Magma chambers emplaced in carbonate substrate: Petrogenesis of skarn and cumulate rocks and implications for CO2 degassing in volcanic areas.

Crystal-rich lithic clasts occurring in volcanic deposits are key tools to understand processes of storage, cooling, and fractionation of magmas in pre-eruptive volcanic systems. These clasts represent snapshots of the magma-chamber-host-rock interface before eruption and provide information on crystallization, differentiation, and degrees of interaction between magma and wall-rock. In this study we have focused on the petrology of clasts of cumulate and skarn rocks from the Colli Albani Volcanic District with the aim of shedding light on magma-carbonate interaction and CO2 emission in volcanic areas. By means of phase relations, bulk-rock chemistry, mineral compositions and isotope data we have identified different types of cumulates and skarns. Cumulates containing either clinopyroxene +/- olivine associated with Cr-bearing spinel or glass + phlogopite have been classed as primitive and differentiated, respectively. Cumulates originate at the interface between either a primitive or differentiated magma and carbonate-bearing wall-rock characterized by the occurrence of CaO-rich melt. Skarns have been classed as exoskarns, characterized by xenomorphic textures and abundant calcite, and endoskarns, characterized by a hypidiomorphic texture, Ca-Tschermak-rich mineral phases, and interstitial glass. Exoskarns formed by means of solid-state reactions in a dolomite-bearing protolith whereas endoskarns crystallized from a silicate melt that experienced exoskarn assimilation. Our study indicates that magma-carbonate interaction is a multi-step process that proceeds beyond the formation of skarn shells. Magma and carbonate rocks, when in contact, continuously interact leading to the formation of exoskarns, endoskarns, cumulates (primitive and differentiated types), and differentiated melts. The geochemical characteristics of the studied endoskarn and cumulate rocks indicate that crustal contamination of the Colli Albani magmas occurs through the simultaneous assimilation of both solid crustal material (dolomite and/or exoskarns) and partially molten crustal material (CaO-rich melt). The oxygen and carbon isotope compositions of calcite in equilibrium with the skarns suggest that dolostone-limestone assimilation and decarbonation are able to provide the massive CO2 release observed in carbonate-hosted magmatic systems, such as Colli Albani.