Water Fugacity Research Articles

Stability of hydrous basaltic melts at low water fugacity: evidence for widespread melting at the lithosphere-asthenosphere boundary

The upper mantle low velocity zone is often attributed to partial melting at the lithosphere-asthenosphere boundary. This implies that basaltic melts may be stable along plausible geotherms due to the freezing point depression in the presence of water and other incompatible impurities. However, the freezing point depression (ΔT) as a function of water content in the near-solidus basaltic melt (cH2O) cannot be precisely determined from peridotite melting experiments because of difficulties in recovering homogeneous basaltic glasses at high pressures. We therefore used an alternative approach to reinvestigate and accurately constrain the ΔT–cH2O relationship for basaltic melts at the low water fugacities that are expected in the upper mantle. Internally heated pressure vessel (IHPV) experiments were performed at water-saturated conditions in the anorthite-diopside-H2O system at confining pressures of 0.02 to 0.2 GPa and temperatures between 940 and 1450 ℃. We determined the water-saturated solidus, and obtained ΔT by combining our data with reports of dry melting temperatures in the anorthite-diopside system. In another series of experiments, we measured water solubility in haplobasaltic melts and extrapolated cH2O to pressures and temperatures of the water-saturated solidus. By combining the results from these two series of experiments, we showed that the effect of water on ΔT was previously underestimated by at least 50 ℃. The new ΔT–cH2O relationship was then used to revise predictions of melt distribution in the upper mantle. Hydrous melt is almost certainly stable beneath extensive regions of the oceanic lithosphere, and may be present in younger and water-enriched zones of the subcontinental mantle.

Experimental constraints on iron and sulfur redox equilibria and kinetics in basaltic melt inclusions

The oxidation states of dissolved iron (Fe) and sulfur (S) in silicate melts were studied experimentally by heating Hawaiian olivines containing S-bearing melt inclusions (MIs) in a 1 atm gas-mixing furnace. The MIs are closed with respect to S and C, but equilibrate with the oxygen fugacity (ƒO2) and water fugacity (ƒH2O) of the furnace. We also conducted experiments using a S-free synthetic Hawaiian MI composition held on Pt-loops. Experiments were run at 1225 °C for 8–96 hr in H2-CO2 gas mixtures corresponding to ƒO2 values that varied over seven orders of magnitude and then drop-quenched into water. Time-series experiments show that MI Fe3+/Fe2+ and S6+/S2− ratios reached constant values within ∼24 hr. The H2O contents of the Pt-loop glasses and the dehydrated MIs overlap and are proportional to the fH2O in the furnace gas. The slope of log10(Fe3+/Fe2+) vs. log10ƒO2 is 0.25±0.02(2σ) for the Pt-loop glasses and 0.23±0.01(2σ) for the quenched MIs; the slopes of the two sets of experiments are consistent with equilibrium having been reached via the reaction FeO + ¼ O2 = FeO1.5. S-bearing quenched MI glasses have Fe3+/Fe2+ ratios that are systematically low compared to ratios measured in the S-free Pt-loop glasses run at the same T and ƒO2; this may reflect an effect of S on Fe3+/Fe2+, although further experiments are needed to explore this possibility. S6+/S2− ratios in the experimental melt inclusions vary systematically as a function of ƒO2, and the measured ratios agree with independent calculations of S oxidation state in basaltic melts.After being held at 1225 °C for 24 or 48 hr, a subset of olivine-hosted MIs were cooled at rates of 1700–40,000 °C/hr and quenched at temperatures between 900 °C and 1150 °C to explore whether Fe and S speciation are modified during cooling. Fe and S speciation in isothermal and cooled MIs held initially at the same ƒO2 overlap within uncertainty, suggesting that the T dependence of the homogeneous reaction 8Fe3+ + S2− = 8Fe2+ + S6+ is small and/or its kinetics are sluggish relative to cooling rates required to quench basaltic liquids to glasses. The centers of unheated, naturally quenched MIs from Papakōlea, Hawaii with syneruptive cooling rates ranging from ∼50 to 12,000 °C/hr were also measured for Fe and S speciation. MIs that experienced the lowest cooling rates have higher Fe3+/Fe2+ and S6+/S2− ratios relative to rapidly cooled inclusions of comparable size. The increase in Fe3+/Fe2+ is due to olivine crystallization on the inclusion walls during cooling and diffusion of the resulting oxidized boundary layer into MI interiors. Diffusive modification is minimized in MIs that have been rapidly cooled and/or are sufficiently large such that the oxidized boundary layer is restricted to a narrow region adjacent to the inclusion walls. Our experiments and their comparison to naturally quenched MIs show that measurements of Fe3+/Fe2+ and S6+/S2− ratios in quenched basaltic glasses and in the centers of rapidly cooled and/or large MIs are likely to be representative of the redox state in the melt at magmatic temperatures prior to quenching to glass.

