The role of carbon and oxygen in cosmic gases: some applications to the chemistry and mineralogy of enstatite chondrites

A new condensation sequence appears if the $\text{C}\text{O}$ ratio in a gas of otherwise solar composition is increased by less than a factor of two. As the ratio increases from the solar value of 0.6 to ? 1 the gas becomes extremely reduced, the condensation temperatures of silicates and oxides are depressed markedly ～ 400 K and a new suite of refractory minerals appears: AIN, CaS, MgS, SiC, TiN, graphite, Si₂N₂O and probably metastable (Fe,Ni)³C. Many of these minerals are unique to enstatite chondrites and may be analogues of the refractory silicates and oxides found in more oxidized meteorites such as Allende.The change in chemistry is related to the stability of CO, the most stable C or O compound at high T. Since the elements occur in a 1:1 ratio in CO, only the element which is in excess is free to form other compounds. But as T decreases CO reacts with H₂ to form graphite, CH₄ or other hydrocarbons thereby freeing O to form H₂O. If equilibrium is maintained oxides and silicates form at about 1000 K ($\text{C}\text{O}\text{> 1}$, P_τ = 10^?4atm) as products of reactions among the carbides, nitrides, sulfides and the gas. The possibility that equilibrium was not maintained among the C-bearing species was also investigated. If either graphite or CH₄ does not form as predicted the stability fields of the reduced minerals expands to lower temperatures. If neither graphite nor CH₄ form as predicted, CO remains stable and the nebular gas is highly reduced at all temperatures.Enstatite chondrites appear to have originated in a region of the nebula where the $\text{C}\text{O}$ ratio was somewhat higher than the solar value. Various fractionation mechanisms are considered. An interesting possibility is that graphite, which is quite refractory under a wide range of conditions, survived the collapse of the solar nebula.     

Sorption of noble gases by solids,with reference to meteorites. I. Magnetite and carbon

To simulate trapping of meteoritic noble gases by solids, 18 samples of Fe₃O₄ were synthesized in a noble gas atmosphere at 350–720 K by the reactions: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (Ne, Ar, Kr, Xe) 3Fe + 4CO → Fe₃O₃ + 4C + carbides (Xe only) Phases were separated by selective solvents (HgCl₂, HCl). Noble gas contents were analyzed by mass spectrometry, or, in runs where 36 d Xe¹²⁷ tracer was used, by γ-counting. Surface areas, as measured by the BET method, ranged from 1 to 400 m²/g. Isotopic fractionations were below the detection limit of 0.5%/m.u.Sorption of Xe on Fe₃O₄ and C obeys Henry's Law between $\text{1 × 10}{}^{\mathrm{?8}}$ and $\text{4 × 10}{}^{\mathrm{?5}}$ atm, but shows only a slight temperature dependence between 650 and 720 K ($\text{ΔH}{}_{\mathrm{sol}}\text{= ?4 ± 2}\text{kcal/mole}$). The mean distribution coefficient K_Xe is $\text{0.28 ± 0.09}$ cc STP/g atm for Fe₃O₄ and only a factor of $\text{1.2 ± 0.4}$ greater for C; such similarity for two cogenetic phases was predicted by Lewis et al. (1977). Stepped heating and etching experiments show that 20–50% of the total Xe is physically adsorbed and about 20% is trapped in the solid. The rest is chemisorbed with $\text{ΔH}{}_{\mathrm{s}}\text{? ?13}\text{kcal/mole}$. The desorption or exchange half-time for the last two components is >10² yr at room temperature.Etching experiments showed a possible analogy to “Phase $\text{Q}$” in meteorites. A typical carbon + carbide sample, when etched with HNO₃, lost 47% of its Xe but only 0.9% of its mass, corresponding to a ~0.6 Å layer. Though this etchable, surficial gas component was more thermolabile than $\text{Q}$ (release $\text{T}$ below 1000°C, compared to 1200–1600°C), another experiment shows that the proportion of chemisorbed Xe increases upon moderate heating (1 hr at 450°C). Apparently adsorbed gases can become “fixed” to the crystal, by processes not involving volume diffusion (recrystallization, chemical reaction, migration to traps, etc.). Such mechanisms may have acted in the solar nebula, to strengthen the binding of adsorbed gases.Adsorbed atmospheric noble gases are present in all samples, and dominate whenever the noble gas partial pressure in the atmosphere is greater than that in the synthesis. Many of the results of Lancet and Anders (1973) seem to have been dominated by such an atmospheric component; others are suspect for other reasons, whereas still others seem reliable. When the doubtful samples of Lancet and Anders are eliminated or corrected, the fractionation pattern—as in our samples—no longer peaks at Ar, but rises monotonically from Ne to Xe. No clear evidence remains for the strong temperature dependence claimed by these authors.     

