Crustal Pressure Research Articles

Susceptibility of Microseismic Triggering to Small Sinusoidal Stress Perturbations at the Laboratory Scale

AbstractSmall transient stress perturbations are prone to trigger (micro)seismicity. In the Earth's crust, these stress perturbations can be caused by various sources such as the passage of seismic waves, forcing by tides, or hydrological seasonal loads. A better understanding of the dynamic of earthquake triggering by stress perturbations is essential to improve our understanding of earthquake physics and our consideration of seismic hazard. Here, we study an experimental sandstone‐gouge‐filled fault system undergoing combined far field loading and periodic stress perturbations (of variable amplitude and frequency) at crustal pressure conditions. Microseismicity—in the form of acoustic emissions (AEs)—strains, and stresses, are continuously recorded in order to study the response of microseismicity as a function of loading rate, amplitude, and frequency of a periodic stress perturbation. The observed AE distributions do not follow the predictions of either a Coulomb failure model, taking into account both constant loading and oscillation‐induced strain rates, or a rate and state model. A susceptibility of the system's AE response to the amplitude of the confinement pressure perturbation is estimated, which highlights a linear relation between confinement pressure amplitude and the AE response amplitude, observations which agree with recent higher frequency experimental results on dynamic triggering. The magnitude‐frequency distribution of AEs is also computed. The Gutenberg‐Richter b‐value oscillates with stress oscillations. Our experiments may help complement our understanding of the influence of low inertia stress phenomena on the distribution of seismicity, such as observations of dynamic triggering and seismicity modulation by tides or hydrological loading.

Formation of HFSE-Rich Glimmerites by Silicate Liquid-Liquid Immiscibility, Suzhou Pluton, Eastern China

Abstract Understanding the formation of high field strength element (HFSE) mineralization remains a challenge. Processes ranging from melt generation to final crystallization have been invoked to play a key role in the enrichment of HFSEs in magmatic-hydrothermal systems, yet the importance of crystal-liquid fractionation, liquid immiscibility, and/or fluid unmixing remains debated. Here, we present results of a study of granites and HFSE-mineralized glimmerites from the granitic Suzhou pluton, eastern China. We provide whole-rock major and trace element compositions, a description of mineral assemblages, and a detailed description of zircon textures, zircon Raman spectra, zircon major and trace element and δ18O compositions, and U-Pb ages. The granites and glimmerites have been dated at ~126 to 124 Ma and are thus coeval. The granites range from biotite and zircon poor to biotite and zircon rich and are in contact with glimmerites. The glimmerites form lenses and layers close to and along the margin of the Suzhou granite, comprising up to ~37 vol % biotite and up to ~2 vol % zircon and other HFSE-rich accessory minerals. The biotite-poor granites contain a single type of zircon (type-A: single crystals, oscillatory zoned, fully crystallized structure, relatively poor in trace elements, and relatively high δ18O), whereas the biotite-rich granites and the glimmerites contain two zircon types (type-A crystals: same features as in the biotite-poor granites; type-B crystals: clustered, unzoned, partially metamict structure, rich in trace elements, and relatively low δ18O). Both granite types are Si, Al, and Na rich, whereas the glimmerites are Fe, Ti, Mn, Mg, Ca, P, F, and HFSE rich and, compositionally, fall off simple fractionation trends. We interpret the textural, mineralogical, and compositional relationships to indicate that the glimmerites are the products of Fe-, F-, and HFSE-rich immiscible melts that unmixed from an alkali-rich, moderately reducing (~QFM + 0.5 and ~QFM + 1.0; QFM = quartz-fayalite-magnetite buffer) Suzhou magma system at low crustal pressure. In addition, we note that the zircon textures and compositions are important recorders of the processes and conditions that led to the HFSE mineralization.

Chemistry in earthquake: the active chemical role of liquid and supercritical waters in microfracturing at depth

The role of water in the nucleation process of an earthquake and its contribution to the mainshock is ascertained by many models in its physical part, as a factor capable of altering the balance of pressures and thus influencing the effective pressure. Assuming that water is present at depth, starting from the observation of its molecular structure at various crustal pressure and temperature conditions, the present paper analyses water’s chemical role in relation with the rock matrix, and its response during microfracturing. The creation of a network of new void spaces produces a decrease of the water pressure. Water may respond at molecular scale differently, depending on its aggregation state. Effectively depressurisation has a limited influence on the liquid water, only if it does not cause the transition to the vapour phase. Conversely, depressurisation causes an instantaneous variation in the intermolecular structure of supercritical water (SCW). Specifically, the nearly total disappearance of its ionic characteristics: that means the severe drop of solubility constants. At the same time, the already low viscosity decreases too: SCW intrudes easily into new fissures. When the microcracks tend to close, SCW reacquires adequate ionic characteristics for the rise in density (isothermal pressurisation); hence, an intense water rock interaction starts with freshly opened surfaces. This process influences actively the subcritical crack growth too, again with differences between liquid and SCW: last one participates only when reacquires density. Summarising, it is likely that water plays a fundamental and active role in determining the rock weakening, once earthquake preparation process begin with the development of microcracks are forming, perhaps playing an active role in determining the main rupture. With different modalities according to its aggregation state.

