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

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 amphibole, 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 data, 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.

Slab Detachment, Mantle Flow, and Crustal Collision in Eastern Sicily (Southern Italy): Implications on Mount Etna Volcanism

“…A high‐velocity V P volume, interpreted as a main solidified intrusive body, has been found/imaged within Mount Etna through tomographic techniques and petrographic studies (Corsaro et al, 2014; Patanè et al, 2006). Indeed, huge intrusive emplacements of plutonic rocks are to be expected along volcanoes involved in contraction (Cas & Wright, 1987).…”

New Insights on Mt. Etna’s Crust and Relationship with the Regional Tectonic Framework from Joint Active and Passive P-Wave Seismic Tomography

“…The origin of the anomaly is interpreted to be old magmatic intrusions, with the HVB representing the main structural feature beneath the volcano, reflecting its intense volcanic history and the accumulation of a very large volume of non-erupted volcanic material. Detailed studies of its composition have been performed; for example, Corsaro et al (2014) analysed gabbroic xenoliths that may be originated from the HVB and inferred the slow solidification of a mafic magma.…”

Dyke intrusion and stress-induced collapse of volcano flanks: The example of the 2018 event at Mt. Etna (Sicily, Italy)

“…The upper part of the HVB has persistent low V P /V S anomalies. Following 23 , the HVB can be interpreted as a massive accumulation of intrusions pervading the sedimentary basement, that repeatedly were emplaced during the recent and past activity of the volcano [see also 26 ]. • Low V P anomalies around the central intrusive mesh (down to 9-12 km depth), representing the sedimentary substratum deformed with southward verging system of thrust nappes of the Apenninic-Maghrebian Chain 27 .…”