Crystal-scale records of the Gakkel Ridge magma plumbing system

• Main Author: Bennett, Emma
• Format: Thesis
• Language: English
• Published: 2019
• Subjects: QE Geology; Arctic; Arctic Ocean; Gakkel Ridge
• Online Access: https://orca.cardiff.ac.uk/id/eprint/126866/; https://orca.cardiff.ac.uk/id/eprint/126866/1/Bennett_PhD_Thesis.pdf; https://orca.cardiff.ac.uk/id/eprint/126866/7/Appendix1-Hand_Specimen_descriptions.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/8/Appendix_2_Mineral_and_glass_EDS.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/9/Appendix3_Samples_Modal.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/10/Appendix_4_Plagioclase_LA-ICP-MS_Data.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/11/Appendix-5-SIMs-Raw_and_Corrected_Volatile.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/12/Appendix_6_Crystal_cargo_database.xlsx; https://orca.cardiff.ac.uk/id/eprint/126866/13/Publication_Form_Benett.pdf

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 ...