Article

Effect of Silica Activity on OH- IR Spectra of Olivine: Implications for Low-aSiO2 Mantle Metasomatism

April 2001
Journal of Petrology 42(4)

April 2001
42(4)

DOI:10.1093/petrology/42.4.721

Authors:

Sergei Matveev

Utrecht University

Chris Ballhaus

University of Bonn

Wayne Taylor

Energy Metals Ltd

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Hydrogen solubility and hydroxyl substitution mechanism in olivine at upper-mantle conditions are not only a function of pressure, temperature, water fugacity and hydrogen fugacity, but are also influenced by silica activity. Olivine synthesized in equilibrium with magnesiow u ¨ stite displays hydroxyl stretching bands in the wavenumber range from 3640 to 3430 cm –1 . In contrast, olivine in equilibrium with orthopyroxene shows absorption bands in a narrower wavenumber range from 3380 to 3285 cm –1 . The two fundamentally different spectra are assigned to hydroxyl in tetrahedral and octahedral sublattices, respectively. Olivine in equilibrium with orthopyroxene is also less capable of incorporating hydroxyl, relative to olivines in equilibrium with magnesiow u ¨ stite, by about a factor of ten. A comparison of spectra obtained as part of this study with hydroxyl spectra of natural mantle olivines shows that the latter display hydroxyl stretching patterns reminiscent of equilibrium with magnesiow u ¨ stite, although undoubtedly olivine in the Earth’s mantle coexists with orthopyroxene. This may be attributed to a metasomatic overprint by a low-silica fluid and/or melt that was in reaction relationship with orthopyroxene. A likely metasomatic agent is a carbonatitic melt. When carbonatitic melts decompose to oxides and CO 2 , they may temporarily impose a low- a SiO 2 environment inherited by the olivine structure. If this suggestion proves true, Fourier transform IR spectroscopy may be used to fingerprint metasomatic episodes in the lithospheric mantle.

Point defect equilibria in hydrous forsterite synthesized at 1100°C and up to 4 GPa

Article

Full-text available

Sep 2019

Water distribution in the deep Earth represents one of the most important topics in the field of geodynamics due to its large impact on the physical and chemical properties of the Earth's mantle, such as electrical conductivity, seismic anisotropy, diffusion, and rheology. In this study, we synthesized hydrous forsterite at 1100 • C and up to 4 GPa with either a piston-cylinder or multianvil apparatus. As a starting material, we used synthetic forsterite, unbuffered by SiO 2 , obtained by thermo-mechanical activation of talc and magnesium carbonate hydroxide. Hydration was carried out using liquid H 2 O as hydrogen source. Samples were polycrystalline in an effort to distribute H 2 O throughout the sample both rapidly and homogeneously. Using the Paterson calibration, we observed total water content concentrations ranging between 100 and 500 ppm wt H 2 O. Multiple absorption bands are found in the frequency range between 3400 and 3650 cm −1 , identifying at least seven peaks in all samples. Vibrational bands were centered at 3476, 3535, 3550, 3566, 3578, 3605, and 3612 cm −1 , in good agreement with experimental studies conducted on both hydrous forsterite and single crystals of olivine. The stronger OH stretching peaks can be attributed to vibrational modes associated with the hydrogarnet defect 4H x Si in which four protons occupy a vacant tetrahedral site. None of the OH bands observed are found at frequencies associated with hydrogen occupying vacant octahedral sites. High-temperature FTIR spectroscopy was used to evaluate the evolution of IR spectra as a function of temperature, up to 500 • C. The complete reversibility of peak absorption vs. temperature in the OH stretching region confirms that no water loss occurred during heating. We observe an overall a decrease in total absorption with increasing temperature, and a prominent decrease in the relative intensities of the higher frequency bands (>3600 cm −1) with respect to lower frequency bands. We have assigned a series of equilibrium expressions based on the variation of relative peak areas with temperature and find that enthalpies of these processes range between 0.047-0.068 eV (4.5-6.5 kJ/mol), very low in comparison to activation energies observed for electrical conduction in hydrous olivine. Major changes in the vibrational spectrum are expected to be related to configurational changes of the same fully protonated hydrogarnet defect species. However, the complexity of the FTIR spectra may also be related to partially protonated defects, such as the associate defect 3H Si + H • i generated by a dissociation reaction of the hydrogarnet species.

Pressure Dependence of Proton Incorporation and Water Solubility in Olivine

Article

Full-text available

Jan 2020

Hydrous forsterite single crystals were synthesized at 8 GPa and 1250 °C using a multianvil apparatus under various SiO2 activity conditions. Electron microprobe and transmission Fourier‐transform infrared spectroscopy analyses show that the Mg/Si atomic ratio increases with increasing water content. This provides new evidence that protons are mainly stored in Si sites in hydrous forsterite. The contribution of Mg sites for proton incorporation can be significant at ≤2.5 GPa with excess SiO2 but is negligible at higher pressures. Protons in olivine under asthenospheric conditions are therefore mostly maintained in Si sites. A summary of experimental studies suggests that the water solubility of forsterite/olivine equilibrated with excess (Mg,Fe)O is substantially higher than that with excess SiO2; however, this difference decreases with increasing pressure and becomes identical at 12–14 GPa. Owing to the low water solubility of olivine under topmost asthenospheric conditions, the geophysically observed low viscosity and low seismic velocity zones cannot be caused by olivine hydration, and the high conductivity anomaly should not be attributed to free H⁺‐controlled proton conduction in olivine.

Coupled inter-site reaction and diffusion: Rapid dehydrogenation of silicon vacancies in natural olivine

Article

Jul 2019
GEOCHIM COSMOCHIM AC

Natural olivine from Zermatt (Switzerland) and Almklovdalen (Norway) were dehydrogenated at 1 bar, 517–1009 °C at various oxygen fugacity conditions. Following experiments, H contents (either bulk, or core-rim profiles) were measured using Fourier transform infrared spectroscopy, and spectra were resolved into Gaussian peaks. The starting olivine contained ∼150 and ∼10 wt. ppm H2O for the Zermatt and Almklovdalen samples respectively, but importantly the initial H distribution in both corresponded to defects with 4H⁺ occupying Si vacancies. Experiments in pure forsterite Padrón-Navarta and Hermann, (2017) showed that this defect diffuses very slowly relative to other H diffusion mechanisms in olivine. Conversely, we show that, in the Almklovdalen samples, H loss from the Si-vacancy defect can be extremely rapid, and approaches the fastest known mechanism of H diffusion (proton-polaron diffusion) in olivine. The rate of dehydrogenation from the Zermatt olivine is slightly slower, more consistent with the diffusivity of hydrogenated M-site vacancies. The sum of all defects, along with the integrated area of the main Si vacancy peak (3612 cm⁻¹) generally shows simple diffusive behaviour, whereas profiles (or maps) resolved into individual peaks reveal complex profile shapes not consistent with a simple out-diffusion mechanism. This behaviour can be modeled as a combination of inter-site reaction and diffusion, whereby H leaves the tetrahedral site, moves into a faster diffusion pathway, then rapidly exits the crystal. The rate of H loss from, or gain into, olivine can therefore be either diffusion limited, reaction limited, or a combination of the two. This may explain current discrepancies between experiments conducted under different conditions and between experimental and natural data, including the recent observation that H stored in Si vacancies in metamorphic olivine may be retentive over millions of years, despite the capability to diffuse rapidly via a different mechanism.

The role of trace elements in controlling H incorporation in San Carlos olivine

Article

Full-text available

Oct 2018
CONTRIB MINERAL PETR

We conducted a series of hydroxylation experiments using mm-sized cuboids cut from six different crystals of San Carlos olivine with a range of trace-element concentrations. The cuboids were pre-annealed and then hydroxylated under identical conditions, ensuring that variation in the amounts of H incorporated depended only on the compositional variables. The pre-anneal was at 1400 °C, atmospheric pressure and an oxygen fugacity equivalent to Δlog FMQ + 1, with the subsequent hydroxylation at 800 °C and 1.5 GPa, for 3 days. Hydrogen was incorporated into all six crystals by the four main substitution mechanisms [Si], [Mg], [Ti] and [triv], with homogeneous H contents in the cores of the crystals, indicating H diffusion rates faster than 10− 11 m²/s. Total H as H2O in the homogeneous cores calculated by summing all the infrared absorbance bands ranges from 13 to 27 wt. ppm. The total H2O in the six pre-annealed crystals is poorly correlated with any measured compositional variable. However, when the H2O associated with individual infrared bands is compared, clear trends emerge. The intensity of absorption bands at 3572 and 3525 cm− 1 are strongly correlated with Ti concentrations, whose range in the six crystals exceeds an order of magnitude. Bands between 3400 and 3300 cm− 1, correlate negatively with Na⁺, but are positively correlated with the difference between molar Cr³⁺ and Na⁺. This highlights a previously unrecognised role for Na in suppressing H incorporation in natural olivines. The results confirm the important role that the trace constituents of olivine play in H incorporation. Two of these trace elements, Na and Ti, tend to be similarly enriched or depleted by partial melting or metasomatism of the mantle, but have opposite effects on H incorporation, with Ti enhancing it but Na suppressing it. Models estimating the effect of H in olivine on mantle rheology must, therefore, consider carefully the availability of these trace elements.

The responses of the four main substitution mechanisms of H in olivine to H 2 O activity at 1050 °C and 3 GPa Progress in Earth and Planetary Science

Article

Full-text available

May 2017

The water solubility in olivine C H 2 O ð Þ has been investigated at 1050 °C and 3 GPa as a function of water activity a H 2 O ð Þ at subsolidus conditions in the piston-cylinder apparatus, with a H 2 O varied using H 2 O–NaCl fluids. Four sets of experiments were conducted to constrain the effect of a H 2 O on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm −1. About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of C H 2 O ∝a H 2 O 2 , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm −1. The band position, polarisation and observed C H 2 O ∝a H 2 O are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by C H 2 O ∝a H 2 O and a 1:1 relationship of molar H 2 O and TiO 2 in these experiments. Sc-doped, enstatite-buffered experiments display a main absorption band at 3355 cm −1 with C H 2 O ∝a H 2 O 0:5 and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at a H 2 O ¼ 1 are applied to the mantle, where in most cases, a low a H 2 O persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing a H 2 O than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-P-fO 2-fH 2 O conditions that may be anticipated in nature. However, the small amounts of H 2 O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent a H 2 O (all buffered by enstatite), confirming the assumption that, thermodynamically, C H 2 O substituting by each mechanism does not depend on the water concentration that substitutes by other mechanisms.

Effect of water on the phase relations in Earth's mantle and deep water cycle

Article

Full-text available

Jan 2007

Water is transported by subducting slabs into the transition zone and lower mantle. Important water carriers to the deep mantle may be serpentine, chlorite, phase A, and superhydrous phase B in peridotite; zoisite, lawsonite, and phengite in basalt; and topaz-OH and phase Egg in sediments. Phase D, stable in peridotite, and the δ-AlOOH phase in the sedimentary component may transport water to at least 1200-1500 km depth. Phase relations in hydrous peridotite show that the phase boundaries of olivine to wadsleyite and ringwoodite to Mg-perovskite 〈 fer-ropericlase phase transitions shift to lower and higher pressures, respectively. Thus, elevation of the 410 km discontinuity and depression of the 650 km discontinuity in subduction zones may be partially affected by water. Water may also control the separation of the basaltic layer of the slab near the 650 km discontinuity. The density crossover that occurs under dry conditions between peridotite and basalt components of slab near 650 km disappears under hydrous conditions due to a significant shift to lower pressures of post-garnet transformation in basalt. Due to high water solubility in wadsleyite and ringwoodite, the transition zone may be a water reservoir in Earth's interior. Recent data show that the transition zone at least locally may contain 0.2-1.5 wt% H2O. In addition, the transition zone may serve as a water absorber for the water circulation system of the mantle. The upper mantle appears to be largely degassed through the action of mid-ocean-ridge and hotspot volcanism. The water storage capacity of the lower mantle as well as hydrogen storage potential of the core are still uncertain. There are several potential dehydration sites in the mantle that may control water circulation through plate tectonics: the mantle wedge above subduction slabs, a region above the 410 km discontinuity, the top of the lower mantle, and the deep lower mantle.

