Crystal-scale control on magmatic mush mobilization and mixing

Abstract

Magmatic systems present an apparent paradox: they exist as long-term, crystal-rich magmas (called mushes) which are mechanically locked, yet crystals in exposed plutons and volcanic deposits with diverse histories indicate magmatic mobilization and mixing processes are common. Geochemical and petrologic analyses of igneous rocks and crystals provide a way to distinguish magmatic events such as intrusion, crystallization, assimilation, and mixing that occurred. These events cannot be directly observed, so previous studies have used experiments and numerical simulations to investigate the dynamics of mobilization and mixing. The high- crystal fractions in mushes require consideration of particle-particle-liquid interactions, which previous continuum and quasi-multiphase models do not recover. This dissertation takes a multi-scale approach to understanding the processes of mush mobilization and mixing. Chapters 2 and 3 present a discrete element method-computational fluid dynamics (DEM-CFD) model of a basalt and olivine magmatic mush subject to intrusion by basaltic magma from below. Chapter 2 demonstrates the crystal-scale control on the system-wide response to the intrusion. The localized mobilization of crystals above the intrusion site produces a region called the mixing bowl, where liquids and crystals are fluidized and mixed. Monitoring the crystals and liquid throughout the intrusion demonstrates the potential for diverse crystal populations to be created in even simple magmatic systems. Chapter 3 quantifies the dispersion of crystals in the simulations from their initial state for a range of intrusion velocities. The crystal dispersion occurs with an exponential relationship with time, and a mixing time scaling produces a single curve for the tested intrusion rates. Extrapolating the results to a realistic magmatic system produces mixing times that agree with those inferred for mixing events occurring in nature. Chapter 4 is a case study of a natural mush, the 1868 picrite eruption of Mauna Loa, Hawaii. Geochemical analyses at the crystal-scale demonstrate the existence of six olivine populations. These populations reflect a diversity of magmatic conditions and processes within the central and rift magmatic systems in Mauna Loa. Also included with this dissertation are three supplementary movies. These movies show the simulation presented in Chapters 2 and 3. Movie 2.1 shows the intrusion of the magma into the mush, Movie 2.2 shows the coordination number of the crystals within the mush, and Movie 2.3 tracks the three pairs of crystals described in the text.