The skarn shell of the 1944 Vesuvius magma chamber. Genesis and P-T-X conditions from melt and fluid inclusion data

Skarn rocks are a component of tephra ejected during the 1944 eruption of V esuvius. Three different types were recognized: 1. melilite-, 2. phlogopite - and 3. periclase-bearing, The presence of well preserved melt and fluid i nclusions and fresh interstitial glass, allowed the reconstruction of the p rocesses occurring during the formation of skarn rocks and the establishmen t of the P-T-X conditions within the magma chamber wall rocks. Skarn formed at temperatures of about 1000 and 800 degrees C in Type-1 and Type-2 respe ctively, and at pressures of about 100 MPa. These rocks record in-situ endo skarn genesis at the interface between magma and carbonate rocks in which b imetasomatic diffusion of constituents down chemical potential gradients be tween magma and carbonate rocks (reciprocal diffusion metasomatism) Lakes p lace. Skarn rocks constitute a transition zone between the carbonate countr y rocks and magma chamber, where the well-defined reaction zones are due to melts infiltrating porous decarbonating rocks. Magmatic melts are modified by addition of Ca and Mg. These melts metasomatize the carbonates inducing skarn reactions. The modified melts during their differentiation may exsol ve hypersaline fluids. Different metasomatising agents (melt, melt + hypers aline fluids and possibly hypersaline fluids) produce the different types o f skarn facies that are transitional to thermometamorphic marbles.