Effect of oxygen fugacity on OH dissolution in olivine under peridotite-saturated conditions: An experimental study at 1.5-7 GPa and 1100-1300 °C

Article

Jan 2016
GEOCHIM COSMOCHIM AC

Xiaozhi Yang

The dissolution of OH in olivine by experimental studies at simulated conditions has attracted increasing interest over the past three decades, and the influence of pressure, temperature and composition has been relatively well constrained. Oxygen fugacity is highly heterogeneous in the upper mantle, on both temporal and spatial scales, and is an important parameter in characterizing many chemical and physical processes in the mantle. However, less attention has been devoted to the effect of oxygen fugacity on OH dissolution in olivine, and the only few available reports on this topic have led to significant inconsistency and debate. In this study, the correlation between oxygen fugacity and OH solubility in Fe-bearing olivine has been systematically investigated by conducting experiments at 1.5-7GPa and 1100-1300°C and under peridotite- and fluid-saturated conditions, with natural gem-quality olivine single crystals and fresh peridotite xenoliths as starting materials and with oxygen fugacity controlled by the Fe-FeO, Ni-NiO and Fe2O3-Fe3O4 oxygen buffer pairs. The water concentrations were determined by polarized analyses using a Fourier-transform infrared spectroscopy. The results show that, at all the experimental conditions, the OH bands at both high frequency (~3650-3450cm-1) and low frequency (~3450-3100cm-1) are prominent. The intensity of OH bands at ~3355 and 3325cm-1 increases positively with oxygen fugacity, suggesting a dominant role of Fe3+ in their incorporation. Under otherwise identical conditions, the water content is gradually enhanced with increasing pressure, temperature or oxygen fugacity. The effect of oxygen fugacity on the enhancement of OH solubility appears not sensitive to temperature (1100-1300°C) at a given pressure, but becomes progressively stronger with increasing pressure from 1.5 to 7GPa given the temperature. Relative to oxygen fugacity buffers, the OH solubility is on average increased by ~50% between Fe-FeO and Ni-NiO, independent of pressure and temperature. The enhanced OH solubility at relatively oxidizing conditions could be explained by the role of oxygen fugacity in changing the point defects of olivine. Experimental work concerning the storage capacity of water in olivine and in the upper mantle must consider the effect of oxygen fugacity. The OH solubility of olivine in the reducing deep upper mantle obtained from experiments under oxidizing conditions would be overestimated. The initial OH information of many natural olivines may have been modified by some recent secondary events, e.g., during their shallow residence and/or transport to the surface, and in principle, olivine equilibrated in the upper mantle may be D-depleted relative to its coexisting minerals. Some preliminary constraints on the water storage capacity in the upper mantle of the Earth and the Mars are also provided.

OH solubility in olivine in the peridotite–COH system under reducing conditions and implications for water storage and hydrous melting in the reducing upper mantle

Article

Dec 2015
EARTH PLANET SC LETT

Xiaozhi Yang

Experimental studies of OH solubility in peridotite minerals are of crucial importance for understanding some key geochemical, geophysical and geodynamical properties of the upper mantle. In reducing depths of the upper mantle, C–O–H fluids are dominated by CH4 and H2O. However, available experimental H-annealing of olivine concerning water storage capacity in the reducing upper mantle has been exclusively carried out by equilibrating olivine with H2O only. In this study, OH solubility in olivine has been investigated by annealing natural olivine crystals under peridotite-bearing and CH4–H2O-present conditions with piston cylinder and multi-anvil apparatus. Experiments were performed at 1–7 GPa and 1100–1350 °C and with oxygen fugacity controlled by Fe–FeO buffer, and OH solubilities were measured from polarized infrared spectra. The olivines show no change in chemical composition during the experiments. The infrared spectra of all the annealed olivines show OH bands in the range 3650–3000 cm−1, at both high (>3450 cm−1) and low (<3450 cm−1) frequency, and the bands at ∼3400–3300 cm−1 are greatly enhanced above ∼3 GPa and 1300 °C. The determined H2O solubility is ∼90–385 ppm for the olivine coexisting with H2O (1–7 GPa and 1100 °C), and is ∼40–380 ppm for the olivine coexisting with CH4–H2O (1–7 GPa and 1100–1350 °C). When CH4 is present in the equilibrium fluid, the H2O solubility is reduced by a factor of ∼2.3 under otherwise identical conditions, indicating a strong effect of CH4 on the partitioning of water between olivine and coexisting fluid. The storage capacity of water in the reducing upper mantle is, modeled with the measured solubility of olivine and available partition coefficients of water between coexisting minerals, up to ∼2 orders of magnitude lower than some previous estimates. Considering the temperature along the geotherm in the reducing oceanic upper mantle, the required H2O concentration to trigger hydrous melting is 250 and 535 ppm at ∼100 and 210 km depth, respectively, and is even larger at greater depths. These values exceed the typical H2O abundance (∼100±50 ppm∼100±50 ppm) in the upper mantle, suggesting that pervasive hydrous melting at reducing depths of the oceanic upper mantle is not likely. Similar arguments may also be casted for the reducing deep upper mantle in the continental regions.

Diffusion of Hydroxylated Si Vacancies in Olivine, and Its Relevance to Si Diffusion

Article

Full-text available

May 2023
GEOCHEM GEOPHY GEOSY

Michael Jollands

The diffusion coefficient associated with the 3,612 cm⁻¹ infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H⁺ (i.e., (4H)Si× ${(4\mathrm{H})}_{\mathrm{S}\mathrm{i}}^{\times }$), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol⁻¹), when both are charge‐compensated solely by H⁺. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine.

Controls on determining the bulk water content of the Moon

Article

Full-text available

Nov 2022

The Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100–1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer.

A combined Fourier transform infrared and Cr K-edge X-ray absorption near-edge structure spectroscopy study of the substitution and diffusion of H in Cr-doped forsterite

Article

Full-text available

Feb 2021
EUR J MINERAL

Single crystals of synthetic Cr-doped forsterite (Cr:Mg2SiO4) containing both Cr3+ and Cr4+ were partially hydroxylated in piston-cylinder apparatuses at 750–1300 ∘C and pressures from 0.5 to 2.5 GPa, with p(H2O) ≈Ptotal. The oxygen fugacity (fO2) was buffered by graphite-water, Ni–NiO, Re–ReO2, Fe2O3–Fe3O4 or Ag–Ag2O, and the silica activity (aSiO2) was buffered by powdered forsterite plus either enstatite (Mg2Si2O6), periclase (MgO) or zircon–baddeleyite (ZrSiO4–ZrO2). Profiles of OH content versus distance from the crystal edge were determined using Fourier transform infrared (FTIR) spectroscopy, and profiles of the oxidation state and coordination geometry of Cr were obtained, at the same positions, using K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The techniques are complementary – FTIR spectroscopy images the concentration and nature of O–H bonds, where Cr K-edge XANES spectroscopy shows the effect of the added H on the speciation of Cr already present in the lattice. Profiles of defect-specific absorbance derived from FTIR spectra were fitted to solutions of Fick's second law to derive diffusion coefficients, which yield the Arrhenius relationship for H diffusion in forsterite: log⁡10D̃[001]=-2.5±0.6+-(224±12+4.0±2.0P)2.303RT, where D̃ is the measured diffusion coefficient in m2 s−1, valid for diffusion parallel to [001] and calibrated between 1000 and 750 ∘C, P and T are in GPa and K, and R is 0.008314 kJK−1 mol−1. Diffusivity parallel to [100] is around 1 order of magnitude lower. This is consistent with previous determinations of H diffusion associated with M-site vacancies. The FTIR spectra represent a variety of Cr-bearing hydrous defects, along with defects associated with the pure Mg–Si–O–H system. It is proposed that all of the defects can form by interaction between the dry lattice, including Cr3+ and Cr4+, and fully hydroxylated M-site vacancies. The initial diffusive wave of hydroxylation is associated with neither reduction nor oxidation of Cr but with Cr4+ changing from tetrahedral to octahedral coordination. Superimposed on the H diffusion and concomitant change in Cr4+ site occupancy, but at a slower rate, producing shorter profiles, is reduction of Cr4+ to Cr3+ and potentially of Cr4+ and Cr3+ to Cr2+. In addition, by comparing FTIR data to trace element contents measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), constraints can be placed on absorption coefficients used for converting absorbance to H2O contents – our data support either wavenumber- or defect-dependent values of absorption coefficients. We estimate absorption coefficients of between 60 200 and 68 200 L mol−1 cm−1 for OH− associated with octahedral Cr3+ and an M-site vacancy and 18 700 to 24 900 L mol−1 cm−1 for two OH− associated with octahedrally coordinated Cr4+ and a Si vacancy (i.e. a “clinohumite-type” point defect).

Explaining the dependence of M-site diffusion in forsterite on silica activity: a density functional theory approach

Article

Full-text available

Dec 2020
PHYS CHEM MINER

Experimentally, silica activity (aSiO2) has been shown to have an effect on Mg diffusion in forsterite, but no fully satisfactory mechanism has yet been proposed. We calculated the effects of aSiO2 and aluminium content (the main contaminant in some recent experimental studies), and their co-effect, on Mg diffusion in forsterite, using thermodynamic minimisations of defect formation energies [calculated using density functional theory (DFT)] and a Monte-Carlo diffusion model. These two variables, in isolation, do not appreciably change the defect concentrations of forsterite and thus do not affect the diffusivity of Mg. However, when elevated together, they cause large increases in the Mg vacancy content and thus can increase the Mg diffusivity by one to six orders of magnitude depending on temperature, with little pressure dependence. This effect is largely independent of Al2O3 concentration above ~ 1 wt. ppm, and thus, for all practical purposes, should occur wherever forsterite is in the presence of enstatite. It is also largely dependent upon configurational entropy and is thus highly sensitive to the chemistry of the crystal. A low concentration of structurally bound hydroxyl groups at low temperatures (1000 K) suppresses this effect in pure forsterite, but it is likely robust in the presence of water either when alternative water sinks (such as Ti or Fe) are present, or at high temperatures (> 1500 K). This effect is also robust in the presence of ferrous iron (or other substitutional Mg defects) at all temperatures. Fe2O3 can operate like Al2O3 in this reaction and should enhance its effect. These findings explain the experimentally observed dependency of Mg diffusion of aSiO2, and elucidate how chemical activity variations in both experiments and natural settings could affect not only the diffusivity of Mg in forsterite, but of olivine-hosted cations in general.

High‐Temperature Evolution of Point Defect Equilibria in Hydrous Forsterite Synthesized at 1100 °C and up to 4 GPa

Article

Full-text available

Sep 2019

B.T.

Water distribution in the deep Earth represents one of the most important topics in the field of geodynamics due to its large impact on the physical and chemical properties of the Earth’s mantle, such as electrical conductivity, seismic anisotropy, diffusion, and rheology. In this study, we synthesized hydrous forsterite at 1100 C° and up to 4 GPa with either a piston-cylinder or multianvil apparatus. As a starting material, we used synthetic forsterite, unbuffered by SiO2, obtained by thermo‐mechanical activation of talc and magnesium carbonate hydroxide. Hydration was carried out using liquid H2O as hydrogen source. Samples were polycrystalline in an effort to distribute H2O throughout the sample both rapidly and homogeneously. Using the Paterson calibration, we observed total water content concentrations ranging between 100 and 500 ppm wt H2O. Multiple absorption bands are found in the frequency range between 3400 and 3650 cm−1, identifying at least seven peaks in all samples. Vibrational bands were centered at 3476, 3535, 3550, 3566, 3578, 3605, and 3612 cm−1, in good agreement with experimental studies conducted on both hydrous forsterite and single crystals of olivine. The stronger OH stretching peaks can be attributed to vibrational modes associated with the hydrogarnet defect 4HSix in which four protons occupy a vacant tetrahedral site. None of the OH bands observed are found at frequencies associated with hydrogen occupying vacant octahedral sites. High-temperature FTIR spectroscopy was used to evaluate the evolution of IR spectra as a function of temperature, up to 500 °C. The complete reversibility of peak absorption vs temperature in the OH stretching region confirms that no water loss occurred during heating. We observe an overall a decrease in total absorption with increasing temperature, and a prominent decrease in the relative intensities of the higher frequency bands (>3600 cm−1) with respect to lower frequency bands. We have assigned a series of equilibrium expressions based on the variation of relative peak areas with temperature and find that enthalpies of these processes range between 0.047–0.068 eV (4.5–6.5 kJ/mol), very low in comparison to activation energies observed for electrical conduction in hydrous olivine. Major changes in the vibrational spectrum are expected to be related to configurational changes of the same fully protonated hydrogarnet defect species. However, the complexity of the FTIR spectra may also be related to partially protonated defects, such as the associate defect 3HSi’ + Hi• generated by a dissociation reaction of the hydrogarnet species.

