Changes in magmatic assemblages and crystal stability as a response of CO2 flushing in basaltic systems have been never directly addressed experimentally, making the role of CO2 in magma dynamics still controversial and object of scientific debate. We conducted a series of experiments to understand the response of magmas from Etna volcano to CO2 flushing. We performed a first experiment at 300 MPa to synthesize a starting material composed of crystals of some hundreds of m and melt pools. This material is representative of an initial magmatic assemblage composed of plagioclase, clinopyroxene and a water undersaturated melt. In a second step, the initial assemblage was equilibrated at 300 and 100 MPa with fluids having different XCO2fl . Our experiments demonstrate that flushing basaltic systems with fluids may drastically affect crystal textures and phase equilibria depending on the amount of H2O and CO2 in the fluid phase. Since texture and crystal proportions are among the most important parameters governing the rheology of magmas, fluid flushing will also influence magma ascent to the Earths surface. The experimental results open new perspectives to decipher the textural and compositional record of minerals observed in volcanic rocks from Mt. Etna, and at the same time offer the basis for interpreting the information preserved in minerals from other basaltic volcanoes erupting magmas enriched in CO2.
Processes of gas escape and changing concentration of volatiles in natural magmas provide the driving force for volcanic eruptions, as volatiles control the physical melt properties and phase equilibria, with far-reaching implications for magma ascent dynamics and eruptive styles (e.g., Sparks and Pinkerton 1978;Papale and Dobran 1994;Dingwell 1998;Papale 1998;Blundy and Cashman 2001;Sparks et al. 2003;Cashman 2004;Webster et al. 2015). Magmatic volatile composition is typically dominated by H 2 O and CO 2 . These two components behave quite differently in the melt phase during ascent, due to their strong pressure-dependent and contrasting solubilities (e.g., Brey 1976;Papale 1997;Webster et al. 1999;Newman and Lowenstern 2002;Vetere et al. 2011;Iacono-Marziano et al. 2012;Witham et al. 2012;Vetere et al. 2014;Shishkina et al. 2014a;2014b). While the central influence of water on volcanic processes has been recognized since decades (e.g., Tuttle and Bowen 1958), the role of CO 2 has only recently been considered to play a central role in magmatic and volcanic dynamics (e.g., Wilson et al. 1980;Spera 1981;1984;Papale and Polacci 1999;Aiuppa et al. 2007;Blundy et al. 2010;Nicotra and Viccaro 2012). In particular, understanding the behavior of mixed H 2 O-CO 2 volatile components in experimental studies has become one of the key to unravel the effects of ascent rates on the kinetics of magma vesiculation and crystallization, along with their implications on eruption mechanisms (e.g., Mourtada-Bonnefoi and Laporte 2002;Cichy et al. 2011;Pichavant et al. 2013;Riker et al. 2015;Fiege et al. 2015). In spite of the increasing interest in reproducing crystallization paths of multicomponent volatile-bearing systems, our knowledge on the concomitant effect of mixed H 2 O+CO 2 fluids on phase relations in alkali-rich basaltic systems is restricted to a more limited number of experimental data (e.g., Freise et al. 2009;Pichavant et al. 2013;2014;Vetere et al. 2015a). These studies primarily focused on the evolution of phase relations and melt composition across a range of physical and chemical conditions (i.e. P, T, H 2 O content), providing information about crystal changes in terms of abundances and size. Previous studies, however, did not directly address changes in magmatic assemblages and crystal stability as a response of fluid flushing which causes local variations in the CO 2 proportion of the magmatic system. The interpretation of textural features of crystals has been, so far, conducted through experimental reproduction of processes of crystal dissolution/resorption or their growth in presence of a pure-H 2 O magmatic fluid phase (Logfren 1980;Nelson and Montana 1989;Tsuchiyama 1985;Tsuchiyama and Takahashi 1983;Kawamoto 1992;Kirkpatrick 1981;Hummer and Rutherford 2002). Thus, the influence of volatile components in addition to water on textural and compositional changes in alkali basalts has been generally neglected. This means that mineral reactions in systems undergoing rapid changes of (CO 2 -bearing) fluid compositions is still not fully understood, precluding in some cases a proper interpretation of natural samples and of related physical processes during magma ascent.
CO 2 flushing from magmas exsolving gas at depth has been found to play a major role in producing highly explosive eruptions at many basaltic systems such as Etna and Stromboli (cf. Aiuppa et al. 2010a;2010b;Aiuppa et al., 2016;Allard 2010;Pichavant at al. 2013;Ferlito et al. 2014). In this context, Mt. Etna represents one of the most intriguing cases. The complex plumbing system of Mt. Etna is persistently fed by primitive, volatile-rich magmas that mix with mafic, degassed magmas stored at lower pressure (Armienti et al. 2007;2013;Ferlito et al. 2008;Viccaro et al. 2010;2014;2015;Nicotra and Viccaro 2012a;Corsaro et al. 2013;Mollo et al. 2015).
Moreover, published data on primitive glass inclusions in olivine crystals indicate that magmas of Mt. Etna can exsolve a gas phase, rich in CO 2 , at pressure higher than 250 MPa (Metrich et al., 2004;Spilliert et al., 2006;Collins et al. 2009), which continuously flush the overlying magma reservoirs, acting as a recurrent trigger mechanism for paroxysmal events (Aiuppa et al., 2007(Aiuppa et al., , 2016;;Nicotra and Viccaro 2012;Ferlito et al. 2014). This means that a major part of the crystallization occurs in regions of the plumbing system where CO 2 is already exsolved, but in which H 2 O activity can change very abruptly as a result of local fluxes of CO 2 -rich fluids in magma reservoirs (cf. Aiuppa et al. 2010b;Patanè et al. 2013).
In this study, we tested experimentally the effect of CO 2 flushing in a partially crystallized Ktrachybasaltic magma representative of eruptive products from the recent activity of Mt. Etna. We demonstrate that the flushing of CO 2 results in variations of mineral stability and phase relations, which are consistent with observations made in rock samples. The
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