CSEDI Collaborative Research: Investigating the Relationship Between Plume Dynamics and ULVZ Geometry

超低速度带（ultra-low velocity zone， ULVZ）是一层很薄（约10公里）的层，位于地幔最下层的某些区域，紧邻外核的边界，其特征是地震波速度急剧下降。波速降低的原因尚不确定，但可能的原因包括小程度的地幔熔融和地幔中的硅酸盐矿物与地核中的铁之间的反应产生的化学非均质性。以前，低分辨率的研究表明了ULVZ的地理范围，暗示了大的区域斑块，然而，最近，由该小组成员进行的高分辨率观测显示了一个潜在的比原先想象的更小的规模结构。在先前被认为包含更大的连续ULVZ层的区域发现了一个小的，更密集的ULVZ材料的孤立口袋。这里提出的工作涉及亚利桑那州立大学和加州大学伯克利分校的地震学家、数值和实验室地球动力学家的合作。这项工作的最终目标是确定起源于核心-地幔边界的上涌地幔柱如何影响ULVZ的局部几何形状，并且一个令人兴奋的可能性是研究地震可探测的ULVZ袋是否可以用作确定几乎地震不可见的地幔柱的来源区域的标记。在地震学方面，将以比以前高得多的分辨率对极低频区进行研究，目的是确定以前认为连续的极低频区是否由更密集区域的孤立袋组成。地球动力学、数值和实验室实验将用于确定观测到的孤立的极空区物质口袋是否与地幔柱有关，如果是的话，极空区物质形态与地幔柱源区相关的流动模式之间的关系是什么？最后，建议的工作将集中在确定动态预测和观测约束是否可以用于区分部分熔化和化学非均质性作为ULVZ原因的相互竞争的假设。

The ultra-low velocity zone (ULVZ) is a thin ( ~10 km) layer in some regions of the lower-most mantle immediately above the boundary with the outer core that is characterized by a dramatic reduction in seismic wave speeds. The cause of this reduction in wave speed is uncertain, but likely possibilities include a small degree of mantle melting and chemical heterogeneity created by reactions between silicate minerals in the mantle with iron in the core. Previous, lower resolution studies that characterized the geographic extent of the ULVZ have hinted at large regional patches, however, recent, higher resolution observations made by members of this group have revealed a potentially smaller-scale structure than originally thought. A small, isolated pocket of more-dense ULVZ material was discovered in a region that was previously thought to contain a much larger, continuous ULVZ layer. Work proposed here involves a collaboration of seismologists and both numerical and laboratory geodynamicists at Arizona State University and the University of California Berkeley. The ultimate goal of this work is to determine how upwelling mantle plumes originating from the core-mantle boundary affect the local geometry of the ULVZ, and one exciting possibility to examine is whether seismically detectable pockets of ULVZ can be used as markers to determine the source region for nearly seismically-invisible mantle plumes. On the seismology front, the ULVZ will be studied at much higher resolution than before with the goal of determining whether previously-thought-continuous ULVZ regions are instead composed of isolated pockets of more-dense regions. Geodynamically, numerical and laboratory experiments will be used to determine whether observed isolated pockets of ULVZ material are related to mantle plumes, and if so, what is the relationship between the morphology of ULVZ material and the flow patterns associated with mantle plume source regions? Finally, proposed work will focus on determining whether the dynamically-predicted and observational constraints can be used to differentiate between competing hypotheses of partial melting and chemical heterogeneity as a cause of the ULVZ.