In Situ Infrared Spectra for Hydrous Forsterite up to 1243 K: Hydration Effect on Thermodynamic Properties

Article

Full-text available

Aug 2019

Hydrogen substitution has significant effect on the physical properties of olivine, the most abundant mineral in the upper mantle. We collected high-temperature polarized Fourier Transform infrared (FTIR) spectra on hydrous forsterite (Mg-pure olivine) crystals, which were synthesized at 12 GPa, 1473–1673 K. The modes at 3612, 3578, 3566, 3551 cm−1 show comparable negative temperature dependence, and the magnitude of (∂vi/∂T)P decreases dramatically with frequency increasing. Whereas, the peak at 3477 cm−1, which is attributed to protonation along the O1...O2 edge on the Si tetrahedron, has a positive temperature dependence. The absorbance intensities of all these OH bands remained almost the same when quenched to room temperature. On the other hand, we also evaluate the hydration effect on the thermodynamic properties (heat capacities). For the anhydrous forsterite sample, the intrinsic anharmonicity could significantly increase the heat capacity by 5~6% when extrapolated to 2000 K. Hydration further increase such difference to ~9%, in both the cases of M-substitution or Si-substitution. Hence, hydration in olivine has significant impact on the anharmonic contribution to the thermodynamic properties, as well as Equations of State and equilibrium isotope fractionation β-factor at high-P,T conditions in the deep mantle.

Ab initio study of water speciation in forsterite: Importance of the entropic effect

Article

May 2018

In this ab initio study, we expand previous investigations of charge-balanced hydrous Mg ((2H) XMg ) and Si ((4H) XSi ) defects in forsterite, the Mg end-member of olivine, to address the relative stability of these two defects. First, we systematically search for (2H) XMg configurations to find possible defect states; second, we include the contribution of vibrational energy and defect configurational entropy in the calculation of formation energies of both defects; third, we address the effect of pressure and temperature simultaneously on their relative stability. Based on these considerations, we demonstrate that hydrous Mg defects ((2H) XMg ) can be stabilized with respect to hydrous Si defects ((4H) XSi ) at relevant mantle conditions and that configurational entropy and vibrational free energy play key roles in this stabilization. Our results reveal that water speciation in olivine is influenced by temperature and pressure. As mantle physical and chemical properties may be affected by the speciation of water in olivine, application of experimental results to the mantle should account for the temperature- and pressure-dependent changes in water speciation.

Petrology of avachites, high-magnesian basalts of Avachinsky volcano, Kamchatka: I. General characteristics and composition of rocks and minerals

Article

Full-text available

Mar 2005

In 1935, A.N. Zavaritskii described high-magnesian [MgO/(MgO + FeO*) = 0.73-0.82] basalts and picrites, which are unique in Kamchatka and were found on Avachinsky volcano and eventually named avachites. This paper systematizes data on the composition of these rocks and presents the results of their detailed mineralogical examination on an electron microprobe (EMPA) and with the use of secondary-ion mass spectrometry (SIMS) and vibrational IR spectrometry. The results thus obtained suggest that avachites are of volcanic genesis and were produced by the crystallization of olivine (Fo91-80), clinopyroxene (Mg# = 92.5-73 mol%), and spinel [Mg# = 18-59 mol%, Cr/(Cr + Al) = 0.82-0.55] from a basaltic (SiO2 ≤ 52 wt%, MgO ∼ 13 wt%) parental melt, whose composition was intermediate between those of island-arc ankaramites and high-Ca boninites. The high-Mg chemistry of the rocks (MgO = 14-20 wt%) is explained by the accumulation of olivine and pyroxene phenocrysts in an evolved basaltic melt (MgO ∼ 5 wt%), which composes groundmass of avachites. The minerals crystallized under pressures of 1.0-0.1 GPa, at temperatures of ≤1380-1050°C, and an oxygen fugacity of ΔQFM = 0.5-2.0. The results of the IR spectroscopy of the olivine suggest that the parental magmas contained ≥0.5 wt% H2O. The parental magmas of avachites were derived at high degrees (>20%) of the partial melting of a mantle source that was depleted more strongly than the source of mid-oceanic ridge basalts (MORB) and was metasomatized by a fluid or melt rich in LREE, Th, Ba, K, and Sr. The typical basaltic andesites of Avachinsky volcano can be genetically related to avachites and could be produced by olivine and pyroxene crystallization from parental melts of similar composition but under pressures varying within a narrower range. The composition of the associated primitive basalts indicates that the parental melts of the volcano were heterogeneous, and magmas of ankaramite composition could contribute to the genesis of volcanic series in Kamchatka.

Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa

Article

Mar 2023
PHYS EARTH PLANET IN

Rates of dehydration in hydrous, high-Fo, magmatic olivines

Article

Nov 2022
GEOCHIM COSMOCHIM AC

Natural olivine from the Etna Fall Stratified eruption (3930 BP) was dehydrated to determine hydrogen diffusivity. Dehydration was carried out in several time-steps, from 0.5 to 40 cumulative hours, in a 1 atm. furnace at 810 and 1000 oC. This starting material has high H2O concentration (∼50 ppm), high forsterite (Fo ∼90%), and high fO2 (∼ NNO +1.2), reflecting characteristics of its host magma. H2O concentration was measured using Fourier transform infrared spectroscopy after each heating step. Spectral peaks were fit with Gaussian curves to determine the contribution from each H⁺ defect. Zonation profiles along each crystallographic direction were modelled using the ‘whole-block’ method to determine bulk and defect-specific H⁺ diffusivity. Our modelling reveals a common apparent diffusivity for all defects, which is faster along a ([100]) than b ([010]) or c ([001]) by more than an order of magnitude. A common defect-specific diffusivity has not been found in experiments on pure forsterite, so it appears that the presence of Fe enables H⁺ from all defects to diffuse via the proton-polaron mechanism (supported by the observed anisotropy). However, the rates that we observe are slower than the proton-polaron mechanism, which supports the recent proposal by Ferriss et al. (2018) that the apparent diffusivity is rate-limited by reactions between H⁺ bound in defect sites and Fe²⁺. A coupled reaction and diffusion process may explain the observed decrease in diffusivity along a over time, as well as an apparent non-zero H2O concentration at the crystal edge. Both of these effects need to be considered when modelling H⁺ diffusion profiles to determine decompression rate – fixing the edge concentration to zero will underestimate the diffusion timescale. Arrhenius laws describing diffusion parallel to a (at the start and end of the experiments), b, and c are developed from the diffusivity at 810 and 1000 oC: Dastart=10-2.94exp-182000RT, Daend=10-4.63exp-154000RT, Db=10-2.35exp-243000RT, Dc=10-4.01exp-187000RT, where R is the gas constant 8.314 (J/mol K); T is the temperature (K), the units of the diffusivities (D) are in m²/s. A comparison of experimental dehydration studies on natural olivines reveals that the diffusivity during dehydration likely depends on how the olivine was experimentally hydrated. To avoid this experimental effect and understand the diffusivity operating during ascent in a volcanic conduit, crystals which are naturally hydrous (such as the Etna olivines in this study) are particularly useful. Despite some ambiguity in the literature owing to different experimental approaches, our results suggest that for the studied range of pressure and temperature, forsterite content is the major control on H⁺ diffusivity, while H2O concentration and the fO2 at which the crystal last equilibrated have a smaller effect within the ranges studied for magmatic olivines.

Modelling hydrogen mobility in forsterite as diffusion coupled to inter-site reaction

Article

Full-text available

Oct 2022
CONTRIB MINERAL PETR

Whilst the diffusivity of hydrogen (H) in forsterite (Mg2SiO4) has been extensively studied, there still remain some puzzling observations. Firstly, experiments measuring ostensibly the same process have provided different results in terms of diffusion coefficients. Secondly, despite H diffusion in forsterite generally being associated with diffusion of M-vacancies charge compensated by 2H⁺, a plethora of H-bearing point defects have been observed, including those associated with Si vacancies, trivalent cations and tetravalent cations in the form of so-called ‘clinohumite-type’ point defects. This has been tentatively associated with some form of inter-site reaction, such as one in which a M-site vacancy associated with 2H⁺ reacts with tetrahedrally coordinated Ti⁴⁺. Equivalent reactions can be constructed to form all point defects mentioned above. Here, we present a series of numerical models in which these processes are simulated. In the models, the mobility of H is described using a diffusion coefficient (D*) for the hydrogenated M-site vacancies and an equilibrium constant (K) for the relevant inter-site reaction(s). Reevaluation of published data shows that the extracted D* and K values are consistent between some different datasets, even in situations where the phenomenological (chemical) diffusion coefficient D~\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tilde{D}$$\end{document}, extracted simply using solutions of Fick’s second law, did not agree. The ‘true’ mobility (D*) of the M-site vacancy associated with 2H⁺ must be between 1 and 2 orders of magnitude greater than previously determined D~\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\tilde{D}$$\end{document} in order to form measurable profiles of all point defects observed by vibrational spectroscopy. Density functional theory calculations of the K of each of the inter-site reactions implemented in our model show good agreement (within an order of magnitude) with those determined experimentally for the reactions forming ‘clinohumite-type’ defects, but considerable disagreement (~ 3 orders of magnitude) for the defects involving trivalent cations, potentially due to assumptions related to binding of different components within individual defects. Overall, the first-order implication is that H diffusion profiles that we observe in natural and experimental samples are unlikely to be formed by simple diffusion alone. These models provide a new methodological framework for further understanding of complex ionic diffusion mechanisms in olivine and the other nominally anhydrous minerals.

Controls on the distribution of hydrous defects in forsterite from a thermodynamic model

Article

Full-text available

Mar 2022
PHYS CHEM MINER

The distribution of hydrogen across different crystallographic sites and point defects in forsterite determines how many properties, such as rheology, conductivity and diffusion are affected by water. In this study, we use lattice dynamics and Density Functional Theory (DFT) to build a thermodynamic model of H-bearing defects in Al,Ti bearing forsterite. From this, the distribution of hydrogen in forsterite as a function of pressure (P), temperature (T), water, Al and Ti concentration is determined. Primarily, hydrogen distribution in forsterite is complex and highly varied in different P, T and composition regimes. Therefore, extrapolation of properties that depend upon water between these different regimes is non-trivial. This extrapolation has often been done by determining exponents which describe how defect-specific defect concentrations or properties dependent upon them vary with water concentration/fugacity. We show here that these exponents can vary radically across common experimental and geophysical P, T and [H2O]bulk ranges as the favoured hydrogen-bearing defects change. In general, at low pressures hydrogen favours Mg vacancies (high temperatures) or complexes with titanium (low temperatures) whilst at high pressures, hydrogen favours Si vacancies regardless of all other conditions. Higher values of [H2O]bulk also favours hydrated Si vacancies. We evaluate these distributions along geotherms and find that hydrogen distribution and thus its effect on forsterite properties is highly varied across the expected conditions of the upper mantle and thus cannot be simply represented. No such distribution of hydrogen has been previously constructed and it is essential to consider this hydrogen distribution when considering the properties of a wet mantle.

Computational studies of point defects and dislocations in forsterite (Mg₂SiO₄) and some implications for the rheology of mantle olivine

Thesis

Jan 2004

Andrew Mark Walker

Convection in the Earths solid mantle, accompanied by long time-scale viscous creep, is the mechanism operating on the global scale that allows internally generated heat to escape from the Earths deep interior. This process drives plate tectonics and, ultimately, almost every other geological process occurring at the Earths surface. On a small scale, the mantle deforms by plastic deformation of its constituent crystals - a process that involves the movement of a range of crystal defects (point defects, dislocations and grain boundaries). An understanding of the defect properties of mantle phases is thus essential to any unified, multi-scale view of the behaviour of the Earth. This thesis describes research on a range of defect phenomena in olivine ([Mg,Fe]₂SiO₄) undertaken using atomic scale computational modelling techniques. Following an outline of the theoretical background, the structure, energetics and properties of a range of point defects are studied using an inter-atomic parameterised potential model and the Mott-Littleton method. A combined electronic structure - parameterised potential embedded cluster approach has also been employed for selected defects. Bulk diffusion occurring by point defect motion is then considered using static lattice techniques with the parameterised model. The diffusion of oxygen is treated in some detail and an explanation of a range of experimental observations is proposed by considering the formation and motion of defects on the oxygen sub-lattice coupled to the redox behaviour of iron. The second part of the thesis concentrates on modelling line defects. First a methodology for studying screw and edge dislocations in structurally complex ionic materials is developed from an earlier approach limited to studies of ionic materials with the rock salt structure. This methodology is then applied to MgO, zeolite A (a technologically significant material) and finally forsterite, the pure magnesium end member of the olivine solid solution.

