Noble Gases and Halogens in Icelandic Basalts
Noble gas and halogen data from a suite of Icelandic samples are presented. Iceland combines hotspot volcanism, a spreading ridge and abundant subglacially erupted samples. This combination allows for samples that erupted under high enough pressures to retain a measurable mantle volatile content, an...
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| Format: | Doctoral or Postdoctoral Thesis |
| Language: | English |
| Published: |
2013
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| Online Access: | https://research.manchester.ac.uk/en/studentTheses/735b855b-bcb5-4528-9dc7-daaf2d7245d6 https://pure.manchester.ac.uk/ws/files/54534713/FULL_TEXT.PDF |
| Summary: | Noble gas and halogen data from a suite of Icelandic samples are presented. Iceland combines hotspot volcanism, a spreading ridge and abundant subglacially erupted samples. This combination allows for samples that erupted under high enough pressures to retain a measurable mantle volatile content, and also display signatures representing interaction between ocean island basalt (OIB) and mid-ocean ridge basalt (MORB) mantle sources.Erupted samples used to determine the mantle's halogen and noble gas content have undergone a degassing process that can alter their volatile composition. An existing disequilibrium degassing model is developed with the modified model taking into account the evolution of the major volatiles over a multi-stage process and the different conditions present during magma ascent and quenching. The modified model allows substantially lower elemental noble gas ratios to be reached under disequilibrium conditions than allowed by the original model. Initial CO2 concentrations, pressure, diffusivity, ascent rate and degree of disequilibrium are shown to be critical parameters for this model. Final degassed noble gas concentrations are most affected by the surface quenching stage of an eruption, whereas noble gas elemental ratios can be primarily determined during magma ascent. In applying this model to MORB and OIB sample suites, the 3He/22Ne ratio of the MORB source mantle is constrained to be lower than 4.4, similar to estimates for the OIB source mantle. Additionally the most straightforward match between the degassing model and OIB helium and neon data suggest the OIB source mantle has 3He concentrations similar to or lower than the MORB source mantle. This finding requires a model for the OIB source mantle in which a high 3He/4He component is added to a helium-poor protolith.Noble gas studies are hampered by the large, isotopically atmospheric component typically found in Icelandic subglacial samples, which can swamp other signatures. Detailed analysis of a volatile rich sample from SW ... |
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