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

富钙矿物可以作为金刚石包裹体出现，这些矿物引起了特别的关注，因为它们的共生体表明岩石圈下或“超深”起源，可能来自过渡带甚至下地幔。一种特别令人感兴趣的矿物是CaSiO 3，其具有walstromite型结构，现在已被命名为breyite。该相偶尔沿着与钾镁矾或钛酸盐结构的CaSi 2 O 5或这两种相一起出现，并且通常被认为代表来自Ca-钙钛矿前体的再平衡产物。然而，这一解释最近受到质疑。或者，钠闪石可以通过CaCO 3与俯冲板片的前沉积物中的SiO2反应形成。这将涉及通过如下反应的还原：CaCO 3 +SiO2 = CaSiO 3 +C+O2，其中钙铝榴石和金刚石同时结晶。本研究旨在通过实验确定在与元素碳平衡的还原条件下形成钙黄长石的可能条件，以探索在金刚石生长过程中捕获这种钙硅酸盐作为夹杂物的可行性。实验将在简单的CaO-SiO2-CO2±H2O系统中在6-10 GPa和1000-1500°C下进行。将研究两种模拟俯冲沉积物中反应的情景：1）还原条件下CaCO 3和SiO2层之间的相互作用，以模拟上述反应; 2）在存在具有低CO2活性的还原COH流体的情况下，CaCO 3和SiO2混合物的反应。钙铝榴石的形成是通过CaCO 3 + SiO2 = CaSiO 3 + CO2反应进行的，只是熔融发生在CO2-端元体系中。具有较低CO2活性的流体可以防止熔融，允许钠闪石结晶，并可能稳定金刚石。在两个实验系列中，氧缓冲组合物将与样品材料物理分离以防止不希望的反应，但仍施加期望的氧逸度。

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.