IR Features of Hydrous Mg2SiO4-Ringwoodite, Unannealed and Annealed at 200–600 °C and 1 atm, with Implications to Hydrogen Defects and Water-Coupled Cation Disorder

Article

Full-text available

May 2020

Three batches of Mg2SiO4-ringwoodites (Mg-Rw) with different water contents (C-H2O = ~1019(238), 5500(229) and 16,307(1219) ppm) were synthesized by using conventional high-P experimental techniques. Thirteen thin sections with different thicknesses (~14–113 μm) were prepared from them and examined for water-related IR peaks using unpolarized infrared spectra at ambient P-T conditions, leading to the observation of 15 IR peaks at ~3682, 3407, 3348, 3278, 3100, 2849, 2660, 2556, 2448, 1352, 1347, 1307, 1282, 1194 and 1186 cm−1. These IR peaks suggest multiple types of hydrogen defects in hydrous Mg-Rw. We have attributed the IR peaks at ~3680, 3650–3000 and 3000–2000 cm−1, respectively, to the hydrogen defects [VSi(OH)4], [VMg(OH)2MgSiSiMg] and [VMg(OH)2]. Combining these IR features with the chemical characteristics of hydrous Rw, we have revealed that the hydrogen defects [VMg(OH)2MgSiSiMg] are dominant in hydrous Rw at high P-T conditions, and the defects [VSi(OH)4] and [VMg(OH)2] play negligible roles. Extensive IR measurements were performed on seven thin sections annealed for several times at T of 200–600 °C and quickly quenched to room T. They display many significant variations, including an absorption enhancement of the peak at ~3680 cm−1, two new peaks occurring at ~3510 and 3461 cm−1, remarkable intensifications of the peaks at ~3405 and 3345 cm−1 and significant absorption reductions of the peaks at ~2500 cm−1. These phenomena imply significant hydrogen migration among different crystallographic sites and rearrangement of the O-H dipoles in hydrous Mg-Rw at high T. From the IR spectra obtained for hydrous Rw both unannealed and annealed at high T, we further infer that substantial amounts of cation disorder should be present in hydrous Rw at the P-T conditions of the mantle transition zone, as required by the formation of the hydrogen defects [VMg(OH)2MgSiSiMg]. The Mg-Si disorder may have very large effects on the physical and chemical properties of Rw, as exampled by its disproportional effects on the unit-cell volume and thermal expansivity.

Investigation of the crystal structure of a low water content hydrous olivine to 29.9 GPa: A high-pressure single-crystal X-ray diffraction study

Article

Dec 2020

Olivine is the most abundant mineral in the Earth’s upper mantle and subducting slabs. Studying the structural evolution and equation of state of olivine at high-pressure is of fundamental importance in constraining the composition and structure of these regions. Hydrogen can be incorporated into olivine and significantly influence its physical and chemical properties. Previous infrared and Raman spectroscopic studies indicated that local structural changes occur in Mg-rich hydrous olivine (Fo  95; 4883-9000 ppmw water) at high-pressure. Water contents of natural olivine are commonly less than 1000 ppmw, it is thus inevitable to investigate the effects of such water contents on the equation of state (EoS) and structure of olivine at high-pressure. Here we synthesized low water content hydrous olivine (Fo95; 1538 ppmw water) at low SiO2 activity and identified that the incorporated hydrogens are predominantly associated with the Si sites. We performed high-pressure single-crystal X-ray diffraction experiments on this olivine to 29.9 GPa. A third-order Birch-Murnaghan equation of state (BM3 EoS) was fit to the pressure-volume data, yielding the following EoS parameters: VT0 = 290.182(1) Å3, KT0 = 130.8(9) GPa, and K′T0 = 4.16(8). The KT0 is consistent with those of anhydrous Mg-rich olivine, which indicates that such low water content has negligible effects on the bulk modulus of olivine. Furthermore, we carried out the structural refinement of this hydrous olivine as a function of pressure to 29.9 GPa. The results indicate that, similar to the anhydrous olivine, the compression of the M1-O and M2-O bonds are comparable, which are larger than that of the Si-O bonds. The compression of M1-O and M2-O bonds of this hydrous olivine are comparable with those of anhydrous olivine, while the Si-O1 and Si-O2 bonds in the hydrous olivine are more compressible than those in the anhydrous olivine. Therefore, this study suggests that low water content has negligible effects on the EoS of olivine, though the incorporation of water softens the Si-O1 and Si-O2 bond.

Jianite: Massive Dunite Solely Made of Virtually Pure Forsterite from Ji'an County, Jilin Province, Northeast China

Article

Full-text available

Feb 2020

A rare massive yellowish-green serpentinized dunite, covering a minimum area up to ~50 m2, has been found in Ji'an County, Jilin Province, Northeast China. It contains primary olivine and secondary serpentine (antigorite) and brucite. Other primary minerals like orthopyroxene, clinopyroxene, and aluminum-rich phase (such as garnet, spinel, and plagioclase), frequently appearing in ultramafic rocks, have not been identified. The olivine is essentially pure forsterite, with an Mg# (100 × Mg/(Mg + Fe)) of ~99.6–99.7. Due to these distinct features, we especially name the protolith of this dunite as jianite (集安岩). The forsterite grains range up to ~2 mm, show clear equilibrium textures such as nearly straight grain boundaries and ~120° dihedral angles at their triple junctions, and display no intragranular or intergranular composition variations. They are extensively ruptured and hydrated (i.e., serpentinized), with the fractures (and the grain boundaries as well) filled by fine-grained antigorite (ideally Mg6(Si4O10)(OH)8) and brucite (ideally Mg(OH)2). These secondary phases are also extremely poor in Fe, indicating a good chemical equilibrium with the forsterite. The serpentinization reaction may have proceeded as forsterite + fluid = antigorite + brucite at temperatures of ~425(25) °C and at relatively low but undetermined pressures. The fluid was likely a B-rich, but Si-poor dilute aqueous fluid, as implied by the trace element characteristics and water-related infrared features of the forsterites in equilibrium. The petrogenesis of the jianite is presently unclear.

CO2 Induced a Small Water Solubility in Orthopyroxene and Its Implications for Water Storage in the Upper Mantle

Article

Full-text available

Feb 2020

To investigate the effect of CO2 on water solubility in orthopyroxene coexisting with H2O‐CO2 as a buffering fluid, high‐pressure experiments were conducted at pressures of 1.5 and 3 GPa and a temperature of 1100 °C. The experiments were performed in a Walker‐type multianvil assembly using natural orthopyroxene with various CO2 to CO2‐H2O molar ratios as a starting material. The water contents were measured by polarized Fourier transform infrared spectrometry. At 1.5 GPa and 1100 °C, the H2O solubility decreased with increasing CO2 content in the fluid. The water solubility c(H2O) could be quantitatively determined based on water fugacity f(H2O) as c(H2O) = 25.21 × f(H2O)1.24. The addition of 57% CO2 dramatically reduced the water solubility in orthopyroxene from 184 to 90 ppm. In contrast, at 3 GPa and 1100 °C, the water solubility did not change with the CO2 content in the starting material because CO2 is unstable in bulk peridotite due to the reaction between CO2 and olivine at pressures exceeding 2.2 GPa. This study confirms that the additional component in the aqueous fluid can change the water activity and fugacity, thereby directly lowering the water storage capacity in mantle minerals. As a result, previous estimates of the maximum water storage capacity in the shallow mantle may be overestimated by a factor of 3.

Bridging the water solubility and ion diffusivity in the mantle silicates by a thermodynamic model

Article

Full-text available

Feb 2020
MINER PETROL

Water solubility in Mg-endmember silicates is suggested to correlate with cation diffusivity. This study proposes a thermodynamic model by introducing the concept of transient Frenkel defect for Mg ions to quantify the relationship between water solubility and Mg diffusivity in mantle silicates. The formation of the Mg transient Frenkel defect is regarded as the escape of Mg from Mg-O polyhedron, and the concentration of Mg transient vacancy is considered as its statistical probability in silicate minerals. The proposed thermodynamic model reveals that the formation energy for Mg transient vacancy (ΔGtr) is approximately ΔG/6 (ΔG is activation energy for Mg diffusion in silicates) for silicates with Mg-O6 octahedra, such as enstatite, forsterite, wadsleyite, and ringwoodite, and 5ΔG/12 for bridgmanite and post-perovskite. This observation implies a possible diffusion mode of Mg through transient vacancies. Our model connects water solubility and ion diffusivity in silicates.

Point defect populations of forsterite revealed by two-stage metastable hydroxylation experiments

Article

Full-text available

May 2019
CONTRIB MINERAL PETR

Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. We tested this method on synthetic forsterite (Fo:${\text{Mg}}_{2} {\text{SiO}}_{4}$) crystals. To control starting point defect structures, Fo crystals were pre-annealed at different temperatures ($1100{-}1500\,{^\circ }{\text{C}}$), silica activity conditions (forsterite–enstatite Fo–En and forsterite–periclase Fo–Per) and oxygen fugacity (0.21 and $10^{-6}$ bars). Then low-temperature hydroxylation (900 °C, 1.5 GPa) of the crystals successfully allowed the decoration with protons of pre-existing point defect structures, as subsequently revealed by infrared spectroscopy. Protons are arranged in three different point defect stoichiometries in Fo, related to Mg and Si vacancies ([Mg] and [Si], respectively) as well as to a trivalent cation-associated substitution mechanism ([triv]). Over the timescale and equilibrium conditions studied, hydroxylation does not reset the point defect structure inherited from pre-anneal. The data further show that the concentrations of [Mg]-, [Si]- and [triv]-hydrated defects are function of pre-anneal silica activity and temperature. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of the crystals revealed diffusion of Al and Fe into the crystals during the pre-annealing, a phenomenon clearly promoted at high ${\text{a}}_{{{\text{SiO}}_{2} }}$. The data confirm a very fast mechanism of Al diffusion in Fo during pre-annealing, and suggest a strong coupling between ${\text{H}}^{+}$ and ${\text{Al}}^{3+}$ during hydroxylation. Overall, they show the strong importance of ${\text{a}}_{{{\text{SiO}}_{2} }}$ and temperature in the incorporation of trace cations in forsterite, and the subsequent effects of incorporation of trace cations on Mg- and Si-related point defects in Fo. The dry point defect population of Fo is determined by interactions between the trace trivalent cations and dry Si and Mg vacancies. Without trace elements, T only has a limited effect on Mg- and Si-related point defect populations. Finally, approaching or potentially slightly exceeding the Fo–En solidus leads to strong changes in the trace element concentration and point defect population in Fo, which may be related to either partial melting or pre-melting effects.

Activation of [100](001) slip system by water incorporation in olivine and the cause of seismic anisotropy decrease with depth in the asthenosphere

Article

Full-text available

Jan 2019

A transition from A-type to E-type fabrics in olivine may be the cause of a decrease in seismic anisotropy with depth in the upper mantle. To better understand upper mantle seismic signals, we investigate the origin of E-type fabrics using a natural olivine by deformation experiments. An olivine crystal was first hydrated at 5 GPa and 1473 K (with 4-60 ppm H2O), or dehydrated at room pressure at 1473 K at an oxygen fugacity near the enstatite-magnesite-olivine-graphite (EMOG) buffer. This hydrated/dehydrated olivine was then sheared in the [100] direction on the (001) plane at pressures of 2 to 5 GPa and temperatures of 1473 or 1573 K. The deformed samples were observed by transmission electron microscopy (TEM) on the (001) plane to determine whether the [100](001) slip system was activated or not. Only c-elongated [100] dislocations were observed for the anhydrous samples, while [100](001) dislocations dominated in the hydrous samples. The dislocation structure of the [100](001) slip system developed under hydrous and relatively low-temperature conditions indicates different slip mechanism which is detected under anhydrous and high-temperature conditions in previous studies. We conclude that the incorporation of water into olivine helps to activate the [100](001) slip system by reducing its Peierls stress. This supports the idea that E-type fabrics can exist under hydrous conditions and that a transition to this fabric may be the cause of seismic anisotropy decrease with depth in the asthenosphere.

Diffusion and solubilities of selected trace elements in olivine

Thesis

Jan 2015

Irina Zhukova

Olivine with major element composition approximately (Mg0.9Fe0.1)2SiO4 is the principal mineral in the Earth's upper mantle and, as a consequence, its chemical and physical properties are of great importance for understanding processes in the deep Earth. Among the most important properties is diffusion, which controls the rates of many mantle processes. The effect of temperature, pressure and oxygen fugacity on olivine diffusion rates has been well studied. But the effects of varying the activities of its major components (i.e., MgO and SiO2) has escaped experimental attention. The aim of this thesis was to provide new information regarding the effect of major cations activities on the diffusion of selected trace elements in olivine. We performed series of experiments on diffusion of +2 and +3 cations in synthetic Mg2SiO4 (forsterite) and natural (Mg0.9Fe0.1)2SiO4 olivine from San Carlos, Arizona, at different temperatures and oxygen fugacities. Activities of SiO2 and MgO or (Mg,Fe)O were controlled in each experiment using mineral buffer assemblages with either excess pyroxene or excess periclase (magnesiowustite). Diffusion rates are significantly higher in experiments carried out at with excess pyroxene (5 times for Rh, 15 times for Ni and Co and 2 orders of magnitude for Al) than for experiments with excess periclase at the same P-T-fO2 conditions. These differences in diffusion rates are due to the control of the activities of SiO2 and MgO on concentrations of octahedral and tetrahedral vacancies in olivine that in turn, strongly affect the diffusion mechanisms of impurities. The results of this study suggest that the rates of diffusion in other minerals may in general be expected to depend on their paragenesis.

