Abstract
The growth of Mount Etna volcano reflects the superimposition of various eruptive centers, the most voluminous of which is the Ellittico, whose stratigraphic sequence is well exposed on the steep walls of Valle del Bove. The uppermost levels of the sequence have been sampled and investigated through a new set of geochemical data on mineral phases and bulk rock. Sampled rocks display a marked bimodality with aphyric banded trachyandesites, which are some of the most evolved and rare products of the entire Etnean succession (SiO2 58–60 wt.%), intercalated in plagioclase rich porphyritic mugearites (SiO2 49–50 wt.%, P.I. 35–40). In this paper, we provide a detailed textural, mineralogical, and chemical characterization of these products, providing a new interpretative model for their genesis and significance in the context of the Etnean system. Our approach discusses, in a critical way, the “classic” fractional crystallization model of magmas, not supported by field evidence, and proposes a novel hypothesis in which the aphyric-banded trachyandesites represent be the primary products of a gas-induced partial melting of hypabyssal sills and dykes. This hypothesis represents a step towards a comprehensive description of igneous systems that takes into account not exclusively the evolution of basaltic melts, but also the role of volatile contributions in governing volcanic behavior.
Highlights
Mount Etna is generally viewed as an open conduit volcano in which magma differentiation takes place “en route” to the surface, in a continuous time-space interval comprised between magma arrival at crustal levels and eruption at the permanently active summit craters [1,2,3,4,5,6,7,8]
The multidisciplinary study of the uppermost sequence of the Ellittico volcanic phase, characterized by the rare outcrop of banded trachyandesitic lavas interbedded within basaltic products, allowed us to investigate both the genetic process and eruptive dynamics of some of the most evolved products erupted by Mount Etna
Textural, mineral, and 3D investigations with mass balances calculations allowed us to model the genetic process for the evolved products of Mount Etna, evidencing that their origin cannot be related to a univocal process
Summary
Mount Etna is generally viewed as an open conduit volcano in which magma differentiation takes place “en route” to the surface, in a continuous time-space interval comprised between magma arrival at crustal levels and eruption at the permanently active summit craters [1,2,3,4,5,6,7,8]. Magma differentiation at crustal level (P < 600 MPa) is driven by polybaric crystallization of cotectic olivine and clinopyroxene, while plagioclase stability and fractionation are strongly constrained by the magmatic volatiles content in the final 300 MPa of magma ascent [8,13,14] For these reasons, the composition of the products erupted during the last two phases of Mount Etna activity, named Ellittico (57–15 ka) and Recent Mongibello (15 ka to present) are generally very homogenous and relatively poorly differentiated, from hawaiitic to trachybasaltic lavas [15,16,17,18,19]. The stratigraphic records show that sporadically Mount Etna volcano has erupted banded lava flows of relatively acidic (i.e., trachyandesitic) composition
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