The volatile contents of melt inclusions and implications for mantle degassing and ocean island evolution

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.