A REVIEW OF RECENT WORK ON THE GREEN TUFF (PANTELLERIA, SICILY CHANNEL RIFT ZONE, ITALY), A COMPOSITIONALLY STRATIFIED PANTELLERITIC TO TRACHYTIC IGNIMBRITE

Recent work on melt inclusions (Romano et al., 2019; Annals of Geophysics) and glass and minerals (Liszewska et al., 2018; Journal of Petrology) acquired from the Green Tuff ignimbrite have advanced our understanding of the structure and pre-eruptive conditions of this strongly zoned magma chamber. The whole rock composition of the Green Tuff ranges from pantellerite (70.1 wt% SiO₂, mol Na+K/Al [P.I.] = 1.86, 1871 ppm Zr) at the base to crystal rich trachyte (60.0 wt% SiO₂, P.I. = 0.95, 205 ppm Zr) at the top, the variation apparently representing an inverted vertical zonation in the pre-eruptive reservoir. Melt inclusion and glass analyses, however, reveal a much greater range, showing that compositional layers in the upper parts of the reservoir, formed by fractional crystallization, were mixed during eruption. Some areas of glass have low Al₂O₃ (5.16-5.46 wt %), high FeO* (9.66-10.02 wt %), and P.I. > 2.5, making them the most evolved melts yet reported from Pantelleria. The new glass data stress how whole-rock analyses do not truly reflect the complete range of melt compositions in the pre-eruptive reservoir. 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. Trachyte liquids are represented only as melt inclusions, which reveal two types: (i) low-Ba, descended from basaltic melts by 60-70 % fractional crystallisation (comenditic trachyte: 63.4 wt% SiO₂, P.I. = 1.10, 265 ppm Zr), and (ii) high-Ba, that might be affected by processes of feldspar dissolution and entrainment of the resulting small-scale melts in some MIs. MIs hosted in the deep-seated trachyte body are H₂O-poor (≤ 1.2 wt %) with respect to the early erupted (and shallower) pantellerite magma (≤ 4.2 wt). Thermodynamic modelling indicates that the temperature range was ~900-700°C, with fO₂ from FMQ-1.5 to FMQ-0.5, and aSiO₂ (relative to quartz saturation) from 0.74-1.00.