Composition Of Aqueous Fluid Research Articles

Thermally induced release and generation of CO2 and C1–C4 hydrocarbons in Opalinus Clay from Mont Terri (Switzerland)

Claystone formations are candidate host rocks for high-level heat-emitting nuclear waste (HLW). Temperatures from 90 to 150 °C at the canister surface are discussed internationally as potential emplacement and storage conditions. The thermal energy emitted from waste containers will be transported into the host rock formation, accelerating chemical reactions including the release of sorbed and dissolved gases and the generation of new gases. This study investigated gas release and generation in Opalinus Clay from Mont Terri (Switzerland) at elevated temperature and pressure conditions relevant for HLW storage and beyond. Hydrous pyrolysis experiments were conducted in Dickson-type flexible gold-titanium reaction cells and gold capsules in the temperature range of 80–345 °C and at 20 MPa. CO2(g) was the predominant product, followed by C1–C4 hydrocarbons, which decrease in abundance with increasing carbon atom number. Neither CO nor H2S was detected. H2 was generated only in high temperature experiments at 315 °C and 345 °C, respectively. A combination of CO2(g) quantification, stable carbon isotopic composition data, thermodynamic calculations and aqueous fluid composition (dissolved ions, pH) demonstrated that ≥80 % of the measured CO2(g) originated from carbonate mineral dissolution. The model calculations also suggest that the fraction of CO2(aq) in DIC increases from ∼50 % at 80 °C to nearly 100 % at higher temperatures. Thermal transformation of organic matter represented an additional source for CO2(g) and was the predominant process yielding the C1–C4 hydrocarbons. Our findings stress the importance of quantitative geochemical data for the safety assessment of potential host rocks for HLW storage. We demonstrated that two sources are involved in gas release and generation at temperatures relevant for HLW storage, e.g., in the Opalinus Clay – organic matter and carbonate minerals. Our data will contribute to numerical modelling studies and the refinement of feature, events, and processes (FEP) catalogues.

Multiple sulfur isotopes evidence deep intra-slab transport of sulfate-rich fluids

Sub-arc mantle oxidation is often attributed to sulfates from seawater transported in aqueous fluids derived from slab dehydration, but sulfur speciation and origin(s) in high-pressure fluids remain weakly constrained. Here, we use the four sulfur stable isotopes (32S, 33S, 34S, 36S) to decipher processes, e.g. mixing of two end-member reservoirs and open-system reaction occurring during sulfur devolatilization, and deduce the nature and origin of sulfur species in aqueous slab-derived fluids. We analyzed high pressure metamorphic rocks from the Monviso massif Lower Shear Zone (LSZ; Western Alps), recording evidence of an interface-parallel, channelized fluid flow near peak burial metamorphic conditions (∼550 °C and 2.7 GPa). We sampled variably metasomatized Fe-Ti metagabbros, hybrid (talc) schists and serpentinites containing sulfides supposed to preserve the aqueous fluid composition, as well as other alpine metagabbro, metabasalt and serpentinites from greenschist to eclogite-facies devoid of the studied metasomatic overprint.The millimeter to centimeter-sized sulfide crystals found in abundance within LSZ lithologies are mainly pyrites containing inclusions of the peak burial mineral assemblage (e.g. garnet, omphacite, rutile) indicating their crystallization at peak conditions or during incipient exhumation. The sulfides in the non-metasomatic metagabbros and metabasalts cover a narrow range in δ’34S of 0.89 ± 0.63 ‰ and Δ’33S of 0.017 ± 0.01 (n = 6) that could be inherited from hydrothermal sulfides formed on the seafloor. In contrast, the LSZ sulfides from the Fe-Ti metagabbros, serpentinites and hybrid schists (n = 24) are enriched in 34S and 33S (bulk δ’34S from 4.91 to 21.32 ‰ and Δ’33S from 0.02 to 0.06 ‰) compared to the sulfides from the non-metasomatic rocks. Mixing between reduced sulfur species cannot explain all the data without requiring an end-member composition as high as δ34S ∼ 22 ‰ and Δ33S ∼ 0.06 ‰, i.e. values rarely recorded by sulfides in the literature. Instead, we propose that this 33S-34S-enrichment results from open-system sulfate reduction to sulfides in aqueous fluids at 550°C, with an initial δ34S ∼ 12 ± 3 ‰ and Δ33S ∼ 0.025 ± 0.010 ‰. Because this signature is different from seawater sulfate, we suggest that sulfate was produced by hydrothermal sulfide oxidation during antigorite breakdown at 650°C, in agreement with Mg, Ni and Cr enrichments in the metasomatized HP mafic lithologies. This model is consistent with relatively oxidized 34S-enriched arc lavas and can potentially explain recycled sulfur with positive δ34S signature recorded by ocean island basalts.

