Phase Equilibrium Constraints on Pre-eruptive Conditions of Recent Felsic Explosive Volcanism at Pantelleria Island, Italy

Abstract

We present experimental phase equilibria carried out on a pantelleritic bulk-rock composition with a peralkalinity index [PI INTRODUCTIONPantelleria island is the type-locality of pantellerite, an iron-and sodium-rich peralkaline rhyolite. Pantellerites, by far the most abundant rock-type at Pantelleria, occur as pyroclastic flows, pumice fall deposits, or as obsidianaceous lavas (Mahood & Hildreth, 1986). After the Plinian eruption of the Green Tuff (45 ka BP), and the related caldera collapse, resurgent felsic magmatism was mainly centered inside the caldera, producing either low-energy explosive eruptions or lava flows.Recent experimental investigations (Scaillet & Macdonald, 2001 and melt inclusion studies (Kovalenko et al., 1988;Webster et al., 1993;Wilding et al Barclay et al., 1996;Gioncada & Landi, 2010) have shown that peralkaline melts may be rather H 2 O-rich, in contrast to previous inferences. These studies have also demonstrated the role of pressure, water, oxygen fugacity, and silica and sodium disilicate activity (aSiO 2 , aNds) on the phase equilibria of felsic peralkaline magmas from the east Kenyan rift zone (e.g. Scaillet & Macdonald, 2001Caricchi et al., 2006). In this study we present the hydrous phase equilibria of a pantellerite with a peralkalinity index PI [ ¼ molar (Na 2 O þ K 2 O)/Al 2 O 3 ] ¼ 1·4, established within the pressure range 25^150 MPa, for temperatures between 680 and 8008C, with oxygen fugacities below the Ni^NiO solid buffer (NNO), and melt water contents (H 2 O melt ) up to 6 wt %. We apply our experimentally derived P^TĤ 2 O melt grid to natural pantellerites to assess the pre-eruptive conditions (T, P,H 2 O melt )ofthemostpowerful event of the post-caldera period, the Fastuca eruption (Orsi et al.,1989). Knowledge of the pre-eruptive conditions sheds light on the nature of the magmatic plumbing system associated with recent felsic volcanism at Pantelleria, with several obvious implications for the eruptive style of pantellerite magmas. The determination of magma storage conditions of pantelleritic magmas can also provide constraints on the more general problem of the origin of silica-oversaturated peralkaline magmas, which has been extensively debated (see Bohrson & Reid, 1997;Civetta et al., 1998;Marshall et al., 1998;Marshall et al. 2009;Scaillet & Macdonald, 2001;Nekvasil et al., 2004;Macdonald et al., 2008). Much of the controversy has arisen because of the lack of modern phase equilibrium constraints for peralkaline magmas, in contrast to their metaluminous or peraluminous counterparts (e.g. Dall' Agnol et al., 1999;Klimm et al., 2003;Bogaerts et al., 2006). GEOLOGICAL AND PETROLOGICAL BACKGROUNDPantelleria (Italy) is an entirely volcanic island ( Fig. 1) located within the Sicily Channel continental rift system, in the northern part of the African plate (e.g. Rotolo et al., 2006). Pantelleria is well known in the petrological literature because its eruptive products represent a typical bimodal suite: a mafic end-member, which consists of mildly alkali basalts, a...

Evidence for extreme fractionation of peralkaline silicic magmas, the Boseti volcanic complex, Main Ethiopian Rift

“…Using the average rim compositions of pyroxene phenocrysts and of matrix glass (below) in B355, the Fe-Mg K D is 0.14. This is in line with values determined for other peralkaline trachytes and rhyolites from Pantelleria, Italy (0.12-0.16 ;Carmichael 1962;Mahood and Stimac 1990;Di Carlo et al 2010). These K D values are lower than those in mafic and intermediate rocks, which are normally in the range 0.20-0.30 (Sisson and Grove 1993;Putirka et al 2003).…”

“…This is in accord with the general situation in peralkaline rhyolites; hedenbergite is the normal pyroxene phenocryst even in the most strongly peralkaline rocks, aegirine-augite being recorded only rarely (Macdonald et al 2011). This is probably related to the relatively low fO 2 at which the magmas evolve, at or close to FMQ (Scaillet and Macdonald 2001White et al 2005White et al , 2009Ren et al 2006;Di Carlo et al 2010). A microphenocryst in B355 has a relatively magnesian core (Ca 44.9 Mg 21.1 Fe 34.0 ), which may represent a highertemperature phase of magmatic evolution or mixing of the pantellerite with a more trachytic magma.…”

“…Using, however, the Na 2 O values calculated from the FK/A ratios, some points may usefully be made. The range of Or contents in the phenocrysts , omitting the anomalous value in B375) approaches the value of Or 36 recorded by Di Carlo et al (2010) as coexisting with the most evolved glass in their experiments on a Pantellerian pantellerite. The range is also close to the minima between the anorthoclase and sanidine solid solution loops at 1 atm.…”

“…The most evolved glass produced experimentally from an Eburru pantellerite had 5.22 wt.% Al 2 O 3 and 12.27 wt.% FeO*, the glass representing 53.8% residual melt (Scaillet and Macdonald 2006). Similarly, Di Carlo et al (2010) recorded that the most evolved experimental glass from a Pantellerian pantellerite had 4.72 wt.% Al 2 O 3 , FeO* 12.8 wt.%, with 30% modal glass.…”

High-resolution 40Ar/39Ar chronostratigraphy of the post-caldera (<20ka) volcanic activity at Pantelleria, Sicily Strait

“…Ar samples from ten key units were selected based on an extensive field-mapping and petrographic survey of the posttrachyte volcanism (Di Carlo et al, 2010;La Felice, 2008;Rotolo et al, 2007). These are mainly found on the northeastern sector of the young caldera ( Fig.…”