Meteoritic component and impact melt composition at the lac à l'Eau Claire (clearwater) impact structures,Quebec

The major and selected trace element composition of the melt at the eastern Lac à l'Eau Claire impact structure can be modelled by 52.3% granodiorite, 15.1% granulite, 25.7% quartz-monzonite plus 6.9% Orgueil (C1-chondrite). A satisfactory fit to melt at the western structure requires 57.4% granulite and 42.6% quartz-monzonite. These models agree with the suggestions of Palmeet al. (1978, Geochim. Cosmochim. Acta42, 313–323) on the type and extent of meteoritic component in the melts.     

The origin of metal clasts in the Bencubbin meteoritic breccia

Bencubbin is a breccia containing metal and silicate clasts, along with occasional chondritic fragments. The breccia is cemented together by a small amount of shock-melted metal-silicate matrix. There is no evidence, however, for complete melting of either metal or silicate clasts after their incorporation within the breccia. The main metal phase occurs as rounded and angular clasts of Fe-Ni. Each clast is chemically homogeneous, but systematic chemical variations between clasts are observed with Ni concentrations varying from ? 5.3 wt% in some clasts up to 7.5 wt% in others. Cobalt concentrations vary between clasts from 0.25 to 0.35 wt% and are positively correlated with Ni. The Co and Ni concentrations are consistent with the metal condensation path (pressure ? 10^-3 atm) predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187). The P vs Ni concentrations are consistent with the metal condensation path (pressure ? 10^?4atm) predicted by Wai et al. (1978, Lunar and Planetary Science IX, pp. 1193–1195). Thus we believe that Bencubbin metal clasts may record chemical information imparted during condensation of the metal from the nebula. Chromium concentrations in Bencubbin metal (0.05–0.30 wt%) greatly exceed concentrations observed in iron meteorites as predicted by Grossman and Olsen (1974, Geochim. Cosmochim. Acta38, 173–187) for metal condensates from the solar nebula. The Cr-Ni trend in Bencubbin, however, is positively correlated with Ni, in contrast to the predicted condensation trend. This unexpected correlation may be the result of subsequent redistribution of Cr between metal and micron-sized troilite blebs. The incorporation of these troilite blebs within the metal clasts is difficult to explain in terms of low temperature (? 700 K) condensation of troilite. The possible explanations for the presence of the troilite may or may not be consistent with an unaltered primitive composition for the metal clasts. High temperature equilibrium condensation of Bencubbin metal, however, is also supported by the low Ga and Ge contents reported by Kallemeynet al. (1978, Geochim. Cosmochim. Acta42, 507–515). Three of the metal clasts were found to contain ?2.3wt% Si in alloy with the metal. The compositions of these clasts are consistent with equilibrium condensation at a pressure of ? 1 atm from a gas of cosmic composition. The Si-rich clasts could also have condensed at lower pressures from a gas with a fractionated $\text{C}\text{O}$ ratio relative to cosmic abundances.     

New data on the kinetics of the dissolution of alkali feldspars at 200°C in CO2 charged water

The kinetics of the dissolution of a (Na. K) sanidine in CO₂-charged water, were studied experimentally at 200°C as a function of the surface area s and of the time t. The molalities of Na⁺ and K⁺ have been plotted against the product st. As for albite and adularia (Lagache, Bull. Soc. Fr. Minéral. Cristallogr. 88, 223–253, 1965), the rates of dissolution of Na² and K⁺ are smooth continuous functions of st, which implies that they are controlled by the composition of the solution.The comparison between the dissolution of pure sodic or potassic feldspars and that of an intermediate feldspar shows that the sanidine dissolves as if it were composed of albite and K-feldspar grains in the proportions corresponding to its composition.Theoretical considerations presented by Helgeson (Geochim. Cosmochim. Acta35, 421–169, 1971; The Feldspars, pp. 184–217, 1972) and Pa?es (Geochim. Cosmochim. Acta37, 2641–2663, 1973) are reviewed: both had suggested, arguing from my first experimental results, that the dissolution could be described by a process of diffusional mass transfer through a surface layer of reaction products.The present experiments do not agree with such an interpretation of the mechanism of dissolution.     

