Hydrothermal Diamond Anvil Cell Research Articles

Molecular H2 in silicate melts

A series of hydrothermal diamond anvil cell experiments was conducted to constrain the equilibrium distribution of molecular H2 between H2O-saturated sodium aluminosilicate melts and H2O at elevated temperatures (600–800 °C) and pressures (317–1265 MPa). The distribution of H2 between the silicate liquid and the aqueous fluid was achieved through real-time monitoring of the H-H stretching vibration under in situ conditions using Raman vibrational spectroscopy. Results show that the solubility of H2 in silicate melts saturated with H2O decreases as the temperature increases, with control exerted by the mole fraction of H2O in the melt. The dissolution of H2 in the hydrous silicate melts appears to follow Henrian behavior, resembling that of an inert, neutral non-polar species. To express species solubility as a function of temperature (T in K) an empirical equation was developed:ln Km/f = 11.4 (±1.3) *1000 / T (K) − 18.7 (±1.1)where Km/f is the equilibrium constant for the reaction H2(g) = H2(melt). This equation was derived by integrating data from the current and prior experimental studies that include silicate melts with varying H2O saturation levels. It should be deemed applicable within the temperature range of 600–1450 °C and pressures ranging from 0.3 to 3 GPa. The implications are extended into developing an understanding of the H partitioning between H2-rich atmospheres blanketing magma oceans in the early history of planetary bodies. For example, transferring H from primordial atmospheric envelopes to the interior of rocky exoplanets may be less efficient than previously believed, which should be considered in models of volatile retention. Experimental data also suggest that minimal amounts of solar nebula H2 are likely to dissolve in the molten surface of primitive objects in the protoplanetary disk (∼10−5 to 10−9 mol faction H2 in melt), contradicting the highly reducing conditions observed in chondrule mineral compositions.

In situ determination of NaCl-H2O isochores up to 900 oC and 1.2 GPa in a hydrothermal diamond-anvil cell

In situ determination of NaCl-H2O isochores up to 900 oC and 1.2 GPa in a hydrothermal diamond-anvil cell

Hydrolysis rate constants of ATP up to 120 °C and 1.6 GPa: Implications for life at extreme conditions

Extreme environments are habitats for a diverse array of microorganisms, namely extremophiles, that have evolved unique biochemical adaptations to their geological setting. Some examples of extremophiles can be found in the subseafloor or near hydrothermal vents on the ocean floor. Cultivated strains of extremophiles have demonstrated the ability to tolerate temperatures (T) up to 122 °C and pressures (P) up to 125 MPa. Organisms depend on the stability of key metabolites, such as adenosine triphosphate (ATP), to survive and reproduce under these conditions. In order to maintain their intracellular ATP levels, living cells must compensate for the abiotic hydrolysis of ATP, which occurs at a particularly rapid rate at high temperatures. The role of high pressures and high temperatures in abiotic hydrolysis of ATP has been rarely investigated despite the potential for this phenomenon to contribute to limit the adaptation of microorganisms to simultaneous extreme temperatures and pressures. This study presents new data on the effect of pressure on the abiotic hydrolysis of ATP to adenosine diphosphate (ADP) at elevated temperatures. In situ Raman spectra were measured at high pressure and high temperature of the hydrolysis of aqueous disodium ATP solutions in two experimental systems: a hydrothermal diamond anvil cell (HDAC) and a gas-pressurized autoclave. These two systems permitted the determination of hydrolysis rate constants of ATP into ADP up to 1670 MPa at 80 °C, 100 °C, and 120 °C. The data exhibited Arrhenian behavior with a slight decrease in activation energy from 0.5 MPa to 140 MPa. The effect of pressure on ATP hydrolysis rate constants was found to be vanishingly low in the so far known vital range up to 125 MPa. Abiotic hydrolysis rates of ATP showed a pronounced increase at higher pressures. For example, at 100 °C, a rise in pressure from 365 MPa to approximately 1670 MPa results in a nearly tenfold increase in the ATP hydrolysis rate constant. When compared with the typical ATP turnover times reported in living cells, the abiotic ATP hydrolysis rates determined in this study provide insights into the pressure and temperature conditions that could be consistent with living microorganisms.

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.

