Evolution of carbonated melt to alkali basalt in the South China Sea

CO2 is considered to play a key role in the melting of the deep upper mantle, and carbonated silicate melts have been widely predicted by partial melting experiments to exist at mantle depths of greater than 80 km. However, such melts have not been shown to exist in nature. Thus, the relationship between CO2 and the origin of silicate melts is highly speculative. Here we present geochemical analyses of rocks sampled from the South China Sea, at the Integrated Ocean Discovery Program Site U1431. We identify natural carbonated silicate melts, which are enriched in light rare earth elements and depleted in Nb and Ta, and show that they were continuously transformed to alkali basalts that are less enriched in light rare earth elements and enriched in Nb and Ta. This shows that carbonated silicate melts can survive in the shallow mantle and penetrate through the hot asthenosphere. Carbonated silicate melts were converted to alkali basaltic melts through reactions with the lithospheric mantle, during which precipitation of apatite accounts for reduction of light rare earth elements and genesis of positive Nb-Ta anomalies. We propose that an extremely thin lithosphere (less than 20 km in the South China Sea) facilitates extrusion of the carbonated silicate melts, whereas a thickened lithosphere tends to modify carbonated silicate melt to alkali basalt.