Crystallization conditions and evolution of magmatic fluids in the Harney Peak Granite and associated pegmatites, Black Hills, South Dakota—Evidence from fluid inclusions

A microthermometric study of inclusions in granites and pegmatites in the Proterozoic Harney Peak Granite system identified four types of inclusions. Type 1 inclusions are mixtures of CO₂ and H₂O and have low salinities, on average 3.5 wt.% NaCl_eq; type 2 inclusions are aqueous solutions of variable salinities, from 0 to 40% wt.% NaCl_eq; type 3 inclusions are carbonic, dominated by CO₂, with no detectable water; and type 4 inclusions consist of 20 to 100% solids, with the remaining volume occupied by a CO₂-H₂O fluid. Many inclusions have a secondary character; however, a primary character can be unambiguously established in several occurrences of the type 1 inclusions. These inclusions were trapped above the solidus and represent the exsolved magmatic fluid. The secondary populations of types 1, 2, and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions. Type 4 inclusions are primary and are interpreted to be trapped, fluid-bearing, complex silicate melts that subsequently solidified or underwent other posttrapping changes.It is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex, Li-bearing Tin Mountain pegmatite were trapped along the two-fluid phase boundary in the system CO₂-H₂O-NaCl_eq. Consequently, the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions—on average 340°C and 2.7 kbar. These conditions indicate that this Li-, Cs-, Rb-, P-, and B-rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust. An internally consistent, empirical solvus surface in P-T-X_CO2 coordinates was generated for the pseudobinary CO₂-(H₂O-4.3 wt.% NaCl_eq) pegmatite fluid system. Distribution coefficients for the major species CO₂, H₂O, NaCl, and CH₄ between the immiscible CO₂-rich and H₂O-rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types.     

Placing constraints on phase equilibria and thermophysical properties in the system MgO-SiO2 by a thermodynamically consistent vibrational method

We use a lattice vibrational technique to derive thermophysical and thermochemical properties and phase equilibria in the system MgO-SiO₂ at pressures and temperatures relevant to Earth’s mantle. The technique is based on an extension of Kieffer’s model to incorporate details of the phonon spectrum, and it includes treatment of intrinsic anharmonicity. We use a least squares inversion technique applied to available experimental data, and show that it results in an accurate representation of thermodynamic properties and sound wave velocities of high-pressure phases in the system MgSiO₃. The vibrational method has been validated against laboratory experimental data in the temperature range between 0 and 2500 K and at pressures between 1 bar and 30 GPa. The technique results in a phase diagram consistent with the majority of thermophysical and thermochemical data. It is shown that intrinsic anharmonicity affects significantly slopes and positions of the phase boundaries. Our analysis indicates inconsistencies in a number of data sets of thermophysical properties for stishovite, majorite and ortho-enstatite necessitating new measurements. For akimotoite elasticity data at high-pressure and high-temperature conditions and 1 bar heat capacity measurements are needed. For stishovite elasticity measurements are necessary to reconcile elasticity data with V-P-T measurements. Additionally V-P-T measurements at pressures higher than 10 GPa are needed, which should be reconciled with V-P-T data at lower pressures. Raman and infrared spectroscopic data are necessary for both clino-enstatite and majorite. Additionally structural data are needed to resolve the discrepancy between values for the degree of disorder in majorite. Volume expansion data for ortho-enstatite are needed and effects causing differences in measured volume expansion should be elucidated.     

Synthetic fluid inclusions in natural quartz. III. Determination of phase equilibrium properties in the system H2O-NaCl to 1000°C and 1500 bars

Phase equilibria in the system H₂O-NaCl have been determined to 1000°C and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion-free Brazilian quartz in the presence of the two coexisting, immiscible H₂O-NaCl fluids at various temperatures and pressures. Petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present, or mixtures of the two. Salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single, homogeneous fluid phase.Results of the present study are consistent with previously published data on the H₂O-NaCl system at lower temperatures and pressures, and indicate that the two-phase field extends well into the P-T range of most shallow magmatic-hydrothermal activity. As a consequence, chloride brines exsolved from many epizonal plutons during the process of “second-boiling” should immediately separate into a high-salinity liquid phase and a lower salinity vapor phase and produce coexisting halite-bearing and vapor-rich fluid inclusions. This observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions, particularly the porphyry copper deposits, in which halite-bearing and coexisting vapor-rich inclusions are commonly associated with the earliest stages of magmatic-hydrothermal activity.     

