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Magma host-rock interactions are inevitable during a magmas journey from its formation to Earth's surface and may account for major changes in magma composition and volatile budget. Here, we focus specifically on the interaction of magma with carbonate host-rocks (limestones and dolostones) at mid-crustal depths (i.e., p = 0.6 GPa), representing the deeper zone of magmatic pre-eruptive systems. The interaction of magma with carbonate-bearing rocks is of particular interest, since their assimilation may add exsolved CO₂ to the magmatic mixture. Especially in settings with silica-undersaturated mafic melt compositions this might be the driver for unusually explosive eruptions.

To date, short-term magma-carbonate interaction experiments have been conducted only with low-viscosity melts (basaltic andesite and shoshonite). To investigate the possibility of carbonate assimilation being a triggering or sustaining (hence, short-term) process for explosive eruptions of more differentiated melts, we conducted magma-carbonate interaction experiments in an end-loaded piston cylinder at 1200°C and 950°C, representing upper and lower T-limits of typical phonolitic Vesuvian magma chambers, and varying interaction times (from 0s to 1h). All experimental samples have subsequently been analysed by Scanning Electron Microscopy (SEM) to monitor the extent of carbonate assimilation for all p-T-t combinations.

By comparing our results to previous works with lower viscosity melts it is evident that melt viscosity exerts a strong control on the efficacy of carbonate assimilation by magmas. The complete dissolution of the limestone or dolostone clast hasn't been achieved within 1h. Instead, we observe over time in the dolomite-bearing experiments the formation of a skarn-like assemblage with the following succession: normal phonolitic glass – Ca-&Mg-enriched glass – clinopyroxene zone – forsterite zone – periclase-bearing thermometamorphosed dolomite. In limestone-bearing experiments this skarn-like zoning pattern is absent and instead we observe over time a much larger area with Ca-enriched glass and mingling (or homogenization) patterns.