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In the present study, we unveil the real significance of mylonitic reworking of the polymetamorphic crystalline basement in the Serre Massif of Calabria (Southern Italy). We use a multidisciplinary approach to comprehend the structural, microstructural and petrologic changes that occurred along a, so far, not much considered shear zone affecting the Variscan lower crustal rocks. It was never before studied in detail, although some late Cretaceous ages were reported for these mylonites, suggesting that this shear zone is of prime importance. Our observations reveal now that the formation of the new structural fabric within the shear zone was accompanied by changes in mineral assemblages, in a dominant compressive tectonic regime. During this tectono-metamorphic event, high-P mylonitic mineral assemblages were stabilized, consisting of chloritoid, kyanite, staurolite, garnet and paragonite, whereas plagioclase became unstable. Average peak P–T conditions of 1.26–1.1 GPa and 572–626 °C were obtained using THERMOCALC software. These data question (i) that the Serre Massif represents an undisturbed continuous section of the Variscan crust, as generally suggested in the literature, and (ii) highlight the role of (eo-)Alpine high-P tectonics in the Serre Massif, recorded within mylonite zones, where the Variscan basement was completely rejuvenated. [ABSTRACT FROM AUTHOR]

Open conduit basaltic volcanoes can be potentially hazardous as the eruptive activity may turn suddenly from a steady state to highly explosive. Unexpected changes in explosion intensity are recurrent at Stromboli volcano, where major explosions and large-scale paroxysms sometimes break off the ordinary, Strombolian activity with little or no warning. Two powerful paroxysmal eruptions took place at Stromboli volcano during the summer 2019, causing widespread fires, consistent damages across the island, injuries and one fatality. Prediction of similar events is really challenging for the modern volcanology, though models propaedeutic to early-warning monitoring systems are not properly assessed yet in many volcanoes worldwide. Here, we present a multi-parametric study that combines petrological and geophysical data to investigate processes generating the two paroxysms. The time information derived by Li enrichments in plagioclase crystals correlates with tilt time series derived by seismometers installed on the island, highlighting the dominant role of shallow conduit processes in triggering the 2019 paroxysmal activity. Our dataset conceives a mechanism of gas slug formation and fast upward migration that finally triggered the eruptions in very limited times. The proposed model questions our capability to forecast such kind of paroxysms in times that are rapid enough to allow mitigation of the associated risk. [ABSTRACT FROM AUTHOR]

Plagioclase peridotites are an important marker of the shallow geodynamic evolution of the lithospheric mantle at extensional settings. Based on low- pressure experiments, a recent study by Fumagalli et al. (2017) defined and calibrated a geobarometer for peridotitic bulk compositions, based on the Forster- ite-Anorthite-Ca-Tschermak-Enstatite (FACE) pressure-sensitive equilibrium. The Suvero plagioclase-bearing peridotites, on which the FACE geobarometer was calibrated, are primarily associated to plagioclase pyroxenites. Assuming that the pyroxenites record the same Pressure-Temperature evolution than the plagioclase peridotites, they represent ideal candidates to test the applicability of the FACE geobarometer on pyroxenitic compositions. As documented in the plagioclase peridotites, the pyroxenites are characterized by the development of fine-grained neoblastic assemblages, indicative of partial recrystallization under plagioclase-facies conditions. Chemical zonations in these neoblastic mineral aggregates suggest equilibration stages at variable pressure and temperature and allowed to document two re-equilibration stages corresponding to the onset of plagioclase-facies recrystallization (830-850°C, 6.9-8.1±0.5 kbar) and a shallower colder re-equilibration (770-790°C, 5.8-5.9±0.5 kbar), respectively. The decompressional evolution reported for pyroxenitic bulk compositions is consistent with the exhumation history documented in the associated Suvero peridotite, although at slightly higher equilibrium pressures (~ 1 kbar). Remarkably, the much lower XCr in pyroxenites reflects in lower Cr incorporation in pyroxenes and, consequently, in significantly higher Ca-Tschermak activity in clinopyroxene that might introduce the systematic pressure overestimation by FACE geobarometer. [ABSTRACT FROM AUTHOR]

