Robert K. Popp

The compositional limits of fluid immiscibility in the system H2ONaClCO2 as determined with the use of synthetic fluid inclusions in conjunction with mass spectrometry

Abstract The compositional limits of fluid immiscibility in the system NaClH2OCO2 were investigated from 500° to 700°C at pressures of 1, 2 and 3 kbar. Synthetic fluid inclusions formed in quartz prisms were equilibrated with high-temperature, high-pressure fluids in hydrothermal pressure vessels. The inclusions were analyzed optically, by mass spectrometry, and by microthermometry. The fields of immiscibility were defined as a function of temperature and pressure. Mass spectrometry was used to validate the existence of the immiscible fluids and to determine the CO 2 H 2 O ratios of the two coexisting phases. Microthermometry, i.e. the measurement of the melting temperature of solid sodium chloride, was used in conjunction with the results of the mass spectrometry measurements to define the tie-lines within the two-phase fields. Isochores and the location of a portion of the isopleth were determined for composition NaCl8.9H2O76.1CO215.0.

Mineral-solution equilibria—V. Solubilities of rock-forming minerals in supercritical fluids

The solubility constants of sixty-nine rock-forming minerals have been computed for temperatures between 400 and 600°C at 1000 and 2000 bar pressure using the free-energy data for aqueous solutes presented in Parts I through IV of this series combined with the thermodynamic properties of minerals from Helgesonet al. (1978). An example describing solution compositions in equilibrium with a spilite is discussed. A computer program for calculating solution compositions in equilibrium with mineral assemblages is included as an appendix.

The determination of phase relations in the CH4H2ONaCl system at 1 kbar, 400 to 600°C using synthetic fluid inclusions

Synthesis of fluid inclusions in the CH4H2ONaCl system was accomplished by subjecting fractured quartz, along with known quantities of CH4, H2O, and NaCl, to a pressure of 1 kbar and temperatures of 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 1 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 4.00 to 600°C.

Oxy-amphibole equilibria in Ti-bearing calcic amphiboles: Experimental investigation and petrologic implications for mantle-derived amphiboles

Abstract An experimental study was carried out to investigate the equilibrium between Fe oxy-component and hydroxy-component in Ti-bearing calcic amphiboles, as described in the dehydrogenation/oxidation reaction Fe2+ + OH- = Fe3+ + O2- + 1/2 H2, for which the equilibrium constant (K) can be expressed as where □ = H-vacancy on the O3 anion position, Φ is the activity coeficient term, and Kx represents the thermodynamic mole fraction term (i.e., the K expressed as mole fractions rather than activities). The variation in Kx was quantified experimentally by annealing experiments on amphiboles of two different compositions: a mantle-derived kaersutite from Greenland, and a crustal pargasite from the Tschicoma Formation from the Jemez Mountains, New Mexico, volcanic complex. The conditions of the experiments ranged from 700.1000 °C, 1.10 kbar, and fH2 from that of the HM to GM solid buffer assemblages. The results, combined with similar data for a titanian pargasite from Vulcanʼs Throne, Arizona (Popp et al. 1995a), define the variation in log Kx as a function of T, P, and amphibole composition as given by the equation: If the T, P, and amphibole composition are known, values of log Kx calculated from the equation predict the equilibrium logfH2 of any experiment to within ~0.1 to 0.3 log units. It is assumed that a similar uncertainty in log fH2 would also to apply to the conditions of formation of natural amphiboles in the same composition range. If log fO₂ at the time of equilibration can be estimated independently for natural samples (e.g., mantle-derived amphiboles), the H2O activity also can be estimated. An alternate approach for estimating H2O activity from amphibole-bearing mantle rocks is to use a variety of H2O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole, and other phases: e.g., 2 tr +2 fo = 5 en + 4 di + 2 H2O. A self-consistent thermodynamic database (THERMOCALC, Holland and Powell 1990) can be used to determine the aH₂O of such univariant H2O-buffering equilibria as a function of P and T. A mantle amphibole assemblage from Dish Hill (sample DH101-E, McGuire et al. 1991) was used to calculate aH₂O using the two different methods. The mean value of log aH₂O determined from seven different dehydration reactions is .1.70, with a 1σ range of ±0.50. That range of water activity is in good agreement with the value of log aH₂O = .1.90 ± 0.3 obtained using the dehydrogenation/oxidation equilibrium, along with an estimate of log fO₂. The use of xenolith amphiboles to infer values of aH₂O in the mantle requires that the H content of the amphibole does not change significantly during ascent or eruption. Changes in H content have significantly different effects on the dehydration and dehydrogenation equilibria, such that, comparison of the aH₂O estimates from the two different methods may permit quantification of H loss.

