Hydrothermal Diamond Anvil Cell Experiments Research Articles

Hyper-enrichment of gold via quartz fracturing and growth of polymetallic melt droplets

Abstract Gold precipitation in hydrothermal systems is traditionally attributed to supersaturation of gold due to decreasing gold complex stability triggered by changes in physicochemical conditions of the ore fluid. However, ultrahigh-grade gold veins in orogenic (shear zone related) gold deposits can contain kilograms per tonne of gold or more, in marked contrast to the typically very low gold concentrations (tens of parts per billion) in fluid. The gold mineral assemblage is commonly restricted to native gold and/or Au-(Ag)-tellurides and occurs in micro-fractures of sheared quartz veins. Textural and compositional characterization of such assemblages, coupled with hydrothermal diamond anvil cell experiments and heating-freezing experiments, provides evidence for an alternative ultrahigh-grade gold enrichment mechanism via growth of polymetallic melt droplets induced by quartz fracturing. We propose that polymetallic melt droplets of Au-Ag-Te-Bi–rich composition form through adsorption-reduction of metal complexes on fractured quartz surfaces, where surface silanol groups and hydrogen serve as reductants. The melt droplets subsequently grow by catalyzing reduction of metal complexes and absorbing metals from fluids percolating in the fractured quartz network. The mobile and reactive polymetallic melt droplets can repeatedly react with the fluid on protracted quartz fracturing and efficiently continue to scavenge gold from multiple pulses of gold-undersaturated ore fluids.

An in-situ experimental HP/HT study on bromine release from a natural basalt

We present the first in-situ partitioning data for bromine between a natural basaltic melt and a coexisting fluid. For this study hydrothermal diamond anvil cell experiments at pressures up to 1.7 GPa were conducted. We combined laser heating to melt the basalt glass with external heating to lower the temperature gradient in the cell and to initiate circulation for the aqueous fluid. Bromine concentrations were measured in-situ with X-ray fluorescence in the basaltic melts, glasses, and in the fluid. From the results we calculated partition coefficients of DBrfluid/melt = 1.19 to 3.92 in the range of 0.4 to 1 GPa for aqueous fluids. Experiments with neon as the surrounding fluid (DBrfluid/melt = 0.38 ± 0.01 at 1.1 GPa) suggest that Br-release from a basalt into volatiles that have no bonding affinity with Br is weak. This should be the case for dry intra-plate volcanic eruptions. From the experimentally gained partition coefficients and from global Br concentration values in melt inclusions of arc magmas, we calculated an annual global Br flux of 23.5–72.9 × 109 g/y.

In-situ redox conditions in hydrothermal diamond-anvil cell experiments using various metal gaskets

Hydrothermal diamond-anvil cells (HDACs) have been widely used in various research fields since they were introduced in 1993. Unlike temperature (T) and pressure (P), the redox states of the samples in HDAC experiments were poorly controlled or measured; they were usually estimated by observing the reduction/disappearance of redox-sensitive mineral(s) (e.g., Mo metal) and corresponding formation and growth of such mineral(s) (e.g., MoO2) without in-situ redox measurement. In this study, we demonstrated the generation of H2 during experiments using various metal gaskets, including inert (Re) and chemically more reactive metals (Ni, Co, Mo, and W), in the pure H2O system under various P-T conditions. A technique for quantitative Raman spectroscopic analysis of H2 in HDAC is designed. Based on this technique, the oxygen fugacity (fO2) values of Mo-MoO2 buffer at the experimental P-T conditions were then calculated from these measured H2 pressures. We demonstrated that steady-state redox control can be maintained at a fixed T between 400 and 500 °C in HDAC experiments using a Mo gasket for durations of up to two hours and for those using a Re gasket with an additional Mo rod in the sample between 400 and 800 °C for durations of up to 90 min, regardless of the considerable changes in sample pressure. This demonstration was established by the balance between the rates of H2 generation and H2 loss from the sample chamber. The loss of H2 enhanced the consumption of H2O and therefore reduced the sample pressures during the experiments.We believe that hydrogen contents in every HDAC experiment, which attempt to control the redox state, need in situ measurement, then estimate the real redox state and compare it to thermodynamic equilibrium prediction. To enhance in situ redox control and measurements, modifications of HDAC are required for improving the quantitative Raman spectroscopic analysis technique in both sensitivity and accuracy.