The transport of bismuth in HCl-bearing aqueous vapour and low-density aqueous supercritical fluids: Implications for natural systems

Hydration by H2O clusters has been shown to be an important means of transporting metal complexes in hydrothermal vapours and low-density supercritical aqueous fluids. However, the effect of hydration on the transport of relatively volatile metal complexes such as those involving bismuth and chloride has not been evaluated. This effect is important to evaluate because bismuth is a key pathfinder element for gold in many intrusion-related gold ore-forming systems, where low-density supercritical aqueous fluids and vapour may play an important role in metal transport. Here, we present the results of experiments investigating the solubility of BiOCl(s) and the speciation of bismuth in HCl-bearing aqueous vapour and low-density supercritical aqueous fluids at temperatures between 250 and 400 °C and pressures of 5 to 296 bar. Two gaseous bismuth species formed in the HCl-bearing fluids (X(HCl) > 0.0005), namely BiCl3(g) and BiCl3(H2O)n(g). Our data clearly show that BiCl3(g) is highly volatile at low water fugacity (between 30 and 110 bar) and acts as an unhydrated gas molecule. At higher water fugacity, the gaseous BiCl3(g) reacts with water to form hydrated BiCl3(H2O)n(g) species. Significantly, the stability of hydrated bismuth chloride species, BiCl3(H2O)n(g), with low hydration numbers is lower than that of the unhydrated species, BiCl3(g), resulting in a decrease in bismuth solubility with increasing f(H2O). However, at a water fugacity great than 30–110 bar, that increases with increasing temperature, the hydration number and the solubility of hydrated species BiCl3(H2O)n(g) increase with increasing water fugacity or the density of the fluid. Consequently, unhydrated gaseous bismuth species increase in importance with increasing temperature and become dominant at temperatures > 450 °C in fluids that have low density (<0.10 g/cm3). Hydrated bismuth chloride species are dominant in fluids having higher density (0.10–0.34 g/cm3). The overarching conclusion of this study is that aqueous low-density supercritical fluids, which exsolve from magmas to form intrusion-related gold deposits, can transport bismuth in concentrations (up to hundreds ppm) sufficient to form economic deposits.

Hydrogen solubility of stishovite provides insights into water transportation to the deep Earth

Abstract. Water dissolved in nominally anhydrous minerals (NAMs) can be transported to deep regions of the Earth through subducting slabs, thereby significantly influencing the physicochemical properties of deep-Earth materials and impacting dynamic processes in the deep Earth. Stishovite, a prominent mineral present in subducting slabs, remains stable at mantle pressures of 9–50 GPa and can incorporate various amounts of water (H+, OH−, and H2O) in its crystal structure. Consequently, stishovite can play a crucial role in transporting water into the deep Earth through subducting slabs. This paper provides a comprehensive review of the research process concerning water (hydrogen) solubility in stishovite. The key factors that govern water solubility in stishovite are summarized as temperature, pressure, water fugacity, and aluminum content. Combined with published results on the dependence of water solubility on the aforementioned parameters, this paper proposes a new equation to describe the solubility of water in Al-bearing stishovite. Calculation results based on this equation suggest that stishovite may effectively accommodate water released from processes such as hydrous mineral breakdown, which could ultimately contribute to the presence of a water-rich transition zone.