This study presents a new petrological–geochemical data set for the Monte Capio and Alpe Cevia mafic–ultramafic sequences, which are exposed in the deepest levels of the Ivrea–Verbano Zone. These sequences are composed of a peridotite core, with dunite in the center, mantled by minor orthopyroxene-dominated pyroxenites and subordinate hornblende gabbronorites. Amphibole is ubiquitous in the peridotites and the pyroxenites (≤ 15 vol % and 10–40 vol %, respectively), and the peridotite–pyroxenite associations are frequently crosscut by amphibole-rich (45–90 vol %) veins/dykes showing sinuous-to-sharp planar boundaries towards host rocks. The whole-rock Mg# [100 × Mg/(Mg + Fetot2+)] decreases from the peridotites to the pyroxenites and the crosscutting amphibole-rich dykes (84–81, 80–77, and 73–66, respectively), consistently with the Mg# variations shown by included orthopyroxene, clinopyroxene, and amphibole. Olivine has relatively low forsterite and NiO amounts (84–78 mol % and ≤ 0.14 wt%), and spinel is characterized by low Cr# [100 × Cr/(Cr + Al)] of 7–24. The anorthite content of plagioclase varies from 91 to 88 mol% in plagioclase-bearing pyroxenites to 91–75 mol% in amphibole-rich dykes. The chondrite-normalized REE patterns of amphibole from peridotites and pyroxenites show nearly flat MREE–HREE, no evident Eu anomaly, and LREE that are slightly depleted to slightly enriched with respect to MREE. Amphibole from the amphibole-rich veins/dykes exhibits slight LREE depletion. Whole-rock and amphibole separates show substantial variations in initial Nd–Sr isotopic compositions (e.g., whole-rock εNd calculated at 290 Ma ranges from − 0.3 to − 4.7), irrespective of the rock-type and of incompatible element amphibole compositions. We propose that the Monte Capio–Alpe Cevia dunites formed by cooling of magma lenses that intruded the lowermost continental crust of the Ivrea–Verbano Zone. The chemically evolved signature of the dunites documents earlier crystallization of chemically primitive dunites at lower levels, or olivine fractionation within the dunites during melt ascent. Associated pyroxene-bearing peridotites show a magmatic evolution ruled by reaction of a melt-poor crystal mush with migrating melts relatively rich in SiO2 and H2O, which developed orthopyroxene and amphibole at the expenses of olivine ± clinopyroxene. These migrating melts may be reconciled with those feeding the crosscutting amphibole-rich veins/dykes, whose compositions suggest formation by chemically evolved H2O-rich basalts with an arc-type incompatible trace-element fingerprint. Unraveling the origin of the Monte Capio–Alpe Cevia pyroxenites is hampered by the complex open-system magmatic evolution, which also included assimilation of material released by basement metasediments and/or involvement of primary melt batches with different compositions. [ABSTRACT FROM AUTHOR]

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, Σ 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 conditions from 1140 to 1200 °C of temperature and from 0.6 to 0.9 GPa of pressure (22–33 km). Phase equilibria constraints and clinopyroxene compositions suggest that the parental magmas of Procida likely contained <2 wt% of H 2 O and were affected by an early stage of fractionation at a depth corresponding to the local Moho (~ 25 km corresponding to ~ 0.8 GPa assuming an average crustal density of 2.8 g/cm3). Based on our results, an early magmatic storage at this level could have caused a deep degassing process able to remove variable proportions of Na and K from the melt. • Phase relations of K-basalt at high-pressure and at anhydrous-hydrous conditions. • Piston cylinder experiments on the primitive Phlegraean magmas. • Differentiation of Phlegraean Volcanic District parental magmas at the Moho depth. • Deep degassing process removes variable proportions of Na and K from the melt. [ABSTRACT FROM AUTHOR]

The igneous complex of Solanas is a small, composite calc-alkaline intrusion emplaced at ~ 300 Ma in the Sàrrabus region (southeastern Sardinia), and consists of olivine gabbronorites, amphibole gabbros, microgabbros, quartz diorites, tonalites, amphibole granodiorites, and biotite granodiorites. Thermobarometry calculations indicate that the Solanas rocks were emplaced at mid-to-upper crustal levels (0.6–4.0 kbar). The intermediate and silicic rocks are metaluminous to weakly peraluminous and are enriched in large ion lithophile elements. The range in the initial Sr and Nd isotopic compositions is small throughout the intrusion despite a large range in silica contents (46.3–73.6 wt% SiO2). The isotopic signatures, mineralogy, and geochemistry suggest that the quartz diorites, tonalites, and granodiorites derived from fractional crystallisation and crustal contamination processes starting from different mafic parental magmas. The origin of tonalites and granodiorites is compatible with removal of plagioclase, hornblende, biotite, apatite and zircon starting from a quartz dioritic magma. The mafic rocks range in composition from primitive to relatively evolved (Mg# 49–70). The olivine gabbronorites and amphibole gabbros have petrographic and geochemical features of arc cumulates derived from different basaltic magmas. The microgabbros have geochemical characteristics similar to high-alumina basalts with fractionated rare-earth element patterns (LaN/YbN = 4.3–6.0), enrichment in large ion lithophile elements (e.g., Rb, Ba, U, and K) and depletion in Nb and Ta compared with the primitive mantle. These characteristics are consistent with partial melting of a mantle source that was enriched by subduction-related fluids. [ABSTRACT FROM AUTHOR]