The intrinsic effects of using rhenium gaskets in hydrothermal diamond anvil cell experiments on background fluorescence, contamination, and redox control

Because of the great stiffness and high melting point of rhenium (Re) metal, Re gaskets are widely used in hydrothermal diamond anvil cell (HDAC) experiments that involve aqueous fluids and even hydrous melts under high pressure–temperature (P–T) conditions. This metal was usually considered chemically inert in hydrothermal experiments, but some concerns have been raised about its reactivity with the fluids. Therefore, this communication addresses the intrinsic effects of using Re gaskets in HDAC experiments, including background fluorescence in Raman spectroscopy, contamination, and redox control. A series of hydrothermal experiments involving rhenium were performed in pure water, H2O-NaCl, and SiO2-COH systems using the HDAC up to 28.9 kbar and 850 °C and fused silica capillary capsule (FSCC) up to 87 bar and 300 °C, and Raman spectroscopy was employed for in situ analysis. The results indicate that (1) strong background fluorescence, often occurring between 200 and 700 °C, is caused by the dissolution of Re gaskets and the formation of aqueous species of Re with a low oxidation state, such as a Re2+cation and eventually, its hydroxide complexes; (2) in addition to the fluorescent Re species and unwanted carbon species produced from the diamond-water reaction catalyzed by Re, a considerable amount of ReO4−, which does not cause fluorescence and shows a dominant Raman peak at ∼970 cm−1, is detected in the fluids either in situ at high temperatures above 700 °C or under cooling conditions, and it can reduce the activity of H2O and the pH of the fluids and interfere with the assignments of Raman peaks of other species in the samples; and (3) the production of oxidizing aqueous species at high temperatures, including ReO4− and CO2, is attributed to the substantial loss of H2 and the corresponding increase in oxygen fugacity. Because of these intrinsic effects of using Re gaskets in HDAC experiments, implications for hydrothermal experiments are discussed with several previously reported examples such as the discrepancy in the solubility of rutile in water under elevated P–T conditions. Based on the results of the current and previous studies, four suggestions have also been made to minimize the unwanted effects of using Re gaskets, including the generation of a reduced experimental condition, a pre-heating treatment of Re gaskets, a gasket pre-indentation approach, and a gold-lined gasket method. Armed with the knowledge of these intrinsic effects as well as the advantages of using different HDAC metal gaskets, the researchers will be able to create appropriate experimental designs.

Redox controls on H and N speciation and intermolecular isotopic fractionations in aqueous fluids at high pressure and high temperature: Insights from in-situ experiments

Aqueous magmatic fluids are essential to the transport of hydrogen (H), carbon (C), and nitrogen (N) from the mantle to the surface, during which changes in pressure, temperature, and redox conditions affect the chemical speciation and intermolecular isotopic fractionations of H, C, and N. Here, we performed a series of hydrothermal diamond-anvil cell experiments to evaluate the role of pressure, temperature, and redox conditions on the speciation and intermolecular fractionations of H and N during the decompression and cooling of aqueous fluids from 780 MPa to 800°C to 150 MPa and 200°C. We used Raman spectroscopy to investigate the distribution and exchange reactions of H and N isotopologues between water, methane, ammonia, and di-nitrogen molecules under changing physicochemical conditions. Our experiments show that upon decompression, a C- and N-bearing fluid will preferentially degas D-rich methane and 15N-rich N2, depleting the residual aqueous fluid in those isotopes. If this fluid precipitates N-rich (i.e., NH4+-bearing) minerals, the observed N isotopic fractionation is opposite to that during N2 degassing, enriching the aqueous fluid in 15N. Because these fractionations result from changes in H, C, and N speciation in the aqueous fluid, their magnitudes depend on redox conditions as well as pressure and temperature. Our new in-situ experimental results are consistent with the large H and N isotopic fractionations observed between water, methane, and ammonia species in aqueous fluids at high pressures and temperatures, although the magnitude of the fractionations in our experiments cannot be quantified. Nonetheless, our results suggest that statistical thermodynamic models likely underestimate isotopic fractionation effects for isotopic molecules under these conditions, and should account for solubility and isotopic effects of the solvent associated with the solvation of water, methane, and ammonia isotopologues in aqueous fluids.This work has significant implications for interpreting isotopic measurements of natural samples from hydrothermal systems because it offers insights into isotopic fractionations in multicomponent and multiphase systems under hydrothermal temperatures and pressures.