Water and Oxygen Fugacity in the Lithospheric Mantle Wedge beneath the Northern Canadian Cordillera (Alligator Lake)

Article

Sep 2018
GEOCHEM GEOPHY GEOSY

Assessing water contents of subduction zone mantle peridotites can gain insight into the compositions of slab-derived fluids/melts and the active margin water cycle. Here eight mantle xenoliths from Alligator Lake (northern Canadian Cordillera) are examined to address these issues. The harzburgites have less water, on average, but are more oxidized (ΔFMQ ~ 0.1) than the lherzolites (ΔFMQ ~ −1.0). The lherzolites have major and trace element compositions close to primitive mantle, while the harzburgite major element and heavy rare earth element compositions are indicative of higher degrees of melt depletion but with light rare earth element-enriched profiles. Correlations between lherzolite pyroxene water contents and bulk rock Ba/Nb and Ba/Yb ratios likely result from interaction with subduction related fluids. The trace element compositions of the harzburgite clinopyroxenes are successfully modeled by melting of a fertile mantle lithosphere and interaction with a carbonatite melt. Correlations between the harzburgite water contents and clinopyroxene Ca/Al ratios and Mg# are also consistent with the influence of carbonatite metasomatism. Metasomatism likely resulted from opening of a slab window beneath the region, detected as a low-velocity seismic anomaly, which heated and mobilized a heterogeneous mantle lithosphere veined with carbonatite. This study confirms that subduction zone mantle lithosphere is not necessarily more water-rich or more oxidized than oceanic lithosphere or other off-cratonic settings. Moreover, local oxidation is not necessarily related to the ingress of subduction zone fluids but can also be related to melting of a heterogeneous lithosphere following heating above a slab window.

Temperature, pressure and water content dependence of dislocation mobility in olivine

Thesis

Jul 2018

Lin Wang

This thesis is a comprehensive study of dislocation in natural olivine. Since olivine is the most abundant and weakest mineral in the Earth’s upper mantle, it is believed that the upper mantle dynamics are controlled by the rheological properties of olivine. Dislocation creep is considered as the most important mechanism for olivine creep at the upper mantle for the following reasons. First, a large amount of strain is accommodated by dislocation creep, and therefore the viscosity of the upper mantle is determined by dislocation creep to a large extent. Second, olivine fabrics, which can reflect mantle flow geometry, are due to dislocation creep. Previous deformation and diffusion experiments gave some information on olivine dislocation creep. However, both of them have limitations. On one hand, the high strain rates in deformation experiments make the application of results from these experiments to natural conditions doubtable. On the other hand, the link between diffusion and dislocation creep is rather indirect. The application of results from diffusion experiments to natural conditions has to be based on certain creep models. In this project, dislocation recovery method was used to study the dislocation motions in natural olivine and investigate upper mantle rheology. In addition, transmission electron microscopy (TEM) was used to make observations to check whether [100](001) slip system exists at hydrous conditions or not.

Water-content dependence of dislocation mobility of olivine in the [100](010) slip system

Thesis

Jan 2018

Stephan Blaha

The aim of this project is to study the water-content dependence of dislocation mobility in the [100](010) slip-system of olivine under upper-mantle conditions, namely at a pressure of 2.75 GPa and a temperature of 1500, using a multi-anvil apparatus. Since olivine is the most abundant mineral of the Earth’s upper mantle, it is considered to control upper mantle rheology. Olivine is therefore targeted in this study. Plastic deformation of olivine is controlled by the motion of defects such as point defects, dislocations, and grain boundaries. In the upper mantle, dislocation creep is considered to be the dominant mechanism. Therefore, understanding of dislocation mobility in olivine is essential to investigate mantle dynamics. Many deformation experiments demonstrated that creep rates dramatically increase by incorporation of water. However, deviatoric stresses and strain rates in deformation experiments are by far higher than those in the upper mantle, and therefore those results should be examined by an independent technique from deformation experiments. Since the dislocation creep rate should be proportional to the dislcation mobility, water-content dependence of dislocation mobility of olivine is estimated by means of the dislocation-recovery technique in this study. In this technique, water-doped olivine crystals are deformed to form dislocations and then annealed under quasi-hydrostatic conditions. The dislocation annihilation rate is obtained by the following equation: k = (1/ρf -1/ρi)/t, where ρi is the dislocation density before the annealing, ρf is the dislocation density after the annealing, k is the dislocation annihilation rate, and t is the duration of the annealing. The water-content dependence of the dislocation mobility is assumed identical to that of the dislocation annihilation rate. For these purposes the experimental setups were developed, which make it possible to dope water in an olivine single crystal, increase the dislocation density of a certain slip-system without the formation of sub-grains, and anneal the samples at quasi-hydrostatic conditions. By hydration experiment, up to 380 wt. ppm of water was successfully doped into olivine single crystals. The water doped olivine crystals were deformed in the [100] direction on the (010) slip plane to produce dislocations with density from 4×1011 to 14×1011 m−2. The deformed olivine crystals were annealed for 1 to 5 hours to decrease the dislocation density by 8 ∼ 50 %. These data gave the dislocation annihilation rates of 10−16.7 to 10−15.6 m2/s. The water-content exponent is found to be 0.17±0.43. Thus, the enhancement of the dislocation mobility by water incorporation is very small, or actually zero within the uncertainty. By examining the FT-IR spectra, it is found that the dislocation mobility is correlated with the intensity ratios of the peaks at 3612 and 3565 cm−1 to those at 3170, 3320 and 3598 cm−1. rather than the total water content. Since the former and latter peaks are considered to be related to the hydration of Si and Mg sites, respectively, the dislocation mobility seems to be enhanced by the hydration of the Si sites, but suppressed by that of the Mg sites. The present results are in striking contrast with the results of deformation experiment, which gave the water-content exponent of 1.2. Although it has been believed that presence of water should strongly enhance dynamic motion in the upper mantle, this paradigm is questioned by the present study together with the water-content dependence of the Si self-diffusion coefficients.

A Subsolidus Olivine Water Solubility Equation for the Earth's Upper Mantle

Article

Full-text available

Nov 2017

The pressure and temperature sensitivity of the two most important hydrous defects in mantle olivine involving Si-vacancies (associated to trace amounts of titanium [TiChu-PD] or exclusively to Si-vacancies [Si]) was investigated at subsolidus conditions in a fluid-saturated natural peridotite from 0.5 to 6 GPa (ca. 20-200 km depth) at 750 to 1050 °C. Water contents in olivine were monitored in sandwich experiments with a fertile serpentine layer in the middle and olivine and pyroxene sensor layers at the border. Textures and mineral compositions provide evidence that olivine completely recrystallized during the weeklong experiments, whereas pyroxenes displayed only partial equilibration. A site-specific water solubility law for olivine has been formulated based on the experiments reconciling previous contradictory results from low and high-pressure experiments. The site-specific solubility laws permit to constrain water incorporation into olivine in the subducting slab and the mantle wedge, as these are rare locations on Earth where fluid-present conditions exist. Chlorite dehydration in the hydrated slab is roughly parallel to the isopleth of 50 ± 20 ppm wt. H2O in olivine, a value which is independent of the pressure and temperature trajectory followed by the slab. Hydrous defects are dominated by [Si] under the relevant conditions for the mantle wedge affected by fluids coming from the slab dehydration (slab-adjacent low viscosity/seismic low-velocity channel, P > 3 GPa). In cold subduction zones at 5.5 km from the slab surface the storage capacity of the mantle wedge at depths of 100-250 km ranges from 400 to 2,000 ppm wt. H2O.

Theoretical Raman spectrum and anharmonicity of tetrahedral OH defects in hydrous forsterite

Article

Apr 2017
EUR J MINERAL

Substitutional mechanisms involving hydrogen incorporation at vacant tetrahedral sites play a major role in water incorporation in olivine. Infrared (IR) absorption spectra of hydrous forsterite samples usually display a cluster of narrow and weakly anharmonic OH stretching bands at wavenumbers above 3500 cm⁻¹. A broader absorption band displaying pronounced temperaturedependent shift and broadening is commonly superimposed to this diagnostic spectrum and was tentatively assigned to interstitial OH groups. A less frequently observed band with similar temperature-dependent characteristics is related to a coupled incorporation of hydrogen and boron at the tetrahedral site. Here, we re-examine these interpretations by computing the theoretical Raman spectrum and investigating the local vibrational properties of OH groups at the tetrahedral site of forsterite. The present results show that the two anharmonic bands are both to be ascribed to the protonated O2site in the clumped (B;H)x Si and the (4H)x Si defects. The peculiar orientation of the corresponding OH groups does not allow H-bond sharing and leads to efficient vibrational phase relaxation of the stretching mode through a hindered rotational mode coupled to the vibrational density of states of the host. The occurrence of interstitial OH groups previously proposed to interpret specific anharmonic bands of the forsterite IR spectrum is highly challenged by this new explanation. These results confirm that, at high pressure and high temperature, hydrogen incorporation in forsterite is essentially dominated by (4H)xSi defects. © 2016 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.

Heterogeneous distribution of water in the mantle beneath the central Siberian Craton: Implications from the Udachnaya Kimberlite Pipe

Article

Full-text available

Oct 2016
GONDWANA RES

The paper presents new petrographic, major element and Fourier transform infrared (FTIR) spectroscopy data and PT-estimates of whole-rock samples and minerals of a collection of 19 relatively fresh peridotite xenoliths from the Udachnaya kimberlite pipe, which were recovered from its deeper levels. The xenoliths are non-deformed (granular), medium-deformed and highly deformed (porphyroclastic, mosaic-porphyroclastic, mylonitic) lherzolites, harzburgite and dunite. The lherzolites yielded equilibration temperatures (T) and pressures (P) ranging from 913 to 1324 °C and from 4.6 to 6.3 GPa, respectively. The non-deformed and medium-deformed peridotites match the 35 mW/m² conductive continental geotherm, whereas the highly deformed varieties match the 45 mW/m² geotherm. The content of water spans 2 ± 1-95 ± 52 ppm in olivine, 1 ± 0.5-61 ± 9 ppm in orthopyroxene, and 7 ± 2-71 ± 30 ppm in clinopyroxene. The amount of water in garnets is negligible. Based on the modal proportions of mineral phases in the xenoliths, the water contents in peridotites were estimated to vary over a wide range from < 1 to 64 ppm. The amount of water in the mantle xenoliths is well correlated with the deformation degree: highly deformed peridotites show highest water contents (64 ppm) and those medium-deformed and non-deformed contain ca. 1 ppm of H2O. The high water contents in the deformed peridotites could be linked to metasomatism of relatively dry diamondiferous cratonic roots by hydrous and carbonatitic agents (fluids/melts), which may cause hydration and carbonation of peridotite and oxidation and dissolution of diamonds. The heterogeneous distribution of water in the cratonic mantle beneath the Udachnaya pipe is consistent with the models of mantle plume or veined mantle structures proposed based on a trace element study of similar xenolithic suits. Mantle metasomatism beneath the Siberian Craton and its triggered kimberlite magmatism could be induced by mantle enrichment in volatiles (H2O, CO2) supplied by numerous subduction zones which surrounded the Siberian continent in Neoproterozoic-Cambrian time.

Hydrogen partitioning between synthetic olivine, orthopyroxene and melt

Conference Paper

Jun 2004
GEOCHIM COSMOCHIM AC

Hydration effects on crystal structures and equations of state for silicate minerals in the subducting slabs and mantle transition zone

Article

Jan 2016

Yu Ye

There are potentially huge amounts of water stored in Earth’s mantle, and the water solubilities in the silicate minerals range from tens to thousands of part per minion (ppm, part per million). Exploring water in the mantle has attracted much attention from the societies of mineralogy and geophysics in recent years. In the subducting slab, serpentine breaks down at high temperature, generating a series of dense hydrous magnesium silicate (DHMS) phases, such as phase A, chondrodite, clinohumite, etc. These phases may serve as carriers of water as hydroxyl into the upper mantle and the mantle transition zone (MTZ). On the other hand, wadsleyite and ringwoodite, polymorphs of olivine, are most the abundant minerals in the MTZ, and able to absorb significant amount of water (up to about 3 wt.% H2O). Hence, the MTZ becomes a very important layer for water storage in the mantle, and hydration plays important roles in physics and chemistry of the MTZ. In this paper, we will discuss two aspects of hydrous silicate minerals: (1) crystal structures and (2) equations of state (EoSs).

