An experimental and theoretical determination of oxygen isotope fractionation in the system magnetite-H2O from 300 to 800°C

Oxygen isotope fractionations have been determined between magnetite and water from 300 to 800°C and pressures between 10 and 215MPa. We selected three reaction pathways to investigate fractionation: (a) reaction of fine-grained magnetite with dilute aqueous NaCl solutions; (b) reduction of fine-grained hematite through reaction with dilute acetic acid; and (c) oxidation of fine iron power in either pure water or dilute NaCl solutions. Effective use of acetic acid was limited to temperatures up to about 400°C, whereas oxide-solution isotope exchange experiments were conducted at all temperatures. Equilibrium ¹⁸O/¹⁶O fractionation factors were calculated from the oxide-water experiments by means of the partial isotope exchange method, where generally four isotopically different waters were used at any given temperature. Each run product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and on a limited basis, high-resolution transmission electron microscopy (HRTEM) and Mössbauer spectroscopy. Results from the microscopic examinations indicate the formation of well-crystallized octahedra and dodecahedra of magnetite where the extent of crystallization, grain size, and grain habit depend on the initial starting material, P, T, solution composition, and duration of the run.The greatest amount of oxygen isotope exchange (∼90% or greater) was observed in experiments where magnetite either recrystallized in the presence of 0.5 m NaCl from 500 to 800°C or formed from hematite reacted with 0.5 m acetic acid at 300, 350 and 400°C. Fractionation factors (10³ ln α_mt-H2O) determined from these partial exchange experiments exhibit a steep decrease (to more negative values) with decreasing temperature down to about 500°C, followed by shallower slope. A least-squares regression model of these partial exchange data, which accounts for analytical errors and errors generated by mass balance calculations, gives the following expression for fractionation that exhibits no minimum: 1000lnα_lmt-lw=−8.984(±0.3803)x+3.302(±0.377)x²—0.426(±0.092)x³ with an R² = 0.99 for 300 ≤ T≤ 800°C (x = 10⁶/T²). The Fe oxidation results also exhibit this type of temperature dependence but shifted to slightly more negative 10³ ln α values; there is the suggestion that a kinetic isotope effect may contribute to these fractionations. A theoretical assessment of oxygen isotope fractionation using β-factors derived from heat capacity and Mössbauer temperature (second-order Doppler) shift measurements combined with known β-factors for pure water yield fractionations that are somewhat more negative compared to those determined experimentally. This deviation may be due to the combined solute effects of dissolved magnetite plus NaCl (aq), as well as an underestimation of β_mt at low temperatures. The new magnetite-water experimental fractionations agree reasonably well with results reported from other experimental studies for temperatures ≥ 500°C, but differ significantly with estimates based on quasi-theoretical and empirical approaches. Calcite-magnetite and quartz-magnetite fractionation factors estimated from the combination of magnetite β’s calculated in this study with those for calcite and quartz reported by Clayton and Kieffer (1991) agree very closely with experimentally determined mineral-pair fractionations.     

High-pressure neutron diffraction study on H–D isotope effects in brucite

A neutron powder diffraction study of hydrogenated and deuterated brucite was conducted at ambient temperature and at pressures up to 9 GPa, using a Paris–Edinburgh high-pressure cell at the WAND instrument of the ORNL High Flux Isotope Reactor. The two materials were synthesized by the same method and companion measurements of neutron diffraction were conducted under the same conditions. Our refinement results show that the lattice-parameters of the a axis, parallel to the sheets of Mg–O octahedra, decrease only slightly with pressure with no effect of H–D substitution. However, the c axis of Mg(OD)₂ is shorter and may exhibit greater compressibility with pressure than that of Mg(OH)₂. Consequently, the unit-cell volume of deuterated brucite is slightly, but systematically smaller than that of hydrogenated brucite. When fitted to a third-order Birch–Murnaghan equation in terms of the normalized unit-cell volume, values of the bulk modulus for hydrogenated and deuterated brucite (K ₀ = 39.0 ± 2.8 and 40.4 ± 1.3 GPa, respectively) are, however, indistinguishable from each other within the experimental errors. The measured effect of H–D substitution on the unit-cell volume also demonstrates that brucite (and other hydrous minerals) preferentially incorporate deuterium over hydrogen under pressure, suggesting that the distribution of hydrogen isotopes in deep-earth conditions may differ significantly from that in near-surface environments.     

