FLUID INCLUSION STUDIES OF EJECTED NODULES FROM PLINIAN ERUPTIONS OF MT SOMMA-VESUVIUS

Mt. Somma-Vesuvius (Naples, Italy) has erupted potassium-rich and sili ca-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 cumu late and subeffusive nodules from the 3800 y.B.P. Avellino, A.D. 79 Po mpei, and A.D. 472 Pollena eruptions. Silicate-melt and liquid-vapor f luid inclusion studies in clinopyroxene from both types of nodules hav e been used to assess the fluids attending crystallization and to plac e constraints on the pressure and temperature of nodule formation. The rmometric and volumetric data from primary and pseudosecondary CO2-H2O and CO2 and coeval silicate-melt fluid inclusions indicate that they were trapped at a pressure of similar to 1 to similar to 2.5 kbar at s imilar to 1200 degrees C. This suggests a crystallization depth of sim ilar to 4 to similar to 10 km. The H2O-bearing fluid inclusions are ab undant from plinian eruptions in contrast to non-plinian eruptions whe re H2O-bearing fluid inclusions were rare. The presence of primary H2O -CO2 fluid inclusions indicates that an immiscible, supercritical H2O- CO2 fluid was in the nodule-forming environment. The H2O-bearing fluid inclusions in plinian nodules may record a higher pre-eruptive H2O co ntent in the bulk magma that is dramatically reflected in the eruption dynamics.