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USGS1992

Phase relations in the system NaCl-KCl-H2O: V. Thermodynamic-PTX analysis of solid-liquid equilibria at high temperatures and pressures

The Gibbs energies of mixing for NaCl-KCl binary solids and liquids and solid-saturated NaCl-KCl-H 2 O ternary liquids were modeled using asymmetric Margules treatments. The coefficients of the expressions were calibrated using an extensive array of binary solvus and solidus data, and both binary and ternary liquidus data. Over the PTX range considered, the system exhibits complete liquid miscibility among all three components and extensive solid solution along the anhydrous binary. Solid-liquid and solid-solid phase equilibria were calculated by using the resulting equations and invoking the equality of chemical potentials of NaCl and KCl between appropriate phases at equilibrium. The equations reproduce the ternary liquidus and predict activity coefficients for NaCl and KCl components in the aqueous liquid under solid-saturation conditions between 673 and 1200 K from vapor saturation up to 5 kbar. In the NaCl-KCl anhydrous binary system, the equations describe phase equilibria and predict activity coefficients of the salt components for all stable compositions of solid and liquid phases between room temperature and 1200 K and from 1 bar to 5 kbar.

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USGS1992

Reply to Dr. Stoesselfs comment on “Reaction paths and equilibrium end-points in solid-solution aqueous-solution systems”

In reply to the Critical Comment of R. K. Stoessell (this issue), limiting activity coefficients of bromide in halite ( γ NaBr ) have been calculated by least-squares fitting of Simons et al.'s (1952) bromide distribution coefficient data for the Na(Cl,Br)-NaOH-H 2 O system at 35°C. Regular and subregular solidsolution model fits give γ NaBr = 7.4 and γ NaBr = 8.8, respectively. The Br contents of halite at equilibrium with seawater at initial halite saturation, calculated from the regular and subregular fits, are 17 ppm and 14 ppm, respectively. A survey of literature data for trace bromide in halite shows a wide spread in distribution coefficients, with lower values ( D Br ≈ 0.01) reported by Bloch and Schnerb (1953), Puchelt et al. (1972), and Lutz (1975), and higher values ( D Br − ≈ 0.03) reported by Braitsch and Herrmann (1963), Kühn (1968), Herrmann (1972), Herrmann (1980), Mccaffrey et al. (1987), valiashko et al. (1976), Valiashko and Lavrova (1976), and Fontes (pers. commun., 1990). The measurement of stoichiometric saturation states for halite (or sylvite) with trace bromide mole-fractions is not practical, given the insensitivity of the measured solubilities on the bromide mole-fractions. Distribution coefficient measurements, with proof of thermodynamic equilibrium, need to be obtained instead, to conclusively determine the thermodynamic-mixing properties of both Na(Cl,Br) and K(Cl,Br) solidsolution series at very low mole-fractions of bromide. The applicability of the stoichiometric saturation concept to the interpretation of precipitation processes is questionable, primarily because the concept requires solid-solutions to behave as one-component solids with fixed composition. Lippmann diagrams are useful in depicting stoichiometric saturation, endmember saturation, and thermodynamic equilibrium states in binary-solid-solution aqueous-solution systems. Lippmann diagrams can contribute a better understanding of these systems, regardless of the concentration of the endmember components.

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USGS1992

Thermodynamics and kinetics of reactions involving vanadium in natural systems: Accumulation of vanadium in sedimentary rocks