Hydrothermal oxidation of Os

Abstract A series of hydrothermal diamond anvil cell experiments was conducted to investigate the mobility of Os and Ir in saline and oxidizing hydrothermal fluids as function of fluid pH at temperatures ranging from 500 °C to 1000 °C and pressures resembling upper mantle/lower crust environments (29–1343 MPa). The composition of reactant fluids was monitored in real time and at in situ conditions by Raman vibrational spectroscopy. Results revealed the formation of Os O volatile aqueous species under oxidizing redox conditions (∼+5 ΔFMQ) and at temperatures exceeding 850 °C. These species were detected in an immiscible phase separated from the homogeneous fluid at low pressure conditions (

Solubility and speciation of iron in hydrothermal fluids

Abstract Iron is among the most abundant elements in Earth's crust and is also a major aqueous solute in a variety of hydrothermal settings, yet major questions remain regarding the solubility and speciation of iron at hydrothermal conditions. Here, we conducted hydrothermal diamond-anvil cell experiments using synchrotron-radiation micro-XRF and XANES analyses as well as Raman spectroscopy, in situ at hydrothermal temperatures and pressures, to characterize the solubility and speciation of iron in fluids buffered by a variety of mineral assemblages. Our experiments included the assemblages hematite-magnetite (HM), fayalite-magnetite-quartz (FMQ), and magnetite-pyrite-pyrrhotite (MPP). Our results indicate highest solubilities of HM in HCl solutions. In sodium chloride solutions of similar molalities, FMQ shows higher Fe solubility than MPP. XANES data are interpreted as preponderance of ferrous iron in all experiments. Comparison of XANES spectra of these solutions with calculated XANES spectra from the literature indicates octahedral FeClx(H2O)6−x2−x (x = 0–3) as predominant Fe(II) species at lower Cl-Fe ratios, and suggests additional tetrahedral FeCl42− or FeCl3(H2O)− at high Cl-Fe ratios. Raman spectra to 600 °C show that the predominant species in ferric iron solutions is FeCl2(H2O)4+ at temperatures less than 100 °C, which transitions to FeCl4− between 100 and 200 °C. An additional Raman band that occurred in some spectra of a H2O + HCl fluid equilibrated with hematite ± magnetite at temperatures greater than 300 °C may originate from an FeCl3(H2O)x (x = 0–3) species. All Raman spectra of ferrous iron solutions show a fairly broad band at about 280 cm−1, which can be interpreted to stem from Fe(II)–Cl vibrations of FeCl3(H2O)x− (x = 0–3) (point group D3h) species or, at low Cl-Fe ratios of about two or less, from octahedral FeClx(H2O)6−x2−x (x = 0–3) species. These results provide important constraints on the hydrothermal mobilization of iron and fluid-rock reactions involving iron- and chloride-bearing fluids.

Aqueous sodium hydroxide (NaOH) solutions at high pressure and temperature: insights from in situ Raman spectroscopy and ab initio molecular dynamics simulations.

Hydrothermal diamond anvil cell experiments in combination with Raman spectroscopy and first principles molecular dynamics simulations were performed to investigate the structure and dynamics of aqueous NaOH solutions for temperatures up to 700 °C, pressures up to 850 MPa and two different solute concentrations. The significant changes observed in the O-H stretching region of the Raman spectra between ambient and supercritical conditions are explained by both dynamic effects and structural differences. Especially important are a Grotthuss-like proton transport process and the decreasing network connectivity of the water molecules with increasing temperature. The observed transfer of Raman intensity towards lower wavenumbers by the proton transfer affects a wide range of frequencies and must be considered in the interpretation of Raman spectra of highly basic solutions. We suggest a deconvolution of the spectra using a model with four Gaussian functions, which are assigned to the molecular H2O and OH- vibrations, and one asymmetric exponentially modified Gaussian (EMG) function, which is assigned to [HO(H2O)n]- vibrations.