Experimental determination of the activity-composition relations and phase equilibria of H2O-CO2-NaCl fluids at 500°C, 500 bars

An understanding of the activity-composition (a-X) relations and phase equilibria of halite-bearing, mixed-species supercritical fluids is critically important in many geological and industrial applications. We have performed experiments on H₂O-CO₂-NaCl fluids at 500°C, 500 bar, to obtain accurate and precise data on their a-X relations and phase equilibria. Two kinds of experiments were performed. First, H₂O-CO₂-NaCl samples were reacted at fixed activities of H₂O = 0.078, 0.350, 0.425, 0.448, 0.553, 0.560, 0.606, 0.678, 0.798, 0.841, and 0.935 to define the tie lines of known H₂O activity in the halite-vapor and vapor-brine fields. Results indicate that fluids with all but the last of these H₂O activities lie in the vapor-halite two-phase region and that a fluid with a_H2O = 0.841 has a composition close to the three-phase (vapor + brine + halite) field. A second set of experiments was performed to determine the solubility of NaCl in parts of the system in equilibrium with halite. Data from these experiments suggest that the vapor corner of the three-phase field lies at H₂O contents above X_H2O = 0.58 and X_NaCl = 0.06, and below X_H2O = 0.75 and X_NaCl = 0.06, which is a significantly more H₂O-rich composition than indicated by existing thermodynamic models.     

Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites

The chemical evolution of seawater during the Phanerozoic is still a matter of debate. We have assembled and critically analyzed the available data for the composition of fluid inclusions in marine halite and for the mineralogy of marine evaporites. The composition of fluid inclusions in primary marine halite reveals two major long-term cycles in the chemistry of seawater during the past 600 myr. The concentration of Mg²⁺, Ca²⁺, and SO₄²⁻ has varied quite dramatically. The Mg²⁺ concentration in seawater during most of the early Paleozoic and Jurassic to Cretaceous was as low as 30 to 40 mmol/kg H₂O; it reached maximum values ≥50 mmol/kg H₂O during the Late Neoproterozoic and Permian. The Ca²⁺ concentration in seawater during the Phanerozoic has reached maximum values two to three times greater than the concentration in seawater today (10.6 mmol/kg H₂O), whereas SO₄²⁻ concentrations may have been as low as 5 to 10 mmol/kg H₂O (a third to a fifth of the modern value) during the Jurassic and Early Paleozoic. The Mg²⁺/Ca²⁺ ratio in seawater ranged from 1 to 1.5 during the early to middle Paleozoic and Jurassic-Cretaceous to a near-modern value of 5.2 during the Late Neoproterozoic and Permian. This change in seawater Mg²⁺/Ca²⁺ ratio is consistent with the notion of alternating “calcite-aragonite seas” recorded in oölites and marine carbonate cements.Several models have been proposed to explain the chemical evolution of seawater. These have invoked significant changes in one or more of the major geochemical processes that control the composition of seawater. The pattern and magnitude of the variations in the composition of seawater proposed in this study are similar to those proposed elsewhere that suggest that seawater fluxes through midocean ridges have played a major role in the evolution of seawater during the past 600 myr. Two Phanerozoic supercycles of the Earth’s exogenic processes were recognized in the literature that are caused by mantle convection and plate activity. The composition of seawater has apparently undergone dramatic secular changes in phase with these supercycles and as a consequence of biological evolution. Analyses of fluid inclusions containing unevaporated seawater and a better understanding of the processes that affect the composition of seawater are needed to refine our understanding of the history of Phanerozoic seawater.     