New gas geothermometers for geothermal exploration—calibration and application

Calibration of five gas geothermometers is presented, three of which used CO₂, H₂S and H₂ concentrations in fumarole steam, respectively. The remaining two use $\text{CO}{}_2\text{H}{}_2$ and $\text{H}{}_2\text{S}\text{H}{}_2$ ratios. The calibration is based on the relation between gas content of drillhole discharges and measured aquifer temperatures. After establishing the gas content in the aquifer, gas concentrations were calculated in steam formed by adiabatic boiling of this water to atmospheric pressure to obtain the gas geothermometry functions. It is shown that the concentrations of CO₂, H₂S and H₂ in geothermal reservoir waters are fixed through equilibria with mineral buffers. At temperatures above 230°C epidote + prehnite + calcite + quartz are considered to buffer CO₂. Two buffers are involved for H₂S and H₂ and two functions are, therefore, presented for the geothermometers involving these gases. For waters containing less than about 500 ppm chloride and in the range 230–300°C pyrite + pyrrholite + epidote + prehnite seem to be involved, but pyrite + epidote + prehnite + magnetite or chlorite for waters above 300°C and waters in the range 230–300°C, if containing over about 500 ppm.The gas geothermometers are useful for predicting subsurface temperatures in high-temperature geothermal systems. They are applicable to systems in basaltic to acidic rocks and in sediments with similar composition, but should be used with reservation for systems located in rocks which differ much in composition from the basaltic to acidic ones. The geothermometry results may be used to obtain information on steam condensation in upflow zones, or phase separation at elevated pressures.Measured aquifer temperatures in drillholes and gas geothermometry temperatures, based on data from nearby fumaroles, compare well in the five fields in Iceland considered specifically for the present study as well as in several fields in other countries for which data were inspected. The results of the gas geothermometers also compare well with the results of solute geothermometers and mixing models in three undrilled Icelandic fields.     

Mineral-solution equilibria—I. An experimental study of complexing and thermodynamic properties of aqueous MgCl2 in the system MgO-SiO2-H2O-HCl

Speciation of aqueous magnesium in the system MgO-SiO₂-H₂O-HCl in supercritical aqueous fluids has been investigated using standard rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method (Frantz and Eugster, 1973. Am. J. Sci.267, 268–286). A concentric double-capsule charge was utilized. The outer gold capsule contained the assemblage talc + quartz + Ag + AgCl + H₂O-MgCl₂ fluid; the inner platinum capsule, Ag + AgCl + H₂O-HCl fluid. During the experiments, and thus $\text{?}{}_{\mathrm{HCl}}$ equilibrated between the two capsules. After quenching, measurement of the chloride concentration in the fluid in the inner capsule and total magnesium in the fluid in the outer capsule defines the concentrations of HCl and Mg that coexist with talc + quartz in the outer capsule. Changes in the measured molality of HCl as a function of the total magnesium concentration at constant P and T were used to identify the predominant species of magnesium in the hydrothermal fluid. Experimental results showed that at 2000 bar, MgCl°₂ is the predominant species above 550°C and Mg²⁺, below 400°C. Data at intermediate temperatures when combined with the dissociation constant for HCl were used to obtain the dissociation constant for MgCl°₂. The results of these experiments were combined with results from experiments using Ag + AgCl in conjunction with the oxygen buffer, hematite-magnetite, to obtain the equilibrium constant for the reaction $\text{1}\text{3}\text{Talc + 2HC1° H}{}_2\text{O MgCl°}{}_2\text{+}\text{4}\text{3}\text{Quartz +}\text{4}\text{3}\text{H}{}_2\text{O}$ from which the difference in Gibbs free energy of MgCl°₂ and HC1° was obtained as a function of temperature at 1000, 1500 and 2000 bar pressure, Solubility constants for brucite. forsterite, chrysotile, and talc were calculated.     

