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.

在地球向宜居行星发展的过程中，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，铁金属的存在将地幔的氧逸度缓冲在低于铁- <s:1> stite氧缓冲层的水平。然而，最早的岩石记录显示，地幔氧逸度高达5个log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在地球内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建模来测试地幔早期氧化的三种机制。一是岩浆海洋结晶过程中FeO发生歧化，形成含Fe2O3矿物和铁金属。地幔中的氧含量可能因为一些形成的铁金属分离到地核而升高。这一机制将通过测量在相当于整个地幔深度的条件下与铁金属平衡的橄榄岩固体上形成的矿物的Fe2O3含量来测试。第二种可能性是，在较低的地幔压力下，FeO歧化发生在硅酸盐熔体中，而不是矿物中，因为随着压力的增加，熔体Fe2O3成分在较低的氧逸度下变得越来越稳定。这将通过测量在控制氧逸度下平衡的熔体Fe2O3含量作为深地幔条件下压力的函数来测试。第三种可能是富Fe2O3或富H2O物质的吸积导致地幔逐渐氧化。然而，这只有在这些氧化区域通过硫化物熔体的分离继续形成地核的情况下才有可能。否则，高度亲铁元素在地幔中就会变得过于丰富。这将通过限制形成地核的硫化物熔体的组成作为氧逸度在反映大部分地球地幔压力下的函数来检验。利用这些结果在岩心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。对地球挥发性吸积和物种形成的影响将进行研究，并确定通过冥古宙地幔的初始氧化还原剖面。