Building of arc crust during Silurian subduction in the southern Paleo Asian Ocean: Constraint from mineralogy of the pyroxenitic-gabbroic suite in the Chinese East Tianshan

Arc lower crustal cumulates are significant in investigation of arc differentiation in the lower level of an island arc. The Silurian pyroxenitic-gabbroic suite in the Chinese East Tianshan represents a lithological assemblage of the basal to lower arc crust. This suite comprises, in the order of evolution, olivine websterite (ol-websterite), olivine and plagioclase clinopyroxenite (ol- and pl-clinopyroxenite), and amphibole gabbro (amph-gabbro). Pyroxenite are dominated by cumulus clinopyroxene with interstitial olivine. Amphibole is present in all these rocks, occurring either as coronas of pyroxene and olivine or as oikocrysts, implying a H 2 O-rich parental melt. Plagioclase occurs as veinlets cutting cumulus clinopyroxene in the pl-clinopyroxenite, revealing that it crystallizes totally later than clinopyroxene. Spinel ranges from chromite in the ol-websterite and ol-clinopyroxenite to pleonaste in other evolved rocks, revealing an aluminous differentiation trend. Olivine in the pyroxenite ranges in Fo from 64 to 86 mol%, averagely 7–15 Mg # units lower than coexisting clinopyroxene (79–93). Clinopyroxene shows a chemical trend from low Al 2 O 3 (<2 wt%) in the ol-websterite to high Al 2 O 3 (up to 6.5 wt%) in the pl-clinopyroxenite. All clinopyroxene has incompatible trace elements similar to those of clinopyroxene from typical arc lower crustal cumulates. All plagioclase is extremely calcic (An >83 mol%, up to 99 mol%), generally coupled with iron-rich olivine (Fo <76 mol%) in the studied rocks. Such distinct mineral compositions are typical of arc lower crustal cumulates and ascribed to the crystallization sequence of hydrous arc magmas at high pressure. The pressure for the formation of the pyroxenitic rocks is estimated to be 6–8 kbar using the two-pyroxene geobarometer, a preferred pressure for clinopyroxene crystallization coupled with high H 2 O content. In the stability field of clinopyroxene crystallization, olivine crystallizes from the interstitial melt but later than clinopyroxene, having unexpectedly lower Fo than Mg # of clinopyroxene. Meanwhile, plagioclase crystallization is totally suppressed with the residual magmas evolving towards highly aluminous, crystallizing spinel with a pure pleonaste composition. Upon saturation in plagioclase with decreasing pressures, plagioclase will acquire extremely calcic compositions due to the aluminous and hydrous nature of the residual arc magmas. The incompatible trace element compositions of the Silurian primitive magmas, estimated from the most primitive clinopyroxene (Mg # = 92–93), show selective enrichment in large ion lithophile elements relative to high field strength elements with an exception of depletion in Ba and low Nb/Ta ratios, implying a magmatic deviation from metasomatized mantle modified by aqueous fluids dehydrated from rutile-bearing slab. The Silurian arc lower crust was built by crystal fractionation and accumulation from hydrous arc basaltic magmas at 6–8 kbar of the lower crustal pressure. • The Silurian magmatic suite has mineral compositions similar to arc lower crust. • This magmatic suite formed from arc magmas under arc lower crustal pressures. • Mineral compositions suggest formation of arc lower crust by crystal fractionation.

Dynamic pressure variations in the lower crust caused by localized fluid-induced weakening

Metamorphism of dry, lower crust within orogens is often localized in shear zones composed of mechanically weaker rocks1–4. Several field-based studies suggest shear zone development is preceded by brittle faults which provide the fluid pathways required for metamorphism and weakening4–12. However, a unifying model which couples long-term geological deformation to fluid migration and metamorphic reactions does not exist. Here, we present a visco-elasto-plastic model where the most pertinent features observed in transformed lower crust emerge from basic mechanical principles during the deformation of a coherent rock volume with associated fluid introduction. These include a strikingly dynamic and heterogeneous pressure distribution in the reacting and deforming rock volumes. Lower crustal pressure variations may reach 1 GPa at the same depth. This will have first order effects on the pattern of fluid migration in the lower crust, and may explain the apparent discrepancies between the relevant tectonic settings and petrologically-inferred burial depths.

Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

AbstractThe transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where the temperature is likely 35°C–200°C. To test the importance of such elevated temperature on the strength and deformability of sandstone, a series of constant strain rate, triaxial deformation experiments were performed on three different water saturated sandstones at either ambient temperature or 150°C. For each sandstone, an effective pressure range was used which spanned both the brittle and ductile deformation regimes, up to a maximum of 120 MPa. In the brittle regime, we observed a temperature‐dependent lowering of the yield stress of between 8% and 17%. Within the ductile regime, we observed an even greater reduction in the yield stress of between 9% and 37%. A further notable observation is that the transition from dilatant, brittle behavior to compactive, ductile behavior tends to occur at a lower effective pressure at elevated temperature. The weakening observed at elevated temperature can be explained by a reduction in fracture toughness, which is shown mathematically to cause greater weakening in the ductile regime than in the brittle regime. The apparent reduction in fracture toughness at elevated temperature is potentially driven by a combination of a reduction in surface energy and, to a minor extent, an increase in subcritical crack growth rate.

Experimental evaluation of a new H2O-independent thermometer based on olivine-melt Ni partitioning at crustal pressure

Abstract An olivine-melt thermometer based on the partitioning of Ni (DNiOl/liq) was hypothesized by Pu et al. (2017) to have a negligible dependence on dissolved water in the melt (and pressure variations from 0–1 GPa), in marked contrast to thermometers based on DMgOl/liq. In this study, 15 olivine-melt equilibrium experiments were conducted on a basaltic glass starting material (9.6 wt% MgO; 353 ppm Ni) to test this hypothesis by comparing the effect of dissolved H2O in the melt on DMgOl/liq and DNiOl/liq on the same set of experiments. Results are presented for six anhydrous experiments at 1 bar, two anhydrous experiments at 0.5 GPa, and seven hydrous experiments at 0.5 GPa. Analyzed olivine and glass compositions in the quenched run products were used to calculate DMgOl/liq and DNiOl/liq values for each experiment, which in turn permit temperature to be calculated with the Mg- and Ni-thermometers calibrated in Pu et al. (2017) on anhydrous, 1-bar experiments from the literature. The Ni-thermometer recovers the temperatures of all 15 experiments from this study with an average deviation of –3 °C, including those with up to 4.3 wt% H2O dissolved in the melt. In contrast, the Mg-thermometer recovers the anhydrous, 1-bar experimental temperatures within +14 °C on average, but overestimates the hydrous experimental temperatures by +49 to +127 °C, with an average of +83 °C. When the Mg-thermometer of Putirka et al. (2007) is applied, which includes a correction for analyzed H2O (≤4.3 wt%) in the quenched melts of the run products, all experimental temperatures are recovered with an average (±1σ) deviation of +7 °C. The combined results show that DNiOl/liq has a negligible dependence on dissolved water in the melt (≤4.3 wt% H2O), which is in marked contrast to the strong dependence of DMgOl/liq on water in the melt. An understanding of why DNiOl/liq is insensitive to dissolved water, unlike DMgOl/liq, is obtained from spectroscopic evidence in the literature, which shows that Ni2+ (transition metal) and Mg2+ (alkaline earth metal) have distinctly different average coordination numbers (predominantly fourfold and sixfold, respectively) in silicate melts and that fourfold-coordinated Ni2+ is unaffected by the presence of dissolved water in the melt. This difference in coordination number explains why DNiOl/liq and DMgOl/liq each have a different dependence on pressure, anhydrous melt composition, and melt water content. Application of the Ni-thermometer of Pu et al. (2017) to five natural samples from the Mexican arc, for which H2O contents (3.6–6.7 wt%) in olivine-hosted melt inclusions are reported in the literature, leads to temperatures that match those obtained from the Putirka et al. (2007) Mg-thermometer that corrects for analyzed H2O contents. This study demonstrates that a thermometer based on DNiOl/liq can be applied to hydrous basalts at crustal depths without the need to correct for dissolved water content or pressure.

Study on the 2D Optimization Simulation of Complex Five-Hole Cutting Blasting under Different Lateral Pressure Coefficients

With the decrease and depletion of shallow coal resources, the depth of mining is increasing. The mechanism of high crustal stress blasting is not clear, and the effect of crustal stress on blasting effect is obvious. The law of the differential detonation is similar without crustal stress. The crustal stress of rock masses increases linearly with the increase in excavation depth, and the influence of crustal stress on blasting effect is great. In order to study the rock-breaking process of complex differential blasting under deep high crustal stress, the instantaneous detonation of each model and the priority detonation of the central hole are numerically simulated. The evolution law of the blasting crack and the method of optimizing blasting effect by differential blasting and increasing the aperture of the priority detonation hole under high crustal stress are put forward. The authors proposed a study on the 2D optimization simulation of complex five-hole cutting blasting under different lateral pressure coefficients of 400 m and 800 m with software ANSYS/LS-DYNA and analyzed the evolution rules of blasting cracks and lateral pressure coefficients. The results show that setting delayed detonation and hole diameter and optimizing blasthole spacing can optimize the rock-breaking effect under high crustal stress and different lateral pressure coefficients.