Proton cyclotron frequency phenomena in the topside ionosphere

Proton cyclotron echoes and spurs are phenomena related to the proton cyclotron frequency discovered on topside sounder ionograms from Canadian Alouette satellites. The echoes and spurs appears on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is obtained by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Aloutte II and ISIS and II ionograms for about sixty satellite passes were scaled to study the charateristics of these phenomena. Generally, proton cyclotron echoes and spurs occured on the ionograms at frequencies below the electron plasma frequency f_N, the echoes predominantly slightly above the electron cyclotron frequency f_H and the spurs just below f_N. They appeared most often when a harmonic of the electron cyclotron frequency nf_H(n = 1, 2, 3, 4) was approximately equal to one of the other characteristic frequencies, that is: (1) nf_H ≈ f_N, (2) nf_H ≈ f_zS, the frequency of the Z wave at the heght of the satellite, and (3) nf_H ≈ f_T, the upper hybrid resonance frequency.Proton cyclotron echoes, spurs and protein cyclotron wave patterns have many features in common in addition to their fundamental relationship with the proton cyclotron frequency. The echoes and spurs are observed most often when when nf_H overlaps one of the other characteristic frequencies, that is: nf_H ≈ f_N, nf_H ≈ f_zS, and nf_H ≈ f_T. The proton cyclotron wave pattern is observed under the first of the three conditions. It appears that the occurence of the phenomena is related to the plama conditions, the geographic location not being important in itself except that reflects different plasma conditions. Although proton cyclotron echoes and spurs were observed more often near the geomagnetic equator, consistent with the results of Matuura and Nishizaki,(8) they still observed at high latitudes even near the north geomagnetic pole.The echoes and spurs occur at frequencies below f_N, the echoes predominantly slightly above f_H and the spurs just below f_N. Generally it is easy to distinguish between the two since usually they appear separately or, if together, often an echo would terminate and a spur begin at a slightly different apparent range. But it is not always easy since sometimes it appeared that a proton cyclotron echo and a spur formed a continuous trace, suggesting that perhaps they may be different manifestations of the same phenomenon. Work is continuing in an attemp to understand the origin of proton cyclotron echoes, spurs, and proton cyclotron wave patterns.     

Wave-particle interaction around the lower hybrid resonance

Wave-particle interaction in the ionosphere is studied theoretically for wave frequencies around the lower hybrid resonance (LHR) frequency. An expression is derived for the growth rate of whistler-mode waves propagating in a magneto-active plasma penetrated by a tenuous beam of nonthermal particles. This expression is an extension of that derived in a previous paper by employing the electrostatic dispersion equation; here, the full-wave dispersion equation is used which reduces to the electrostatic one for large values of refractive index.     

Pressure effects on the reduced partition function ratio for hydrogen isotopes in water

We have developed a simple, yet accurate theoretical method for calculating the reduced isotope partition function ratio (RIPFR) for hydrogen of water at elevated pressures. This approach requires only accurate equations of state (EOS) for pure isotopic end-members (H₂O and D₂O), which are available in the literature. The effect of pressure or density on the RIPFR of water was calculated relative to that of ideal-gas water at infinitely low pressure for the temperature range from 0 to 527 °C. For gaseous and low-pressure (ca. ?15 MPa) supercritical phases of water, the RIPFR increases slightly (1-1.3‰) with pressure or density in a fashion similar to those of many other geologic materials. However, in liquid and high-pressure (>20 MPa) supercritical phases, the RIPFR of water decreases (0.5-6‰) with increasing pressure (or density) to 100 MPa. This rather unique phenomenon is ascribed to the inverse molar volume isotope effects (MVIE) of liquid and high-density supercritical waters, V (D₂O) > V (H₂O), while other substances including minerals show the normal MVIE. These theoretical predictions were experimentally confirmed by Horita et al. [Horita, J., Cole, D.R., Polyakov, V.B., Driesner, T., 2002. Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite-water at elevated temperatures. Geochim. Cosmochim. Acta66, 3769 - 3788.] for the system brucite-water. Although the P-T ranges for the EOS of normal and heavy waters are rather limited, our modeling indicates that the RIPFR of water continues to decrease with pressure above 100 MPa. The method developed here can be applied to any other geologic fluids, if accurate EOS for their isotopic end-members is available. These results have important implications for the interpretation of high-pressure isotopic partitioning in the Earth, the outer planets, and their moons.     

