A thermodynamic and geochemical evaluation of magmatic processes beneath the Samoan Islands and the thermodynamics of crystalline garnet solutions
This dissertation attempts to grapple with the origin and evolution of chemical heterogeneity reflected in lavas erupted in ocean island settings. The approach taken here is to combine analytical geochemistry with thermodynamic, phase equilibria modeling to place better constraints on the composition of heterogeneities contributing to the mantle source region beneath the Samoan Islands, located in the South Pacific Ocean. Additionally, inherent in thermodynamic, phase equilibria modeling are the underlying liquid and crystalline solution models that are built from thermophysical and experimental data. These liquid and solid solution formulations must be adequately calibrated over a broad region of pressure, temperature, and compositional space to be able to successfully model a range of geologically relevant magmatic systems. Therefore, this dissertation also presents a newly calibrated ternary garnet solution model with applications to high pressure igneous processes in addition to updates to the Magma Chamber Simulator (MCS) - a thermodynamically constrained code that allows for simulations of complex magmatic processes, like simultaneous magma mixing, assimilation, and fractional crystallization (RAFC).
This dissertation consists of 5 chapters: (1) analysis of Sr and Nd isotopic heterogeneity in individual clinopyroxenes from a single Samoan lava, (2) major and trace element characteristics of clinopyroxene-hosted melt inclusions from the same lava as in Chapter 1, (3) a new calibration of a solid solution model for ternary garnets applicable to high-pressure igneous processes, (4) applications and new model developments related to major element systematics of combined RAFC processes in the MCS, and (5) the formulation and applications of trace element systematics of combined RAFC processes in the MCS.
Chapters 1 and 2 reveal new insights into the composition and evolution of the enriched mantle 2 (EM2) reservoir, thought to be the result of ancient, recycled continentally derived sediments incorporated into the Samoan mantle plume source. Given the new Sr and Nd isotopic constraints from clinopyroxenes within a single EM2-derived lava and binary mixing theory, EM2 derived mantle melts beneath Samoa are calculated to be trachytic in composition. Fortuitously, trachytic melt inclusions hosted in these clinopyroxenes from the same lava are remarkably similar to the independently calculated trachytic-derived endmember, suggesting they could be direct evidence of EM2-derived mantle melts. However, thermodynamic simulations of mantle sources that could give rise to these trachytic-melts show their formation is not so simple and more work need be done to evaluate their formation and evolution.
Chapter 3 introduces a newly calibrated ternary garnet solid solution model that uses the most up-to-date melt-present phase equilibria data to constrain a model relevant to high pressure igneous processes. The model presented here is shown to be a significant improvement over previously published models. This model is obtained through Bayesian statistical techniques that allow for a comprehensive evaluation of errors among all constrained solution parameters. Using this calibration procedure, the model presented here achieves a pressure-dependent miscibility gap along the pyrope-grossular binary join, in agreement with recent experiments that is not captured by any previously published models. Chapters 4 and 5 present updates and newly formulated techniques to track the major and trace element and isotopic systematics of open-system processes, pertinent to the first two chapters of this study. Notable features include new geochemical visualizations of magma evolution, the addition of 30 distinct magmatic recharge events, and supplemental calculator that interfaces with the MCS that tracks changes among trace elements and isotopes during combined RAFC process. These types of changes ultimately allow for a detailed quantification of complex magmatic process occurring beneath volcanos.