The stratified layer at the core-mantle boundary caused by barodiffusion of oxygen, sulphur and silicon
The stratified layer at the core-mantle boundary caused by barodiffusion of oxygen, sulphur and silicon
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DOI：
10.1016/j.pepi.2012.11.001
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发表时间：
2013-02-01
影响因子：
2.3
通讯作者：
Davies, C. J.
中科院分区：
文献类型：
--
作者：
Gubbins, D.;Davies, C. J.
气压扩散是轻元素沿压力梯度向下迁移的趋势。在地球的外核，这种效应会导致地核-地幔边界（CMB）下化学稳定层的形成。相对于其他效应，气压扩散一直被认为是不重要的，但在这里，我们表明它在CMB中占主导地位，并导致一个大约100公里厚的层，富含轻元素并稳定分层。气压扩散不仅改变了控制核心中轻元素分子扩散的方程，而且改变了CMB的边界条件，使其变为非零的成分梯度，这是以往研究所忽略的一点。这个数学问题与最近提出的轻元素从地幔进入地核的迁移具有相同的形式；如果考虑到外核的所有轻元素，而不是像以往的研究那样只考虑驱动对流的轻元素，那么气压扩散的影响是可比较的。因此，我们得出结论，在核心的顶部可能存在一个实质性的稳定层，而不受任何穿越CMB的质量通量的影响。我们利用由第一性原理计算得到的扩散常数，求解了在CMB下面的薄层中氧、硫和硅在整个核心历史上的气压扩散方程。该层的下边界被定义为中性稳定的水平，其中稳定的气压扩散梯度等于和相反的不稳定梯度与岩心混合良好的块状浮力源相关。我们假设宇宙微波背景中没有质量通量，并发现气压扩散所施加的成分梯度如此之大，以至于它的稳定密度梯度在任何时候都无法被任何不稳定梯度所克服。因此，轻层在岩心形成后立即在岩心顶部发育；对扩散方程的求解表明，它的厚度增长到100公里数量级。最终厚度对用于指定堆芯混合良好区域的不稳定梯度的堆芯冷却模型非常不敏感。我们考虑了各种不稳定机制，并认为层积的强度足以抑制层内几乎所有的径向运动，尽管层积存在的结论性证据只能来自观测。其成分的变化非常大，足以产生地磁效应和地震速度异常，而这些异常可能已经被探测到，而且可能已经被探测到。三种轻元素的扩散参数不同，导致它们在层中的相对浓度不同，使层相对于硫或硅富氧。(C) 2012 Elsevier B.V.版权所有
Barodiffusion is the tendency of light elements to migrate down a pressure gradient. In the Earth's outer core, this effect can lead to the development of a chemically stable layer beneath the core-mantle boundary (CMB). Barodiffusion has so far been considered unimportant relative to other effects, but here we show that it dominates at the CMB and leads to an order-100 km-thick layer that is rich in light elements and stably stratified. Barodiffusion changes not only the equations governing molecular diffusion of light elements in the core but also the boundary condition at the CMB to a non-zero compositional gradient, a point missed by previous studies. The mathematical problem has the same form as the recently-proposed migration of light elements passing from the mantle into the core; the effect of barodiffusion is comparable provided all light elements in the outer core are included, not just the light element driving the convection as in previous studies. We therefore conclude that a substantial stable layer can exist at the top of the core independent of any mass flux across the CMB. We solve the relevant diffusion equations in a thin layer beneath the CMB for barodiffusion of oxygen, sulphur and silicon over the whole history of the core using diffusion constants obtained from first principles calculations. The lower boundary of the layer is defined to be the neutrally stable level where the stabilising barodiffusive gradient is equal and opposite to the destabilising gradients associated with buoyancy sources in the well-mixed bulk of the core. We assume no mass flux across the CMB, and find the compositional gradient imposed by barodiffusion to be so large that its stable density gradient could not be overcome by any destabilising gradient at any time. The light layer therefore develops at the top of the core immediately after core formation; solving the diffusion equations shows it to grow to a thickness of order 100 km. The final thickness is remarkably insensitive to the model of core cooling used to specify the destabilising gradients in the well-mixed region of the core. We consider a variety of instability mechanisms and argue that the stratification is strong enough to inhibit virtually all radial motion within the layer, although conclusive evidence for the existence of stratification can only come from observations. The variation in composition is sufficiently strong to produce geomagnetic effects and seismic velocity anomalies of a fraction of a percent that could be, and may already have been, detected. Differences in the diffusion parameters for the three light elements cause differences in their relative concentrations in the layer, leaving the layer oxygen-rich relative to sulphur or silicon. (C) 2012 Elsevier B.V. All rights reserved.