A critical review of thermodynamic data for aqueous and solid V species is presented to evaluate dissolution, transport, and precipitation of V under natural conditions. Emphasis is given to results of experimental studies of V chemistry, especially those for which the experimental conditions are near those found in nature. Where possible, data are obtained for or corrected to the reference conditions of 298.15K, 1 atm (1.01325 bar) and zero ionic strength. Vanadium [IV] (V IV ) and vanadium[V] (V V ) are the most soluble forms of V in nature, and their complexes with fluoride, sulfate, and oxalate may act to increase V solubility under oxidizing conditions. Because redox behavior is of fundamental importance to understanding natural V chemistry, the kinetics of reduction of V IV to V III H 2 S were studied. Although H 2 S is predicted from thermodynamic data to be capable of reducing V IV to V III , this reaction has not been demonstrated experimentally. Experiments were carried out under conditions of temperature (45°C), pH (3.6–6.8), ionic strength (0.05–0.1 m), and V concentrations (9.8–240 μmolar) likely to be found in nature. Because the reaction is very slow, H 2 S concentrations in excess of natural conditions were used (8.1 × 10 −4 to 0.41 atm). The results show that V IV is reduced to V III under a variety of conditions. The rate increases with increasing pH, but is not appreciably affected by ionic strength (as represented by the concentration of KCl, which was used as the supporting electrolyte in all cases). Prior to initiation of the reaction, there is an induction period, the length of which increases with increasing KCl concentration or decreasing pH. Attempts to model the reaction mechanism by numerical methods have failed to produce a satisfying fit of the results, indicating partial reaction orders, a complex mechanism, or involvement of a variety of intermediate species. The results of the thermodynamic and kinetic studies were applied to understanding the genesis of V deposits such as those commonly found on the Colorado Plateau. Vanadium in these sandstone-hosted deposits is present mostly in the reduced oxidation state, V III . Because of the insolubility of V III oxyhydroxides, it is likely that a more oxidized form of V (either [IV] or [V]) was transported to the site of mineralization, and that the V was reduced in situ and subsequently precipitated. A probable reductant is hydrogen sulfide; the presence of pyrite cogenetic with the V minerals documents the presence of H 2 S during mineralization. The experiments described here show that H 2 S could have reduced V IV to V III , and thus led to the formation of these deposits.

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USGS1992

Sulfur isotopic disequilibrium and fluid-rock interaction during metamorphism of sulfidic black shales from the Waterville-Augusta area, Maine, USA

Sulfur isotope ratios of pyrite (py) and pyrrhotite (po) from regionally metamorphosed graphitic sulfidic schists and related rocks from south-central Maine, USA, were analysed using SO 2 and SF 6 techniques. There is a broad range in δ 34 S values for both pyrite and pyrrhotite at most outcrops, up to 8%. and overall the values are isotopically light, averaging ~ −27‰ for the entire data set, suggesting that the rocks have not been grossly isotopically disturbed by regional metamorphism from their inferred organic-rich sedimentary origins. At all temperatures from chlorite to sillimanite grades, sulfide analysed from veins and blebs within the schists show predominantly disequilibrium fractionations ranging from Δ 34 S py-po −3.0 to +3.5‰ "> Δ 34 S py-po −3.0to+3.5‰ , as do matrix sulfides from rocks that attained temperatures <500°C. Only at T > 500° C do matrix pyrite-pyrrhotite pairs with polygonal or aligned granoblastic microstructures approach isotopic equilibrium at millimeter to centimeter scales, suggesting that the process that favoured equilibration was recrystallization accompanying metamorphism and deformation. This disequilibrium may be a function of preferential interaction of one of the phases with an infiltrating fluid, but the lack of any systematic trends in the data, particularly with both negative and positive Δ 34 S py-po at some outcrops, does not permit ready identification of fluid sources, fluxes, or compositions. By combining published fluid fluxes for the area and a knowledge of the mass of sulfur contained in the rocks and the inferred infiltrating fluid, it appears that sulfur should have been homogenized over at least 10's to 1000's of meters, if equilibrium had been attained between rock sulfides and an infiltrating fluid of constant composition. That this did not occur was probably due to lack of equilibration between sulfides and the fluid but may also have arisen because of channelling of fluid flow along rather than across layers, or a lack of fluid infiltration through this unit.

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USGS1992

Dissolution of aragonite-strontianite solid solutions in nonstoichiometric Sr (HCO3)2-Ca (HCO3)2-CO2-H2O solutions