A novel early-warning method for atypical outbursts disasters in mines: extraction of indexes from gas concentration data for the early warning of atypical outbursts

Accurate identification and forecasting of typical outbursts in mines remain scientific problems needed to be tackled. Based on the related data of the occurrence of atypical outbursts, this study employed theoretical analysis, laboratory test, mathematical treatment, and commercial test for investigating the physical and mechanical properties of coal that resulted in an atypical outburst, the quantitative conditions of the occurrence of atypical outbursts and the extraction of indexes from gas concentration data for early warning. Results show that the physical and mechanical properties of the coal rock where an outburst occurred fell in between the properties of the coal rock resulting in a rock burst and the properties of the coal rock resulting in coal and gas outburst, and an atypical outburst was easily triggered under high crustal and high gas pressure environment; gas extraction leads to the enhancement of coal rock mass brittleness and bump proneness. Based on specific physical and mechanical characteristics of the coal that results in an atypical outburst, the related early-warning information in gas concentration data that can reflect the evolution process of atypical outburst disasters were mathematically extracted, and the mathematical model of early-warning indexes was established.

Decoupled Ca and Fe + Mg content of S-type granites: An investigation on the factors that control the Ca budget of S-type granites

The compositional variation of S-type granites is thought to derive from source-controlled processes whereby magmas acquire their composition as a result of entrainment of variable proportions of a peritectic-rich residual mineral assemblage into the melt. A corollary of this model is that the entrained phases keep the stoichiometric proportions they have in the partially melted source. These proportions are controlled by the composition of the subsolidus minerals. Consequently, it is expected that the ratio between maficity (FeOt + MgO) and other compatible major elements in granites would be the same as those of the source. This is true for Ti but not for Ca which shows systematically higher Ca:maficity ratios. In order to explore the factors that cause this discrepancy, thermodynamic calculations were used to study the partial melting of a representative metapelite and a metagreywacke under a range of crustal pressure and temperature conditions. The modelling included segregation of melt or magma in pulses, disequilibrium melting of plagioclase (in which only a fraction of this phase participates in the melting system and produces two populations of plagioclase, the unreacted cores with the original, Na-rich composition and a second one with a Ca-rich composition) and the entrainment of up to 40 wt% minerals into the melt. This entrained assemblage consists of two fractions, one containing the minerals that are most likely to be surrounded by melt in the source and representing 80 wt% of the total assemblage, and another one consisting of the phases that are most likely to make up the solid framework, with the minerals within each fraction keeping stoichiometric proportions. The modelled magmas failed to reproduce the observed high Ca:maficity ratios. This indicates that there are other processes that increase the Ca content of granites. Several processes were evaluated, including the presence of apatite, enhanced availability of subsolidus plagioclase in the melting system, disequilibrium Ca-rich melts and enhanced entrainment of Ca-rich plagioclase. The last three processes were found to increase the Ca content but only enhanced entrainment of Ca-rich plagioclase increases the Ca:maficity ratio to values similar to those of the granites. A possible explanation for the preferential entrainment of Ca-rich plagioclase is that the density of this phase is different to that of garnet and ilmenite and similar to the density of the melt. Consequently, a significant fraction of the plagioclase in granites may actually be of residual origin. Other relevant findings of this study are that metagreywackes seem to be the main source of S-type granites, as previous investigations suggested, and that cordierite-rich granitoids represent mineral-rich magmas that are formed only from metapelites at pressures below 0.75 GPa. This investigation emphasizes the importance of source-controlled processes in the composition of S-type granites and indicates that entrainment of a mineral assemblage into the melt is not a simple and straightforward process as previously thought.

