Rates, Sources and Magmagenesis of Alkalic Lavas at the Edge of an Intraplate Hotspot: A Multi-tracer Study of the Youngest Volcanics on Oahu, Hawaii

知识价值。在中板块热点的晚期碱性岩浆活动是一种难以理解的现象。地球化学模型表明岩浆的成因主要是由交代岩石圈地幔的熔体所主导，而大多数地球物理模型则需要软流圈地幔的浮力上升流。这两种模型都不能预测火山沉积物中时间、成分和地质的全部观测结果。这一建议解决了多个悬而未决的问题，为发生在活跃的中板块热点火山主要位置下游的碱性复兴火山活动提供了模型。是什么原因导致了晚期碱复性火山活动的岩浆产生？恢复活力的火山主要是由岩石圈还是软流圈融化形成的？岩石圈岩性可能参与了古老的还是年轻的成岩作用？来源或熔化过程的非均质性的长度和时间尺度是什么？什么样的岩浆管道条件使得喷发的熔岩在短时间和长度尺度上具有极端的成分不均匀性？夏威夷瓦胡岛的火奴鲁鲁系列火山是研究碱恢复火山作用的原因和条件的一个案例。本项目重点研究其最后两个喷发序列（陆上坦塔罗斯裂谷和海底/陆上科科裂谷），因为它们在檀香山系列火山活动（以及其他地方）中表现出极端的成分范围，有很强的地质控制（沿着每个裂谷的多个喷口几乎是同时期的火山活动），喷发年龄足够年轻（80 ka），以限制230Th-238U和231Pa-235U不平衡的火成岩过程的速率和时间。此外，样品是新鲜的，以前已经很好地表征。u系列不平衡可以区分熔体和源中的同步和近期前磁化学分馏，以确定它们发生的时间。通过再结晶珊瑚捕虏体确定科科裂谷带火山活动年龄的试点项目也对熔岩进行了初步的u系列分析，显示了广泛的230 - 238u不平衡。相对年轻，距离活跃热点（340公里）很远，以及其他属性使这些火山喷发成为详细研究岩浆活动来源和类型的特殊例子，这些火山活动实际上是板块内碱性火山活动的类型地点。该项目将涉及手头已有的样本，包括杰克逊、克莱格和弗雷研究的遗留样本，由史密森学会提供，以及戴夫·克莱格提供的潜艇科科样本。该项目将主要利用现有的分析专业知识（u系列，Sr-Nd-Pb-Os同位素，主要和微量元素）和UH实验室的仪器，但也包括231Pa方法的开发，用于本研究和后续研究。更广泛的影响。该项目将有助于发展美国新的研究能力，用于测量231Pa-235U放射性不平衡，以研究火山系统。它还将促进分析方法向新仪器的过渡和早期职业博士后研究人员的发展。该项目的结果将在同行评审的文献中传播，纳入大学课程，通过获奖的夏威夷火山学中心网站（由PI Rubin创建/维护）向公众提供，并添加到国际知名的哈瑙马湾海滩公园的科科裂谷火山活动的游客中心展示。

Intellectual Merit. Late-stage, alkalic magmatism at mid-plate hotspots is an enigmatic occurrence that is poorly understood. Geochemical models suggest that magma genesis is strongly dominated by melts of metasomatised lithospheric mantle, whereas most geophysical models require buoyant upwelling in the asthenospheric mantle. Neither model predicts the full range of observations of time, composition and geology in the volcanic deposits. This proposal addresses multiple outstanding questions to inform models for alkalic rejuvenation volcanism, occurring far downstream of the primary locus of active mid-plate hot spot volcanism.1. What causes the magma production that feeds late stage alkalic rejuvenation volcanism?2. Are rejuvenation volcanics formed primarily by melting the lithosphere or the asthenosphere?3. Are the lithospheric lithologies possibly involved in petrogenesis old or young?4. What are the length and temporal scales of heterogeneity in sources or melting processes?5. What magmatic plumbing conditions allow extreme compositional heterogeneity in erupted lavas over short time and length scales?The Honolulu Series volcanics on Oahu, Hawaii serves as a case study of the causes and conditions of alkalic rejuvenation volcanism. This project focuses on its last two eruption sequences (the subaerial Tantalus rift and the submarine/subaerial Koko rift) because they exhibit the extreme compositional range within Honolulu Series volcanism (and elsewhere), there isstrong geologic control (nearly contemporaneous volcanism at multiple vents along each rift), and eruption ages young enough (80 ka) to constrain rates and timing of igneous processes with 230Th-238U and 231Pa-235U disequilibria. Moreover, samples are fresh and previously have been well characterized.U-series disequilibria can distinguish syn- and recent premagmagenetic chemical fractionation in melts and sources to determine when they occurred. A pilot project to determine the age of Koko rift zone volcanism from recrystallized coral xenoliths also produced preliminary U-series analyses of lavas that show extensive 230Th-238U disequilibria. The relative youth, great distance from the active hotspot (340 km), and other attributes make these eruptions exceptional examples for a detailed study about sources and styles of magmatism at what is effectively a type-locality for intra plate alkalic volcanism. The project will involve samples already in hand, including legacy samples studied by Jackson, Clague and Frey, provided by the Smithsonian Institution and submarine Koko samples provided by Dave Clague. The project will primarily utilize existing analytical expertise (U-series, Sr-Nd-Pb-Os isotopes, major and trace elements) and instrumentation in the UH laboratories, but also includes 231Pa methods development to be used in this and subsequent research.Broader Impacts. This project will contribute to development of new US research capabilities for measuring 231Pa-235U radioactive disequilibrium to study volcanic systems. It will also facilitate transition of analytical methods to new instrumentation and development of an early career post-doctoral researcher. Results of this project will be disseminated in peer-reviewed literature, incorporated in university courses, made available to the public via the award winning Hawaii Center for Volcanology website (created/maintained by PI Rubin), and added to a visitors center display about Koko Rift volcanism for the internationally renowned Hanauma Bay beach park.