Magma evolution at La Fossa volcano (Vulcano Island, Italy) in the last 1000 years: evidence from eruptive products and temperature gradient experiments

Abstract

The intense explosive and effusive volcanic activity of the last 1000 years at La Fossa volcano (Vulcano Island, Italy) was characterized by the eruption of magmas ranging in composition from latites to trachytes and rhyolites, as well as K-rich trachytes. Evidence of syn-eruptive mixing among these magmas is frequently observed in the form of magmatic enclaves and bands in lava flows and pyroclastic products. The petrological and volcanological diversity of the erupted materials suggests that complex differentiation processes occurred in the shallow part of the plumbing system. With the aim to reconstruct the magmatic feeding system and to identify the differentiation processes behind such a petrologic complexity, we analysed lavas and pyroclastic products representative of the recent eruptive sequences at La Fossa and combined the petrochemical features with thermo-barometric calculations, geochemical modelling and temperature gradient experiments. Thermo-barometric calculations indicate that the K-rich trachytic magma crystallized at lower pressure (160 ± 54 MPa) compared to the latitic (307 ± 47 MPa) and trachytic (208 ± 30 MPa) magmas. Differentiation modelling suggests that both trachytic and rhyolitic compositions can be obtained through differentiation of a common latitic magma, essentially by varying the plagioclase/ sanidine ratio. Temperature gradient experiments, performed at the conditions inferred for the shallow plumbing system of La Fossa volcano (150 MPa and 1050-900 °C), indicate different paths of melt differentiation that overall produce an increase of the SiO 2 /K 2 O ratio with the increasing H 2 O in the system (from 0 to 4 wt.%). This is consistent with the origin of K-rich trachytes at lower pressure and lower H 2 O content. In turn, the formation of crystal-poor rhyolites is explained by the segregation of the interstitial melt formed in a latitic-trachytic crystal mush, favoured by the second boiling of the melt and consequent exsolution of a fluid phase.

A Crystal Mush Perspective Explains Magma Variability at La Fossa Volcano (Vulcano, Italy)

“…The recent (i.e., in the last 1000 years) volcanic sequence of La Fossa volcano on the island of Vulcano, Italy represents a study case suitable to investigate the relationships between a mush-dominated crustal plumbing system and the geochemical variability of the erupted magmas. Volcanic products are in fact characterized by remarkable chemical heterogeneities, either in the form of mingled crystal-rich, latitic-trachytic magmatic enclaves in rhyolitic lavas and pyroclasts, or as mingled and partially mixed lati-trachytic and rhyolitic domains in pyroclasts and lavas [19][20][21][22]. Notably, these features are observed both at the scale of the eruptive sequence and within single eruptive units [19,22] and they have been attributed to mixing processes between intermediate mafic and rhyolitic melts, according to different petrogenetic models.…”

“…Ref [24,25] proposed instead that the crystal-rich enclaves occurring in the same eruption can be reconciled with the injection of the latitic magma into the more evolved rhyolitic magma. Ref [26], and more recently [21,27], suggested that crystal-rich enclaves occurring in the Pietre Cotte lava are the result of remobilization of crystal-rich cumulates, derived from a crystal mush. Following this latter interpretation, the association between crystal-poor rhyolites and (heterogeneous) crystal-rich enclaves at La Fossa can provide the ultimate evidence of the twofold role of crystal mushes in the origin of differentiated silicic magmas and their eventual segregation to feed volcanic eruptions in small arc systems.…”

Tracking metal evolution in arc magmas: Insights from the active volcano of La Fossa, Italy

“…The plumbing system of La Fossa is polybaric, dominated by fractional crystallization, crustal assimilation and magma mixing processes (Costa et al, 2020;De Astis et al, 2013;Peccerillo et al, 2006). These occur in a shoshonitic to latitic magma chamber at about 15 km deep, fed by shoshonitic basaltic magmas, and in shallower and smaller trachytic to rhyolitic reservoirs at 5-2 km deep (Costa et al, 2020;De Astis et al, 2013;Fulignati et al, 2018). Basaltic magmas have been only erupted at about 50 ka.…”

“…t2:3 neglected crustal assimilation processes. The model employs bulk distribution coefficient for Zr, Cu and Pb, calculated for each step of the differentiation process using partition coefficients derived from the literature and the relative proportions of crystallizing phases for Vulcano (Costa et al, 2020; see Supplementary Material S1 for details of the model). The sulfide phase/silicate melt partition coefficients for Pb and Cu were calculated according to Li and Audétat (2015) (Supplementary Material S1).…”

“…5). The trachyte-rhyolite step requires a high percentage (60 vol%) of fractionated solid (Costa et al, 2020), strongly increasing the final concentration of incompatible elements and possibly explaining the large spread in Zr values measured in rhyolites.…”