Ar-Ar dating and petrogenesis of the Early Miocene Taşkapı-Mecitli (Erciş-Van) granitoid, Eastern Anatolia Collisional Zone, Turkey

The Early Miocene Taşkapı-Mecitli granitoid that is located in the northern section of the Eastern Anatolia Collision Zone has typical I-type, metaluminous and calk-alkaline characteristics. It also contains mafic microgranular / magmatic enclaves (MMEs). New Ar-Ar dating results show that the age of the Taşkapı-Mecitli granitoid is ∼23 Ma and it crystallised in the Early Miocene, in contrast to its previously known Cretaceous age. Identical crystallisation ages (∼23 Ma), similar mineral assemblages and geochemical compositions, and indistinguishable isotopic compositions of MMEs and host rocks imply that the MMEs are most consistent with a cumulate origin formed at earlier stages of the same magmatic system that produced the Taşkapı-Mecitli granitoid.MELTS modelling suggests that magma of the Taşkapı-Mecitli granitoid was the result of fractionation under a crustal pressure of 4 kbar, with a H2O content of 1.5%. EC-AFC model calculation reveals that the Taşkapı-Mecitli granitoid includes from 0.5% to 2% crustal assimilation rates. These rates indicate that crustal contamination can be negligible when compared to fractional crystallisation in the evolution of the magma beneath the Taşkapı-Mecitli granitoid. The partial melting model calculations and MORB-normalised trace element concentrations of the least evolved samples of the Taşkapı-Mecitli granitoid are consistent with those of mafic melts obtained from partial melting of interacting mantle- lower crust with a melting degree of 18%. The age (23 Ma) of the post- or syn-collisional Taşkapı-Mecitli granitoid suggests that the collision between Arabian and Eurasian plates could be before/around ∼23 Ma (Late Oligocene to Early Miocene).

Experimental Investigation on Permeability and Mechanical Deformation of Coal Containing Gas under Load

Coalbed effective permeability is widely used as a primary index to evaluate gas‐drainage effect in CBM exploitation field. However, it seems to be difficult to obtain by the reason of dynamic change in close relationship with crustal stress, methane pressure, porosity, and adsorption. Due to their dissimilar adsorption properties and tectonic deformation degrees, different types of coal containing gas have various stress‐strain and gas seepage curves. The paper presents the experimental investigations of the dynamic relationship between coal permeability and deformation under load. In this work, stress‐strain and permeability investigations were performed using anthracite lump with a vitrinite reflectance of about 3.24% at various pressures and temperatures. The permeability (including the initial, minimum, and maximum) decreased with increasing temperature. At a constant confining pressure, the strains in different directions almost all increased with increasing axial stress and decreased with increasing pore methane pressure during the prefracture stage. At a constant pore pressure, the compression strength of the coal specimens increased approximately linearly during the prefracture stage and sharply decreased during the postfracture stage, while the permeability decreased rapidly and then increased slowly during the prefracture and remained stable during the postfracture stage. The permeability of the coal specimens mainly depended on the inner fissures. The permeability was greater during the postfracture than that during the prefracture stage. At the same temperature, the gas seepage curve of each coal specimen could be divided into three sections: decreasing, increasing, and constant sections. The necessary time for the permeability to reach a steady state increased as the confining and pore pressures increased. At high confining pressures (i.e., 6 MPa and 8 MPa), no significant differences between the methane seepage velocities of the specimens were evident, and their seepage curves were similar to prefracture. However, clear differences were observable at the postfracture stage. The seepage abilities of the coal specimens were more sensitive to stress than temperature in the same condition.

Amphibole and apatite insights into the evolution and mass balance of Cl and S in magmas associated with porphyry copper deposits

Chlorine and sulfur are of paramount importance for supporting the transport and deposition of ore metals at magmatic–hydrothermal systems such as the Coroccohuayco Fe–Cu–Au porphyry–skarn deposit, Peru. Here, we used recent partitioning models to determine the Cl and S concentration of the melts from the Coroccohuayco magmatic suite using apatite and amphibole chemical analyses. The pre-mineralization gabbrodiorite complex hosts S-poor apatite, while the syn- and post-ore dacitic porphyries host S-rich apatite. Our apatite data on the Coroccohuayco magmatic suite are consistent with an increasing oxygen fugacity (from the gabbrodiorite complex to the porphyries) causing the dominant sulfur species to shift from S2− to S6+ at upper crustal pressure where the magmas were emplaced. We suggest that this change in sulfur speciation could have favored S degassing, rather than its sequestration in magmatic sulfides. Using available partitioning models for apatite from the porphyries, pre-degassing S melt concentration was 20–200 ppm. Estimates of absolute magmatic Cl concentrations using amphibole and apatite gave highly contrasting results. Cl melt concentrations obtained from apatite (0.60 wt% for the gabbrodiorite complex; 0.2–0.3 wt% for the porphyries) seems much more reasonable than those obtained from amphibole which are very low (0.37 wt% for the gabbrodiorite complex; 0.10 wt% for the porphyries). In turn, relative variations of the Cl melt concentrations obtained from amphibole during magma cooling are compatible with previous petrological constraints on the Coroccohuayco magmatic suite. This confirms that the gabbrodioritic magma was initially fluid undersaturated upon emplacement, and that magmatic fluid exsolution of the gabbrodiorite and the pluton rooting the porphyry stocks and dikes were emplaced and degassed at 100–200 MPa. Finally, mass balance constraints on S, Cu and Cl were used to estimate the minimum volume of magma required to form the Coroccohuayco deposit. These three estimates are remarkably consistent among each other (ca. 100 km3) and suggest that the Cl melt concentration is at least as critical as that of Cu and S to form an economic mineralization.