Petrology of avachites, high-magnesian basalts of avachinsky volcano, Kamchatka: II. Melt inclusions in olivine

Article

Full-text available

Jul 2005

The composition and crystallization conditions of the parental melts of avachites were elucidated by studying melt inclusions in olivine (Fo 85.8-90.7) phenocrysts. The melt inclusions captured during the crystallization of primitive magmas subsequently reequilibrated with their host minerals and became partly recrystallized and decrepitated. The diffusion-controlled reequilibration of the melt inclusions with the olivine occurred at temperatures close to ∼1100°C and was associated with the crystallization of daughter phases: olivine, high-Ca pyroxene, and spinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high Al contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase absence from the daughter phase assemblage of the inclusions. Magma decompression induced the partial decrepitation of the melt inclusions, a process that was associated with the escape of fluid components (CO2 and H2O) and variable amounts of the residual silicate material from the inclusions. The initial compositions of the melt inclusions, which were reconstructed using techniques of experimental homogenization and modeling, show broad ranges in the contents of major and trace elements. Compared with the composition of the rocks, the compositions of inclusions in the olivine Fo > 90% are higher in CaO, A12O 3, and Na2O at lower concentrations of SiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring in many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline-normative ankaramite magmas, and the significant crustal contamination of these magmas during their fractionation can be spread more widely than is currently assumed in models for island-arc petrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization of early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the magmatic chambers at different depths (from 5 to 30 km). During two (or more) crystallization stages, olivine-pyroxene cumulates were produced, remobilized, and transported to the surface by the differentiated hypersthene-normative magmas. Avachites are hybrid cumulative rocks, which were produced in a long-lived open magmatic system.

The origins of olivine fabric transitions and their effects on seismic anisotropy in the upper mantle

Thesis

Jan 2012

Sushant Shekhar

Convecting mantle plays a central role in the thermal and geochemical evolution of the Earth. It provides the principal force responsible for major geological features such as mountains and ocean basins. Plate tectonics and its violent consequences such as earthquakes and volcanoes are all manifestations of the dynamics of the convective mantle. Shearing forces generated by mantle convection leads to lattice preferred orientation (LPO) of the major upper mantle mineral phases. LPO that develops in this way is thought to be the principal cause behind seismic anisotropy in the upper mantle, which can consequently be used to chart convective flow of the mantle. Strong changes in seismic anisotropy occur in the top 300 km of the upper mantle where olivine is the principal mineral. In this study a solid media high pressure deformation apparatus, called the deformation-DIA or D-DIA, has been used to deform aggregates of San Carlos olivine in simple shear geometry at pressures between 3 and 8.5 GPa and temperatures from 1300-1500°C. As part of this project a high pressure and temperature solid-media cubic assembly was developed to facilitate these experiment that employed alumina pistons cut at 45° to shear the sample but minimized cold deformation of the sample by employing initially porous alumina in the sample column. Once stable high pressures and temperature were reached the cubic assembly was deformed by compressing two vertically oriented anvils of the D-DIA, while the four horizontally oriented anvils were maintained at a constant loading force. This assembly shortening led to shearing of the olivine sample. Recovered samples were analyzed for fabric development employing electron backscattered diffraction (EBSD) and microstructure was observed using transmission electron microscopy (TEM). Experiments were performed at each pressure and temperature as a function of strain rate and H2O content. In dry olivine deformation experiments performed at slower strain rates an A-type fabric dominated at all pressures and temperatures, implying deformation by dislocation glide through the (010)[100] slip system. At higher strain rates evidence for the B-type fabric was observed, suggesting increased activity of the (010)[001] slip system at higher stresses. Recrystallization grains size and dislocation densities were used to estimate stresses in the samples and a good correlation was observed between strain rate and estimated flow stresses. Dry experiments from 8.5 GPa and 1500°C exhibited no LPO, which may be an indication for deformation through diffusion accommodated grain boundary sliding at these conditions. No indication was found that pressure influences the dominant slip system in olivine, in contrast to previous studies. It is considered that previously reported incidences of pressure effects can in fact be attributed to the development of higher stresses in experiments performed at higher pressures. Fabrics in H2O bearing olivine deformed at similar conditions revealed the overriding dominance of the C-type fabric, developed through action of the (100)[001] slip system. Variations in pressure, temperature and strain rate had little influence on this fabric development. TEM observations confirmed the presence of dislocations with slip systems consistent with the development of the macroscopic fabrics. Viscoplastic self consistent modeling was employed to understand the development of fabric in the samples and to estimate the relative contributions of variations slip systems to the developed fabrics. These results are used to construct an olivine fabric map which is found to be consistent with some previous studies at lower pressures. It is argued that the decrease in seismic anisotropy observed in the top 300 km of the upper mantle cannot originate from a pressure induced change in the dominant olivine deformation fabric. Instead it is argued that changes in the H2O content of olivine with depth cause a shift in the dominant fabric from A-type to C-type, with a possible excursion through the E-type fabric, dominant slip system (001)[100], which was, however, not observed in this study. Modeling is used to show that this variation in fabric with depth can cause the observed weakening the seismic anisotropy in the upper mantle if the olivine H2O content increases from below 100 ppm at 50 km to 250 ppm at 300 km. Rather than implying an increased in the H2O content of the mantle with depth, however, it is argued that this change in olivine H2O content can be caused by changes in the H2O olivine-pyroxene partition coefficients with depth, for a fixed bulk mantle H2O content of 200 ppm. Similar deformation experiments performed on a peridotite assemblage at 8.5 GPa and 1300°C indicate identical olivine fabrics to those observed in monomineralic experiments at the same conditions. Fabrics for diopside and enstatite were found to be similar to those found in previously performed lower pressure experiments. Experiments on a piezoelectric single crystal of GaPO4 were performed in the D-DIA and 6-ram MAVO press at high pressures in order to measure charge on the crystal developed through the application of deviatoric stresses. Electrical charges were measured through the use of an operational amplifier. Experiments performed at room temperature using a developed cubic assembly were successful in measuring quantifiable electrical charges resulting from the advancement of the deformation anvils by as little as 0.5 µm. Although the piezoelectric constant for this material is not yet calibrated at high pressures, stresses were estimated from the measured charges and measureable values were in the range 4-350 MPa.

Pinter et al 2015

Data

Full-text available

Aug 2015

Experimental study of water solubility in iron rich olivine

Article

Jan 2022

In situ reinvestigation of the reaction phase A plus high-pressure clinoenstatite to forsterite plus water in the system MgO-SiO2-H2O (MSH)

Article

Full-text available

Apr 2022
EUR J MINERAL

The dehydration reaction of phase A + high P clinoenstatite to forsterite + water was experimentally investigated at water-saturated conditions in the pressure range between 7.0 and 10.0 GPa by in situ reversal runs in a multi-anvil press at the synchrotron source of PETRA III in Hamburg. By using closed watertight X-ray transparent Ti capsules, its position is determined by reversal brackets at 8.3 GPa (700–760 °C), 8.6 GPa (700–740 °C), and 9.8 GPa (750–800 °C); thus, the equilibrium of the reaction corresponds ideally to the data reported by Wunder (1998). Optical investigations of the quenched product phases show strong grain coarsening of phase A and clinoenstatite, whereas nucleated forsterite from the breakdown of the aforementioned phases is very fine grained. This corresponds to recent experimental observations that the grain size of phases formed in hydration reactions are significantly larger than those from dehydration reactions. In addition, we performed three time-dependent in situ experiments at 9–10 GPa and 800–870 °C and monitored the reaction progress every 10 min to determine the kinetics of the forsterite formation from phase A + high P clinoenstatite. The growth of forsterite at these P –T conditions, already visible after 10 min, confirms the results of the bracketing experiments. However, the reaction is extremely slow, and even after more than 3 h, significant amounts of phase A and high P clinoenstatite are still present. This is in contradiction to other dehydration reactions of former experimental studies, e.g. the fast dehydration of serpentine, which completely dehydrates within 3 h, even at much lower temperatures, closely overstepping serpentine stability. Despite its reaction sluggishness, which would contradict the concept of earthquake initiation, the observed formation of nano-sized forsterite as a dehydration product may still indicate the potential of this reaction to cause mechanical instabilities and, thus, seismicity within cold subduction zones at depths of the Earth’s mantle. Additionally, at depths exceeding serpentine dehydration, the phase A + high P /low P clinoenstatite breakdown to forsterite + water might induce geochemical and geophysical processes, including the formation of low-velocity zones within the overlying mantle wedge from the large amounts of fluid liberated by this water line reaction. After the breakdown of antigorite, the assemblage phase A + clinoenstatite might act as a bridge to transport water to larger depths during cold subduction, followed by the formation of other hydrous high P phases.

Diffusion rates of hydrogen defect species associated with site-specific infrared spectral bands in natural olivine

Article

Mar 2022
EARTH PLANET SC LETT

We investigated hydrogen transport in naturally occurring, iron-bearing samples of San Carlos olivine that were hydrogenated at confining pressures of 200 or 300 MPa and 1173 to 1303 K or dehydrogenated at room pressure and 1191 to 1358 K. Chemical diffusion coefficients were determined from diffusion profiles for individual O-H-stretching bands from series of infrared spectra in orthogonal directions across each sample. Within experimental uncertainty, the diffusivities associated with all the individual bands are in good agreement with one another in both the hydrogenation and the dehydrogenation experiments. Hydrogenation proceeds by two diffusion mechanisms, as reported previously. The faster process involves interstitial diffusion of protons coupled with a counter-flux of polarons, with proton diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along the olivine [100] direction than along [010] and [001], consistent with the anisotropy reported for proton diffusion and conductivity in olivine. The slower process involves interstitial proton diffusion coupled with a parallel flux of metal vacancies, with vacancy diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along [001] than along [100] and [010], consistent with the anisotropy previously reported for the diffusion of metal cations in olivine. Diffusivities from our new dehydrogenation experiments are identical in both magnitude and anisotropy to those determined in our earlier hydrogenation experiments. This agreement demonstrates the validity of studies that used the results of our hydrogenation experiments to analyze dehydrogenation profiles in olivine xenocrysts and olivine in mantle xenoliths to determine rates of magma ascent from the source regions in Earth's interior.

The distribution of hydrogen/water in Al and Ti-containing forsterite: A thermodynamic model

Preprint

Apr 2021

Low hydrogen concentrations in Dharwar cratonic lithosphere inferred from peridotites, Wajrakarur kimberlites field: Implications for mantle viscosity and carbonated silicate melt metasomatism

Article

Full-text available

Oct 2020
PRECAMBRIAN RES

Hydrogen as an atomic impurity in mantle minerals is recurrently proposed as a key element impacting significantly on many mantle properties and processes such as melting temperature and mechanical strength. Nevertheless, interpretation based on the natural samples remains weak as we do not have yet a robust world-wild database for hydrogen concentrations in mantle minerals and rocks. Here, we report the first hydrogen concentrations in nominally anhydrous minerals from a rare selection of ultramafic rocks and minerals embedded in Mesoproterozoic Wajrakarur kimberlites (Eastern Dharwar craton, India). Based on key chemical elements, we demonstrate that olivine, pyroxenes and garnet from the Dharwar craton are of mantle origin. We quantify the hydrogen concentrations using Fourier transform infrared spectroscopy (FTIR) and mineral-specific FTIR calibrations. Calculated hydrogen concentrations are, in average, 18 ppm wt H2O in olivine, 70 ppm wt H2O in orthopyroxene and 207 ppm wt H2O in clinopyroxene. Garnet has highly variable hydrogen concentration ranging from 0 to 258 ppm wt H2O, probably influenced by nano-scale inclusions. The average of clean garnet spectra yields 14.5 ppm wt H2O. The reconstructed hydrogen bulk concentrations of Dharwar peridotites yields 40-8+10 ppm wt H2O. This value is two to five times lower than the estimated hydrogen concentration in the lithospheric mantle, and agree well with the lower range of hydrogen bulk concentration from the current data base for the upper mantle minerals transported by kimberlites from other cratons (e.g., South Africa, Siberia). The low hydrogen concentration in mantle minerals, together with petrological and geochemical evidence of carbonated silicate melt metasomatism in Dharwar cratonic lithospheric mantle, suggest that these xenoliths are possibly related to proto-kimberlite melts with low water activity prior to being transported to the surface by the Mesoproterozoic Wajrakarur kimberlites. These observations, valid to a depth of ~165-km, suggest that cratonic lithosphere beneath the Dharwar craton may not be particularly indicative of an abnormal hydrogen-rich southern Indian lithosphere in the late Archean and that hydroxylic weakening in olivine would induced a negligible effect on the mantle viscosity of Indian subcontinent.