Hydrogen deficiency in mantle-derived phlogopites

Abstract The substitution mechanisms of Fe and Ti have been determined in phlogopite megacrysts from an ultramafic lamprophyre dyke from the Okenyenya igneous complex, northwestern Namibia. Mica separates were heat-treated from 800 to 900 °C. 1 atm to 10 kbar. and fH2 from that of the IQF solid- state buffer to that of ah. Iron oxidation states and H2O contents of the ran products were determined using 57Fe Mossbauer spectroscopy and a vacuum fusion, U-fumace manometry system, respectively. The least-squares fit between the univalent anion content (OH + F) and molar Fe3+ atoms per formula unit (apfu) has a negative slope with a high correlation coefficient and. at the 95% confidence level, is consistent with the Fe-oxy reaction, Fe2+ + OH-= Fe3+ + O2-+ 1/2 H2 By adding O2- to the univalent anion content in 2:1 molar proportions to the Ti. the total anion content in the OH site of the natural phlogopite is. at the 95% confidence level, close to the theoretical value of 4.0 (O = 24 apfu) for the mica structure. It is proposed that the total H deficiency in the natural phlogopite can be explained by both Fe- and Ti-oxy substitution mechanisms. Principal Components Analysis carried out on exchange components for the experimentally treated phlogopite confirms the operation of the oxy-substitution mechanisms. Both oxy-substitutions dominate in the compositions of natural igneous micas from a variety of tectonic environments. The dehydrogenation of Fe oxy-components in micas from silicic lavas may be a source of water that can be liberated into crustal melts and play an important role in the mechanism for initiating volcanic eruptions.

The effect of NaCl on bunsenite solubility and Ni-complexing in supercritical aqueous fluids

Abstract Experiments were carried out using rapid-quench hydrothermal techniques to determine the solubility of bunsenite (NiO) in supercritical H2O-NaCl-NiCl2 solutions in the range 550 to 750°C, at 2 kbar. The solubility of NiO was enhanced by the formation of complexes of higher stoichiometry than NiCl20 in the presence of increasing concentrations of aqueous NaCl. The results are consistent with the formation of the NiCl3− complex in the fluid over the entire temperature range, as a result of the reaction NiCl20(aq) + Cl−(aq) = NiCl3−(aq). The variation in values of the equilibrium constant for the above reaction is given by logKeq = 3.40(±0.83) − 2365 (±758)/T(°K). Despite the increase in solubility that results in the NaCl-bearing system, nickel should still remain a relatively immobile element, as compared to Ca, Mg, Fe and Mn, in supercritical aqueous Cl-bearing fluids, provided the activity of nickel in the coexisting solid mineral assemblage is not unusually high.

Amphibole equilibria in mantle rocks: Determining values of mantle aH2O and implications for mantle H2O contents

