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Publication - Professor Tim Elliott

    Investigating ocean island mantle source heterogeneity with boron isotopes in melt inclusions

    Citation

    Walowski, KJ, Kirstein, LA, De Hoog, JC, Elliott, TR, Savov, IP, Jones, RE & , 2019, ‘Investigating ocean island mantle source heterogeneity with boron isotopes in melt inclusions’. Earth and Planetary Science Letters, vol 508., pp. 97-108

    Abstract


    Recycling of the lithosphere via subduction drives the trace element and isotopic heterogeneity of the mantle, yet, the inventory of volatile elements in the diverse array of mantle reservoirs sampled at ocean islands remains uncertain. Boron is an ideal tracer of volatile recycling because it behaves similarly to volatiles during high-temperature geochemical reactions and carries a distinctive isotope signature into the mantle, but is subsequently little-influenced by degassing on return to the surface. Furthermore, B-rich recycled lithologies will have a strong influence on typical upper mantle compositions characterized by low B concentrations (<0.2 μg/g and δ
    11
    B −7.1 ± 0.9‰). Here, we present and compare the B abundances and isotope compositions, together with the volatile element contents (H
    2
    O, CO
    2
    , and Cl) of basaltic glasses and olivine-hosted melt inclusions from two different ocean island localities (La Palma, Canary Islands, and Piton de Caille, La Réunion Island). Our results suggest that olivine hosted melt inclusions are protected from contamination during ascent and provide more robust estimates of primary mantle source δ
    11
    B than previous bulk rock studies. We find that the δ
    11
    B of the La Réunion samples (−7.9 ± 0.5‰ (2σ)) overlaps with the recently defined MORB datum, indicating that the depleted upper-mantle and ‘primitive mantle’ reservoirs are indistinguishable with respect to δ
    11
    B, or that B concentrations are sufficiently low that they are diluted by partial melting in the uppermost mantle. In contrast, the La Palma samples, notable for their radiogenic Pb isotope ratios, are characterized by δ
    11
    B values that are distinctly isotopically lighter (−10.5 ± 0.7‰ (2σ)) than La Réunion or MORB. We suggest these isotopically light values are derived from significantly dehydrated recycled materials preserved in the La Palma mantle source region, in keeping with their lower B/Zr and H
    2
    O/Ce. This work therefore provides strong new support for subduction zone processing as a mechanism for generating radiogenic Pb isotopic signatures and volatiles heterogeneities in the mantle.

    Full details in the University publications repository