Rapid fCO2 rise in the northern Barents Sea and Nansen Basin

Maps of surface water fugacity of CO2 (fCO2) over eastern Fram Strait, south-western Nansen Basin, and the north-western Barents Sea (73–84°N, 5–46°E) from September 1997 to December 2020 were made and used to investigate seasonal and temporal trends. The mapping utilized a neural network technique, the self-organizing map (SOM), that was trained with different combinations of satellite/observational/model data of sea surface temperature (SST), sea surface salinity (SSS), mixed layer depth (MLD), chlorophyll a (Chl a), sea ice concentration, and atmospheric mole fraction of CO2 (xCO2). The trained SOM was labelled with available surface ocean fCO2 data, and the labelled SOM was subsequently used to map the fCO2. The produced maps reveal that fCO2 in northern Barents Sea, at the border of the Nansen Basin, has increased significantly over the last decades by between 4.2 and 5.5 ± 0.6–1.1 µatm yr−1 over the winter to summer seasons. These rates are twice the rate of atmospheric CO2 increase, which was about 2 µatm yr−1. The spatial pattern coincides with the strongest decreases in sea ice concentration as well as with a salinification of the surface water. The former allows for a prolongation of the air-sea CO2 flux with resultant oceanic CO2 uptake in previously ice-covered waters, and the latter is caused by a shift from Arctic Water dominance to more saline waters containing more dissolved inorganic carbon, most likely of Atlantic Water origin although brine-release influenced deep water may also contribute.

Amphibole stability, water storage in the mantle, and the nature of the lithosphere-asthenosphere boundary

Amphibole could potentially be an important host of water in the upper mantle. Moreover, the decomposition of amphibole has been invoked as a possible cause of the lithosphere-asthenosphere boundary. However, amphibole stability has been experimentally studied mostly under water-saturated conditions, which are unrealistic for most of the mantle that may contain only traces of water. Experiments with low nominal water contents yielded controversial results and were properly hampered by problems in controlling water activity. We have solved this problem with a novel experimental approach. We carried out piston cylinder experiments from 900 to 1350°C and 2 to 4.5 GPa using a peridotitic composition coexisting with an excess H2O-N2 fluid phase. The dilution by inert N2 was used to precisely control water fugacity to values realistic for the upper mantle. Numerous reversed experiments were carried out to circumvent problems with the metastable formation of amphibole. Our data show a dual effect of water fugacity on the stability of amphibole. With decreasing water activity, the stability field is simultaneously displaced to lower pressures, and expanded to higher temperatures. This behavior is due to two different decomposition reactions with dehydration involving only solid phases at low temperature, but melting at high temperature. Along a continental geotherm, amphibole will never be stable for typical upper mantle water contents. However, for a mantle containing 150 – 200 ppm of water, traces of amphibole may form in a narrow pressure interval along an oceanic geotherm. Here, amphibole may contribute significantly to bulk water storage, although most of the water still resides in nominally anhydrous minerals. However, even if amphibole is stable in a restricted depth range, it cannot account for the lithosphere-asthenosphere boundary, since decomposition proceeds through a solid-state reaction and does not involve melting for realistic mantle water contents.

Hydrogen Partitioning Between Olivine and Orthopyroxene: Implications for the Lithosphere‐Asthenosphere Structure

