Transfer of oxygen to Earth's core from a long-lived magma ocean
Transfer of oxygen to Earth's core from a long-lived magma ocean
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DOI：
10.1016/j.epsl.2020.116208
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发表时间：
2020-05-15
影响因子：
5.3
通讯作者：
Alfe, Dario
中科院分区：
文献类型：
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作者：
Davies, Christopher J.;Pozzo, Monica;Alfe, Dario
在核幔边界（CMB）金属和硅酸盐之间的化学相互作用现在被认为导致氧气转移到地球的液体核心。确定这种转移的性质和程度对于限制堆芯形成的条件、CMB下方稳定分层区域的起源以及堆芯内可能的氧化物沉淀是重要的。以前的FeO转移模型认为是一个固体地幔;然而，几条证据表明，最低的地幔可能在地核形成完成后很长一段时间仍然保持在其固相线之上，这将允许更快的质量转移。我们调查这种情况下，通过开发一个随时间变化的模型之间的FeO交换扩散分层层的核心和一个长寿的熔融岩浆海洋的顶部。核心FeO浓度，C-FeO（-（m）over bar），根据CMB半径r（cmb）处随时间变化的质量通量而变化，该质量通量取决于底部（C-FeO（-（m）over bar）（r（cmb）））和顶部（C-FeO（大块）（m）over bar（r（）））的FeO浓度。核-岩浆海洋演化的耦合是由于C-FeO（-（m）over bar）（r（cmb））和C-FeO（-（m）over bar）（r（cmb））通过分配系数P=C-FeO（-（m）over bar）（r（cmb））/C-FeO（-（m）over bar）（r（cmb））联系起来的。C-FeO（体）（m）相对于棒（r（））在无结晶（NC）模型中保持不变，并根据Labrosse等人（2007）的基底岩浆海洋模型在中间结晶（MOC）模型中演变，一般用于解释FeO损失到核心。在第一个1 Gyr，FeO的转移在所有的模式与>= 10%的FeO在岩浆海洋和P >= 5产生纯FeO成分的CMB，稳定分层层60 - 80公里，占15 - 50%的总现今核心氧含量。在NC模型中，岩浆海洋在4 Gyr内没有完全冻结，此时稳定层达到120 - 150 km，FeO转移可以解释地核中所有的现代O。然而，在MOC模型中，FeO损失到核心导致岩浆海洋在核心形成后的前1-3个Gyrs完全冻结。我们的研究结果表明，目前的核心组合物可能不会提供一个强大的约束模型的核心形成和FeO可以沉淀在顶部的核心。(C)2020年，任作家。由爱思唯尔公司出版
Chemical interactions between metal and silicates at the core-mantle boundary (CMB) are now thought to lead to transfer of oxygen into Earth's liquid core. Establishing the nature and extent of this transfer is important for constraining the conditions under which the core formed, the origin of a stably stratified region below the CMB and the possible precipitation of oxides within the core. Previous models of FeO transfer have considered a solid mantle; however, several lines of evidence suggest that the lowermost mantle could have remained above its solidus long after core formation was complete, which would allow much faster mass transfer. We investigate this scenario by developing a time-dependent model of FeO exchange between a diffusive stratified layer at the top of the core and a long-lived molten magma ocean. Core FeO concentration, C-FeO(-(m) over bar), is evolved subject to a time-dependent mass flux at the CMB, radius r(cmb), which depends on the FeO concentration at the bottom (C-FeO(-(m) over bar) (r(cmb))) and top (C-FeO(bulk)(m) over bar (r())) of the chemical boundary layer above the CMB. Coupled core-magma ocean evolution arises because C-FeO(-(m) over bar) (r(cmb)) and C-FeO(-(m) over bar)(r(cmb)) are linked through the partition coefficient P=C-FeO(-(m) over bar) (r(cmb))/C-FeO(-(m) over bar) (r(cmb)). C-FeO(bulk)(m) over bar (r()) is held constant in No Crystallization (NC) models and evolves in Middle-Out Crystallization (MOC) models according to the basal magma ocean model of Labrosse et al. (2007), generalised to account for FeO loss to the core. In the first 1 Gyr, FeO transfer in all models with >= 10% FeO in the magma ocean and P >= 5 produces pure FeO compositions at the CMB, stably stratified layers of 60 - 80 km and accounts for 15 - 50% of the total present-day core oxygen content. In NC models the magma ocean does not completely freeze in 4 Gyr, in which time the stable layer reaches 120 - 150 km and FeO transfer can account for all of the present-day O in the core. However, in MOC models FeO loss to the core causes the magma ocean to completely freeze in the first 1-3 Gyrs following core formation. Our results suggest that the present-day core composition may not provide a strong constraint on models of core formation and that FeO could have precipitated at the top of the core. (C) 2020 The Authors. Published by Elsevier B.V.