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RESEARCH ARTICLEXenopumice erupted on 15 October 2011 offshore of El Hierro(Canary Islands): a subvolcanic snapshot of magmatic,hydrothermal and pyrometamorphic processesS. Del Moro 1 & A. Di Roberto 2 & S. Meletlidis 3 &M. Pompilio 2 & A. Bertagnini 2 & S. Agostini 4 & F. Ridolfi 1 &A. Renzulli 1Received: 23 December 2014 /Accepted: 21 May 2015 /Published online: 31 May 2015# Springer-Verlag Berlin Heidelberg 2015Abstract On15October2011,asubmarineeruptionoffshoreof El Hierro Island gave rise to floating volcanic products,known as xenopumices, i.e., pumiceous xenoliths partly min-gled and coated with the juvenile basanitic magma. Over thelast few years, no consensus in the scientific community inexplaining the origin of these products has been reached. Inorder to better understand the formation of xenopumice, wepresentatextural,mineralogical,andgeochemicalstudyofthepossible magmatic, hydrothermal, and pyrometamorphic pro-cesses, which usually operate in the plumbing systems of ac-tive volcanoes. We carried out a comprehensive SEM inves-tigation and Sr-Nd-Pb isotope analyses on some samples rep-resentative of three different xenopumice facies. All the datawere compared with previous studies, new data for El Hierroextrusives and a literature dataset of Canary Islands igneousand sedimentary rocks. In the investigated xenopumices, weemphasize the presence of restitic magmatic phases as well ascrystallization of minerals (mainly olivine+pyroxene+magnetite aggregates) as pseudomorphs after pre-existingmafic phenocrysts, providing evidence of pyrometamorphisminduced by the high-T juvenile basanitic magma. In addition,we identify veins consisting of zircon+REE-oxides+mulliteassociated with Si-rich glass and hydrothermal quartz, whichindicate the fundamental role played by hydrothermal fluidcirculation in the xenopumice protolith. The petrological dataagree with a pre-syneruptive formation of the xenopumice,when El Hierro basanite magma intruded hydrothermally al-tered trachyandesite to trachyte rocks and triggered local par-tial melting. Therefore, the El Hierro xenopumice represents asnapshot of the transient processes at the magma-wall rockinterface, which normally occurs in the feeding system ofactive volcanoes.Keywords Xenopumice . ElHierro . Hydrothermalprocess .Pyrometamorphism . SubvolcanicIntroductionIn this paper, we are investigating the products erupted duringthe onset of El Hierro 2011–2012 submarine eruption, knownas xenopumices or “restingolites” (Troll et al 2012;Rodriguez-Losada et al. 2014), sampled on the 15th October2011 floating offshore from the small village of La Restingaalong the south coast of the island (Fig. 1). They consist ofwhitish to grey, highly vesiculated xenoliths, with SiO 2 con-tents ranging from 62 to 71 wt.% (Meletlidis et al. 2012; Trollet al. 2012; Sigmarsson et al. 2013; Rodriguez-Losada et al.2014). They are plastically convoluted with thin bands andcoated by a few-centimeter-thick crust ofthe juvenile basanitemagma that fed the eruption (Fig. 2).Editorial responsibility: J.E. Gardner* A. Renzullialberto.renzulli@uniurb.it1Dipartimento di Scienze della Terra, della Vita e dell’Ambiente,Università degli Studi di Urbino Carlo Bo, Campus Scientifico,“Enrico Mattei”, Via Cà le Suore 2-4, I-61029 Urbino, Italy2Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Viadella Faggiola 32, I-56126 Pisa, Italy3Centro Geofísico de Canarias, Instituto Geográfico Nacional, c/LaMarina, 20, 38001 Santa Cruz de Tenerife, Spain4Istituto di Geoscienze e Georisorse - CNR, Via Moruzzi 1,I-56124 Pisa, ItalyBull Volcanol (2015) 77: 53DOI 10.1007/s00445-015-0940-0