The December 2018 eruption at Etna volcano: a geochemical study on melt and fluid inclusions

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/4He 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

Experimental Crystallization of a High-K Arc Basalt: the Golden Pumice, Stromboli Volcano (Italy)

International audienceThe near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 49·4 wt % SiO2, 1·85 wt % K2O, 7·96 wt % MgO) from the present-day activity of Stromboli (Aeolian Islands, Italy) has been experimentally investigated between 1050 and 1175°C, at pressures from 50 to 400 MPa, for melt H2O concentrations between 1·2 and 5·5 wt % and {Delta}NNO ranging from –0·07 to +2·32. A drop-quench device was systematically used. AuPd alloys were used as containers in most cases, resulting in an average Fe loss of 13% for the 34 charges studied. Major crystallizing phases include clinopyroxene, olivine and plagioclase. Fe–Ti oxide was encountered in a few charges. Clinopyroxene is the liquidus phase at 400 MPa down to at least 200 MPa, followed by olivine and plagioclase. The compositions of all major phases and glass vary systematically with the proportion of crystals. Ca in clinopyroxene sensitively depends on the H2O concentration of the coexisting melt, and clinopyroxene Mg-number shows a weak negative correlation with {Delta}NNO. The experimental data allow the liquidus surface of PST-9 to be defined. When used in combination with melt inclusion data, a consistent set of pre-eruptive pressures (100–270 MPa), temperatures (1140–1160°C) and melt H2O concentrations is obtained. Near-liquidus phase equilibria and clinopyroxene Ca contents require melt H2O concentrations <2·7–3·6 and 3 ± 1 wt %, respectively, overlapping with the maximum frequency of glass inclusion data (2·5–2·7 wt % H2O). For olivine to crystallize close to the liquidus, pressures close to 200 MPa are needed. Redox conditions around {Delta}NNO = +0·5 are inferred from clinopyroxene compositions. The determined pre-eruptive parameters refer to the storage region of golden pumice melts, which is located at a depth of around 7·5 km, within the metamorphic arc crust. Golden pumice melts ascending from their storage zone along an adiabat will not experience crystallization on their way to the surface

High-resolution 40Ar/39Ar chronostratigraphy of the post-caldera (<20 ka) volcanic activity at Pantelleria, Sicily Strait

The island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major calderaforming eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of the latest eruptions. The new 40Ar/39Ar data capture a long-term (N15 ka) decline in eruption frequency with a shift in eruptive pace from 3.5 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12–14 ka. This shift in extraction frequency occurswithoutmajor changes in eruptive style, and is paralleled by a subtle trend of decreasingmelt differentiation index. We speculate that this decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steadystate compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal evidence for a short-term silicic eruption

Phase equilibrium constraints on the production and storage of peralkaline silicic magmas: insights from Kenya and Pantelleria.

The origin of peralkaline silicic rocks is still obscure and stands perhaps as one of the last major unsettled issues in classic igneous petrology. The debate goes back to the end of the 18th century and despite intensive petrological, geochemical and laboratory efforts the consensus has yet to emerge as to which mechanisms produce peralkaline derivatives. Bowen (1937) first proposed that the shift from metaluminous to peralkaline field was due to extensive fractionation of calcic plagioclase. Perhaps the best illustration of such an hypothesis is provided by the Boina rock series in the Ethiopian rift studied by Barberi et al. (1975). However, such an hypothesis still awaits experimental confirmation. A different view has been expressed for the origin of peralkaline rhyolites erupted in the central part of the Kenya Rift Valley. There, a partial melting of crustal protoliths has been advocated on the basis of geochemical arguments (Macdonald et al., 1987). The origin of peralkaline rocks at Pantelleria, the type locality of peralkaline rhyolites, is also a matter of debate. Mahood et al (1990) have proposed an origin via partial melting of Fe-rich differentiates of transitional basalts, whilst Civetta et al. (1998) have argued that pantellerites could be produced via extensive fractionation of their putative parent basalts. The diversity of opinions reflects in part that, presumably, there is not only one mechanism at work. But it is also due to the fact that most experimental studies devoted to the clarification of this problem have failed in producing decisive arguments during more than one century of intense debate