Origin of deep carbonate reservoir in northeastern Sichuan Basin: New insights from in-situ hydrothermal diamond anvil cell experiments

In situ Raman analysis on the segregated near-equilibrium carbonate-fluid interaction at elevated temperatures (room temperature−260 °C) and pressures (13-812 MPa) in a hydrothermal diamond anvil cell (HDAC) reveals the preservation mechanism of porosity in deep carbonate reservoirs in the northeastern Sichuan Basin. The carbonate-fluid interaction was investigated by separately heating carbonate minerals and rocks with four different acid solutions (saturated CO2 and H2S solutions, HCl, CH3COOH) in a sealed sample chamber. A minor continuous precipitation with increasing temperatures and pressures was observed during the experiments which caused minor sample volume change. The closed system is a preservation of pores and burial dissolution may not be the dominant diagenesis in the origin of porosity. Thin section photomicrographs observations in Changxing and Feixianguan Formations demonstrate that eogenetic pores such as moldic or intragranular pores with late small euhedral minerals, intergranular, intercrystal and biological cavity pores are the main pore types for the reservoirs. Early fast deep burial makes the porous carbonate sediments get into the closed system as soon as possible and preserves the pores created in the early diagenetic stage to make significant contribution to the deep reservoir quality. The anomalous high porosity at a given depth may come from the inheritance of primary pores and eogenetic porosity is fundamental to carbonate reservoir development. The favorable factors for deep reservoir origin include durable meteoric leaching, early fast deep burial, early dolomitization, etc. This deep pores preservation mechanism may be of great importance to the further exploration in deep carbonate reservoirs in the northeastern Sichuan Basin.

Olivine + Quartz + Water ± HCl At Mid-Crustal Conditions: Controls On the Growth of Fibrous Talc As Determined From Hydrothermal Diamond Anvil Cell EXPERIMENTS

Abstract Many talc deposits are formed by metasomatism of dolostone or ultramafic rocks during metamorphism or hydrothermal alteration. In these deposits, talc commonly exhibits a platy habit; rarely, it forms fibers. In previous studies, the origin of fibrous talc has been explained by the secondary replacement of fibrous amphiboles during retrograde metamorphism. In this study, experiments have been performed in the MgO–FeO–SiO 2 –H 2 O–HCl system using a hydrothermal diamond anvil cell (HDAC). San Carlos olivine (Fo 90 ) + Brazilian quartz + aqueous fluid [deionized water or 0.1–10 m m (milli-molal) HCl] were used as starting materials and were reacted for approximately 2 to 6 hours at temperatures of 400–655 °C and pressures of 0.1–1.2 GPa. All experiments produced talc as the only new phase, but with both fibrous and platy habits. The talc fibers (length:width ≥ 25:1; average length ∼40 μm) grew on the surface of the olivine and on the gasket walls. Maximum talc growth rates in these experiments ranged from 10 –11 to 10 –13 moles per second or 10 –5 to 10 –6 moles per second per square meter. Experiments in this study demonstrate that the growth of fibrous talc does not require a fibrous precursor. Experimental results suggest that the mechanism for talc fiber growth is related to initial nucleation density of talc on the substrate and the degree of supersaturation. Conditions that appear to inhibit the growth of fibrous talc include a large surface area of olivine relative to quartz; an increase in the density of the aqueous fluid; and the chemical isolation of olivine by talc armoring. Decreases in aqueous fluid density and lower olivine surface area relative to quartz are consistent with the production of fibrous talc.

