An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part I: Calibration and Solubility Models |
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Authors: | DIXON JACQUELINE EABY; STOLPER EDWARD M; HOLLOWAY JOHN R |
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Institution: | 1 DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CA 91125, USA
2 DEPARTMENTS OF CHEMISTRY AND GEOLOGY, ARIZONA STATE UNIVERSITY TEMPE, AZ 85287, USA |
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Abstract: | Experiments were conducted to determine the solubilities ofH2O and CO2 and the nature of their mixing behavior in basalticliquid at pressures and temperature relevant to seqfloor eruption.Mid-ocean ridge basaltic (MORB) liquid was equilibrated at 1200°Cwith pure H2O at pressures of 176717 bar and H2OCO2vapor at pressures up to 980 bar. Concentrations and speciationof H2O and CO2 dissolved in the quenched glasses were measuredusing IR spectroscopy. Molar absorptivities for the 4500 cm1band of hydroxyl groups and the 5200 and 1630 cm1 bandsof molecular water are 067±003, 062±007,and 25±3 l/mol-cm, respectively. These and previouslydetermined molar absorptivities for a range of silicate meltcompositions correlate positively and linearly with the concentrationof tetrahedral cations (Si+Al). The speciation of water in glass quenched from vapor-saturatedbasaltic melt is similar to that determined by Silver &Stolper (Journal of Petrology 30, 667709, 1989) in albiticglass and can be fitted by their regular ternary solution modelusing the coefficients for albitic glasses. Concentrations ofmolecular water measured in the quenched basaltic glasses areproportional to f H2O in all samples regardless of the compositionof the vapor, demonstrating that the activity of molecular waterin basaltic melts follows Henry's law at these pressures. Abest fit to our data and existing higher-pressure water solubilitydata (Khitarov et al., Geochemistry 5, 479492, 1959;Hamilton et al., Journal of Petrology 5, 2139, 1964),assuming Henrian behavior for molecular water and that the dependenceof molecular water content on total water content can be describedby the regular solution model, gives estimates for the Vo, mH2Oof 12±1 cm3/mol and for the 1-bar water solubility of011 wt%. Concentrations of CO2 dissolved as carbonate in the melt forpure CO2-saturated and mixed H2O-CO2-saturated experiments area simple function of fCO2 These results suggest Henrian behaviorfor the activity of carbonate in basaltic melt and do not supportthe widely held view that water significantly enhances the solutionof carbon dioxide in basaltic melts. Using a Vo, mr of 23 cm3/mol(Pan et al., Geochimica et Cosmochimica Acta 55, 15871595,1991), the solubility of carbonate in the melt at 1 bar and1200°C is 05 p.p.m. Our revised determination of CO2solubility is 20% higher than that reported by Stolper &Holloway (Earth and Planetary Science Letters 87, 397408,1988). KEY WORDS: mid-ocean ridge basalts; water and carbon dioxide solubility; experimental petrology |
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