Synthetic strontianite-aragonite solid-solution minerals were dissolved in CO 2 -saturated non-stoichiometric solutions of Sr(HCO 3 ) 2 and Ca(HCO 3 ) 2 at 25°C. The results show that none of the dissolution reactions reach thermodynamic equilibrium. Congruent dissolution in Ca(HCO 3 ) 2 solutions either attains or closely approaches stoichiometric saturation with respect to the dissolving solid. In Sr(HCO 3 ) 2 solutions the reactions usually become incongruent, precipitating a Sr-rich phase before reaching stoichiometric saturation. Dissolution of mechanical mixtures of solids approaches stoichiometric saturation with respect to the least stable solid in the mixture. Surface uptake from subsaturated bulk solutions was observed in the initial minutes of dissolution. This surficial phase is 0–10 atomic layers thick in Sr(HCO 3 ) 2 solutions and 0–4 layers thick in Ca(HCO 3 ) 2 solutions, and subsequently dissolves and/or recrystallizes, usually within 6 min of reaction. The initial transient surface precipitation (recrystallization) process is followed by congruent dissolution of the original solid which proceeds to stoichiometric saturation, or until the precipitation of a more stable Sr-rich solid. The compositions of secondary precipitates do not correspond to thermodynamic equilibrium or stoichiometric saturation states. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of solid solutions on surfaces of aragonite and strontianite single crystals immersed in Sr(HCO 3 ) 2 and Ca(HCO 3 ) 2 solutions, respectively. In Sr(HCO 3 ) 2 solutions, the XPS signal from the outer ~ 60 Å on aragonite indicates a composition of 16 mol% SrCO 3 after only 2 min of contact, and 14–18 mol% SrCO 3 after 3 weeks of contact. The strontianite surface averages approximately 22 mol% CaCO 3 after 2 min of contact with Ca(HCO 3 ) 2 solution, and is 34–39 mol% CaCO 3 after 3 weeks of contact. XPS analysis suggests the surface composition is zoned with somewhat greater enrichment in the outer ~25 Å (as much as 26 mol% SrCO 3 on aragonite and 44 mol% CaCO 3 on strontianite). The results indicate rapid formation of a solid-solution surface phase from subsaturated aqueous solutions. The surface phase continually adjusts in composition in response to changes in composition of the bulk fluid as net dissolution proceeds. Dissolution rates of the endmembers are greatly reduced in nonstoichiometric solutions relative to dissolution rates observed in stoichiometric solutions. All solids dissolve more slowly in solutions spiked with the least soluble component ((Sr(HCO 3 ) 2 )) than in solutions spiked with the more soluble component (Ca(HCO 3 ) 2 ), an effect that becomes increasingly significant as stoichiometric saturation is approached. It is proposed that the formation of a non-stoichiometric surface reactive zone significantly decreases dissolution rates.

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USGS1992

The influence of pH on biotite dissolution and alteration kinetics at low temperature

Biotite dissolution rates in acidic solutions were determined in fluidized-bed reactors and flowthrough columns. Biotite dissolution rates increased inversely as a linear function of pH in the pH range 3–7, where the rate order n = −0.34. Biotite dissolved incongruently over this pH range, with preferential release of magnesium and iron from the octahedral layer. Release of tetrahedral silicon was much greater at pH 3 than at higher pH. Iron release was significantly enhanced by low pH conditions. Solution compositions from a continuous exposure flow-through column of biotite indicated biotite dissolves incongruently at pH 4, consistent with alteration to a vermiculite-type product. Solution compositions from a second intermittent-flow column exhibited elevated cation release rates upon the initiation of each exposure to solution. The presence of strong oxidizing agents, the mineral surface area, and sample preparation methodology also influenced the dissolution or alteration kinetics of biotite.

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USGS1992

Experimental study of iron-chloride complexing in hydrothermal fluids

Mineral assemblage solubilities were measured in cold-seal pressure vessels as a function of pressure, temperature, and potassium chloride concentration in order to determine the nature and thermodynamic properties of iron-chloride complexes under hydrothermal conditions. The assemblage pyritepyrrhotite-magnetite was used to buffer ƒS 2 "> ƒS 2 and ƒO 2 "> ƒO 2 , and K + H + "> K + H + ratios were buffered at reasonable geologic values using the assemblage potassium feldspar-muscovite (or andalusite)-quartz. The pressure-temperature ranges were 0.5-2.0 kbar and 300–600°C, and initial fluid compositions ranged from 0.01–2.0 molal KCl. With all other factors constant, the concentration of iron in solution increases with increasing temperature, with decreasing pressure, and with increasing total potassium chloride concentration. Changes in iron concentrations as a function of KCl concentration, in conjunction with charge balance, mass action, and mass balance constraints on the system, place constraints on the stoichiometry of the important iron-chloride complexes under each of the experimental conditions. Using least-squared linear regression fits to determine these slopes, the calculations yield values for the average ligand numbers that are in the range 1.2-1.9, with uncertainties ranging from ±0.1-0.6 at the several PT conditions considered. The slopes of the regressed fits to the data suggest that both FeCl + and FeCl 2 0 are important in the experimental fluids, with FeCl 2 0 becoming dominant at the higher temperatures. Theoretical calculations, however, indicate that FeCl + does not contribute significantly to the solubility. Because of the large uncertainties associated with some of the calculated average ligand numbers, we base our data analysis on the theoretical calculations. A statistical analysis is applied to the solubility data in order to determine the values and uncertainties of the dissociation constant for FeCl 2 0 that best fit the data at each of the experimental pressures and temperatures. The calculated stability of FeCl 2 0 increases with increasing temperature and total chloride concentration, and with decreasing pressure. The values of the dissociation constant of FeCl 2 0 that are calculated in this study are in moderately good agreement with FeCl 2 0 dissociation constants from other studies of iron-chloride complexing in supercritical fluids. Differences are likely due to different assumptions made concerning activity coefficients of aqueous species. Log k d values for full dissociation of FeCl 2 0 at 0.5 kbar—300°C—and at 1 kbar—400, 500, and 600°C, respectively—are −3.75 ± 0.40, −6.25 ± 0.10, −9.19 ± 0.44, and −13.29 ± 0.09.

