Assessing the inner core nucleation paradox with atomic-scale simulations
Assessing the inner core nucleation paradox with atomic-scale simulations
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DOI：
10.1016/j.epsl.2018.11.019
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发表时间：
2019-02-01
影响因子：
5.3
通讯作者：
Alfe, Dario
中科院分区：
文献类型：
--
作者：
Davies, Christopher J.;Pozzo, Monica;Alfe, Dario
我们研究了在地核中心附近冻结液态铁和铁合金所需的条件。通常认为，一旦周围核心温度福尔斯下降到地球中心铁合金的熔化温度以下，内核就开始生长;然而，需要额外的（欠）冷却来克服与产生固液界面相关的能量障碍。基于经典成核理论（CNT）的预测已经估计了类似于1000 K的所需过冷度，这与类似于100 K Gyr（-1）的预测的核心冷却速率不一致。这种明显的矛盾被称为“内核成核悖论”。在这里，我们解决三个主要的不确定性，在应用CNT内核成核使用原子尺度模拟。首先，我们模拟冻结在Fe和Fe-O液体在核心条件下自我一致地约束CNT方程所需的所有参数。第二，我们通过直接计算观察Fe和Fe-O液体中冻结事件的等待时间来测试CNT的基本有效性。第三，我们研究了应用于原子模拟的波动强迫的影响，这已被建议作为一种手段，以显着降低能垒。我们的研究结果是一致的CNT在计算上可访问的参数制度，虽然误差估计的等待时间可以达到50%的测量在最大的过冷温度。使用CNT外推到内核条件产生纯铁系统的730 +/- 20 K和Fe-O系统的675 +/- 35 K的估计过冷度。对应于O（10）GPa的大压力变化的力使这些值减小了大约100 K。虽然我们的过冷度估计值明显低于以前的估计值，但它们还不足以解决内核成核悖论。(C)2018作者由爱思唯尔公司出版
We investigate the conditions required to freeze liquid iron and iron alloys near the centre of Earth's core. It is usually assumed that inner core growth begins once the ambient core temperature falls below the melting temperature of the iron alloy at Earth's centre; however, additional (under)cooling is required to overcome the energy barrier associated with creating a solid-liquid interface. Predictions based on Classical Nucleation Theory (CNT) have estimated a required undercooling of similar to 1000 K, which cannot be reconciled with predicted core cooling rates of similar to 100 K Gyr(-1). This apparent contradiction has been called the 'inner core nucleation paradox'. Here we address three major uncertainties in the application of CNT to inner core nucleation using atomic-scale simulations. First, we simulate freezing in Fe and Fe-O liquids at core conditions to self-consistently constrain all parameters required by the CNT equations. Second, we test the basic validity of CNT by directly calculating the waiting time to observe freezing events in Fe and Fe-O liquids. Third, we investigate the influence of wave-like forcings applied to the atomic simulations, which have been suggested as a means to significantly reduce the energy barrier. Our results are consistent with CNT in the computationally accessible parameter regime, though error estimates on the waiting time can reach 50% of the measurement at the largest undercooling temperatures. Using CNT to extrapolate to inner core conditions yields estimated undercooling of 730 +/- 20 K for the pure iron system and 675 +/- 35 K for the Fe-O system. Forcings corresponding to large pressure variations of O(10) GPa reduce these values by similar to 100 K. While our undercooling estimates are significantly lower than previous estimates they are not low enough to resolve the inner core nucleation paradox. (C) 2018 The Authors. Published by Elsevier B.V.