Mafic enclaves in the rhyolitic products of Lipari historical eruptions; relationships with the coeval Vulcano magmas (Aeolian Islands, Italy)

Abstract

The recent finding of mafic enclaves in the Rocche Rosse (RR) lava flow, the last magmatic product on Lipari (Aeolian Islands, Italy) (AD 1230±40), opens the possibility to investigate in detail the most recent magmatic system of the island, an important issue for the volcanic hazard assessment of the area. The RR lava flow is an aphyric rhyolitic coulée consisting of grey and black pumice and black and grey obsidian. Enclaves have ellipsoidal to spheroidal shape and vary from mm-sized in the central portion of the flow, to cm-sized, at the top and in the flow front, where they are also more abundant. Enclaves are shoshonitic-latitic (group A) and trachytic (group B) in composition. The mineralogy of group A consists of dominant clinopyroxene crystals with minor abundance of feldspar (plagioclase > K-feldspar), olivine and biotite, while group B is composed of feldspar (K-feldspar> plagioclase) with minor clinopyroxene, olivine and biotite. Geochemical modeling suggests that the host rhyolitic rocks could be the product of AFC (Assimilation plus Fractional Crystallization) of a magma compositionally similar to the associated shoshonitic-latitic enclaves, which, in turn, could be obtained, through an AFC process, from the primitive melts erupted as olivine hosted melt inclusions during the last 15 ka at Vulcano. The already-known last 42 ka relationship between Lipari and Vulcano Islands is here reinforced until historical time, especially for the last 1 ka. The geochemical and petrological overlap between Lipari and Vulcano is interpreted to reflect the existence of a similar magmatic system underneath the two islands. The nearly aphyric RR rhyolites are interpreted to be the products of a superheated (temperature far above the liquidus) and initially water-undersaturated magma that underwent degassing close to the surface inhibiting microlite crystallization.

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…Approximately 400 years prior to lava effusion the Monte Pilato cone erupted explosively, producing over 200 m thick pumice deposits (Pichler, 1980). Whole rock chemistry is the same for explosive and effusive products (Gioncada et al, 2003;Cabrera et al, n.d.), except for a marked decrease in volatile content from maximum 0.9 wt.% H 2 O measured in glassy rinds of obsidian pyroclasts to~0.2 wt.% in obsidian lava (Gottsman & Dingwell, 2001;Davì et al, 2010;Cabrera et al, n.d.).…”

“…1), represents the final, effusive stage of the most recent eruptive activity from the Monte Pilato pumice cone on Lipari Island, which ended in 1220 ± 30 AD (Tanguy et al, 2003) and produced a 2 km long, 100 m thick, almost aphyric rhyolitic pumice and obsidian flow (Davì et al, 2010). Approximately 400 years prior to lava effusion the Monte Pilato cone erupted explosively, producing over 200 m thick pumice deposits (Pichler, 1980).…”

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

“…Bagdassarov and Dingwell 1992;Smith 1996;Stevenson et al 1996;Manga et al 1998;Griffiths 2000;Pistone et al 2016). However, the Rocche Rosse lava flow is generally a crystal-free glass (with some exceptions; Davì et al 2009Davì et al , 2010, thus crystal content is excluded from calculations. The amount of dissolved gases, preserved as bubbles in the lava flow, can also affect viscosity and flow behaviour (e.g.…”

“…However, knowledge of these influences on the apparent viscosity of a bubbly lava is limited (Stein and Spera 1992;Quane and Russell 2005;Pistone et al 2012;Chevrel et al 2013), and they are therefore not included in viscosity calculations here. The estimated flow surface viscosity was 10 8 to 10 10 Pa s, using rhyolite glass chemistry from Bullock (2015); eruptive temperature range estimates (750 to 900°C) of Cas and Wright (1987), Gottsmann and Dingwell (2001) and Davì et al (2010); and the water content (0.08-0.25 wt%) of Shields et al (2016). Deformation is heterogeneous and multi scale, varying greatly and non-systematically across flow (Shields et al 2016).…”

“…Previous work on the Rocche Rosse has focused on fragmentation and transport mechanisms of the associated pyroclastic sequence (Dellino and La Volpe 1995), the thermal history of flow frontal ramps, i.e. ridges of fold structures extending downward into the flow by tens of metres (Gottsmann and Dingwell 2001), magmatic history (Gioncada et al 2003(Gioncada et al , 2005Davì et al 2009Davì et al , 2010 and textural heterogeneity (Gonnermann and Manga 2003;Clay et al 2013;Shields et al 2016). Ramp structures of the Rocche Rosse were also documented by Hall (1978) and Cas and Wright (1987), inferring a compressive regime and build-up of lava behind a brecciated flow front, hindering flow advance.…”