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USGS1992

Interoceanic variation in the rare earth, major, and trace element depositional chemistry of chert: Perspectives gained from the DSDP and ODP record

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~ 11% NASC, Atlantic chert ~ 17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for ∑REE, approximations of La ex (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest La ex (85% of La total ) and SHL chert the least (38% of La total ). As shown by interelement associations, this La ex is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert CeCe * ⪡ 1 and La n Yb n ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., La n Yb n ~ 0.4), which increases the La n Yb n ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and La n Yb n "> La n Yb n ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce Ce * ~1 "> CeCe * ~1 and inherited La n Yb n "> La n Yb n values of ~1.2–1.4. Ce Ce * "> ~1.2–1.4.CeCe * does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce Ce * "> CeCe * does not correlate with P 2 O 5 concentrations, even though phosphatic phases may be an important REE carrier. This and previous studies of the large-scale controlling parameters of sedimentary REEs across ocean basins collectively indicate that REE indices of depositional regime (e.g., Ce Ce * "> CeCe * , La n Yb n "> La n Yb n , La ex ) are reproducible in a variety of sediment and rock lithologies, ages, and ocean basins, and present a coherent tool for paleoceanographic and tectonic basin reconstructions.

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USGS1992

Sulfur, carbon, and oxygen isotope variations in submarine hydrothermal deposits of Guaymas Basin, Gulf of California, USA

Sulfur, carbon, and oxygen isotope values were measured in sulfide, sulfate, and carbonate from hydrothermal chimney, spire, and mound samples in the southern trough of Guaymas Basin, Gulf of California, USA. δ 34 S values of sulfides range from −3.7 to 4.5%. and indicate that sulfur originated from several sources: 1. (1) dissolution of 0‰ sulfide contained within basaltic rocks, 2. (2) thermal reduction of seawater sulfate during sediment alteration reactions in feeder zones to give sulfide with positive δ 34 S , and 3. (3) entrainment or leaching of isotopically light (negative- δ 34 S ) bacteriogenic sulfide from sediments underlying the deposits. δ 34 S of barite and anhydrite indicate sulfur derivation mainly from unfractionated seawater sulfate, although some samples show evidence of sulfate reduction and sulfide oxidation reactions during mixing within chimneys. Oxygen isotope temperatures calculated for chimney calcites are in reasonable agreement with measured vent fluid temperatures and fluid inclusion trapping temperatures. Hydrothermal fluids that formed calcite-rich chimneys in the southern trough of Guaymas Basin were enriched in 18 O with respect to seawater by about 2.4‰ due to isotopic exchange with sedimentary and/or basaltic rocks. Carbon isotope values of calcite range from −9.6 to −14.0‰ δ 34 C pDB , indicating that carbon was derived in approximately equal quantities from the dissolution of marine carbonate minerals and the oxidation of organic matter during migration of hydrothermal fluid through the underlying sediment column. Statistically significant positive, linear correlations of δ 34 S , δ 34 C , and δ 18 O of sulfides and calcites with geographic location within the southern trough of Guaymas Basin are best explained by variations in water/rock ( w r "> wr ) ratios or sediment reactivity within subsurface alteration zones. Low w r "> wr ratios and the leaching of detrital carbonates and bacteriogenic sulfides at the southern vent sites result in relatively high δ 13 C and low δ 34 S in chimney carbonates and sulfides, respectively. In the north, where the depletion of alkalis in vent fluids indicates higher w r "> wr ratios, positive δ 34 S and more negative δ 13 c are due to increased contributions from organic matter oxidation and sulfate reduction reactions.