The state of Zr and Hf in chloride hydrothermal fluids from in situ X-ray absorption spectroscopy

In order to quantify the hydrothermal mobility of Zr and Hf in chloride fluids we performed an investigation of the speciation of these metals by means of in situ X-ray absorption spectroscopy (XAS). The experiments included registration of Zr K-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS), and Hf L1-edge (XANES) and Hf L3-edge (EXAFS) spectra. The capillary method, when the experimental solution together with baddeleyite ZrO2(cr) or HfO2(cr) is sealed inside a silica glass capillary, was used. The spectra were recorded between 320 °C/600 bar and 480 °C/2200 bar (Zr) and at 370 °C/2100 bar (Hf) after the stationary solubility of the solid phase was attained. The aqueous fluid composition varied from 3.8 m HCl to 3.5 m HCl/3.2 m CsCl and 3.3 m HCl/2.2 m NaCl (Zr), and was set to 7.7 m HCl in the case of Hf. The composition of the dominant complexes was determined by means of EXAFS fitting and theoretical XANES modeling as ZrCl3(OH)2−/ ZrCl5(H2O)− and HfCl4OH−/ HfCl5(H2O)−. In Zr complexes the metal-to-ligand distances varied between 2.10 ± 0.06 Å - 2.16 ± 0.06 Å (OH−), 2.22 ± 0.03 Å - 2.32 ± 0.07 Å (OH2), and 2.46 ± 0.02 Å - 2.49 ± 0.02 Å (Cl−) depending on the fluid PT-compositional parameters. In Hf-bearing species the average HfO distance is 2.07 ± 0.12 Å, and the HfCl distance is 2.43 ± 0.03 Å. Both the distances are slightly shorter compared to the interatomic distances in Zr complexes. Higher number of Cl ligands and shorter metal-to-ligand distance determined for Hf complexes mean higher affinity of Hf to oxygen-bearing ligands and chloride ion, which can result in separation of Zr and Hf in hydrothermal processes. Reactions of the dissolution of Zr- and Hf-bearing minerals imply that both ionic associate HCl(aq) and ion Cl− are necessary for the formation of the negatively charged complexes. The region, that best satisfy these conditions, is located between ca. 300 and 450 °C. At higher temperature the neutral Zr/Hf-OH-Cl complexes can support the hydrothermal mobility of these metals. The complexation mechanisms determined for Zr and Hf demonstrate that deep chloride fluids can play an important role in mobilization and separation of these metals in subduction zones and in the fluid-rock interaction processes.

Influence of divalent ion concentration on the adhesion behaviour of sulfonate self-assembled monolayers (SAM)

Surfactant-assisted oil recovery is usually employed for the production of residual oil after primary and secondary recovery techniques have been exhausted. The loss of injected surfactants via adsorption on to the porous media surfaces impede the efficiency of the procedure and greatly impact process economics. To address this issue, the choice of surfactant and aqueous fluid composition are varied. In particular, the divalent ion concentration is a property that potentially influences surfactant adsorption significantly. It is this aspect which has been addressed in this study. In this study, atomic force microscope (AFM). tips were functionalised with a self-assembled monolayer (SAM) of the alkanethiol, sodium 11-mercaptoundecanesulfonate (otherwise known as MUS) with the –SO3-headgroups oriented outwards towards the aqueous solutions, this is a model for the well-studied surfactant, sodium dodecyl sulfate, SDS. The adhesion forces between these “surfactant-functionalised” tips and Bandera Brown sandstone surfaces were measured in brine solutions of varying CaCl2 concentrations. Observations deviate from previous studies where adsorption of surfactant headgroups in the bulk solution is enhanced via Ca2+ bridging with the surface. Rather, measured adhesion forces decreased with increase in CaCl2 concentration. The results are reversible and also occurs in a reducing environment when the iron in the Bandera brown is reduced for iron (III) to iron (II). The observed behaviour is interpreted in terms of Ca2+ preferentially bridging between neighbouring headgroups on the tip due to the dense packing of molecules non-representative of respective bulk behaviours and acting as a barrier to adhesive contact. Thus, this supposed micellization at the tip aided by the formation of Ca2+ salt bridges between alternate headgroups more closely mimics a surfactant micelle, rather than dispersed individual monomers. The findings of this study provide insight into nanoscale anionic surfactant interactions in general and within sandstone reservoirs in particular where surfactant adsorption is driven largely by monomers rather than micelles. Investigating surfactant adsorption using microscopic techniques such as AFM allow for the determination of interactions at mineral-fluid interfaces and constitute an effective screening methodology for ascertaining enhanced oil recovery process feasibility.

