Pressure and temperature dependence of H solubility in forsterite: An implication to water activity in the Earth interior
Pressure and temperature dependence of H solubility in forsterite: An implication to water activity in the Earth interior
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DOI：
10.1016/j.epsl.2008.01.035
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发表时间：
2008-04
影响因子：
5.3
通讯作者：
E. Bali;N. Bolfan-Casanova;K. Koga
中科院分区：
文献类型：
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作者：
E. Bali;N. Bolfan-Casanova;K. Koga
在MgO-SiO2-H2O系统中，在顽火辉石饱和的条件下，利用多砧装置在2.5、6和9 GPa，1000 ~ 1400 °C的温度范围内进行了实验，并确定了OH在镁橄榄石中的溶解度随压力和温度的变化关系。用偏振傅里叶变换红外光谱法测定了镁橄榄石中OH的含量。结果表明，OH在镁橄榄石中的溶解度仅在2.5GPa时随温度的升高而增大。在6和9 GPa下，OH溶解度在1175至1250 °C的温度下达到最大值，这取决于压力，然后在更高的温度下降低。这种行为是由硅酸盐组分溶解引起的流体中水活度的变化来解释的。利用在亚固相线样品上测量的OH浓度，我们确定了镁橄榄石中OH掺入的热力学参数，如内能和熵的变化。我们发现ΔE为37.1±6.7 kJ/mol，ΔS为82.8±6.8 J/mol/K。这些参数与10.6 cm 3/mol的Δ Vsolids一起用于计算真实的流体（即水+硅酸盐）的逸度，并将其与纯水的逸度进行比较。我们的研究结果意味着，与假设水是纯流体的模型相比，对地幔深度大于80 km且高于~1250 °C的水储存能力的估计应大幅降低。
We performed experiments at 2.5, 6 and 9 GPa, and temperatures ranging from 1000 to 1400 °C in enstatite saturated conditions in the MgO–SiO2–H2O system using a multi-anvil apparatus, and determined the dependence of OH solubility in forsterite as a function of pressure and temperature. The abundance of OH in forsterite was determined using polarized Fourier transform infrared spectroscopy. The results show that OH solubility in forsterite increases with temperature only at 2.5 GPa. At 6 and 9 GPa, the OH solubility reaches a maximum at temperatures of 1175 to 1250 °C, depending on pressure, and then decreases at higher temperatures. Such behaviour is explained by the change of water activity in the fluid due to dissolution of silicate component. Using OH concentrations measured on subsolidus samples we determined the thermodynamic parameters of OH incorporation in forsterite, such as change in internal energy and entropy. We find almost constant ΔE of 37.1±6.7 kJ/mol and ΔS of 82.8±6.8 J/mol/K. These parameters together with a ΔVsolidsof 10.6 cm3/mol are used to calculate the fugacity of the real fluid (i.e. water+silicates) and compare it to the fugacity of pure water. Our results imply that estimates of water storage capacity at mantle depths greater than 80 km and above ~1250 °C should be substantially reduced compared to models that assume that water is a pure fluid.