An electrochemical study of Ni2+, Co2+, and Zn2+ ions in melts of composition CaMgSi2O6

Cyclic voltammetry has been done for Ni²⁺, Co²⁺, and Zn²⁺ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.32 ± .01}\text{V}$, $\text{E}{}^1\text{(}\text{Co}{}^{\mathrm{2+}}\text{Co}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.45 ± .02}\text{V}$, and $\text{E}{}^1\text{(}\text{Zn}{}^{\mathrm{2+}}\text{Zn}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.53 ± .01}\text{V}$. The electrode reactions are rapid, with first order rate constants of the order of 10^?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10^?6 cm²/sec for Ni²⁺, 3.4 × 10^?6 cm²/sec for Co²⁺, and 3.8 × 10^?6 cm²/sec for Zn²⁺ at 1500°C. The value of $\text{E1}$ ($\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0$, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{, 1525°C}$) was not observed to vary with composition over the range CaO · MgO · 2SiO₂ to CaO·MgO·3SiO₂ or from 1.67 CaO·0.33MgO·2SiO₂ to 0.5 CaO·1.5MgO·2SiO₂. The value for the diffusion coefficient for Ni²⁺ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO₂ to CaO · MgO · 3SiO₂. This is consistent with a mechanism by which Ni²⁺ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni²⁺ species discharging at the cathode regardless of the SiO₂ concentration in the melt.     

Light hydrocarbons in Red Sea brines and sediments

Light hydrocarbon (C₁-C₃) concentrations in the water from four Red Sea brine basins (Atlantis II, Suakin, Nereus and Valdivia Deeps) and in sediment pore waters from two of these areas (Atlantis II and Suakin Deeps) are reported. The hydrocarbon gases in the Suakin Deep brine (T = ~ 25°C, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 85‰}$, $\text{CH}{}_4\text{=}\text{~ 71}\text{1}$) are apparently of biogenic origin as evidenced by $\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3$) ratios of ~ 1000. Methane concentrations (6–8 μl/l) in Suakin Deep sediments are nearly equal to those in the brine, suggesting sedimentary interstitial waters may be the source of the brine and associated methane.The Atlantis II Deep has two brine layers with significantly different light hydrocarbon concentrations indicating separate sources. The upper brine (T = ~ 50°C, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 73‰}$, CH₄ = ~ 155 μl/l) gas seems to be of biogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~1100}$], whereas the lower brine (T = ~ 61°C, $\text{Cl}{}^{\mathrm{?}}\text{= ~ 155‰}$, CH₄ = ~ 120μl/l) gas is apparently of thermogenic origin [$\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{) = ~ 50}$]. The thermogenic gas resulting from thermal cracking of organic matter in the sedimentary column apparently migrates into the basin with the brine, whereas the biogenic gas is produced in situ or at the seawater-brine interface. Methane concentrations in Atlantis II interstitial waters underlying the lower brine are about one half brine concentrations; this difference possibly reflects the known temporal variations of hydrothermal activity in the basin.     