Methane: An equation of state with application to the ternary system H2O-CO2-CH4

The following hardsphere modified Redlich-Kwong (HSMRK) equation of state was obtained by least squares fitting to available P-V-T data for methane (P in bars; T in Kelvins; v in cm³ mol^?1; b = 60.00 cm³ mol^?1; R = 83.14 cm³barmol^?1K^?1): $\text{P}\text{RT(1 + y + y}{}^2\text{?y}{}^3\text{v(1?y)}{}^3\text{)}\text{-}\text{c(T) + d(T)}\text{v}\text{+}\text{e(T)}\text{v}{}^2\text{/v(v + b)T}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$$\text{y =}\text{b}\text{4v}$c(T) = 13.403 × 10⁶ + (9.28 × 10⁴)T + 2.7 T²d(T) = 5.216 × 10⁹ ? (6.8 × 10⁶)T + (3.28 × 10³)T²e(T) = (?2.3322 × 10¹¹) + (6.738 × 10⁸)T + (3.179 × 10⁵)T² For the P-T range of experimental data used in the fit (50 to 8600 bars and from 320 to 670 K), calculated volumes and fugacity coefficients for CH₄ relative to experimentally determined volumes and fugacity coefficients have average percent deviations of 0.279 and 1.373, respectively. The HSMRK equation, which predicts linear isochores over a wide P-T range, should yield reasonable estimates of fugacity coefficients for CH₄ to pressures and temperatures well outside the P-T range of available P-V-T data. Calculations for the system H₂O-CO₂-CH₄, using the HSMRK equations for H₂O and CO₂ of Kerrick and Jacobs (1981) and the HSMRK equation for CH₄ of this study, indicate that compared to the binary H₂O-CO₂ system, small amounts of CH₄ in the ternary system H₂O-CO₂-CH₄ slightly increases the activity of H₂O, and significantly decreases the activity of CO₂.     

Phase relations in the CH4-H2O-NaCl system at 2 kbar, 300 to 600°C as determined using synthetic fluid inclusions

Synthesis of fluid inclusions in the CH₄-H₂O-NaCl system was accomplished by subjecting fractured quartz or fluorite, along with known quantities of CH₄, H₂O, and NaCl, to a pressure of 2 kbar and temperatures of 300, 400, 500, or 600°C, in sealed Au capsules. Under the elevated P-T conditions, some of the fractures healed, trapping fluids as inclusions. Microthermometric measurements conducted on the fluid inclusions show that at 2 kbar and 400 to 600°C, there are very broad regions of fluid unmixing in the CH₄-H₂O-NaCl system. For those bulk fluid compositions that lie in the two-phase (i.e., immiscible fluids) field, the high-density phase is enriched in NaCl, whereas the low-density phase is enriched in CH₄. For any given bulk composition, the degree of NaCl enrichment in the high-density phase increases, whereas the degree of CH₄ enrichment in the low-density phase decreases, as temperature increases from 400 to 600°C. Our experimental constraints on the size of the two-phase field are generally consistent with results generated using the equation-of-state GEOFLUIDS (available at http://geotherm.ucsd.edu/geofluids/). However, when comparing the compositions of coexisting immiscible fluids, as determined experimentally vs. calculated using GEOFLUIDS, we find that some relatively small but probably significant differences exist between our experiments and this equation of state.     

Thermodynamic modeling of binary CH4–CO2 fluid inclusions

An equation of state (EOS) explicit in Helmholtz free energy has been improved to calculate the PVTx and vapor–liquid phase equilibrium properties of CH₄–CO₂ fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs recommended by NIST for pure components (CH₄ and CO₂) and contains a simple generalized departure function presented by Lemmon and Jacobsen (1999). Comparison with experimental data available indicates that the EOS can calculate both vapor–liquid phase equilibrium and volumetric properties of this binary fluid system with accuracy close to that of experimental data up to high temperature and pressure within full range of composition. The EOS of CH₄–CO₂ fluid, together with the updated Gibbs free energy model of solid CO₂ (dry ice), is applied to calculate the CH₄ content (x_CH4) and molar volume (V_m) of the CH₄–CO₂ fluid inclusion based on the assumption that the volume of an inclusion keeps constant during heating and cooling. $V_{\text{m}}-x_{{\text{CH}}_4}$ diagrams are presented, which describe phase transitions involving vapor, liquid and CO₂ solid phases of CH₄–CO₂ fluid inclusions. Isochores of CH₄–CO₂ inclusions at given $x_{{\text{CH}}_4}$ and V_m can be easily calculated from the improved EOS.     