Cooling rate based on schreibersite growth for the Emery mesosiderite

The Emery mesosiderite contains large Ni-rich grains of the phosphide schreibersite, which have exsolved from kamacite. Computer simulation of diffusion-controlled growth of this schreibersite indicates that exsolution occurred during cooling at the rate of 0.1°C/Myr. This determination supports the cooling rate estimated for mesosiderites by Powell (Geochim. Cosmochim. Acta33, 789–810), using taenite-kamacite data.     

Clinopyroxene-matrix partitioning of K,Rb, Cs,Sr and Ba

Extremely pure samples of clinopyroxene phenocrysts from two volcanic rocks have been analyzed for K, Rb, Cs, Sr and Ba. In conjunction with matrix concentrations, partition coefficients are obtained which are in the range 0.001–0.004 for K, Rb, Cs and Ba. These values are lower than those in the literature by factors of 6–100 but are in good agreement with values determined experimentally at pressures of 15–30 kb by Shimizu (Geochim. Cosmochim. Acta38, 1974). Values for partition coefficients measured on separates of impure or cloudy pyroxenes from these same rocks were higher and similar to those in the literature. We suggest this effect is related to ‘trapping,’ during crystal growth, of liquid which is enriched in the larger ions (such as Rb and Cs) due to lack of diffusion equilibrium in the liquid. Partition coefficient values for olivine and plagioclase from one of these same rocks were also determined.     

Pressures of formation of iron meteorites from sphalerite compositions

The FeS content of sphalerite, a minor phase in some meteorites, is strongly dependent on pressure when the sphalerite is in equilibrium with troilite. We have determined FeS contents for sphalerite in Bogou, Gladstone, Sardis and Odessa ; these, together with published data on Odessa and Campo del Cielo, have been used to calculate pressures of formation of meteorites, assuming that FeS-diffusion in sphalerite ceases at 350°C. Calculated pressures range from 0.2 to 3.1 kbar, corresponding to formation at centres of chondritic objects from 140 to 410 km in radius, or metallic objects of from 50 to 200 km radius. Formation at shallower depths would require the objects to have been correspondingly larger.All meteorites in this study are members of Ga-Ge group I. Inverse correlation between Ge content and pressure of formation suggests formation at various depths in a compositionally zoned (fractionated?) object. Comparison between our pressure estimates and radii estimated from cooling rates (Frickeret al., 1970, Geochim. Cosmochim. Acta34, 475–492) suggests that Odessa, Bogou and possibly other Group I meteorites formed in a single object with a radius between 400 and 180 km and an overall composition richer in metal than average chondrites.     

Comment on “Hydrolysis of neptunium(V) at variable temperatures (10-85 °C)” by L. Rao, T.G. Srinivasan, A.Yu. Garnov, P. Zanonato, P. Di Bernardo, and A. Bismondo

In a recent study [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830.] the hydrolysis of Np(V) was investigated at 10-85 °C by absorption spectroscopy, potentiometry, and microcalorimetry along the titration of Np(V) solutions with tetramethylammonium hydroxide up to pH 10. However, there is strong evidence that the precautions to avoid competing reactions with carbonate were not sufficient and that the measured effects are not caused by the formation of Np(V) hydroxide complexes but primarily by the formation of Np(V) carbonate complexes. The reported equilibrium constants, enthalpies, entropies, and heat capacities for the complexes NpO₂OH(aq) and are severely in error and must not be used for the geochemical modeling of neptunium. If the hydrolysis constants reported by Rao et al. [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830] are used to calculate neptunium solubilities in alkaline solutions relevant for nuclear waste repositories, the Np(V) concentrations are overestimated by orders of magnitude.     

