Metal-silicate mixing by large Earth-forming impacts
Metal-silicate mixing by large Earth-forming impacts
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DOI：
10.1016/j.epsl.2021.116888
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发表时间：
2021-04-14
影响因子：
5.3
通讯作者：
Dalziel, Stuart B.
中科院分区：
文献类型：
--
作者：
Landeau, Maylis;Deguen, Renaud;Dalziel, Stuart B.
地球化学和同位素观测限制了地球形成的时间、温度和压力。然而，为了充分解释这些观察结果，我们必须了解形成地球的碰撞后金属和硅酸盐之间的混合和平衡程度。最近的流体动力学实验提供了这种混合的初步估计，但它们完全忽略了行星形成撞击器的惯性。在这里，我们使用实验室实验来研究浓液体体积对较轻液体池的影响，以建立混合的比例定律，作为冲击器速度、尺寸、密度和局部重力的函数。我们的实验重现了冲击模拟中观察到的陨石坑过程。它们还会产生小尺度的湍流，接近行星撞击的动态状态。在每个实验中，我们都观察到早期以撞击为主的阶段，包括陨石坑的形成、陨石坑塌陷成向上的喷射流，以及喷射流的塌陷。后来，我们观察到一股浮力热气流向下传播。我们量化了撞击和随后的热阶段对混合的贡献。我们的实验结果与理论计算一起表明，射流的塌陷产生了大部分撞击引起的混合。我们发现射流惯性和冲击器浮力之间的比率控制着混合。应用于地球的形成，我们预测直径小于 100 公里的撞击体会达到完全化学平衡，但对于月球形成的巨大撞击来说，只有部分平衡。通过我们考虑冲击器惯性的新比例，金属和硅酸盐之间的质量传递比之前的估计大二十倍。这将从同位素数据推导出来的吸积时间尺度减少了多达十倍，并且根据亲铁元素推导出来的平衡压力减少了多达两倍。 (C) 2021 Elsevier B.V. 保留所有权利。
Geochemical and isotopic observations constrain the timing, temperature and pressure of Earth's formation. However, to fully interpret these observations, we must know the degree of mixing and equilibration between metal and silicates following the collisions that formed the Earth. Recent fluid dynamical experiments provide initial estimates of this mixing, but they entirely neglect the inertia of planet-building impactors. Here we use laboratory experiments on the impact of a dense liquid volume into a lighter liquid pool to establish scaling laws for mixing as a function of the impactor speed, size, density and the local gravity. Our experiments reproduce the cratering process observed in impact simulations. They also produce turbulence down to small scales, approaching the dynamical regime of planetary impacts. In each experiment, we observe an early impact-dominated stage, which includes the formation of a crater, its collapse into an upward jet, and the collapse of the jet. At later times, we observe the downward propagation of a buoyant thermal. We quantify the contribution to mixing from both the impact and subsequent thermal stage. Our experimental results, together with our theoretical calculations, indicate that the collapse of the jet produces much of the impact-induced mixing. We find that the ratio between the jet inertia and the impactor buoyancy controls mixing. Applied to Earth's formation, we predict full chemical equilibration for impactors less than 100km in diameter, but only partial equilibration for Moon-forming giant impacts. With our new scalings that account for the impactor inertia, the mass transfer between metal and silicates is up to twenty times larger than previous estimates. This reduces the accretion timescale, deduced from isotopic data, by up to a factor of ten and the equilibration pressure, deduced from siderophile elements, by up to a factor of two. (C) 2021 Elsevier B.V. All rights reserved.