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The TiO2 and MgO contents in the rocks from present-day plume (Iceland and Hawaii) and island arc (Sunda and Aegean Arcs) settings (A) and from Archean cratons (B). The chemical composition data of these rocks are from the GeoRoc database. The samples from Hekla (Iceland) (15), Afar hotspot (East Africa), and Agung (Sunda Arc) (19) are shown in A. The Inset in A illustrates the typical fractionation path inducing the TiO2 variations in plume lavas (18) and island arc lavas (14).
