The deep Earth oxygen cycle: Mass balance considerations on the origin and evolution of mantle and surface oxidative reservoirs
The deep Earth oxygen cycle: Mass balance considerations on the origin and evolution of mantle and surface oxidative reservoirs
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地球深层氧循环：地幔和地表氧化储层起源和演化的质量平衡考虑
DOI：
10.1016/j.epsl.2023.118311
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发表时间：
2023
影响因子：
5.3
通讯作者：
Hirschmann, Marc M.
中科院分区：
文献类型：
--
作者：
Hirschmann, Marc M.
地球深部的氧循环解决了地幔和外逸层之间氧化物种的起源和长期交换问题，并创造了内部和表面环境演变的地球化学背景。块状硅酸盐地球（BSE）的氧化还原能力可以用其氧化还原收支RB来衡量，RB是相对于地幔中氧化还原敏感元素的主要价态的参考状态。82±4%（1.9±0.5× 1023摩尔）的氧化还原能力存在于地幔中，18±4%（4.2±0.5× 1022摩尔）存在于外逸层中。地幔中氧化态物质的强烈流出（3.8±0.7× 1013 mol/yr）可在1.1± 0.2Ga时补充外逸层储层，这需要通过俯冲作用对氧化还原能量进行有效的长期再循环。在不确定性范围内，最近（最近200 Ma）地幔RB流出和俯冲流入（4.9±1.3× 1013 mol/yr）是平衡的，但流出可能超过地球历史早期的流入。CO2占RB流出量的66±20%，但仅占流入量的24±13%。这主要是因为共轭的间隔反应;一个在浅地幔中以Fe 2 O3为代价产生碳酸盐，另一个通过有机碳固定和氧化风化的组合从表面的CO2产生Fe 2 O3。地幔氧逸度fO 2与氧化还原质量平衡的标度关系为Δ logK（Fe 3+ Fe 3+）橄榄岩→ 1 4 Δ logK（fO 2）地幔。因此，从太古代到元古代地幔氧逸度的长期演化（fO 2约为1.5个对数单位）表明太古代地幔的Fe 3+/FeT比值小于0.02，而不是现代的0.04±0.01。大洋玄武岩来源于比洋中脊更大的氧化还原平衡源，这部分表现为较高的源Fe 3+/FeT比值，但更重要的是，更高的源CO2浓度。结合起来，这些要求在上地幔深部的地幔柱的Fe 3+/Fe T比显着大于亏损地幔。BSE的氧化物存量来源于大气中的H2 O分解导致H2逃逸，以及地幔深部的FeO分解导致Fe向地核的损失。在深岩浆海洋中的FeO沉淀作用不可避免地产生BSE RB的重要部分，但可能需要额外的贡献。进一步的铁损失可能是从硼镁石结晶，然后通过基底岩浆海洋的铁逃逸。由此产生的深部氧化层的逐渐混合可以解释3 - 2Ga火成岩地幔源区的长期氧化作用。H2 O的净化有助于氧化的表面物种的积累，但不是地幔氧化能力的重要来源，因为氧化流入的机制在数量上是不够的。
The deep Earth oxygen cycle addresses the origin and long-term exchange of oxidized species between the mantle and exosphere and creates the geochemical context in which the interior and surface environment have evolved. The redox power of the bulk silicate Earth (BSE) can be measured in terms of its redox budget, RB, relative to a reference state of the predominant valences of redox-sensitive elements in the mantle. 82±4%(1.9±0.5× 10 23 moles) of the redox power resides in the mantle and 18±4%(4.2±0.5× 10 22 moles) in the exosphere. Vigorous outfluxes of oxidized species from the mantle (3.8±0.7× 10 13 mol/yr) can replenish the exosphere reservoir in 1.1±0.2 Ga, which requires efficient long-term recycling of redox power via subduction. Within uncertainties, recent (last 200 Ma) mantle RB outfluxes and subduction influxes (4.9±1.3× 10 13 mol/yr) are balanced, but outfluxes likely exceeded influxes earlier in Earth history. CO 2 is 66±20% of the RB outfluxes but only 24±13% of the influxes. This is largely because of conjugate intervalence reactions; one in the shallow mantle creates carbonate at the expense of Fe 2 O 3 and the other creates Fe 2 O 3 from CO 2 on the surface by a combination of organic carbon fixation and oxidative weathering. Scaling of mantle oxygen fugacity, f O2, to redox mass balance is approximately Δ log(Fe 3+ Fe⊤) peridotite≈ 1 4 Δ log(f O 2) mantle. Consequently, inferences of secular evolution of mantle oxygen fugacity from the Archaean to the Proterozoic, amounting to about 1.5 log units in f O2, imply that the Archaean mantle had an Fe 3+/Fe T ratio< 0.02, rather than the modern value of 0.04±0.01. Oceanic basalts derive from sources with greater redox budgets than mid-ocean ridges, and this is partly expressed as higher source Fe 3+/Fe T ratios, but more importantly, as greater source CO 2 concentrations. In combination, these require that plumes in the deep upper mantle have Fe 3+/Fe T ratios significantly greater than the depleted mantle. The oxidative inventory of the BSE originated by a combination of H 2 O disproportionation in the atmosphere, leading to H 2 escape, and FeO disproportionation in the deep mantle, with loss of Fe to the core. FeO disproportionation in a deep magma ocean inevitably produces a significant fraction of the BSE RB, but additional contributions are likely required. Further Fe loss could be from bridgemanite crystallization followed by Fe escape through a basal magma ocean. Gradual mixing of the resulting deep oxidized layer may account for secular oxidation of the mantle source regions of igneous rocks from 3 to 2 Ga. H 2 O disproportionation assisted in accumulation of the oxidized surface species, but was not a significant source of mantle oxidative power, as mechanisms of oxidative influx are quantitatively insufficient.
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