Robert J. Bodnar

Revised equation and table for determining the freezing point depression of H2O-Nacl solutions

Salinities of H[sub 2]O-salt inclusions are most often determined by measuring the melting temperature of ice in the inclusion and then referring this value to an equation or table describing the relationship between salinity and freezing-point depression. Generally, data for the system H[sub 2]O-NaCl are used to determine an NaCl-equivalent salinity, owing to lack of information concerning the salts (or other electrolytes) actually contributing to the freezing-point depression. The equation most often used to determine the salinity of H[sub 2]O-salt inclusions from freezing measurements is that of Potter et al (1978), which is based on a regression of data available in the literature at that time. More recently, Hall et al (1988) experimentally redetermined the ice-melting temperatures of H[sub 2]O-NaCl-KCl solutions having compositions ranging from pure water to the ternary eutectic and to each of the two binary (H[sub 2]O-NaCl and H[sub 2]O-KCl) eutectics.

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.

Synthetic fluid inclusions. V. Solubility relations in the system NaCl-KCl-H2O under vapor-saturated conditions

Vapor-saturated solubility relationships in the system NaCl-KCl-H2O have been determined by experimentally synthesizing fluid inclusions in quartz in the presence of known brine compositions and then measuring the dissolution temperatures of halite and/or sylvite daughter crystals within the inclusions using a microscope equipped with a heating stage. These data, along with other literature values have been used in a stepwise multiple regression routine to generate a series of equations describing vapor-saturated solubility relations within the halite, sylvite and hydrohalite stability fields. These equations, together with a recently published equation for the ice stability field (Hallet al., 1987), have been used to construct the complete vapor-saturated solubility surface in the NaCl-KCl-H2O ternary system. The diagram may be used in the interpretation of microthermometric data to determine the compositions of fluid inclusions approximated by the NaCl-KCl-H2O system. For the NaCl-H2O binary system, the ternary halite field expression reduces to Wt.% NaCl = 26.242 + 0.4928Ψ + 1.42Ψ2 − 0.223Ψ3 + 0.04129Ψ4 + 0.006295Ψ5− 0.001967Ψ6 + 0.0001112Ψ7 (Ψ = T°C/100, where 0.1° ≤T°C ≤801°C) which describes halite solubilities along the three-phase halite + liquid + vapor (H + L + V) curve. Similarly, sylvite solubilities along the three-phase sylvite + liquid + vapor (S + L + V) curve are described by the equation Wt.% KC1 = 21.886 + 20.28Ψ - 9.603Ψ2 + 4.078Ψ3 - 0.8724Ψ4 + 0.09174Ψ5 − 0.003776Ψ6 (Ψ= T°C/100, where −10.7° ≤ T°C ≤ 770°C). Solubility data obtained from synthetic fluid inclusions are in good agreement with recently published data for the KC1-H2O and NaCl-H2O binary systems but are at variance with some earlier works.

Can economic porphyry copper mineralization be generated by a typical calc‐alkaline melt?

Numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that “typical” calc-alkaline magmas contain sufficient copper, chlorine, and water to produce economic porphyry copper mineralization. Neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles. The most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper, the ratio of initial water in the melt to the water saturation level, and the initial chlorine/water ratio of the melt. The absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume. The bulk salinity of the exsolved fluid may vary from < 2.0 wt.% NaCl to saturation levels (84 wt.% NaCl at 700°C) indicating that boiling is not necessary to produce high salinity brines. At appropriate P-T-XNaCl conditions the magmatic aqueous fluid separates into a saline liquid, which transports most of the copper, and a low-salinity vapor. The salinities of the two immiscible phases are governed by the P-T conditions, while the bulk fluid salinity determines the mass fractions of liquid and vapor formed. Pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid. This results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt. As copper is transported from the melt by the earliest exsolving fluids in deep (2.0 kbar) systems and by late exsolving fluids in shallow (0.5 kbar) systems, the relative timing of pressure quenching/aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures. Model results incorporating petrologic constraints determined for Yerington, Nevada, are in good agreement with observed mineralization.