Characterization of CO2CH4H2O fluid inclusions by microthermometry and laser Raman microprobe spectroscopy: Inferences for clathrate and fluid equilibria

Microthermometry (MT) and laser Raman microprobe (LRM) spectroscopy (at room temperature and at about 0°C) were done on 33 synthetically produced CO₂CH₄H₂O fluid inclusions in quartz (from R. Bodnar and M. Sterner). At room temperature, the inclusions consist of an aqueous liquid, a CO₂CH₄ supercritical carbonic fluid and (in most cases) graphite. In all these inclusions, the melting temperature for solid CO₂ is less than that for the homogenization of the vapor bubble in the carbonic fluid.A method is described whereby MT data for CO₂CH₄H₂O inclusions can be projected within the CO₂CH₄ binary phase diagram to infer CO₂:CH₄ ratios in the carbonic fluid (<2 to > 35 mole% CH₄ in the inclusions under study). This method takes into account the formation of CO₂CH₄ clathrate hydrate during MT analysis. Unless clathrate formation is properly considered, errors arise in the determination of the bulk CO₂CH₄ ratio. For the inclusions in our study, these errors are on the order of 5 to 8 mole% CH₄. Our interpretation of the MT data indicates that CH₄ is preferentially partitioned into the clathrate over the coexisting carbonic fluid, in contradiction to the prediction from Parrish and Prausnitz's (1972) model for clathrate equilibria. Comparison of LRM analyses on the bulk carbonic fluid (in the absence of clathrate) and the residual carbonic fluid (in the presence of clathrate) confirm the preferential partitioning of CH₄ into the clathrate. LRM analyses of the clathrate itself indicate that CH₄ occupies both types of cage sites in the clathrate structure, whereas CO₂ may only occupy one site. Two by-products of the combined LRM and MT analyses of the same inclusions are derivation of empirical ratios of Raman quantification factors for high-density CO₂CH₄ fluids and the ability to determine CO₂:CH₄ ratios of inclusions whose MT data lie near the critical region for CO₂CH₄. Thus, the joint use of LRM and MT techniques provides information that could not be obtained by either technique alone.     

Phase equilibria in carbonic systems,and their application to freezing studies of fluid inclusions

Addition of CH₄ to CO₂ lowers the temperatures at which phase changes occur with respect to those in the unary system CO₂. At high density and high a melting interval of solid CO₂ can be expected. Rearrangement of currently available theoretical and experimental data permits bulk compositions of carbonic fluid inclusions to be determined from the final melting temperature of CO₂ and the degree of filling at that temperature. Homogenization temperatures of CO₂-CH₄ inclusions can be expressed in terms of equivalent CO₂-densities, permitting estimates of P-T relations using isochores in the unary system CO₂.     

Synthetic fluid inclusions XVIII: Experimental determination of the PVTX properties of H2O-CH4 to 500 °C, 3 kbar and XCH4 ?4 mol.%

The pressure-volume-temperature-composition (PVTX) properties of H₂O-CH₄ were determined from the bubble point curve to 500 °C and 3 kbar for compositions ?4 mol.% CH₄ using the synthetic fluid inclusion technique. H₂O-CH₄ inclusions were produced by loading known amounts of Al₃C₄ and H₂O into platinum capsules along with pre-fractured and inclusion-free quartz cores. During heating the Al₃C₄ and H₂O react to produce CH₄, and the H₂O-CH₄ homogeneous mixture was trapped as inclusions during fracture healing at elevated temperature and pressure. The composition of the fluid in the inclusion was confirmed using the weight loss technique after the experiment and by Raman spectroscopic analysis of the inclusions.Homogenization temperatures of the inclusions were determined and the results were used to construct iso-T_h lines, defined as a line connecting the formation temperature and pressure with the homogenization temperature and pressure. The pressure in the inclusion at the homogenization temperature was calculated from the Duan equation of state (EOS). The slope (ΔP/ΔT) of each iso-T_h line was calculated and the results fitted to a polynomial equation using step-wise multiple regression analysis to estimate the slope of the iso-T_h line as a function of the homogenization temperature and composition according to:

(ΔP/ΔT)=a+b·m+c·m⁴+d·(T_h)²+e·m·T_h+f·m·(T_h)⁴,     

Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex

The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C.Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO₂ (gas or liquid or both at room temperature) through CO₂ + H₂O ± CH₄ mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism.The density and fluid composition data are derived from direct observations of phase changes between ? 180 and + 380°C and from the application of published experimental data in the system CH₄-CO₂-H₂O-NaCl. The most dense, pure CO₂ inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P-T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO₂ + H₂O inclusions. There is evidence of immiscible separation of CO₂-rich and H₂O-rich fluids at temperatures at least as high as 375°C.     

