Recycled carbonate-bearing silicate sediments in the sources of circum-Mediterranean K-rich lavas: evidence from Mg-Zn isotopic decoupling

The high flux of subaerial volcanic CO₂ emissions around the circum-Mediterranean region requires the involvement of an unusually carbon-rich reservoir, but the origin of which is still unclear. Here, we aim to resolve this problem by analyzing Mg and Zn isotopes for the widely distributed mafic potassic to ultrapotassic lavas in this region. These K-rich lavas have lower δ²⁶Mg but similar δ⁶⁶Zn compared to mid-ocean ridge basalts (MORB). No known magmatic processes can explain the isotopic data, which must therefore be characteristics of the mantle sources. Recycled carbonate sediments are capable of explaining the low δ²⁶Mg, but they typically also have high δ⁶⁶Zn. Thus, the low δ²⁶Mg but unfractionated δ⁶⁶Zn of these K-rich lavas define “Mg-Zn isotopic decoupling” which has not yet been observed for other types of mantle-derived lavas. The carbonate-bearing silicate sediments analyzed here possess low δ²⁶Mg and MORB-like δ⁶⁶Zn, which can account for the Mg-Zn isotopic decoupling. Therefore, the nature of recycled materials (carbonates vs. carbonate-bearing silicate sediments) in the mantle can be distinguished by the coupling or decoupling of Mg and Zn isotopes of mantle-derived magmas. The input flux of carbon from the sediments to the lithospheric mantle is estimated to be ∼8.1 Mt/yr, and ∼22.4 Mt/yr of CO₂ emissions are predicted, which fit well with the observed output flux of 20.1 ± 13.4 Mt/yr. Our results demonstrate that recycled crustal carbon stored in the lithospheric mantle is an important source for the extensive subaerial volcanic CO₂ emissions in the circum-Mediterranean region.