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Olivine, the most abundant mineral in the upper mantle, plays a key role in controlling the thermodynamic properties in the Earth's and planetary interiors, like the temperature distribution along the adiabatic geotherm. In this study, we conducted simultaneous high-pressure (P) and high-temperature (T) Raman measurements on a San Carlos olivine sample in an externally heated diamond-anvil cell (DAC). The intrinsic anharmonic parameters, ai, are calculated as functions of both pressure and temperature, and the isochoric (CV) and isobaric (CP) heat capacities are computed at various P-T conditions with the anharmonic correction. The harmonic heat capacities are CV = 807.7 J/kg/K and CP = 815.4 J/kg/K at ambient conditions, with anharmonic contribution of ΔC = 7.9 J/kg/K. Relative to the previous vibrational measurements conducted at high-P or high-T conditions, this simultaneous high-P-T experiment indicates that the anharmonic contribution to heat capacities is overestimated if the anharmonic parameters (ai) are treated as constants, as done previously. The pressure effect is marginal on the intrinsic anharmonic contribution to thermodynamic properties, whereas it has a much more significant effect on the external anharmonicity (thermal expansivity). The pressure dependence of CP (dCP/dP, in J/kg/K/GPa) increases from –3.14 at 300 K to –1.94 at 700 K, and then decreases smoothly to –5.03 at 1800 K. Combining the derived high-P-T capacity with a reliable P-V-T equation of state (EoS) for olivine, we further modeled the thermodynamic Grüneisen parameter, γ(P-T). The Grüneisen parameter is important for the connection between isothermal and adiabatic compressions of minerals, which can be decreased by approximately 5% with the anharmonic correction at high temperatures. The modeled adiabatic bulk modulus and bulk sound velocity can be expressed as: KS(T,P) (GPa) = 127.5(1) + 4.32(5)·P – 0.018(1)·(T – 300) and VΦ(T-P) (km/s) = 6.22(2) + 0.069(3)·P – [3.74(15)–0.075(13)·P]·10–4·(T – 300). The adiabatic temperature gradient, dTS/dP, which is almost independent of pressure, equals 13.40(16) and 12.35(16) K/GPa in the harmonic and anharmonic models, respectively. This study provides a useful example for modeling the radial temperature distribution in adiabatic planetary mantles. [ABSTRACT FROM AUTHOR]

Jeffbenite (Mg3Al2Si3O12) is a tetragonal phase found in so far only in superdeep diamonds, and its thermoelastic parameters are a prerequisite for determining entrapment pressures as it is regarded as a potential indicator for superdeep diamonds. In this study, the thermoelastic properties of synthetic Fe3+‐jeffbenite were measured up to 33.7 GPa and 750 K. High‐temperature static compression data were fitted, giving (∂KT0/∂T)P = −0.0107 (4) GPa/K and αT = 3.50 (3) × 10−5 K−1. The thermoelastic properties and phase stability are applied to modeling isomekes, or P‐T paths intersecting possible conditions of entrapment in diamond. We calculate that under ideal exhumation, jeffbenite entrapped at mantle transition zone conditions will exhibit a high remnant pressure at 300 K (Pinc) of ∼5.0 GPa. Elastic geobarometry on future finds of jeffbenite inclusions can use the new equation of state to estimate entrapment pressures for this phase with still highly uncertain stability field in the mantle. Plain Language Summary: Ongoing superdeep diamonds research is providing new insights into the Earth's deep mantle. Natural superdeep diamonds and its inclusions can show compelling evidence for retrograde conversion from the lower mantle or transition zone precursors; along with carbonate melt‐peridotite reactions. Jeffbenite with a composition of Mg3Al2Si3O12, found in so far only in superdeep diamonds can be regarded as a potential indicator mineral for superdeep diamonds. Recent synthesis of Fe3+‐rich jeffbenite provides an opportunity for in‐situ measurements to study the thermodynamic properties of jeffbenite at deep‐mantle conditions. Thus, in this study, we explored the high pressure and temperature stability and thermoelastic properties of Fe‐bearing jeffbenite up to 33.7 GPa and 750 K. The thermoelastic data and phase stability were measured and the results are applied to modeling the isomekes, or P‐T paths intersecting possible conditions of entrapment and along which the pressure on the inclusion is equal to the external pressure on the diamond host. Our finding can be applied to determining entrapment pressures in such diamond inclusions in future finds and its primary or retrograde history is essential in understanding mantle dynamics and the hidden consequences of plate tectonics. Key Points: We measure the phase stability and thermoelastic properties on synthetic Fe‐bearing jeffbeniteThe thermoelastic data are applied to modeling isomekes and the remnant pressure (Pinc) is determinedThe smaller thermal expansivity of jeffbenite gives rise to a broader pressure and temperature stability field in the upper lower mantle [ABSTRACT FROM AUTHOR]