Purines and triazines in the Murchison meteorite

Two samples of the Murchison C2 chondrite were examined for organic nitrogen compounds, using mass spectrometry as well as paper and thin-layer chromatography. Under mild extraction conditions (water or formic acid) only aliphatic amines and C₂-C₆ alkylpyridines were seen; the latter may be contaminants. Under drastic extraction conditions (hot, 3–6 M HCl or CF₃COOH), a variety of basic nitrogen compounds appeared, in the following amounts (ppm): adenine (15), guanine (5), melamine (20), cyanuric acid (20–30), guanylurea (30–45), urea (25), etc. Apparently these compounds are present mainly in macromolecular material, and are released only upon acid hydrolysis.These findings support our earlier identifications of these compounds in the Orgueil meteorite. They also suggest that the recent failure by Folsomeet al. (Nature232, 108–109, 1971; Geochim. Cosmochim. Acta37, 455–465, 1973) to find purines or triazines in carbonaceous chondrites was due to inadequate extraction conditions: water and formic acid, rather than HCl. Conversely, we were unable to detect the principal compound class reported by Folsome et al.: 4-hydroxypyrimidines.     

Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

The apparent inconsistency in calcite-water fractionation does occur between the arithmetic combination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2003) An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. Geochim. Cosmochim. Acta67, 387-399] and the experimental determination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2005) Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite and water: a low-temperature experimental study. Am. Mineral90, 1121-1130]. To resolve this issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on oxygen isotope exchange between water and the minerals of interest. The question is whether or not a term of mineral-water interaction should be taken into account when calculating mineral-water 10³ln α factors by an arithmetic combination between theoretical 10³ln β factors for mineral and water, respectively. The hydrothermal experiments of Hu and Clayton [Hu G.-X., and Clayton R.N. (2003) Oxygen isotope salt effects at high pressure and high temperature, and the calibration of oxygen isotope geothermometers. Geochim. Cosmochim. Acta67, 3227-3246] demonstrate the absence of isotope salt effect on the oxygen isotope fractionation between calcite and water, and this abnormal behavior reasonably explains the so-called inconsistency in the calcite-water fractionations of Zhou and Zheng (2003, 2005). We argue that the mineral-water correction is still necessary for calculation of fractionations in mineral-water systems. New experimental data for oxygen isotope fractionations involving dolomite and cerussite are consistent with the calculations of Zheng [Zheng Y.-F. (1999a) Oxygen isotope fractionation in carbonate and sulfate minerals. Geochem. J.33, 109-126], but also shed light on the assumptions used in modifying the increment method. We argue that the modified increment method has developed into a theoretical mean of predictive power for calculation of oxygen isotope fractionation factors for crystalline minerals of geochemical interest.     

The role of reaction affinity and secondary minerals in regulating chemical weathering rates at the Santa Cruz Soil Chronosequence, California