俯冲带及转换带中含水硅酸盐矿物的晶体结构和状态方程

Article

Mar 2016

宇叶

Dehydration of melt inclusions in olivine and implications for the origin of silica-undersaturated island-arc melts

Article

Apr 2019
EARTH PLANET SC LETT

Primary subduction-related magmas build up modern continental crust and counterbalance massive recycling of crustal material into the deep mantle occurring at this tectonic setting. Melt inclusions in Mg-rich olivine are believed to be the best probes of primary subduction-related melts. However, unexpectedly, most of such inclusions are SiO2-undersaturated, in contrast to predominantly SiO2-saturated island-arc rocks. The origin of these melts has been explained by melting of amphibole-bearing pyroxenites in the lower crust or upper mantle. The current models fail, however, to explain the high abundance of SiO2-undersaturated melts as well as their compositional difference with host rocks for the major elements but not for incompatible trace elements. Here we report results from the investigation of rocks and melt inclusions in olivine from Klyuchevskoy volcano in Kamchatka. We show that experimental re-hydration of SiO2-undersaturated melt inclusions in olivine Fo85−90 at 300 MPa pressure and 1200°C causes a concomitant enrichment of melt in H2O and SiO2 so that re-hydrated inclusions (4–5 wt% H2O) become as silica-saturated as primitive Klyuchevskoy rocks. An experimental dehydration of previously re-hydrated inclusions also resulted in coupled depletion of melt in H2O and SiO2. The estimated stoichiometry of SiO2 and H2O gain/loss is consistent with dissolution/crystallization of metal-defect olivine on inclusion walls. The migration mechanism of water is controlled by hydrogen diffusion in the octahedral metal (Mg, Fe) vacancies through olivine structure as confirmed by FTIR spectroscopy. We conclude that the previously reported SiO2-undersaturated composition of many melt inclusions from hypersthene-normative island-arc rocks can be explained by the coupled loss of up to several weight percent of H2O and SiO2 from the initially trapped primitive SiO2-saturated hydrous melts. Thus, SiO2-undersaturated melt inclusions may not be representative of primitive island-arc magmas. The discovery of the coupled SiO2 and H2O loss from inclusions allowed us to propose a method for reconstruction of the initial water content even for completely dehydrated inclusions. The results of this study may indicate that the majority of primitive island-arc inclusions have not preserved their initial H2O content, and that primary arc melts contain on average ≥4 wt% H2O. The higher H2O concentration in primary arc melts implies the existence of a ‘crustal filter’ controlling the water content, which can be preserved in melt inclusions, and also the lower mantle melting temperatures and higher output H2O fluxes in subduction zones than previously estimated based on direct determination of H2O in potentially dehydrated melt inclusions.

Sorptive equilibrium profile of fluoride onto aluminum olivine [(Fe x Mg 1− x ) 2 SiO 4 ] composite (AOC): Physicochemical insights and isotherm modeling by non-linear least squares regression and a novel neural-network-based method

Article

Jun 2018

A novel aluminum/olivine composite (AOC) was prepared by wet impregnation followed by calcination and was introduced as an efficient adsorbent for defluoridation. The adsorption of fluoride was modeled with one-, two- and three-parameter isotherm equations by non-linear regression to demonstrate the adsorption equilibrium. The FI was the best-fitted model among the two-parameter isotherms with a R² value of 0.995. The three-parameter models were found to have better performance with low values of the error functions and high F values. The neural-network-based model was applied for the first time in the isotherm study. The optimized model was framed with eight neurons in hidden layer with a mean square of error of 0.0481 and correlation coefficient greater than 0.999. The neural-based model has the better predictability with a higher F value of 9484 and R² value of 0.998 compared to regression models, exhibiting the F value and the R² in the range of 86–3572 and 0.835–0.995, respectively. The material characterization established the formation of the aluminum oxide, silicate, etc. onto the olivine which is conducive of the removal of fluoride by the formation of aluminum fluoride compounds, such as AlF3 in the spent material after defluoridation.

Hydrogen incorporation mechanisms in forsterite: New insights from 1 H and 29 Si NMR spectroscopy and first-principles calculation

Article

Mar 2017

The presence of water (hydrogen) in nominally anhydrous mantle minerals may have profound effects on their physical properties (e.g., electrical conductivity, diffusivity, rheology), and these effects are expected to depend on how water is incorporated in the crystal structure. For olivine, the most abundant upper mantle mineral, despite extensive studies, mostly using vibrational spectroscopy, the interpretations are still not well constrained. To provide better understanding on this issue, we carried out a comprehensive H-1 and Si-29 NMR study on an Mg2SiO4 forsterite sample containing about 0.5 wt% H2O synthesized at 12 GPa and 1200 degrees C, complemented by Raman measurement and first-principles calculation of the geometry, stability, and NMR parameters of model structures. The Raman spectra contain relatively sharp O-H stretching bands near 3612, 3579, and 3567 cm(-1) and a broader band near 3547 cm(-1), similar to previous reports. The H-1 static and MAS NMR data revealed that there are two main populations of protons in the hydrous forsterite structure, one experiencing strong (H-1H)-H-1 homonuclear dipolar couplings and contributing to a broad peak near 2.4 ppm, and another with weaker dipolar couplings and contributing to a narrower peak near 1.2 ppm in the MAS NMR spectrum at 30 kHz. Two-dimensional H-1 CRAMPS-MAS NMR measurements confirmed that the two proton components belong to the same phase and the contrast in MAS NMR peak width is largely due to difference in the strength of H-1-H-1 homonuclear dipolar couplings. In addition, there is also a very weak, narrow H-1 MAS NMR peak near 7.3 ppm (contributing to < 0.1% of the total intensity) due to protons that are more remote from the two main components. First-principles calculation confirmed that the two main proton components can be attributed to the hydrogarnet-like substitution mechanism of four H ions for one Si [(4H) Si] in a tetrahedral site of olivine, but unlike hydrogarnet with one of the protons pointing away from the tetrahedral center and located in an adjacent interstitial site, thus experiencing weaker dipolar couplings than those in the vicinity of the vacant tetrahedron; the very weak narrow peak near 7.3 ppm can be attributed to the substitution mechanism of two H ions for one Mg in an M1 site [(2H)M-1] of forsterite. The H-1-Si-29 CP-MAS NMR spectra revealed both a broad peak encompassing the position for OH defect-free forsterite (-61.7 ppm) and a narrower peak at higher frequency (-60.9 ppm). First-principles calculation indicates that these peaks are accountable by the same models as for the H-1 NMR data. Thus, this study has provided unambiguous evidence supporting that hydrogen is incorporated in forsterite at relatively high-pressure dominantly as (4H) Si defects, with (2H) M1 defects playing only a very minor role. The much larger H-1 chemical shift for protons associated with the latter (than the former) is correlated with stronger hydrogen bonding for the latter, which in turn reflects difference in bonding environments of the OH groups (with the latter bonded to a Si, and the former only bonded to Mg). Similar correlation applies to the O-H stretching frequency. The (4H) Si defects are responsible for the observed high-frequency O-H stretching bands (> 3450 cm(-1)), and the (2H) M1 defects give lower frequencies (undetected here due to low abundance, but most likely near 3160-3220 cm(-1) as previously reported) in vibrational spectra. These results can serve as a guide for (re-) interpretation of infrared and Raman spectroscopic data on hydrous olivine produced under different pressure and silica activity conditions, and require reconsideration of any models for the effects of water on physical properties of olivine based on different interpretations of such data. This study also demonstrated the usefulness of the combined solid-state NMR and first-principles calculation approach in unraveling the hydrogen incorporation mechanisms in nominally anhydrous minerals.

Titanium-hydroxyl defect-controlled rheology of the Earth's upper mantle

Article

Oct 2016
EARTH PLANET SC LETT

Experiments were conducted with hydrous olivine to investigate the defect responsible for the influence of water (hydrogen structurally incorporated as hydroxyl) on the olivine rheology. Solution–gelation derived Fo90 olivine doped with nominally 0.04–0.1 wt.% TiO2 was first hot-pressed and then deformed in platinum capsules at 300 MPa confining pressure and temperatures from 1200–1350°C. The water content was not buffered so that deformation occurred at water-undersaturated conditions. Due to the enhanced grain growth under hydrous conditions, the samples were at least a factor of three more coarse-grained than their dry counterparts and deformed in powerlaw creep at differential stresses as low as a few tens of MPa. Since all experiments were conducted at the same confining pressure, the essentially linear relationship between strain rate and water content was for the first time determined independently of an activation volume. Infrared spectra are dominated by absorption bands at 3572 and 3525 cm⁻¹. These bands also predominate in infrared spectra of natural olivine, and can only be reproduced experimentally in the presence of titanium. In contrast to the previous interpretation of the hydrous rheology in terms of intrinsic point defects, the experiments show that extrinsic defects (impurities) in natural olivine play the dominant role for water weakening at the water contents expected for most of the upper mantle.

The Role of Water in the Deformation of Olivine Single Crystals

Article

Full-text available

Jan 1985

Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100° to 1300°C and 300- and 1500-MPa confining pressure, with the oxygen fugacity around the specimen constrained neat the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10-1-0 m2 s-1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300-MPa confining pressure. Crystals hydrothermally treated at 1300°C, 300-MPa confining pressure, and 10-5 s-1 strain rate as a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigation suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the ``wet'' experiments. First-order calculations suggest a stress exponent of n~2.5 and a dislocations in the ``wet'' experiments. First-order calculations suggest a stress exponent of n~2.5 and a water fugacity exponent of m~1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.

Metasomatism in Two Natural Peridotites: Effects of Low-aSiO2 Melts

Article

Full-text available

Jan 1994

Enno Zinngrebe

Rheology of Synthetic Olivine Aggregates: Influence of Grain Size and Water

Article

Full-text available

Jan 1986

The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied using a gas-medium high-pressure deformation apparatus. The plasticity is found to depend both on grain size and water fugacity, as well as on stress and temperature. At high stress and coarse grain size, the rheology is insensitive to grain size and the stress exponent is around three to four. At low stress and fine grain size, the rheology is highly sensitive to grain size and the stress exponent is close to one. At 1573 K, 300 MPa confining pressure, and strain rate of 10 to the -5th/s, the transition between the two regimes occurs at a stress of about 60 MPa and a grain size of 40 microns under 'wet' conditions and about MPa and 25 microns under 'dry' conditions. Water has a weakening effect in both regimes which is primarily an intragranular effect in the grain size insensitive regime.

Wallace, M. E. & Green, D. H. An experimental determination of primary carbonatite magma composition. Nature 335, 343-346

Article

Full-text available

Sep 1988

Carbonatites are uncommon carbonate-rich rocks usually found in continental intra-plate regions and often associated with rifting. There has been much debate as to whether carbonatite magmas are primary melts derived from partial melting of mantle peridotite, or are formed by exsolution of an immiscible carbonate melt fraction from phonolitic or nephelinitic magmas. Our experiments on the phase relationships of carbonate and amphibole-bearing peridotite (containing 0.3% H2O and 0.5–2.5% CO2) show that sodic dolomitic carbonatite magma coexists with an amphibole lherzolite assemblage in a field ranging from 21 to 30 kbar and 930 to 1,080 °C, spanning a pressure and temperature interval between the solidus and the amphibole breakdown and melting curve. Thus primary carbonatite melts may occur under suitable geothermal conditions. The nature of the peridotite solidus and of the melting reactions differ considerably from published models1–3. The carbonatite melt composition, determined by a series of 'sandwich' experiments, was found to be rich in Na, Mg, Ca and Fe, with a small dissolved silicate content. This melt quenches to an assemblage of dolomite and Na–Mg carbonate minerals, producing textures similar to those preserved in samples from Oldoinyo Lengai4, Homa mountains, Tanzania5 and Kaiserstuhl, Germany6.

Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al 2 O 3-SiO 2-CO 2 from 3 to 7 GPa

Article

Full-text available

Apr 1998

We have experimentally determined the soli-dus position of model lherzolite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 (CMAS.CO 2) from 3 to 7 GPa by locating isobaric invariant points where liquid coex-ists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus re-actions at the solidus involving carbonate generates two invariant points, I 1A and I 2A , which mark the transition from CO 2 -bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO 2 , we ®nd I 1A at 2.6 GPa/1230 °C and I 2A at 4.8 GPa/1320 °C. The variation of all phase composi-tions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbon-atitic with SiO 2 contents of <6 wt%, CO 2 contents of ~ 45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocar-bonatites may well be an example of the eruption of such melts directly from their mantle source region as evi-denced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO 2 system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO 2 -rich and silica poor. The widespread evidence for the presence of CO 2 in both the oceanic and continental upper mantle implies that such low degree SiO 2 -poor carbonatitic melts are com-mon in the mantle, despite the rarity of carbonatites themselves at the Earth's surface.

Über (OH)-Gruppen in Olivin

Article

Jan 1969

A. Beran

Friction Effects and Pressure Calibration in a Piston-Cylinder Apparatus at High Pressure and Temperature

Article

Jul 1966
J GEOPHYS RES

The pressure of a piston-cylinder apparatus was calibrated at a temperature of 1100°C. The calibration is based on the quartz-coesite phase transition. Pressure losses are considerable and a correction of −11% at 1100°C and 35 kb is indicated for a compression run with talc as the pressure-transmitting medium. This correction was evaluated by comparing results obtained with talc and silver chloride pressure-transmitting mediums.

