite (Fe 2 O 3 ) rather than undergoing an isochemical phase transition. This result, combined with literature data indicates (1) that this reaction occurs at ~9.5–11 GPa and 973–1673 K, and (2) these two phases should recombine at yet higher pressures to produce an h -Fe 3 O 4 phase.

Beschreibung: The thermal expansion of anhydrous Mg 2 SiO 4 wadsleyite and forsterite was comprehensively studied over the temperature ranges 297–1163 and 297–1313 K, respectively, employing X-ray powder diffraction. Experiments were carried out with two separately synthesized samples of wadsleyite (numbered z626 and z627), for which room temperature unit-cell volumes differed by 0.05%, although the determined thermal expansions were identical within error. The high-temperature thermal expansions of wadsleyite and forsterite were parameterized on the basis of the first-order Grüneisen approximation using a Debye function for the internal energy. Values for hypothetical volume at T = 0 K, Debye temperature and Grüneisen parameter are 536.86(14) Å 3 , 980(55) K, 1.28(2) and 537.00(13) Å 3 , 887(50) K, 1.26(1) for z626 and z627, respectively, with the bulk modulus fixed to a literature determination of 161 GPa. For forsterite, the respective values are 288.80(2) Å 3 , 771(9) K, and 1.269(2) with a constrained bulk modulus of 125 GPa. These quantities are in good agreement with literature values obtained independently from sound velocity and heat capacity measurements, giving strong support to the applicability of Grüneisen theory in describing the thermal expansion of wadsleyite and forsterite. In addition, high-temperature structural variations were determined for wadsleyite from Rietveld analysis of the X-ray diffraction data. The pronounced anisotropy in thermal expansion of wadsleyite with a more expandable c -axis, similar to the compressional anisotropy, arises from specific features of the crystal structure consisting of the pseudolayers of MgO 6 octahedra parallel to the a - b plane with cross-linking Si 2 O 7 dimers along the c -axis. Although anisotropic compression and expansion originate from the same structural features, the details of structural changes with pressure differ from those caused by temperature. The longest Mg-O bonds, which are roughly parallel to the c -axis in all three octahedral sites of wadsleyite, dominate the compression, but these bonds do not exhibit the largest expansivities.

Beschreibung: We present 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg 1–x Fe x )(Si 1–x Al x )O 3 (x = 0.01, 0.025, and 0.05). 27 Al MAS-NMR spectra of the perovskite samples indicate that Al 3+ replaces both Si 4+ in the octahedral site and Mg 2+ in the larger 12-coordinated site. NMR signal loss caused by paramagnetic interactions is often a severe complication when performing NMR on materials containing Fe 2+,3+ ; however, careful measurement of signal loss and comparison to total Fe content in these samples sheds light on the nature of Al and Fe incorporation. NMR signal loss for the pyroxenes is linearly related to total Fe content as would be expected in the case of uncorrelated substitution of randomly distributed Al and Fe. However, 27 Al signal loss for the perovskite samples increases only slightly between samples with x = 0.01 and 0.025 indicating similar coordination of Al by Fe and non-random distribution. Complete signal loss at Fe/(Fe + Mg) = 0.05 suggests the upper limit of Fe 2+ and Fe 3+ concentration at which useful NMR data can be obtained for this system.

Beschreibung: Multiple substitution mechanisms for hydrogen in -(Mg,Fe) 2 SiO 4 , 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% H 2 O 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 Mg Si ''+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.