COMPOSITIONAL AND THERMODYNAMIC VARIABILITY IN A STRATIFIED MAGMA CHAMBER: EVIDENCE FROM THE GREEN TUFF IGNIMBRITE (PANTELLERIA, ITALY)

The Green Tuff Ignimbrite, Pantelleria, is compositionally zoned from pantellerite at the base to comenditic trachyte at the top, the variation apparently corresponding to an inverted vertical zonation in the pre-eruptive reservoir. The main phenocryst assemblages are alkali feldspar + olivine + clinopyroxene + ilmenite + apatite in the trachytes and alkali feldspar + aenigmatite + clinopyroxene ± quartz in the rhyolites. Thermodynamic modelling indicates that the temperature range was ~900-700°C, fO₂ FMQ-1.5 to FMQ-0.5 and aSiO₂ (relative to quartz saturation) 0.74-1.00. Melt water contents ranged from ~1 wt % in the trachytes to ~4 wt % in the pantellerites. The new glass data stress how whole-rock analyses do not truly reflect the complete range of melt compositions in the magma chamber. Previous studies have demonstrated that compositional zonation in the reservoir is the result of progressive magma differentiation (i.e., fractional crystallization.) However, the considerable chemical variability of the matrix glasses in the more evolved rocks shows that compositional layers in the upper parts of the reservoir were mixed during eruption. The proportion of rhyolitic to trachytic melts increased towards the top of the reservoir. Some areas of glass have low Al₂O₃ contents (5.16-5.46 wt %) and high FeO* contents (9.66-10.02 wt %), making them the most evolved melts yet reported from Pantelleria. The trachytes contain >40% modal phenocrysts, which with relatively high Ba contents and positive Eu anomalies, are considered to have originated in a feldspar-accumulitic layer. Phenocrysts in the trachytes are commonly heavily resorbed, inferred to be a result of heating by influxes of intermediate composition magmas, which, however, were not erupted. It is argued that magma of intermediate composition were present in the Green Tuff reservoir but were efficiently trapped in a crystal-rich layer below the Green Tuff magmas, which was eventually erupted during a resurgent phase as the Montagna Grande Trachyte.