On the solvation properties of supercritical electrolyte solutions

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the effect of anions (Cl−, F−) and cations (Mg+2, Na+) on the molecular structure of high temperature/-pressure fluids, and to express the extent of hydration as a function of solute's speciation and concentration at temperatures ranging from 400 to 800°C and pressures of 1.7 to 30.1kbar. Results reveal that hydrogen-bonding environments for H2O molecules in the first hydration shell of anions evolve from pure OH⋯O to dynamic mixtures with OH⋯X− (F−, Cl−). The distribution of the “OH⋯X−” structures (OH⋯O, OH⋯X−) is proportional to the concentration of dissolved anions; strongly dependent, however, on ionic speciation. To this end, the OH⋯O structures inside the hydration shell of F− are weaker than the OH⋯Cl− and OH⋯O in Cl−bearing solutions. On the contrary, cations (Na+, Mg2+) appear to impose a minimal effect on the Raman spectra of the OH⋯O structures. The thermodynamic stability of “OH⋯Cl−” environments shows a strong correlation with the activity of H2O, indicative of the localized effect of the Cl− electric field that decouples the hydration from the bulk water molecules. The enthalpy change required to rupture these H-bonding environments (ΔHOH⋯Cl−) provides a measure for the extent of ion hydration and constrains the solvation properties of supercritical electrolyte solutions. For example, the dominant presence of “OH⋯Cl− “ environments lowers the concentration of isolated water dipoles and contributes to the decrease in the dielectric constant of supercritical electrolyte solutions, which in turn, could suppress the solubility of hydrophobic non-polar aqueous species, such as SiO2(aq).

Determination of pressure from measured Raman frequency shifts of anhydrite and its application in fluid inclusions and HDAC experiments

A new geobarometry was derived from the quantified relationships among Raman vibrational frequencies of anhydrite, pressure and temperature, as determined from in-situ micro-Raman spectroscopy of natural anhydrite crystals measured at p–T conditions up to 560°C and 1400MPa by using a hydrothermal diamond anvil cell (HDAC). With this geobarometry, the pressure in HDAC experiments and in anhydrite-bearing fluid inclusions can be determined directly from the ν1, 1016, ν3, 1128 and ν3, 1160 Raman frequency shifts of anhydrite at high p–T conditions relative to their values measured at ambient conditions. The pressure can be determined to an accuracy of better than 30MPa based on the attainable accuracy of ±0.1cm−1 for the fitted ν1 Raman peak positions, provided the measured spectra are calibrated using the emission peak of an external fluorescent light source. The feasibility and reliability of this geobarometry were verified by rebuilding the p–T history of two fluid inclusions from the ν1 frequency shifts of anhydrite daughter minerals from room to high temperatures, and by measuring the phase-transition pressures of calcite–CaCO3(II)–CaCO3(III) sequence at ambient temperature in a HDAC experiment using anhydrite as a Raman pressure sensor.

Experimental study on the pseudobinary H2O + NaAlSi3O8 at 600–800 °C and 0.3–2.4 GPa

The pseudobinary H2O+NaAlSi3O8 was studied at compositions between 35 and 66 mass% NaAlSi3O8, at temperatures between 600 and 804°C, and at pressures ranging from 0.3 to 2.41GPa using a hydrothermal diamond-anvil cell (HDAC), cold-seal hydrothermal pressure vessels (CSPVs), and a piston-cylinder apparatus (PC). We propose a revised pseudosection for bulk compositions near 40 mass% NaAlSi3O8, the critical composition at 750°C obtained from the HDAC experiments. The corresponding critical pressure was 1.56GPa, as determined directly using zircon as Raman spectroscopic pressure sensor. Pressures at the melt-fluid homogenization temperature and along six isochoric P–T paths obtained from the Raman spectrum of zircon were close to or slightly lower than the pressure calculated from the EoS of H2O based on the vapor dissolution or ice melting temperature, which demonstrates that the effect of albite or NaAlSi3O8 melt dissolution is small. In an experiment up to 700°C and 2.28GPa, no clear evidence was found from optical observation for the hypothetical reappearance of melt-fluid immiscibility at higher pressure.The results consistently point to incongruent behavior at elevated pressures by the formation of paragonite. The HDAC experiments showed that paragonite nucleation was generally sluggish and occurred usually within several hours to days. For the critical composition of 40 mass% NaAlSi3O8, the temperature limit of paragonite stability was determined to a pressure of 1.2GPa using CSPVs and a PC and found to increase significantly with pressure. There was no discernible difference in its P–T location from that of the paragonite breakdown reaction to corundum, albite, and water reported in the literature for pressures to 0.7GPa. The paragonite stability limit intersected the critical curve at 764°C, 1.53GPa. Hence, our experiments indicate the existence of three previously not recognized stability fields in pseudosections for bulk compositions near 40 mass% NaAlSi3O8, namely (1) paragonite+albite+fluid, (2) paragonite+melt+fluid, and (3) paragonite+fluid. Formation of a substantial fraction of paragonite at these NaAlSi3O8 concentrations implies that the aqueous fluid must have a peralkaline composition, which has important implications for mass transfer in the crust an upper mantle, e.g. enhancement of the solubility of high field strength elements.

