Determining the water dissociation constant from the relationship between water solubility in ringwoodite and oxygen fugacity

最近在钻石中发现的环木石包裹体含有超过1.4%的水，这表明地幔过渡带是含水的，至少在局部是这样。一个悬而未决的问题是，过渡地带是全球富水还是局部富水。地幔过渡带的氧逸度被认为非常低：正好低于铁-维氏体缓冲层(IW)。热力学研究表明，由于水被分解为氢和氧，流体中水的比例随着氧逸度的降低而减少。流体中水含量的减少应该会降低环木石中的最大水含量，因此富水和高度还原的条件可能是不相容的。如果过渡带的大部分处于减少的条件下，过渡带就不可能是富水的，这使得富水条件成为当地的特征。为了验证这一假设，我们将在过渡带条件下测定环木中最大含水率作为氧逸度的函数。我们将在以下三个体系中测量环木中的最大含水率：1)Mg2SiO4环木+H2O-H2流体，2)Mg2SiO4环木+钻石+H2O-CH4流体，以及3)天然橄榄石+钻石+H2O-CH4环木。水化实验将在19.5 Gpa的压力和1400K的温度下使用大容量多砧压机进行。将采用改进的双胶囊技术来平衡样品、氧气缓冲液和流体：将样品和水装载到内囊中，在内外胶囊之间装载缓冲液和水。第一个系统将使用11种氧缓冲液，分别为Pt-PtO_2、Fe_3O_4-Fe_2O_3、Ni-NiO、Re-ReO_2、FeO-Fe_3O_4、Mg_2SiO_4-MgSiO_3-MgCO_3-C、Mo-MoO_2、Fe-Fe(X)O、Cr-Cr2O_3、Si-SiO_2和Zr-ZrO_2。第二个系统将使用六种缓冲液：镁硅酸镁-镁硅酸-镁碳酸盐，钼-钼，铁-铁(X)O，铬-铬，硅-二氧化硅和锆-氧化锆。第三个系统将使用Fe-FexO、Mo-MoO2、Re-ReO2、Ni-NiO2和FexO-Fe3O4五种缓冲液。根据水含量与氧逸度的关系，研究过渡区的水含量。

The recently discovered ringwoodite inclusion in a diamond was found to contain more than 1.4% water, which suggests that the mantle transition zone is hydrous, at least locally. An open question is whether the transition zone is water-rich globally or locally. Oxygen fugacity in the mantle transition zone is considered to be very low: just below the iron-wustite (IW) buffer. Thermodynamic investigation suggests that the fraction of water in fluids decreases with decreasing oxygen fugacity due to the dissociation of water into hydrogen and oxygen. This decrease in the water fraction in fluids should decrease the maximum water content in ringwoodite, and therefore the water-rich and highly reduced conditions could be incompatible. If the majority of the transition zone were under reduced conditions, the transition zone could not be water-rich, making the water-rich condition a local feature. In order to prove this hypothesis, we will determine the maximum water content in ringwoodite as a function of oxygen fugacity under transition-zone conditions.We will measure the maximum water content in ringwoodite in the following three systems: 1) Mg2SiO4 ringwoodite + H2O-H2 fluid, 2) Mg2SiO4 ringwoodite + diamond + H2O-CH4 fluid, and 3) ringwoodite with natural olivine composition + diamond + H2O-CH4. Hydration experiments will be conducted at a pressure of 19.5 GPa and a temperature of 1400 K using a large-volume multi-anvil press. A modified double-capsule technique will be adopted to equilibrate the sample, oxygen buffer, and fluid: the sample and water will be loaded in the inner capsule, and a buffer and water will be loaded between the inner and outer capsules. Eleven oxygen buffers of Pt-PtO2, Fe3O4-Fe2O3, Ni-NiO, Re-ReO2, FeO-Fe3O4, Mg2SiO4-MgSiO3-MgCO3-C, Mo-MoO2, Fe-Fe(x)O, Cr-Cr2O3, Si-SiO2, and Zr-ZrO2 will be used for the first system. Six buffers of Mg2SiO4-MgSiO3-MgCO3-C, Mo-MoO2, Fe-Fe(x)O, Cr-Cr2O3, Si-SiO2, and Zr-ZrO2 will be used for the second system. Five buffers of Fe-FexO, Mo-MoO2, Re-ReO2, Ni-NiO2, and FexO-Fe3O4 will be used for the third system. The water content in the transition zone will be investigated based on the relationship between water content and oxygen fugacity.