Identifying the mechanism for the early oxidation of the Earths interior

在地球向宜居行星发展的过程中，最重要的事件之一是在核心形成期间或刚刚形成之后，地幔的氧化状态迅速增加。在核形成过程中，铁金属的存在会将地幔的氧逸度缓冲到低于铁-钨铁矿氧缓冲的水平。然而，最早的岩石记录显示，地幔的氧逸度高出5个对数单位。这一氧化事件的原因和时间不仅与大气的演化有关，而且对地幔的早期分化以及地球内部积累和保留的挥发性元素的比例也有影响。在这个项目中，将通过高压和高温实验和模拟来测试地幔早期氧化的三种机制。首先，FeO的歧化作用发生在岩浆海洋结晶形成含铁矿物和铁金属的过程中。地幔中的氧含量可能是由于形成的一些铁金属分离到地核而增加的。这一机制将通过测量橄榄岩固相线上形成的矿物的Fe2O3含量来测试，在相当于整个地幔深度的条件下，与铁金属保持平衡。第二种可能性认为，FeO歧化发生在低地幔压力下的硅酸盐熔体中，而不是矿物中，这是因为熔体中的Fe2O3组分在低氧逸度下随着压力的增加而变得越来越稳定。这将通过测量在受控氧逸度下平衡的熔体Fe2O3含量作为深部地幔条件下压力的函数来进行测试。第三种可能是富Fe_2O_3或H_2O物质的堆积导致地幔的逐渐氧化。然而，只有在这些氧化区域通过分离硫化物熔体继续形成核心的情况下，这才是可能的。否则，地幔中高度亲铁的元素就会变得过于丰富。这一点将通过限制形成核心的硫化物熔体的组成作为氧逸度的函数来进行测试，这些氧逸度反映了大部分地幔的压力。利用这些结果，在岩芯形成和岩浆海洋结晶的最后阶段的模型中，将确定主要的氧化机制。将研究挥发性物质在地球上的吸积和物种形成的含义，并确定穿过赫甸地幔的初始氧化还原剖面。

One of the most important events in the Earths development towards a habitable planet was the rapid increase in the oxidation state of the mantle during or just after core formation. During core formation the presence of iron metal would have buffered the oxygen fugacity of the mantle at a level below the iron-wüstite oxygen buffer. However, the earliest rock record reveals a mantle oxygen fugacity up to 5 log units higher. The cause and timing of this oxidation event are not only relevant to the evolution of the atmosphere but have implications for the early differentiation of the mantle and the proportions of volatile elements accreted and retained within the Earth. In this project three mechanisms for the early oxidation of the mantle will be tested through high pressure and temperature experiments and modelling. The first is that disproportionation of FeO occurred during the crystallisation of a magma ocean to form Fe2O3 bearing minerals and iron metal. The oxygen content of the mantle could have been raised as some of the iron metal that formed separated to the core. This mechanism will be tested by measuring the Fe2O3 content of minerals forming on the peridotite solidus in equilibrium with iron metal at conditions equivalent to the entire depth of the mantle. The second possibility proposes that FeO disproportionation occurred within silicate melts, rather than minerals, at lower mantle pressures as a result of melt Fe2O3 components becoming increasingly stable at lower oxygen fugacities as pressure increases. This will be tested by measuring melt Fe2O3 contents equilibrated at controlled oxygen fugacities as a function of pressure to deep mantle conditions. The third possibility is that the accretion of either Fe2O3 or H2O rich material led to the gradual oxidation of the mantle. This was only possible, however, if core formation continued from these oxidised regions through the separation of a sulphide melt. Otherwise highly siderophile elements would have become over abundant in the mantle. This will be tested by constraining the composition of core forming sulphide melts as a function of oxygen fugacity at pressures reflecting much of the Earths mantle. Using these results in models for the final stages of core formation and magma ocean crystallisation the dominant oxidation mechanism will be determined. The implications for volatile accretion and speciation in the earth will be examined and the initial redox profile through the Hadean mantle determined.