A systematic assessment of the diamond trap method for measuring fluid compositions in high-pressure experiments

Abstract A variety of experimental techniques have been proposed to measure the composition of aqueous fluids in high-pressure experiments. In particular, the “diamond trap method,” where the fluid is sampled in the pore space of diamond powder and analyzed by laser-ablation ICP-MS after the experiment, has become a popular tool. Here, we carried out several tests to assess the reliability of this method. (1) We prepared several capsules loaded with fluid of known composition and analyzed the fluid by laser-ablation ICP-MS, either (a) after drying the diamond trap at ambient condition; (b) after freezing and subsequent freeze-drying; and (c) after freezing and by analyzing a frozen state. Of these methods, the analysis in the frozen state (c) was most accurate, while the results from the other two methods were poorly reproducible, and the averages sometimes deviated from the expected composition by more than a factor of 2. (2) We tested the reliability of the diamond trap method by using it to measure mineral solubilities in some well-studied systems at high pressure and high temperature in piston-cylinder runs. In the systems quartz-H2O, forsterite-enstatite-H2O, and albite-H2O, the results from analyzing the diamond trap in a frozen state by laser-ablation ICP-MS generally agreed well with the expected compositions according to literature data. However, in the systems corundum-H2O and rutile-H2O, the data from the analysis of the diamond trap were poorly reproducible and appeared to indicate much higher solubilities than expected. We attribute this not to some unreliability of the analytical method, but instead to the fact that in these systems, minor temperature gradients along the capsule may induce the dissolution and re-precipitation of material during the run, which causes a contamination of the diamond trap by solid phases. (3) We carried out several tests on the reliability of the diamond trap to measure fluid compositions and trace element partition coefficients in the eclogite-fluid system at 4 GPa and 800 °C using piston-cylinder experiments. The good agreement between “forward” and “reversed” experiments—with trace elements initially either doped in the solid starting material or the fluid—as well as the independence of partition coefficients on bulk concentrations suggests that the data obtained are reliable in most cases. We also show that the rate of quenching/cooling has little effect on the analytical results, that temperature oscillations during the run can be used to enhance grain growth, and that well-equilibrated samples can be obtained in conventional piston-cylinder runs. Overall, our results suggest that the diamond trap method combined with laser-ablation ICP-MS in frozen state yields reliable results accurate within a factor of two in most cases; however, the precipitation of accessory minerals in the diamond trap during the run may severely affect the data in some systems and may lead to a gross overestimation of fluid concentrations.

Experimental determination of barite dissolution and precipitation rates as a function of temperature and aqueous fluid composition

Barite dissolution and precipitation rates were investigated in closed system reactors, in which total aqueous NaCl concentrations ranged from 0 to 1.5molal, pH from 2 to 9, and temperature from 25 to 90°C. Measured barite dissolution and precipitation rates exhibited a reaction order of 0.2 and 1, respectively, with respect to the barite saturation state. Although these different reaction orders suggest distinctly different mechanisms for dissolution and precipitation, the rates for both processes approach equilibrium with a similar slope on a rate versus saturation state plot, consistent with the concept of micro-reversibility. Barite dissolution rate constants increase as a linear function of the square root of ionic strength but vary only slightly with pH. The dissolution rate dependence on temperature is consistent with an activation energy of 25± 2kJmol−1. Barite dissolution and precipitation rates are not significantly affected by the presence of aqueous calcium, magnesium or strontium. The rates measured in the study were generated in fluids similar to those found in sedimentary basins, ocean floor sediments and oil field reservoirs so the data may provide close estimates for the reactivity of barite during a variety of natural and industrial processes.

Composition of the slab-derived fluids released beneath the Mariana forearc: Evidence for shallow dehydration of the subducting plate

In cold subduction systems, the downgoing plate is thought to mostly dehydrate (≥90% water) beneath the arc front. However, the composition of aqueous fluids released from the shallow subducting slab is poorly constrained because the occurrence of melt inclusions is rare in forearcs. The Southeast Mariana Forearc Rift (SEMFR) is a recently discovered site where extensionally induced magmatism occurred in Pliocene time unusually close to the trench. SEMFR basalts sampled fluids derived from the subducted plate at ∼30 to 100 km deep. Here, we use SEMFR basalts to obtain new insights into the composition of shallow slab-derived fluids by evaluating major element, trace element and volatile contents of olivine-hosted melt inclusions and glassy rinds. Olivine-hosted melt inclusions and their host glassy rinds both contain at least ∼2 wt% H2O, representing minimum estimates for magmatic water content. Melt inclusions have the highest volatile and alkali ratios (i.e., H2O/Ce=6000–19000; Rb/Th=200–2600; Cs/Th=0.4–20) yet recorded in glasses from subduction zones (arc magmas have mean H2O/Ce<2700; Rb/Th<40; Cs/Th<2). Our results indicate that shallow slab-derived fluids are water- and alkali-rich as compared to the deeper fluids released beneath the arc system. Shallow subduction outfluxes peak at ∼70–80 km slab depth, demonstrating that significant slab dehydration occurs beneath the forearc.