The transition zone of the Earth's upper mantle is characterized by three discontinuities in seismic wave velocity profiles. One of these at about a depth of 520 km is assigned to the transformation of wadsleyite (β-) to ringwoodite [γ-(Mg, Fe).sub.2SiO.sub.4] (e.g., Shearer in J Geophys Res 101:3053-3066, 1996 (See CR32)). The exact location, width, and other properties of that discontinuity are affected by a multitude of parameters. The present study specifically focuses on the effect of water, iron content, and redox environment on the depth of the phase transition. We performed high-pressure experiments in a multi-anvil apparatus at 1200 °C with variation in Mg-Fe compositions (0.10 < x.sub.Fe < 0.24), water contents (0 < [Formula omitted] < 2 wt%), and the redox environment [using different buffers: Fe/FeO (reducing), Re/ReO.sub.2 (oxidizing)]. Run products were investigated using electron microprobe and Fourier transform infrared spectroscopy to obtain the composition including the hydroxyl concentration of coexisting phases. Mossbauer (MB) spectroscopy and electron energy loss spectroscopy as well as single-crystal X-ray diffraction were applied to gain insight into the Fe.sup.3+ content and incorporation mechanisms. Under hydrous and reducing conditions, the wadsleyite-ringwoodite boundary shifts by 0.5 GPa to higher pressures accompanied by a broadening of the region of coexisting wadsleyite and ringwoodite. In contrast, under hydrous and oxidizing conditions, the two-phase field gets narrower and the shift of the two-phase field to higher pressure is amplified. Thus, the stability field of wadsleyite is extended to higher pressure, most likely due to the higher water and Fe.sup.3+ content in the wadsleyite structure compared to ringwoodite. Based on results from MB spectroscopy and single-crystal X-ray diffraction, we infer that Fe.sup.3+ in wadsleyite is incorporated as a spinelloid component and stabilizes wadsleyite to higher pressures. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1163-2) contains supplementary material, which is available to authorized users., Author(s): Maria Mrosko[sup.1] , Monika Koch-Müller[sup.1] , Catherine McCammon[sup.2] , Dieter Rhede[sup.3] , Joseph R. Smyth[sup.4] , Richard Wirth[sup.1] Author Affiliations: (1) Sektion 3.3, Chemie und Physik der Geomaterialien, Deutsches [...]