In order to explore the reasons for the apparent discrepancy between laboratory and field weathering rates and to determine the extent to which weathering rates are controlled by the approach to thermodynamic equilibrium, secondary mineral precipitation, and flow rates, a multicomponent reactive transport model (CrunchFlow) was used to interpret soil profile development and mineral precipitation and dissolution rates at the 226 ka Marine Terrace Chronosequence near Santa Cruz, CA. Aqueous compositions, fluid chemistry, transport, and mineral abundances are well characterized [White A. F., Schulz M. S., Vivit D. V., Blum A., Stonestrom D. A. and Anderson S. P. (2008) Chemical weathering of a Marine Terrace Chronosequence, Santa Cruz, California. I: interpreting the long-term controls on chemical weathering based on spatial and temporal element and mineral distributions. Geochim. Cosmochim. Acta72 (1), 36-68] and were used to constrain the reaction rates for the weathering and precipitating minerals in the reactive transport modeling. When primary mineral weathering rates are calculated with either of two experimentally determined rate constants, the nonlinear, parallel rate law formulation of Hellmann and Tisserand [Hellmann R. and Tisserand D. (2006) Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar. Geochim. Cosmochim. Acta70 (2), 364-383] or the aluminum inhibition model proposed by Oelkers et al. [Oelkers E. H., Schott J. and Devidal J. L. (1994) The effect of aluminum, pH, and chemical affinity on the rates of aluminosilicate dissolution reactions. Geochim. Cosmochim. Acta58 (9), 2011-2024], modeling results are consistent with field-scale observations when independently constrained clay precipitation rates are accounted for. Experimental and field rates, therefore, can be reconciled at the Santa Cruz site.Additionally, observed maximum clay abundances in the argillic horizons occur at the depth and time where the reaction fronts of the primary minerals overlap. The modeling indicates that the argillic horizon at Santa Cruz can be explained almost entirely by weathering of primary minerals and in situ clay precipitation accompanied by undersaturation of kaolinite at the top of the profile. The rate constant for kaolinite precipitation was also determined based on model simulations of mineral abundances and dissolved Al, SiO₂(aq) and pH in pore waters. Changes in the rate of kaolinite precipitation or the flow rate do not affect the gradient of the primary mineral weathering profiles, but instead control the rate of propagation of the primary mineral weathering fronts and thus total mass removed from the weathering profile. Our analysis suggests that secondary clay precipitation is as important as aqueous transport in governing the amount of dissolution that occurs within a profile because clay minerals exert a strong control over the reaction affinity of the dissolving primary minerals. The modeling also indicates that the weathering advance rate and the total mass of mineral dissolved is controlled by the thermodynamic saturation of the primary dissolving phases plagioclase and K-feldspar, as is evident from the difference in propagation rates of the reaction fronts for the two minerals despite their very similar kinetic rate laws.     

Silicon isotopic fractionation of CAI-like vacuum evaporation residues

Calcium-, aluminum-rich inclusions (CAIs) are often enriched in the heavy isotopes of magnesium and silicon relative to bulk solar system materials. It is likely that these isotopic enrichments resulted from evaporative mass loss of magnesium and silicon from early solar system condensates while they were molten during one or more high-temperature reheating events. Quantitative interpretation of these enrichments requires laboratory determinations of the evaporation kinetics and associated isotopic fractionation effects for these elements. The experimental data for the kinetics of evaporation of magnesium and silicon and the evaporative isotopic fractionation of magnesium is reasonably complete for Type B CAI liquids (Richter F. M., Davis A. M., Ebel D. S., and Hashimoto A. (2002) Elemental and isotopic fractionation of Type B CAIs: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta66, 521-540; Richter F. M., Janney P. E., Mendybaev R. A., Davis A. M., and Wadhwa M. (2007a) Elemental and isotopic fractionation of Type B CAI-like liquids by evaporation. Geochim. Cosmochim. Acta71, 5544-5564.). However, the isotopic fractionation factor for silicon evaporating from such liquids has not been as extensively studied. Here we report new ion microprobe silicon isotopic measurements of residual glass from partial evaporation of Type B CAI liquids into vacuum. The silicon isotopic fractionation is reported as a kinetic fractionation factor, α_Si, corresponding to the ratio of the silicon isotopic composition of the evaporation flux to that of the residual silicate liquid. For CAI-like melts, we find that α_Si = 0.98985 ± 0.00044 (2σ) for ²⁹Si/²⁸Si with no resolvable variation with temperature over the temperature range of the experiments, 1600-1900 °C. This value is different from what has been reported for evaporation of liquid Mg₂SiO₄ (Davis A. M., Hashimoto A., Clayton R. N., and Mayeda T. K. (1990) Isotope mass fractionation during evaporation of Mg₂SiO₄. Nature347, 655-658.) and of a melt with CI chondritic proportions of the major elements (Wang J., Davis A. M., Clayton R. N., Mayeda T. K., and Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO-MgO-SiO₂-CaO-Al₂O₃-TiO₂-REE melt system. Geochim. Cosmochim. Acta65, 479-494.). There appears to be some compositional control on α_Si, whereas no compositional effects have been reported for α_Mg. We use the values of α_Si and α_Mg, to calculate the chemical compositions of the unevaporated precursors of a number of isotopically fractionated CAIs from CV chondrites whose chemical compositions and magnesium and silicon isotopic compositions have been previously measured.     

