Phase equilibria of Pantelleria trachytes (Italy): constraints on pre-eruptive conditions and on the metaluminous to peralkaline transition in silicic magmas.

Abstract

Pantelleria Island is the type locality of pantellerite, an iron and alkali-rich rhyolite (P.I=molar Na2O+K2O/Al2O3 >1.05). Peralkaline rhyolites (i.e pantellerite and comendite) and trachytes usually represent the felsic end-members in continental rift systems (e.g., Pantelleria, Tibesti, Ethiopia, Afar, Kenya, Bain and Range, South Greenland) and in oceanic sland settings (Socorro Is., Easter Is., Iceland and Azores). The origin of peralkaline rhyolites in the different tectonic settings is still a matter of debate and three hypotheses have been suggested: (a) crystal fractionation of alkali-basalt in a shallow reservoir to produce a trachyte which subsequently gives rise to a pantellerite (e.g., Barberi et al., 1975, Mungall & Martin 1995, Civetta et al., 1998,) (b) partial melting of cumulate gabbros to form a trachyte which then produces pantellerite (e.g., Lowestern & Mahood 1991; Bohrson & Reid 1997), (c) partial melting of different lithospheric sources fluxed by volatiles which add the excess alkalies to the melt (Bailey & Macdonald, 1975, 1987). Recent petrological work has helped to define the temperature range and redox conditions of pantellerite magmas (Scaillet & Macdonald 2001, 2003, 2006; White et al., 2005, 2009; Di Carlo et al., 2010,) as well as their pre-eruptive volatile contents (e.g., Gioncada & Landi 2010, Neave et al., 2012, Lanzo et al., 2013). In contrast little is known about the conditions of storage and evolution of the associated trachytes. At Pantelleria, trachytes and pantellerites constitute most of the outcropping rocks, the former being erupted dominantly as lava flows while pantellerites are erupted either explosively or effusively. We have experimentally investigated the phase relationship of two representative trachytes from Pantelleria island in order to shed light on their pre-eruptive conditions (pressure, temperature, H2Omelt, oxygen fugacity) and define their liquid lines of descent toward more evolved compositions. We have established the phase relationships over a P-T fO2-H2Omelt range of T=750-950°C, P=0.5-1.5 kbar, fO2=~FMQ and XH2Ofluid (H2O/H2O+CO2, in moles) between 1 and 0. By comparing the experimental phase assemblages, abundances and compositions with the natural products we set constraints on the storage conditions of trachytic magmas at Pantelleria and more generally, on the putative parent-daughter relationship between trachytic and pantelleritic magmas. Our results lay thebasis to understand the long debated petrological issue regarding the link 94 between silica oversaturated peralkaline and metaluminous magmas.