High pressure and temperature fluid fugacities

Article

Apr 1987
GEOCHIM COSMOCHIM AC

High-temperature and high-pressure shock-wave data on fluids (pure species) have been combined with low-temperature and low-pressure data to generate a corresponding state equation in the virial format in reduced pressure and temperature for many species. The equation is then modified to obtain a similar equation of state for H/sub 2/O. The fugacities of the pure species in the C-H-O system can be calculated to a temperature of 3000 K and to a pressure of 1 megabar. However, dissociation of the pure species may invalidate the data over certain pressure-temperature ranges.

Diffusion of hydrogen in olivine: Implications for water in the mantle

Article

Jan 1990

To investigate the kinetics of diffusion of hydrogen in olivine, single crystals from San Carlos in Arizona have been annealed at temperatures between 800° and 1000°C under hydrothermal conditions at a confining pressure of 300 MPa. The hydrogen diffusivities were determined for the [100], [010], and [001] directions from concentrations profiles for hydroxyl in the samples. These profiles were obtained from infrared spectra taken at 100-mum intervals across a thin slice which was cut from the central portion of each annealed crystal. The rate of diffusion is anisotropic, with fastest transport along the [100] axis and slowest along the [010] axis. The fit of the data to an Arrhenius law for diffusion parallel to [100] yields an activation enthalpy of 130+/-30 kJ/mol with a preexponential term of (6+/-3)×10-5 m2 s-1. For diffusion parallel to [001], as there are insufficient data to calculate the activation enthalpy for diffusion, we used the same value as that for diffusion parallel to [100] and determined a preexponential term of (5+/-4)×10-6 m2 s-1. The diffusion rate parallel to [010] is about 1 order of magnitude slower than along [001].The measured diffusivities are large enough that the hydrogen content of olivine grains which are millimeters in diameter will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800°C. As xenoliths ascending from the mantle remain at high temperatures (i.e.,>1000°C) but experience a rapid decrease in pressure, and hence hydrogen fugacity, olivine grains may dehydrate during ascent. By comparison, slow rates of carbon diffusion (Tingle et al. 1988) suggest that carbon will not be lost from olivine during ascent. Thus, low hydrogen contents within olivine and within fluid inclusions in olivine cannot be taken as support for low water contents in the mantle.

The Solubility of Quartz in H2O in the Lower Crust and Upper Mantle

Article

Nov 1994
GEOCHIM COSMOCHIM AC

Craig E. Manning

The solubility of quartz in H 2 O has been determined experimentally from 5 to 20 kb and 500 to 900°C. The results double the pressure range over which the molality of aqueous silica ( m SiO 2( aq ) ) has been determined and lead to more accurate estimates of quartz solubility in H 2 O below 5 kb because of the rapid-quench methods employed. At constant temperature, log m SiO 2( aq ) increases with increasing pressure and ( log m SiO 2( aq ) / t 6 P ) T decreases with increasing pressure. Comparison of the new data with previous low-pressure experiments demonstrates that isothermal values of log m SiO 2( aq ) increase linearly with increasing log m H 2 O between 200 and 900°C. This observation was used to derive the following expression for the equilibrium constant ( K ) of the reaction QUARTZ = SiO 2(aq) where log K = log m SiO 2( aq ) . The equation agrees well with previous results, while accurately reproducing measured quartz solubilities over a much wider range in pressure and temperature, from 25°C and 1 bar to the conditions of this study. If the isothermal variation of log mSiO 2( aq ) with log H 2 O is assumed to be linear, the results can be extrapolated to > 20 kb. The equation allows evaluation of aqueous silica transport in Barrovian metamorphic belts, subduction zones, and metasomatized magma source-regions in the mantle.

Electrical conduction in olivine

Article

Jan 1989

This paper reports detailed measurements of electrical conductivitysigma and thermoelectric effect /ital S/ in the mineral olivine and in syntheticforsterite as functions of temperature in the range from 1000/degree/ to1500 /degree/C and oxygen partial pressure in the range from 10/sup /minus/10/ to10â´ Pa. The two most striking observations are strong conductivityanisotropy in forsterite and a sign change in /ital S/ in olivine at 1390 /degree/C.These results are interpreted to show that both materials have mixed ionic andextrinsic electronic conduction under these conditions. On the basis ofthese interpretations, we infer that forsterite conductivity is dominatedby electronic conduction in the /ital a/ and /ital b/ directions and probably bymovement involving magnesium vacancies in the /ital c/ direction, wherefar higher /ital P//sub Oâ/-independent conductivity is observed. Olivineappears to show mixed conduction under all the circumstances observed; at low temperature, electron holes dominate but are superseded bymagnesium vacancies at high temperatures./copyright/ American Geophysical Union 1989

Oxygen Diffusion in Olivine

Article

Apr 1989

The self-diffusion of oxygen in San Carlos olivine single crystals has been measured along the [001] direction. The diffusion coefficient can be written D* = D*0 exp(-E/RT)(pO2/pO) , where E is the activation energy, R is the gas constant, T, is the temperature in K, pO2 is the oxygen partial pressure, pO is the room pressure, m is a numerical coefficient, and D*O is the preexponential term. A data inversion method has been used to determine the three parameters and yields log (D*O) = -5.2 (±3.6) with D*O in m2/s, E = 318 (±17) kJ/mol, m = 0.34 (±0.02); D* = 6.7 × 10-6 exp(-38 000/T)(pO2/pO)0.34 m2/s; the number in parentheses is the standard deviation. D* is related to the self-diffusion coefficient D by the relation D* = fD, with f close to 1.0. An attempt to show pipe diffusion in predeformed samples, with a dislocation density of about 2 × 1011 m-2, along dislocation cores perpendicular to the specimen surface, showed no contribution to the main bulk diffusion. We conclude that in olivine, Fo≃90, within the temperature range investigated, DFe, DMg >> DOx >> DSi as has already been demonstrated in Fo100. We infer that the oxygen diffusion occurs via an interstitial mode and that these defects carry one negative charge. -from Authors

Water in minerals? A peak in the infrared

Article

Jun 1984

The study of water in minerals with infrared spectroscopy is reviewed with emphasis on natural and synthetic quartz. Water can be recognized in minerals as fluid inclusions and as isolated molecules and can be distinguished from hydroxide ion. The distinction between very small inclusions and aggregates of structurally bound molecules is difficult. New studies of synthetic quartz using near-infrared spectroscopy are reported. These demonstrate that water molecules are the dominant hydrogen containing species in synthetic quartz but that this water is not in aggregates large enough to form ice when cooled.

Tracerdiffusion of the Fe-cations in olivine (Fe x Mg1- x )2SiO4 (III)

Article

Oct 1984

Tracerdiffusion coefficients D Fe* (and D Mg*) are presented for olivines of composition (Fex Mg1−x )2SiO4 at T=1,130° C as a function of x, and oxygen activity, a O2. Since the oxygen activity dependence of D Fe* (D Mg*) and that of the cation vacancy concentration are almost identical, it is concluded that a vacancy diffusion mechanism is operative in the octahedrally coordinated cation sublattices. From D Fe* and D Mg*, the chemical diffusion coefficient $\bar D$ can be calculated. The calculated $\bar D$ is in agreement with $\bar D$-values obtained by Boltzmann-Matano analysis of interdiffusion experiments. In addition, correlation factors are evaluated from the tracerdiffusion data in order to calculate selfdiffusion coefficients.

Solubility of water in the α, β and γ phases of (Mg,Fe)2SiO4

Article

May 1996

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 C OH = 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.

On the Nonstoichiometry and Point Defects of Olivine

Article

Mar 1983

This paper presents the point-defect thermodynamics for fayalite and olivine solid solutions (Fe x Mg1–x )2SiO4. By means of thermogravimetry, the metal-to-oxygen ratio of these silicates has been determined as a function of oxygen potential, compositionx and temperature. Experiments were performed in the range of 1,000 CpO2 p_{O_2 } and aSiO2 a_{SiO_2 } . The cation vacancy concentration shows a pO2 1/5p_{O_2 }^{1/5} -dependence (forxpO2 p_{O_2 } almost exponentially with compositionx. In the composition range studied here, the silicates show an oxygen excess, and FeO is more soluble in the olivine than SiO2.

OH defects in forsterite

Article

Sep 1995

Polarized FTIR spectra of near endmember forsterite single crystals from Pamir, Tadzikistan show the existence of sharp strongly pleochroic absorption bands in the region of the OH stretching fundamental. Bands centered at 3674/3624, 3647/3598 and 3640/ 3592 cm-1 are attributed to OH dipoles oriented parallel to [100]. An OH band doublet at 3570/3535 cm-1 shows both, a strong absorption parallel to [100] and a strong component parallel to [001]. On the basis of the pleochroic scheme and under the assumption of vacancies on Si- and M-sites it is proposed that O1 is partially replaced by OH defects pointing to the vacant Si-site. O3 is donator oxygen of OH dipoles lying near the O3-O1 tetrahedral edge or roughly pointing to a vacant M2-site. Also O2 can act as donator oxygen of an OH group oriented along the O2-O3 edge of a vacant M1 octahedron. The splitting of the bands is explained by the presence of Fe2+ in cation sites surrounding the OH defects.

A model of the OH positions in olivine, derived from infrared-spectroscopic investigations

Article

Feb 1983

Polarized infrared (IR) spectroscopy of olivine crystals from Zabargad, Red Sea shows the existence of four pleochroic absorption bands at 3,590, 3,570, 3,520 and 3,230 cm–1, and of one non pleochroic band at 3,400 cm–1. The bands are assigned to OH stretching frequencies. Transmission electron microscopy (TEM) shows no oriented intergrowths in this olivine; it is concluded that OH is structural. On the basis of the pleochroic scheme of the absorption spectra it is proposed that [O(OH)3] and [O2(OH)2] tetrahedra occur as structural elements, assuming that the vacancies are on Si sites. If M2 site vacancies were assumed [SiO3(OH)] and [SiO2(OH)2] tetrahedra occur as structural elements.

The natural occurrence of hydroxide in olivine

Article

Jan 1987

Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/106Si, whereas olivines from basalts contain the least at 3 H/106Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H2 in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm−1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H2O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.

Experimental evidence for a hydrous transition zone in the early Earth's mantle

Article

Aug 1996
EARTH PLANET SC LETT

Partition coefficients of H2O between β and γ phases of olivine stoichiometry and coexisting ultra mafic melt have been estimated to be > 0.1 ± 0.04 (1σ) and 0.04, respectively; based on experiments at 15–16.5 GPa, 1300–1500°C in a hydrous KLB-1 peridotite system. The high H2O contents of β (1.5–3 wt%) and γ phases (0.7 wt%) would form a reservoir for H2O after cooling and crystallization of a hydrous magma ocean. Subsequent upwelling of this hydrous mantle would release H2O at the β phase-olivine boundary near 400 km depth, inducing partial melting of the peridotite to produce hydrous ultramafic magma. Most subducting hydrous minerals dehydrate at pressures shallower than 6.5 GPa if the down-dragged hydrous peridotite follows a P-T path hotter than 900°C at 8 GPa and cannot re-hydrate the transition zone. Therefore, the above proposed partial melting would gradually deplete the H2O reservoir, which is consistent with the decrease in the activity of ultramafic magmatism and the apparent degree of melting of komatiites from the Archean to the Mesozoic.

Silicon Diffusion in San Carlos Olivine

Article

Dec 1990
PHYS EARTH PLANET IN

The diffusion of 30Si in single crystals of San Carlos olivine has been measured. To achieve diffusion experiments the olivine samples were coated with a thin Mg2SiO4 layer and then heated between 1130°C and 1530°C under controlled oxygen partial pressures in the range 10−5 to 10−1 Pa. The annealing durations ranged from 1 h 30 min to 150 h, yielding diffusion lengths of 150–425 10−10 m. Diffusion profiles were analysed using the α-RBS technics (Rutherford Backscattering Spectroscopy of α-particles). The results show that the silicon diffusion coefficient obeys the law with E = 291 ± 15 kJ mole−1, m ∼ −0.19 ± 0.1 and ln(D0) = − 29.3 ± 1 where D is in m2 s−1, pO2 is the oxygen partial pressure, p0 the room pressure and R the gas constant. We deduce from these results that DSi ⪡ DOx ⪡ DMg, DFc in the T–pO2 domain investigated, but extrapolation towards low pO2 and temperatures suggests that it is not impossible for DSi to be larger than DOx owing to opposite pO2 dependences. The negative value of m indicates a probable interstitial mechanism for silicon diffusion. Results show that silicon diffusion in San Carlos olivine is enhanced by ∼ 30 relative to pure forsterite in the T–pO2 range studied.

Effect of Silica Activity on OH- IR Spectra of Olivine: Implications for Low-aSiO2 Mantle Metasomatism

Abstract

No full-text available

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