Effect of water on the phase relations of primitive K-basalts: Implications for high-pressure differentiation in the Phlegraean Volcanic District magmatic system

Abstract

Phase relations, compositions, and modes of two natural primitive alkaline basalts were determined through experiments performed at 0.8 GPa, temperatures between 1000 and 1310 °C, and nominal water (H 2 O i) contents varying from < 1 (natural water rock content) to 6 wt.%. The two natural samples used as starting material are high-Mg, moderate-K, alkaline basalt scoria clasts (Mg# = 0.68-0.66,  alkalis = 4.4 wt.%) dispersed within the hydromagmatic tuff of the Solchiaro eruption (Procida Island, southern Italy). These samples are considered representative of parental magmas feeding the Phlegraean Volcanic District, which includes the Campi Flegrei caldera and the Procida and Ischia islands. The composition of the experimental phases has been compared with that of natural rocks to constrain the early stages of crystallization of poorly differentiated magmas from the Phlegraean Volcanic District. Our study shows that the liquidus temperature at H 2 O i < 1 wt% is ~1300 °C whereas with water contents between 1.5 and 3 wt.%, and between 4 and 6 wt.%, the apparent liquidus temperature decreases of about 50-100 °C, respectively. Under natural water conditions (<1 wt%), the liquidus phase is clinopyroxene, followed by olivine and Cr-spinel and, then, by plagioclase. The increase of H 2 O content in the system enhances olivine stability along with clinopyroxene at the liquidus temperature. The 4-6 wt.% water bearing experiments show evidence of H 2 O saturation at 1100 °C, with olivine being replaced by orthopyroxene later joined by pargasitic amphibole and minor amounts of oxide starting from 1080 °C. The composition of glasses in the experiments with low H 2 O i content is consistent with the trachybasaltic products of Procida; glasses obtained in the runs with 1.5-3 wt.% water resemble the trend formed by the shoshonitic products of Campi Flegrei. The water-saturated experiments produced variably alkali-depleted residual glasses whose composition converges toward the subalkaline field. The compositions of the experimental clinopyroxenes match well those found in the Campi Flegrei products for which we have estimated crystallization

Geochemical and Sr–Nd isotopic features of the Zaro volcanic complex: insights on the magmatic processes triggering a small-scale prehistoric eruption at Ischia island (south Italy)

“…Mafic enclaves instead, being depleted in radiogenic Sr and unradiogenic Nd, could derive from a less enriched source, similar to the one that generated the Solchiaro melt at Procida, in which the Sr-and Nd-signature was less considerably modified. Probably, the magma that generated the mafic enclaves has changed its isotopic signature very little in the upper crust, suggesting that this magma stationed for a short time at a mid-low storage region, as also suggested for Solchiaro mafic magmas (Moretti et al 2013;Perinelli et al 2019;Bonechi et al 2020). Ascent and emplacement of magmas at shallower depth, where they evolve to trachyte and phonolite, could be controlled by the intersection of NE-SW transverse and NW-SE normal regional fault systems (Acocella and Funiciello 2006;Moretti et al 2013).…”

New Insights Into the Recent Magma Dynamics Under Campi Flegrei Caldera (Italy) From Petrological and Geochemical Evidence

“…The reported melt‐inclusion‐based temperatures are of about 1,100–1,200°C for trachybasaltic‐shoshonitic melts and of about 950°C for trachytic melts (references in Figure 2 and Table 1). These values are consistent with pressures and temperatures calculated using MELTS (Ghiorso & Sack, 1995) thermodynamic approach (e.g., Fowler et al., 2007; Pappalardo et al., 2004, 2008; Pappalardo & Mastrolorenzo, 2010, 2012), geothermobarometry (e.g., Balcone‐Boissard et al., 2016; Forni et al., 2018; Masotta et al., 2013; Mollo & Masotta, 2014) as well as phase‐equilibrium experiments (e.g., Dolfi & Trigila, 1978; Fabbrizio & Carroll, 2008; Perinelli et al., 2019; Pichavant et al., 2014; Scaillet et al., 2008; Stabile & Carroll, 2020), constrained with data from erupted products and summarized in Table 1.…”