Experimental constraints on phase relations in a multilevel magmatic system: the Phlegraean Volcanic District (South Italy) case study

The Phlegraean Volcanic District (PVD) is an active area that hosts about 1,5 million people within its surroundings. This makes the area one of the most dangerous on the Earth. The activity of the PVD (including Campi Flegrei and the islands of Ischia and Procida) produced dramatic explosive events in the Mediterranean area during the last 200 ky, and many studies have been performed to investigate the magmatic differentiation processes responsible of the compositionally largely variable products emitted during PVD activity. However, these studies have mostly followed a petrological and geochemical approach, whereas very few experimental studies have been carried out to understand and constrain origin and evolution of PVD magmas, and only limited to differentiated compositions. Here we present an experimental study conducted on a rock of primitive composition, probably representing the mantle derived parental magmas feeding the PVD, outcropping in the area, a K-basalt from the Solchiaro eruption. This rock has been used as starting material for an experimental study addressed to unravel the differentiation mechanisms acting in the PVD plumbing system. The experiments were performed in a piston–cylinder apparatus at three different pressures, 200, 400 and 800 MPa based on the current knowledge of the PVD plumbing system structure. The ranges of temperature and water concentration investigated were 1050 - 1300 °C and 1-7 wt%, respectively. These experiments allows us to reconstruct the phase relations of Solchiaro K-basalt in the P-T-H2O space. At 200 MPa and H2O content lower than 4 wt%, olivine is the liquidus phase followed by chromium spinel, clinopyroxene, and finally plagioclase. At 400 MPa and H2O lower than 3 wt%, olivine is still the liquidus phase but chromium spinel also occurs; then, clinopyroxene and plagioclase crystallize. Interestingly, at 800 MPa and water content below 1.5 wt% , chromium spinel and clinopyroxene replace olivine as liquidus phases; whereas for water content above 1.5 wt% all the three phases occur on the liquidus . This suggests the equilibrium of primary PVD magmas with a wehrlitic source at pressure corresponding to the crust-mantle boundary. Moreover, our 800 MPa experiments corroborate results from a recent study on melt inclusions constraining the amount of water in the PVD primary magma at values up to 2 wt%. The high pressure experiments, showing olivine on the liquidus only for H2O content in the melt above 1.5 wt% suggest that in a magmatic system characterized by water content below 1.5 wt%, the occurrence of olivine as liquidus phase can possibly occur by the increase of XCO2 in the magma. Experiments to investigate the role of XCO2 on the phase relations of PVD primitive magmas are in progress.