Element partitioning during precipitation of aragonite from seawater: A framework for understanding paleoproxies

This study presents the results from precipitation experiments carried out to investigate the partitioning of the alkaline earth cations Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ between abiogenic aragonite and seawater as a function of temperature. Experiments were carried out at 5 to 75 °C, using the protocol of Kinsman and Holland [Kinsman, D.J.J., Holland, H.D., 1969. The coprecipitation of cations with CaCO₃ IV. The coprecipitation of Sr²⁺ with aragonite between 16 and 96 °C. Geochim. Cosmochim. Acta33, 1-17.] The concentrations of Mg Sr and Ba were determined in the fluid from each experiment by inductively coupled plasma-mass spectrometry, and in individual aragonite grains by secondary ion mass spectrometry. The experimentally produced aragonite grains are enriched in trace components (“impurities”) relative to the concentrations expected from crystal-fluid equilibrium, indicating that kinetic processes are controlling element distribution. Our data are not consistent with fractionations produced kinetically in a boundary layer adjacent to the growing crystal because Sr²⁺, a compatible element, is enriched rather than depleted in the aragonite. Element compatibilities, and the systematic change in partitioning with temperature, can be explained by the process of surface entrapment proposed by Watson and Liang [Watson, E.B., Liang, Y., 1995. A simple model for sector zoning in slowly grown crystals: implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks. Am. Mineral.80, 1179-1187] and Watson [Watson, E.B., 1996. Surface enrichment and trace-element uptake during crystal growth. Geochim. Cosmochim. Acta60, 5013-5020; Watson, E.B., 2004. A conceptual model for near-surface kinetic controls on the trace-element and stable isotope composition of abiogenic calcite crystals. Geochim. Cosmochim. Acta68, 1473-1488]. This process is thought to operate in regimes where the competition between crystal growth rate and diffusivity in the near-surface region limits the extent to which the solid can achieve partitioning equilibrium with the fluid. A comparison of the skeletal composition of Diploria labyrinthiformis (brain coral) collected on Bermuda with results from precipitation calculations carried out using our experimentally determined partition coefficients indicate that the fluid from which coral skeleton precipitates has a Sr/Ca ratio comparable to that of seawater, but is depleted in Mg and Ba, and that there are seasonal fluctuations in the mass fraction of aragonite precipitated from the calcifying fluid (“precipitation efficiency”). The combined effects of surface entrapment during aragonite growth and seasonal fluctuations in “precipitation efficiency” likely forms the basis for the temperature information recorded in the aragonite skeletons of Scleractinian corals.     

Chromite alteration processes within Vourinos ophiolite

The renewed interest in chromite ore deposits is directly related to the increase in Cr price ruled by international market trends. Chromite, an accessory mineral in peridotites, is considered to be a petrogenetic indicator because its composition reflects the degree of partial melting that the mantle experienced while producing the chromium spinel-bearing rock (Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993). However, the understanding of chromite alteration and metamorphic modification is still controversial (e.g. Evans and Frost in Geochim Cosmochim Acta 39:959–972, 1975; Burkhard in Geochim Cosmochim Acta 57:1297–1306, 1993; Oze et al. in Am J Sci 304:67–101, 2004). Metamorphic alteration leads to major changes in chromite chemistry and to the growth of secondary phases such as ferritchromite and chlorite. In this study, we investigate the Vourinos complex chromitites (from the mines of Rizo, Aetoraches, Xerolivado and Potamia) with respect to textural and chemical analyses in order to highlight the most important trend of alteration related to chromite transformation. The present study has been partially funded by the Aliakmon project in collaboration between the Public Power Corporation of Greece and Institute of Geology and Mineral Exploration of Kozani.     

