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，我们将测试关于核幔边界（CMB）小尺度（10至100公里）非均质性（称为超低速度带（ULVZs））起源的假设。最下端地幔是多种构造的宿主，在许多尺度长度上表现出非均质性。可以说，在过去的15年里，关于最下层地幔最有趣的地震发现是在CMB附近被称为ulvz的间歇性薄块，这些薄块的地震速度极低，表明存在10到40公里厚的区域性基底地幔层。ulvz影响了地幔动力学的许多方面，据推测它们是地幔柱的根源，进入地幔的核心物质区域，全球岩浆海洋的残留物，极反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，了解超高压粒子的起源和性质不仅具有学术意义，而且会对一系列一阶地球问题产生影响。尽管如此，ulvz的起源仍然没有解决，一些基本问题，如部分熔体和化学非均质作为ulvz的来源仍然存在争议。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ULVZ模型都将导致ULVZ的具体速度变化、纵波与横波速度比和密度变化。例如，ulvz的部分熔体源将导致p - s波速比为1:3，而成分源将产生VP/VS~1-2。目前我们对超低震区结构和下地幔物质性质的了解还不足以区分这两种模型，我们将采用新的地震探针来更好地解析超低震区存在的速度和密度结构。此外，我们将通过第一性原理计算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ULVZ材料的弹性性质。确定无超低震带的区域对于理解地幔流与超低震带之间的联系至关重要。提高地震覆盖范围，我们将获得CMB的地图，显示ULVZ区域，它们的地震速度和密度。利用基于矿物物理结果的正演建模，我们将能够彻底测试ulvz的不同起源模型。

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 thick regional 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.