AbstractHydrogen solubility was determined in olivine and orthopyroxene under water‐saturated conditions at P = 3–5 GPa and T = 1373–1573 K. For olivine, polycrystalline samples were prepared from San Carlos olivine, and for orthopyroxene synthetic samples were prepared from oxide mixture containing 1.5–5 wt% of Al2O3. Olivine and orthopyroxene were placed next to each other and annealed under various pressure and temperature conditions for 3–5 hr. Hydrogen content was measured across each sample by FTIR spectroscopy. Under the water‐saturated conditions, the hydrogen solubility in olivine increases with pressure and temperature similar to previous results. Hydrogen solubility in Al2O3‐bearing orthopyroxene also increases with temperature and pressure for a fixed Al2O3 content. Based on these observations we calculated the partition coefficients of hydrogen between orthopyroxene and olivine assuming the fugacity dependence of hydrogen solubility in olivine and Al2O3‐bearing orthopyroxene reported by previous studies. We find that the partition coefficient depends weakly on temperature but strongly on pressure and water fugacity. Our results are extended to an open system where Al2O3 content in orthopyroxene changes with pressure and temperature. At relatively low pressures and low water fugacity (in the lithosphere (shallower than ∼50 km)), the partition coefficient is high and a majority of hydrogen is present in orthopyroxene. Consequently, the influence of water on the bulk physical properties is small. In contrast, at higher pressures and higher water fugacity (in the asthenosphere), the partition coefficient is smaller and a substantial amount of hydrogen is present in olivine. Consequently, hydrogen has a strong effect on the bulk properties of the asthenosphere reducing viscosity and increasing electrical conductivity.

Convective Melting and Water Behavior around Magmatic-Hydrothermal Transition: Numerical Modeling with Application to Krafla Volcano, Iceland

Abstract Water is an essential component of rhyolitic magmas and nearly universally, silicic magmatism in the upper crust includes a transition from magma to water-saturated roof rocks. We have numerically simulated the effect of the addition of geothermal fluids to an intruded rhyolitic sill from the hydrothermal system contained within the porous felsitic roof rocks. Water uptake in the melt proceeds via its thermodynamically estimated saturation with a partial melt, corresponding to the fugacity of hydrothermal water in the melt-fluid zone at particular T–P-${X}_{H_2O}$ conditions. It is assumed that the exchange occurs until the melt fraction increases to the threshold melt fraction value εb ≈ 0.3–0.45. In this approximation, the amount of added water is the product of its solubility and the critical melt fraction εb. In two series of numerical experiments run at pressures of 200 and 50 MPa, the interaction of water-filled porous felsite with near liquidus rhyolite magma resulted in water absorption, induced partial melting creating a narrow several meters wide mushy zone, and sluggish convection below that distributed water across the intruded sill. At P = 200 MPa, the addition of about 1.5 wt% water results in stronger volume convection, causing the melting rate to increase to 20 m/year. However, the addition of &amp;lt;0.22 wt% water induced no melting on the magma/contact mush interface, and the intruded sill crystallizes without convection. We apply the results of these numerical experiments to hot and dry rhyolites of the Yellowstone hot spot track magmas and then to the 2009 AD rhyolite sampled by the IDDP-1 exploration well in Krafla (Iceland). An active contact between the hydrothermal system within felsite and hot 963°C rhyolite magma was accidentally crossed at the depth of 2100 m, with a very thin (&amp;lt;30 m) transition providing information for a partial verification of our theoretical model. With the parameters observed in 2009, including the water concentrations in the melt (1.8 wt%) and felsite (0.92 wt%) and the high temperature of the intruded magma (945°C–980°C), we obtained slow melting of the preheated felsite roof at a rate of about 1 m/year. This seems reasonable if the 2009 magma was intruded during the 1975–1984 Krafla Fires eruption. We additionally present new δD values (−118‰) and H2Otot (1.6–2.0 wt%) concentration and reinterpret published δD and H2Otot values for felsitic fragments from IDDP-1; we find these to be lower than the δD in the mantle-derived magmas or hydrothermal systems. We demonstrate that, in theory, the formation of a fluid with such a low δD can be provided by the addition of low-δD water from OH-bearing minerals in hydrothermally altered roof rock. This may happen during dehydration of epidote from the altered roof rocks, and, alternatively, may also proceed by the mechanism of thermal diffusion (the Soret effect) through the partially molten/hydrothermal transition zone controlled by fluid fugacity. The high-temperature gradient in the contact zone between magma and the geothermal system of about 15–17°/m with conditions at the cold end close to the critical point for the aqueous fluid further decreases the expected δD value at the hot end of the contact zone to less than −110‰.

