RESEARCH ARTICLE Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano Rosa Anna Corsaro & Silvio Giuseppe Rotolo & Ornella Cocina & Gianvito Tumbarello Received: 24 May 2013 /Accepted: 12 October 2013 /Published online: 5 December 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract Various xenoliths have been found in lavas of the 1763 (“La Montagnola”), 2001, and 2002–03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphi- bole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm 3 . P wave velocities (V P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric da- ta, and density-dependent V P with recent literature data on 3D V P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3–13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the “solidification front”, a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna’ s plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 “La Montagnola”, 2001 and 2002–03 eruptions. Keywords Cognate xenoliths . Gabbro . Geobarometry . Rock density . P-wave velocity . Mt. Etna Introduction The presence of cognate xenoliths in volcanic rocks is quite common in many tectonic settings, such as oceanic ridges, hotspots and subduction zones. Some cognate xenoliths have a mantle or high-pressure origin, while others form in a shallower context (Holness et al. 2007 and references therein). Magmatic xenoliths have also been studied in products of active Italian volcanoes, namely Stromboli (Renzulli and Santi 1997; Mattioli et al. 2003; Laiolo and Cigolini 2006; Corazzato et al. 2008; Tibaldi et al. 2009) and, to a lesser extent, Mt. Etna (Lo Giudice and Ritmann 1975; Aurisicchio and Scribano 1987; Andronico et al. 2005; Corsaro et al. 2007). At Mt. Etna, cognate xenoliths are infrequent and occur exclusively within lavas/pyroclasts of rare eruptions, originally named “eccentric” (Rittmann 1965) and more recently “deep dyke-fed” (DDF, Corsaro et al. 2009b Editorial responsibility: G. Giordano Electronic supplementary material The online version of this article (doi:10.1007/s00445-013-0772-8) contains supplementary material, which is available to authorized users. R. A. Corsaro (*) : O. Cocina Istituto Nazionale di Geofisica e Vulcanologia (INGV), Osservatorio Etneo, Sezione di Catania, Piazza Roma, 2, 95125 Catania, Italy e-mail: corsaro@ct.ingv.it S. G. Rotolo Dipartimento di Scienze della Terra e del Mare (DISTeM), Università di Palermo, Via Archirafi, 22, 90123 Palermo, Italy S. G. Rotolo Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo, Via U. La Malfa, 153, 90146 Palermo, Italy G. Tumbarello Eni SpA, E&P Division, Via Emilia, 1, San Donato Milanese, 20097 Milan, Italy Bull Volcanol (2014) 76:772 DOI 10.1007/s00445-013-0772-8