Pressure determination in HDAC experiments with haplogranite glass plus water by laser interferometry

The pressure in a series of Hydrothermal Diamond Anvil Cell (HDAC) experiments with water plus haplogranite was determined via in-situ observation of the alpha to beta quartz transformation in the range 130–900MPa and 600–800°C, by means of laser interferometry.The pressures measured at the alpha-beta quartz transformation temperature are found to be similar or less than the values estimated using the EOS of pure water (including the error) for runs where the transition temperature is ≤665°C. On the contrary, at ~780°C the pressure in the HDAC is ~100MPa greater than that estimated using pure water isochore.A simplified model of haplogranite dissolution in water for an isochoric sample chamber is proposed to explain the negative sign and the variation of the observed discrepancy between measured and calculated pressure.We suggest that, beyond the change in the intrinsic properties of the fluid phase with increasing solute concentration, two factors control the P–T path in the HDAC: (1) hydration of the glass prior to the glass transition; and (2) changing volume of the aqueous pressure medium.The measurements made in this study demonstrate that laser interferometry on quartz inversion is an effective tool for precise pressure determination in HDAC experiments aimed at the investigation of physical and chemical properties of quartz-saturated hydrous melts in the mid and upper continental crust (i.e., P<1GPa).

Effects of fluid and melt density and structure on high-pressure and high-temperature experimental studies of hydrogen isotope partitioning between coexisting melt and aqueous fluid

Hydrogen isotope partitioning (as H2O and D2O) between silicate-saturated aqueous fluid and water-saturated aluminosilicate melt has been determined with vibrational spectroscopy (Raman and infrared) in situ with the samples at high temperature and pressure by using a hydrothermal diamondanvil cell (HDAC) for sample containment. To assess the effects of pressure and, therefore, different silicate speciation in fluids and melts, on the D/H partitioning behavior, two pressure/temperature experimental trajectories (450-800 °C/155-754 MPa, and 450-800 °C/562-1271 MPa) were used. In these temperature and pressure ranges, the fluid/melt partition coefficients are temperature (and pressure) dependent with the average enthalpy change, △H = -6.6 ± 15 kJ/mol and -10.3 ± 1.1 kJ/mol for H2O and D2O, respectively. The △H-values for the lower-pressure trajectory (and, therefore, lower fluid density) were 15-20% higher than for the higher-pressure (and higher fluid density) trajectory. The (D/H) ratios of fluids and melts, (D/H)fluid and (D/H)melt, are also temperature dependent with a small negative DH for (D/H)fluid (average: -2.4 ± 0.8 kJ/mol) and a positive △H-value for (D/H)melt (2.3 ± 1.4 kJ/mol). The (D,H) exchange equilibrium between fluid and melt is also temperature (and pressure) dependent so that for the low-density P/T trajectory, the △H = -4.2 ± 0.6 kJ/mol, whereas for the higher-density trajectory, △H = -5.4 ± 0.7 kJ/mol. The difference between the H2O and D2O fluid/melt partition coefficients and the temperature- and pressure-dependent D/H fractionation behavior in and between hydrous silicate melts and silicate-saturated aqueous fluid in part is because pressure increases with increasing temperature in the HDAC experiments and the volume difference between fluid and melt differ for H2O and D2O. In addition, the silicate speciation in fluids and melts are temperature and pressure dependent, which also leads to significantly temperature- and pressuredependent D/H fractionation within and between silicate melts and fluids at high temperature and pressure. In the Earth’s deep crust and upper mantle, hydrogen isotope partitioning between condensed phases and aqueous fluid can differ substantially from that between condensed phases and pure H2O because the aqueous fluid in the Earth’s interior is a concentrated silicate solution wherein the silicate speciation affects the isotope partitioning.

