Insights into Hawaiian Magma Storage and Melt/Crust Interaction from Geochemical and Petrologic Investigation of Xenoliths from Hualalai and Mauna Kea Volcanoes

建造夏威夷群岛的火山等火山喷发出的熔岩成分变化为我们提供了对地球内部产生岩浆的过程的洞察。从岩浆成分的变化推断出的地幔不同部分的成分变化也提供了对板块构造和地幔对流等过程如何塑造地球内部的长期演化的洞察，这最终也影响了地球表面环境和生命本身的演化。然而，岩浆在地表喷发之前，岩浆的储存、混合和与地壳岩石的相互作用可以影响岩浆的成分。这些过程也会影响火山喷发是否具有爆炸性，从而影响火山灾害。这个项目将研究夏威夷熔岩喷发带到地表的结晶聚集体(或积聚体)的组成。这些堆积物很可能是在夏威夷岩浆深处形成的，暂时储存起来，后来被夏威夷岩浆带到地表。这些结晶堆积体的成分记录了岩浆储存的深度和过程，如岩浆冷却和结晶、离散岩浆批次的混合以及岩浆上升过程中围岩的同化。这项研究的结果将使我们更好地了解夏威夷火山的所谓“管道系统”，使我们能够更好地解释在喷发的熔岩中观察到的成分，并可能帮助我们解释为什么许多夏威夷火山似乎在非爆炸性喷发(低火山危险)和爆炸性喷发(高得多的火山危险)之间交替。这项研究的成果将被纳入外展讲座和展览，特别是“热门科学酷谈”计划和一年一度的“探索德克萨斯大学”开放日，这两个项目都吸引了当地大量的小学、初中和高中年龄的儿童来到该机构。该项目还将支持研究生和本科生的培训和发展，为他们在地质科学领域的职业生涯做好准备。该项目将研究最近来自华莱火山和莫纳克亚火山的屏蔽期后熔岩中包裹体的地球化学变化。这些深成岩样，包括太平洋洋壳的稀有碎片以及夏威夷屏蔽层和屏蔽层后岩浆的堆积体，对夏威夷岩浆储存和运输系统的时间演化和空间变化提供了强有力的约束，并将使我们能够评估熔体/岩石圈、熔体/地壳和熔体/建筑物相互作用在夏威夷熔岩中产生化学和同位素特征(或散布)的潜在作用。矿物主要元素组成和岩石学模拟将结合支持微量元素、放射性成因(Sr-ND-Pb-Hf-Os)和稳定的氧同位素数据，以约束产生不同包体的母岩浆(屏蔽玄武岩或屏蔽后碱性玄武岩)的性质，以及这些岩浆聚集和分馏的深度。这将提供夏威夷岩浆管道如何随时间演变的4-D图像，以及它是否在Kea和Loa趋势火山之间变化。将同位素和微量元素数据与矿物主要元素数据相结合，将使我们能够研究夏威夷捕虏体的总体同位素组成和同位素变异性如何随着形成深度、熔体分馏程度或火山阶段的变化而变化。要解决的具体问题包括：1)夏威夷岩浆在多深的范围内形成池塘和分馏？在火山活动的不同阶段(例如，在Kea趋势火山和Loa趋势火山之间)，或者在个别火山内部(例如，屏蔽和后屏蔽火山作用)，岩浆存储深度是否显著不同？2)火山建筑物内的地壳同化、熔融/岩石圈相互作用或“自同化”在多大程度上影响夏威夷熔岩的最终成分，以及这在夏威夷屏蔽阶段、后屏蔽阶段和复兴阶段熔岩之间有何不同？3)岩浆混合和均一化在抑制来自离散熔体批次的小规模不均匀方面起到什么作用？深成包体能否提供对夏威夷羽流不均匀性质和长度尺度的更多洞察？这项研究有望对我们理解夏威夷岩浆的储存、混合和同化过程做出重大贡献。

Compositional variations in lavas erupted from volcanoes such as those that have built the Hawaiian Islands provide insights into the processes that generate magma in Earth's interior. Compositional variations in different portions of Earth's mantle inferred from variations in magma composition also provide insights into how processes such as plate tectonics and mantle convection have shaped the long-term evolution of Earth's interior, which ultimately also influences the evolution of Earth's surface environment and life itself. However, magma storage, mixing, and interaction with crustal rocks can affect the compositions of magmas before they erupt at the surface. These processes can also influence whether volcanic eruptions are explosive or not, and thus influence volcanic hazards. This project will study the compositions of crystalline aggregates (or cumulates) brought to the surface by erupting lava flows in Hawaii. These cumulates are probably formed at depth within Hawaiian magma is stored temporarily and are later were carried to the surface by Hawaiian magmas. The compositions of these crystalline cumulates provide a record of the depths of magma storage and processes such as magma cooling and crystallization, mixing of discrete magma batches, and assimilation of the surrounding rock during magma ascent. The results of this study will provide a better understanding of the so-called "plumbing system" of Hawaiian volcanoes, allow us to better interpret the compositional observed in erupted lavas, and may help us explain why many Hawaiian volcanoes appear to alternate between periods of non-explosive eruptions (low volcanic hazard) and explosive eruptions (much higher volcanic hazard). Results of this research will be incorporated into outreach lectures and exhibits, specifically the "Hot Science Cool Talks" program and the annual "Explore University of Texas" open house, both of which draw a large number of local elementary-, middle- and high-school-age children to the institution. This project will also support the training and development of both graduate and undergraduate students, preparing them for careers in the Geologic Sciences.This project will examine geochemical variations in xenoliths hosted in recent post-shield-stage lavas from Hualalai and Mauna Kea volcanoes. These plutonic samples, which include rare fragments of Pacific oceanic crust as well as cumulates of Hawaiian shield- and post-shield magmas, provide powerful constraints on the temporal evolution and spatial variation of Hawaiian magma storage and transport systems, and will allow us to evaluate the potential roles of melt/lithosphere, melt/crust, and melt/edifice interaction in generating chemical and isotopic signatures (or scatter) in Hawaiian lavas. Mineral major element compositions and petrologic modeling will be combined with supporting trace element, radiogenic (Sr-Nd-Pb-Hf-Os) and stable O isotope data to constrain the nature of parental magmas (shield tholeiities or post-shield alkali basalts) that produced different xenolith populations, and the depths at which these magmas ponded and fractionated. This will provide a 4-D picture of how Hawaiian magmatic plumbing evolves over time, and whether it varies between Kea- and Loa-trend volcanoes. Integration of isotopic and trace element data with mineral major element data will allow us to examine how both the overall isotopic compositions and isotopic variability in Hawaiian xenoliths varies as a function of formation depth, degree of melt fractionation, or volcanic stage. Specific questions to be addressed including 1) Over what range of depths do Hawaiian magmas pond and fractionate? Does magma storage depth vary significantly from volcano to volcano (e.g., between Kea-trend and Loa-trend volcanoes) or within individual volcanoes during different stages of volcanism (e.g., shield and post-shield volcanism)? 2) To what extent does crustal assimilation, melt/lithosphere interaction, or "self assimilation" within the volcanic edifice affect the final compositions of Hawaiian lavas, and how does this vary between Hawaiian shield-stage, post-shield-stage, and rejuvenation-stage lavas? 3) What role does magma mixing and homogenization play in damping small-scale heterogeneities from discrete melt batches? Can plutonic xenoliths provide additional insight into the nature and length-scales of Hawaiian plume heterogeneities This research is expected to contribute significantly to our understanding of Hawaiian magma storage, mixing, and assimilation processes.