High‐pressure and high‐temperature Raman spectroscopic study of zircon as a pressure scale in hydrothermal DACs

AbstractRaman spectra of zircon have recently been used as a pressure scale for studies of geological fluids at high temperatures and high pressures using diamond anvil cells (DACs). The zircon scale is advantageous in high chemical stability and the large pressure response of the B1g mode. Despite its excellent applicability, the calibration of the scale has been carried out only in a narrow pressure–temperature range, especially under limited high‐temperature and high‐pressure conditions. In this study, the pressure and temperature dependence of the Raman modes of synthetic zircon was investigated up to 9.5 GPa and from room temperature to 776 K using an externally heated diamond anvil cell. Ruby and gold were used as the reference pressure scales. The Raman shift of the B1g mode for the antisymmetric stretching of the SiO4 structure in zircon showed a linear pressure dependence of 5.48(4) cm−1/GPa up to 8 GPa at room temperature, in agreement with the previous studies. Measurements under high‐pressure and high‐temperature conditions confirmed that the pressure dependence up to 9.5 GPa along the isotherms from 373 to 675 K was consistent with the room‐temperature value; the wavenumbers can be well deduced from the sum of the individual effects of pressure and temperature, obtained at ambient temperature and pressure, respectively. A comparison of the zircon scale with the c‐BN Raman spectroscopic scale confirmed that the pressures determined with these scales were in reasonable agreement. The present results provide a confident use of the zircon Raman spectroscopic scale in a wider pressure–temperature range than previous studies for the internally consistent pressure determination.

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.

Rhenium solubility and speciation in aqueous fluids at high temperature and pressure

In order to characterize rhenium transport via infiltration of fluids in the Earth's interior, the solubility and solution mechanisms of ReO2 in aqueous fluids were determined to 900 °C and about 1710 MPa by using an externally–heated hydrothermal diamond anvil cell. In order to shed light on how Re solubility and solution mechanisms in aqueous fluids can be affected by interaction of Re with other solutes, compositions ranged from the comparatively simple ReO2–H2O system to compositionally more complex Na2O–ReO2–SiO2–H2O fluids. Fluids in the ReO2–SiO2–H2O, SiO2–H2O, Na2O–SiO2–H2O, and Na2O–ReO2–H2O systems also were examined. The presence of Na2O enhances the ReO2 solubility so that in Na2O–ReO2–H2O fluids, for example, Re solubility is increased by a factor of 10–15 compared with the Re solubility in Na2O-free ReO2–H2O fluids. The SiO2 component in ReO2–SiO2–H2O causes reduction of ReO2 solubility compared with ReO2–H2O fluids. The ReO2 solubility in the Na-bearing Na2O–ReO2–SiO2–H2O fluids is greater than that in fluids in both the ReO2–H2O and ReO2–SiO2–H2O systems. Rhenium is dissolved in aqueous fluid as ReO4-complexes with Re in fourfold coordination with oxygen. Some, or all, of the oxygen in these complexes is replaced by OH-groups depending on whether Na2O also is present. It is proposed that during dehydration of hydrated subduction zone mineral assemblages in the upper mantle, the alkali/alkaline earth ratio of the source of the released aqueous fluid affects the extent to which Re (and other HFSE) can be transported into an overlying peridotite mantle wedge. The infiltration of such fluids will, in turn, affect the Re content (and Re/Os ratio) of magma formed by partial melting of this peridotite wedge.

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.

Transition from carbonatitic magmas to hydrothermal brines: Continuous dilution or fluid exsolution?

Carbonatites are the most important primary sources for the rare earth elements (REEs). While fractional crystallization of carbonate minerals results in the enrichment of volatiles, alkalis, and REEs in the remaining melts, the transition from carbonatitic magmas to hydrothermal brines remains unclear. Here, we investigated the pressure-temperature-composition (P-T-X) properties of the Na2CO3-H2O system up to 700°C and 11.0 kbar using a hydrothermal diamond anvil cell and a Raman spectrometer. Our results show that Na2CO3 becomes increasingly soluble under high P-T conditions, leading to the disappearance of melt-fluid immiscibility and the continuous transition from Na2CO3 melts to hydrothermal brines under deep crustal conditions. Given the abundance of Na2CO3 in highly evolved carbonatitic systems, we suggest that the continuous melt-fluid transition in deep-seated carbonatites results in REEs being sufficiently concentrated in the brine-melts to form economic ore bodies, whereas in shallow systems, REEs preferentially partition into carbonatitic magmas over synmagmatic brines and disperse in carbonatite rocks that underwent limited fractionation.

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.