H/D methane isotopologues dissolved in magmatic fluids: Stable hydrogen isotope fractionations in the Earth's interior

A series of hydrothermal diamond-anvil cell experiments have been conducted to evaluate the role of supercritical water on the isotopic equilibrium between H/D methane isotopologues at 600-800 °C and 409-1622 MPa. Raman spectroscopy was deployed to investigate the distribution of H/D isotopic molecules formed during hydrothermal decomposition of Si5C12H36 in H2O-D2O aqueous solutions. To this end, the intensities of the fundamental vibrational C-H and C-D modes of deuteromethanes were employed to determine the thermodynamic properties of isotope exchange reactions between H/D isotopologues and to constrain the methane D/H molar ratios. By adjusting the initial volume ratios of silane/H2O-D2O, reactions in the CH4-D2O-H2O system were monitored for gaseous and supercritical-water phases. Discreet differences between the equilibrium constants, describing the relationship between the CH3D-CH2D2-CHD3-CH4 species dissolved in supercritical water or present as a homogeneous gas phase, are revealed. The bulk D/H methane composition in the liquid-system is also twice that of the D/H molar ratios recorded in the gas-bearing system. Accordingly, condensedphase isotope effects are inferred to play a key role on the evolution of H/D isotopologues, likely induced by differences in the solubility of the isotopic molecules driven by the excess energy/entropy developed during mixing of non-polar species in the H2O-D2O structure. Our experiments show that isotope fractionation effects need to account for the presence of condensed matter (e.g., melts, magmatic fluids), even at conditions at which theoretical models suggest minimal (or nonexistent) isotope exchange, but comparable to those within the Earth’s interior.

Thermal reduction of molybdite and hematite in water and hydrogen peroxide-bearing solutions: Insights on redox conditions in Hydrothermal Diamond Anvil Cell (HDAC) experiments

The oxygen fugacity in Hydrothermal Diamond Anvil Cell (HDAC) experiments was determined by means of direct observation of the thermal reduction of molybdite or hematite in water and water–H2O2 solutions. HDAC experiments were conducted with and without rhenium gasket in order to evaluate the effect of rhenium on oxygen fugacity of the contained sample. In experiments where pure water and no Re gasket were used, i.e., when the sample charge is loaded into a laser-drilled recess in the lower anvil, the Log(fO2) was shown to be −19.55±0.13 over a wide range of temperature and pressure conditions (335–500°C, 15.0–136.3MPa — pressure determined using EOS of water by Wagner and Pruss, 2002), i.e., HM+8 to HM−0.5, at the limit temperatures. In contrast, in experiments where a Re gasket was used to contain the sample, the Log(fO2) is slightly more reducing (−20.60±0.04), e.g., equals to HM+6 at 350°C, but is significantly higher than that predicted at a given temperature using the Re–ReO2 buffer. Experiments with hydrogen peroxide showed a non-linear increase of oxygen fugacity with increasing molarity of H2O2 from 0.10 to 0.25M. Results indicate that the oxygen fugacity in water dominated HDAC runs is imposed by the fluid medium, and that hydrogen peroxide may be used to increase oxygen fugacity of water-dominated fluid media in HDAC experiments.

Ammonium in aqueous fluids to 600 °C, 1.3 GPa: A spectroscopic study on the effects on fluid properties, silica solubility, and K-feldspar to muscovite reactions