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USGS1992

Garnet/high-silica rhyolite trace element partition coefficients measured by ion microprobe

Garnet/liquid trace element partition coefficients have been measured in situ by ion microprobe in a rhyolite from Monache Mountain, California. Partition coefficients are reported for La, Ce, Nd, Sm, Dy, Er, Yb, Sc, Ti, V, Cr, Sr, Y, and Zr. The in situ analyses avoid the problem of contamination of the garnet phase by trace element-rich accessory minerals encountered in traditional bulk phenocryst/matrix partitioning studies. The partitioning pattern for the rare earth elements (REEs, excluding Eu) is smooth and rises steeply from the light to the heavy REEs with no sharp kinks or changes in slope, unlike patterns for garnet /silicic liquid REE partitioning determined by bulk methods. This difference suggests that the previous determinations by bulk methods are in error, having suffered from contamination of the phenocryst separates.

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USGS1992

Rare earth, major, and trace element composition of Monterey and DSDP chert and associated host sediment: Assessing the influence of chemical fractionation during diagenesis

Chert and associated host sediments from Monterey Formation and Deep Sea Drilling Project (DSDP) sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments. The primary compositional contrast between chert and host sediment is a greater absolute SiO 2 concentration in chert, often with final SiO 2 ≥ 98 wt%. This contrast in SiO 2 (and Si Al "> SiAl ) potentially reflects precursor sediment heterogeneity, diagenetic chemical fractionation, or both. SiO 2 concentrations and Si Al "> SiAl ratios in chert are far greater than in modern siliceous oozes, however and often exceed values in acid-cleaned diatom tests. Compositional contrasts between chert and host sediment are also orders-of-magnitude greater than between multiple samples of the host sediment. Calculations based on the initial composition of adjacent host, observed porosity reductions from host to chert and a postulated influx of pure SiO 2 , construct a chert composition which is essentially identical to observed SiO 2 values in chert. Thus, precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest. In order to assess the extent of chemical fractionation during diagenesis, we approximate the precursor composition by analyzing host sediments adjacent to the chert. The SiO 2 concentration contrast seems caused by biogenic SiO 2 dissolution and transport from the local adjacent host sediment and subsequent SiO 2 reprecipitation in the chert. Along with SiO 2 , other elements are often added (with respect to Al) to Monterey and DSDP chert during silicification, although absolute concentrations decrease. The two Monterey quartz chert nodules investigated, in contrast to the opal-CT and quartz chert lenses, formed primarily by extreme removal of carbonate and phosphate, thereby increasing relative SiO 2 concentrations. DSDP chert formed by both carbonate/phosphate dissolution and SiO 2 addition from the host. Manganese is fractionated during chert formation, resulting in MnO Al 2 O 3 "> MnOAl2O3 ratios that no longer record the depositional signal of the precursor sediment. REE data indicate only subtle diagenetic fractionation across the rare earth series. Ce Ce ∗ "> CeCe* values do not change significantly during diagenesis of either Monterey or DSDP chert. Eu Eu ∗ "> EuEu* decreases slightly during formation of DSDP chert. La n Yb n "> LanYbn is affected only minimally as well. During formation of one Monterey opal-CT chert lens, REE Al "> REEAl ratios show subtle distribution changes at Gd and to a lesser extent near Nd and Ho. REE compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment, indicating that the paleoenvironmental REE signature is not obscured by diagenetic overprinting.

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USGS1992

The Cretaceous-Tertiary extinction: A lethal mechanism involving anhydrite target rocks

The Chicxulub Crater, Yucatan, Mexico, is a leading contender as the site for the impact event that caused the Cretaceous-Tertiary (K-T) extinctions. A considerable thickness of anhydrite (CaSO 4 ) forms part of the target rock. High temperatures resulting from impact would drive SO 2 off from the anhydrite. Hundreds of billions of tonnes of sulfuric acid aerosol would thus enter the stratosphere and cause considerable cooling of the Earth's surface, decrease photosynthesis by orders of magnitude, deplete the ozone layer, and permit increased UV radiation to reach the Earth's surface. Finally, the aerosol would fall back to Earth as acid rain and devastate land and some lacustrine biota and near-surface marine creatures. The presence of anhydrite in the Chicxulub target rock may thus help explain the many extinctions observed at the K-T boundary.

