月幔条件下水在橄榄石中扩散的实验研究

Water Diffusion in Olivine under Lunar Mantle Conditions

利用高温高压实验技术,对月幔条件下水在橄榄石中的扩散行为开展实验模拟研究,考察氧逸度、压力和温度对水沿橄榄石晶体不同晶轴扩散速率的影响。实验结果表明:在高氧逸度条件下水在橄榄石中的扩散速率比低氧逸度条件下更高;扩散速率与温度正相关,与压力负相关;水沿橄榄石[100]轴的扩散速率较高,沿[001]轴的扩散速率较低,且随着压力的升高,扩散的各向异性减弱。月幔条件下,即使未完全饱和时橄榄石中的羟基含量仍超过10-4,因此橄榄石可成为月球深部水的重要储库。通过对比岩浆上升及喷发速率与水在橄榄石熔体包裹体中的扩散速率可知,熔体包裹体在岩浆上升过程中不会出现水的丢失,而在岩浆喷发过程中极有可能由于扩散作用而丢失大量的水。因此,前人根据橄榄石熔体包裹体所推测的月幔水含量有可能仅是下限值。研究工作为准确推演月球演化历史提供基础科学依据。

High-pressure water diffusion experiments in olivine crystal were conducted in a piston-cylinder press in the present work to investigate systematically the diffusion coefficients of water in view of changes of pressure,temperature and oxygen fugacity. It was found that diffusion coefficients increase with elevated temperatures and decreased pressures,and become relatively larger at high oxygen fugacity. The rate of the diffusion along [100]axis is faster than that along [001]axis and the anisotropy becomes weaker with the increase of the pressure. The measured hydroxyl concentrations in the olivine under lunar mantle conditions are higher than 10-4,thus indicating that the olivine could be a major water reservoir in the deep lunar mantle. By comparing the diffusion rate of the water in the olivine melt inclusions with the magma ascent and the eruption rates,we found that the water in the melt inclusions in the olivine xenocrysts will be well maintained during the magma ascent,whereas water will diffuse out of the xenocrysts during the magma eruption process. The estimated water concentration in the lunar mantle based on the melt inclusion data could be the lower limit. Our work provides significant thermodynamic parameters for exploring the moon evolution history.