Basalt genesis and mantle structure studied through Th-isotopic geochemistry

The relative importance of the degree of partial melting and of the composition of the source; is investigated for basalts of various origins on the basis of observed Th isotopic variations. If the oceanic island basalts (OIB) and the mid-oceanic ridge basalts (MORB) are not mutually contaminated, the Th isotopic tracers show that the difference between MORB and OIB is due to a difference in the composition of their sources rather than in the degree of partial melting. The characteristics of the island arc volcanics are completely different from those for the oceanic basalts, thereby implying a difference in both their origin and their geochemical process of petrogenesis.