Ternary diagrams combining crustal thickness (tc) and geochemical constraints (Fpxmelt) to identify the allowable lithology combinations beneath Iceland. Each apex of the ternary represents an endmember lithology in the mantle: lherzolite (lz), harzburgite (hz) and pyroxenite (px, for (a) the KG1 composition from Kogiso et al., 1998, for (b) the G2 composition from Pertermann and Hirschmann, 2003). The three endmember lithologies are mechanically mixed in variable proportions and three-lithology melting calculations performed to fill in the ternary space for Fpxmelt and tc. Background colours in these diagrams correspond to Fpxmelt determined for model runs with Tp = 1480 °C. The dashed white line marks the lithology combinations melting to produce a tc = 20 km, the solid white line and grey shaded region mark the lithology combinations reproducing the observed Fpxmelt. The point of intersection of the solid and dashed white lines is the lithology mixture able to match both crustal thickness and geochemical constraints (in (a) this is lz71hz17px12, in (b) lz70hz22px8).

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