Both seismic observations of dense low shear velocity regions and models of magma ocean crystallization and mantle dynamics support enrichment of iron in Earth’s lowermost mantle. Physical properties of iron-rich lower mantle heterogeneities in the modern Earth depend on distribution of iron between coexisting lower mantle phases (Mg,Fe)O magnesiowüstite, (Mg,Fe)SiO₃ bridgmanite, and (Mg,Fe)SiO₃ post-perovskite. The partitioning of iron between these phases was investigated in synthetic ferrous-iron-rich olivine compositions (Mg_0.55Fe_0.45)₂SiO₄ and (Mg_0.28Fe_0.72)₂SiO₄ at lower mantle conditions ranging from 33–128 GPa and 1900–3000 K in the laser-heated diamond anvil cell. The resulting phase assemblages were characterized by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy. The exchange coefficient between bridgmanite and magnesiowüstite decreases with pressure and bulk Fe# and increases with temperature. Thermodynamic modeling determines that incorporation and partitioning of iron in bridgmanite are explained well by excess volume associated with Mg-Fe exchange. Partitioning results are used to model compositions and densities of mantle phase assemblages as a function of pressure, FeO-content and SiO₂-content. Unlike average mantle compositions, iron-rich compositions in the mantle exhibit negative dependence of density on SiO₂-content at all mantle depths, an important finding for interpretation of deep lower mantle structures.

中文翻译：

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富铁下地幔非均质性中亚铁分配的组成和压力效应

稠密的低剪切速度区域的地震观测以及岩浆海洋结晶和地幔动力学模型都支持地球最低地幔中铁的富集。在现代地球富含铁的下地幔的异质性的物理性质依赖于铁的分布共存下地幔相（镁，铁）O之间镁方，（镁，铁）的SiO ₃ bridgmanite，或（Mg，Fe）的的SiO ₃后钙钛矿。在富含亚铁的合成橄榄石成分（Mg _0.55 Fe _0.45）₂ SiO ₄和（Mg _0.28 Fe _0.72）₂ SiO _4中研究了铁在这些相之间的分配。在较低的地幔条件下，激光加热的钻石砧单元中的温度范围为33–128 GPa和1900–3000K。通过原位X射线衍射和非原位透射电子显微镜相结合来表征所得的相集合。桥锰矿与菱镁矿之间的交换系数随压力和Fe＃的增加而降低，随温度的升高而增加。热力学模型确定，桥煤中铁的结合和分配可以通过与Mg-Fe交换相关的过量体积很好地解释。划分结果用于模拟地幔相组合物的组成和密度随压力，FeO含量和SiO ₂含量的变化。与普通地幔成分不同，地幔中的富铁成分表现出对SiO的密度负相关性_2-在所有地幔深度的含量，这是解释深部下部地幔结构的重要发现。