Collaborative Research: Developing New Models of Oceanic Magmatism and Source Heterogeneity Using the 8 degree 20' N Seamounts as Windows into the Sub-Ridge Mantle

合作研究：利用北纬 8 度 20 英尺海山作为进入亚脊地幔的窗口，开发大洋岩浆作用和源异质性的新模型

• 批准号: 2001331
• 负责人: Virginia Dorsey Wanless
• 金额: $ 16.21万
• 依托单位: Boise State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-05-01 至 2025-04-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2001331&HistoricalAwards=false
• 关键词: Collaborative; Research; Developing; New; Models

Eruptions of lava form submarine volcanoes, known as seamounts, near mid-ocean ridge spreading centers. These lavas can provide important information about what Earth’s uppermost (~ 120 miles) mantle is composed of and how it melts to form the majority of Earth’s crust. At present no studies have examined long chains of seamounts adjacent to mid-ocean ridges to address these fundamental questions. In this study, we plan to analyze over 400 well located lava samples from the 8°20'N seamount chain. The chain was previously sampled and mapped in 2016 and 2018 using the submersible Alvin and autonomous vehicle Sentry. The geochemical, mineralogical, and age data to be obtained under this project will allow us to determine the origin of the magmas that feed the seamounts and nearby ridge. The data will also tell us how those magmas evolve to form oceanic lavas, and how the volcanism is related to regional tectonic processes. The study will promote advances in the field of geochemistry, marine geology, and geodynamics. This project will provide significant mentorship and training for students at a variety of levels. It will fully or partially support three graduate students at Florida and Boise State and will provide training for several undergraduates in geochemical techniques and interpretation, and support senior research projects. The project will also support education and diversity by funding two female scientists, one a first-generation college graduate, toward obtaining their PhDs in geochemistry. Both principal investigators are heavily involved in outreach and education at the K-12 levels in Florida and Idaho.Sparse sampling and geochemical analyses of seamounts scattered throughout the northeastern Pacific suggest the sub-ridge mantle associated with the East Pacific Rise (EPR) has far greater compositional variability then is preserved in lavas erupted on-axis. While the relative homogeneity of axial lavas has resulted in intriguing generalizations about magma differentiation and mixing processes at mid-ocean ridges (MOR), seamount studies have had a significant impact on our understanding of the source compositions for oceanic basalts, the scales of mantle heterogeneity, and mantle melting systematics at MOR. However, many of these studies have focused on dredged lavas from individual seamounts with very few combining in situ sampling and high-resolution mapping from a single chain oriented perpendicular to the ridge axis. To fill this gap in knowledge about the sub-ridge mantle, two research cruises in 2016 and 2018 investigated the 8°20’N seamount chain that extends ~200km west of the EPR. The 8°20’N seamount chain was unexplored and virtually unsampled before our highly successful OASIS cruises. Geophysical, observational, and preliminary geochemical data from this nearly continuous chain of volcanoes led us to unexpected discoveries about their construction and revealed extreme compositional variability in the near MOR mantle that was previously only surmised from regional seamount studies in the northeast Pacific. The location, orientation, and preliminary data from the 8°20'N seamount chain make it an ideal a natural laboratory to investigate source heterogeneity and melting systematics in the near-ridge mantle. We propose to study the petrologic processes and mantle sources feeding MOR and near-axis volcanoes though comprehensive geochemical analyses of lavas and melt inclusions from the 8°20’N seamounts. Site-specific sampling using HOV Alvin and extensive, fine-scale mapping with AUV Sentry along the seamount chain, coupled with our previous comprehensive investigations of the 8°- 10°N EPR provide an exceptional opportunity to constrain geodynamic and geochemical models of mantle melting near MOR and determine the extent and length scales of mantle heterogeneity. We propose to collect a wide spectrum of petrologic and geochemical data including Sr, Nd, Pb, and Os isotopes, melt inclusion compositions, volatile contents, and 40Ar/39Ar age dates of lavas from the extensive collection of samples we have from the 8°20’N seamount chain. These results will be combined with numerical models to produce a petrologic model of mantle melting and magmatic processes in the near-ridge mantle. This extensive off-axis seamount chain provides a rare opportunity to probe the compositions of the mantle near MOR and to understand how magmas are spatially and temporally distributed in the shallow mantle and crust. Results from this research have implications for the generation of the majority of Earth’s crust.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.