Effect of water content and side chain on hydroxide transport in quaternary ammonium functionalized covalent organic frameworks as anion exchange membranes

Covalent organic framework (COF) is an excellent candidate of ion transport material for its outstanding properties, yet the correlation between the structures and ion transport in COF is still to be clarified. Here we systematically investigate the effect of water content and side chain on the diffusion of hydroxide in several COFs functionalized with quaternary ammonium (QA) groups via molecular dynamics simulations. The water uptakes of the systems are determined by computing the adsorption isotherm curves at different water fugacity. Different from the previous assumption, we find that the length of the side chain does not change the radius of the channel, which consequently has little impact on the diffusion of hydroxide either. Instead, hydroxide diffusion is mainly determined by the strength of association between the cations and anions. Moving the QA groups from the side chain to the aromatic ring on the framework of the COF dramatically reduces hydroxide diffusion, which is attributed to the significantly enhanced association. Removing a fraction of tethers reduces hydroxide diffusion probably due to the broken of regularity of the hopping sites for the anions. Those results shed light on the interplay between several key parameters and the anion diffusion in the COF-based anion exchange membranes, which provide guidance for the future design.

Silicates chemistry as indicators of physicochemical and geothermometry conditions on porphyry ore system: A case study of the Haftcheshmeh Cu–Mo deposit, NW Iran

The Haftcheshmeh porphyry Cu–Mo deposit is located in the northwestern part of the Arasbaran Metallogenic Zone (AMZ), NW Iran. The Cu–Mo mineralization in this deposit is spatially and temporally related to late Oligocene gabbrodiorite and early Miocene granodiorite stocks. The chemistry of biotite, amphibole, and plagioclase in the ore–related granodiorite and gabbrodiorite intrusions was investigated to determine the physicochemical conditions of crystallization (temperature, pressure, oxygen fugacity, water activity, and fluorine–chlorine activity). Mg–biotite, Mg–hornblende, andesine/oligoclase and quartz are the main minerals in these rocks, associated with zircon, apatite, titanite, rutile and Fe–Ti oxides as accessory minerals; plus secondary biotite, K–feldspar, epidote, chlorite and muscovite as secondary minerals. The chemical compositions of the Mg–biotite and Mg–hornblende minerals reveal that the Oligo–Miocene magmas were most likely generated from mantle–derived oxidized I–type magma that was only weakly to moderately contaminate by crustal materials. The log(fH2O/fHF) and log(fH2O/fHCl) for the Mg–rich and relatively F–rich magmatic biotite indicate an oxidized, water– and halogen–rich melt. The halogen and water fugacities, show that the Mg–rich biotites crystallized under conditions that were similar in terms of pressure, temperature, flour, and chlorine content. Al–in–hornblende barometry and hornblende––plagioclase thermometry suggest that the Mg–hornblendes in the Haftcheshmeh granodiorite and gabbrodiorite intrusions crystallized under similar P–T conditions (from 0.11 to 0.96 kbar and 712 to 1060 °C, respectively). Oxygen fugacities obtained from the amphibole–only chemometer suggest that the gabbrodiorite and granodiorite intrusions have crystallized from oxidized (NNO + 1.82 and NNO + 2.92) magmas during amphibole crystallization. The presented method for fingerprinting magmas via trace element chemistry in biotite, amphibole and plagioclase by microprobe studies, demonstrate grouping of V, Cr, Ni and Nb concentrations in these silicates, despite the presence of large variation and zonation in other trace elements (Ba, Sr, Pb, Zn, P, Cl, F) that indicate complex petrogenetic processes. Our results show that trace elements in such silicates can be used to fingerprint of magma process.

Partial dehydration of brucite and its implications for water distribution in the subducting oceanic slab

