Li Mingming, McNamara Allen K, Garnero Edward J, Yu Shule
Arizona State University, School of Earth and Space Exploration, PO Box 871404, Tempe, AZ, 85287-1404, USA.
Michigan State University, Department of Earth and Environmental Sciences, Natural Science Building, East Lansing, MI, 48824, USA.
Nat Commun. 2017 Aug 2;8(1):177. doi: 10.1038/s41467-017-00219-x.
The Earth's lowermost mantle large low velocity provinces are accompanied by small-scale ultralow velocity zones in localized regions on the core-mantle boundary. Large low velocity provinces are hypothesized to be caused by large-scale compositional heterogeneity (i.e., thermochemical piles). The origin of ultralow velocity zones, however, remains elusive. Here we perform three-dimensional geodynamical calculations to show that the current locations and shapes of ultralow velocity zones are related to their cause. We find that the hottest lowermost mantle regions are commonly located well within the interiors of thermochemical piles. In contrast, accumulations of ultradense compositionally distinct material occur as discontinuous patches along the margins of thermochemical piles and have asymmetrical cross-sectional shape. Furthermore, the lateral morphology of these patches provides insight into mantle flow directions and long-term stability. The global distribution and large variations of morphology of ultralow velocity zones validate a compositionally distinct origin for most ultralow velocity zones.Ultralow velocity zones are detected on the core-mantle boundary, but their origin is enigmatic. Here, the authors find that the global distribution and large variations of morphology of ultralow velocity zones are consistent with most having a compositionally-distinct origin.
地球最下层地幔的大型低速区在核幔边界的局部区域伴有小规模的超低速带。大型低速区被认为是由大规模成分不均一性（即热化学堆）造成的。然而，超低速带的起源仍然难以捉摸。在此，我们进行了三维地球动力学计算，以表明超低速带的当前位置和形状与其成因有关。我们发现，最炽热的最下层地幔区域通常位于热化学堆内部深处。相反，超致密的成分不同的物质堆积沿着热化学堆的边缘呈不连续斑块状出现，且具有不对称的横截面形状。此外，这些斑块的横向形态为地幔流动方向和长期稳定性提供了线索。超低速带的全球分布和形态的巨大变化证实了大多数超低速带起源于成分不同的物质。在核幔边界探测到了超低速带，但其起源仍是个谜。在此，作者们发现超低速带的全球分布和形态的巨大变化与大多数超低速带起源于成分不同的物质这一观点相符。
