Hydrogen grain boundary diffusion in olivine and its contribution to conductivity anomaly

在1300 ℃下，以铁-浮氏体（IW）缓冲和Ni-NiO（NNO）缓冲为反应介质，用气体混合炉制备了橄榄石（MgO、SiO2和Fe 2 O3的混合氧化物）和5wt% Pt粉的氧传感器。采用扩散偶法研究了橄榄石的氧化还原动力学。在1GPa、1100-1300 oC条件下，利用活塞-圆筒装置进行了不同时间的扩散实验。回收后，按照Faul等人（2019）描述的方法计算氧逸度曲线。扩散系数采用Boltzmann-Matano方法计算。用FTIR测定了扩散后的水含量，除一对外，其余对的水含量均< 10 wt. %。ppm.橄榄石的氧化还原过程可以看作是一个扩散控制的过程。氧化橄榄石中的扩散系数具有明显的氧逸度依赖性，而还原橄榄石中的扩散系数具有较小的氧逸度依赖性。计算出的活化焓为235 kJ/mol，这是可比的Fe-Mg在橄榄石中的扩散。但是，幅度要高出2-4个数量级。由于激活焓和幅度是可比的与镁空位有关的含水缺陷的扩散，它是可能的，即使是极少量的水可以占主导地位的橄榄石氧化还原动力学。计算的扩散长度表明，板片对还原地幔的影响是非常有限的。因此，氧化板片在保持干燥的条件下，可能在进一步俯冲的过程中，给深部地幔带来氧化还原平衡。

Olivine (MgO, SiO2 and Fe2O3 oxide mixture) with 5 wt% Pt powder used as oxygen sensor was synthesized using gas mixture furnace at iron-wustite (IW) buffer and Ni-NiO (NNO) buffered condition at 1300 oC. Diffusion couple method is adopted to investigate the redox kinetics of olivine. Diffusion experiments were conducted for different durations at 1 GPa, 1100-1300 oC using piston cylinder apparatus. After recovery, oxygen fugacity profiles were calculated following the method described by Faul et al. (2019). Diffusion coefficients were obtained using Boltzmann-Matano method. Water content after diffusion was measured using FTIR.Except one couple, the water content of all the other couples shows < 10 wt. ppm. Redox process in olivine could be viewed as a diffusion-controlled process. Diffusion coefficient in oxidized olivine shows obvious oxygen fugacity dependence, while that in reduced one shows small dependence. The activation enthalpy is calculated to be 235 kJ/mol, which is comparable with that for Fe-Mg diffusion in olivine. However, the magnitude is 2-4 orders higher. Since the activation enthalpy and magnitude are comparable with that of diffusion of hydrous defects related to Mg-vacancies, it is possible that even extremely small amount of water could dominate redox kinetics in olivine. The diffusion length calculated indicate that the influence exerted by slab on the reduced mantle is very limited. Therefore, it is possible that once subducted more, the oxidized slab can bring redox budget to deep mantle as it keeps dry condition.