Crystal-scale records of the Gakkel Ridge magma plumbing system

The global mid-ocean ridge system is the site of 75% of Earth’s volcanism. Whilst mid-ocean ridges have been studied for decades, ultraslow-spreading ridges, such as the Gakkel Ridge have received comparably little attention. These ultraslow-spreading ridges are important because, in addition to representing a spreading ridge end-member, they are also volumetrically significant and make up ~36% of the global mid-ocean ridge system. Despite this, we have very few constraints on the physical nature of the underlying magma plumbing systems or the magmatic processes occurring within them. To address this knowledge gap, this thesis presents the first systematic and quantitative analysis of crystal cargo textures and compositions of ultraslow-spreading mid-ocean ridge basalt (Gakkel Ridge, Arctic Ocean). These data are used to infer the physiochemical conditions present within the Gakkel Ridge magma plumbing system. Using the textures and compositions of >1800 plagioclase crystals, the Gakkel Ridge crystal cargo is shown to be complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. These textures, combined with basalt modal crystal contents, indicate that the crystal cargo is not generally phenocrystic in origin, and that undercooling, magma mixing, decompression and mush disaggregation are important processes occurring within the magma plumbing system. To constrain the depths of crystallisation within the Gakkel Ridge magma plumbing system, volatile contents of olivine- and plagioclase-hosted melt inclusions have been determined. The volatile contents of plagioclase-hosted melt inclusions correspond to significantly higher crystallisation pressures (mean 270 MPa) than olivine-hosted melt inclusions (mean 145 MPa). The higher pressures recorded in plagioclase-hosted inclusions, which correspond to crystallisation as deep as 16.4 km below the seafloor, are consistent with both the proposed thickness of the Gakkel Ridge lithosphere and pressures reconstructed from glass compositions. Contrary to previous studies using olivine-hosted melt inclusions alone, these results demonstrate that mid-ocean ridge volcanoes, at least at ultraslow-spreading ridges, have magmatic roots deep in the lithospheric mantle. The observed dichotomy in pressure recorded in olivine vs. plagioclase indicates that a multi-mineral melt inclusion approach is required to constrain the full range of crystallisation depths present in magmatic systems. Examination of along axis changes in mineral textures, mineral and melt geochemistry, melt inclusion crystallisation pressures, crystal content and ridge bathymetry demonstrates that the nature of the plumbing system and processes occurring within it are variable. There are no systematic changes with decreasing spreading rate along-axis. Instead, variations in the extent of partial melting and the degree of melt focussing along the base of the lithosphere play a key role in influencing the nature of magma plumbing systems at the Gakkel Ridge. Taken together, this thesis builds on our growing understanding of the complexity of processes occurring within mid-ocean ridge magmatic systems. It demonstrates that, contrary to the often-held view that mid-ocean ridges are nominally simple and have shallow magma plumbing systems, the observed textural and compositional complexity and crystallisation depths attest to the presence of vertically extensive plumbing systems within which complex magmatic processes occur