Magma genesis and evolution of source composition during the weakening of Caroline mantle plume activity

The Caroline seamount chain consists of Chuuk (14.8～4.3 Ma), Pohnpei (8.7～＜1 Ma), Kosrae (2～1 Ma) islands and a series of seamounts as the result of late-stage mantle plume. Geochemical variations in the seamount chain can deepen the understanding of late activity of the mantle plume. The whole rock major- and trace-elements, electron probe mineral analyses of the samples from Chuuk and Kosrae islands were conducted, and the results were compared with published data of Pohnpei. Kosrae and Chuuk islands are composed of nephelinites and alkaline basalts, reflecting typical ocean-island alkaline basalts in trace element patterns. Olivine phenocrysts in the samples are Ni-enriched but Ca-Mn–depleted, which is similar to olivines from Hawaiian OIB (ocean island basalt), suggesting the existence of pyroxenite in the mantle source. The presence of carbonate melt inclusions in the olivine phenocryst (Fo=85 mol%) of Kosrae nephelinite indicates that CO₂ plays an important role in mantle melting and magma generation. The average La/Sm ratio of volcanic rocks gradually increases from Chuuk, Ponape, to Kosrae, which may reflect the decreasing degree of mantle melting during the weakening of the Caroline hot spot activity. In addition, the Nb/Nb^* ratio decreases with the increase of La/Sm, Sm/Yb ratios and the decrease of SiO₂, indicating the enhancing effect of CO₂ due to the decrease in mantle melting degree. Therefore, the continuous geochemical changes of volcanic rocks from Chuuk, Ponape, Kosrae islands are caused by the gradual weakening of Caroline mantle plume activity, during which CO₂ plays an increasingly obvious role in genesis of volcanic rocks.