Vapor pressures and evaporation coefficients for melts of ferromagnesian chondrule-like compositions

To determine evaporation coefficients for the major gaseous species that evaporate from silicate melts, the Hertz-Knudsen equation was used to model the compositions of residues of chondrule analogs produced by evaporation in vacuum by Hashimoto [Hashimoto A. (1983) Evaporation metamorphism in the early solar nebula-evaporation experiments on the melt FeO-MgO-SiO₂-CaO-Al₂O₃ and chemical fractionations of primitive materials. Geochem. J. 17, 111-145] and Wang et al. [Wang J., Davis A. M., Clayton R. N., Mayeda T. K., Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO-MgO-SiO₂-CaO-Al₂O₃-TiO₂ rare earth element melt system. Geochim. Cosmochim. Acta 65, 479-494], in vacuum and in H₂ by Yu et al. [Yu Y., Hewins R. H., Alexander C. M. O’D., Wang J. (2003) Experimental study of evaporation and isotopic mass fractionation of potassium in silicate melts. Geochim. Cosmochim. Acta 67, 773-786], and in H₂ by Cohen et al. [Cohen B. A., Hewins R. H., Alexander C. M. O’D. (2004) The formation of chondrules by open-system melting of nebular condensates. Geochim. Cosmochim. Acta 68, 1661-1675]. Vapor pressures were calculated using the thermodynamic model of Ghiorso and Sack [Ghiorso M. S., Sack R. O. (1995) Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures. Contrib. Mineral. Petrol. 119, 197-212], except for the late, FeO-free stages of the Wang et al. (2001) and Cohen et al. (2004) experiments, where the CMAS activity model of Berman [Berman R. G. (1983) A thermodynamic model for multicomponent melts, with application to the system CaO-MgO-Al₂O₃-SiO₂. Ph.D. thesis, University of British Columbia] was used. From these vapor pressures, evaporation coefficients (α) were obtained that give the best fits to the time variation of the residue compositions. Evaporation coefficients derived for Fe_(g), Mg_(g), and SiO_(g) from the Hashimoto (1983) experiments are similar to those found by Alexander [Alexander C. M. O’D. (2004) Erratum. Meteoritics Planet. Sci. 39, 163] in his EQR treatment of the same data and also adequately describe the FeO-bearing stages of the Wang et al. (2001) experiments. From the Yu et al. (2003) experiments at 1723 K, α_Na = 0.26 ± 0.05, and α_K = 0.13 ± 0.02 in vacuum, and α_Na = 0.042 ± 0.020, andα_K = 0.017 ± 0.002 in 9 × 10⁻⁵ bar H₂. In the FeO-free stages of the Wang et al. (2001) experiments, α_Mg and α_SiO are significantly different from their respective values in the FeO-bearing portions of the same experiments and from the vacuum values obtained at the same temperature by Richter [Richter F. M., Davis A. M., Ebel D. S., Hashimoto A. (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta 66, 521-540] for CMAS compositions much lower in MgO. When corrected for temperature, the values of α_Mg and α_SiO that best describe the FeO-free stages of the Wang et al. (2001) experiments also adequately describe the FeO-free stage of the Cohen et al. (2004) H₂ experiments, but α_Fe that best describes the FeO-bearing stage of the latter experiment differs significantly from the temperature-corrected value derived from the Hashimoto (1983) vacuum data.     

The role of magnesium in the crystal growth of calcite and aragonite from sea water

The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7–10 mole % MgCO₃ in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability.From the work of Katz (Geochim. Cosmochim. Acta37, 1563–1586, 1973), Plummer and Mackenzie (Amer. J. Sci. 273, 515–522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO₃ in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO₃ are less stable, and those containing less than 8.5 mole% MgCO₃ are more stable than aragonite plus Ca and Mg in sea water.     

