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首页> 外文期刊>Contributions to Mineralogy and Petrology >H_2O diffusion in peralkaline to peraluminous rhyolitic melts
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H_2O diffusion in peralkaline to peraluminous rhyolitic melts

机译：H_2O在高碱性中扩散到高铝流纹体熔体中

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The diffusion of water in a peralkaline and a peraluminous rhyolitic melt was investigated at temperatures of 714-1,493 K and pressures of 100 and 500 MPa. At temperatures below 923 K dehydration experiments were performed on glasses containing about 2 wt percent H_2O, in cold seal pressure vessels. At high temperatures diffusion couples of water-poor (<0.5 wt percent H_2O_t) and water-rich (approx 2 wt percent H_2O_t) melts were run in an internally heated gas pressure vessel. Argon was the pressure medium in both cases. Concentration profiles of hydrous species (OH groups and H_2O molecules) were measured along the diffusion direction using near-infrared (NIR) microspectroscopy. The bulk water diffusivity (D_(H_2O_t)) was derived from profiles of total water (C_(H_2O_t)) using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between D_(H_2O_t) and C_(H_2O_t). Both methods consistently indicate that D_(H_2O_t) is proportional to C_(H_2O_t) in this range of water contents for both bulk compositions, in agreement with previous work on metaluminous rhyolite. The water diffusivity in the peraluminous melts agrees very well with data for metaluminous rhyolites implying that an excess of A1_2O_3 with respect to alkalis does not affect water diffusion. On the other hand, water diffusion is faster by roughly a factor of two in the peralkaline melt compared to the metaluminous melt. The following expression for the water diffusivity in the peralkaline rhyolite as a function of temperature and pressure was obtained by least-squares fitting: log D_(H_2O_t)~(1 wt percent) = (-7.09 + - 0.15) - (4,788 + - 166) + (0.56 + - 0.21) X P/T where D_(H_2O_t)~(1 wt percent) is the water diffusivity at 1 wt percent H_2O_t in m~2/s, T is the temperature in K and P is the pressure in MPa. The above equation reproduces the experimental data (14 runs in total) with a standard fit error of 0.15 log units. It can be employed to model degassing of peralkaline melts at water contents up to 2 wt percent.
机译：在714-1493 K的温度和100和500 MPa的压力下研究了水在高碱性和高铝质流纹岩熔体中的扩散。在低于923 K的温度下，在冷密封压力容器中对含约2 wt％H_2O的玻璃进行脱水实验。在高温下，在内部加热的气压容器中进行了贫水（<0.5 wt％H_2O_t）和富水（大约2 wt％H_2O_t）的扩散对。在两种情况下，氩气都是压力介质。使用近红外（NIR）显微技术沿扩散方向测量了含水物（OH基和H_2O分子）的浓度分布。使用改良的Boltzmann-Matano方法以及假设D_（H_2O_t）和C_（H_2O_t）之间的函数关系的拟合，从总水（C_（H_2O_t））的分布图得出总水扩散率（D_（H_2O_t））。两种方法一致地表明，对于两种本体组合物，在此水含量范围内，D_（H_2O_t）与C_（H_2O_t）成比例，这与先前有关金属流纹岩的研究一致。高铝熔体中水的扩散率与金属流纹岩的数据非常吻合，这意味着相对于碱而言过量的Al_2O_3不会影响水的扩散。另一方面，与碱金属熔体相比，在碱金属熔体中水的扩散速度快大约两倍。通过最小二乘法拟合，得出了碱金属流纹岩中水扩散率与温度和压力的关系的以下表达式：log D_（H_2O_t）〜（1 wt％）=（-7.09 +-0.15）-（4,788 +- 166）+（0.56 +-0.21）XP / T其中D_（H_2O_t）〜（1 wt％）是在1 wt％H_2O_t下以m〜2 / s为单位的水扩散率，T是以K为单位的温度，P是压力以MPa为单位。上面的方程式再现了实验数据（总共运行14次），标准拟合误差为0.15 log个单位。它可以用于模拟高碱熔体在水含量高达2 wt％时的脱气。

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