The temperature dependence of Raman intensity of aqueous species (SO42− and H3PO40): Implication for in situ fluid composition investigation at elevated temperature

Raman spectroscopy is a crucial technology in geoscience and often employed in in situ quantification experiments to investigate fluid compositions at elevated temperatures. Raman spectroscopic quantification is fundamental to understanding the hydrothermal behaviors of aqueous species, such as SO42− and H3PO40. Considering that SO42− and PO43− are closely associated with metal transportation and deposition, investigating the hydrothermal behaviors of these species, via Raman spectroscopic quantification, can give an assistance for the comprehension of relative ore-forming processes. During Raman spectroscopic quantification, the accuracy of the calibration coefficient (k), which bridges the concentration of a species (Cspecies) and its Raman peak area (Anormalized) in the form of Cspecies = k · Anormalized, is the key to accurate measurement of fluid composition. For high-temperature experiments, quantification of fluid composition is complicated because the coefficient k varies with varying temperatures. In this study, the temperature dependence of the calibration coefficients is retrieved, based on experiments on solutions containing SO42− and H3PO40 using the fused silica capillary capsules. The results show a well-established linear relationship between the k and 1/T (absolute temperature in Kelvin) for both SO42− and H3PO40. The reported linear k − 1/T relationship simplifies Raman-based fluid composition investigation at elevated temperatures: the k values throughout the whole temperature range of interest can be retrieved by measuring the room-temperature Raman intensity of a standard sample (i.e., with known composition) and one single data point at high temperature. We speculate that this linear k − 1/T relationship method may be established for other species (e.g., carbonate ions) and can be applied for experiments using other technics (e.g., hydrothermal diamond anvil cell) or retrieving the compositions of natural or synthetic fluid inclusions. For example, the empirical linear relationship can facilitate the measurements of gas concentrations (e.g., CO2) in natural fluid inclusions.

Toward quantitative experiment using hydrothermal diamond anvil cell: Solubility of sylvite up to 1.6 GPa

Potassium chloride is a common component in geofluids and sylvite is a broadly observed daughter mineral in fluid inclusions along with halite, especially in skarn- and porphyry-type deposits. Interpretation of fluid inclusion microthermometric results requires knowledge of high pressure and temperature (P-T) solubility of salts, which is, however, incomprehensive for sylvite in the KCl-H2O system. In this study, the high P-T solubility of sylvite is determined within the compositional (salinity) range of 38–62 wt% KCl, using a new method that applies the three-dimensional reconstruction function of confocal laser fluorescence microscopy (CLFM) to hydrothermal diamond anvil cell (HDAC) experiments. This method is used to retrieve the volumes of phases (i.e., sylvite, aqueous solution and bubble) in the sample chamber of HDAC and thus to achieve quantitative analysis of sylvite solubility at elevated P-T. Combining experimental results in this study with those published in previous literature, the P-T dependence of the solubility of sylvite is retrieved and applied to data interpretation of microthermometric results. For inclusions containing both halite and sylvite daughter minerals, the commonly used approach for evaluating fluid salinity and P is to simplify the inclusions as the binary NaCl-H2O system. This simplification would lead to underestimation of fluid salinities by up to ~34% and homogenization P by up to 0.25 GPa, for fluid inclusions having relatively high T of daughter mineral dissolution, which are typical of magmatic-hydrothermal ore deposits.

Hydrolysis rate constants of ATP determined in situ at elevated temperatures

Life can be found even in extreme environments, e.g., near black smokers on the ocean floor at temperatures up to ca. 120 °C and hydrostatic pressures of 40 MPa. To maintain vital reactions under these hostile conditions, extremophiles are interacting with the surrounding geochemical system. In this context, the stabilities of the essential energy-storing adenosine triphosphate (ATP) and adenosine diphosphate (ADP) are of fundamental importance because reactions involving adenosine phosphates constrain the conditions at which carbon-based life can exist and might be also crucial information for the search of extra-terrestrial life. Adenosine phosphates react by non-enzymatic hydrolysis, which is kinetically enhanced at high temperatures. If these abiotic hydrolysis processes are too rapid, they will most likely prevent metabolisms from relying on ATP.Here, we report on an approach used in experimental geochemistry, i.e., in situ Raman spectroscopic analyses of a fluid phase at high temperature using a hydrothermal diamond anvil cell. This combination allowed the investigation of the hydrolysis of Adenosine Tri-Phosphate (ATP) in aqueous solution at pH 3 and 7 and at temperatures of 80, 100 and 120 °C, extending the so far measured temperature range substantially. We observed Arrhenian behaviour over this temperature interval. The rate constants at 120 °C were 4.34 × 10−3 s−1 at pH 3 and 2.91 × 10−3 s−1 at pH 7. This corresponds to ATP half-lives of a few minutes. These high decomposition rates of ATP suggest that organisms must have developed a mechanism to counteract this fast reaction at high temperatures to maintain the vital processes.