The P-V̄-T-X-f O2 evolution of H2O-CO2-CH4-bearing fluid in a wolframite vein: Reconstruction from fluid inclusion studies

Aqueous-carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite (Fe_.95Mn_.05 WO₄) vein within the low-grade metamorphic aureole of the Borne granite (French Massif Central) have been studied by microthermometry and Raman spectrometry. The bulk V?-X properties of the aqueous-carbonaceous inclusions have been derived using the equation of state of Heyenet al. (1982) for the low-temperature CO₂-CH₄ system. A P-T path has been proposed for their trapping using the equations of state of Jacobs and Kerrick (1981a) for the H₂O-CO₂-CH₄ system. Two main episodes were reconstructed for the history of the aqueous-carbonaceous fluid. (1) Primary H₂O-CO₂-CH₄ vapourrich inclusions in quartz indicated the early circulation of a low-density fluid (65 mole% H₂O-34 mole% CO₂-1 mole% CH₄ and traces of N₂: d = 0.35 gcm^?3) at around 550° ± 50°C and 700 ± 100 bar. Fluid cooled approximately isobarically to 450°-400°C and was progressively diluted by H₂O with a concomitant increase in density. The fO₂ of the H₂OCO₂-CH₄ fluid, estimated from the equilibrium CO₂ + 2H₂O CH₄ + 2O₂, first ranged from 10^?22 to 10^?27 bar, close to the Q-F-M buffer. Within analytical errors, these values were consistent with the presence of graphite in equilibrium with the fluid. (2) A drop in PCO₂, and therefore a drop in fO₂, was recorded by the secondary liquid-rich inclusions in quartz. The inclusions, formed at and below 400°C, were composed of H₂O and CH₄ only, and fO₂ at that stage was below that fixed by the graphite-fluid equilibrium. This second episode in the fluid-rock system could be explained by the drop of temperature below the blocking temperature of the graphite-fluid equilibrium. According to this interpretation, the blocking of the graphite-fluid equilibrium occurred at T ≥ 370°C and probably at 400°C on account of the pressure correction. Mass spectrometric data show that ferberite contains H₂O, CO₂ and CH₄ in fluid inclusions, which lie in the gap of the V?-X properties of the aqueouscarbonaceous fluid in quartz. Deposition of ferberite probably occurred at around 400°C, the previously inferred blocking temperature, resulting from either the drop in PCO₂, the drop fO₂ and/or the related pH-increase.It is concluded that the existence of a blocking-temperature for the graphite-fluid chemical equilibrium may be a critical factor for maintaining a stable fluid pressure gradient in geothermal systems occurring under greenschist facies conditions in graphite-bearing rocks.     

Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

Fluid inclusions in quartz veins within Proterozoic metamorphic rocks in the Black Hills, South Dakota, were examined by microthermometry and Raman spectroscopy to assess the evolution of fluid compositions during regional metamorphism of organic-rich shales and late-orogenic magmatism, both of which were related to the collision of the Wyoming and Superior crustal blocks. Fluid inclusions occur in veins that began to be generated before or during regional compression and metamorphism that reached at least garnet-grade conditions, and in veins within the aureole of the Harney Peak Granite (HPG), where temperatures reached second-sillimanite grade conditions. Early veins in the schists have undergone recrystallization during heating and deformation that modified the composition of early CH₄ or CO₂ and N₂-dominated inclusions. These fluids were apparently trapped under conditions of immiscibility with a saline aqueous fluid phase. They are interpreted to represent components generated during maturation of organic matter and dehydration of phyllosilicates during incipient metamorphism at reducing fO₂ conditions. Most inclusions in the quartz veins are, however, secondary CO₂-bearing. They imply a transition to higher fO₂ conditions with increasing temperature of regional metamorphism. The fO₂ conditions may have been controlled by the mineral assemblage in the host metapelites. The prevalence of bimodal distributions of trapped CO₂-N₂ and aqueous endmembers in the biotite and garnet zones also suggests that two immiscible fluid phases existed during the regional metamorphism.In the aureole of the HPG, graphite was evidently consumed by influx of magmatic fluids. CO₂-H₂O fluid inclusions dominate, but they have significantly less N₂ than inclusions at lower metamorphic grades. All inclusions define secondary trails in mostly unstrained quartz. The bimodality of inclusion compositions is not as well defined as at lower grades, with many inclusions containing intermediate CO₂-H₂O compositions. This suggests that a single fluid phase existed at the high temperatures in the granite aureole, but then unmixed during cooling. A set of late quartz veins with graphitized and tourmalinized selvages in the granite aureole contains CH₄-bearing inclusions with little N₂. The existence of CH₄ in these inclusions is attributed to complexing of magmatic B with hydroxyl anions taken from the CO₂-H₂O fluid phase, effectively causing reduction in fO₂ and promoting precipitation of graphite.     

