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

合作研究：东非裂谷下方岩石圈滴水活力的地球化学和地球动力学研究

• 批准号: 1753748
• 负责人: Wendy Nelson
• 金额: $ 8.39万
• 依托单位: Towson University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2022-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1753748&HistoricalAwards=false
• 关键词: Collaborative; Research; Geochemical; geodynamic; investigation

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.

东非裂谷下岩石圈滴注岩浆活动的地球化学和地球动力学研究大陆裂谷是威尔森旋回的一个基本组成部分，它定义了板块构造的地质范式。然而，在这种背景下，岩浆活动、岩石圈减薄和地壳伸展之间的关系却没有得到很好的约束。pi将通过研究乌干达西部Bufumbira和Toro Ankole火山场的碱性基性熔岩，评估岩石圈滴流岩浆作用作为成功的大陆裂谷的触发因素的作用。越来越多的证据表明，最下层岩石圈的密度不稳定可能会由于区域热应力或机械应力而发生突然的局部不稳定。岩石圈的这种物理减薄，发生在20-50公里的尺度上，增强了岩浆作用和伸展破坏的可能性，即成功的大陆裂谷，从而控制了大陆边缘的最终形状。该工作将提出的与水滴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水滴和随后的地壳伸展的地球物理模拟结合起来。这项工作对理解大陆伸展的意义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。在区域范围内，这项工作对东非裂谷沿线的国家具有高度重要性：岩浆活动不能维持地热能源生产所必需的浅层长寿命岩浆房。在人力资源方面，本项目支持来自广泛教育机构的女性研究人员以及地球化学和地球动力学的研究生和本科生，并为寻求在该领域就业的地质学本科生提供地球物理短期课程。岩石圈地幔是构造板块构造、破坏和分裂的基础——从字面上和概念上都是如此。岩浆和流体对岩石圈地幔的改造影响了它的组成，进而影响了浮力。如果岩石圈底部变得更致密，岩石圈的底部就会分离和下沉，导致小体积的玄武岩火山活动，岩石圈变薄，并可能发生裂谷作用。熔岩的地球化学数据表明，岩石圈滴液驱动东非裂谷若干地区的火山活动。此外，来自该地区的捕虏体显示出熔体和流体的明显交代作用，岩石圈的移除被用来解释最近隆升的时间和速率。为了验证这一假设，岩石圈滴液的作用将通过以下方式进行评估：1)获得熔岩和辉石岩捕虏体的详细岩石学和矿物尺度成分数据；2)计算熔岩和捕虏体记录的物理条件；3)将这些计算应用于二维和三维地球动力学模型，以确定岩石圈滴液构造和熔融是否解释了岩石本身。岩石圈滴式融化有点矛盾，因为寒冷的下降岩石圈不应该融化，除非它通过传导加热的速度比下降过程中被压缩的速度快。可能需要对称滴水的额外复杂性，例如裂谷期间的边缘驱动对流或附近的上升流羽流。该项目将促进包括一所REU机构在内的三所美国大学之间的合作，并为来自乌干达的研究人员提供机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Petrogenesis of primitive lavas from the Toro Ankole and Virunga Volcanic Provinces: Metasomatic mineralogy beneath East Africa's Western Rift

托罗安科莱和维龙加火山省原始熔岩的岩石成因：东非西部裂谷下的交代矿物学

• DOI: 10.1016/j.lithos.2021.106192
• 发表时间: 2021
• 期刊: Lithos
• 影响因子: 3.5
• 作者: Pitcavage, Erica;Furman, Tanya;Nelson, Wendy R.;Kalegga, Peggy K.;Barifaijo, Erasmus
• 通讯作者: Barifaijo, Erasmus