Biotite surface chemistry as a function of aqueous fluid composition

The chemical composition and charge of the biotite near-surface, in contact with NaCl bearing aqueous solutions at 25°C from pH 1 to 12, have been derived via zeta potential measurements and potentiometric titrations performed for 20 and 60min in batch reactors. Zeta potential measurements yielded an isoelectric point of pH 3.0 (±0.2) and batch potentiometric titrations yielded a pH of immersion of 9.66 (S.D. 0.24). From batch potentiometric titrations we determined both the proton consumption and the metal release from the biotite surface as a function of pH. Potassium removal from the near-surface of biotite is only slightly dependent on pH with a minimum of ∼6atomsnm−2 removed at the immersion pH, corresponding to an average depletion depth of ∼1.5nm. In contrast, the release of Mg, Al and Fe is strongly pH-dependent as those metals are preferentially removed from the biotite surface at pH less than 9 (Mg) and 4 (Al, Fe). The average depletion depth of Mg, Al, and Fe increases with decreasing pH, reaching on average ∼2nm at pH ∼1. The removal of K, Mg, Al, and Fe is not charge conservative, resulting in a relative negative charge in the biotite near-surface. Taken together, our results indicate that the composition of the biotite surface varies dramatically as a function of pH. At basic conditions, the biotite near-surface is K depleted and likely hydrogen enriched. At near-neutral conditions, the biotite near-surface is comprised of only the Si and Al tetrahedral, and the Fe(II) octahedral framework, following the removal of both alkali metals and Mg. Finally, at acidic conditions, the biotite near-surface is comprised exclusively of a remnant Si, O and H framework. The results of these experiments give an indication of the composition and charge of the biotite surface in the natural environment, following contact with water, for example in the vadose zone, and can help us understand weathering reactions in these systems.

The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas

We have studied the solubility of W- and Mo-oxide in aqueous fluids at 2.61GPa, 600 to 800°C at variable controlled oxygen fugacity conditions, relevant to subduction zones. We observed that the solubility of W-oxide is only slightly dependent on fO2 between conditions buffered by Co–CoO and Re–ReO2. In contrast, Mo-oxide solubility is strongly dependent on both oxygen fugacity and fluid salinity under the same conditions. At 2.61GPa, in the fO2 range between Co–CoO and Re–ReO2 their solubility can be described by the following equations:logW=0.07logfO2−4.72×1000/T+4.43andlogMo=0.44logfO2+0.42logNaCl−1.8×1000/T+4.80where W, Mo and NaCl are concentrations in molalities, fO2 is oxygen fugacity and T is temperature in Kelvin. We also studied the solubility of Mo and W in mantle minerals coexisting with Mo- and W-oxide and combined these data with the fluid solubilities to calculate fluid/mineral partition coefficients of W and Mo. Both W and Mo are incompatible in the structure of major eclogite minerals (garnet and clinopyroxene) in the presence of aqueous fluid, but they are compatible in rutile. Thus, the presence or absence of rutile strongly affects the mobility of these elements in subduction zones. From the partition coefficients we calculated the composition of aqueous fluid released from an N-MORB eclogite source. We found that the Mo–Ce–W–Th–U systematics of arc magmas can be modeled by the mixing of depleted mantle with an aqueous fluid phase that was released from a rutile-bearing N-MORB source in fO2-conditions between FMQ-1.6 and FMQ+3 having a maximum salinity of 20wt% NaClequiv. If rutile contents in residual eclogite are independently determined, redox conditions, salinity and amount of fluid added to the source region of melting can be tightly constrained by plotting Mo/Ce, W/Th, and Mo/W vs. U/Th. In particular, the W/Th ratio is a sensitive indicator of the amount of fluid, since W always very strongly partitions into the fluid, nearly independent of oxygen fugacity and salinity. The Mo/W ratio is particularly sensitive to redox conditions, while U/Th reflects both salinity and redox state.