Rutile solubility and titanium coordination in silicate melts

The solubility of rutile has been determined in a series of compositions in the K₂O-Al₂O₃-SiO₂ system ($\text{K}{}^1\text{=}\text{K}{}_2\text{O}\text{(K}{}_2\text{O}$ + Al₂O₃) = 0.38–0.90), and the CaO-Al₂O₃-SiO₂ system ($\text{C}{}^1\text{=}\text{CaO}\text{(CaO + Al}{}_2\text{O}{}_3\text{)}\text{= 0.47–0.59}$). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the $\text{K}{}^1$ ratio. For example, at 1475°C the amount of TiO₂ required for rutile saturation varies from 9.5 wt% ($\text{K}{}^1\text{= 0.38}$) to 11.5 wt% ($\text{K}{}^1\text{= 0.48}$) to 41.2 wt% ($\text{K}{}^1\text{= 0.90}$). In the CAS system at 1475°C, rutile solubility is not a strong function of $\text{C}{}^1$. The amount of TiO₂ required for saturation varies from 14 wt% ($\text{C}{}^1\text{= 0.48}$) to 16.2 wt% ($\text{C}{}^1\text{= 0.59}$).The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K⁺ in excess of that needed to charge balance Al³⁺. The suggested stoichiometry of this complex is K₂Ti₂O₅ or K₂Ti₃O₇. In CAS melts, the data suggest that Ca²⁺ in excess of A1³⁺ is not as effective at complexing with Ti as is K⁺. The greater solubility of rutile in CAS melts when $\text{C}{}^1$ is less than 0.54 compared to KAS melts of equal $\text{K}{}^1$ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiK_β x-ray emission spectra of KAS glasses ($\text{K}{}^1\text{= 0.43–0.60}$) with 7 mole% added TiO₂, rutile, and Ba₂TiO₄, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba₂TiO₄, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules.     

Fluid-mineral equilibria in a hydrothermal system,Roosevelt hot springs,Utah

The availability of fluids and drill cuttings from the active hydrothermal system at Roosevelt Hot Springs allows a quantitative comparison between the observed and predicted alteration mineralogy, calculated from fluid-mineral equilibria relationships. Comparison of all wells and springs in the thermal area indicates a common reservoir source, and geothermometer calculations predict its temperature to be higher ($\text{288°C ± 10°}$) than the maximum measured temperature of 268°C.The composition of the deep reservoir fluid was estimated from surface well samples, allowing for steam loss, gas release, mineral precipitation and ground-water mixing in the well bore. This deep fluid is sodium chloride in character, with approximately 9700 ppm dissolved solids, a pH of 6.0, and gas partial pressures of O₂ ranging from 10^?32 to 10^?35 atm, CO₂ of 11 atm, H₂S of 0.020 atm and CH₄ of 0.001 atm.Comparison of the alteration mineralogy from producing and nonproducing wells allowed delineation of an alteration pattern characteristic of the reservoir rock. Theoretical alteration mineral assemblages in equilibrium with the deep reservoir fluid, between 150° and 300°C, in the system Na₂O-K₂O-CaO-MgO-FeO-Fe₂O₃-Al₂O₃-H₄SiO₄-H₂O-H₂S-CO₂-HCl, were calculated. Minerals theoretically in equilibrium with the calculated reservoir fluid at $\text{>240°C}$ include sericite, K-feldspar, quartz, chalcedony, hematite, magnetite and pyrite. This assemblage corresponds with observed higher-temperature ($\text{>210°C}$) alteration assemblage in the deeper parts of the producing wells. The presence of montmorillonite and mixed-layer clays with the above assemblage observed at temperatures <210°C corresponds with minerals predicted to be in equilibrium with the fluid below 240°C.Alteration minerals present in the reservoir rock that do not exhibit equilibrium with respect to the reservoir fluid include epidote, anhydrite, calcite and chlorite. These may be products of an earlier hydrothermal event, or processes such as boiling and mixing, or a result of errors in the equilibrium calculations as a result of inadequate thermochemical data.     

Estimation of the dielectric constant of H2O from experimental solubilities of quartz,and calculation of the thermodynamic properties of aqueous species to 900°C at 2 kb