Hydrous minerals within the subducting oceanic slab are important hosts for water. Clarification of the stability field of hydrous minerals helps to understand transport and distribution of water from the surface to the Earth’s interior. We investigated the stability of brucite, a prototype of hydrous minerals, by means of electrical conductivity measurements in both open and closed systems at 3 GPa and temperatures up to 1300 K. Dramatic increase of conductivity in association with characteristic impedance spectra suggests that partial dehydration of single-crystal brucite in the open system with a low water fugacity occurs at 950 K, which is about 300 K lower than those previously defined by phase equilibrium experiments in the closed system. By contrast, brucite completely dehydrates at 1300 K in the closed system, consistent with previous studies. Partial dehydration may generate a highly defective structure but does not lead to the breakdown of brucite to periclase and water immediately. Water activity plays a key role in the stability of hydrous minerals. Low water activity (aH2O) caused by the high wetting behavior of the subducted oceanic slab at the transition zone depth may cause the partial dehydration of the dense hydrous magnesium silicates (DHMSs), which significantly reduces the temperature stability of DHMS (this mechanism has been confirmed by previous study on super hydrous phase B). As a result, the transition zone may serve as a ‘dead zone’ for DHMSs, and most water will be stored in wadsleyite and ringwoodite in the transition zone.

Assessment of Quartz Grain Growth and the Application of the Wattmeter to Predict Quartz Recrystallized Grain Sizes

AbstractRelationships between the recrystallized grain size and stress are investigated for experimentally deformed water‐added quartz aggregates. For stresses ≥100 MPa there is a variation in the measured recrystallized grain size for a given stress. This variation correlates with a change in the c‐axis fabric in general shear experiments, where samples with larger recrystallized grain sizes for a given stress have dominantly prism &lt;a&gt; c‐axis fabrics and samples with smaller recrystallized grain sizes for a given stress have dominantly basal &lt;a&gt; c‐axis fabrics. The dislocation creep flow law also changes at conditions where these two c‐axis fabrics form (Tokle et al., 2019, https://doi.org/10.1016/j.epsl.2018.10.017). Using the wattmeter model (Austin &amp; Evans, 2007, https://doi.org/10.1130/G23244A.1), different piezometric relationships are quantified for samples that develop prism &lt;a&gt; and basal &lt;a&gt; c‐axis fabrics, respectively. The wattmeter model is sensitive to grain growth kinetics; a new grain growth law for quartz is formulated based on reanalysis of microstructures in samples from previous work. The activation enthalpies and water fugacity exponents for our grain growth law and dislocation creep flow laws are the same within error, suggesting the recrystallized grain size versus stress relationships are nearly independent of temperature and water fugacity, consistent with laboratory observations. The wattmeters successfully predict the recrystallized grain size versus stress relationships of all quartzite samples from experiments with added water. These results support the use and extrapolation of the wattmeter model for both experimental and geologic conditions to investigate the stress state and grain size evolution of quartz rich rocks.

An experimental study of the solubility of rare earth chloride salts (La, Nd, Er) in HCl bearing water vapor from 350 to 425 °C

The solubilities of the rare earth chlorides REECl3, where REE = (La, Nd, Er), were measured in HCl bearing water vapor from 350 to 425 °C with water partial pressures ranging from 8 to 170 bar. Solubility data were fit to the Pitzer-Pabalan quasi-chemical model in order to extract thermodynamic parameters for the formation of the gaseous REE-chloride-water clusters REECl3(H2O)n. The data show that the solubility of the REE chlorides are orders of magnitude higher than salts such as NaCl or CuCl at low water fugacities, despite their sublimation energies being substantially higher. This enhanced solubility is likely due to the high enthalpy associated with binding a single water molecule to form the species REECl3(H2O), with derived enthalpies ranging from −378 to −465 kJ/mol. Addition of further water molecules to form higher order clusters (n > 1) involves enthalpy changes of ~−20 kJ/mol, and are in effect thermodynamically suppressed over the temperature range 350–425 °C. Despite the enhanced solubility of small REECl3 water clusters, simulations of boiling processes demonstrate that the REE show highly conservative behavior, partitioning strongly into the dense aqueous phase. Not surprisingly, the presence of phosphates in the system makes this effect even more pronounced, completely immobilizing the REE. This would reduce transport in both the vapor and aqueous phase to negligible levels. However, we suggest that vapor phase transport of the REE may play a significant role in systems having a relatively low partial pressure of water (below the saturation point), where the relatively high stability of the first hydrated REE chloride clusters (REECl3(H2O) and REECl3(H2O)2) will give a preference for gas transport of the REE relative to other elements. This can likely happen in systems involving a gas/melt exchange in fumarolic exhalations, where water vapor discharges at relatively low (close to atmospheric) pressures.