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USGS1993

Radium isotope geochemistry of thermal waters, Yellowstone National Park, Wyoming, USA

Radium isotope activities ( 226 Ra, 228 Ra, and 224 Ra), chemical compositions, and sulfur isotope ratios in sulfate were determined for water samples from thermal areas in Yellowstone National Park, Wyoming. Activities of 226 Ra in these waters range from <0.2 to 37.9 dpm/kg. Activity ratios of 228 Ra 226 Ra "> 228Ra226Ra range from 0.26 to 14.2, and those of 224 Ra 228 Ra "> 224Ra228Ra range from 0.73 to 3.1. Radium concentrations are inversely correlated with aquifer equilibration temperatures (estimated from dissolved silica concentrations), while[ Ra Ba "> RaBa ] aq and 228 Ra 226 Ra "> 228Ra226Ra activity ratios depend upon U Ba "> UBa and Th U "> ThU ratios in aquifer rocks. Major controls on Ra concentration in Yellowstone thermal waters are inferred to be 1. (1) barite saturation (at Morris Geyser Basin, Mammoth Hot Springs, and other northern areas) and 2. (2) zeolitewater ion exchange (at Upper Geyser Basin). The data are consistent with a model in which 3. (1) radium and barium are supplied to water by bulk dissolution of aquifer rock, and 4. (2) chemical equilibration of water with rock is rapid relative to the 1602 year half-life of 226 Ra. The 228 Ra 226 Ra "> 228Ra226Ra activity ratios of the waters may in some cases reflect surface enrichments of 232 Th and/or may indicate that α-recoil input of 228 Ra is rapid relative to water-rock chemical equilibration. Activity ratios of 224 Ra 228 Ra "> 224Ra228Ra indicate a nearly ubiquitous 224 Ra excess that generally increases with decreasing pH. Near-surface (≤100 m) thermal water flow velocities at Mammoth Hot Springs are estimated from 224 Ra 228 Ra "> 224Ra228Ra variation to be ≥ 1 m h −1 .

Wyoming↗

USGS1993

Oxygen buffering of Kilauea volcanic gases and the oxygen fugacity of Kilauea basalt

Volcanic gases collected during episode 1 of the Puu Oo eruption along the east rift zone of Kilauea Volcano, Hawaii, have uniform C-O-H-S-Cl-F compositions that are sharply depleted in CO 2 . The CO 2 -poor gases are typical of Type II volcanic gases ( gerlach and Graeber , 1985) and were emitted from evolved magma stored for a prolonged period of time in the east rift zone after releasing CO 2 -rich gases during an earlier period of temporary residence in the summit magma chamber. The samples are remarkably free of contamination by atmospheric gases and meteoric water. Thermodynamic evaluation of the analytical data shows that the episode 1 gases have equilibrium compositions appropriate for temperatures between 935 and 1032°C. Open- and closed-system equilibrium models of species distributions for the episode 1 gases show unequivocally that coexisting lavas buffered the gas oxygen fugacities during cooling. These models indicate that the f O 2 buffering process occurs by transfer of oxygen from the major species in the gas phase (H 2 O, CO 2 , SO 2 ) to the lava during cooling and that the transfer of oxygen also controls the fugacities of several minor and trace species (H 2 , CO, H 2 S, S 2 , Cl 2 , F 2 ), in addition to O 2 during cooling. Gas/lava exchanges of other components are apparently insignificant and exert little influence, compared to oxygen exchange, during cooling. Oxygen transfer during cooling is variable, presumably reflecting short-term fluctuations in gas flow rates. Higher flow rates restrict the time available for gas/lava oxygen transfer and result in gases with higher equilibrium temperatures. Lower flow rates favor f O 2 -constrained equilibration by oxygen transfer down to lower temperatures. Thus, the chemical equilibrium preserved in these gases is a heterogeneous equilibrium constrained by oxygen fugacity, and the equilibrium temperatures implied by the compositions of the gases reflect the temperatures at which gas/lava oxygen exchange ceased. This conclusion challenges the common assumption that volcanic gases are released from lava in a state of chemical equilibrium and then continue equilibrating homogeneously with falling temperature until reaction rates are unable to keep pace with cooling. No evidence is found, moreover, that certain gas species are kinetically more responsive and able to equilibrate down to lower temperatures than those of the last gas/lava oxygen exchange. Homogeneous reaction rates in the gas phase are apparently slow compared to the time it took for the gases to move from the last site of gas/lava equilibration to the site of collection. An earlier set of data for higher temperature CO 2 -rich Type I volcanic gases, which come from sustained summit lava lake eruptions supplied by magma that experienced substantially shorter periods of crustal storage, shows f O 2 buffering by oxygen transfer up to 1185°C. Oxygen fugacity measurements in drill holes into ponded lava flows suggest that buffering by oxygen transfer may control the f O 2 of residual gases down to several hundred degrees below the solidus in the early stages of cooling. Although the details of the f O 2 buffering mechanisms for oxygen transfer are unknown, the fact that f O 2 buffering is effective from molten to subsolidus conditions suggests that the reaction mechanisms must change with cooling as the reactants change from predominantly melt, to melt plus crystals, to glass plus crystals. Mass balance calculations suggest that redox reactions between the gas and ferrous/ferric iron in the lava are plausible mechanisms for the oxygen transfer and that the f O 2 of the gases is buffered by sliding ferrous/ferric equilibria in the erupting lavas. Contrary to expectations based on models predicting the oxidation of basalt by H 2 and CO escape during crustal storage, CO 2 -rich Type I gases and CO 2 -poor Type II gases have identical oxygen fugacities despite greatly different crustal storage and degassing histories. Volcanic gas data give a tightly constrained log f O 2 of NNO − 0.5 (±0.05) for subaerially erupted Kilauea basalt from liquidus to solidus temperatures, consistent with recent f O 2 determinations for the mantle source regions of ocean island basalts. Because the oxygen fugacities of volcanic gases emitted by subaerial lavas imply that the f O 2 of Kilauea basalt is unchanged during crustal storage, Kilauea basalt either arrives in the crust with an oxygen fugacity between NNO and FMQ, or it develops an oxygen fugacity in this range immediately upon arrival in the summit chamber.