The system NaClCaCl2H2O: I. The ice liquidus at 1 atm total pressure

Abstract Phase relations in the ice-stable field of the system NaClCaCl 2 H 2 O have been determined under 1 atm total pressure along the NaClH 2 O and CaCl 2 H 2 O binaries and along five pseudobinaries with constant NaCl /( NaC 1+ CaCl 2 ) weight ratios. The results are in excellent agreement with published data along the NaClH 2 O binary but show large discrepancies when compared to previous determinations of the ice liquidus along the CaCl 2 H 2 O binary and in the NaClCaCl 2 H 2 O ternary. Eutectic temperatures and compositions are −21.20°C (±0.01 °) and 23.2 wt% for the NaClH 2 O binary and −49.95°C (±0.15°) and 30.33 wt% for the CaCl 2 H 2 O binary. The minimum temperature reached during our experiments in the ternary system was −34°C; therefore, we do not report isotherms below −35°C or the ternary eutectic temperature. At moderate to high salinities, isotherms cross the ice sub-field at lower total salt concentrations than previously reported. Fluid inclusion salinities determined from previously published freezing-point data may be in error by as much as 2 wt% total salt. In addition, NaCl /( NaCl + CaCl 2 ) weight ratios estimated from hydrohalite- and ice-melting temperatures may be in error by as much as 0.15.

Synthetic fluid inclusions in natural quartz I. Compositional types synthesized and applications to experimental geochemistry

Synthetic fluid inclusions having a wide range of geologically applicable compositions in the C-O-H-S-Na-K-Ca-Cl-Cu-Fe system have been formed by healing fractures in inclusion-free natural quartz and by precipitating new quartz overgrowths on an original “seed” crystal. Inclusion types synthesized include: 1. (1) liquid-rich, pure H2O inclusions of primary and secondary origin, 2. (2) pure H2O inclusions with the critical density, 3. (3) liquid-rich inclusions containing undersaturated aqueous solutions of NaCl or KCl or CaCl2, or mixtures of the three salts, 4. (4) H2O-NaCl inclusions containing halite daughter minerals, 5. (5) H2O-NaCl-KCl inclusions containing halite and sylvite daughter minerals, 6. (6) H2O-CO2 inclusions of various compositions containing liquid H2O and either CO2 vapor or CO2 liquid, or both, at 25°C, 7. (7) H2O-CO2-NaCl inclusions containing an aqueous phase, liquid and vapor CO2, and halite at 25°C, 8. (8) C-O-H inclusions containing liquid H2O, a CO2-CH4 liquid phase and graphite daughter minerals at 25°C, 9. (9) H2O-NaCl inclusions that contain a chalcopyrite daughter mineral, and 10. (10)inclusions representing trapping of the coexisting, immiscible fluids in the H2O-NaCl, H2O-CO2-NaCl, and Na-C-O-H systems. The inclusions exhibit uniform phase ratios at room temperature, and the temperatures of various phase changes within individual inclusions agree with those predicted from experimental and theoretical data, indicating that the inclusion fluid has the same composition and density as the parent solution. These ‘miniature autoclaves’ thus may be used to study various problems related to fluid inclusion research, to calibrate analytical equipment used to study natural inclusions, and to study phase equilibria, solubility and PVT relations of a variety of chemical systems.

Jarosite as an indicator of water-limited chemical weathering on Mars.

The Mars Exploration Rover Opportunity identified the ferric sulphate mineral jarosite and possible relicts of gypsum at the Meridiani Planum landing site. On Earth, jarosite has been found to form in acid mine drainage environments, during the oxidation of sulphide minerals, and during alteration of volcanic rocks by acidic, sulphur-rich fluids near volcanic vents. Jarosite formation is thus thought to require a wet, oxidizing and acidic environment. But jarosite on Earth only persists over geologically relevant time periods in arid environments because it rapidly decomposes to produce ferric oxyhydroxides in more humid climates. Here we present equilibrium thermodynamic reaction-path simulations that constrain the range of possible conditions under which such aqueous alteration phases are likely to have formed on Mars. These calculations simulate the chemical weathering of basalt at relevant martian conditions. We conclude that the presence of jarosite combined with residual basalt at Meridiani Planum indicates that the alteration process did not proceed to completion, and that following jarosite formation, arid conditions must have prevailed.