Aqueous fluid composition in CI chondritic materials: Chemical equilibrium assessments in closed systems

Solids of nearly solar composition have interacted with aqueous fluids on carbonaceous asteroids, icy moons, and trans-neptunian objects. These processes altered mineralogy of accreted materials together with compositions of aqueous and gaseous phases. We evaluated chemistry of aqueous solutions coexisted with CI-type chondritic solids through calculations of chemical equilibria in closed water–rock–gas systems at different compositions of initial fluids, water/rock mass ratios (0.1–1000), temperatures (<350°C), and pressures (<2kbars). The calculations show that fluid compositions are mainly affected by solubilities of solids, the speciation of chlorine in initial water–rock mixtures, and the occurrence of Na-bearing secondary minerals such as saponite. The major species in modeled alkaline solutions are Na+, Cl−, CO32-,HCO3-, K+, OH−, H2, and CO2. Aqueous species of Mg, Fe, Ca, Mn, Al, Ni, Cr, S, and P are not abundant in these fluids owing to low solubility of corresponding solids. Typical NaCl type alkaline fluids coexist with saponite-bearing mineralogy that usually present in aqueously altered chondrites. A common occurrence of these fluids is consistent with the composition of grains emitted from Enceladus. Na-rich fluids with abundant CO32-,HCO3-, and OH− anions coexist with secondary mineralogy depleted in Na. The Na2CO3 and NaHCO3 type fluids could form via accretion of cometary ices. NaOH type fluids form in reduced environments and may locally occur on parent bodies of CR carbonaceous chondrites. Supposed melting of accreted HCl-bearing ices leads to early acidic fluids enriched in Mg, Fe and other metals, consistent with signs of low-pH alteration in chondrites. Neutralization of these solutions leads to alkaline Na-rich fluids. Sulfate species have negligible concentrations in closed systems, which remain reduced, especially at elevated pressures created by forming H2 gas. Hydrogen, CO2, and H2O dominate in the gaseous phase, though the abundance of methane cannot be fairly estimated.

Evidence for channelized external fluid flow and element transfer in subducting slabs (Raspas Complex, Ecuador)

Fluids released at great depth by dehydrating subducting plates play a major role in mass transport and are responsible for metasomatism and partial melting in the overlying mantle wedge. To investigate the fluid-flow regime and element mobility within a subducting oceanic lithosphere, the Raspas Complex in Ecuador has been selected where an ophiolite association of blueschists, eclogites, metapelites and ultramafitites is exposed. This ophiolite association stands out by high-pressure zoisite veins, associated metasomatized zoisite eclogites and deserpentinized, pseudo-spinifex textured chlorite harzburgites indicating fluid flow and element mobility at a depth of about 60km. Oscillatory zoning in vein zoisite and garnet rims in the metasomatized eclogites is explained by cyclic fluid expulsion during high-pressure deserpentinization after overstepping of the antigorite breakdown reaction. Fluid inclusion studies reveal a homogeneous low-salinity aqueous fluid composition with minor CH4 and CO2 which is in accord with open-system fluid infiltration derived from an external source. The deserpentinized chlorite harzburgite is a potential source for the low-salinity aqueous fluid. The association of zoisite veins with zoisite eclogites which are enriched in LREE, MREE, Pb, Sr, HFSE, Th, and U, compared to MORB-type eclogites, documents the metasomatic effects and the fluid mobility of a large range of trace elements. In addition, the fluid-mobile trace elements B, Rb, Pb, and Sr are enriched in fluid inclusions in omphacite. Most trace elements cannot be derived from the serpentinites but might be explained by leaching from metabasites and metapelites in zones of intense fluid–rock interaction. A garnet–amphibole rock, deficient in LREE, MREE and Sr, could represent such a leached metabasite. B, Rb, Th and U could be derived from metapelites. By channelized fluid flow and high fluid flux, the fluid-mobile trace elements might be transported into the mantle wedge.

Effect of the heterotrophic bacterium Pseudomonas reactans on olivine dissolution kinetics and implications for CO2 storage in basalts

This work is aimed at quantification of forsteritic olivine (Fo92) dissolution kinetics in batch and mixed-flow reactors in the presence of aerobic gram-negative bacteria (Pseudomonas reactans HK 31.3) isolated from an instrumented well located within a basaltic aquifer in Iceland. The release rate of mineral constituents was measured as a function of time in the presence of live and dead cells in constant-pH (4–9), bicarbonate-buffered (0.001–0.05M), nutrient-rich and nutrient-free media in batch reactors at 0–30atm of CO2 partial pressure (pCO2). In batch reactors at 30atm pCO2, 0.1M NaCl and 0.05M NaHCO3 the rates were weakly affected by the presence of bacteria. In nutrient media, the SEM observation of reacted grains revealed the presence of biofilm-like surface coverage that does not modify Mg and Si release rate at the earlier stages of reaction but significantly decreased the dissolution after prolonged exposure.Olivine dissolution rates measured in flow-through reactors are not affected by the presence of dead and live bacteria at pH ⩾9 in 0.01M NaHCO3 solutions. In circumneutral, CO2-free solutions at pH close to 6, both live and dead bacteria increase the dissolution rate, probably due to surface complexation of exudates and lysis products. In most studied conditions, the dissolution was stoichiometric with respect to Mg and Si release and no formation of secondary phases was evidenced by microscopic examination of post-reacted grains. Obtained results are consistent with known molecular mechanism of olivine dissolution and its surface chemistry. Overall, this work demonstrates negligible effect of P. reactans on olivine reactivity under conditions of CO2 storage in the wide range of aqueous fluid composition.

