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

智力上的功绩。板块中部热点地区的晚期碱性岩浆作用是一种神秘的现象，人们对此知之甚少。地球化学模型表明，岩浆成因强烈地受交代岩石圈地幔熔体的控制，而大多数地球物理模型需要软流圈地幔中的浮力上涌。这两种模型都不能全面预测火山沉积的时间、成分和地质情况。该提案涉及多个悬而未决的问题，以提供发生在板块中部活动热点火山主要轨迹下游的碱性复活火山作用模型的信息。是什么原因导致岩浆的产生为晚期碱性复兴火山作用提供了支持？2.复兴火山岩主要是由岩石圈或软流圈融化形成的？3.岩石圈的岩性可能涉及成岩作用是古老的还是年轻的？4.岩浆来源或熔融过程中的不均一性的长度和时间尺度是什么？什么岩浆管道条件允许喷发的熔岩在短时间和长度尺度上具有极大的成分不均一性？夏威夷瓦胡岛的火奴鲁鲁系列火山是碱性复兴火山作用的原因和条件的研究案例。该项目重点研究最后两个喷发序列(陆上坦塔卢斯裂谷和海底/陆上科科裂谷)，因为它们在火奴鲁鲁系列火山活动(和其他地方)内表现出极端的成分范围，有强烈的地质控制(每个裂谷沿线的多个喷口几乎同时发生火山活动)，并且喷发年龄足够年轻(80ka)，通过230°-238U和231PA-235U的不平衡来限制火成岩作用的速率和时间。此外，样品是新鲜的，以前已经得到了很好的表征。U系列不平衡可以区分熔体和源岩中同步的和最近的磁前化学分馏，以确定它们发生的时间。从重结晶珊瑚包体中确定科科裂谷带火山活动年龄的试点项目也产生了对熔岩的初步U系列分析，显示出广泛的230-238U不平衡。相对年轻，距离活动热点很远(340公里)，以及其他属性使这些喷发成为详细研究板内碱性火山活动类型地点的岩浆活动来源和类型的例外例子。该项目将涉及已有的样本，包括由史密森学会提供的杰克逊、克莱格和弗雷研究的遗留样本，以及由戴夫·克莱格提供的潜艇科科样本。该项目将主要利用UH实验室现有的分析专业知识(铀系列、锶-钕-铅-铯同位素、常量元素和痕量元素)和仪器，但也包括将在本次和后续研究中使用的231PA方法开发。该项目将有助于开发新的美国研究能力，以测量231Pa235U的放射性不平衡，以研究火山系统。它还将促进分析方法向新仪器的过渡，并促进职业生涯早期的博士后研究人员的发展。该项目的结果将在同行评议的文献中传播，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花间湾海滩公园的游客中心展示关于科科裂谷火山活动。

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.