The evolution of silicic magmatism in the post-caldera volcanism of the Phlegrean Fields, Italy

In Chapter 1, post-Caldera magmatism of the Phlegrean Fields was investigated based on pre-eruption storage conditions of Agnano Monte Spina. The volatile contents of melt inclusions in phenocrysts were determined by FTIR spectroscopy. Most melt inclusions contain only water though some contain CO2 as carbonate. Solubility experiments for H2O and CO2 in trachytic glass were used to constrain melt inclusion pressure. Non-CO 2 bearing melt inclusions indicated magma storage pressures of 22--54 MPa (1.1--2.7 km deep). Geothermometry yielded pre-eruption magma temperatures of 936--945°C. An oxygen fugacity of NNO + 1.3 was determined from ferrous-ferric iron contents of the matrix glass. Although Agnano Monte Spina magmas are trachytes, sanidine phenocrysts contain phonolitic melt inclusions. The sanidine grew in a phonolitic melt and was admixed into the trachytic magmas prior to eruption.; Experimental constraints on Agnano Monte Spina magmas were determined in Chapter 2. Water-saturated and water-undersaturated (CO2-bearing) experiments were performed on layers D1 and B1 trachytes. Experimental conditions ranged from 775 to 1050°C, 50 to 200 MPa, NNO and ReReO2 buffers, and XH2O of 0.5 and 0.25 in the volatile phase. Replication of natural glass and mineral compositions required T = 900--950°C, 50--100 MPa, and XH2O of 0.5.; Three new TiO2-in-glass geothermometers for potassic magmas were calibrated in Chapter 3. The calibrations are for different degrees of alkalinity: one for metaluminous melts and two for peralkaline melts. These geothermometers were used to examine the crystallization temperatures of phenocryst populations. At Agnano Monte Spina, melt inclusions in sanidine record temperatures ∼50°C lower than those in pyroxene. Apparently, sanidine phenocrysts were admixed into Agnano Monte Spina trachytes prior to eruption. Temperatures of 890--921°C are calculated for the grey magma matrix glass of Vesuvius 79 A.D. Sanidine-hosted melt inclusions in the grey magma record temperatures 100°C lower than the grey matrix glass. The grey magma entrained the sanidine from a different magma prior to eruption. Indeed, the addition of sanidine grains to magmas prior to eruption may be a common process in potassic magmatic systems.