Magma–carbonate country rock interaction can provide H2O during magma ascent: Results from fluid and melt inclusions in skarn xenoliths from Breccia Museo, Campi Flegrei (Southern Italy)

In this paper, the results of a study on fluid inclusions (FIs) and melt inclusions (MIs) hosted in skarn-bearing minerals sampled in the Breccia Museo deposits at Campi Flegrei (Southern Italy) are reported and discussed to investigate magma‑carbonate interactions. The Campi Flegrei shallow magma chamber fluid environment has been interpreted as similar to that documented in the magmatic-hydrothermal systems associated with porphyry copper deposits. Skarns form along the top and sides of the magma chamber in the brittle-plastic transition zone, where magmatic fluids accumulate through magmatic vesiculation and magma interacts with carbonate country rocks. Melt inclusions, here named saline‑carbonate-melt inclusions (SCMI), trapped in olivine, provide crucial insights into the melt that forms in this setting. Heating/cooling experiments on SCMI show that they trapped a homogeneous melt that, on cooling, undergoes instantaneous unmixing, leading to the formation of three immiscible liquids: silicate, carbonate and hydrosaline (brine). The melt behavior on micrometer scales, in SCMI, is assumed reproduces what happens on a large scale, this means that in the transition zone melt remains homogeneous at T > 980 °C and instantly unmixes when cooled below 790 °C. To account for the instant unmixing and the absence of CO₂ in SCMI shrinkage bubbles, we propose that at high T the reaction of CaCO₃ with H₂O (by magma second boiling) produces Ca(OH)₂ and H₂CO₃ that dissolve in the homogeneous melt and prevent the formation of CO₂. At lower T by unmixing, CaCO₃ re-forms, releasing H₂O. The carbonate plays an essential role as it removes at high T, one mole of CO₂ from homogeneous melt and simultaneously releases two moles of H₂O at lower T when unmixing occurs. We argue that, during magma ascent, this water supply can facilitate the upward propagation of dyke to the surface and can enhance explosivity, during an ongoing eruption.