40Ar/39Ar dating of the trachytic crystal mush of the Green Tuff ignimbrite, Pantelleria island

The Green Tuff (GT) is the last of nine ignimbrite eruptions associated with the peralkaline volcanic complex of Pantelleria island (Sicily Channel, Italy). Well known in the scientific literature for its petrological and eruptive peculiarities, the GT eruption withdrew a shallow chemically zoned magma reservoir, consisting of a deeper and a late-erupted trachytic crystal mush (P = 1-1.5 kbar, T = 925 C°, H2Omelt = 1.5-2.5 wt.%) to a shallower pantelleritic magma batch (P = 0.5-1-2 kbar, T = 730 °C; H2Omelt = 4.0-4.5 wt.%) (Romano et al., 2018, 2019). In this work we focus on the trachytic top member, which is characterized by high porphyricity (> 35 vol.% Na, K-feldspars phenocryst), high Ba concentrations and positive Eu anomalies both in the groundmass glass and in cpx-hosted H2O-poor melt inclusions as well (Romano et al., 2018, 2019). To better constrain the Green Tuff eruption and to stress the relationship between trachytic and pantelleritic members, we performed new 40Ar/39Ar age determinations on anorthoclases phenocrysts (mean composition: Ab70Or25An5; Romano et al., 2018). Irradiation with fast neutrons along the flux monitor FCT (Fish Canyon Tuff) at the Oregon State TRIGA Reactor was followed by a long degassing at low temperature, a necessary step to discard any extraneous surface-bound 40Ar. Single crystal laser ablation was followed by the isotopic ratios measurements of argon using a Helix SFT (2013) mass spectrometer housed at Institut des Sciences de la Terre d’Orléans (CNRS-ISTO). Despite accurate hand-picking selection, xenocrysts cannot be avoided, but their effect can be mitigated using the single grain fusion approach to provide a robust eruption age. Pantelleritic members (basal fallout deposit and ignimbritic member) were previously dated at 45.7 ± 1.0 ka (2σ; Scaillet et al., 2013). Here, we obtained a statistically indistinguishable weighted mean age of 43.9 ± 0.5 ka (1σ), with a Mean Square Weighted Deviation (MSWD) of 1.34. This age confirms previous dating and point to variable (lesser) xenocrysts contaminations relative to the other GT members. This is unexpected on the premise that more 40Ar should be stored in crystals accumulating at the bottom of the pantellerite magma chamber due to feldspar fractionation and accumulation in the magma mush. Compared to the age of the pantelleritic members, our new data basically supports field evidence, confirming no stasis took place between the plinian/sub-plinian phase of the eruption (basal fallout deposit and ignimbrite) and the late extrusion of the trachytic ignimbrite top member representing the deeper portion of the Green Tuff reservoir.