In situ spectroscopic study of the thermodynamic properties of water-bearing magmas

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The goal of this research was to quantify the thermodynamic properties of water dissolved in silicate melts. Water is a major volatile component dissolved in magma at depth, and is capable of nearly complete exsolution from the melt (but being sustained in the melt as vapor bubbles) when the magma rises to near the surface. This causes a progressive increase of the volume per unit mass of the rising magma and a modification of the cooling path. Knowledge of the thermodynamic properties of water in silicate melts is critical for modeling these effects, and thus fundamental for understanding the dynamics of magma migration and eruption. Two major advances in the quantification of the thermodynamic properties of water in silicate melts have been achieved through this research: (1) High temperature infrared absorption peaks due to OH group and H2O molecule were measured in two basalt glasses (one containing 1.3 wt% water, one containing 2.9 wt% water; the anhydrous composition remains unch … More anged) as a function of temperature between 〜 25℃ and 575℃. The concentrations of OH group and H2O molecule were determined using the room-temperature calibrations of the molar absorption coefficients. The results show that both OH group and H2O molecule are equilibrium species in the melts, which is consistent with the idea that water is incorporated in a silicate melt through homogeneous reaction H2O molecule + O = 2OH. The product is favored by increase of temperature above the glass transition, and the homogeneous reaction equilibria in these two melts show a good agreement with each other; an equation 1n K = -4070(±470)/T + 3.68(±0.60) reproduces both. The 1n K values obtained in the melts are not distinguishable from those obtained by previous studies in rhyolitic melts within experimental uncertainty. This suggests that there is little influence of composition on the calorimetric properties of the homogeneous reaction equilibria in natural magmatic melts. (2) Heat of solution of water in rhyolite melts was thermodynamically modeled, on the basis of the temperature dependence of water solubility in the melts obtained from infrared spectroscopy of the quenched glasses. In the model developed here, water was assumed to be dissoleved through a coupled reaction H2O molecule melt + O melt = 2OH melt (homogeneous reaction) and H2O vapor = H2O molecule melt (heterogeneous reaction), and ideal mixing of these melt components was assumed. A non-linear multiple regression to the solubility dataset converged upon ΔH° homo (P, T) = 25.8±11.8 kJ, ΔS° homo (P, T) = 6.0±8.7 J/K, and, ΔH° hetero (1bar, T) = -25.3±4.8 kJ/mol. From these optimum values, the following interpretations can be made: The heat of solution is approximately zero at 0.1 MPa, and increases with pressure below 〜 20 MPa, where the absolute value of the heat is negative. At pressures between 〜 20 MPa and 100 MPa, the heat of solution roughly levels out (〜 -7±6 kJ/mol at 850℃) as a result of the balance of the exothermic heterogeneous reaction and endothermic homogeneous reaction. Although error of estimation is large, the present results suggest that at pressures of 10 to 100 MPa (the pressure range important to closed system magma bubbling), the calorimetric effect of bubbling causes a temperature drop of only <10℃. Less