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

东非裂谷之下岩石圈滴状岩浆活动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，该旋回定义了板块构造的地质范例。然而，在这种背景下，岩浆作用、岩石圈减薄和地壳伸展之间的关系受到的限制很小。PIS将通过研究乌干达西部BufumBira和Toro Ankole火山田的碱性镁铁质熔岩，评估岩石圈滴状岩浆活动作为大陆裂谷成功触发因素的作用。越来越多的证据表明，岩石圈最下部的密度不稳定可以经历突然的局部不稳定，以响应区域热应力或机械应力。这种发生在20-50公里范围内的岩石圈物理减薄，既增强了岩浆作用，也增强了伸展破坏的可能性，即成功的大陆裂谷作用，从而控制了大陆边缘的最终形状。这项工作结合了对拟议的与水滴有关的熔岩及其伴生的地幔包体的详细地球化学分析，以及对岩石圈水滴和随后的地壳伸展的地球物理模拟。这项工作对理解大陆伸展的影响代表了一种新的方法来预测新的海洋盆地的位置和产生它们所需的力量。在区域范围内，这项工作对东非裂谷沿线国家非常重要：滴状岩浆作用不能维持浅层长寿命岩浆室，而浅层岩浆室是可行的地热生产所必需的。在人力资源方面，该项目支持来自各种教育机构的女研究人员以及地球化学和地球动力学的研究生和本科生，并为在该领域寻求职业的地质学本科生提供地球物理短期课程。从字面上和概念上讲，岩石圈地幔对构造板块的建造、破坏和分裂都是基础性的。岩浆和流体对岩石圈地幔的改造会影响其组成，进而影响浮力。如果岩石圈的底部变得更致密，岩石圈的底部可能会分离并下沉，导致少量的玄武岩火山活动，岩石圈变薄，并可能发生裂谷。关于熔岩的地球化学数据表明，岩石圈的水滴驱动了东非裂谷几个部分的火山活动。此外，该地区的捕虏体显示出明显的熔体和流体交代作用，并援引了岩石圈的去除来解释最近抬升的时间和速率。为了验证这一假设，将通过以下方式评估岩石圈水滴的作用：1)获得有关熔岩和辉石岩包体的详细岩石学和矿物尺度成分数据，2)计算熔岩和包体记录的物理条件，3)将这些计算应用于2D和3D地球动力学模型，以确定岩石圈水滴构造和熔融是否解释了岩石本身。岩石圈的水滴融化有些自相矛盾，因为冷的下降的岩石圈不应该融化，除非它通过传导被加热的速度比它在下降过程中变得更快地压缩。可能需要对称滴水的额外复杂性，例如裂谷过程中的边缘驱动对流或附近的上升羽流。该项目将促进3所美国大学之间的合作，包括一所REU机构，并为来自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优势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.