Subducted Carbon From Mantle Plume in Mid-Ocean Ridge Basalts

Abstract

Deciphering the Earth's deep carbon cycle, from mantle plumes to mid-ocean ridges, remains incompletely understood. In this study, we analyze the magnesium isotope composition of basalts collected from the South Mid-Atlantic Ridge (SMAR), which have been influenced by the off-axis Saint Helena plume originating from the core-mantle boundary. The magnesium isotope composition of SMAR basalts falls within a similar range (−0.22 to −0.32‰; average −0.25‰ ± 0.03‰) to that of known global oceanic basalts. However, isotope mixing calculations suggest that the lighter magnesium isotope composition in the SMAR basalts is due to the incorporation of approximately 5%–10% recycled carbonate material carried by the Saint Helena plume into the SMAR asthenosphere. This finding not only highlights the interaction between ridges and off-axis plumes but also proposes a comprehensive model for the Earth's deep carbon cycle, spanning from the subduction zone through the core-mantle boundary to the mid-ocean ridge system.