Evolution of the magma system of Pantelleria (Italy) from 190 ka to present

The eruptive history of Pantelleria has been marked by the eruption of nine peralkaline ignimbrites, with inter-ignimbrite episodes from small, local volcanic centres. New whole-rock geochemical data are presented for seven ignimbrites and used with published data for younger units to track compositional changes with time. From»190 ka, silicicmagmatismwas dominated by comenditic trachyte to comendite compositions, evolving along generally similar liquid lines of descent (LLOD). The final ignimbrite, the Green Tuff (»46 ka), was tapped from a compositionally zoned pantelleritic upper reservoir to a trachytic mush zone. Younger (20–7 ka) silicic magmatism has been relatively small scale, with compositions similar to the earliest pre-Green Tuff pantelleritic ignimbrite (Zinedi). These data suggest that the comenditic reservoirs may have been emplaced at deeper levels than the pantelleritic reservoirs. While both types of series evolved along similar LLOD dominated by fractionation of alkali feldspar, it is the fractionation of iron that determines whether comendite or pantellerite is produced. The deeper reservoirs were more oxidizing and wetter, thus leading to the crystallization of magnetite and therefore the fractionation of iron

Geochemical constraints on basalt petrogenesis in the Strait of Sicily Rift Zone (Italy): Insights into the importance of short lengthscale mantle heterogeneity

Igneous activity from the late Miocene to historic time (most recently 1891 CE) in the Strait of Sicily has created two volcanic islands (Pantelleria and Linosa) and several seamounts. These volcanoes are dominated by transitional (ol + hy-normative) to alkaline (ne-normative) basaltic lavas and scoriae; volcanic felsic rocks (peralkaline trachyte-rhyolite) crop out only on Pantelleria. Although most likely erupted through continental crust, basalts demonstrate no evidence of crustal contamination and are geochemically similar to oceanic island basalts (OIB). Despite their isotopic similarities, there are considerable compositional differences with respect to major and trace element geochemistry both between and within the two islands that are due to short-length scale mantle heterogeneity beneath the region as well as variability in partial melting and magma storage conditions. Published geophysical surveys suggest that lithospheric thickness beneath both islands is ~60 km; this is consistent with the results of our geochemical modelling (59\u201360 km), which also suggest mantle potential temperatures between 1415 and 1435 \ub0C, similar to those documented in other continental passive rifts. Trace element and isotopic data reveal that the asthenosphere beneath the Strait of Sicily is heterogenous at both interisland (100s of km) and intra-island (10s of km) scales. Although there is some compositional overlap between the two major synthems at Linosa, in general the older magmas (Arena Bianca, 700 ka) formed as a result of ~5% partial melting of a depleted MORB mantle (DMM) source enriched with a relatively small amount of recycled MORB material, whereas the younger magmas (Monte Bandiera, 530 ka) formed as a result of ~2% partial melting of a similar mantle source. Pantelleria magmas formed from a higher degree (~6%) of partial melting of a DMM source with a relatively greater amount of recycled MORB material and possibly other components. Geochemical modelling also suggests the older magmas on Linosa differentiated at a much shallower level (~8 km) than the younger magmas (~25 km, at or below the base of the crust) prior to eruption. Magmas stored in higher-level reservoirs were effectively homogenized and preserve a narrower compositional range than magmas sourced from depth. Data for the seamounts are scarce and compromised by significant seawater alteration; thus, these volcanic centers cannot be modelled but based on comparative geochemistry with the islands are likely the result of even smaller (&lt; 2%) degrees of partial melting beneath thicker (&gt; 60 km) lithosphere. Despite the geophysical similarities between the two islands in terms of lithospheric thickness and crustal thinning, melt productivity has been greater at Pantelleria, producing a much larger island and sustaining felsic magmatism, which we hypothesize may ultimately be entirely due to the local occurrence of much more fusible mantle

Geochemical Constraints on Mantle Sources and Basalt Petrogenesis in the Strait of Sicily Rift Zone (Italy): Insights into the Importance of Short Lengthscale Mantle Heterogeneity.

