Wang Duojun, Mookherjee Mainak, Xu Yousheng, Karato Shun-ichiro
Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China.
Nature. 2006 Oct 26;443(7114):977-80. doi: 10.1038/nature05256.
It is well known that water (as a source of hydrogen) affects the physical and chemical properties of minerals--for example, plastic deformation and melting temperature--and accordingly plays an important role in the dynamics and geochemical evolution of the Earth. Estimating the water content of the Earth's mantle by direct sampling provides only a limited data set from shallow regions (<200 km depth). Geophysical observations such as electrical conductivity are considered to be sensitive to water content, but there has been no experimental study to determine the effect of water on the electrical conductivity of olivine, the most abundant mineral in the Earth's mantle. Here we report a laboratory study of the dependence of the electrical conductivity of olivine aggregates on water content at high temperature and pressure. The electrical conductivity of synthetic polycrystalline olivine was determined from a.c. impedance measurements at a pressure of 4 GPa for a temperature range of 873-1,273 K for water contents of 0.01-0.08 wt%. The results show that the electrical conductivity is strongly dependent on water content but depends only modestly on temperature. The water content dependence of conductivity is best explained by a model in which electrical conduction is due to the motion of free protons. A comparison of the laboratory data with geophysical observations suggests that the typical oceanic asthenosphere contains approximately 10(-2) wt% water, whereas the water content in the continental upper mantle is less than approximately 10(-3) wt%.
众所周知，水（作为氢的来源）会影响矿物的物理和化学性质——例如，塑性变形和熔点——因此在地球动力学和地球化学演化中起着重要作用。通过直接采样来估算地球地幔的含水量，只能提供来自浅层区域（深度小于200千米）的有限数据集。诸如电导率之类的地球物理观测被认为对含水量敏感，但尚未有实验研究来确定水对橄榄石（地球地幔中最丰富的矿物）电导率的影响。在此，我们报告一项关于橄榄石集合体在高温高压下电导率对含水量依赖性的实验室研究。通过在4吉帕斯卡压力下、873至1273开尔文温度范围内、含水量为0.01至0.08重量百分比的条件下进行交流阻抗测量，确定了合成多晶橄榄石的电导率。结果表明，电导率强烈依赖于含水量，但仅适度依赖于温度。电导率对含水量的依赖性，用一个以自由质子运动为导电机制的模型来解释最为合适。将实验室数据与地球物理观测结果进行比较表明，典型的大洋软流圈含水量约为10⁻²重量百分比，而大陆上地幔的含水量则小于约10⁻³重量百分比。
