Origin of ultra-low velocity zones at the core mantle boundary

利用新的地震学观测和矿物物理约束，我们将检验关于核幔边界(CMB)称为超低速带(ULVZ)的小尺度(10至100公里)非均质性来源的假说。最下面的地幔是各种各样的结构的宿主，并在许多尺度长度上显示出不均质性。可以说，过去15年中关于最低地幔的最耐人寻味的地震发现是在CMB附近断断续续地发现极低地震速度的薄片，被称为ULVZ，表明存在10至40公里的基础地幔。ULVZ影响着地幔动力学的许多方面，人们推测它们是地幔热柱的根源，进入地幔的核心物质区域，全球岩浆海洋的残留物，在极地逆转期间对磁极路径的影响，以及化学上独特的外来物质。因此，了解ULVZ的起源和性质不仅具有学术意义，而且会对一系列地球一级问题产生影响。尽管如此，ULVZ的起源仍未得到解决，部分熔融与化学不均匀作为ULZs来源的基本问题仍存在争议。为了验证关于ULVZ起源的假设，我们将使用矿物-物理和地震学相结合的方法。所提出的每一种模型都会导致ULVZ的比速变化、P波与S波的速度比以及密度的变化。例如，ULVZ的部分熔融成因将导致P-S波速比为1：3，而成分成因将产生VP/VS~1-2。目前，我们对ULVZ结构和下地幔物质性质的了解还不足以区分这些模型，我们将利用新的地震学探测来更好地解决ULVZ存在的速度和密度结构。此外，我们将从第一性原理计算中确定钙钛矿和后钙钛矿的弹性性质，作为成分、压力和温度的函数，以了解潜在的ULVZ材料的弹性性质。识别缺乏ULVZ的区域对于了解地幔流动与ULZs之间的联系至关重要。提高地震覆盖率，我们将得到CMB的地图，表明ULVZ地区，它们的地震速度和密度。使用基于矿物物理结果的正演模拟，我们将能够彻底测试ULVZ的不同原产地模型。

Using new seismological observations together with mineral physics constraints, we will test hypotheses as to the origin of small scale (10 to 100 kms) heterogeneities called ultra-low velocity zones (ULVZs) at the core mantle boundary (CMB). The lowermost mantle is host to a wide variety of structures and shows heterogeneities on many scale lengths. Arguably the most intriguing seismic discovery of the last 15 years regarding the lowermost mantle is that of intermittent thin patches of extremely reduced seismic velocities near the CMB dubbed ULVZs, indicative of the existence of a 10 to 40 km basal mantle layer. ULVZs influence many aspects of mantle dynamics and it has been speculated they are the roots of mantle plumes, areas of core material entering the mantle, remnants of a global magma ocean, an influence on the path of the magnetic poles during polar reversals, and chemically distinct exotic material. Therefore, understanding the origin and properties of ULVZs is not of just academic interest, but impacts on a wide range of first-order Earth issues. Nonetheless, the origin of ULVZs remains unsolved and fundamental questions such as partial melt vs. chemical heterogeneity as source for ULVZs are still debated. To test hypotheses on the origin of ULVZs we will use a combined mineral-physical and seismological approach. Each of the proposed ULVZ models will lead to specific velocity changes, P-wave to S-wave velocity ratios and density changes for ULVZs. For instance, a partial melt origin of ULVZs will lead to a P-to-S wave velocity ratio of 1:3, while a compositional origin creates VP/VS~1-2. Currently our knowledge about ULVZ structure and lower mantle material properties is not sufficient to differentiate between these models and we will employ new seismological probes to better resolve the existent velocity and density structure of ULVZ. Furthermore, we will determine the elastic properties of perovskite and post-perovskite, as function of composition, pressure and temperature from first principle calculations to understand the elastic properties of potential ULVZ material. Identification of regions devoid of ULVZs is crucial to understand the connection between mantle flow and ULVZs. Improving the seismic coverage we will obtain a map of the CMB indicating ULVZ regions, their seismic velocities and densities. Using forward modeling based on the mineral-physics results we will be able to thoroughly test different models of origin for ULVZs.