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

夏威夷群岛等火山喷发出的熔岩的成分变化，为我们了解地球内部岩浆的形成过程提供了线索。从岩浆组成的变化推断出的地幔不同部分的成分变化，也为板块构造和地幔对流等过程如何塑造了地球内部的长期演化提供了见解，这些过程最终也影响了地球表面环境和生命本身的演化。然而，岩浆的储存、混合以及与地壳岩石的相互作用会影响岩浆在地表喷发之前的成分。这些过程还可以影响火山喷发是否为爆炸性喷发，从而影响火山危害。该项目将研究由夏威夷火山喷发的熔岩流带到地表的结晶聚集体（或堆积物）的组成。这些堆积物可能形成于夏威夷岩浆暂时储存的深处，后来被夏威夷岩浆带到地表。这些结晶堆积物的成分提供了岩浆储存深度和过程的记录，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周围岩石的同化。这项研究的结果将使我们更好地理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（低火山危险性）和爆炸性喷发（高火山危险性）之间交替出现。这项研究的结果将纳入外展讲座和展览，特别是“热门科学酷谈话”计划和年度“探索德克萨斯大学”开放日，这两个项目都吸引了大量当地的小学，初中和高中年龄的孩子到该机构。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Hualalai和Mauna Kea火山最近的后盾期熔岩中捕获的捕虏体的地球化学变化。这些深成岩样品，包括太平洋地壳的稀有碎片以及夏威夷盾状和后盾状岩浆的累积，为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/大厦相互作用在夏威夷熔岩中产生化学和同位素特征（或散射）的潜在作用。矿物主元素组成和岩石学建模将结合辅助微量元素、放射性成因（Sr-Nd-Pb-Hf-Os）和稳定O同位素数据，以约束产生不同捕虏体种群的母岩浆（盾构玄武岩或后盾构碱玄武岩）的性质，以及这些岩浆的沉积和分选深度。这将提供夏威夷岩浆管道如何随时间演变的4d图像，以及它是否在Kea和loa走向的火山之间有所不同。同位素和微量元素数据与矿物主元素数据的整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化如何随着地层深度、熔体分馏程度或火山阶段的变化而变化。需要解决的具体问题包括：1)夏威夷岩浆在多大的深度范围内聚集和分馏？不同火山（例如，Kea-trend火山和Loa-trend火山）之间的岩浆储存深度是否有显著差异，或者在火山作用的不同阶段（例如，盾状火山和后盾状火山）中单个火山的岩浆储存深度是否有显著差异？2)地壳同化、熔体/岩石圈相互作用或火山大厦内的“自我同化”在多大程度上影响夏威夷熔岩的最终组成，这在夏威夷盾构阶段、后盾构阶段和复兴阶段的熔岩之间有何不同？3)岩浆混合和均质化在抑制离散熔体批次的小尺度非均质性中起什么作用？深成捕虏体能否为夏威夷岩浆柱非均质性的性质和长度尺度提供更多的见解？这项研究有望对我们了解夏威夷岩浆的储存、混合和同化过程作出重大贡献。

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.