Aqueous fluids at elevated pressure and temperature

AbstractThe general major component composition of aqueous fluids at elevated pressure and temperature conditions can be represented by H2O, different non‐polar gases like CO2 and different dissolved metal halides like NaCl or CaCl2. At high pressure, the mutual solubility of H2O and silicate melts increases and also silicates may form essential components of aqueous fluids. Given the huge range of P–T–x regimes in crust and mantle, aqueous fluids at elevated pressure and temperature are highly variable in composition and exhibit specific physicochemical properties. This paper reviews principal phase relations in one‐ and two‐component fluid systems, phase relations and properties of binary and ternary fluid systems, properties of pure H2O at elevated P–T conditions, and aqueous fluids in H2O–silicate systems at high pressure and temperature. At metamorphic conditions, even the physicochemical properties of pure water substantially differ from those at ambient conditions. Under typical mid‐ to lower‐crustal metamorphic conditions, the density of pure H2O is , the ion product Kw = 10−7.5 to approximately 10−12.5, the dielectric constant ε = 8–25, and the viscosity η = 0.0001–0.0002 Pa sec compared to , Kw = 10−14, ε = 78 and η = 0.001 Pa sec at ambient conditions. Adding dissolved metal halides and non‐polar gases to H2O significantly enlarges the pressure–temperature range, where different aqueous fluids may co‐exist and leads to potential two‐phase fluid conditions under must mid‐ to lower‐crustal P–T conditions. As a result of the increased mutual solubility between aqueous fluids and silicate melts at high pressure, the differences between fluid and melt vanishes and the distinction between fluid and melt becomes obsolete. Both are completely miscible at pressures above the respective critical curve giving rise to so‐called supercritical fluids. These supercritical fluids combine comparably low viscosity with high solute contents and are very effective metasomatising agents within the mantle wedge above subduction zones.

An evaluation of reactive fluid flow and trace element mobility in subducting slabs

Permeabilities in the subducting slab appear to be too low and dihedral angles between fluid and relevant minerals too high to allow for porous flow, hence fluid channelization is critical for the understanding of subduction zone fluid fluxes. In this review we will outline how fluid channelization controls reaction rates and element redistributions during metamorphism of the subducting plate as well as trace element compositions of subduction-related fluids during flow. Channelized fluid flow predicts that from a rock point of view, most formerly subducted material will show only very limited evidence for fluid flow, consistent with the rarity of observed high fluid fluxes in subduction-related rocks. Aqueous fluid produced by dehydration reactions will not percolate through large rock volumes, but rather will be carried away from the dehydration sites by a veining network. Indeed evidence for significant aqueous-fluid fluxes have been found in high-pressure veins with adjacent selvages. In such selvages, large lithophile elements (LILE's) generally show the highest mobilities, followed by light (L) rare earth elements (REE) and then heavy (H) REE. Compared to high field strength elements (HFSE), even Th shows higher mobilities. From a fluid point of view, equilibrium between aqueous fluid and surrounding rock will only be approached at sites of fluid production and mineral reaction. However, this fluid can be significantly modified while moving upwards through a veining network where the wallrocks are out of equilibrium with the fluid. In a subducting slab, such reactive fluid flow can preferentially dissolve minerals and release their trace elements (e.g. Ba in phengite, Th and La in monazite). The degree of change in aqueous-fluid composition will depend on the amount of fluid–mineral surface interaction. The chemical exchange reactions will not be possible to model by trace element partition coefficients alone, instead future models need to incorporate kinetic parameters such as surface reaction rates.

An evaluation of synthetic fluid inclusions for the purpose of trapping equilibrated, coexisting, immiscible fluid phases at magmatic conditions