熔岩喷发形成海底火山，称为海山，位于大洋中脊扩张中心附近。这些熔岩可以提供关于地球最上面(约120英里)的地幔是由什么组成的重要信息，以及它是如何熔化形成地壳的大部分的。目前，还没有研究对大洋中脊附近的长链海山进行研究，以解决这些根本问题。在这项研究中，我们计划分析来自8°20‘N海山链的400多个位置良好的熔岩样本。这条链之前在2016年和2018年使用潜水器阿尔文和自动驾驶车辆哨兵进行了采样和绘制。根据该项目将获得的地球化学、矿物学和年龄数据将使我们能够确定供给海山和附近山脊的岩浆的来源。这些数据还将告诉我们这些岩浆是如何演化形成海洋熔岩的，以及火山作用是如何与区域构造过程有关的。这项研究将促进地球化学、海洋地质学和地球动力学领域的进展。该项目将为不同层次的学生提供重要的指导和培训。它将完全或部分支持佛罗里达和博伊西州立大学的三名研究生，并将为几名本科生提供地球化学技术和解释方面的培训，并支持高级研究项目。该项目还将通过资助两名女科学家获得地球化学博士学位来支持教育和多样性，其中一名女科学家是第一代大学毕业生。两位主要研究人员都积极参与佛罗里达州和爱达荷州K-12级别的推广和教育。对散布在整个东北太平洋的海山进行的分散采样和地球化学分析表明，与东太平洋隆起(EPR)相关的次山脊地幔的成分变化比轴线上喷发的熔岩中保存的成分变化要大得多。虽然轴向熔岩的相对均质性导致了关于大洋中脊(MOR)岩浆分异和混合过程的有趣概括，但海山研究对我们理解大洋玄武岩的来源成分、地幔不均匀的尺度以及大洋中脊的地幔熔融系统学产生了重大影响。然而，其中许多研究侧重于从个别海山挖出的熔岩，很少结合现场采样和从垂直于海脊轴线的单链进行高分辨率测绘。为了填补关于亚脊地幔的这一认识空白，2016年和2018年的两次研究巡航调查了EPR以西约200公里处的8°20‘N海山链。在我们非常成功的绿洲巡航之前，8°20‘N海山链从未被勘探过，也几乎没有取样过。来自这一几乎连续的火山链的地球物理、观测和初步地球化学数据使我们对它们的结构有了意想不到的发现，并揭示了近莫尔地幔的极端成分变异性，这以前只是从东北太平洋的区域性海山研究中推测出来的。8°20‘N海山链的位置、方位和初步数据使其成为研究近山脊地幔中物源不均一性和熔融体系学的理想天然实验室。我们建议通过对8°20‘N海山熔岩和熔融包裹体的综合地球化学分析，研究Mor火山和近轴火山的岩石学过程和地幔来源。使用HOV Alvin的特定地点采样和AUV Sentry沿海山链的广泛精细制图，加上我们之前对8°-10°N EPR的全面调查，提供了一个难得的机会来约束MOR附近地幔熔融的地球动力学和地球化学模型，并确定地幔不均匀的程度和长度尺度。我们建议从8°20‘N海山链的大量样品中收集广泛的岩石学和地球化学数据，包括锶、钕、铅和Os同位素、熔体包裹体成分、挥发分含量和熔岩的40Ar/39Ar年龄。这些结果将与数值模型相结合，产生地幔熔融和近山脊地幔中岩浆作用的岩石学模型。这种广泛的离轴海山链提供了一个难得的机会来探索莫尔附近的地幔成分，并了解岩浆是如何在浅地幔和地壳中的时空分布的。这项研究的结果对大部分地球地壳的形成有影响。这一裁决反映了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。