The redox state of the mantle during and just after core formation
The redox state of the mantle during and just after core formation
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DOI：
10.1098/rsta.2008.0147
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发表时间：
2008-11-28
期刊：
影响因子：
5
通讯作者：
Rubie, D. C.
中科院分区：
文献类型：
--
作者：
Frost, D. J.;Mann, U.;Rubie, D. C.
相对于球粒陨石，亲铁元素在地幔中被耗尽，这是与形成核心的富铁金属平衡的结果。金属硅酸盐分配系数的测量表明，相对于从当前地幔FeO含量推断出的条件，轻微亲铁元素（如Cr， V）的地幔消耗必须发生在更多的还原条件下。这意味着随着吸积的进行，地幔的氧化态（即FeO含量）随着时间的推移而增加。现今上地幔的氧逸度比与形成核心的铁金属平衡时所施加的水平高好几个数量级。这是由于地幔中Fe2O3含量的增加，这可能发生在地球历史的前1ga。在这里，我们探索了可能导致地幔FeO和Fe2O3含量增加的分选机制，而吸积物质的氧化态保持不变（均质吸积）。利用测量的O和Si的金属硅酸盐分配系数，我们模拟了岩浆海洋中随着吸积的进行而逐渐变深的核幔平衡。该模型表明，由于Si进入地核，地幔会逐渐被氧化。然而，地幔FeO含量和氧逸度的增加受到以下事实的限制：O在高温下也会进入地核，从而降低了地幔的FeO含量。（Mg, Fe）（Al, Si） O-3钙钛矿是下地幔的主要矿物，即使在金属铁存在的情况下，其对Fe2O3的亲和力也很强。由于上地幔在岩心形成时缺乏Fe2O3， FeO会歧化生成Fe2O3（在钙钛矿中）和Fe金属。一些歧化铁金属在地核的损失会使剩余的地幔中Fe2O3含量丰富，如果整个地幔都被均质化，那么上地幔的氧逸度就会提高到现在的水平。
Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal silicate partition coefficients show that mantle depletions of slightly siderophile elements ( e. g. Cr, V) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe2O3 content of the mantle that probably occurred in the first 1 Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe2O3 contents to increase while the oxidation state of accreting material remained constant ( homogeneous accretion). Using measured metal silicate partition coefficients for O and Si, we have modelled core mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg, Fe)(Al, Si) O-3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe2O3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe2O3 during core formation, FeO would have disproportionated to produce Fe2O3 ( in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe2O3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.