MgSi[O.sub.3] akimotoite is stable relative to majoritegarnet under low-temperature geotherms within steeply or rapidly subducting slabs. Two compositions of Mg-akimotoite were synthesized under similar conditions: Z674 (containing about 550 ppm wt [H.sub.2]O) was synthesized at 22 GPa and 1,500°C and SH1101 (nominally anhydrous) was synthesized at 22 GPa and 1,250°C. Crystal structures of both samples differ significantly from previous studies to give slightly smaller Si sites and larger Mg sites. The bulk thermal expansion coefficients of Z674 are (153-839 K) of [a.sub.1] = 20(3) x [10.sup.-9] [K.sup.-2] and [a.sub.0] = 17(2) x [10.sup.-6] [K.sup.-1], with an average of [α.sub.0] = 27.1(6) x [10.sup.-6] [K.sup.-1]. Compressibility at ambient temperature of Z674 was measured up to 34.6 GPa at Sector 13 (GSECARS) at Advanced Photon Source Argonne National Laboratory. The second-order Birch-Murnaghan equation of state (BM2 EoS) fitting yields: [V.sub.0] = 263.7(2) [Å.sup.3], [K.sub.T0] = 217(3) GPa (K fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: [α.sub.a] = 8.2(3) and [α.sub.c] = 10.68(9) ([10.sup.-6][K.sup.-1]); [β.sub.a] = 11.4(3) and [β.sub.c] = 15.9(3) ([10.sup.-4] GPa). Hydration increases both the bulk thermal expansivity and compressibility, but decreases the anisotropy of structural expansion and compression. Complementary Raman and Fourier transform infrared (FTIR) spectroscopy shows multiple structural hydration sites. Low-temperature and high-pressure FTIR spectroscopy (15-300 K and 0-28 GPa) confirms that the multiple sites are structurally unique, with zero-pressure intrinsic anharmonic mode parameters between -1.02 x [10.sup.-5] and +1.7 x [10.sup.-5] [K.sup.-1], indicating both weak hydrogen bonds (O-H ... O) and strong OH bonding due to long O ... O distances. Keywords Akimotoite * Crystal structure * Thermal expansion * Compressibility * Anisotropy * FTIR, Introduction Akimotoite (ilmenite-type MgSi[O.sub.3]) is stable at pressures of 18- 25 GPa and up to about 2,000 K in the MgSi[O.sub.3] system, with wadsleyite plus stishovite stable at lower pressures, [...]

Two samples of hydroxyl-clinohumite, sample SZ0407B with approximate composition Mg8674(14) [Fe0.374.sub.(4)][([Si.sub.0.99(1)][O.sub.4]).sub.4][(OH).sub.2] and sample SZ0411B with composition [Mg.sub.9][(Si[O.sub.4]).sub.4][(OH).sub.2], were synthesized at 12 GPa and 1,250°C coexisting with olivine. Unit-cell parameters determined by single-crystal X-ray diffraction are given as follows: a = 4.7525(4) Å, b = 10.2935(12) Å, c = 13.7077(10) Å, α = 100.645(9)°, V = 659.04(9) [Å.sup.3] for SZ0407B, and a = 4.7518(6) Å, b = 10.2861(12) Å, c = 13.7008(9) Å, α = 100.638(9)°, V = 658.15(9) [Å.sup.3] for SZ0411B. Single-crystal X-ray intensity data were collected for crystal structure refinements of both samples. Relative to the pure-Mg sample, Fe decreases M3-OH bond lengths by ~0.010(3) Å, consistentwithsome ferriciron ordering into M3. Raman spectroscopy shows two strong bands in the lattice-mode region at 650 and 690 [cm.sup.-1] in the Fe-bearing sample, which are not observed in the pure-Mg sample. Spectra in the [H.sub.2]O region show at least five bands, which are deconvolved into seven distinct O-H-stretching modes. Thermal expansion measurements were carried out for both samples from 153 to 787 K by single-crystal X-ray diffraction. The average a-, b-, c-axial and volumetric thermal expansion coefficients ([10.sup.-6] [K.sup.-1]) are 10.5(1), 12.3(2), 12.5(2) and 34.9(5) for SZ0407B, respectively, and 11.1(1), 12.6(3), 13.7(3), 36.8(6) for SZ0411B, respectively. After heating, the unit-cell parameters were refined again for each sample at ambient condition, and no significant changes were observed, indicating no significant oxidation or dehydration during the experiment. For the DHMS phases along the brucite-forsterite join, linear regression gives a systematic linear decrease in expansivity with increasing density. Further, substitution of ferrous iron into these structures decreases thermal expansivity, making the Fe-bearing varieties slightly stiffer. Keywords Clinohumite * Crystal structure * Thermal expansion * Iron effect, Introduction The dense hydrous magnesium silicate (DHMS) minerals along the brucite (Br)-forsterite (Fo) join have been widely studied and proposed as potential carriers of water into the Earth's mantle(McGetchinetal., 1970;Wunder [...]

