The Icelandic mantle is lithologically and chemically heterogeneous. This had led to the identification of geochemically enriched and depleted domains within the Icelandic mantle. Enriched domains have been prescribed to the presence of recycled oceanic lithosphere in the form of pyroxenite. Halogens are highly incompatible and fluid mobile elements which are concentrated in the Earth's crust, particularly in altered oceanic crust and serpentinised oceanic lithosphere from hydrothermal alteration. During subduction these halogen-rich domains are returned to the mantle in the down-going slab. Therefore halogens are potentially excellent tracers of recycled oceanic lithosphere in the mantle. This thesis uses the halogen concentration of melt inclusions and basaltic glasses from Iceland and the Reykjanes Ridge to address this potential. Firstly, the influence of pyroxenite-derived melts to melt compositions across Iceland is assessed. Pyroxenite-derived melts have the greatest influence in the flank zones of Iceland, far from the rift and plume centre. Concentrations of halogens measured in melt inclusions and glasses from Iceland are highest in the flank zones. Additionally, the highest halogen concentrations in glasses from the Reykjanes Ridge are in those closest to Iceland and correlate with enriched melt compositions. Therefore halogen concentrations appear to be a promising tracer of the influence of recycled oceanic lithosphere, with higher concentrations indicating higher influence on erupted melt compositions. Ratios of halogens to elements of similar compatibility show a different behaviour to the concentrations. The influence of recycled oceanic lithosphere has no influence on these ratios and they cannot be used as tracers of its contribution. Heterogeneity in the ratios can be used to identify recycled mantle domains with differing ratios. Differences in the ratios may be caused by individual recycled lithologies retaining unique halogen contents which can be identified in high density study of particular locations. On a global scale halogens are likely recycled throughout MORB and OIB mantle domains. Recycled oceanic lithosphere is likely present in the upper mantle and recycling is not confined to entrainment of residue slabs at the core-mantle boundary. Over time this recycling has created mantle halogen ratios which are similar to that of MORB. Thus on a global scale mantle halogen ratios are consistent, however smaller scale studies with high sampling density are able to identify heterogeneities which may reflect heterogeneity in subducted lithologies.
- mantle heterogeneity
- melt inclusion
Halogen heterogeneity in the Icelandic mantle source
Waters, E. (Author). 1 Aug 2022
Student thesis: Phd