Keywords:carbonated silicate melt, reactive evolution, alkali basalt, hotspot, Caroline seamount chain
The deep Earth’s interior is a large reservoir of carbon and deep-rooted oceanic hotspots usually produce CO2-rich magmas. The melting of mantle with CO2 is shown to produce Si-undersaturated alkalinn melts, and, thus, CO2 may play a key role in the origin of these magmas. However, volatiles tend to be lost during natural volcanism, and it is usually difficult to identify the role of these volatiles during the genesis of these lavas. Here we report geochemical analyses of a suite of volcanic rocks formed by the deep-rooted Caroline hotspot in the Pacific. The most primitive magmas have depletion of SiO2 and high field strength elements and enrichment of rare earth elements that are in concert with mantle-derived primary carbonated melts. The carbonated melts show compositional variations that indicate reactive evolution within the overlying mantle lithosphere, during which the evolving melts obtained depleted components from the lithospheric mantle. The carbonated melts were de-carbonated and modified to oceanic alkali basalts by precipitation of perovskite, apatite and ilmenite that significantly decreased the concentrations of rare earth elements and high field strength elements. These magmas also experienced a late stage of non-reactive fractional crystallization after the reactive evolution was completed. We propose that the carbonated melts would experience two stages, reactive and un-reactive, of evolution during their transport through in thick oceanic lithospheric mantle. The mantle lithosphere plays a key role in de-carbonation and conversion of deep-sourced carbonated melts to alkali basalts. This work was financially supported by the National Natural Science Foundation of China (91858206, 41876040).