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

最近在一颗钻石中发现的林伍德石内含物中含有超过1.4%的水，这表明地幔过渡带至少在局部是含水的。一个悬而未决的问题是，过渡区是全球还是局部水资源丰富。地幔过渡带的氧逸度被认为是非常低的：就在铁-浮氏体（IW）缓冲层之下。热力学研究表明，流体中水的分数随着氧逸度的降低而降低，这是由于水分解成氢和氧。流体中水部分的这种降低应降低林伍德石中的最大水含量，因此富水和高度还原的条件可能是不相容的。如果过渡区的大部分处于还原条件下，则过渡区不可能是富水的，从而使富水条件成为局部特征。为了证明这一假设，我们将确定过渡带条件下林伍德石中的最大含水量作为氧逸度的函数。我们将在以下三个系统中测量林伍德石中的最大含水量：1）Mg_2SiO_4林伍德石+H_2 O-H_2流体，2）Mg_2SiO_4林伍德石+金刚石+H_2 O-CH_4流体，（3）天然橄榄石成分+金刚石+H_2 O-CH_4的林伍德石。水合实验将在19.5 GPa的压力和1400 K的温度下使用大体积多砧压机进行。将采用改良的双胶囊技术来平衡样品、氧气缓冲液和液体：样品和水将装载在内胶囊中，缓冲液和水将装载在内胶囊和外胶囊之间。第一个系统将使用Pt-PtO 2、Fe 3 O 4-Fe 2 O3、Ni-NiO、Re-ReO 2、FeO-Fe 3 O 4、Mg 2SiO 4-MgSiO 3-MgCO 3-C、Mo-MoO 2、Fe-Fe（x）O、Cr-Cr2 O3、Si-SiO2和Zr-ZrO 2十一种氧缓冲剂。第二个系统将使用Mg 2SiO 4-MgSiO 3-MgCO 3-C、Mo-MoO 2、Fe-Fe（x）O、Cr-Cr2 O3、Si-SiO2和Zr-ZrO 2六种缓冲剂。第三个系统将使用Fe-FexO、Mo-MoO 2、Re-ReO 2、Ni-NiO 2和FexO-Fe 3 O 4五种缓冲液。根据水含量与氧逸度之间的关系，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.