Ppm Wt H2O Research Articles

Measurement of water contents in olivine using Raman spectroscopy

We have measured the water contents in forsterites and olivines synthesized in the multi-anvil press using confocal Raman spectroscopy. These samples were previously characterized for water contents by polarized FTIR and contain from 75 to 1300 ppm wt H2O. We find that both forsterite and olivine follow the same trend in water content vs. integrated Raman OH/Si intensity. In addition three synthetic enstatites also display a linear trend in water vs. OH/Si integrated Raman intensity but with a different slope than for olivine, indicating that the calibration for measuring water by Raman is matrix dependent. Three glasses of different compositions (two rhyolites and one basalt) and different water contents were also analyzed. Comparison with the forsterites and olivines shows that the Raman cross-section of these glasses is very different and their intensities must be corrected by different factors. Therefore, to be able to use glasses as external calibrants, prior knowledge of their behavior compared to well-characterized NAM standards is necessary.

Crystal structure, Raman and FTIR spectroscopy, and equations of state of OH-bearing MgSiO3 akimotoite

MgSiO3 akimotoite is stable relative to majorite-garnet 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 H2O) 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 1 = 20(3) × 10−9 K−2 and a 0 = 17(2) × 10−6 K−1, with an average of α 0 = 27.1(6) × 10−6 K−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 0 = 263.7(2) A3, K T0 = 217(3) GPa (K′ fixed at 4). The anisotropies of axial thermal expansivities and compressibilities are similar: α a = 8.2(3) and α c = 10.68(9) (10−6 K−1); β a = 11.4(3) and β c = 15.9(3) (10−4 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 × 10−5 and +1.7 × 10−5 K−1, indicating both weak hydrogen bonds (O–H···O) and strong OH bonding due to long O···O distances.

Low strength of Earth’s uppermost mantle inferred from tri-axial deformation experiments on dry olivine crystals

We have performed tri-axial compression experiments on single crystals of San Carlos olivine with various orientations at temperatures relevant for the uppermost mantle, between 800 and 1090°C. The experiments were carried out at a confining pressure of 300MPa in a high-resolution gas-medium mechanical testing apparatus at various constant strain rates (from 7×10−6s−1 to 1×10−4s−1); they yield differential stresses ranging from 88 to 754MPa. Unpolarized infrared spectroscopy analyses indicate that hydrogen concentration in the olivine lattice is very low (<0.5ppm wt H2O) both before and after deformation. Transmission electron microscopy confirms plastic deformation by dislocation glide. [001] glide dominates regardless of the orientation of the crystals, even if very marginal [100] glide has also been activated in some samples. Orientation mapping by electron backscatter diffraction highlights significant local deviations from the original orientation in some samples associated with bending and deformation bands. These strain localizations suggest a heterogeneous mechanical behavior in this temperature range, which is favored by the strong mechanical anisotropy of the olivine crystal. The present experiments confirm that previous published high-temperature flow laws (i.e., power flow law) overestimate the strength of lithospheric mantle, since all samples deformed under stresses significantly lower than predicted by these flow laws. Based on the present results and on a compilation of previously published data on olivine deformation between 500 and 1000°C, we propose a new semi-empirical exponential flow law applicable to the uppermost mantle: ε̇=1±0.2×106exp-450±60×103RT1-σ15±41/22where T is the absolute temperature, R is the gas constant, and σ is the differential stress in GPa. Subsequently, small fractions of melt or incorporation of hydrogen in olivine may not be required to reconcile natural observations indicating a limited strength of Earth’s uppermost mantle lithosphere (<1GPa) and experimental data on the rheology of olivine.

Diffusion and solubility of hydrogen and water in periclase

Cylinders of synthetic periclase single crystals were annealed at 0.15–0.5 GPa and 900–1200 °C under water-saturated conditions for 45 min to 72 h. Infrared spectra measured on the quenched products show bands at 3,297 and 3,312 cm−1 indicating VOH− centers (OH-defect stretching vibrations in a half-compensated cation vacancy) in the MgO structure as a result of proton diffusion into the crystal. For completely equilibrated specimens, the OH-defect concentration, expressed as H2O equivalent, was calculated to 3.5 wt ppm H2O at 1,200 °C and 0.5 GPa based on the calibration method of Libowitzky and Rossmann (Am Min 82:1111–1115, 1997). This value was confirmed via Raman spectroscopy, which shows OH-defect-related bands at identical wavenumbers and yields an H2O equivalent concentration of about 9 wt ppm using the quantification scheme of Thomas et al. (Am Min 93:1550–1557, 2008), revised by Mrosko et al. (Am Mineral 96:1748–1759, 2011). Results of both independent methods give an overall OH-defect concentration range of 3.5–9 (+4.5/−2.6) ppm H2O. Proton diffusion follows an Arrhenius law with an activation energy Ea = 280 ± 64 kJ mol−1 and the logarithm of the pre-exponential factor logDo (m2 s−1) = −2.4 ± 1.9. IR spectra taken close to the rims of MgO crystals that were exposed to water-saturated conditions at 1,200 °C and 0.5 GPa for 24 h show an additional band at 3,697 cm−1, which is related to brucite precipitates. This may be explained by diffusion of molecular water into the periclase, and its reaction with the host crystal during quenching. Diffusion of molecular water may be described by logDH2O (m2 s−1) = −14.1 ± 0.4 (2σ) at 1,200 °C and 0.5 GPa, which is ~ 2 orders of magnitude slower than proton diffusion at identical P-T conditions.

Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite

The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14-16 GPa and T = 1230-1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (fO₂) was investigated using three metal-oxide buffers (Mo-MoO2, Ni-NiO, and Re-ReO2). The effect of water fugacity (fH₂₂O) was studied using two different capsule assemblies (“nominally dry” and “dry” assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/106Si (<3 wt ppm H2O) and 23 000 H/106Si (1400 wt ppm H2O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 (“3620”), 3480 (“3480”), and 3205 cm-1 (“3205”) and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm−1. The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to “3620,” “3480,” and “3205” were found to be highly dependent on both fH₂₂O and fO₂₂. Assuming COH,Group O = 2[(2H)xM] (COH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of “3620” and “3480” was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are fH₂O and fO₂ exponents, respectively), whereas that of “3205” was consistent with q = 1/2 and r = -1/12. These results suggest that “3620” and “3480” correspond to HM′ whereas “3205” corresponds to H·, respectively, under the charge neutrality condition of [FeM′] = 2[VM″ ].

Mechanisms of OH defect incorporation in naturally occurring, hydrothermally formed diopside and jadeite

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H2O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm−1. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm−1 and contains about 197 wt ppm H2O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm−1 implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.

Water partitioning at 660 km depth and evidence for very low water solubility in magnesium silicate perovskite

Water partitioning experiments were performed in MgO ± FeO ± Al2O3‐SiO2‐H2O systems at 24 GPa and 1400°C and 1750°C in the multi‐anvil press. In high‐temperature experiments, water was not detected in near‐infrared spectra of perovskite. In samples synthesized at 1400°C, in Al‐free systems, (Mg,Fe)SiO3 perovskite coexisting with hydrous ringwoodite contained 1–2 ppm wt H2O. The partition coefficient Dringwoodite/perovskite of water between perovskite and ringwoodite varies between ∼1050 and 1400. In the presence of aluminium at 1400°C, perovskite coexisted with superhydrous phase B. Raman analysis of the samples reveals the presence of superhydrous phase B and brucite as small inclusions in the perovskite, explaining the peaks near 3690, 3410 and 3350 cm−1, frequently seen in the infrared spectra of aluminous perovskites, but which are absent in perfectly clear regions. Consequently, the water solubility in aluminous perovskite appears to be very low. The partition coefficient Dmagnesiowüstite/perovskite varies between 60 and 75 in Fe‐ and Al‐bearing systems, respectively.

Hydrogen in some natural garnets studied by nuclear reaction analysis and vibrational spectroscopy

A suite of 11 gem-quality, optically completely clear garnet crystals with a broad variety of compositions in the space of the end members pyrope–almandine–spessartine–grossular–andradite–goldmanite were analyzed for trace amounts of “water” by nuclear reaction analysis, NRA, based on the reaction 1H(15N, αγ)12C, and by single-crystal absorption spectroscopy in the νOH vibrational range using microscope-FTIR-spectroscopic methods. The aim was to establish a calibration of the highly sensitive IR method with high areal resolution for “water” determination in garnets, by studying garnets of a wide compositional range, and to check for compositional dependencies of the integral molar absorptivities of the “water” component, ɛint[1molH 2 O −1cm−2], in the nominally “water”-free garnets. The results of NRA show a broad variation of water contents in the range (14 ± 3) to (950 ± 80) wt ppmH 2 O, the values being low and very high for the garnet solid solutions (PyrAlm)SS and close-to-end-member GrossSS, respectively. There were no indications of inhomogeneities in the OH distribution, except possibly for one of the garnets (grossular, variety hessonite, from Tanzania). The quantitative evaluation of the complex νOH spectra, which showed similar shape only for members of the (PyrAlm)SS, yielded integral absorption coefficients, αint (cm−2), which allowed the calculation of integral molar absorptivities, ɛint, using the “water” values of NRA. The ɛint values obtained varied in a wide range but with no obvious correlation with the composition of the garnet except for the extremely high values, in the 104 range, of the two specimen with compositions close to end-member grossular. In all other garnets, ɛint was in the 103 range with an average of ɛint=3630±1580[1molH 2 O −1cm−2]. Therefore, this value is proposed for the use in routine “water” determinations of compositionally different garnets by the micro-IR method, except for garnets near to end-member grossular.

Transport of water into the lower mantle: Role of stishovite

When subjected to lower‐mantle pressures and temperatures, natural “anhydrous” basalt containing 0.2 wt.% H2O forms a phase assemblage in which SiO2 stishovite is a significant carrier of hydrogen (up to 500 ppm H2O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO3 perovskite appears to be not (upper bound of 50 ppm (wt) H2O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H2O in residual stishovite increases from ∼100 to ∼400 ppm by weight upon partially melting the high‐pressure mineral assemblage at 28–60 GPa. We infer that the trace concentration of Al within residual stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The “anhydrous” component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted stishovite potentially returning ∼102 times the amount of water present in today's atmosphere.

Solubility of water in the α, β and γ phases of (Mg,Fe) 2 SiO 4

The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)2SiO4 was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/106Si (1200 wt ppm H2O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/106Si (24,000 wt ppm H2O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/106Si (27,000 wt ppm H2O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation COH = A(T)f nH2Oexp(−PΔV/RT), where A(T) = 1.1 H/106Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10–6 m3/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone.