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

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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.