We report data that allow us to evaluate the method of trapping immiscible, saline aqueous fluids (i.e., vapor and brine in the NaCl-KCl-HCl-FeCl2-AuHCl2-H2O system) as synthetic fluid inclusions in pre-fractured quartz cores in order to quantify the concentrations of Au, Fe, K, and Na, among coexisting three-phase, immiscible fluids (i.e., haplogranite melt, brine, and vapor) at magmatic conditions. Coexisting vapor and brine were trapped experimentally at 800 °C and 100-110 MPa as synthetic fluid inclusions in both quartz microfractures and quenched silicate melt (i.e., glass), and also sampled indirectly using the recovered quenched aqueous fluid. Quartz-hosted and glass-hosted brine inclusions were analyzed by laser-ablation inductively-coupled-plasma mass spectrometry (LAICPMS) and instrumental neutron activation analysis (INAA), respectively. Quenched aqueous fluid from each experiment containing a quartz core was recovered and analyzed by atomic absorption spectrophotometry (AAS). The composition of aqueous fluids trapped as quartz-hosted inclusions, glass-hosted inclusions, and those recovered after quench yield consistent and precise data, at the 2σ uncertainty level, for the elements of interest. The overlapping Au, Fe, K, and Na concentrations in aqueous fluids trapped and analyzed via three entirely different instrumental techniques (i.e., LAICPMS, INAA, and AAS) suggest strongly that quartz microfractures heal on a slow enough time scale to permit entrapment of fully equilibrated aqueous fluids at our experimental PTX conditions. The data evince clearly that the chemical composition of fluids in quartz microfractures at the time of self-healing represents equilibrium conditions; hence, synthetic fluid inclusions in experiments with low thermal gradients across the charge provide a reasonable estimate of fluid composition at least at the experimental conditions examined in this study.

Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: Implications for element transfer in subduction zones

High-pressure (HP) and ultra-high pressure (UHP) terranes are excellent natural laboratories to study subduction-zone processes. In this paper we give a brief theoretical background and we review experimental data and observations in natural rocks that constrain the nature and composition of the fluid phase present in HP and UHP rocks. We argue that a fluid buffered by a solid residue is compositionally well defined and is either an aqueous fluid (total amount of dissolved solids < 30 wt.%) or a hydrous melt (H 2O < 35 wt.%). There is only a small temperature range of approximately 50–100 °C, where transitional solute-rich fluids exist. A review of available experimental data suggest that in felsic rocks the second critical endpoint is situated at 25–35 kbar and ∼ 700 °C and hence must be considered in the study of UHP rocks. Despite this, the nature of the fluid phase can be constrained by relating the peak metamorphic conditions of rocks to the position of the wet solidus even if the peak pressure exceeds the pressure where the wet solidus terminates at the second critical endpoint. Transitional solute-rich fluids are expected in UHP terrains ( P > 30 kbar) with peak temperatures of about 700 ± 50 °C. At higher temperatures, hydrous granitic melts occur whereas at lower temperatures aqueous fluids coexists with eclogite-facies minerals. This argument is complemented by evidence on the nature of the fluid phase from high-pressure terrains. We show that in the diamond-bearing, high-temperature UHP rocks from the Kokchetav Massif there are not only hydrous felsic melts, but probably also carbonate and sulfide melts present. Hydrous quartzo-feldspathic melts are mainly produced in high temperature UHP rocks and their composition is relatively well constrained from experiments and natural rocks. In contrast, constraining the composition of aqueous fluids is more problematic. The combined evidence from experiments and natural rocks indicates that aqueous fluids liberated at the blueschist to eclogite facies transition are dilute. They contain only moderate amounts of LILE, Sr and Pb and do not transport significant amounts of key trace elements such as LREE, U and Th. This indicates that there is a decoupling of water and trace element release in subducted oceanic crust and that aqueous fluids are unable to enrich the mantle wedge significantly. Instead we propose that fluid-present melting in the sediments on top of the slab is required to transfer significant amounts of trace elements from the slab to the mantle wedge. For such a process to be efficient, top slab temperature must be at least 700–750 °C at sub-arc depth. Slab melting is likely to be triggered by fluids that derive from dehydration of mafic and ultramafic rocks in colder (deeper) portions of the slab.

Comment on K. Mibe, T. Fujii, and A. Yasuda (2002), “Composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the earth’s mantle,” Geochimica et Cosmochimica Acta 66,2273–2285

Comment on K. Mibe, T. Fujii, and A. Yasuda (2002), “Composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the earth’s mantle,” Geochimica et Cosmochimica Acta 66,2273–2285

Response to the comment by R. Stalder on “composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the earth’s mantle”

Response to the comment by R. Stalder on “composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the earth’s mantle”

Composition of aqueous fluid coexisting with mantle minerals at high pressure and its bearing on the differentiation of the Earth’s mantle

In order to understand the role of aqueous fluid on the differentiation of the mantle, the compositions of aqueous fluids coexisting with mantle minerals were investigated in the system MgO-SiO 2-H 2O at pressures of 3 to 10 GPa and temperatures of 1000 to 1500°C with an MA8-type multianvil apparatus. Phase boundaries between the stability fields of forsterite + aqueous fluid, forsterite + enstatite + aqueous fluid, and enstatite + aqueous fluid were determined by varying the bulk composition at constant temperature and pressure. The composition of aqueous fluid coexisting with forsterite and enstatite can be defined by the intersection of these two phase boundaries. The solubility of silicate components in aqueous fluid coexisting with forsterite and enstatite increases with increasing pressure up to 8 GPa, from about 30 wt% at 3 GPa to about 70 wt% at 8 GPa. It becomes almost constant above 8 GPa. The Mg/Si weight ratio of these aqueous fluids is much higher than at low pressure (0.2 at 1.5 GPa) and almost constant (1.2) at pressures between 3 and 8 GPa. At 10 GPa, it becomes about 1.4. Aqueous fluid migrating upward through the mantle can therefore dissolve large amounts of silicates, leaving modified Mg/Si ratios of residual materials. It is suggested that the chemical stratification of Mg/Si in the Earth may have been formed as a result of aqueous fluid migration.