Hydrous albite magmas at lower crustal pressure: new results on liquidus H2O content, solubility, and H2O activity in the system NaAlSi3O8–H2O–NaCl at 1.0 GPa

The system albite–H2O serves as an important model for the generation of granitic magmas, yet relatively few experimental investigations have focused on phase relations at high pressure. This study reports new experimental results, at 1.0 GPa and 690–1050 °C, on the temperature and liquid composition at vapor-saturated melting, the H2O content of undersaturated silicate liquids in equilibrium with albite, the solubility of albite in H2O–NaCl fluids immediately below the solidus, and the activity of H2O in hydrous NaAlSi3O8 liquids along the liquidus. Albite melts and dissolves congruently at all temperatures and salinities. In the NaCl-absent system, the temperature of vapor-saturated melting of low albite, confirmed by X-ray diffraction, is 695 ± 5 °C and the liquid composition is 18.14 ± 1.35 wt% H2O. The temperature dependence of the fluid-undersaturated liquidus curve in the system NaAlSi3O8–H2O varies with H2O wt% (\(w_{{{\text{H}}_{2} {\text{O}}}}\)) according to $$T = - 2.0331 \times 10^{ - 3} w_{{{\text{H}}_{2} {\text{O}}}}^{3} + 1.6497w_{{{\text{H}}_{2} {\text{O}}}}^{2} {-} 58.963w_{{{\text{H}}_{2} {\text{O}}}} + 1235.5\,^{ \circ } {\text{C}}$$ indicating positive curvature in temperature-composition coordinates and a dry melting temperature of ~1235 °C. At 690 °C, immediately below the solidus, albite solubility decreases drastically with NaCl content of the fluid phase, from 8.8 ± 0.6 wt% in the NaCl-free fluid to ˂2 % at NaCl concentration of only 10 mol%. Experiments determining the activity of H2O (\(a_{{{\text{H}}_{2} {\text{O}}}}\)) in liquids at vapor-saturated melting exploited low Cl solubility in liquids and low albite solubility in the presence of H2O–NaCl fluids. The maximum Cl content of quenched glasses, only 0.95 wt%, and very low albite solubility together make possible H2O activity measurement in melts equilibrated with NaCl–H2O solutions. When combined with activity data for H2O–NaCl fluids, experimentally determined \(a_{{{\text{H}}_{2} {\text{O}}}}\) along the liquidus is described by $$T = - {469.16a_{{H_{2} O}}^{L}}^{{\frac{1}{2}}} {-} 93.382a_{{H_{2} O}}^{L} + 1235.5\;^\circ C.$$ At 1 GPa, H2O activities in hydrous albitic melts in H2O–NaCl fluids agree with those in the presence of H2O–CO2 fluids, provided appropriate fluid mixing data are used. Our results constrain an asymmetric regular solution model for the mixing of Na1/8Al1/8Si3/8O and H2O, yielding Margules parameters of W Ab = 25.56 ± 0.54 and \(W_{{{\text{H}}_{2} {\text{O}}}} = \, 10.50\; \pm \;0.74{\text{ kJ}}/{\text{mol}}\). The results imply critical mixing of liquid and vapor at ~1100 °C. Using existing speciation models, our results imply that H2O dissolved in hydrous albitic liquids is almost entirely in the form of molecular H2O (OH− concentration is negligible) at temperatures near vapor-saturated melting. The results provide a more complete experimental foundation for modeling the system NaAlSi3O8–H2O at high pressure.

Straddling the tholeiitic/calc-alkaline transition: the effects of modest amounts of water on magmatic differentiation at Newberry Volcano, Oregon

