Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt. Etna Volcano

Manuela Nazzari,Piergiorgio Scarlato,Carmelo Ferlito,Ben Ellis,Silvio Mollo,Vanni Tecchiato,Flavio Di Stefano,Olivier Bachmann

doi:10.3390/min9020126

Abstract

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface.

Highlights

IntroductionMt. Etna volcano (Sicily, Italy; Figure 1a) is the largest volcano in Europe, and one of the most active and most intensely monitored on Earth
Photomicrographs samples were collected withwith the backscattered electron (BSE) mode
Phenocrysts, microphenocrysts and microlites are discriminated on the basis of the longest size dimensions >0.3, 0.3–0.1, and

Summary

Introduction

Mt. Etna volcano (Sicily, Italy; Figure 1a) is the largest volcano in Europe, and one of the most active and most intensely monitored on Earth. The volcanic edifice is 1200 km wide and. The greater part of the stratovolcano was constructed by overlapping products with a. Na-alkaline affinity and principally erupted in the last 220 ka (e.g., Reference [1]). Magma dynamics are frequently referred to the pulsatory upward migration of H2 O-rich magmas and/or fluxes of abundant volatiles from depth [2,3,4,5,6]. The explosive activity of the volcano mostly involves gas-charged

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