Hydrous wadsleyite ([beta]-[Mg.sub.2]Si[O.sub.4]) with 2.8 wt% water content has been synthesized at 15 GPa and 1250[degrees]C in a multi-anvil press. The unit-cell parameters are: a = 5.6686(8), b = 11.569(1), c = 8.2449(9) [Angstrom], [beta] = 90.14(1)[degrees], and V = 540.7(1) [[Angstrom].sup.3], and the space group is I2/m. The structure was refined in space groups Imma and I2/m. The room-pressure structure differs from that of anhydrous wadsleyite principally in the increased cation distances around O1, the non-silicate oxygen. The compression of a single crystal of this wadsleyite was measured up to 61.3(7) GPa at room temperature in a diamond anvil cell with neon as pressure medium by X-ray diffraction at Sector 13 at the Advanced Photon Source, Argonne National Laboratory. The experimental pressure range was far beyond the wadsleyite-ringwoodite phase-transition pressure at 525 km depth (17.5 GPa), while a third-order Birch-Murnaghan equation of state (EoS) [[V.sub.0] = 542.7(8) [[Angstrom].sup.3], [K.sub.TO] = 137(5) GPa, K' = 4.6(3)] still fits the data well. In comparison, the second-order fit gives [V.sub.0] = 542.7(8) [[Angstrom].sup.3], [K.sub.T] = 147(2) GPa. The relation between isothermal bulk modulus of hydrous wadsleyite [K.sub.TO] and water content [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is: KTO = 171 ( 1)-12(1 ) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (up to 2.8 wt% water). The axial-compressibility [[beta].sub.c] is larger than both [[beta].sub.a] and [[beta].sub.b], consistent with previous studies and analogous to the largest coefficient of thermal expansion along the c-axis. Keywords: Compressibility, hydrous wadsleyite, neon, orthorhombic DOI: 10.2138/am.2010.3533

We present a detailed [sup.29]Si NMR spectroscopic study of isotopically enriched samples of forsterite and of anhydrous and hydrous wadsleyite and ringwoodite ([alpha], [beta], and [gamma] phases of [Mg.sub.2]Si[O.sub.4]), which complement previous extensive studies of these minerals by XRD and vibrational spectroscopy, [sup.VI]Si 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 ([sup.29]Si{[sup.1]H } 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.sup.+] substitutes into [Mg.sup.2+] 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 '[T.sub.1]' 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. Keywords: NMR spectroscopy, forsterite, wadsleyite, ringwoodite, water in mantle

Single-crystal X-ray diffraction has been used to measure the thermal expansion coefficients of forsterite, wadsleyite, ringwoodite, and their hydrous forms at ambient pressure, from temperatures as low as 133 K to as high as 919 K. Second-order polynomial fitting to ln(V/[V.sub.0]) vs. T was applied to derive the expansion coefficients in the form of [alpha] = [a.sub.l] x T + [a.sub.0]. The single crystal of anhydrous wadsleyite persisted up to 859 K. Hydrous forsterite was observed to dehydrate at 919 K, whereas hydrous wadsleyite started to dehydrate at 655 K. The crystal of ringwoodite with 0.20 wt% broke down at 911 K, and the two ringwoodite samples with 0.74 and 2.4 wt% [H.sub.2]O, were observed to break down with an irreversible unit-cell expansion at 808 and 606 K, respectively. From room temperature to high temperatures in this study, the mean thermal volume expansion coefficients are 36.4(5) and 38.1 (9) x [10.sup.-6] [K.sup.-1], respectively, for anhydrous and hydrous forsterite; 28.5(5) and 35.8(8) x [10.sup.-6] [K .sup.-1], respectively, for the anhydrous and hydrous wadsleyite; 30.6(9), 35.2(15), and 34.9(7) x [10.sup.-6] [K.sup.-1] for the samples ofringwoodite with 0.2, 0.74, and 2.4 wt% [H.sub.2]O, respectively. Thus, forsterite, wadsleyite, and ringwoodite all have larger thermal expansion coefficients in their hydrous forms than in their anhydrous forms. Keywords: Thermal expansion, forsterite, wadsleyite, ringwoodite, single crystal

