Broad plumes rooted at the base of the Earth's mantle beneath major hotspots
Broad plumes rooted at the base of the Earth's mantle beneath major hotspots
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DOI：
10.1038/nature14876
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发表时间：
2015-09-03
期刊：
影响因子：
64.8
通讯作者：
Romanowicz, Barbara
中科院分区：
文献类型：
--
作者：
French, Scott W.;Romanowicz, Barbara
热上升流岩石柱植根于地幔深处被认为是热点火山的可能起源，但这一想法是激烈辩论的主题（1，2）。根据地球动力学计算，纯热成因的羽流应该包括薄尾，只有几百公里宽（3），很难用标准地震层析成像技术探测到。在这里，我们描述了一种全地幔地震成像技术的使用-将精确的波场计算与整个地震波形中包含的信息相结合（4）-揭示了许多突出热点下方宽（而不是薄），准垂直管道的存在。这些管道从地核-地幔边界延伸到地表以下约1,000公里处，其中一些管道水平偏转，好像被卷入更活跃的上地幔环流。在地幔底部，这些管道植根于剪切速度大大降低的斑块中，在夏威夷、冰岛和萨摩亚的情况下，对应于已知的大型超低速带的位置（5-7）。这种对应关系清楚地确立了这些带和地幔柱之间的连续联系。我们还发现，成像的管道比经典的热羽流尾部要宽得多，这表明它们的寿命很长（8），并且可能具有热化学起源（9-11）。它们的垂直方向表明，在1 000公里深处的背景环流非常缓慢。我们的研究结果将为地幔粘性分层的研究提供约束条件，并指导热化学对流的进一步研究。
Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate(1,2). On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide(3), and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique-combining accurate wavefield computations with information contained in whole seismic waveforms(4)-that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones(5-7). This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived(8), and may have a thermochemical origin(9-11). Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection.