Rapid Core Formation in Terrestrial Planets by Percolative Flow: In-Situ Imaging of Metallic Melt Migration Under High Pressure/Temperature Conditions
Rapid Core Formation in Terrestrial Planets by Percolative Flow: In-Situ Imaging of Metallic Melt Migration Under High Pressure/Temperature Conditions
复制标题
DOI：
10.3389/feart.2018.00077
复制
发表时间：
2018-06-12
影响因子：
2.9
通讯作者：
Potts, Nicola J.
中科院分区：
文献类型：
--
作者：
Berg, Madeleine T. L.;Bromiley, Geoffrey D.;Potts, Nicola J.
核心的形成在地球上留下了持久的地球化学特征。为了限制地球的组成，我们必须充分了解新形成的地球及其附生的天体的分化过程。在早期太阳系中，富铁熔体通过固体硅酸盐的渗流被认为是地体分化和核心形成的一种机制。然而，由于缺乏关于富铁熔体迁移速度的数据，到目前为止，渗流对岩心形成的贡献还无法评估。在这里，我们使用一个新的实验设计来研究一个模拟系统中的织构变化，该系统在3 Gpa的多晶h-BN中熔化Au，与早期太阳系的核心形成有关。利用高分辨率、原位X射线层析成像和快速二维射线成像相结合的方法，我们首次获得了高PT下熔体迁移速度的直接数据。熔体迁移受熔体迁移和基质压实过程中压差变化的驱动，具有高度的多变性和阶段性。较小规模的熔融过程代表了熔体沿现有熔体网络的迁移，提供了相对较快的速度，为0.6-60µms(-1)。使用非原位实验比较了模拟体系中的熔体网络和硅酸盐中的富铁熔体。熔体迁移的两个相互竞争的过程是熔体沿晶界的渗流和熔体注入引起的水力压裂。通常，这两个过程都在实验和自然系统中被注意到，尽管每种机制的相对重要性是可变的。使用一个简单的熔体在多孔介质中流动的模型，这里确定的迁移速度解释了地球大小的物体在10(1)-10(3)Myr范围内、亚微米直径熔体带或几个Myr或微米大小的熔体带内的完全分化。这与行星体核形成的地球化学所推断的快速时间尺度是一致的，这意味着渗流作用可能对地球上的核分化做出了重要贡献。
Core formation has left a lasting geochemical signature on the Earth. In order to constrain the composition of the Earth we must fully understand the processes by which newly formed Earth, and the bodies which accreted to it, differentiated. Percolation of iron-rich melt through solid silicate has been invoked as a mechanism for differentiation and core formation in terrestrial bodies in the early solar system. However, to date the contribution of percolation to core formation cannot be assessed due to the absence of data on Fe-rich melt migration velocities. Here we use a novel experimental design to investigate textural changes in an analog system, Au melt in polycrystalline h-BN, at 3 GPa, relevant to core formation in the early solar system. Using a combination of high resolution, in-situ X-ray tomography and fast 2-D radiographic imaging, we obtain the first direct data on melt migration velocities at high PT. Melt migration is highly variable and episodic, driven by variations in differential pressure during melt migration and matrix compaction. Smaller scale melt processes, representing migration of melt along pre-existing melt networks, give comparatively fast velocities of 0.6-60 mu ms(-1). Ex-situ experiments are used to compare melt networks in analog systems to Fe-rich melt in silicates. Two competing processes for melt migration are percolation of melt along grain boundaries, and hydraulic fracturing induced by melt injection. Typically, both processes are noted in experimental and natural systems, although the relative importance of each mechanism is variable. Using a simple model for melt flow through a porous media, migration velocities determined here account for full differentiation of Earth-sized bodies within 10(1)-10(3) Myr, for submicron diameter melt bands, or within a few Myr or micron-sized melt bands. This is consistent with rapid timescales inferred from geochemistry for core formation in planetesimals, implying that percolation may have had an important contribution to core differentiation in the Earth.