Quantifying the lithological and thermal properties of the mantle using basalt chemistry

As the primary flux of material from the mantle to the surface, the basalts erupted at mid-ocean ridges (MORB) are a key resource for investigating the mantle's chemical composition. However, despite the large volumes of oceanic lithosphere returned to the mantle by subduction, it has proven difficult using basalt chemistry alone to quantify this material's involvement in melt production. Even more enigmatic is the signal of refractory material in the source, which may barely melt if other more fusible lithologies are present and so be difficult to identify from many common chemical tracers. Here we demonstrate how combining thermodynamic models of melting, the density of phase assemblages at high pressure and geochemical observations, can allow the proportion of refractory and enriched material in the mantle source to be estimated and place limits on mantle potential temperature. We focus on determining the abundance of recycled material in the mantle beneath Iceland, where we have excellent geophysical and geochemical constraints on the melting process and the chemical variability in the mantle. Firstly, the lithologies contributing to melting are identified by quantitative comparison of the major element composition of erupted basalts to a database of experimental partial melts (Shorttle and Maclennan, 2011). Secondly, a mass balance is calculated between the endmember basalt compositions and the fully mixed melt to obtain the relative proportion, by mass, of enriched and depleted melts. A three-lithology melting model is then developed (peridotite-harzburgite-basalt), which uses the appropriate melting parametrisations to account for the differences in productivity between each lithology. The melting model enables the calculated abundance of the different endmember melt compositions to be projected back into mass fractions of solid mantle domains. Applying this method to Iceland demonstrates that ~10% of the source is recycled basaltic material and at least 20% must be highly refractory and essentially un-melting. Combining geophysical constraints with the modelled high pressure densities of the three lithologies' assemblages constrains excess mantle temperature beneath Iceland to be at least 150°C. We extend the crustal thickness and geochemical constraints south along the Reykjanes ridge to show that Iceland represents a long wavelength lithological and thermal anomaly in the mantle - and that both lithology and temperature must be varying along ridge to match observations. Density modelling shows that the proportion of recycled basaltic material carried in the Iceland plume is near the limit of what maintains plume buoyancy in the shallow mantle. References: Shorttle O. and Maclennan J. Compositional trends of Icelandic basalts: Implications for short-lengthscale lithological heterogeneity in mantle plumes. Geochemistry, Geophysics and Geosystems, 12(11):Q11008, 2011.