A 17-year dataset of surface water fugacity of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; along with calculated pH, aragonite saturation state and air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes in the northern Caribbean Sea

Abstract. A high-quality dataset of surface water fugacity of CO2 (fCO2w)1, consisting of over a million observations, and derived products are presented for the northern Caribbean Sea, covering the time span from 2002 through 2018. Prior to installation of automated pCO2 systems on cruise ships of Royal Caribbean International and subsidiaries, very limited surface water carbon data were available in this region. With this observational program, the northern Caribbean Sea has now become one of the best-sampled regions for pCO2 of the world ocean. The dataset and derived quantities are binned and averaged on a 1∘ monthly grid and are available at http://accession.nodc.noaa.gov/0207749 (last access: 30 June 2020) (https://doi.org/10.25921/2swk-9w56; Wanninkhof et al., 2019a). The derived quantities include total alkalinity (TA), acidity (pH), aragonite saturation state (ΩAr) and air–sea CO2 flux and cover the region from 15 to 28∘ N and 88 to 62∘ W. The gridded data and products are used for determination of status and trends of ocean acidification, for quantifying air–sea CO2 fluxes and for ground-truthing models. Methodologies to derive the TA, pH and ΩAr and to calculate the fluxes from fCO2w temperature and salinity are described.

English

The Ikare area is underlain by rocks of the migmatite-gneiss-quartzite complex and older granite lithologic groups and charnockites. Categorization of these rocks in the literature and published map is generalized and the rocks are undifferentiated. Though reported in few literature as an area with rocks composed of granulite facie grade, the nature of transition is not well documented. Field and petrographic studies were carried out to properly characterize the different rock types, evaluate the migmatization processes and elucidate the nature of amphibolite to granulite facie transition. Major rock types recognized and mapped includes grey gneiss, charnockitic gneiss, granite gneiss, pelitic gneiss, porphyritic granite and quartzite. Minor rock types are the mafic and ultramafic components which are made up of basic schist and amphibolites. The area suffered at least four episodes of deformation (D1, D2, D3 and D4) and at least three episodes of metamorphism (M1, M2 and M3). Petrographic studies further show that the mineral assemblage within the quartzo-feldspathic rocks changed from hydrous to anhydrous phases as one traverses from the southern part of the study area to the northern part. Reaction textures indicate that the transition zone between the amphibolite and granulite facie rocks is achieved by a prograde dehydration reaction while rehydration reactions took place during the waning retrograde stage. Anatexis and metamorphic differentiation were mainly responsible for the migmatization process in Ikare area while the charnockitization process was a product of dehydration reactions aided by reduction in water fugacity due to influx of CO2. Key words: Structure, migmatites, granulite facies, deformational episodes, metamorphism, charnockitic gneiss. &nbsp;

Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

AbstractInfrared absorption measurements of molecular water in sheared Cambrian quartzites in the footwall to the Moine thrust reveal a decrease in water content from 4080 to 1570 ppm with increasing recrystallization traced toward the overlying thrust at the Stack of Glencoul in northwest Scotland. These results are contrary to the expected correlation between shear strain and water content for quartz deformed by dislocation creep and water-weakening processes. The observed inverse correlation indicates that fluid inclusions and hydrous defects within grains were lost by mobile grain boundary sweeping and grain boundary diffusion. Although reduced water contents might lead to hardening as chemical weakening is diminished, quartz mylonites in the immediate footwall (5 mm) to the thrust are characterized by intense strain localization and contain the least water, and there is little evidence of shear zone widening. Water weakening appears to have been important throughout the quartz mylonites, controlled by the presence of water, not by water concentration. Fluids present within relict inclusions and at grain boundaries may have governed the high water fugacities critical for water weakening.

