The volatile contents of melt inclusions and implications for mantle degassing and ocean island evolution
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The amount of volatile elements dissolved in silicate melts is a controlling factor in a range of geologic processes, which include hazardous volcanic eruptions, economically-significant ore-forming systems, and global-scale volatile fluxes, which contribute to planetary evolution. While melt volatile contents are important, estimating the origin and fate of volatiles distributed within magmas is challenging because volatiles exsolve from the melt during eruption and are transferred into the atmosphere. Therefore, the stratigraphic record of volcanic and intrusive deposits does not contain direct information regarding the pre-eruptive volatile content of the melt. However, melt inclusions trapped within growing phenocrysts present an opportunity to sample the melt before it has completely degassed. Analysis of melt inclusions is challenging owing to a range of processes which occur after the melt inclusion is trapped and which overprint the original texture and composition of the inclusion at the time of entrapment. Thus, efforts to accurately determine the current composition of the melt inclusion sample and then infer the original composition of the trapped melt which that inclusion represents require a combination of microanalytical, numerical, and/or experimental methods. In Chapter 1, we present a pedagogical approach for estimating the processes that affect the CO2 content of a magma from its origin during melting a C-bearing source material to its exsolution into a free fluid phase during crystallization and degassing. In Chapter 2, we explore different experimental, microanalytical, and numerical methods which may be used to estimate the CO2 contents of melt inclusions that contain fluid bubbles and describe the advantages and disadvantages of each approach. In Chapter 3, we apply some of the methods discussed in the previous chapters to estimate the pre-eruptive volatile content of Haleakala Volcano (Maui) and assess different melting mechanisms that may be active in the Hawaiian plume.
General Audience Abstract
Volcanoes are features which form on the Earth’s surface and are located above regions where material melts tens of kilometers (or more) below the surface. The process of melting is studied through laboratory experimentation, and therefore it is possible to estimate the composition of deep subsurface material based on the compositions of volcanic rocks which can be sampled on the Earth's surface. This sub-discipline of geologic research is called "igneous petrology." A fundamental problem in igneous petrology is estimating the volatile content of the Earth's deep interior. Volatile elements are those elements such as hydrogen and carbon, which are stable as gasses in the atmosphere rather than in the mineral components of a rock. It is thought that the gasses produced from volcanic vents, of which the compositions are well known, represent volatile elements which were originally present as dissolved components in the melt. Experiments performed on volcanic rocks have demonstrated that volatile elements can be dissolved in melts at high pressures corresponding to depths within the Earth's crust, and these elements exsolve from the melt when it approaches the surface -- similar to how CO2 can be dissolved in a carbonated beverage, which bubbles out when the beverage is opened. The only geologically-persistent features which preserves the pre-eruptive volatile content of a melt (i.e. how much gas was dissolved before eruption) are droplets of melt which are accidentally trapped within crystals that grow from the melt as it cools near the Earth's surface -- these are called "melt inclusions." While melt inclusions are useful in this regard, they are challenging to apply to geologic problems because they undergo a range of physical and chemical changes after they are trapped, which can alter their composition from the original composition of the melt that was trapped. This dissertation concerns the theory used to infer how volatile elements are distributed within the deep Earth, analytical and numerical methods used to gather relevant information from melt inclusion samples, and an application of these methods to investigate the volatile content of the mantle below Hawaii. Chapter 1 describes a framework for systematically determining the amount of CO2 distrubuted within a given volcanic setting. Chapter 2 compares different methods used to estimate the original volatile content of melt inclusions from Kamchatka, which have formed fluid bubbles -- a common feature present in melt inclusions. Chapter 3 applies the methods described in the first two chapters to estimate how volatile elements are distributed within the Earth's mantle below Hawaii, and how the process of melting transfers them to the Earth's atmosphere.
- Doctoral Dissertations