The crystal structure of hydrous pure magnesium forsterite ([Mg.sub.2]Si[O.sub.4]) containing 8900 ppmw [H.sub.2]O, synthesized at 12 GPa and 1250[degrees]C, has been refined. The major hydration mechanism appears to be M1 cation vacancy with protonation of the O1-O2 octahedral edge of M1. Raman spectra up to 41 GPa show strong coupling between the two [A.sub.g] modes (824.4and 856.2 [cm.sup.-1]). Mode Gruneisen parameters [[gamma].sub.i], related to Mg(2)[O.sub.6] translation mode decrease relative to anhydrous forsterite. This is attributed to the fact that the M2 site shows full occupancy compared to the M1 site, and the vacancy predominantly occurs at the M1 site. Pressure dependencies of four OH vibrations in the region 3548-3615 [cm.sup.-1] suggest that positional ordering of hydrogen ion (proton) takes place with increasing pressure. The OH mode at 3615 [cm.sup.-1] shows complex response as a function of pressure: the ([partial derivative]v/[partial derivative]P) slope changes from -0.43 below l0 GPa to 1.97 between 10 and 20 GPa, and does not show clear pressure dependence above 20 GPa. Single-crystal X-ray data were used to assign the OH band to the structural sites, O1 and O2. The proton is closer to O2 than O1. The distance between the proton and O2 is 0.96 [Angstrom]. In view of the empirical relation between OH-stretching frequencies and O ... O distances (Libowitzky 1999), the pressure dependence of the OH mode (3615 [cm.sup.-1]) is well correlated with the O1-O2 distance and the degree of hydrogen bonding. Keywords: Forsterite, X-ray diffraction, Raman spectroscopy, hydration mechanism, high pressure

We present here high-resolution [sup.29]Si MAS NMR data for synthetic samples of forsterite ([alpha]- [Mg.sub.2]Si[O.sub.4]), wadsleyite ([beta]), and ringwoodite ([gamma]). Enrichment to >99% [sup.29]Si provides greatly enhanced signal-to-noise 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 ([sup.vi]si) 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 [sup.iv]Si 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 [sup.29]Si 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. Keywords: NMR spectroscopy, forsterite, ringwoodite, wadsleyite, high-pressure studies

A suite of pure magnesian wadsleyite ([beta]-[Mg.sub.2]Si[O.sub.4]) samples containing 0.005, 0.38, 1.18, and 1.66 wt% [H.sub.2]O was studied by single-crystal X-ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry. Separate compressibility experiments were carried out to 9.6 GPa to determine the effects of hydration on isothermal equations of state. Crystal-structure refinements at ambient conditions show cation vacancies order onto the M3 site. The most hydrous sample (1.6 wt% [H.sub.2]O) displayed monoclinic symmetry with [beta] = 90.090(7)[degrees], whereas the samples with lower content were statistically orthorhombic. The density of wadsleyite decreases with increasing water content at STP according to the empirical relation, [rho] = 3.470(2) -0.046(2) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] g/[cm.sup.3] (with [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] in wt% [H.sub.2]O). Bulk moduli and pressure derivatives of wadsleyite are [K.sub.T0] = 173(5) GPa, [K.sub.0]' = 4.1(15) for 0.005 wt% [H.sub.2]O; [K.sub.T0] = 161(4) GPa, [K.sub.0]' = 5.4(11) for 0.38 wt% [H.sub.2]O; [K.sub.T0] = 158(4) GPa, [K.sub.0]' = 4.2(9) for 1.18 wt% [H.sub.2]O; and [K.sub.T0] = 154(4) GPa, [K.sub.0]' = 4.9(11) for 1.66 wt% [H.sub.2]O. Variation of the bulk modulus of wadsleyite with water content is non-linear, which may be attributable to softening of the structure by ordering of vacancies onto two non-equivalent M3 sites (M3a and M3b) and an accompanying dilution of orthorhombic symmetry. Keywords: Wadsleyite, bulk modulus, equation of state, nominally anhydrous minerals, mantle Transition Zone

Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth's lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals ate small needles up to approximately 20 x 20 x 100 [micro]m in size. Electron microprobe chemical analysis indicates about 11.7 wt% [Al.sub.2][O.sub.3]. FTIR spectra indicate 7500 ppmw [H.sub.2]O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives a = 18.1876(7) [Angstrom]; b = 8.7352(7) [Angstrom]; c - 5.1789 (5) [Angstrom], V- 822.79(11) [[Angstrom].sup.3], which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, [([Mg.sub.095,0.05]).sup.M2], [([Mg.sub.0.79][Al.sub.0.21]).sup.M1], [([Al.sub.0.25][Si.sub.0.75]).sup.T2] [Si.stup.T1] [O.sub.6]. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions. Keywords: Orthopyroxene, crystal synthesis, high-pressure studies, X-ray data, crystal structure

Raman spectra in the range 80 to 4000 [cm.sup.-1] of wadsleyite II ([Fo.sub.90] with 2.0 wt% [H.sub.2]O and [Fo.sub.88] with 2.7 wt% [H.sub.2]O) have been measured in a diamond-anvil cell with solid rare-gas pressure-transmitting media to 51.4 GPa at room temperature. The ambient Raman spectrum of wadsleyite II is closely similar to wadsleyite modified with bands in frequency regions where the Si[O.sub.4] tetrahedral and OH stretching vibrations of hydrous ringwoodite occur. The most intense, characteristic wadsleyite II modes at 709 and 911 [cm.sup.-1] ([Si.sub.2][O.sub.7] and Si[O.sub.3] symmetric stretching vibrations, respectively) shift continuously to 51.4 GPa showing no evidence for a change in the crystal structure. A striking feature in the high-pressure Raman spectra of wadsleyite II is a significant growth in intensity in the mid-frequency range (300-650 [cm.sup.-1] at [10.sup.-4] GPa and 400-750 [cm.sup.-1] at 51.4 GPa) under compression accompanied by the appearance of new Raman modes near 40 GPa, perhaps a result of resonance electronic Raman scattering. In the OH stretching frequency range, the Raman spectrum of wadsleyite II exhibits at least six modes and their high-pressure behavior agrees with that of [Fo.sub.90] hydrous wadsleyite: OH stretching modes above 3530 [cm.sup.-1] remain approximately constant up to at least 21.8 GPa whereas OH modes at frequencies Keywords: Hydrous wadsleyite II, Raman spectroscopy, diamond-anvil cell, transition zone, high pressure

The single-crystal elastic properties of phase A have been investigated by Brillouin spectroscopy at ambient conditions. The Voigt-Reuss-Hill average for the adiabatic bulk and shear moduli are [K.sub.s] = 106(1) GPa and [mu] = 61(1) GPa, respectively. The present acoustic measurements resolve discrepancies between the bulk moduli obtained in earlier compression studies. The axial compressibility of the hexagonal (P[6.sub.3]) structure is highly anisotropic with the c-axis, which is perpendicular to the distorted close-packed layers, approximately 21% stiffer than the a-axis, in agreement with previous compression studies. The hydration of forsterite to phase A decreases the bulk and shear moduli by about 18 and 21%, respectively, while both compressional [V.sub.p] and shear [V.sub.s] wave velocities decrease by about 7%. These results suggest that water could be identified seismologically if phase A is present in abundance in cold subducted slabs. Keywords: Single-crystal elasticity, phase A, Brillouin spectroscopy, water in subduction zones

