Tracing Subducted Carbonates in Earth's Mantle Using Zinc and Molybdenum Isotopes

Abstract

Although carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high-precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high δ⁶⁶Zn values (+0.39 to +0.50‰) of these lavas require involvement of recycled carbonates in the mantle source. Variable δ⁹⁸Mo compositions (−0.39 to +0.27‰) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between δ⁶⁶Zn and δ⁹⁸Mo. Modeling reveals that involvement of ≤5% carbonate-bearing oceanic crust fully resolves the observed δ⁶⁶Zn–δ⁹⁸Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined δ⁶⁶Zn–δ⁹⁸Mo data sets for mantle-derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science.