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This study focus on the Mt Etna December 2018 eruption with the aim of investigating the geochemical characteristics of the feeding magma. New data on major and trace element geochemistry of olivine-hosted melt inclusions (MI) in volcanic products are presented together with the noble gas geochemistry of fluid inclusions (FI) in olivines. The noble gas geochemistry of fluid inclusions (FIs) in olivines was also investigated. The major element composition of MIs is variable from tephrite/trachybasalt to phonotephrite/basaltic trachyandesite, with SiO2 = 45.51-52.72 wt%, MgO = 4.01-6.02 wt%, and CaO/Al2O3 = 0.34-0.72. Trace element patterns of MIs present a typical enrichment in LILE and LREE, depletion in HFSE, and relatively fractionated REE patterns: (La/Lu) N= 18.8-41.08, with Eu/Eu* = (0.5-1.8). Positive anomalies in Sr (Sr/Sr* = 0.8-2.3) and Ba can be ascribed to the assimilation of plagioclase-rich cumulates in the magmatic reservoir. The variable Ba/La (9.8-15.8), K/Nb (260-1037), Ce/Nb (1.9-3.4), Rb/La (0.4-1.6), and Ba/Nb (10.8-25.8) ratios reveal mixing between two types of end-member magmas comparable to those emitted from 1) the 2001 Upper Vents and 2002-03 Northern Fissures (Type-1) and 2) the 2001 Lower Vents and 2002-03 Southern Fissures (Type-2), respectively. Type-2 represents a magma that was under the influence of a crustal component, whereas Type-1 is compatible with a HIMU-MORB-type heterogeneous mantle source. It appears that the 2018 MIs have captured the two different types of magmas, and the lack of homogenization may imply a very fast ascent (a few months). Compatible with the contemporary presence of primordial HIMU-MORB and crust-contaminated end-members are the data on noble gases from FI that highlighted an ³He/[sup 4]He value of 6.5-6.6Ra. The hypothesis of two different types of magmas, identified by the trace element geochemistry in MIs, is, thus, reinforced by helium isotopic data on FI of the 2018 eruption together with data from other Etnean eruptions and allows the inference of a bicomponent magma mixing. [ABSTRACT FROM AUTHOR]

Various xenoliths have been found in lavas of the 1763 ('La Montagnola'), 2001, and 2002-03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm. P wave velocities (V), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V with recent literature data on 3D V seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3-13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the 'solidification front', a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna's plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 'La Montagnola', 2001 and 2002-03 eruptions. [ABSTRACT FROM AUTHOR]

• Magma in Etna's fall stratified eruption was extremely volatile-rich (9600 ppm CO 2). • Magma ascended from storage at the Moho to the surface in <1–5 days. • High volatiles and fast ascent likely caused explosive intensity of this picritic magma. The 3930 BP Fall Stratified (FS) eruption at Mt. Etna is a rare example of a highly explosive eruption of primitive (picritic) magma directly from the mantle. The eruption produced ash plumes up to an estimated 20 km height, leading to a volcanic explosivity index (VEI) 4 (subplinian). Given its volatile-rich and primitive nature, the FS magma may have ascended rapidly from great depths to avoid fractionation and mixing within the extensive plumbing system beneath Etna. To determine the pressures from which the FS magma derived, we perform rehomogenization experiments on melt inclusions hosted in Fo 90–91 olivines to resorb shrinkage bubbles and determine the initial H 2 O and CO 2 in the melt. With measured CO 2 concentrations of up to 9600 ppm, volatile solubility models yield magma storage pressures of 630–800 MPa. These correspond to depths of 24–30 km, which are comparable to the seismologically estimated Moho. Therefore, the magma's high CO 2 concentration must come from carbon in the mantle (likely from subducted carbonates), as opposed to assimilation of shallow (<10 km) crustal carbonates. Diffusion modeling of H 2 O and forsterite zonation profiles in clear, euhedral, and crystallographically oriented olivines indicates rapid ascent of magma directly from its source region to the surface. Forsterite profiles exhibit a narrow rim of growth zoning but no detectable diffusional zoning, reflecting maximum ascent times of 1–5 days. Eighteen measured H 2 O profiles result in remarkably uniform decompression rates of 0.47 MPa/s (95% confidence interval of 0.16–1.28 MPa/s), which is among the fastest measured for basaltic-intermediate magmas. These decompression rates indicate that the final stage of magma ascent over the region in which H 2 O degasses (between the surface and ∼ 15 km) occurred extremely fast at ∼ 17.5 m/s. This eruption may provide a link between primary magma composition and eruption intensity: we propose that the unusually explosive nature of this picritic eruption was driven by high H 2 O and CO 2 concentrations, which led to continuously rapid ascent without stalling, all the way from the Moho. [ABSTRACT FROM AUTHOR]

