Experimental investigation of reduction in the CaO-SiO2-CO2 system with application to the formation of Breyite inclusions in diamonds

富钙矿物可以以包裹体钻石的形式出现，这些矿物引起了特别的关注，因为它们的共生体暗示着可能来自过渡带或甚至下地幔的亚岩石圈或“超深”成因。其中一种特别令人感兴趣的矿物是具有闪锌矿结构的硅酸钙，现在已被命名为水镁石。这一相偶尔与钙钛矿结构的CaSi2O5或两者同时出现，通常被认为是钙钙钛矿前驱体重新平衡的产物。然而，这种解读最近受到了质疑。此外，俯冲岩板原沉积物中的二氧化硅与CaCO3反应也可形成水镁石。这将涉及通过如下反应进行还原：CaCO3+SiO_2=CaSiO_3+C+O_2，在这种反应中，闪锌矿和钻石同时结晶。本研究的目的是在实验上确定在与元素碳平衡的还原条件下形成水镁石的可能条件，以探索在金刚石生长过程中将这种硅酸钙作为包裹体捕获的可行性。实验将在6-10 Gpa和1000-1500℃的简单CaO-SiO_2-CO_2±H_2O体系中进行。将研究两个模拟俯冲沉积物中反应的情景：1)还原条件下CaCO3和SiO_2层之间的相互作用以模拟上述反应；以及2)CaCO_3和SiO_2混合物在还原的COH流体存在下的反应，该流体具有低CO2活度。通过CaCO3+SiO_2=CaSiO_3+CO_2反应可形成Breyite，但在CO_2-端元体系中发生熔融。二氧化碳活度较低的流体可能会阻止熔融，使水辉石结晶，并有可能稳定钻石。在这两个实验系列中，氧气缓冲组件将与样品材料物理分离，以防止不必要的反应，但仍可施加所需的氧逸度。

Ca-rich minerals can occur as inclusions diamond and these have attracted special attention since their parageneses suggest a sub-lithospheric or “superdeep” origin, potentially from the transition zone or even lower mantle. One mineral of particular interest is CaSiO3 with the walstromite-type structure, which has now been named breyite. This phase occasionally occurs along with larnite or titanite-structured CaSi2O5, or both phases and has often been considered to represent a re-equilibration product from a Ca-perovskite precursor. However, this interpretation has recently been questioned. Alternatively, breyite could form by reaction of CaCO3 with SiO2 in former sediments of subducted slabs. This would involve reduction via a reaction like: CaCO3+SiO2 = CaSiO3+C+O2, where breyite and diamond crystallize simultaneously. This study aims to experimentally determine the possible conditions of breyite formation under reducing conditions in equilibrium with elemental carbon in order to explore the feasibility of trapping this Ca-silicate as an inclusion during diamond growth. Experiments will be performed at 6-10 GPa and 1000-1500°C in the simple CaO-SiO2-CO2±H2O system. Two scenarios simulating reactions in subducting sediment will be investigated: 1) interaction between CaCO3 and SiO2 layers under reducing conditions to model the reaction mentioned above; and 2) reaction of CaCO3 and SiO2 mixtures in the presence of a reduced COH fluid that has a low CO2 activity. Breyite could form via the reaction CaCO3 + SiO2 = CaSiO3 + CO2, except that melting occurs in the CO2-endmember system. Fluids with a lower CO2 activity could prevent melting, allow breyite to crystallize, and potentially stabilize diamond as well. In both experiment series, the oxygen buffering assemblage will be physically separated from the sample material to prevent unwanted reaction, but yet impose the desired oxygen fugacity.