Impulsive supply of volatile-rich magmas in the shallow plumbing system of Mt. Etna volcano

Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O -undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H2O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume

An improved clinopyroxene-based hygrometer for Etnean magmas and implications for eruption triggering mechanisms

We have refined the clinopyroxene-based hygrometer published by Armienti et al. (2013) for a better quantitative understanding of the role of H2O in the differentiation of Etnean magmas. The original calibration data set has been significantly improved by including several experimental clinopyroxene compositions that closely reproduce those found in natural Etnean products. To verify the accuracy of the model, some randomly selected experimental clinopyroxene compositions external to the calibration data set have been used as test data. Through a statistic algorithm based on the Mallows' CP criterion, we also check that all model parameters do not cause data overfitting, or systematic error. The application of the refined hygrometer to the Mt. Etna 2011-2013 lava fountains indicates that most of the decreases in H2O content occur at P < 100 MPa, in agreement with melt inclusion data suggesting abundant H2O degassing at shallow crustal levels during magma ascent in the conduit and eruption to the surface

An integrated P-T-H2O-lattice strain model to quantify the role of clinopyroxene fractionation on REE+Y and HFSE patterns of mafic alkaline magmas: Application to eruptions at Mt. Etna

A correct description and quantification of the geochemical behaviour of REE+Y (rare earth elements and Y) and HFSE (high field strength elements) is a key requirement for modeling petrological and volcanological aspects of magma dynamics. In this context, mafic alkaline magmas (MAM) are characterized by the ubiquitous stability of clinopyroxene from mantle depths to shallow crustal levels. On one hand, clinopyroxene incorporates REE+Y and HFSE at concentration levels that are much higher than those measured for olivine, plagioclase, and magnetite. On the other hand, the composition of clinopyroxene is highly sensitive to variations in pressure,temperature, and melt-water content, according to exchange-equilibria between jadeite and melt, and between jadeite/Ca-Tschermak and diopside-hedenbergite. As a consequence, the dependence of the partition coefficient on the physicochemical state of the system results in a variety of DREE+Y and DHFSE values that are sensitive to the magmatic conditions at which clinopyroxenes nucleate and grow. In order to better explore magma dynamics using clinopyroxene chemical changes, an integrated P-T-H2Olattice strain model specific to MAM compositions has been developed. The model combines a set of refined clinopyroxene-based barometric, thermometric and hygrometric equations with thermodynamically-derived expressions for the lattice strain parameters, i.e., the strain-free partition coefficient (D0), the site radius (r0), andthe effective elastic modulus (E). Through this approach, it is found that the incorporation of REE+Y and HFSE into M2 and M1 octahedral sites of clinopyroxene is determined by a variety of physicochemical variables that may or may not change simultaneously during magma differentiation. The applicability of the P-T-H2O-lattice strain model to natural environments has been verified using clinopyroxene-melt pairs from a great number of volcanic eruptions at Mt. Etna volcano (Sicily, Italy). DREE+Y and DHFSE values recovered by the model have been used as input data to quantify fractional crystallization processes in natural MAM compositions. Results from calculation illustrate that the concentration of REE+Y and HFSE in the magma is primary controlled by the geochemical evolution of clinopyroxene in terms of major cation exchange-equilibria and trace cation lattice strain properties.Published32-563V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici4V. Processi pre-eruttiviJCR Journa