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.

In situ determination of magnesite solubility and carbon speciation in water and NaCl solutions under subduction zone conditions

The dissolution behavior of carbonates in subduction zone fluids has not been well constrained. In this study we investigated the solubility of magnesite in pure water and NaCl solutions (with up to 19 wt% NaCl) in situ with a hydrothermal diamond anvil cell, and determined carbon speciation in the fluid by Raman spectroscopy. The solubility of magnesite in pure water falls in the range of 0.01–0.05 mol/kg at 0.7–2.4 GPa and 635–940 °C, enhanced strongly by temperature. Least squares fitting of experimental data leads to the following empirical expression for magnesite solubility in pure water: CMgCO3=(964510±41362)exp(−6953±690T)exp[0.0200±0.0098T(P−1)] where CMgCO3 is given in μg/g, T is the temperature in K, and P is the pressure in bar, respectively. The solubility of MgCO3 is about an order of magnitude lower than that of CaCO3 at 0.7–2.4 GPa, 640–940 °C. The solubility enhancement factor by NaCl (m/m° with m and m° being magnesite solubility in NaCl solution and water, respectively) presents as a parabolic trend with the mole fraction of NaCl in the range of 0–0.07, with a maximum amplification of 5.2 at XNaCl = 0.035, which is different from the continuously increase of solubility with salinity increasing at high salinity conditions in previous studies and suggests the dissolution reaction of magnesite in dilute NaCl solution is different. Despite slight contamination of CH4 formed by the reaction of the diamond anvils, we were able to identify CO32− and HCO3- to be the aqueous carbon species, HCO3- was predominant over CO32− in the range of 200–800 °C and 1.9–3.8 GPa and its proportion was affected by temperature, but not affected by pressure at 400–600 °C. Our experimental data suggest that in the absence of melting, only a small amount of magnesite can be mobilized by the slab-released fluid at subarc depths.

Refinement of the α–β quartz phase boundary based on in situ Raman spectroscopy measurements in hydrothermal diamond‐anvil cell and an evaluated equation of state of pure H2O

AbstractIn this study, we determined the α–β quartz phase transition temperatures (Ttrs; up to ~924°C for a total of 23 obtained under 0.1 MPa and elevated pressures) in a hydrothermal diamond‐anvil cell (HDAC) at H2O pressures up to ~1310 MPa by using in situ Raman spectroscopy. When compared with the commonly used α‐quartz Raman band near 465 cm−1 (at 24°C and 0.1 MPa), the one near 128 cm−1 is much more sensitive for detecting Ttr. The corresponding phase transition pressure (Ptr) at each measured Ttr was calculated from three equations of state (EoS) of pure H2O. The corresponding three α–β quartz phase boundaries were compared with those derived from previous experimental results. The best fitted one can be represented by: Ptr (±5.6 MPa) = 0.0008·Ttr2 + 2.8056·Ttr − 1877.5, where 574°C ≤ Ttr ≤ 889°C with R2 = 0.9998. Our results agree, within experimental uncertainties, with most of previous experimental data, and our newly determined α–β quartz Ptr–Ttr phase boundary can be used as a reliable pressure calibrant in HDAC. Furthermore, the experimental procedures used in this study, including the use of Raman spectroscopy criterion for Ttr determination, can be easily applied to obtain improved EoS of geologically important aqueous solutions containing salt(s) at pressure–temperature conditions up to ~1.2 GPa–900°C.

Origin of pegmatitic melts from granitic magmas in the formation of the Jiajika lithium deposit in the eastern Tibetan Plateau

