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

含水岩浆热力学性质的原位光谱研究

• 批准号: 11640480
• 负责人: YAMASHITA Shigeru
• 金额: $ 2.24万
• 依托单位: Okayama University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 1999
• 资助国家: 日本
• 起止时间: 1999 至 2000
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11640480/
• 关键词: magma; water; silicate melt; thermodynamics; infrared spectroscopy; 高温高圧実験; 反応エネルギー; 含水マグマ; 相平衡; 溶解反応エネルギー

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

这项研究的目的是量化水在硅酸盐熔体中的热力学性质。水是溶解在岩浆深处的主要挥发性成分，当岩浆上升到接近地表时，水能够几乎完全从熔体中逸出(但以蒸汽泡的形式存在于熔体中)。这会导致上升岩浆的单位质量体积逐渐增加，冷却路径也会发生改变。了解硅酸盐熔体中水的热力学性质对于模拟这些效应至关重要，因此也是理解岩浆迁移和喷发动力学的基础。通过本研究，在定量研究硅酸盐熔体中水的热力学性质方面取得了两大进展：(1)测定了两种玄武岩玻璃(一种含1.3wt%水，一种含2.9wt%水；无水组分仍为…)中由于羟基和H2O分子引起的高温红外吸收峰角度更大)作为~25℃到575℃之间的温度的函数。用摩尔吸收系数的室温校正方法测定了羟基和H2O分子的浓度。结果表明，熔体中的OH基团和H2O分子均为平衡物种，这与水通过H2O分子+O=2OH均相反应进入硅酸盐熔体的观点是一致的。玻璃化转变以上温度的升高有利于产物的生成，两种熔体的均相反应平衡符合方程1nK=-4070(±470)/T+3.68(±0.60)。在实验不确定度范围内，在熔体中获得的1n K值与以前在流纹岩熔体中所获得的值是无法区分的。这表明组成对天然岩浆熔体中均相反应平衡的量热性质影响不大。(2)根据淬火玻璃红外光谱得到的流纹岩熔体中水的溶解度随温度的变化规律，对流纹岩熔体中水的溶解热进行了热力学模拟。在该模型中，假设水通过耦合反应H2O分子熔体+O熔体=2OH熔体(均相反应)和H2O蒸气=H2O分子熔体(异相反应)溶解，并假设这些熔体组分的理想混合。对溶解度数据的非线性多元回归得到ΔH°Homo(P，T)=25.8±11.8kJ，ΔS°Homo(P，T)=6.0±8.7J/K，ΔH°Homo(1bar，T)=-25.3±4.8kJ/Mol。根据这些最佳值，可以作出如下解释：在0.1 Mpa时，溶解热近似为零，当压力低于~20 Mpa时，溶解热随压力的增加而增加，其中绝对值为负。在~2 0~10 0 Mpa压力范围内，由于放热多相反应和吸热均相反应的平衡，溶液热基本趋于平稳(85 0℃时为-7±6kJ/m ol)。尽管估算误差很大，但目前的结果表明，在10~100℃的压力范围内(这是封闭系统岩浆鼓泡的重要压力范围)，鼓泡的量热效应只会导致&lt；10 Mpa的温降。较少

项目成果

Yamashita, S: "Heat of solution of water in rhyolite melt : constraints from water solubility measurements"EOS. 81. S433-S433 (2000)

Yamashita，S：“流纹岩熔体中水的溶解热：水溶性测量的限制”EOS。

Yamashita, S.: "Heat of solution of water in rhyolite melt: constraints from water solubility measurements."EOS. 81. S433 (2000)

Yamashita, S.：“流纹岩熔体中水的溶解热：水溶性测量的限制。”EOS。

山下 茂: "珪酸塩ガラス中の水の顕微赤外分光分析〜マグマ研究への応用"Jasco Report. 41・2. 15-18 (1999)

Shigeru Yamashita：“硅酸盐玻璃中水的微红外光谱分析 - 在岩浆研究中的应用”Jasco Report 41・2（1999）。

Yamashita, S: "Experiment study of the effect of temperature on water solubility in natural rhyolite melt to 100 MPa"Journal of Petrology. 40. 1497-1507 (1999)

Yamashita，S：“温度对 100 MPa 天然流纹岩熔体中水溶性影响的实验研究”岩石学杂志。

Yamashita, S.: "Micro-IR spectroscopy of water-bearing silicate glasses: an application to experimental petrology."Jasco Report. 41(in Japanese). 15-18 (1999)

Yamashita, S.：“含水硅酸盐玻璃的微红外光谱：在实验岩石学中的应用。”Jasco 报告。