An experimental study of the interaction of water with granitic melt

Citation

Ihinger, Phillip Dean (1991) An experimental study of the interaction of water with granitic melt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1ds0-5n95. https://resolver.caltech.edu/CaltechTHESIS:03012011-153233991

Abstract

The nature of water in granitic melt is investigated through a variety of experimental and analytical techniques. The knowledge of the presence of two dissolved melt species (hydroxyl and molecular water groups), and the precise determination of their concentrations with infrared spectroscopy, is used in conjunction with vacuum extraction and hydrogen isotopic techniques to determine, (1) the solubility of water in granitic melts as a function of pressure at 850°C, (2) the speciation of water in granitic melt as a function of temperature, and (3) the fractionation factors which govern the partitioning of hydrogen isotopes between water vapor and granitic melt. Natural obsidian starting materials were held at 850°C and pressures ranging from 200 to 1600 bars in the presence of excess vapor. Samples were rapidly quenched to glasses and their water contents were determined using vacuum manometry and infrared spectroscopy. The results of these experiments demonstrate a progressive increase in solubility with pressure, but suggest that increasing dissolution of water has no influence on the total volume of the melt (the partial molar volume of water in these low pressure melts is near zero). The solubility measurements can be used to model shallow level eruptive cycles which involve crystal fractionation, volatile saturation, onset of eruption, and influx of new magma to repeat the cycle. Preliminary experiments on the temperature dependence of water solubility suggest a dependence as high as 1 wt % per 200°C at 1600 bars. This result supports the suggestion that mafic intrusion into silicic magma chambers induces supersaturation of the felsic liquid and subsequent explosive eruption. Hydrated obsidian chips were held at temperatures ranging from 450 to 600°C to determine the equilibrium speciation of water in the melt. These data show distinct nonideal behavior, In K_(eq) decreases with increasing water content (K_(eq) = the equilibrium constant for the reaction involving dissolved water species). Relaxation analyses are used to demonstrate that these rapidly-quenched glasses preserve their high-temperature speciation. This analysis has been used to demonstrate that melts held at 850°C do not preserve their equilibrium speciation on quench. Intersection of the equilibrium isotherms with the fictive temperature curve recorded by the 850°C samples provides identification of the last-equilibrated temperature for the quenched glasses. This information, coupled with the speciation recorded in glasses quenched at a slower rate, is used to extract the viscosity of hydrous rhyolitic liquids as a function of temperature. This formulation is compared to empirical methods for determining melt viscosities. The equilibrium speciation data is used to formulate a regular solution model to determine the speciation of water in rhyolitic melts at any temperature and water content. The hydrogen isotopic composition of water evolved from quenched granitic melts is compared to the isotopic composition of their equilibrium vapors. The bulk partitioning of hydrogen isotopes between melt and vapor varies as a regular function of the total dissolved water content, suggesting that two independent fractionation factors control their equilibrium. The fractionation factor between vapor and hydroxyl groups (≈1.040 ± 0.05 ‰) is significantly greater than the fractionation between vapor and the molecular water species (≈1.00 ± 0.10 ‰). These fractionation factors can be used to explain the degassing trends measured in natural igneous environments. The magma chamber beneath Mono Craters, CA may have experienced a significant amount of closed system degassing before undergoing open system degassing late in its history.

Thesis Files