Abstract H2O can affect the thermophysical properties of the mantle, and nominally anhydrous mantle minerals, such as olivine, pyroxenes, and garnet, may be an important reservoir of mantle H2O. However, the H2O content of nominally anhydrous mantle minerals now at the Earth’s surface may not always reflect mantle values. It is, therefore, desirable to develop different techniques to estimate mantle H2O contents, or values of the activity of H2O (ɑH₂O) at the conditions of equilibration in the mantle. To examine the potential of amphibole equilibria to determine values of mantle ɑH₂O, the chemical compositions of co-existing amphibole, olivine, two-pyroxenes, and spinel from a mantle xenolith, sample DH101E of McGuire et al. (1991), were used to estimate values of pressure (P), temperature (T), and ɑH₂O. A value of ɑH₂O was estimated from pargasite dehydration equilibria using chemical compositions of minerals as the basis for estimating activities of end-members in the natural phases (e.g., the activity of forsterite in olivine). These calculations were performed with the THERMOCALC software package and, at an estimated maximum T and P of 900 °C and 20 kbar, they yield an estimated value of ɑH₂O ≈ 0.02 for sample DH101E. The application of oxy-amphibole equilibrium, as described by Popp et al. (2006a, 2006b), using the composition of the amphibole in DH101E yields a value of the log of the hydrogen fugacity (fH₂) of -1.37. This value of fH₂ together with the estimated log fO₂ of -9.9 yields a value of ɑH₂O ≈ 0.0005 for sample DH101E. The lower estimated ɑH₂O compared to that estimated from dehydration equilibria may reflect a slight loss of H from amphibole in the post-formation environment, but both types of amphibole equilibria are consistent with a low value of ɑH₂O. Values of mantle ɑH₂O can be used to predict the H2O content of mantle olivines. At 900 °C and 20 kbar, the olivine in a sample that equilibrates at ɑH₂O <0.04, as estimated for sample DH101E, should contain <10 wt ppm H2O. This value is consistent with the lower end of the range of measured H2O contents of mantle olivines (≈4-400 wt ppm). Thus, estimates of values of ɑH₂O from amphibole equilibria can produce useful predictions of both the activity of H2O as well as the H2O content of nominally anhydrous mantle minerals.

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

Abstract 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.

Solubility and complexing of Ni in the system NiO-H2O-HCl

Abstract The solubility of bunsenite (NiO) in Cl-bearing fluids in the range of 450°–700°C, 1–2 kbar was determined using the Ag + AgCl acid buffer technique. Based on the results of the experiments, it is concluded that the associated NiCl 0 2 complex is the dominant Ni species in the fluid over the entire temperature-pressure range investigated. The temperature dependence of the equilibrium constant for the reaction NiO ( s ) + 2 HCl 0 ( aq ) = NiCl 0 2 ( aq ) + H 2 O is given by log K = −4.17(±0.55) + 4629(±464)/ T ( K ) at 1 kbar, and log K = −4.75(±0.91) + 5933(±756)/ T ( K ) at 2 kbar. The calculated difference in standard state Gibbs free energy of formation between NiCl 0 2 and 2HCl 0 in kcal is G 0 ( NiCl 0 2 ) − 2 G 0 ( HCl 0 ) = −20.77(±2.22) + 0.03264(±0.0026) T ( K ), at 1 kbar and G 0 ( NiCl 0 2 ) − 2 G 0 ( HCl 0 ) = −25.01(±1.35) + 0.03264(±0.0016) T ( K ) at 2 kbar. Comparison of the solubilities of Ni end-member minerals with those of Ca, Mn, Fe, and Mg indicates that nickel minerals generally are the least soluble at a given temperature and pressure. The relatively low solubility of Ni end-member minerals, combined with the relatively low concentration of Ni in most rocks, should result in a quite low mobility of Ni in hydrothermal fluids.

An experimental investigation of the quartz, Na-K, Na-K-Ca geothermometers and the effects of fluid composition

Abstract To test the possible effect of different fluid compositions on some standard geothermometry techniques, experiments were conducted in which a rhyolite from the Presidio Bolson area of West Texas was interacted with fluids of two different compositions (0.1 M NaCl and 0.01 M NaHCO3). The temperature range was 100–500°C, pressure was 1000 bars, water/rock mass ratios were 6:1 and 5:1, and the duration of the experiments ranged from 12 to 130 days. Results showed that the quartz geothermometer worked well in the experimental system up to temperatures of 400°C. The results were not affected by differences in the major anionic species. The Na-K geothermometer gave temperatures an average of 76°C lower than the experimental temperatures, regardless of fluid type. The experimental data from this study agree well with previous experimental work in feldsparquartz systems. The Na-K-Ca geothermometer did not work well for experiments using 0.01 M NaHCO3 but did work well for experiments using 0.1 M NaCl. Benjamin et al. (1983) concluded that the Na-K-Ca geothermometer is based on alteration reactions rather than feldspar exchange; however, no evidence for alteration reactions was observed in this study.