Magma storage, evolution, and mixing within Kīlauea volcano’s lower east rift zone

Abstract

Kīlauea volcano is well known for its frequent effusive eruptions both within the summit caldera and on its flanks along two rift zones. Recent activity within the East Rift Zone, the more active of the two, has damaged and destroyed communities located on the more distal portion of the rift, referred to as the Lower East Rift Zone. A pattern of rift zone eruptions beginning with cooler, more evolved lavas and then transitioning to more mafic compositions over time has been recognized at Kīlauea. The evolved lavas are derived from pockets of magma stored within the rift zone that differentiate in isolation between eruptions and are then remobilized by new intrusions. The eruption that occurred in the Lower East Rift Zone in 2018 involved the remobilization of two different evolved lavas: a differentiated basalt at the beginning of the eruption and then an andesite thirteen days later along a separate fissure line. The first chapter of this dissertation provides background on Hawaiian volcanism and the history of the Lower East Rift Zone. The second chapter focuses on the andesite, and mafic enclaves that were found within it. We analyzed the textural and geochemical properties of the enclaves and host lavas and compared them to other historical lavas from the Lower East Rift Zone to determine the source of the enclaves. We concluded that the source was the basalt that simultaneously mixed to form homogenous basaltic-andesite at other portions of the same fissure and erupted contemporaneously from adjacent fissures. We developed a computational model for enclave thermal equilibration within the andesite to explain how the preservation of enclaves and homogenization of the same two magmas could occur along the same fissure as a result of locally varying mixing percentages of the andesite and basalt. Chapter 3 focuses on the differentiated basalt erupted at the beginning of the 2018 eruption. We used whole rock, mineral, and glass major and trace element compositions to evaluate a potential source for this magma and concluded that it was most likely a remnant of the magma involved in the beginning of the 1955 eruption. The small compositional difference between the connected magmas was used to suggest a slow cooling rate for the magma body of roughly ~0.1 °C/year. Chapter 4 considers a magma that did not erupt in the Lower East Rift Zone and instead was encountered in the subsurface during geothermal drilling. This magma, a rhyolite, is the most evolved material ever sampled at Kīlauea, and its discovery was the first instance of a melt being encountered during drilling. We characterize the petrography and geochemistry of this sample and consider the possible origins of the magma. This unique sample, collected at a precisely known depth, is utilized to perform a spot-check on the accuracy of petrologic tools used to estimate magma storage pressures and the models for temperature and water content they depend on. Chapter 5 provides a summary of the broader conclusions of this work and some suggestions for future avenues of investigation. The work described in this dissertation contributes to an enhanced understanding of processes during magma storage and magma mixing within Kīlauea’s rift zones and to a broader discussion of how to leverage the best tools for making interpretations about the origins of magmas and pre-eruptive storage conditions.