Isotopic Evolution of Saline Lakes in the Low-Latitude and Polar Regions

Isotopic fractionations associated with two primary processes (evaporation and freezing of water) are discussed, which are responsible for the formation and evolution of saline lakes in deserts from both low-latitude and the Polar regions. In an evaporative system, atmospheric parameters (humidity and isotopic composition of water vapor) have strong influence on the isotopic behavior of saline lakes, and in a freezing system, salinity build-up largely controls the extent of freezing and associated isotope fractionation. In both systems, salinity has a direct impact on the isotopic evolution of saline lakes. It is proposed that a steady-state “terminal lake” model with short-term hydrologic and environmental perturbations can serve as a useful framework for investigating both evaporative and freezing processes of perennial saline lakes. Through re-assessment of own work and literature data for saline lakes, it was demonstrated that effective uses of the isotope activity compositions of brines and salinity-chemistry data could reveal dynamic changes and evolution in the isotopic compositions of saline lakes in response to hydrologic and environmental changes. The residence time of isotopic water molecules in lakes determines the nature of responses in the isotopic compositions following perturbations in the water and isotope balances (e.g., dilution by inflow, water deficit by increased evaporation, and/or reduction in inflow). The isotopic profiles of some saline lakes from the Polar regions show that they switched the two contrasting modes of operation between evaporative and freezing systems, in response to climate and hydrological changes in the past.     

Experimental and theoretical study of pressure effects on hydrogen isotope fractionation in the system brucite-water at elevated temperatures

A detailed, systematic experimental and theoretical study was conducted to investigate the effect of pressure on equilibrium D/H fractionation between brucite (Mg(OH)₂) and water at temperatures from 200 to 600°C and pressures up to 800 MPa. A fine-grained brucite was isotopically exchanged with excess amounts of water, and equilibrium D/H fractionation factors were calculated by means of the partial isotope exchange method. Our experiments unambiguously demonstrated that the D/H fractionation factor between brucite and water increased by 4.4 to 12.4‰ with increasing pressure to 300 or 800 MPa at all the temperatures investigated. The observed increases are linear with the density of water under experimental conditions. We calculated the pressure effects on the reduced partition function ratios (β-factor) of brucite (300-800 K and P ≤ 800 MPa) and water (400-600°C and P ≤ 100 MPa), employing a statistical-mechanical method similar to that developed by Kieffer (1982) and a simple thermodynamic method based on the molar volumes of normal and heavy waters, respectively. Our theoretical calculations showed that the reduced partition function ratio of brucite increases linearly with pressure at a given temperature (as much as 12.6‰ at 300 K and 800 MPa). The magnitude of the pressure effects rapidly decreases with increasing temperature. On the other hand, the β-factor of water decreases 4 to 5‰ with increasing pressure to 100 MPa at 400 to 600°C. Overall D/H isotope pressure effects combined from the separate calculations on brucite and water are in excellent agreement with the experimental results under the same temperature-pressure range. Our calculations also suggest that under the current experimental conditions, the magnitude of the isotope pressure effects is much larger on water than brucite. Thus, the observed pressure effects on D/H fractionation are common to other systems involving water. It is very likely that under some geologic conditions, pressure is an important variable in controlling D/H partitioning.