High pressure optical and near infrared spectra of a single crystal of ringwoodite with composition [([Mg.sub.0.90][Fe.sub.0.10]).sub.2]Si [O.sub.4] were measured to 21.5 GPa. The spectrum at ambient pressure shows a strong band at 12 265 [cm.sup.-1] with two shoulders at 8678 [cm.sup.-1] and 17 482 [cm.sup.-1]. The bands at 12 265 [cm.sup.-1] and at 8678 [cm.sup.-1] are due to spin-allowed crystal field transitions of octahedral [Fe.sup.2+], while the band at 17 482 [cm.sup.-1] is most likely due to [Fe.sup.2+] [right arrow] [Fe.sup.3+] charge transfer. The absorption edge due to ligand-to-metal charge transfer occurs close to 30 000 [cm.sup.-1]. With increasing pressure, both the crystal field and the charge transfer bands shift to higher frequencies. Whereas this is expected for the crystal field bands, this blue shift is surprising for an intervalence charge transfer band. Moreover, neither the crystal field nor the charge transfer bands broaden or intensify significantly with pressure. These results have major implications for radiative heat transfer in the Earth's mantle. It has commonly been assumed that radiative heat transfer is blocked in the mantle, because it was believed that the red shift and the increased intensity of charge transfer bands with pressure would effectively make mantle minerals opaque throughout the near infrared and visible range. Our results demonstrate that this effect does not occur for ringwoodite with a Mg/Fe ratio realistic for the Earth's mantle. Quite to the contrary, the mean free path of photons in ringwoodite actually increases with pressure, because the absorption bands move away from the maximum of the blackbody radiation.

The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm [H.sub.2]O. The silica phases surrounding coesite, however, show varying amounts of [H.sub.2]O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (< 100 ppm [H.sub.2]O, red-violet CL); 'mosaic' quartz, which is actually chalcedony (up to 0.4 wt% [H.sub.2]O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% [H.sub.2]O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm [H.sub.2]O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Seven separate samples of hydrous ringwoodite with compositions ranging from F[O.sub.100] to F[O.sub.89] 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 [degrees]C. The crystals were characterized by single-crystal X-ray diffraction, electron microprobe, IR and Mossbauer 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. Mossbauer 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.sup.-1] with maxima ranging from 3105 for the pure magnesian samples to 3150 [cm.sup.-1] for the F[O.sub.89] 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.

Wadsleyite [[Beta]-[(Mg,Fe).sub.2]Si[O.sub.4]] is known to accept variable amounts of H and may be an important reservoir of H in the transition zone of the mantle. The crystal structure of [Fo.sub.94.6] hydrous wadsleyite ([Mg.sub.1.730][Fe.sub.0.098] [Al.sub.0.008][Si.sub.0.0991][H.sub.0.355][O.sub.4]) synthesized at 1400 [degrees] C and 17 GPa was refined in space group I2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater than 4[Sigma]. Unit cell parameters are a = 5.6715(7), b = 11.582(2), c = 8.258(1) [Angstrom], and [Beta] = 90.397(9) [degrees]. The final R(F) for all reflections was 0.026 ([R.sub.w] = 0.024); goodness of fit was 1.68. An H position was located on the nonsilicate O atom (O1). Partial occupancy of a normally vacant tetrahedral site adjacent to M3 was observed. This is postulated to be the result of Si moving from M3 to the adjacent tetrahedral void on decompression. Deviation from orthorhombic symmetry appears to result from ordering of H, (Mg, Fe), and possibly Si, within the two nonequivalent M3 sites.

