Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity
Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity
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DOI：
10.1016/j.epsl.2012.08.016
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发表时间：
2012-11-01
影响因子：
5.3
通讯作者：
Ritsema, J.
中科院分区：
文献类型：
--
作者：
Davies, D. Rhodri;Goes, S.;Ritsema, J.
位于非洲和太平洋下地幔深处的两个大的低剪切速度省（llsvp）通常被解释为热但化学密度高的“堆”，它们与地幔环流隔离了数亿年。这种解释在很大程度上取决于四个地震观测：(i)它们的横波速度异常对于纯热结构来说被认为太大了；（ii）其边缘的横波速度梯度是尖锐的；(3)横波与纵波速度异常比高；(4)剪切速度异常与体声速度异常呈反相关关系。然而，利用300 Myr板块运动历史驱动的可压缩全球地幔环流模型和从物理结构转换为地震结构的热力学方法，我们表明观测到的下地幔剪切波速异常不需要，而且很可能与大规模的化学“堆”不相容。规定的4000 K的核幔边界温度与目前的估计一致，结合非弹性地震对温度的敏感性，确保了纯热的llsvp，通过俯冲历史强烈集中在非洲和太平洋之下，可以调和观测到的横波速度异常和梯度。相比之下，即使考虑到有限的层析分辨率，包含地幔底部密集化学“堆”的模型的剪切波速异常也比层析模型S40RTS强得多，其中“堆”仅占地幔体积的3%。我们的研究结果还表明，在钙钛矿后存在的情况下，剪切和纵波速度异常之间的高比值以及剪切和体声速度异常之间的相关性不能用于区分深度的热非均质性和成分非均质性：在所有计算中，反相关只发生在钙钛矿后稳定场内。综上所述，这意味着尽管地幔中一定存在相当大的化学非均质性，但不需要大量的、连贯的“堆”来调和这里所检验的地震观测结果。事实上，我们的研究结果表明，如果这些地区存在化学非均质性，其动力学和地震意义远不如以前推断的那样重要。(C) 2012 Elsevier B.V.版权所有
Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wave-speed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anti-correlation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred. (C) 2012 Elsevier B.V. All rights reserved.