Experimental investigation of the redox conditions at which carbonate minerals and melts transform to graphite or diamond in Earths mantle

地球内部的氧化还原状态将影响地幔和幔源岩浆中挥发性元素的形态。碳是以这种方式受到影响的主要元素之一，因为在还原条件下它形成石墨或金刚石，在氧化条件下形成碳酸盐（或含CO2）矿物和熔体。在一系列的高温高压实验中，我们将研究这种氧化过程发生的氧化还原条件，从而绘制出地幔中含碳物种的稳定场。我们将在橄榄岩和榴辉岩的整体组成中研究这种氧化过程。一些研究表明，地幔可能会随着深度的增加而减少。这意味着元素碳（石墨或金刚石）的氧化可能发生在上涌岩石中，氧化产物是含碳酸盐的岩浆。我们将集中在这个“氧化还原熔融”的过程中，这可能标志着在地幔熔融的开始，并控制熔融开始的熔融分数。同样，我们将研究碳酸盐矿物在俯冲带中随着深度增加而减少，特别是在俯冲洋壳内，这可能对碳在地幔中的停留时间产生直接影响。

The redox state of the Earth’s interior will influence the speciation of volatile elements both in the mantle and in mantle-derived magmas. Carbon is one of the principal elements to be affected in this way because under reducing conditions it forms graphite or diamond, and under oxidising conditions carbonate (or CO2-bearing) minerals and melts. In a series of high pressure and temperature experiments we will study the redox conditions under which this oxidation process occurs, thus mapping out the stability fields of carbon-bearing species in the mantle. We will study this oxidation process both in peridotitic and eclogitic bulk compositions. Several studies have indicated that the mantle may become more reduced with depth. This means that the oxidation of elemental carbon (graphite or diamond) may occur in up welling rocks where the oxidised product is a carbonate bearing magma. We will focus on this “redox melting” process, which may mark the onset of melting in the mantle and control the melt fraction at which melting commences. Similarly we will study the reduction of carbonate minerals with increasing depth in subduction zones, and particularly within subducted oceanic crust, which may have a direct influence over the residence time of carbon in the mantle.