An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymium (III) fluoride was investigated at temperatures of 150, 200 and 250 °C, saturated water vapor pressure, and a total fluoride concentration (HF°_aq + F⁻) ranging from 2.0 × 10⁻³ to 0.23 mol/l. The results of the experiments show that Nd³⁺ and NdF²⁺ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF²⁺ and a solubility product for NdF₃. The solubility product of NdF₃(logK_sp=loga_Nd³⁺+3loga_F^-) is −24.4 ± 0.2, −22.8 ± 0.1, and −21.5 ± 0.2 at 250, 200 and 150 °C, respectively, and the formation constant of NdF²⁺(logβ=loga_NdF²⁺-loga_Nd³⁺-loga_F^-) is 6.8 ± 0.1, 6.2 ± 0.1, and 5.5 ± 0.2 at 250, 200 and 150 °C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF²⁺ and the solubility of NdF₃.The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta59, 4329-4350.], which indicate that NdF²⁺ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta66, 4311-4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta70, 983-992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated.     

New high-precision measurements of the isotopic composition of atmospheric argon

Earth’s atmosphere is used as a standard reference gas for mass spectrometric determinations of argon (Ar) isotopes used principally in geochronology. There are three published independent determinations of the Ar isotope composition of modern atmosphere that differ subtly. We have made new high-precision measurements of Ar isotope ratios of five different sources of air using a high-sensitivity multi-collector noble gas mass spectrometer in order to distinguish between them. The isotope ratios, corrected only for backgrounds, reside on a inverse square-root mass law fractionation line that passes through the air value proposed by Lee et al. [Lee J. Y., Marti K., Severinghaus J. P., Kawamura K., Yoo H. S., Lee J. B. and Kim J. S. (2006) A redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta70, 4507-4512]. They are distinct from both the other proposed compositions and provide the first independent confirmation of the atmospheric Ar isotope composition. We suggest that the revised values should now be in routine employment.     

Accretion and differentiation of carbon in the early Earth.

The abundance of C in carbonaceous and ordinary chondrites decreases exponentially with increasing shock pressure as inferred from the petrologic shock classification of Scott et al. [Scott, E.R.D., Keil, K., Stoffler, D., 1992. Shock metamorphism of carbonaceous chondrites. Geochim. Cosmochim. Acta 56, 4281-4293] and Stoffler et al. [Stoffler, D., Keil, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845-3867]. This confirms the experimental results of Tyburczy et al. [Tyburczy, J.A., Frisch, B., Ahrens, T.J., 1986. Shock-induced volatile loss from a carbonaceous chondrite: implications for planetary accretion. Earth Planet. Sci. Lett. 80, 201-207] on shock-induced devolatization of the Murchison meteorite showing that carbonaceous chondrites appear to be completely devolatilized at impact velocities greater than 2 km s-1. Both of these results suggest that C incorporation would have been most efficient in the early stages of accretion, and that the primordial C content of the Earth was between 10(24) and 10(25) g C (1-10% efficiency of incorporation). This estimate agrees well with the value of 3-7 x 10(24) g C based on the atmospheric abundance of 36Ar and the chondritic C/36Ar (Marty and Jambon, 1987). Several observations suggest that C likely was incorporated into the Earth's core during accretion. (1) Graphite and carbides are commonly present in iron meteorites, and those iron meteorites with Widmanstatten patterns reflecting the slowest cooling rates (mostly Group I and IIIb) contain the highest C abundances. The C abundance-cooling rate correlation is consistent with dissolution of C into Fe-Ni liquids that segregated to form the cores of the iron meteorite parent bodies. (2) The carbon isotopic composition of graphite in iron meteorites exhibits a uniform value of -5% [Deines, P., Wickman, F.E. 1973. The isotopic composition of 'graphitic' carbon from iron meteorites and some remarks on the troilitic sulfur of iron meteorites. Geochim. Cosmochim. Acta 37, 1295-1319; Deines, P., Wickman, F.E., 1975. A contribution to the stable carbon isotope geochemistry of iron meteorites. Geochim. Cosmochim. Acta 39, 547-557] identical to the mode in the distribution found in diamonds, carbonatites and oceanic basalts [Mattey, D.P., 1987. Carbon isotopes in the mantle. Terra Cognita 7, 31-37]. (3) The room pressure solubility of C in molten iron is 4.3 wt% C. Phase equilibria confirm that the Fe-C eutectic persists to 12 GPa, and thermochemical calculations for the Fe-C-S system by Wood [Wood, B.J., 1993. Carbon in the core. Earth Planet. Sci. Lett. 117, 593-607] predict that C is soluble in Fe liquids at core pressures. The abundance of 36Ar in chondrites decreases exponentially with increasing shock pressure as observed for C. It is well known that noble gases are positively correlated and physically associated with C in meteorites [e.g. Otting, W., Zahringer J., 1967. Total carbon content and primordial rare gases in chondrites. Geochim. Cosmochim. Acta 31, 1949-1960; Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta, 42, 1775-1797]. This suggests a mechanism by which primordial He and other noble gases may have incorporated into the Earth during accretion. The abundance of He in the primordial Earth required to sustain the modern He flux for 4 Ga (assuming a planetary 3 He/4 He; Reynolds et al. [Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta 42, 1775-1797] is calculated to be > or = 10(-8) cm3 g-1. This minimum estimate is consistent with a 1-10% efficiency of noble gas retention during accretion and the observed abundance of He in carbonaceous chondrites (10(-5) to 10(-4) cm3 g-1 excluding spallogenic contributions).     