Melting experiments have been performed at 1 bar (anhydrous) and 1- and 2-kbar H2O-saturated conditions to study the effect of water on the differentiation of a basaltic andesite. The starting material was a mafic pumice from the compositionally zoned tuff deposited during the ~75 ka caldera-forming eruption of Newberry Volcano, a rear-arc volcanic center in the central Oregon Cascades. Pumices in the tuff of Newberry caldera (TNC) span a continuous silica range from 53 to 74 wt% and feature an unusually high-Na2O content of 6.5 wt% at 67 wt% SiO2. This wide range of magmatic compositions erupted in a single event makes the TNC an excellent natural laboratory in which to study the conditions of magmatic differentiation. Our experimental results and mineral–melt hygrometers/thermometers yield similar estimates of pre-eruptive H2O contents and temperatures of the TNC liquids. The most primitive (mafic) basaltic andesites record a pre-eruptive H2O content of 1.5 wt% and a liquidus temperature of 1,060–1,070 °C at upper crustal pressure. This modest H2O content produces a distinctive fractionation trend that is much more enriched in Na, Fe, and Ti than the calc-alkaline trend typical of wetter arc magmas, but slightly less enriched in Fe and Ti than the tholeiitic trend of dry magmas. Modest H2O contents might be expected at Newberry Volcano given its location in the Cascade rear arc, and the same fractionation trend is also observed in the rim andesites of the rear-arc Medicine Lake volcano in the southern Cascades. However, the Na–Fe–Ti enrichment characteristic of modest H2O (1–2 wt%) is also observed to the west of Newberry in magmas erupted from the arc axis, such as the Shevlin Park Tuff and several lava flows from the Three Sisters. This shows that modest-H2O magmas are being generated directly beneath the arc axis as well as in the rear arc. Because liquid lines of descent are particularly sensitive to water content in the range of 0–3 wt% H2O, they provide a quantitative and reliable tool for precisely determining pre-eruptive H2O content using major-element data from pumices or lava flows. Coupled enrichment in Na, Fe, and Ti relative to the calc-alkaline trend is a general feature of fractional crystallization in the presence of modest amounts of H2O, which may be used to look for “damp” fractionation sequences elsewhere.

Constraints on the mobilization of Zr in magmatic-hydrothermal processes in subduction zones from in situ fluid-melt partitioning experiments

The partitioning of Zr between high P-T aqueous fluids and melts has been investigated in situ in the haplogranite-H2O and haplogranite-(F)-H2O systems to assess the mobilization of high field strength elements (HFSE) in magmatic-hydrothermal processes in subduction zones. The partition coefficients Df/Zmr were determined from Zr concentrations measured in situ by synchrotron X-ray fluorescence (SXRF) in both aqueous fluids and F-free or F-bearing hydrous haplogranite melts equilibrated in diamond-anvil cells at 575 to 800 °C and 0.3 to 2.4 GPa. This experimental approach eliminates the need for internal or external calibrations of the SXRF signal and/or post-mortem analysis of the melt phase, hence decreasing the total uncertainties on Df/Zmr below 16%. Above 0.6 GPa, Zr partitions favorably into the hydrous silicate melt in both F-free and F-bearing systems, with Df/Zmr that range between 0.19 ± 0.02 and 0.38 ± 0.03. However, the relatively high Df/Zmr values indicate that alkali-silica rich aqueous fluids generated by metamorphic devolatilization may contribute significantly to the recycling of HFSE in subduction zones. The efficient uptake of Zr (and likely other HFSE) by subduction zone fluids, regardless of their nature (aqueous fluid, hydrous melt, or supercritical fluid), supports the idea that the typical HFSE depletion recorded in arc magmas does not result from their incompatibility in water-rich slab-derived fluids but most probably originates from complex fluid-melt-rock interactions occurring at the slab interface and within the mantle wedge. At shallow crustal pressure conditions (800 °C and 0.3 GPa), Zr partitions reversely into the aqueous fluid in the presence of fluorine (Df/Zmr = 1.40 ± 0.10) as observed for Nb at similar conditions by Webster et al. (1989). The enrichment of the aqueous phase in HFSE (Zr, Nb) at shallow crustal conditions is likely related to the enhanced peralkalinity of low pressure, F-bearing aqueous fluid with temperature, that provides the favorable conditions for their mobilization via the formation of HFSE-O-Si/Na clusters. This mechanism may control the enrichment in HFSE (and plausibly other rare metals such as REE) in early magmatic fluids exsolved from granitic melts, leading to the formation of HFSE-enriched aggregates in shallow magmatic-hydrothermal environments (e.g., Strange Lake and Thor Lake Nechalacho deposit, Canada; Galineiro complex, Spain).

