Identifying the mechanism for the early oxidation of the Earths interior

确定地球内部早期氧化的机制

• 批准号: 277057085
• 负责人: Professor Dr. Daniel J. Frost
• 金额: --
• 依托单位: Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/277057085?language=en
• 关键词: Identifying; mechanism; early; oxidation; Earths

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.

在地球向宜居星球发展的过程中，最重要的事件之一是在地核形成期间或之后地幔氧化态的迅速增加。在地核形成过程中，铁金属的存在会缓冲地幔的氧逸度，使其低于铁-方铁矿的氧缓冲层。然而，最早的岩石记录显示地幔氧逸度高达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和地球内积累和保留的挥发性元素的比例有影响。在该项目中，将通过高压和高温实验和建模来测试地幔早期氧化的三种机制。第一种是在岩浆海洋结晶过程中，FeO发生结晶作用，形成含Fe_2 O_3矿物和铁金属。地幔中的氧含量可能会随着一些形成的铁金属分离到地核而升高。将通过测量在相当于地幔整个深度的条件下与铁金属平衡的橄榄岩固相线上形成的矿物的Fe 2 O3含量来测试这种机制。第二种可能性提出，FeO结晶发生在硅酸盐熔体，而不是矿物，在下地幔压力的结果熔体Fe 2 O3成分变得越来越稳定，在较低的氧逸度，随着压力的增加。这将通过测量熔体Fe 2 O3含量进行测试，该含量在受控氧逸度下平衡，作为压力到深部地幔条件的函数。第三种可能是富Fe_2 O_3或富H_2 O物质的增生导致地幔逐渐氧化。这是唯一可能的，但是，如果核心的形成继续从这些氧化区域通过分离的硫化物熔体。否则，高亲铁元素在地幔中会变得过于丰富。这将通过限制核心形成硫化物熔体的组成作为反映地球地幔大部分压力下氧逸度的函数来测试。利用这些结果在模型的核心形成和岩浆海洋结晶的最后阶段的主导氧化机制将被确定。挥发性的吸积和物种形成在地球上的影响将被检查和确定通过冥古宙地幔的初始氧化还原剖面。