We report a structure refinement ofa [P2.sub.1]/m ferromagnesian cummingtonite crystal with Fe/(Mg + Fe) = 0.37 at 140 K; this crystal was inverted from the C2/m structure, which is stable at room temperature. The phase-transition temperature is [approximately]240 K. The most significant structural changes associated with the C2/m-to-[P2.sub.1]/m phase transition are as follows: (1) one crystallographically independent O-rotated silicate chain becomes two, with the A chain S rotated and the B chain O rotated; (2) the M4 site collapses, with a marked decrease (0.020 [Angstrom]) in the average M4-O6 bond distance; and (3) the largest decrease in the isotropic displacement factor is for the M4 cation. From 295 to 140 K, total changes of 14 and 11 [degrees] occur in the O5-O6-O5 kinking angles of the A and B chains, respectively. The volume of the M1 octahedron decreases more than that of the other three M[O.sub.6] polyhedra, whereas the volume of the M3 octahedron is essentially unchanged. All four M[O.sub.6] polyhedra become more regular as a result of the C2/m-to-[P2.sub.1]/m transformation. These results provide new evidence that the effective size of the M4 cation controls the relative stability of the C2/m and [P2.sub.1]/m structures. The results also provide a new explanation for the unresolved broadening of the cummingtonite Raman spectra measured at low temperatures by Wang et al. (1988).

The intrinsic anharmonicity plays an important role in the thermodynamic properties of minerals at the high-temperature conditions of the mantle. To investigate the effect of iron on the thermodynamic properties of olivine, the most abundant mineral in the upper mantle, we collected in situ high-temperature and high-pressure Raman spectra of natural Fo89Fa11 and synthetic Fo58Fa42 samples. Fo58Fa42 dissociates to enstatite + quartz + Fe2O3(+Fe) at 893 K. All the Raman-active modes systematically shift to lower frequencies at elevated temperatures, whereas to higher frequencies with increasing pressure. The Ag mode at ~960 cm-1 is more sensitive to the variations of temperature and pressure than other internal modes. The crystal-field splitting of the vibrational energy states becomes slightly weakened at high temperatures but strengthened at elevated pressures. We calculated the isobaric (γiP) and isothermal (γiT) mode Grüneisen parameters for these olivine samples. The intrinsic anharmonic parameters (ai) are negative for both the lattice and internal vibrations, and our calculations indicate that the intrinsic anharmonicity makes positive contributions to the thermodynamic properties of olivine at high temperatures, such as the internal energy (U), heat capacities (CV and CP), and entropy (S). Iron incorporation further increases the magnitudes of these anharmonic contributions. In addition, the Fe effect on the intrinsic anharmonicity may also apply to other thermodynamic properties in olivine, such as equations of state and equilibrium isotopic fractionations, which are important in constraining physical and chemical properties of the upper mantle. [ABSTRACT FROM AUTHOR]

The structure of rutile-type compounds shows no substantial changes with the addition of H, as proved by the comparative analysis of the detailed X-ray structures of a synthetic anhydrous crystal and a hydrous rutile crystal. The structures of the two types of synthetic anhydrous and hydrous rutile crystals were based on the single-crystal x-ray data. The a/c ratios and the O position were also nearly the same for both the structures.

A hypothetical, ordered model of hydrous wadsleyite has the formula Mg7Si4O14(OH)2 and belongs to a space group Pmmb. Nonequivalent O1 sites have two H atoms. IR and Raman O-H stretching absorptions exhibit strong C-axis polarization. Hydration relieves the underbonding of O1 and overbonding of bridging O2. The model predicts greater distortion of silicate tetrahedra on partial hydration, while full hydration of O1 decreases the distortion. Trivalent octahedral cations Fe3+, Cr3+ and Al3+ exhibit higher solubility in wadsleyite than in olivine under hydrous conditions. The model prediction of maximum water content suggests that several oceans of H2O may be present in the transition zone of the mantle.

Three-dimensional, single-crystal X-ray diffraction data is used to define the crystal structures of a synthetic H-bearing aluminous stishovite containing nearly 1.6 weight% of Al2O3, and a pure-silica stishovite. Analysis of the results of the experiments verify that Al is interchangeable with Si in octahedral sites in conditions simulating the conditions of earth's mantle.