Experimental quartz solubilities in H₂O (Anderson and Burnham, 1965, 1967) were used together with equations of state for quartz and aqueous species (Helgesonet al., 1978; Walther and Helgeson, 1977) to calculate the dielectric constant of H₂O () at pressures and temperatures greater than those for which experimental measurements (Heger, 1969; Lukashovet al., 1975) are available ($\text{0.001 ? P ? 5}\text{kb}$ and $\text{0 ? T ? 600°C}$). Estimates of computed in this way for 2 kb (which are the most reliable) range from 9.6 at 600°C to 5.6 at 800°C. These values are 0.5 and 0.8 units greater, respectively, than corresponding values estimated by Quist and Marshall (1965), but they differ by <0.3 units from extrapolated values computed from Pitzer's (1983) adaptation of the Kirkwood (1939) equation. The estimates of generated from quartz solubilities at 2 kb were fit with a power function of temperature, which was then used together with equations and data given by Helgeson and Kirkham (1974a,b, 1976) Helgesonet al. (1981), and Helgeson (1982b, 1984) to calculate Born functions, Debye Hückel parameters, and the thermodynamic properties of Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, and other aqueous species of geologic interest at temperatures to 900°C.     

Complex formation in the ternary system Mg(II)-CO2-H2O

The carbonato and hydrogencarbonato complexes of Mg²⁺ were investigated at 25 and 50° in solutions of the constant ClO₄^? molality (3 M) consisting preponderantly of NaClO₄. The experimental data could be explained assuming the following equilibria: Mg²⁺ + CO_2B + H₂O ag MgHCO⁺₃ + H⁺, $\text{log}{}^1\text{β}{}_1\text{= ?7.64}{}_4\text{± 0.01}{}_7\text{(25°), ?7.46}{}_2\text{± 0.01}{}_1\text{(50°)}$, Mg²⁺ + 2 CO_2g + 2 H₂Oag Mg(HCO₃)⁰₂ ± 2 H⁺, $\text{log}{}^1\text{β}{}_2\text{= ?15.00 ± 0.14 (25°)}$, ?15.37 ± 0.39 (50°), Mg²⁺ + CO_2g + H₂Oag MgCO⁰₃ + 2 H⁺, $\text{log}{}^1\text{k}{}_1\text{= ?15.64 ± 0.06 (25°)}$,?15.23 ± 0.02 (50°), with the assumption γ_MgCO3⁰ = γ_Mg(HCO3)⁰2, ΔG⁰(I = 0) for the reaction MgCO⁰₃ + CO_2g + H₂O = Mg(HCO₃)⁰₂ was estimated to be ?3.9₁ ± 0.8₆ and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.     

Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H2O-CO2-NaCl on phase relations in geologic systems: Equation of state for H2O-CO2-NaCl fluids at high pressures and temperatures

Fluid inclusion analyses leave little doubt that solutions containing large concentrations of H₂O, CO₂, and electrolytes are involved in a wide range of geologic processes. Although the miscibility gap in the system H₂O-CO₂ occurs only at low temperatures, experimental data reported by Takenouchi and Kennedy (1965) and Gehrig (1980) indicate that the addition of 6 weight percent NaCl relative to H₂O + Nacl extends the region of immiscibility in the system H₂O-CO₂-NaC] to ≥700°C at 500 bars and mole fractions of CO₂ (X_CO2) ? 0.1. In contrast, addition of 20 weight percent NaCl relative to H₂O + NaCl at 700°C and 500 bars expands the miscibility gap to X_CO2 ? 0.2. At 2000 bars, addition of 20 and 35 weight percent NaCl relative to H₂O + NaCl causes the miscibility gap to extend to ~500° and ~700°C, respectively, at . The existence of the immiscible region in this high-pressure/temperature environment has a profound effect on temperatures of equilibration for metamorphic mineral assemblages (Bowers and Helgeson, 1983). To determine the extent to which nonideality in the ternary system affects these equilibria, the modified Redlich-Kwong (MRK.) equation of state was fit to pressure-volume-temperature data taken from Gehrig (1980) along pseudobinaries for which is constant. Fugacity coefficients of the components were then generated from the fugacity coefficient analog of the MRK equation of state and these coefficients were used together with solubility data to determine the compositions of the coexisting immiscible phases. The tie lines connecting the coexisting phases shift in orientation from nearly parallel to the H₂O-CO₂ binary at low temperatures to almost perpendicular to this binary at high temperatures.     

Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite,South Africa

The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ¹³C P-type diamonds tend to have inclusions lower in SiO₂ (ol), Al₂O₃ (opx, gt), Cr₂O₃, MgO, $\text{Mg}\text{(Mg + Fe)}$ (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ¹³C E-type diamonds tend to have inclusions lower in SiO₂, Al₂O₃ (gt, cpx), MgO, $\text{Mg}\text{(Mg + Fe)}$ (gt), Na₂O, K₂O, TiO₂ (cpx) and higher in CaO, $\text{Ca}\text{(Ca + Mg)}$ (gt, cpx).Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.     

Chemical fractionations in meteorites—VII. Cosmothermometry and cosmobarometry

The fractional condensation of Bi, Cd, In, Pb and Tl from a cooling gas of cosmic composition is calculated. Predicted absolute and relative abundances of the elements are in good to excellent agreement with the analytical data. This strongly suggests that the presently observed abundances were established at the time of accretion. There is no need to invoke non-equilibrium during condensation or element redistribution after accretion to explain the observations. The elements may therefore be used as cosmothermometers to predict accretion temperatures.Calculated accretion temperatures fall in the range of 420 to 540°K. But a large percentage of each chrondrite group (H, L, LL and E) fall within much narrower intervals, ≤20°K. This implies that the bulk of each group accreted over a narrow temperature range which is consistent with their uniform oxidation states and $\text{O}{}^{18}\text{O}{}^{16}$ ratios. In fact, temperatures inferred from the oxidation state and oxygen isotopes are in excellent agreement with the trace element data.The condensation curves of all these elements are pressure-dependent but are confined to fall in the temperature interval 400 to 600°K owing to the absence of Fe₃O₄ and the presence of FeS in ordinary chrondrites. Absolute upper and lower limits on the total pressure can thus be deduced: 10^?3 to 10^?6 atm. In addition, the condensation curves for Bi and In cross over at T = 462°K and P_t = 2 × 10^?5 atm. The observed relative abundances of Bi and In suggest the L-group formed at slightly lower and the H-group at slightly higher P and T.     

Thermochemical study of glasses in the system CaMgSi2O6-CaAl2SiO6

High temperature solution calorimetry of glasses in the system CaMgSi₂O₆ (Di)-CaAl₂SiO₆ (CaTs) show them to have negative enthalpies of mixing with a regular enthalpy parameter, W_H, of -11.4 ± 0.7 kcal. Negative heats of mixing between alumina-rich and alumina-poor glasses seem to be a general phenomenon in aluminosilicates and are not confined only to glassy systems containing anorthite as a component. The thermodynamic behavior of glasses in the system SiO₂-Ca_0.5;AlO₂-CaMgO₂ appears to vary in a smooth fashion, with small positive heats of mixing near SiO₂ and substantial negative heats of mixing for other compositions. The exothermic behavior with increasing $\text{A1}\text{(Al + Si)}$ may be related to local charge balance of M²⁺ and Al³⁺. The negative heats of mixing in MgCaSi₂O₆-CaAl₂SiO₆, MgCaSi₂O₆-CaAl₂Si₂O₈ and NaAlSi₃O₈-CaAl₂Si₂O₈ glasses are in contrast to the positive heats of mixing found in MgCaSi₂O₆-CaAl₂SiO₆ (pyroxene) and NaAlSi₃O₈-CaAl₂Si₂O₈ (high plagioclase) crystalline solid solutions.     

Diffusion of hydrogen through platinum membranes at high pressures and temperatures

The diffusion of hydrogen through platinum membranes has been measured at 450, 500, 550 and 600°C at 2000 bar pressure, using the hydrogen sensor technique. Ag + AgCl + 3 M HC1 was the starting solution inside the platinum tube. Hydrogen diffuses out of the platinum tube into a system containing Fe₂O₃ + Fe₃O₄ + H₂O; that is, a solution with a fixed hydrogen fugacity. After quench, the drop in f_H2 inside the platinum tube was calculated from measurements of pH and chloride molality. f_H2 is initially roughly proportional to $\text{t}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. Diffusion constants were calculated from these data by numerical integration, and the results can be expressed by logD (cm²/sec) = ? 5489.6/T, K - 4.648.     

