Fluid inclusion studies of ejected nodules from plinian eruptions of Mt. Somma-Vesuvius

Abstract

Mt. Somma-Vesuvius (Naples, Italy) has erupted potassium-rich and silica-undersaturated products during a complicated history of plinian and non-plinian events. Coarse-grained cognate nodules are commonly found in the pyroclastics and are upper crustal in origin. We examined cumulate and subeffusive nodules from the 3800 y.B.P. Avellino. A.D. 79 Pompei, and A.D. 472 Pollena eruptions. Silicate-melt and liquid-vapor fluid inclusion studies in clinopyroxene from both types of nodules have been used to assess the fluids attending crystallization and to place constraints on the pressure and temperature of nodule formation. Thermometric and volumetric data from primary and pseudosecondary CO 2-H 2O and CO 2 and coeval silicate-melt fluid inclusions indicate that they were trapped at a pressure of ∼1 to ∼2.5 kbar at ∼1200°C. This suggests a crystallization depth of ∼4 to ∼10 km. The H 2O-bearing fluid inclusions are abundant from plinian eruptions in contrast to non-plinian eruptions where H 2O-bearing fluid inclusions were rare. The presence of primary H 2O-CO 2 fluid inclusions indicates that an immiscible, supercritical H 2O-CO 2 fluid was in the nodule-forming environment. The H 2O-bearing fluid inclusions in plinian nodules may record a higher pre-eruptive H 2O content in the bulk magma that is dramatically reflected in the eruption dynamics.