Aluminum speciation in aqueous fluids at deep crustal pressure and temperature

We investigated aluminum speciation in aqueous fluids in equilibrium with corundum using in situ Raman spectroscopy in hydrothermal diamond anvil cells to 20kbar and 1000°C. We have studied aluminum species in (a) pure H2O, (b) 5.3m KOH solution, and (c) 1m KOH solution. In order to better understand the spectral features of the aqueous fluids, we used ab initio simulations based on density functional theory to calculate and predict the energetics and vibrational spectra for various aluminum species that are likely to be present in aqueous solutions. The Raman spectra of pure water in equilibrium with Al2O3 are devoid of any characteristic spectral features. In contrast, aqueous fluids with 5.3m and 1m KOH solution in equilibrium with Al2O3 show a sharp band at ∼620cm−1 which could be attributed to the [Al(OH)4]1- species. The band grows in intensity with temperature along an isochore. A shoulder on the high-frequency side of this band may be due to a hydrated, charge neutral Al(OH)3·H2O species. In the limited pressure, temperature and density explored in the present study, we do not find any evidence for the polymerization of the [Al(OH)4]1- species to dimers [(OH)2-Al-(OH)2-Al(OH)2] or [(OH)3-Al-O-Al(OH)3]2−. This is likely due to the relatively low concentration of Al in the solutions and does not rule out significant polymerization at higher pressures and temperatures. Upon cooling of Al-bearing solutions to room temperatures, Raman bands indicating the precipitation of diaspore (AlOOH) were observed in some experiments. The Raman spectra of the KOH solutions (with or without dissolved alumina) showed a sharp OH stretching band at ∼3614cm−1 and an in-plane OH bending vibration at ∼1068cm−1, likely related to an OH− ion with the oxygen atom attached to a water molecule by hydrogen bonding. A weak feature at ∼935cm−1 may be related to the out-of-plane bending vibration of the same species or to an OH species with a different environment.

Origin of wehrlite cumulates in the Moho transition zone of the Neoproterozoic Ras Salatit ophiolite, Central Eastern Desert, Egypt: crustal wehrlites with typical mantle characteristics

Ultramafic cumulates, mainly crustal true wehrlites, were discovered and described in the mantle–crust transition zone (MTZ) and the extremely lower layered gabbro sequence of the Ras Salatit ophiolite, Central Eastern Desert, Egypt. They form either boudinaged lensoidal tabular bodies or interdigitated layers often concordant with the planolinear fabrics of the Ras Salatit ophiolite rocks. The contact between wehrlites and the host MTZ dunite or layered gabbro is razor sharp, lobate and/or sinuous, without chilled margins or any visible deformations. The Ras Salatit wehrlites are orthopyroxene-free and composed mainly of olivine and clinopyroxene. They are texturally equilibrated and show a characteristic poikilitic texture. Crystallization order of the Ras Salatit wehrlites is olivine/spinel followed by clinopyroxene with the absence of plagioclase. Olivine and clinopyroxene of the Ras Salatit wehrlites are compositionally uniform and conspicuously high in Mg#, mostly around 0.93 and 0.92, respectively. Moreover, the clinopyroxene shows low Ti and Al contents coupled with marked depletion in LILE. The calculated melt in equilibrium with clinopyroxene from the Ras Salatit wehrlites is largely similar to lavas from the Izu-Bonin forearc. Given the above characteristics, the Ras Salatit wehrlites were produced by crystal accumulation from a hydrous depleted basaltic/tholeiitic melt corresponding to temperatures between 1,000 and 1,100°C at the oceanic crustal pressure (~2 kbar). The involved hydrous tholeiitic melt has been probably formed by fluid-assisted partial melting of a refractory mantle source (similar to the underlying harzburgites) in a somewhat shallow sub-arc environment.

Long term surface deformation of Soufrière Hills Volcano, Montserrat from GPS geodesy: Inferences from simple elastic inverse models

Campaign and continuous GPS geodetic measurements on Soufrière Hills Volcano, Montserrat are reported from 1995 to 2009, spanning three dome growth and repose episodes. Uniform elastic half‐space inversions were used to examine how crustal pressure sources evolved by inverting subsets of all available 3D site data for any given episode using a single Mogi‐source. Changes in network topology were also examined. The average best‐fitting single Mogi model yields X = 0.3 ± 0.5 km, Y = 0.8 ± 0.4 km, and a depth of Z = 10.4 ± 2.1 km (1‐σ, Z positive down), relative to the center of the model domain. The mean Mogi depth for effusive (deflation) versus repose (inflation) is different and significant at &gt;90% confidence, yielding Z = 11.4 ± 2.0 km and 9.3 ± 1.6 km, respectively. A vertical, prolate ellipsoid improves the fit at &gt;95% confidence over a single Mogi source or stacked, two‐source model for the 2003–2005 repose and 2005–2007 dome growth episodes.

Dual reservoir structure at Soufrière Hills Volcano inferred from continuous GPS observations and heterogeneous elastic modeling

Most inversions of geodetic data for volcanic systems assume homogeneous media. Here we examine the effect of inhomogeneous media using axisymmetric finite element models of Soufrière Hills Volcano (SHV), to show that homogeneous elastic crustal models underestimate both the depth and excess pressure of the deformation source. Continuous GPS data from 1999 to 2009 are then used to infer that the SHV system has multiple crustal pressure sources. For a dual reservoir volcanic model, a deep reservoir located within the mid crust undergoes volume changes approximately an order of magnitude larger than those of the shallow reservoir.