The behavior of ammonium, NH 4 +, in aqueous systems was studied based on Raman spectroscopic experiments to 600 °C and about 1.3 GPa. Spectra obtained at ambient conditions revealed a strong reduction of the dynamic three-dimensional network of water with addition of ammonium chloride, particularly at small solute concentrations. The differential scattering cross section of the ν 1-NH 4 + Raman band in these solutions was found to be similar to that of salammoniac. The Raman band of silica monomers at ∼780 cm −1 was present in all spectra of the fluid at high temperatures in hydrothermal diamond-anvil cell experiments with H 2O ± NH 4Cl and quartz or the assemblage quartz + kyanite + K-feldspar ± muscovite/tobelite. However, these spectra indicated that dissolved silica is less polymerized in ammonium chloride solutions than in comparable experiments with water. Quantification based on the normalized integrated intensity of the H 4SiO 4 0 band showed that the silica solubility in experiments with H 2O + NH 4Cl was significantly lower than that in equimolal NaCl solutions. This suggests that ammonium causes a stronger decrease in the activity of water in chloridic solutions than sodium. The Raman spectra of the fluid also showed that a significant fraction of ammonium was converted to ammonia, NH 3, in all experiments at temperatures above 300 °C. This indicates a shift towards acidic conditions for experiments without a buffering mineral assemblage. The estimated pH of the fluid was ∼2 at 600 °C, 0.26 GPa, 6.6 m initial NH 4Cl, based on the ratio of the integrated ν 1-NH 3 and ν 1-NH 4 + intensities and the HCl 0 dissociation constant. The NH 3/NH 4 + ratio increased with temperature and decreased with pressure. This implies that more ammonium should be retained in K-bearing minerals coexisting with chloridic fluids upon high- P low- T metamorphism. At 500 °C, 0.73 GPa, ammonium partitions preferentially into the fluid, as constrained from infrared spectroscopy on the muscovite and from mass balance. The conversion of K-feldspar to muscovite proceeded much faster in experiments with NH 4Cl solutions than in comparable experiments with water. This is interpreted as being caused by enhancement of the rate-limiting alumina solubility, suggesting complexation of Al with NH 4. Nucleation and growth of mica at the expense of K-feldspar and NH 4 +/K + exchange between fluid and K-feldspar occurred simultaneously, but incorporation of NH 4 + into K-feldspar was distinctly faster than K-feldspar consumption.

Extreme alkali bicarbonate- and carbonate-rich fluid inclusions in granite pegmatite from the Precambrian Rønne granite, Bornholm Island, Denmark

Our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the Precambrian Ronne granite, Bornholm Island, Denmark revealed extremely alkali bicarbonate- and carbonate-rich inclusions. The solid phases (daughter crystals) are mainly nahcolite [NaHCO3], zabuyelite [Li2CO3], and in rare cases potash [K2CO3] in addition to the volatile phases CO2 and aqueous carbonate/bicarbonate solution. Rare melt inclusions contain nahcolite, dawsonite [NaAl(CO3)(OH)2], and muscovite. In addition to fluid and melt inclusions, there are primary CO2-rich vapor inclusions, which mostly contain small nahcolite crystals. The identification of potash as a naturally occurring mineral would appear to be the first recorded instance. From the appearance of high concentrations of these carbonates and bicarbonates, we suggest that the mineral-forming media were water- and alkali carbonate-rich silicate melts or highly concentrated fluids. The coexistence of silicate melt inclusions with carbonate-rich fluid and nahcolite-rich vapor inclusions indicates a melt-melt-vapor equilibrium during the crystallization of the pegmatite. These results are supported by the results of hydrothermal diamond anvil cell experiments in the pseudoternary system H2O–NaHCO3–SiO2. Additionally, we show that boundary layer effects were insignificant in the Bornholm pegmatites and are not required for the origin of primary textures in compositionally simple pegmatites at least.

Diamond dissolution and the production of methane and other carbon-bearing species in hydrothermal diamond-anvil cells

Raman analysis of the vapor phase formed after heating pure water to near critical (355–374 °C) temperatures in a hydrothermal diamond-anvil cell (HDAC) reveals the synthesis of abiogenic methane. This unexpected result demonstrates the chemical reactivity of diamond at relatively low temperatures. The rate of methane production from the reaction between water and diamond increases with increasing temperature and is enhanced by the presence of a metal gasket (Re, Ir, or Inconel) which is compressed between the diamond anvils to seal the aqueous sample. The minimum detection limit for methane using Raman spectroscopy was determined to be ca. 0.047 MPa, indicating that more than 1.4 nanograms (or 8.6 × 10 −11 mol) of methane were produced in the HDAC at 355 °C and 30 MPa over a period of ten minutes. At temperatures of 650 °C and greater, hydrogen and carbon dioxide were detected in addition to methane. The production of abiogenic methane, observed in all HDAC experiments where a gasket was used, necessitates a reexamination of the assumed chemical systems and intensive parameters reported in previous hydrothermal investigations employing diamonds. The results also demonstrate the need to minimize or eliminate the production of methane and other carbonic species in experiments by containing the sample within a HDAC without using a metal gasket.