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USGS1993

An extremely low UPb source in the Moon: U Th Pb, Sm Nd, Rb Sr, and 40Ar 39Ar isotopic systematics and age of lunar meteorite Asuka 881757

We have undertaken U Th Pb, Sm Nd, Rb Sr, and 40 Ar 39 Ar "> 40 Ar 39 Ar isotopic studies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare basalts. The Pb Pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 ± 28 Ma, which is similar to the U-Pb (3850 ± 150 Ma) and Th-Pb (3820 ± 290 Ma) internal isochron ages. The Sm-Nd data for the mineral separates yield an internal isochron age of 3871 ± 57 Ma and an initial 143 Nd 144 Nd "> 143 Nd 144 Nd value of 0.50797 ± 10. The Rb-Sr data yield an internal isochron age of 3840 ± 32 Ma ( λ( 87 Rb) = 1.42 × 10 −11 yr −1 "> λ( 87 Rb) = 1.42 × 10 −11 yr −1 ) and a low initial 87 Sr 86 Sr "> 87 Sr 86 Sr ratio of 0.69910 ± 2. The 40 Ar 39 Ar "> 40 Ar 39 Ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 ± 12 Ma. We interpret these ages to indicate that the basalt crystallized from a melt 3.87 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr system and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following interpretation is also possible; the meteorite crystallized at 3.94 Ga (the Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2–3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-forming event (Imbrium?). The age span for VLT basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least ~500 million years. The impact event that thermally reset the K-Ar system of Asuka 881757 must have been post-Imbrium (perhaps Orientale) in age. The lead isotopic composition of Asuka 881757 is nonradiogenic compared with typical Apollo mare basalts and the estimated 238 U 204 Pb "> 238 U 204 Pb (μ) value for the basalt source is 10 ± 3. This source-μ value is the lowest so far measured for lunar rocks. A large positive ϵ Nd value (7.4 ± 0.5) and the time averaged 147 Sm 144 Nd "> 1 47 147 Sm 144 Nd "> Sm 144 Nd ratio for the basalt source are similar to those for some Apollo 12, 15, and 17 basalts, suggesting a LREE-depleted mantle, which is consistent with the global magma ocean hypothesis. The U-Th-Pb, Sm-Nd, and Rb-Sr data on Asuka 881757 suggest that the basalt was derived from a low U Pb "> UPb , low Rb Sr "> RbSr , and high Sm Nd "> SmNd source region, mainly composed of olivine and orthopyroxene with minor amounts of plagioclase (or clinopyroxene) and with sulfides enriched in volatile chalcophile elements. The basalt source may be deep in origin and different in chemistry from those previously estimated from studies of Apollo and Luna mare basalts, indicating heterogeneous sources for mare basalts.

Geochimica et Cosmochimica Acta↗

USGS1993

Ikaite precipitation by mixing of shoreline springs and lake water, Mono Lake, California, USA

