Collaborative Research: Geochemical and Geodynamic investigation of lithospheric drip viability beneath the East African Rift

东非裂谷下方岩石圈滴水岩浆作用的地球化学和地球动力学研究大陆裂谷是威尔逊旋回的基本组成部分，威尔逊旋回定义了板块构造的地质范式。然而，在这种情况下，岩浆作用、岩石圈减薄和地壳伸展之间的关系却很难受到约束。 PI 将通过研究乌干达西部布富姆比拉 (Bufumbira) 和托罗安科莱 (Toro Ankole) 火山田的碱性镁铁质熔岩，评估岩石圈滴水岩浆作用作为成功大陆裂谷触发因素的作用。越来越多的证据表明，岩石圈最底层的密度不稳定性可能会因区域热应力或机械应力而发生突然的局部失稳。岩石圈的这种物理变薄（发生在 20-50 公里的范围内）增强了岩浆活动和伸展破坏（即成功的大陆裂谷）的可能性，从而控制了大陆边缘的最终形状。这项工作将与滴水相关的熔岩及其伴随的地幔捕虏体的详细地球化学分析与岩石圈滴水和随后的地壳伸展的地球物理模型结合起来。这项工作对理解大陆扩张的影响代表了一种预测新洋盆位置以及产生它们所需力的新方法。在区域范围内，这项工作对东非裂谷沿线国家非常重要：滴水岩浆作用无法维持浅层长寿命岩浆库，而岩浆库是可行的地热能生产所必需的。在人力资源方面，该项目为来自各个教育机构的女性研究人员以及地球化学和地球动力学领域的研究生和本科生提供支持，并为寻求该领域职业的地质学本科生提供地球物理学短期课程。从字面上和概念上讲，岩石圈地幔是构造板块构造、破坏和分裂的基础。岩浆和流体对岩石圈地幔的改变会影响其成分，进而影响浮力。如果岩石圈底部变得更致密，岩石圈底部可能会分离和下沉，导致小体积玄武岩火山活动、岩石圈变薄和潜在的裂谷。熔岩的地球化学数据表明，岩石圈滴水推动了东非裂谷几个地区的火山活动。此外，该地区的捕虏体显示出明显的熔体和流体交代作用，并且岩石圈的去除已被用来解释最近隆起的时间和速率。为了检验这一假设，将通过以下方式评估岩石圈滴水的作用：1）获取熔岩和辉石岩捕虏体的详细岩石学和矿物规模成分数据，2）计算熔岩和捕虏体记录的物理条件，3）将这些计算应用于 2D 和 3D 地球动力学模型，以确定岩石圈滴水构造和熔融是否可以解释岩石本身。岩石圈滴熔有点自相矛盾，因为冷下降的岩石圈不会熔化，除非它通过传导加热的速度比下降过程中压缩的速度更快。对称滴落可能需要额外的复杂性，例如裂谷期间的边缘驱动对流或附近的上升羽流。该项目将促进包括 REU 机构在内的 3 所美国大学之间的合作，并为乌干达的研究人员提供机会。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Geochemical and geodynamic investigation of lithospheric drip magmatism beneath the East African RiftContinental rifting is a fundamental part of the Wilson Cycle that defines the geological paradigm of plate tectonics. In this context, however, the relationships among magmatism, lithospheric thinning and crustal extension are poorly constrained. The PIs will evaluate the role of lithospheric drip magmatism as a trigger for successful continental rifting by studying alkaline mafic lavas from the Bufumbira and Toro Ankole volcanic fields in western Uganda. A growing body of evidence suggests that density instabilities in the lowermost lithosphere can undergo abrupt localized destabilization in response to regional thermal or mechanical stress. This physical thinning of the lithosphere, occurring on a scale of 20-50 km, enhances both magmatism and the likelihood of extensional failure, i.e., successful continental rifting, and thus controls the ultimate shape of continental margins. The work couples detailed geochemical analysis of proposed drip-related lavas and their accompanying mantle xenoliths with geophysical modeling of lithospheric drip and the ensuing crustal extension. The implications of this work to the understanding of continental extension represent a new approach to predicting the locus of new ocean basins and the forces required to produce them. At a regional scale, this work is of high importance to nations along the East African Rift: drip magmatism does not sustain the shallow long-lived magma chambers that are necessary for viable geothermal energy production. In terms of human resources, this project supports female researchers from a wide range of educational institutions as well as graduate and undergraduate students in geochemistry and geodynamics, and brings a short-course in geophysics to geology undergraduates who seek careers in this field. The lithospheric mantle is foundational - literally and conceptually - to the construction, destruction and division of tectonic plates. Modification of the lithospheric mantle by magma and fluids affects its composition and, by extension, buoyancy. If the bottom of the lithosphere is made denser, the base of the lithosphere can detach and sink, leading to small-volume basaltic volcanism, lithospheric thinning, and potentially rifting. Geochemical data on lavas suggest lithospheric drips drive volcanism in several parts of the East African Rift. Also, xenoliths from this area show significant metasomatism by melts and fluids, and lithospheric removal has been invoked to explain the timing and rates of recent uplift. In order to test this hypothesis, the role of lithospheric drips will be evaluated by: 1) obtaining detailed petrographic and mineral-scale compositional data on lavas and pyroxenite xenoliths, 2) calculating the physical conditions recorded by lavas and xenoliths, and 3) applying those calculations to 2D and 3D geodynamic models to whether lithospheric drip tectonics and melting explain the rocks themselves. Lithospheric drip melting is somewhat paradoxical, because cold descending lithosphere should not melt unless it is heated through conduction more rapidly than it becomes compressed during descent. It may be that additional complexity to symmetrical dripping is required, such as edge-driven convection during rifting or a nearby upwelling plume. This project will promote collaboration between 3 U.S. universities, including an REU institution, and provide opportunity for researchers from Uganda.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.