Extreme short scale mantle heterogeneity and its significance for probing mantle plume physics

Many geochemical and geophysical investigations of mantle structure are based upon an assumption of compositional homogeneity on lengthscales between ~100 km and the grain scale. However, it has been known for decades that the isotopic composition of basalts from individual volcanic systems can vary over limited spatial and temporal scales. More recently, investigation of the Pb-isotopic composition of olivine hosted melt inclusions in Icelandic basalts has revealed extreme compositional variation within the convecting mantle source regions of individual eruptions[1]. The relationship between isotopic variations and major element compositions on Iceland, and the nature of the response of Icelandic mantle melting to glacial unloading, indicates that the isotopic heterogeneity of the mantle is correlated with mineralogical variations [2,3]. Geochemically enriched domains are more fusible than their depleted counterparts and may reflect the presence of recycled basalt in the convecting mantle. With constraints on the composition and prevalence of these mantle heterogeneities, it is then possible to improve models of mantle temperature variations and plume fluxes under the north Atlantic [2,4]. [1] Maclennan, J. (2008) Lead isotope variability in olivine-hosted melt inclusions from Iceland. Geochimica et Cosmochimica Acta, 72 (16). pp. 4159-4176. [2] Shorttle, Oliver and Maclennan, John (2011) Compositional trends of Icelandic basalts: Implications for short-lengthscale lithological heterogeneity in mantle plumes.Geochemistry Geophysics Geosystems, 12 . Q11008. [3] Sims, K.W.W., J. Maclennan, J. Blichert-Toft, E.M. Mervine, J. Blusztajn and K Grönvold, Short length scale mantle heterogeneity beneath Iceland probed by glacial modulation of melting, Earth Planet. Sci. Lett., 379, 146-157, 2013 [4] Jones, S.M., Murton, B.J., Fitton, J.G., White, N.J., Maclennan, J. and Walters, R.L., A joint geochemical-geophysical record of time-dependent mantle convection south of Iceland, Earth Planet. Sci. Lett., 386, 86-97, 2014.