Granitic pegmatites may be genetically related to granites. However, it is still debatable how pegmatitic melts originated from granitic magmas. For example, a possible mechanism of melt–melt immiscibility for the origin of pegmatitic melt has not been well documented. The Jiajika Li deposit in the eastern Tibetan Plateau is a world-class deposit and is hosted in pegmatites, which occurred as dikes surrounding their source granite (two-mica granite). This paper concerns melt inclusions (MIs) in quartz from the granite–marginal pegmatite (GMP) at the roofward contact of the two-mica granite with the country-rock schist, as the MIs from such pegmatite are considered to represent the original pegmatitic melt and hence are ideal for investigating the origin of pegmatitic melts from granitic magmas. The MIs can be classified into H2O–CH4- and H2O–CO2-bearing types, both containing solid phases, including muscovite, albite, and apatite; the H2O–CO2-bearing MIs are usually associated with the H2O–CO2 fluid inclusions. To prevent leakage or decrepitation of these MIs during heating, external pressures were applied by using a hydrothermal diamond-anvil cell. The compositions of muscovite inside the unheated MIs and glass inside the MIs, which was quenched from a homogenous state, were analyzed using electron microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). During heating, the H2O–CH4-bearing MIs were homogenized into an H2O-saturated melt mostly at 750–800 °C, while the H2O–CO2-bearing MIs, when similarly treated, showed H2O–CO2-oversaturated entrapment features. The H2O–CH4-bearing MIs are considered to be trapped from the original pegmatitic melt containing ∼12 wt% H2O with up to ∼1000 ppm Li, ∼1500 ppm Rb, and ∼1800 ppm Cs, at pressures of ∼550–700 MPa. Based on previous MI-homogenization experiments, it has been reported that the microthermometry features and compositions (e.g., H2O contents) of the H2O–CH4-bearing MIs in the GMP quartz are similar to those in an H2O-rich melt, which was immiscible with an H2O-poor melt inside the heated MIs in quartz from the two-mica granite. Accordingly, in this study, we demonstrate that pegmatitic melts could be originated via melt–melt immiscibility of granitic magma, and that the initial exsolution of the pegmatitic melt from granitic magma can occur on a very small scale, as tiny droplets whose sizes may be similar to those of the observed MIs in granite. In addition to the enrichment of Li, B, and P, the enhanced H2O content under low oxygen fugacity condition in the granitic magma, as well as the decrease in pressure following the magma emplacement, could promote the melt–melt immiscibility.

Electrical Conductivity of KCl‐H2O Fluids in the Crust and Lithospheric Mantle

AbstractEvidence from diamond inclusions suggests that KCl‐bearing fluids may be abundant in the subcratonic mantle and these fluids may also cause local anomalies of high electrical conductivity. We therefore measured the electrical conductivity of aqueous fluids containing 6.96, 0.74 and 0.075 wt% KCl using a hydrothermal diamond anvil cell to 2.5 GPa and 675°C, and with a piston‐cylinder apparatus to 5 GPa and 900°C. We found that below 550°C, increasing pressure generally decreases solution conductivity, while at higher temperatures the effect is opposite. However, at high pressures, the conductivity of KCl in H2O is smaller than for NaCl, possibly due to a hydration shell collapse. The experimental data are described by two numerical models. The first model (R2 = 0.999), is preferable for crustal conditions: log σ = −2.03 + 25.0 × T−1 + 0.923 × log c + 0.990 × log ρ + log Λ0, where σ is the conductivity in S/m, T is temperature in K, c is KCl concentration in wt%, ρ is the density of pure water (in g/cm3) at given pressure and temperature. Λ0 is the limiting molar conductivity of KCl (in S·cm2·mol−1): Λ0 = 1,377 – 1,082 × ρ + 6,883 × 102 × T−1 – 2,471 × 105 × T−2. The second model (R2 = 0.986), is applicable to the lithospheric mantle: log σ = −1.52 – 357 × T−1 + 0.865 × log c + 1.72 × log ρ + log Λ0, with the same equation for Λ0. The latter model shows that already traces of KCl‐bearing aqueous fluid may account for high conductivity anomalies in the subcratonic mantle.

Raman Spectroscopic Studies of Pyrite at High Pressure and High Temperature

Variations in the Raman spectra of pyrite were studied from 113 to 853 K at room pressure with a Linkam heating and freezing stage, and for 297–513 K and pressures up to 1.9 GPa with a hydrothermal diamond anvil cell. All observed frequencies decreased continuously with an increase in temperatures up to 653 K at ambient pressure. Hematite began to form at 653 K, all pyrite had transformed to hematite (H) at 688 K, and the hematite melted at 853 K. An increase in temperature at every initial pressure (group 1: 0.5 GPa, group 2: 1.1 GPa, group 3: 1.7 GPa, group 4: 1.9 GPa), showed no evidence for chemical reaction or pyrite decomposition. Two or three Raman modes were observed because of crystal orientation or temperature-induced fluorescence effects. The pressure groups showed a decreasing trend of frequency with gradual heating. The interaction of pressure and temperature led to a gradual decrease in Ag and Eg mode at a lower pressure (0.5 GPa and 1.1 GPa) than other pressure groups. Pressure and temperature effects are evident for groups 1 and 2; however, for groups 3 and 4, the temperature shows a larger effect than pressure and leads to a sharp decrease in Ag and Eg modes.

In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature

The dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg H2O–1] Na2CO3 and 0.47 m NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3 and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H2O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1 (silicate center having a single bridging oxygen atom) in the spectra of the Na2CO3 solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3 and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.