Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano

Various xenoliths have been found in lavas of the1763 (“La Montagnola”), 2001, and 2002–03 eruptions at Mt.Etna whose petrographic evidence and mineral chemistryexclude a mantle origin and clearly point to a cognate nature.Consequently, cognate xenoliths might represent a proxy toinfer the nature of the high-velocity body (HVB) imagedbeneath the volcano by seismic tomography. Petrographyallows us to group the cognate xenoliths as follows: i) gabbroswith amphibole and amphibole-bearing mela-gabbros, ii)olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole,and iv) Plg-rich leuco gabbros. Geobarometry estimatesthe crystallization pressure of the cognate xenoliths between1.9 and 4.1 kbar. The bulk density of the cognate xenolithsvaries from2.6 to 3.0 g/cm3. P wave velocities (VP), calculatedin relation to xenolith density, range from 4.9 to 6.1 km/s. Theintegration of mineralogical, compositional, geobarometric data,and density-dependent VP with recent literature data on 3DVP seismic tomography enabled us to formulate the firsthypothesis about the nature of the HVB which, in the depthrange of 3–13 km b.s.l., is likely made of intrusive gabbroicrocks. These are believed to have formed at the “solidificationfront”, a marginal zone that encompasses a deep region (>5 kmb.s.l.) of Mt. Etna’s plumbing system, within which magmacrystallization takes place. The intrusive rocks were afterwardsfragmented and transported as cognate xenoliths by thevolatile-rich and fast-ascending magmas of the 1763 “LaMontagnola”, 2001 and 2002–03 eruptions.