Mg-Zn isotopes track the role of recycled sediment composition in shifting silica-saturated to silica-unsaturated ultrapotassic rocks from the Central Mediterranean

To investigate the role of carbonate recycling in generating ultrapotassic magmatic rocks, we carried out detailed Mg[sbnd]Zn isotope analyses of silica-saturated to silica-unsaturated ultrapotassic volcanic rocks from the Central Mediterranean basin. These rocks have lower Mg isotopic compositions (δ26Mg = −0.40‰ to −0.21‰, n = 20) than the mantle and exhibit MORB-like Zn isotopic compositions (δ66Zn = 0.23‰ to 0.30‰, n = 16). Postmagmatic alteration, crustal assimilation, magmatic differentiation, and diffusion processes were found to have insignificant effects on Mg[sbnd]Zn isotope variations of the ultrapotassic rocks. Based on the relationships between δ26Mg and major and trace element contents and Nd isotopic compositions, the Mg isotope variations cannot be induced by adding carbonated eclogite to the mantle source. Instead, the low Fe/Mn, Hf/Hf⁎, Ti/Ti⁎ and high Ca/Al ratios and good correlations of Hf/Sm vs. Ca/Al and La/Yb vs. Ti/Eu suggest that the light Mg isotopic compositions of the ultrapotassic rocks were caused by carbonate melt metasomatism. In addition, the combined Mg[sbnd]Zn isotopic compositions suggest that the metasomatic agent is recycled carbonate-bearing silicate sediments. Silica-unsaturated ultrapotassic rocks have slightly lower δ26Mg values than silica-saturated rocks, which is consistent with the greater amounts of carbonates in the recycled carbonate-bearing silicate sediments. The highly radiogenic Sr and unradiogenic Nd isotopic compositions of the studied rocks and two end-member mixing models further document that the mantle sources contained silicate-rich components, possibly Italian basements. The transition from silica-saturated to silica-unsaturated ultrapotassic rocks corresponds to the transformation of sediments from metapelites to carbonated metapelites. Therefore, by integrating Mg–Zn–Sr–Nd isotopic compositions, this study constrains the composition of subducted sediments and documents the important role of carbonate melt metasomatism in shifting the compositions of ultrapotassic rocks from silica-saturated to silica-unsaturated.