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

本研究的目的是量化的水溶解在硅酸盐熔体的热力学性质。水是溶解在岩浆深处的主要挥发性成分，并且当岩浆上升到接近表面时，能够从熔体中几乎完全出溶（但在熔体中以蒸汽泡的形式存在）。这将导致体积的每单位质量的上升岩浆和修改的冷却pathway. Knowledge的水在硅酸盐熔体的热力学性质的逐步增加是至关重要的模拟这些影响，从而了解岩浆迁移和喷发的动力学基础。通过本研究，在硅酸盐熔体中水的热力学性质定量化方面取得了两个主要进展：（1）在两种玄武岩玻璃（一种含水1.3wt%，一种含水2.9wt%;无水组成保持在1.3wt%，另一种含水2.9wt%;无水组成保持在1.3wt%，一种含水2.9wt%，一种含水 ...更多信息 在125 ℃和575℃之间变化）。OH基团和H2O分子的浓度使用摩尔吸光系数的室温校准来确定。结果表明，在熔体中，OH基团和H2O分子均为平衡态，这与水通过H2O分子+ O = 2 OH的均相反应进入硅酸盐熔体的观点相一致。温度升高到玻璃化转变温度以上有利于产物的生成，两种熔体的均相反应平衡表现出良好的一致性，方程1 n K = -4070（±470）/T + 3.68（±0.60）再现了两者。在熔体中获得的1 n K值是无法区分的流纹质熔体中的实验不确定性内通过以前的研究所获得的。这表明，在天然岩浆熔体中，成分对均相反应平衡的量热性质影响不大。(2)根据淬火玻璃的红外光谱得到的流纹岩熔体中水溶解度的温度依赖性，建立了流纹岩熔体中水溶解热的热力学模型。在这里开发的模型中，水被假定为溶解通过耦合反应H2O分子熔体+ O熔体= 2 OH熔体（均相反应）和H2O蒸汽= H2O分子熔体（非均相反应），并假设这些熔体组分的理想混合。溶解度数据集的非线性多元回归收敛于ΔH° homo（P，T）= 25.8±11.8 kJ，ΔS° homo（P，T）= 6.0±8.7 J/K，ΔH° hetero（1bar，T）= -25.3 ~4.8 kJ/mol。根据这些最佳值，可以做出以下解释：溶解热在0.1 MPa时近似为零，并且随着压力的增加而增加，低于200 MPa时，热量的绝对值为负值。在压力为100 ~ 200 MPa范围内，由于多相反应放热与均相反应吸热平衡，溶解热大致趋于稳定（850℃时为100 ~ 706 kJ/mol）。虽然估计误差很大，但目前的结果表明，在10至100 MPa的压力范围内（压力范围对封闭系统岩浆起泡很重要），起泡的量热效应引起的温度下降仅<10℃。少

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