Hydrogen incorporation in plagioclase

We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700–850 °C, 0.5 GPa, and fO2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on fO2 between IW and NNO, but experiments at 800–850 °C on other compositions (with 0.3–0.5 wt% FeO) demonstrate that H solubility is enhanced by a factor of ∼2–3 at IW compared to NNO, consistent with previous experiments by Yang (2012a) on An58. By analogy with synthetic hydrogen feldspar (HAlSi3O8), we infer that the predominant mechanism for H incorporation in Pl is through bonding to O atoms adjacent to M-site vacancies, and we propose likely O sites for H incorporation based on MO bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low fO2is explained by the formation of defect associates resulting from the reduction of Fe3+ in tetrahedral sites to Fe2+, allowing additional H to be incorporated in adjacent M-site vacancies. This mechanism counteracts the expected effect of water fugacity on H solubility. We also speculate on possible substitutions of H on tetrahedral vacancies, as well as coupled H-F substitution. Enhanced incorporation of H in Pl at low fO2 may have implications for estimating the water content of the lunar magma ocean. However, mechanisms unrelated to low fO2 are needed to explain high H contents in terrestrial Pl xenocrysts, such as those found in basalts from the Basin and Range.

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

AbstractTo 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.

The Influence of Water on the Strength of Olivine Dislocation Slip Systems

AbstractThe nature of lattice‐preferred orientation (LPO) in olivine‐rich rocks strongly influences many important physical properties of Earth's upper mantle. Different LPO types have been observed to develop in deformation experiments on olivine‐rich rocks carried out at different water fugacity conditions. The development of the different LPO types has been attributed to dislocation slip systems in olivine having different sensitivities to water fugacity, but this hypothesis has not been directly tested. To measure the influence of water fugacity on the relative strengths of olivine dislocation slip systems, we carried out a series of deformation experiments on olivine single crystals under either anhydrous or hydrous conditions. The crystals were oriented to activate either the (010)[100], (001)[100], or (100)[001] dislocation slip systems using a direct shear geometry, which allows for isolation of single slip systems, in contrast to the multiple systems activated in experiments carried out in compression. Post‐deformation electron backscatter diffraction analyses reveal orientation gradients consistent with deformation occurring via the motion of dislocations on the activated slip systems. Crystals in all of the investigated orientations exhibit hydrolytic weakening, but crystals oriented to activate the (001)[100] slip system exhibit the largest degree of weakening. These results are consistent with a water‐induced change in LPO in olivine‐rich rocks deforming by dislocation creep. The rheological data obtained from the experiments can be used to improve models of LPO evolution in Earth's mantle, which is critical for imaging the structure of Earth's interior and predicting the movement of Earth's tectonic plates.

Strain localization by shear heating and the development of lithospheric shear zones

Analogue and numerical models show that strong or weak domains in a deforming ductile material cause stress concentrations that may promote strain localization. Such domains commonly occur in the lithosphere through variations in composition or mineral fabric. Here we use a 2D plane-stress, non-Newtonian, viscous model to explore how strain localization develops from an initial isolated weak inclusion. We use a temperature-dependent rheological law for which the material weakens as a result of work done by shear converted to heat. The progress of strain localization follows a power-law growth that is strongly non-linear and may be regarded as an instability. Although this localization mechanism is ultimately limited by thermal diffusion, this parametrization permits a robust criterion for the conditions in which localized shear zones can form within the lithosphere. Shear zones in the lower crust are typically depicted as the downward continuation of faults. We argue that the depth-extent of narrow shear zones within the lithosphere is limited by the stability criteria that we infer from 2D numerical experiments. When applied to the rheological laws for common lithospheric minerals, the combination of temperature and stress-dependence provides a direct means of predicting the depth below which the localization instability does not occur. For an olivine based rheology, the maximum depth at which rapid localization is expected is in the range of ~20 to 60 km, depending on heat flow, strain-rate and water fugacity. We apply our calculations to two major continental strike-slip zones, the San Andreas Fault and North Anatolian Fault, and compare our predicted maximum localization depths with published seismological images. Strain localization in the lower crust requires a dry rheology comparable to plagioclase. Observations that imply localized strain in the uppermost mantle beneath these fault zones are consistent with the localization criteria and the rheological properties of dry olivine.