Metastable ikaite (CaCO 3 ·6H 2 O) forms abundantly during winter months along the south shoreline of Mono Lake where shoreline springs mix with lake water. Ikaite precipitates because of its decreased solubility at low temperature and because of orthophosphate-ion inhibition of calcite and aragonite. During the spring some of the ikaite is transformed to anhydrous CaCO 3 and is incorporated into tufa, but most is dispersed by wave action into the lake where it reacts to form gaylussite (Na 2 Ca(CO 3 ) 2 · 5H 2 O). Spring waters have low pH values, are dominantly Ca-Na-HCO 3 , have low radiocarbon activities, and are mixtures of deep-seated geothermal and cold groundwaters. Chemical modeling reveals that precipitation of CaCO 3 can occur over a broad range of mixtures of spring and lake water with a maximum production occurring at 96% spring water and 4% lake water. Under these conditions all the Ca and a significant fraction of the CO 3 of the precipitate is spring supplied. A radiocarbon age of 19,580 years obtained on a natural ikaite sample supports this conclusion. With the springs supplying a large and probably variable portion of the carbonate, and with apparent 14 C age of the carbonate varying from spring to spring, tufa of similar actual antiquity may yield significantly different 14 C dates, making tufa at this location unsuitable for absolute age dating by the radiocarbon method.

Geochimica et Cosmochimica Acta↗

USGS1993

Solubility-product constant and thermodynamic properties for synthetic otavite, CdCO3(s), and aqueous association constants for the Cd(II)-CO2-H2O system

Considerable disparity exists in the published thermodynamic data for selected species in the Cd(II)-CO 2 -H 2 O system near 25°C and 1 atm pressure. Evaluation of published experimental and estimated data for aqueous cadmium-carbonate species suggests an association constant, pK , of −3.0 ± 0.4 for CdCO 3 0 , about −1.5 for CdHCO 3 + , and −6.4 ± 0.1 for Cd(CO 3 ) 2 2− ( T = 298.15 K ; P = 1 atm; I = 0). Examination of all available data for cadmium-hydrolysis species and β - Cd ( OH ) 2( s ) ) confirms that the consistent set of constants presented by Baes and Mesmer ( Hydrolysis of Cations , 1976) is the best available. The solubility of synthetic otavite, CdCO 3(s) , has been measured in KClO 4 solutions where I ≤ 0.1 M . We calculated pK sp = 12.1 ± 0.1 ( T = 25.0° C ; P = 1 atm; I = 0) from measured concentrations of Cd 2+ , measured P C 02 and pH, our selected set of equilibrium constants, and activity corrections estimated using the Davies equation. Values at 5 and 50°C were 12.4 ± 0.1 and 12.2 ± 0.1, respectively. Based on the new solubility data and the CODATA key values for Cd 2+ and CO 3 2− , a new set of thermodynamic properties is recommended for otavite: ΔG f 0 = −674.7 ± 0.6 kJ / mol ; ΔH f 0 = −751.9 ± 10 kJ / mol ; S 0 = 106 ± 30 J/mol K; and ΔG r 0 for the reaction Cd 2+ + CO 3 2− ⇌ CdCO 3 ( s ) is −69.08 ± 0.57 kJ / mol.

Geochimica et Cosmochimica Acta↗

USGS1993

Calculation of the vapor-saturated liquidus for the NaCl-CO2-H2O system

The polybaric liquidus surface for the H 2 O-rich corner of the NaCl-CO 2 -H 2 O ternary is calculated, relying heavily on 1. (1) a Henry's law equation for CO 2 in brines (modified from Drummond, 1981), 2. (2) the assumption that the contributions of dissolved NaCl and CO 2 in lowering the activity of H 2 O are additive, and 3. (3) data on the CO 2 clathrate solid solution (nominally CO 2 · 7.3H 2 O, but ranging from 5.75 to 8 or 9 H 2 O) from Bozzo et al. (1975). The variation with composition of the activity of CO 2 ·7.3H 2 O, or any other composition within the clathrate field, is small, thereby simplifying the calculations appreciably. Ternary invariant points are 1. (1) ternary eutectic at −21.5°C, with ice + clathrate + hydrohalite NaCl-·H 2 O + brine m NaCl = 5.15, m co 2 = 0.22 + vapor P total ≈ P co 2 = 5.7 atm; 2. (2) peritectic at −9.6°C, with clathrate + hydrohalite + liquid CO 2 + brine m NaCl = 5.18, m co 2 = 0.55 + vapor ( P total ≈ P co 2 = 26.47 atm ); and 3. (3) peritectic slightly below +0.1 °C, with halite + hydrohalite + liquid CO 2 + brine ( m NaCl ≈ 5.5, m co2 ≈ 0.64 ) + vapor ( P total ≈ P co 2 ≈ 34 atm). CO 2 isobars have been contoured on the ternary liquidus and also on the 25°C isotherm. An important caveat regarding the application of this information to the interpretation of the freezing-thawing behavior of fluid inclusions is that metastable behavior is a common characteristic of the clathrate.

Geochimica et Cosmochimica Acta↗