The lithosphere-asthenosphere boundary: integrated modeling of scattered wave observations and mantle dynamics

区分岩石圈和软流圈的物理和化学因素并不是唯一已知的，它们在不同构造环境之间的变化也不是唯一已知的。然而，岩石圈-软流圈边界（LAB）地震速度梯度的高分辨率成像有可能限制温度、成分（耗尽和挥发性含量）和熔体分数的组合，这些组合可能会产生LAB。例如，在北美东部宽带站观测到的P-to-S散射波（Ps）揭示了LAB速度梯度（在不到11公里的范围内，横波速度下降3%至11%），这种速度梯度过于强烈和急剧，无法单独用温度来解释，这表明了成分的影响（更枯竭、干燥的岩石圈与更少枯竭、水化的软流圈），或者可能是软流圈中存在部分融化。该项目的目的是更好地确定岩石圈向软流圈过渡的清晰度，主导这种过渡的因素组合，以及它们在不同构造背景下的相对影响的变化。该方法是将LAB散射波观测对速度梯度的约束与地幔流动和熔融模型相结合。具体来说，PI是：在各种大陆和海洋环境下，使用Ps和Sp相来约束LAB剪切速度梯度；对地幔流动和熔融过程进行数值模拟，计算地幔温度范围、主元素损耗、挥发性富集和部分熔融过程；利用地震速度与地幔成分和温度之间的现有关系，确定哪些条件与观测到的地震速度梯度一致。一个有待检验的总体假设是，拉布拉多下异常陡峭的地震速度梯度可能是由绝热减压融化形成的小程度部分熔融造成的，并被渗透性较低的下层岩石圈所困。不太陡峭的梯度可以单独用温度或挥发性含量来解释。该项目最初将侧重于三个案例：北美东部、非洲南部和不同年龄的海洋岛屿。之所以选择这些区域，是因为它们包含对LAB梯度成像所必需的长期宽带站的分布，并且因为它们允许评估地幔流、成分、温度和融化之间的特定相互作用。在北美东部和非洲南部，有待检验的观点是，岩石圈厚度的快速变化驱动了地幔上涌，通过绝热减压产生了小程度的部分融化。就海洋岛屿而言，该项目将检查需要融化的LAB梯度是否发生在年轻的海洋岛屿附近，而与岩石圈脱水和枯竭一致的LAB梯度，或仅与温度一致的LAB梯度，更有可能出现在最古老的海洋岛屿下方。这项工作得益于研究生和本科生研究人员的参与，反过来也有助于他们的教育。

The physical and chemical factors that distinguish the Earth's lithosphere from its asthenosphere are not uniquely known, and neither is their variation between different tectonic environments. However, high resolution imaging of seismic velocity gradients at the lithosphere-asthenosphere boundary (LAB) has the potential to constrain the combinations of temperature, composition (depletion and volatile content), and melt fraction that could create the LAB. For example, P-to-S scattered waves (Ps) observed at broadband stations in eastern North America have revealed LAB velocity gradients (a 3 to 11% shear-wave velocity drop over less than 11 km) that are too strong and sharp to be explained by temperature alone, suggesting compositional effects (a more depleted, dry lithosphere over a less depleted, hydrated asthenosphere) or perhaps the presence of partial melt in the asthenosphere.The goal of this project is to better determine the sharpness of the transition from the lithosphere to the asthenosphere, the combination of factors dominate this transition, and the variance of their relative influence between different tectonic settings. The approach is to integrate constraints on velocity gradients at the LAB from scattered wave observations with models of mantle flow and melting. Specifically, the PI's are: using Ps and Sp phases to constrain LAB shear velocity gradients in a variety of continental and oceanic settings; numerically modeling mantle flow and melting to calculate ranges of temperature, major element depletion, volatile enrichment, and partial melting near the LAB; determining which conditions are consistent with the observed seismic velocity gradients, using existing relationships between seismic velocity and mantle composition and temperature.An over-arching hypothesis to be tested is that exceptionally steep seismic velocity gradients beneath the LAB may be caused by a small degree of partial melt formed through adiabatic decompression melting, and trapped by a less permeable lower lithosphere. Less steep gradients may be explained by temperature or volatile content alone. The project will initially focus on three cases: eastern North America, southern Africa, and ocean islands of varying age. These regions were chosen because they contain the distributions of long-term broadband stations necessary for imaging LAB gradients, and because they permit the evaluation of specific interactions between mantle flow, composition, temperature, and melting. In eastern North America and southern Africa, the idea to be tested is that mantle upwelling driven by rapid changes in lithospheric thickness generates a small degree of partial melt through adiabatic decompression. In the case of ocean islands, the project will examine whether LAB gradients requiring melt occur near young ocean islands, and LAB gradients consistent with dehydration and depletion of the lithosphere, or with temperature alone, are more likely beneath the oldest ocean islands.This work is benefiting from the participation of a graduate student and undergraduate researchers and is in turn contributing to their education.