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

板内热点边缘碱性熔岩的速率、来源和岩浆成因：夏威夷欧胡岛最年轻火山的多示踪剂研究

• 批准号: 0838271
• 负责人: Kenna Rubin
• 金额: $ 29.77万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-01-15 至 2013-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838271&HistoricalAwards=false
• 关键词: Rates; Sources; Magmagenesis; Alkalic; Lavas

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.

智力优势。中板块热点的晚期碱性岩浆活动是一个谜，人们对此知之甚少。地球化学模型表明，岩浆成因强烈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。这两种模型都不能预测火山沉积物的时间、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，为碱性复兴火山作用的模型提供信息，发生在活跃的中板块热点火山作用的主要地点的下游。是什么导致了岩浆的产生，为晚期碱性复兴火山作用提供了原料？2.再生火山岩主要是由岩石圈还是软流圈熔融形成的？3.岩石圈岩性可能参与成岩作用是古老的还是年轻的？4.源或融化过程的不均匀性的长度和时间尺度是什么？5.什么样的岩浆管道条件允许喷发熔岩在短时间和长度尺度上具有极端的成分不均匀性？夏威夷奥陶勒盖的檀香山系列火山岩是碱性复活火山作用的成因和条件的一个研究实例。本项目的重点是它的最后两次喷发序列（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作用中表现出极端的成分范围（和其他地方），有很强的地质控制，（几乎同时代的火山活动在多个喷口沿着每个裂谷），火山喷发年龄较年轻（80 ka），可以用~（230）Th-~（238）U和~（231）Pa-~（235）U不平衡来限制岩浆作用的速率和时间。此外，样品是新鲜的，以前已经得到很好的characteristics. U系列不平衡可以区分同期和最近的岩浆前的化学分馏熔体和来源，以确定它们何时发生。一个试点项目，以确定科科裂谷带火山作用的再结晶珊瑚捕虏体的年龄也产生了初步的U系列分析的熔岩，显示广泛的230 Th-238 U不平衡。相对年轻，距离活动热点（340公里），和其他属性，使这些火山喷发的特殊例子，详细研究的岩浆活动的来源和风格，在什么是有效的板内碱性火山作用的典型位置。该项目将涉及已经掌握的样本，包括史密森学会提供的杰克逊、Clague和Frey研究的遗留样本和Dave Clague提供的潜艇Koko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、Sr-Nd-Pb-Os同位素、主要和微量元素）和仪器，但也包括将用于本研究和后续研究的231 Pa方法开发。 该项目将有助于发展美国测量231 Pa-235 U放射性不平衡的新研究能力，以研究火山系统。它还将促进分析方法向新仪器的过渡，并促进早期职业博士后研究人员的发展。该项目的成果将在同行评议的文献中传播，纳入大学课程，通过屡获殊荣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纳乌玛湾海滩公园的游客中心展示科科裂谷火山活动。