Pressure Dependence of Proton Incorporation and Water Solubility in Olivine
Pressure Dependence of Proton Incorporation and Water Solubility in Olivine
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DOI：
10.1029/2019jb018813
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发表时间：
2020-01
期刊：
影响因子：
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通讯作者：
H. Fei;T. Katsura
中科院分区：
文献类型：
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作者：
H. Fei;T. Katsura
在不同的SiO_2活度条件下，用多面顶压机在8 Gpa和1250℃下合成了水合镁橄榄石单晶。电子探针和透射式傅里叶变换红外光谱分析表明，随着水分含量的增加，镁硅原子比增大。这为质子主要储存在水合镁橄榄岩中的硅位提供了新的证据。当≤超过2.5GPa时，镁位置对质子结合的贡献可能是显著的，但在较高的压力下可以忽略不计。因此，软流圈条件下橄榄石中的质子主要保持在硅位。实验研究表明，镁橄榄石/橄榄石在过量(Mg，Fe)O平衡时的水溶解度明显高于过量SiO_2时的溶解度，但这一差异随着压力的增加而减小，并在12-14 Gpa时趋于一致。由于橄榄石在软流圈顶部条件下的低溶解度，地球物理观测到的低粘度和低地震速度带不是橄榄石水化作用造成的，高电导率异常不应归因于橄榄石中自由H+控制的质子传导。
Hydrous forsterite single crystals were synthesized at 8 GPa and 1250 °C using a multianvil apparatus under various SiO2 activity conditions. Electron microprobe and transmission Fourier‐transform infrared spectroscopy analyses show that the Mg/Si atomic ratio increases with increasing water content. This provides new evidence that protons are mainly stored in Si sites in hydrous forsterite. The contribution of Mg sites for proton incorporation can be significant at ≤2.5 GPa with excess SiO2 but is negligible at higher pressures. Protons in olivine under asthenospheric conditions are therefore mostly maintained in Si sites. A summary of experimental studies suggests that the water solubility of forsterite/olivine equilibrated with excess (Mg,Fe)O is substantially higher than that with excess SiO2; however, this difference decreases with increasing pressure and becomes identical at 12–14 GPa. Owing to the low water solubility of olivine under topmost asthenospheric conditions, the geophysically observed low viscosity and low seismic velocity zones cannot be caused by olivine hydration, and the high conductivity anomaly should not be attributed to free H+‐controlled proton conduction in olivine.