Near Continent Intraplate Magmatism in the Atlantic: Implications for Mantle Dynamics and Melting

大西洋近大陆板内岩浆作用：对地幔动力学和融化的影响

• 批准号: 1565614
• 负责人: Esteban Gazel
• 金额: $ 33.5万
• 依托单位: Virginia Polytechnic Institute and State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-04-01 至 2018-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1565614&HistoricalAwards=false
• 关键词: Near; Continent; Intraplate; Magmatism; Atlantic

The processes that result in volcanism away from active plate boundaries are not explained by the current plate tectonics theory, and thus remain a missing piece in our understanding of the internal dynamics and evolution of our planet. Localized upwelling of hot mantle known as "mantle plumes" successfully explains locations like Hawaii and Yellowstone. However, there are many other sites worldwide like Cape Verde and the Canaries islands that require further research to understand the mechanisms behind volcanic activity. The team's goal is to elucidate the mechanisms that produce volcanoes away from plate margins using a collaborative approach that combines data from geochemical and geodynamical studies. The investigators, including international collaborators, will collect new geochemical data including volatiles (C, H, S, etc.) from Cape Verde and the Canaries, using these locations as natural laboratories to study the deep Earth. They will use melt inclusions contained within the host crystals to collect volatile data to answer the proposed fundamental questions. These results can also be incorporated in assessment of volcanic hazards. Additionally, they will provide new CO2 and H2O data that will help constrain volatile output for intraplate magmatism, and especially increase our understanding of the deep Earth carbon cycle. The results from this project will flow directly into the teaching of the principal investigator, and will be disseminated to a broad audience through outreach activities at the Geosciences Museum at Virginia Tech, the Virginia Science Festival, and through different media sources to the public. The goal of this project is to integrate petrological and geochemical data from Cape Verde and the Canaries with geodynamic models to broaden our understanding of the composition and dynamical processes in the mantle that can give rise to intraplate magmatism. The mantle plume model has generally been accepted as a plausible mechanism for intraplate volcanism, but not all locations that are considered "hotspots" have the seismic and geochemical signatures to back up a deep-rooted source. Small-scale convection (e.g., edge-driven convection, EDC) has been proposed as an alternative mechanism for a subset of near continent locations. The investigators propose to test mantle plume vs. EDC as mechanisms for intraplate volcanism using Cape Verde and the Canaries as natural laboratories. These locations were selected for their position near a continental margin but away from plate boundaries, making them ideal to test our working hypotheses. Also, these locations provide easy access to abundant primitive tephras to be used for melt-inclusion studies and volatile determinations, which are necessary for an improved determination of melting conditions. Finally, Cape Verde and the Canaries are the only two oceanic settings where carbonatites have been reported. Preliminary work on Cape Verde and the Canaries suggest that some samples (including the carbonatites) from these locations melted from a carbonated mantle source, indicating the importance of intraplate volcanism for mantle outgassing, a missing link in the deep global water and carbon cycles. Thereby, the team will address these fundamental questions: 1) How do the dynamics of mantle upwelling control the melting behavior of the source through the effects of temperature and upwelling rate? 2) What is the source composition of these Atlantic hotspots, and how does it affect the dynamics of mantle upwelling as well as petrological determinations of source temperatures, the size of the swell and the seismic expression of mantle upwelling? 3) What are the volatile budgets of intraplate magmas and their role in the global water and carbon cycles, and how do volatiles modulate mantle dynamics by reducing viscosity? 4) What are the diagnostic geochemical and petrological signatures of the plume model vs. EDC?

目前的板块构造理论无法解释导致火山活动远离活动板块边界的过程，因此仍然是我们对地球内部动力学和演化的理解中缺失的一部分。被称为“地幔柱”的热地幔的局部上涌成功地解释了像夏威夷和黄石公园这样的地方。然而，世界上还有许多其他地方，如佛得角和加那利群岛，需要进一步研究，以了解火山活动背后的机制。该团队的目标是利用结合地球化学和地球动力学研究数据的协作方法，阐明远离板块边缘产生火山的机制。包括国际合作者在内的研究人员将收集新的地球化学数据，包括挥发物（C、H、S等）。从佛得角和加那利群岛，利用这些地点作为自然实验室，研究地球深处。他们将使用包含在主晶体中的熔融包裹体来收集挥发性数据，以回答所提出的基本问题。这些结果也可用于火山灾害的评估。此外，他们将提供新的CO2和H2O数据，这将有助于限制板内岩浆活动的挥发性输出，特别是增加我们对地球深部碳循环的理解。该项目的成果将直接用于首席研究员的教学，并将通过弗吉尼亚理工大学地球科学博物馆的外联活动、弗吉尼亚科学节以及通过不同的媒体向公众传播。 该项目的目标是将来自佛得角和加那利群岛的岩石学和地球化学数据与地球动力学模型结合起来，以扩大我们对可能引起板内岩浆活动的地幔成分和动力学过程的了解。地幔柱模型被普遍认为是板内火山活动的合理机制，但并非所有被认为是“热点”的地点都有地震和地球化学特征来支持一个根深蒂固的来源。小尺度对流（例如，边缘驱动对流，EDC）已被提出作为一个替代机制的一个子集的近大陆的位置。研究人员建议使用佛得角和加那利群岛作为天然实验室，测试地幔柱与EDC作为板内火山作用的机制。这些地点被选择为靠近大陆边缘但远离板块边界的位置，使它们成为测试我们工作假设的理想选择。此外，这些位置提供了容易获得丰富的原始火山岩，用于熔体包裹体研究和挥发分测定，这是必要的改进熔化条件的测定。最后，佛得角和加那利群岛是仅有的两个有碳酸岩报告的海洋环境。对佛得角和加那利群岛的初步研究表明，这些地区的一些样品（包括碳酸岩）是从碳酸化的地幔源熔融而来的，表明板内火山活动对地幔脱气的重要性，这是全球深部水和碳循环中缺失的一环。因此，该团队将解决这些基本问题：1）地幔上涌的动力学如何通过温度和上涌速率的影响控制源的熔融行为？2)这些大西洋热点的源成分是什么，它如何影响地幔上涌的动力学以及源温度的岩石学测定，膨胀的大小和地幔上涌的地震表达？3)板内岩浆的挥发性收支是什么，它们在全球水和碳循环中的作用是什么，挥发性物质如何通过降低粘度来调节地幔动力学？4)羽流模型与EDC的诊断地球化学和岩石学特征是什么？