Serpentinization of cumulate ultramafic rocks from the North Arm Mountain massif of the Bay of Islands ophiolite

Partially serpentinized dunites and wehrlites comprise the bulk of the cumulate ultramafic unit at the North Arm Mountain massif of the Bay of Islands ophiolite complex, Newfoundland. In a suite of 59 dunites and werhlites from the base of the unit, the serpentinized portions consist of lizardite + chrysotile + brucite + (accessory) magnetite. The ratio of $\text{(}\text{lizardite}\text{+}\text{chrysotile}\text{)}\text{to brucite}\text{= ~8:2 (}\text{weight percent}\text{)}$. Petrographic observations show that most serpentinization occurred at the expense of olivine; only limited amounts of clinopyroxene were serpentized. An estimated volume increase of 32% accompanied serpentinization of the peridotites. Reconstructions of the primary modal proportions of wehrlites (made taking this volume increase into account) contain an average of 6% more clinopyroxene and 6% less olivine than do modal reconstructions that ignore the volume increase. Mass balance calculations provide no clear evidence for appreciable metasomatism of Al₂O₃, CaO, FeO, MgO, or SiO₂ during Serpentinization. The presence of brucite, the evidence that most serpentinization occurred at the expense of olivine, and the lack of appreciable metasomatism, suggest that the primary reaction that controlled serpentinization of the peridotites is: $\text{2Mg}{}_2\text{SiO}{}_2\text{+ 3H}{}_2\text{O ? Mg}{}_3\text{Si}{}_2\text{O}{}_5\text{(OH)}{}_4\text{+ Mg(OH)}{}_2$. olivine added serpentine brucite     

Alkali-free tourmaline in the system MgO-Al2O3-B2O3-SiO2-H2O

An end member of the tourmaline series with a structural formula □(Mg₂Al)Al₆(BO₃)₃[Si₆O₁₈](OH)₄ has been synthesized in the system MgO-Al₂O₃-B₂O₃-SiO₂-H₂O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B₂O₃, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B₂O₃ tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al₂O₃-B₂O₃-SiO₂-H₂O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions.The (3a)-vacant tourmaline has the space group R3m with $\text{a =15.90}\text{A}\text{?}$, $\text{c = 7.115}\text{A}\text{?}$, and $\text{V = 1557.0}\text{A}\text{?}{}^3$. However, these values vary at room temperature with the pressure-temperature conditions of synthesis by $\text{±0.015}\text{A}\text{?}\text{in a}$, $\text{±0.010}\text{A}\text{?}\text{in c}$, and $\text{±4.0}\text{A}\text{?}{}^3\text{in V}$, probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are n_o = 1.631(2), n_E = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca.     

The system pargasite-H2O-CO2: a model for melting of a hydrous mineral with a mixed-volatile fluid—I. Experimental results to 8 kbar

The stability of the amphibole pargasite [NaCa₂Mg₄Al(Al₂Si₆))O₂₂(OH)₂] in the melting range has been determined at total pressures (P) of 1.2 to 8 kbar. The activity of H₂O was controlled independently of P by using mixtures of H₂O + CO₂ in the fluid phase. The mole fraction of H₂O in the fluid ($\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$) ranged from 1.0 to 0.2.At P < 4 kbar the stability temperature (T) of pargasite decreases with decreasing $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at constant P. Above P ? 4 kbar stability T increases as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ is decreased below one, passes through a T maximum and then decreases with a further decrease in $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$. This behavior is due to a decrease in the H₂O content of the silicate liquid as $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ decreases. The magnitude of the T maximum increases from about 10°C (relative to the stability T for $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{= 1}$) at P = 5 kbar to about 30°C at P = 8 kbar, and the position of the maximum shifts from $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.6}$ at P = 5 kbar to $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}\text{? 0.4}$ at P = 8 kbar.The H₂O content of liquid coexisting with pargasite has been estimated as a function of at 5 and 8 kbar P, and can be used to estimate the H₂O content of magmas. Because pargasite is stable at low values of $\text{X}{}_{\mathrm{H}}{}_2\text{O}{}^{\mathrm{1fl}}$ at high P and T, hornblende can be an important phase in igneous processes even at relatively low H₂O fugacities.