The role of carbon from recycled sediments in the potassic magmatism of the Central Mediterranean region: evidence for carbonate metasomatism in the mantle

The Central Mediterranean region is one of the most important region on Earth for studying subduction-related potassic and ultrapotassic magmatisms, deriving from partial melting of the metasomatised lithospheric mantle wedge. Mediterranean ultrapotassic igneous rocks are intimately associated in space and time with shoshonites and calc-alkaline igneous rocks and genetically related with the mechanisms that driven the geodynamic evolution of the region. The petrological, geochemical, and isotopic features of the erupted rocks are thought to be derived from the recycling within the mantle wedge of subducted sediments characterised by variable amounts of pelitic and carbonate components. Accordingly, subduction drags a large amount of CO2 into the Earth’s interior, which is partly returned to the atmosphere by volcanism. Subducted CO2 may dramatically affect the equilibria among peridotitic minerals (olivine vs. pyroxenes) changing their stability fields and hence their modal abundances. Trace elements in olivine in subduction-related mafic alkaline ultrapotassic rocks from Central Mediterranean region are used as a proxy to define mantle wedge mineralogy and metasomatic processes. Minor element concentrations, and in particular the high Li and low Ti of all the olivines, confirm a major role for recycled sediment in the generation of Italian ultrapotassic magmas. The distinct contents of Ni, Mn, and Ca in olivine reflect the bimodal character of silica-rich and silica-poor ultrapotassic Italian rocks and constrain two distinct mineralogical reactions between metasomatic agents and peridotite. Olivine chemistry from silica-saturated rocks reflects the reaction of silicate melts with the ambient mantle, with consequent consumption of olivine in favour of orthopyroxene. In contrast, the low-Ni, high-Mn/Fe of olivine crystallised from silica-undersaturated leucitites require a mantle source enriched in olivine (and clinopyroxene) compared to orthopyroxene, as a result of the interaction between the ambient peridotitic mantle and CaCO3-rich metasomatic agents. The change from silica-oversaturated lamproites to silica-undersaturated leucitites and thus the difference in the olivine composition is due to a change in composition of the subducting sediment from pelitic to carbonate-rich. The results of this study provide new insights into how CO2 is recycled via subduction processes deep into the mantle.