Abstract
The island of Pantelleria is one of the best known localities of bimodal mafic-felsic magmatism (alkali basalt and trachyte-pantellerite). Among the felsic rocks, the coexistence in a single eruption of products of both trachyte and pantellerite compositions is limited to few occurrences, the Green Tuff (GT) ignimbrite being one of these. The GT is compositionally zoned from pantellerite (70.1 wt% SiO 2 , mol Na+K/Al = 1.86, 1871 ppm Zr) at the base to crystal-rich (>30 vol%) comenditic trachyte (63.4 wt% SiO 2 , mol Na+K/Al = 1.10, 265 ppm Zr) at the top, although the pantellertic compositions dominate the erupted volume. We present here new data on melt inclusions (MIs) from the pantellerite portions of the GT eruption and, most importantly, from the trachyte member, which have not been studied in-situ by previous work focused on the GT. We document the first occurrence of trachytic melt inclusions in the late-erupted member, whose importance resides in the fact that trachytes were known mostly as crystal-rich lavas or ignimbrites, all variably affected by crystal accumulation. Besides the obvious inferences on the interplay between parental-derivative magmas, this evidence adds also some helpful elements in understanding zoning of silicic and peralkaline (i.e. low-viscosity) magma chambers. Trace elements compositions of MIs reveal that trachyte melts are of two types : (i) a low-Ba , directly descending from basaltic melts by 60-70 % of fractional crystallisation, and (ii) a 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 H 2 O-poor (≤ 1.2 wt %) with respect to the early erupted (and shallower) pantellerite magma (≤ 4.2 wt %), raising the possibility that either trachyte magma was H 2 O-undesaturated, or clinopyroxene hosted melt inclusions which suffered consistent H 2 O loss.
Highlights
The Green Tuff (GT) is a low-aspect ratio rheomorphic ignimbrite that covers the island of Pantelleria (83 km2), and is the most studied and representative eruption of the peralkaline volcano of Pantelleria
The rationale of our choice is in that MIs offer the possibility to avoid some pitfalls that may affect the whole rock chemistry, such as crystal accumulation/depletion, which are phenomena both widely recorded in ignimbrites, provided that melt inclusions were not modified by post-entrapment processes
The Green Tuff ignimbrite inundated the whole island of Pantelleria with crystal-poor pantellerites and, at the end of the eruption, with a crystal-rich trachyte, which flowed in a pyroclastic density current in the south-west sector of the island
Summary
The Green Tuff (GT) is a low-aspect ratio rheomorphic ignimbrite that covers the island of Pantelleria (83 km2), and is the most studied and representative eruption of the peralkaline volcano of Pantelleria. = Na+K/Al molar ratio) of the erupted products, from the initial pantelleritic pumice fallout and the following pyroclastic currents that progressively inundated the island, to the final erupted trachytes. These latter are restricted in their areal occurrence (and volume as well) to the south west of the island. The rationale of our choice is in that MIs offer the possibility to avoid some pitfalls that may affect the whole rock chemistry, such as crystal accumulation/depletion, which are phenomena both widely recorded in ignimbrites, provided that melt inclusions were not modified by post-entrapment processes
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