C—O—H体系高温高压流体相组成的热力学研究

本文在C－O－H体系流体相平衡基础上,利用现有的热力学数据和新的p－V－T资料,在pT=∑pi假定下,计算了高温、高压条件下流体相组成。结果表明,该体系主要存在五种流体相,在不同温压条件下各流体相所占比例不同。在相对较低的温、压条件下,CH4是体系中占主要的流体相（约占70％）,且随温、压和氧逸度的升高,它所占比例却明显降低,所获结果为探讨无机成因天然气形成的可能性、存在的量比和稳定存在的物理化学条件提供了充分的理论依据。     

Cordierite gneisses of southern Kerala,India: petrology,fluid inclusions and implications for crustal uplift history

The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO₂ (0.80–0.95 g/cm³) as the dominant fluid phase, with traces of probable CH₄ (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO₂ and CO₂-H₂O (±CH₄?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics.     

An algorithm for finding composition,molar volume and isochors of CO2-CH4 fluid inclusions from Th and Tfm (for Th < Tfm)

A modified Redlich-Kwong equation of state is used to calculate the solubility of CO₂ in methane at various temperatures and pressures. From the solubility of CO₂ in CH₄ at the triple point and at final melting (T_h < T_fm), and the molar volume of solid CO₂, the volume of solid at the triple point, and the molar volume of the inclusion can be calculated using a mass balance. The pressure at the melting point is calculated from the equation of state.The algorithm predicts composition, molar volume, pressure at final melting and the isochor pressure (for a given temperature of trapping) for CO₂-CH₄ fluid inclusions for the case T_h < T_fm, given T_h, T_fm and experimental data on P_h and d_co2 (solid) at T_h.     

Methane-rich fluid inclusions in skarn near the giant REE–Nb–Fe deposit at Bayan Obo, Northern China

We report fluid inclusion data for skarn, formed at the contact between Hercynian granitoids and dolomite of the Proterozoic Bayan Obo Group, in the vicinity of Bayan Obo REE–Nb–Fe deposit, Inner Mongolia, China. Three types of fluid inclusions are identified: two-phase CH₄-rich, three-phase liquid–vapour–solid and two-phase aqueous inclusions. Using microthermometry and laser Raman microprobe analysis to calculate isochores for CH₄-bearing inclusions, we estimate fluid trapping conditions at T=280 to 344 °C and P<1 to 2.3 kbar. Such conditions are compatible with formation of CH₄ inclusions as a result of reaction between graphite in the country rocks (black slate sequence) and fluids derived from magma. The lack of carbonaceous material in the inclusions supports the hypothesis that CH₄ was generated during fluid migration rather than by in situ reaction. In contrast to the skarn, and despite the fact that similar graphite-bearing slates are found in the host rocks, no CH₄-bearing inclusions have been so far reported from Bayan Obo REE ores. We therefore conclude that the skarn-forming fluids in the contact aureole of the Hercynian granitoids were not involved at any stage in the formation of the Bayan Obo deposit.     

