Abstract Interaction between magma and crustal carbonate at active arc volcanoes has recently been proposed as a source of atmospheric CO 2 , in addition to CO 2 released from the mantle and subducted oceanic crust. However, quantitative constraints on efficiency and timing of these processes are poorly established. Here, we present the first in situ carbon and oxygen isotope data of texturally distinct calcite in calc-silicate xenoliths from arc volcanics in a case study from Merapi volcano (Indonesia). Textures and C-O isotopic data provide unique evidence for decarbonation, magma-fluid interaction, and the generation of carbonate melts. We report extremely light δ 13 C PDB values down to −29.3‰ which are among the lowest reported in magmatic systems so far. Combined with the general paucity of relict calcite, these extremely low values demonstrate highly efficient remobilisation of crustal CO 2 over geologically short timescales of thousands of years or less. This rapid release of large volumes of crustal CO 2 may impact global carbon cycling.