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.

地球向宜居行星发展过程中最重要的事件之一是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，铁金属的存在会将地幔的氧逸度缓冲在低于铁方铁矿氧缓冲的水平。然而，最早的岩石记录显示地幔氧逸度高达 5 个对数单位。这种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积累和保留的挥发性元素的比例也有影响。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。第一个是，在岩浆海洋结晶过程中，FeO 发生歧化，形成含 Fe2O3 的矿物和铁金属。当一些形成的铁金属分离到地核时，地幔的氧含量可能会升高。该机制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 Fe2O3 含量来测试。第二种可能性提出，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。这将通过测量在受控氧逸度下平衡的熔体 Fe2O3 含量作为深部地幔条件压力的函数来测试。第三种可能性是富含 Fe2O3 或 H2O 物质的堆积导致地幔逐渐氧化。然而，只有通过硫化物熔体的分离，从这些氧化区域继续形成核心，才有可能实现这一点。否则，地幔中的高度亲铁元素就会变得过于丰富。这将通过限制形成核心的硫化物熔体的成分作为在反映大部分地幔的压力下氧逸度的函数来进行测试。在核心形成和岩浆海洋结晶的最后阶段的模型中使用这些结果，将确定主要的氧化机制。将研究对地球中挥发性吸积和物种形成的影响，并确定穿过冥古宙地幔的初始氧化还原剖面。