Synthetic fluid inclusions: XII. The system H2ONaCl. Experimental determination of the halite liquidus and isochores for a 40 wt% NaCl solution

Abstract The slopes of the liquidus and lines of constant liquid-vapor homogenization temperature (iso-Th) in P-T space for a 40 wt% NaCl bulk composition in the H2O-NaCl system were determined using synthetic fluid inclusions. Inclusions were synthesized in the one-phase field at 350°–800°C and 1–6 kbar, and the temperatures of liquid-vapor homogenization and halite dissolution were determined on a heating/cooling stage. The pressure along the liquidus corresponding to a measured halite dissolution temperature [Tm (halite)] was determined from the intersection of the inclusion iso-Th line in the one-phase field with the measured Tm (halite). The P-T position of the iso-Th line in the one-phase field was determined from the measured liquid-vapor homogenization temperature [Th (L-V)] of a given inclusion and the known experimental formation temperature and pressure for that same inclusion. The slope of the 40 wt% halite liquidus determined from synthetic fluid inclusions is −5.5°C/kbar. This value is in good agreement with an extrapolation of the data of Gunter et al. (1983) and cooling data of Koster van Groos (1991) to lower salinities, but shows considerable disagreement with the extrapolated heating data of Koster van Groos (1991). Over the salinity range 27–100 wt% NaCl, the slope of the halite liquidus predicted by the results from this study combined with data of Milne (1978), Gunter et al. (1983), and cooling data of Koster Van Groos (1991) is given by dT dP (°C/kbar) = − 38.38 + 0.90S − 0.0029S 2 , where “S” is the salinity in wt% NaCl. Microthermometric data combined with the known temperatures and pressures of formation were used to construct lines of constant liquid-vapor homogenization temperature (iso-Th) for fluid inclusions with a salinity of 40 wt% NaCl. The slopes of the iso-Th lines for inclusions which homogenize by vapor-bubble disappearance decrease smoothly with increasing Th, from 14.1 bars/°C for inclusions which homogenize at 323°C to 5.52 bars/°C for inclusions with homogenization temperatures of 650°C. For inclusions which homogenize by halite dissolution (Th

Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction

Experimental reequilibration of synthetic 10 wt% NaCl-H2O inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the P-T path followed by the inclusion after formation. These differences combined with other geological information may be used to determine whether the sample (rock) followed a dominantly isothermal or isobaricP-T path following entrapment. The intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of theP-T path from the original isochore for the inclusion. Thus, fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradeP-T path, but also the magnitude of departure of this path from one that is isochoric.

Synthetic fluid inclusions: VIII. Vapor-saturated halite solubility in part of the system NaCl-CaCl2-H2O, with application to fluid inclusions from oceanic hydrothermal systems

Abstract Halite solubility along part of the vapor-saturated liquidus in the system NaCl - CaCl 2 - H 2 O has been determined using the synthetic fluid inclusion technique. Data allow the construction of liquidus isotherms for temperatures up to 500°C and bulk compositions containing >60 wt% total salt and as much as 25 wt% CaCl 2 . Combined with previous data for the binary system NaCl - H 2 O and for the ternary system NaCl - CaCl 2 - H 2 O in the low-salinity, low-temperature region, a preliminary ternary phase diagram can be constructed that remains incomplete only in the CaCl 2 -rich region. Results are applied to the interpretation of saline fluid inclusions from quartz veins in oceanic metagabbros, and can be applied to many other natural inclusions containing aqueous solutions with NaCl and CaCl 2 the dominant solutes. Microthermometric measurements at equilibrium of the melting temperature of ice [ Tm (ice)] and of the dissolution temperature of halite [ Tm (halite)] are sufficient to determine the bulk composition of the NaCl - CaCl 2 - H 2 O fluid.