Mineral-aqueous fluid partitioning of trace elements at 900°C and 2.0 GPa: Constraints on the trace element chemistry of mantle and deep crustal fluids

To constrain the trace element composition of aqueous fluids in the deep crust and upper mantle, mineral-aqueous fluid partition coefficients ( D min/fluid) for U, Th, Pb, Nb, Ba, and Sr have been measured for clinopyroxene, garnet, amphibole, and olivine in experiments at 2.0 GPa and 900°C. Clinopyroxene-and garnet-fluid partition coefficients are similar for Nb (0.01-0.7) and Ba (∼10 −4-10 −5 ), whereas values of D cpx/fluid for Sr (0.5−4), Th (0.6–9), and Pb (0.04–0.09) are ∼1Ox (Th, Pb) to ∼1000x (Sr) higher than D garnet/fluid . At the same f O 2 (FMQ + 1), garnet-fluid partition coefficients for U are ∼10x higher than those for clinopyroxene. Amphibole-fluid partition coefficients are uniformly high (-I) for all elements studied, and, with the exception of Ba, interelement fractionations are similar to clinopyroxene. The olivine-fluid partition coefficient for Nb is similar to values measured for the other silicates, whereas D olivine/fluid for U, Th, Pb, Sr, and Ba are significantly lower. Clinopyroxene and garnet partition coefficients follow Henry's Law up to ∼300 ppm of either Ba, Pb, or Sr in the fluid. Both the major-element chemistry of clinopyroxene and fluid have some influence on partitioning, with the magnitude of these effects varying according to element type. Although clinopyroxene concentrations of Pb, Ba, and Sr were found to be homogeneous, core-to-rim decreases in wt% Al 2O 3 were found to correlate with reductions in the concentrations of Nb, U and Th, and hence D cpx/fluid. Both increases in solute content and the addition of NaCl to fluids lower the measured partition coefficients. A decrease in experiment f O 2 reduces D Th/D U for clinopyroxene, which is consistent with the compatibility of U 4+ relative to U 6+ in the clinopyroxene structure. Comparison of mineral/ fluid partition coefficients with mineral /basaltic melt values from the literature reveal notable distinctions in partitioning behavior for fluids vs. melts. Mineral-melt and mineral-fluid partitioning for elements such as Ba, Pb, and Sr are similar, but in contrast, U, Th, and Nb are more strongly partitioned into silicate melts than aqueous fluids. Such differences may provide a means of discerning the products of melt- vs. fluid-mediated metasomatism. Bulk eclogite- and lherzolite-aqueous fluid partition coefficients, calculated from mineral/aqueous fluid values, are used to illustrate how partitioning data can constrain (1) the trace element composition of fluids that may be a product of dehydration of basaltic oceanic crust and (2) the effect of the subarc mantle on trace element fractionation processes. The silicate assemblage produced during basalt dehydration (garnet + cpx ± amphibole) does not selectively deplete the coexisting fluid in Nb relative to the other elements studied, nor is Nb preferentially withdrawn from the fluid by passage through an amphibole lherzolite mantle. Results, therefore, reaffirm the notion that residual rutile is necessary to selectively deplete slab-derived fluids in high field strength elements. Calculations also indicate that fluids with excess [ 238U ] relative to [ 230Th] may be produced during dehydration of basaltic oceanic crust, and such excesses are retained or enhanced during transit through the mantle wedge, provided that mildly oxidizing conditions prevail. Slab-derived fluids can therefore produce the requisite low ratios of high-field-strength/large-ion-lithophile elements (such as Nb/Th) and [ 238U]/[ 230Th] > 1 in the source regions of island arc basalts by metasomatism of the mantle wedge. In addition to constraints on the composition of the fluid liberated during slab dehydration, our data allow us to estimate the trace element composition of the material returned to the deep mantle during subduction. Calculations indicate that, following dehydration, the U/Pb ratio in basaltic crust is increased and Rb/Sr is likely to be dramatically reduced. Subduction and prolonged aging of this material produces an isotopic reservoir with the characteristics of the HIMU component sampled by some oceanic island basalts.