Igneous activity from the late Miocene to historic time (most recently 1891 ce) in the Strait of Sicily has created two islands (Pantelleria and Linosa) and several seamounts. These volcanoes are dominated by transitional (ol+hy-normative) to alkaline (ne-normative) basaltic lavas and scoriae; peralkaline felsic rocks (trachyte-rhyolite) crop out only on Pantelleria. Although most likely erupted through continental crust, basalts demonstrate no evidence of crustal contamination and are geochemically similar to oceanic island basalts (OIB). Despite their isotopic similarities, there are considerable compositional differences with respect to major and trace element geochemistry both between and within the two islands that are due to short-length scale mantle heterogeneity beneath the region as well as variability in partial melting and magma storage conditions. Published geophysical surveys suggest that lithospheric thickness beneath both islands is ~60 km; this is consistent with the results of our geochemical modelling (59-60 km), which also suggest mantle potential temperatures between 1415-1435°C, similar to other documented continental passive rifts. Although there is some compositional overlap between the three synthems at Linosa, in general the older magmas (Arena Bianca, 700 ka) formed as a result of ~5% partial melting of a depleted MORB mantle (DMM) source enriched with a relatively small amount of recycled MORB material, which differentiated in a shallow-level (~8 km) magma chamber prior to eruption whereas the younger magmas (Monte Bandiera, 530 ka) formed as a result of ~2% partial melting of a similar mantle source, which differentiated in a magma chamber at or below the base of the crust (~25 km). Pantelleria magmas formed from a higher degree (~6%) of partial melting of a DMM source enriched with a relatively greater amount of recycled MORB material with possibly other components. Data for the seamounts are scarce and compromised by significant seawater alteration; thus, these volcanic centers cannot be modelled but based on comparative geochemistry with the islands are likely the result of even smaller (60 km) lithosphere. Magmas stored in the higher-level chamber were more effectively homogenized and preserve a narrower compositional range. Despite the geophysical similarities between the two islands in terms of lithospheric thickness and crustal thinning, melt productivity has been greater at Pantelleria, producing a much larger island and sustaining felsic magmatism, which may ultimately be entirely due to the local occurrence of much more fusible mantle

Volcanological evolution of Pantelleria Island (Strait of Sicily) peralkaline volcano: a review

Pantelleria volcano has a particularly intriguing evolutionary history intimately related to the peralkaline composition of its explosively erupted magmas. Due to the stratigraphic complexity, studies over the last two decades have explored either only the pre-Green Tuff ignimbrite volcanism or the post-Green Tuff activity. We here focus on the whole evolutionary history, detailing the achievements since the first pioneering studies, in order to illustrate how the adoption and integration of progressively more accurate methods (40Ar/39Ar, paleomagnetism, petrography, and detailed field study) have provided many important independent answers to unresolved questions. We also discuss rheomorphism, a distinct feature at Pantelleria, at various scales and possible evidence for multiple, now hidden, caldera collapses. Although the evolutionary history of Pantelleria has shown that each ignimbrite event was followed by a period of less intense explosivity (as could be the present-day case), new geochronological and geochemical data may indicate a long-term waning of volcanic activity

Ascent of Stromboli yellow pumice magmas : experimental simulation at P<=4 KB.

Stromboli volcano is characterised by a persistent, mildly explosive activity producing a crystal-rich HK- basaltic scoria. The normal activity is periodically interrupted by more energetic explosions during which a crystal-poor HK basaltic pumice is emitted (yellow pumice), often intermingled with the crystal-rich scoria. We experimentally investigated the ascent path of the yellow pumice from the inferred depth of segregation ˜12 km to a very shallow level, where it interacts with the already degassed resident magma