Abstract: The major elements, trace elements and Sr and Nd isotopes of selected Etnean primitive rocks (<15kyBP) were studied in order to characterize their mantle source. The noble-gas geochemistry of fluid inclusions in minerals from the same lavas was also investigated. The major element compositions of whole rocks and minerals showed that these products are among the most primitive at Mt. Etna, comprising 6.3–17.5wt.% MgO. The variable LREE (Light Rare Earth Elements) enrichment relative to MORB (Mid-Ocean Ridge Basalt) (Lan/Ybn =11–26), together with the patterns of certain trace-element ratios (i.e., Ce/Yb versus Zr/Nb and Th/Y versus La/Yb), can be attributed to varying degrees of melting of a common mantle source. Numerical simulations performed with the MELTS program allowed the melting percentages associated with each product to be estimated. This led us to recalculate the hypothetical parental trace-element content of the Etnean mantle source, which was common to all of the investigated rocks. The characteristics of the Sr, Nd and He isotopes confirmed the primitive nature of the rocks, with the most-depleted and primitive lava being that of Mt. Spagnolo (SPA; 143Nd/144Nd=0.512908 87Sr/86Sr=0.703317–0.703325 and 3He/4He=7.6 Ra), and highlighted the similarity of the mantle sources feeding the volcanic activity of Mt. Etna and the Hyblean Plateau (a region to the south of Mt. Etna and characterized by older magmatism than Mt. Etna). The coupling of noble gases and trace elements suggests an origin for the investigated Etnean lavas from melting of a Hyblean-like mantle, consisting of a two-component source where a peridotitic matrix is veined by 10% pyroxenite. A variable degree of mantle contamination by crustal-like fluids, probably related to subduction, is proposed to explain the higher Sr-isotope and lower Nd-isotope values in some rocks (143Nd/144Nd up to 0.512865 and 87Sr/86Sr up to 0.703707). This process probably occurred in the source prior to magma generation, refertilizing some portions of the mantle. Accordingly, the estimated degree of melting responsible for each magma appears to be related to its 87Sr/86Sr enrichment. In contrast, the decoupling between 3He/4He and 87Sr/86Sr ratios requires the occurrence in the crustal reservoirs of further processes capable of shifting the He isotope ratio towards slightly more radiogenic values, such as magma aging or a contribution of shallow fluid. Therefore, different residence times in the Etnean reservoir and/or various rates of magma ascent could be key parameters for preserving the original He isotope marker of the Etnean mantle source. [Copyright &y& Elsevier]

During its activity, Mt. Etna underwent repeated changes of eruptive style and rock composition. Some of these variations are ascribed to the particular geodynamic setting in which Mt. Etna appears, namely that of the African subduction under the European plates. Presently, its volcanic activity consists of effusive and explosive eruptions marked by high gas fluxes. The increase in volatile and alkali contents of Etnean magmas that occurred in the last decades are mainly ascribed to variations in the mantle source(s).Olivine hosted melt inclusions were analysed in products of the last 15 ky for their chemical composition, volatile content and Fe speciation, in order to interpret the chemical variability and the oxidation state of Etnean magmas as well as and their possible evolution. Samples were selected from the most primitive products of picritic composition (Mg#= 67–70, Fo89–91) of the Fall Stratified (FS) eruption, the Mt. Spagnolo eccentric lavas (Mg#= 52–64, Fo82–88) and from more recent 2002–2013 eruptive products (Mg#=33–53, Fo68–83). Crystal fractionation and degassing processes were modelled at temperatures of 1050–1300 °C, pressures <500 MPa, and oxygen fugacity between 1 and 2 log units above the nickel-nickel oxide buffer, in order to interpret melt inclusions data. The variability of the major elements contents in the MIs, characterized by the decrease of MgO (1-12 wt%) and CaO/Al2O3 ratio (up to 1.5) and the increase of alkalis (1–6 wt% K2O; 1-8 wt% Na2O), suggests a continuous differentiation trend, which is mostly reproduced by the fractional crystallization of olivine + spinel + clinopyroxene ± plagioclase, in order of appearance. Similarly, the volatiles contents are extremely variable, with the highest H2O (up to 6 wt%) and CO2 (~0.6 wt%) contents found in FS magma, suggesting minimum entrapment depths up to 12-16 km below sea level. The variability of the S content (up to 0.43 wt% in the older Mt. Spagnolo inclusions) does not show a strong correlation with other volatiles content nor with the decrease of pressure, suggesting other influencing parameters, such as the variation of redox conditions.Fe3+/ΣFe ratios (0.15-0.36) by X-ray Absorption Near Edge Spectroscopy (XANES), obtained for some of these melt inclusions, embayments and matrix glasses, are weakly correlated with their volatile content, but strongly correlated with some major elements, particularly with respect to MgO and K2O (markers of melt differentiation). We argue that the differentiation process plays a major role in determining the observed Fe3+/ΣFe ratios, and therefore in constraining the oxidation state of Etnean magmas (NNO-1< fO2< NNO+2).The modelling of fractional crystallization and degassing processes concurs to suggest that the chemical variability of Etnean magmas, during the last 15 ky, can be mostly ascribed to melt differentiation due to fractional crystallization (and degassing) of a highly oxidized, primitive and hydrous FS magma-type. Such a process occurs along highly variable P-T-fO2 paths, accompanied by reduction of the oxidation state, and occasionally by mixing processes, crustal assimilation and volatile flushing. [ABSTRACT FROM AUTHOR]