An isopiestic study of aqueous NaBr and KBr at 50 °C: Chemical equilibrium model of solution behavior and solubility in the NaBr-H2O, KBr-H2O and Na-K-Br-H2O systems to high concentration and temperature

The isopiestic method has been used to determine the osmotic coefficients of the binary solutions NaBr-H₂O (from 0.745 to 5.953 mol kg⁻¹) and KBr-H₂O (from 0.741 to 5.683 mol kg⁻¹) at the temperature t = 50 °C. Sodium chloride solutions have been used as isopiestic reference standards. The isopiestic results obtained have been combined with all other experimental thermodynamic quantities available in literature (osmotic coefficients, water activities, bromide mineral’s solubilities) to construct a chemical model that calculates solute and solvent activities and solid-liquid equilibria in the NaBr-H₂O, KBr-H₂O and Na-K-Br-H₂O systems from dilute to high solution concentration within the 0-300 °C temperature range. The Harvie and Weare [Harvie C., and Weare J. (1980) The prediction of mineral solubilities in naturalwaters: the Na-K-Mg-Ca-Cl-SO₄-H₂O system from zero to high concentration at 25 °C. Geochim. Cosmochim. Acta44, 981-997] solubility modeling approach, incorporating their implementation of the concentration-dependent specific interaction equations of Pitzer [Pitzer K. (1973) Thermodynamics of electrolytes. I. Theoretical basis and general equations. J. Phys. Chem.77, 268-277] is employed. The model for binary systems is validated by comparing activity coefficient predictions with those given in literature, and not used in the parameterization process. Limitations of the mixed solutions model due to data insufficiencies are discussed. This model expands the variable temperature sodium-potassium model of Greenberg and Moller [Greenberg J., and Moller N. (1989) The prediction of mineral solubilities in natural waters: a chemical equilibrium model for the Na-K-Ca-Cl-SO₄-H₂O system to high concentration from 0 to 250 °C. Geochim. Cosmochim. Acta53, 2503-2518] by evaluating Br⁻ pure electrolyte and mixing solution parameters and the chemical potentials of three bromide solid phases: NaBr-2H₂O (cr), NaBr (cr) and KBr (cr).