Post-spreading volcanism triggered by CO2 along the South China Sea fossil spreading axis

The post-spreading seamount chain formed along the fossil ridge of the South China Sea (SCS) consists of carbonated silicate melts and alkaline basalts. These temporally and spatially linked samples give an intriguing case where CO2-rich magmas and associated basaltic rocks occurred upon the ultra-thin oceanic lithosphere, and thus they could carry robust information on the generation of alkaline magma via melting of carbonated mantle source. Here we present magnesium isotopic data (delta Mg-26), and elemental and radiogenic isotope data for the lavas from the on-axis SCS seamount chain. Both the early-and late-stage samples classified based on their formation ages exhibit remarkable delta Mg-26 variations (-0.53 parts per thousand to -0.11 parts per thousand and - 0.38 parts per thousand to -0.23 parts per thousand, respectively). The low-delta Mg-26 signatures found in two groups of the SCS samples indicate a recycled origin for the source carbon inventory. However, the low-delta Mg-26 endmember of the early-stage samples is characterized by low SiO2 and enrichment in rare earth elements (REEs), whereas the late-stage samples display a low-delta Mg-26 end member that produced high-SiO2 and REE-depleted melts. The Mg-Nd isotope systematics suggest that the early stage lavas were yielded by CO2-enhanced melting of sediment-rich oceanic crust (epsilon(Nd) = 4.2), whereas the late stage lavas have sourced the residual and carbon-deficient oceanic crust (epsilon(Nd) = 8.7). Our model demonstrates that the melting of carbonated component can produce compositionally distinct carbonatitic and silicate melts in sequence due to different solidus temperatures. Owe to the relatively thin lithosphere, these two types of melt could individually appear along the fossil ridge of the SCS.