Stability of methane in reduced C–O–H fluid at 6.3 GPa and 1300–1400°C

The composition of a reduced C–O–H fluid was studied by the method of chromatography–mass spectrometry under the conditions of 6.3 GPa, 1300–1400°C, and fO₂ typical of the base of the subcratonic lithosphere. Fluids containing water (4.4–96.3 rel. %), methane (37.6–0.06 rel. %), and variable concentrations of ethane, propane, and butane were obtained in experiments. With increasing fO₂, the proportion of the CH₄/C₂H₆ peak areas on chromatograms first increases and then decreases, whereas the CH₄/C₃H₈ and CH₄/C₄H₁₀ ratios continually decrease. The new data show that ethane and heavier HCs may be more stable to oxidation, than previously thought. Therefore, when reduced fluids pass the “redox-front,” carbon is not completely released from the fluid and may be involved in diamond formation.     

Thermodynamic modeling of binary CH4-H2O fluid inclusions

This work reports the application of thermodynamic models, including equations of state, to binary (salt-free) CH₄-H₂O fluid inclusions. A general method is presented to calculate the compositions of CH₄-H₂O inclusions using the phase volume fractions and dissolution temperatures of CH₄ hydrate. To calculate the homogenization pressures and isolines of the CH₄-H₂O inclusions, an improved activity-fugacity model is developed to predict the vapor-liquid phase equilibrium. The phase equilibrium model can predict methane solubility in the liquid phase and water content in the vapor phase from 273 to 623 K and from 1 to 1000 bar (up to 2000 bar for the liquid phase), within or close to experimental uncertainties. Compared to reliable experimental phase equilibrium data, the average deviation of the water content in the vapor phase and methane solubility in the liquid phase is 4.29% and 3.63%, respectively. In the near-critical region, the predicted composition deviations increase to over 10%. The vapor-liquid phase equilibrium model together with the updated volumetric model of homogenous (single-phase) CH₄-H₂O fluid mixtures (Mao S., Duan Z., Hu J. and Zhang D. (2010) A model for single-phase PVTx properties of CO₂-CH₄-C₂H₆-N₂-H₂O-NaCl fluid mixtures from 273 to 1273 K and from 1 to 5000 bar. Chem. Geol.275, 148-160), is applied to calculate the isolines, homogenization pressures, homogenization volumes, and isochores at specified homogenization temperatures and compositions. Online calculation is on the website: http://www.geochem-model.org/.     

Phase relations in high-pressure metapelites in the system KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) with application to natural rocks

Using a previously published, internally consistent thermodynamic dataset and updated models of activity–composition relations for solid solutions, petrogenetic grids in the model system KFMASH (K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O) and the subsystems KMASH and KFASH have been calculated with the software THERMOCALC 3.1 in the P–T range 5–36 kbar and 400–810 °C, involving garnet, chloritoid, biotite, carpholite, talc, chlorite, staurolite and kyanite/sillimanite with phengite, quartz/coesite and H₂O in excess. These grids, together with calculated AFM compatibility diagrams and pseudosections, are shown to be powerful tools for delineating the phase equilibria and P–T conditions of pelitic high-P assemblages for a variety of bulk compositions. The calculated equilibria and mineral compositions are in good agreement with petrological observation. The calculation indicates that the typical whiteschist assemblage kyanite–talc is restricted to the rocks with extremely high X_Mg values, decreasing X_Mg in a bulk composition favoring the stability of chloritoid and garnet. Also, the chloritoid–talc paragenesis is stable over 19–20 kbar in a temperature range of ca. 520–620 °C, being more petrologically important than the previously highlighted assemblage talc–phengite. Moreover, contours of the calculated Si isopleths in phengite in P–T and P–X pseudosections for different bulk compositions extend the experimentally derived phengite geobarometers to various KFMASH assemblages.     

Size effect in metastable water

We experimentally determined the maximum tension in synthetic fluid inclusions from the difference between the temperatures of homogenization (T _h) and spontaneous vapor nucleation (T _n). At temperatures of 100–200°C, liquid water may exist at negative pressures of up to 100–150 MPa. Owing to an increase in surface tension, the effect is even more significant in salt solutions and occurs at higher temperatures. A decrease in the linear dimension of fluid phase by an order of magnitude and, correspondingly, a three orders of magnitude decrease in volume (which is proportional to R ₃) increase the maximum tension by ～25MPa. Tension in the liquid phase of water-salt systems may be higher than ~200 MPa without cavitation. Metastability of water and salt solutions in small-sized vacuoles generates stresses in the fluid-mineral system resulting in high solubilities of solid phases. An increase in volume due to coalescence of small inclusions or vanishing of metastability results in an abrupt decrease in supersaturation.