The Panarea Volcanic Group (PVG) is a group of emergent islands rising from the truncated cone of an underwater edifice in the eastern sector of the Aeolian Island Arc in the Tyrrhenian Sea, Italy. Selected lava units from the main island of Panarea and some of the nearby islets were analysed for their major and trace element compositions to the dataset available in the literature. Major mineral phases were identified as plagioclase ± clinopyroxene ± orthopyroxene ± olivine ± amphibole ± mica. The lavas of this study range from andesite to rhyolite with major element compositions equivalent to previously published data. Pyroxene geobarometry suggests a polybaric distribution to crystal fractionation, beginning at the Moho, and continuing to a shallow magma reservoir, at approximately 0.8 km depth. A plot of Nd/Nd vs. Sr/Sr show the compositions of Panarea overlap with the compositions of the eastern and central Aeolian Arc, while Pb/Pb vs. Pb/Pb do not overlap, but fall between the central and eastern arc values. As major and trace element concentrations, and isotope compositions of the lavas of this study overlap most consistently with lava compositions from the central and eastern Aeolian Arc, indicating Panarea should be considered an 'intermediate' volcano in the arc. [ABSTRACT FROM AUTHOR]

Lava flows and pyroclastic deposits from strombolian fallout related to the activity of the Rivi and Capo volcanoes, which are representative of early subaerial volcanoes on Salina (Aeolian Islands), have been investigated through a geological-petrological approach. Our geological field survey shows that Rivi and Capo volcanoes are part of a single N50°E aligned volcanic complex, here named Rivi-Capo Volcanic Complex (RCVC). Stratigraphically specific rock sampling has allowed reconstruction of the magma feeding processes through time. Whole rock major element compositions, together with core-to-rim profiles of plagioclase and clinopyroxene crystals, show a general evolution toward more basic compositions through the three formations constituting the Capo volcano and within the Rivi center. MELTS simulations and mass balance modeling suggest that the RCVC rocks are the result of fractional crystallization of plagioclase, clinopyroxene, and olivine (ca. 45 % of solid removed) from a primary magma. In addition to fractional crystallization, continuous recharge and mixing with more basic magma coming from deeper parts of the magmatic plumbing system contributed to the final volcanic rock compositions. Our textural and microanalytical data on plagioclase and clinopyroxene crystals allow the definition of a multilevel magmatic storage system with reservoirs at ~20 and ~3 km below sea level. When processes of magma differentiation, ascent, and storage are considered together with the stratigraphic position of each sample, a history of continuous modification of the RCVC plumbing system can be constructed. Volcanism may have been characterized by fissure-type eruptions during the early stages (Lower Capo, Lower Rivi, and Middle Capo Formations), gradually changing later to central-type volcanism (Upper Capo and Upper Rivi Formations). [ABSTRACT FROM AUTHOR]

Abstract: The European Union Site of Community Importance Pineta San Vitale (Ravenna, Italy) is located in a complex lagoon system nearby the Adriatic Sea. This area is affected by relevant environmental pressures related to subsidence, groundwater salinization and industrial contamination. The occurrence of rare earth elements (REE), and the characterization of processes that control their migration in the water–soil system were performed for implementing the knowledge of the present environmental status in the area. Nine soil profiles were opened in the Pineta San Vitale and each horizon was characterized. In the parent material, dominant quartz, followed by plagioclase, mica, chlorite and calcite were recognized by XRD. Topsoil samples were characterized by neutral or sub-acid pH, relatively high organic carbon content (LOI up to 43wt.%; TOC: 38÷61g/kg), and low carbonate (11÷20g/kg CaCO3). Subsoil horizons were sub-alkaline or moderately alkaline, rich in carbonate (up to 114g/kg CaCO3), and depleted in organic matter. The electrical conductivity (EC: 0.2÷4.2 mS/cm) values along the soil profiles did depend on the water table fluctuations and the salinity of groundwater. Sequential extraction tests on two soil profiles showed the highest Y and ΣREE occurrence in the residual fraction of soils, with values generally increasing at depth, and total amounts similar to values estimated in the upper continental crust (UCC). Poor fractionation of REE was observed when the bulk soil horizon was considered, but different REE patterns occurred in the organic, oxide, and carbonate soil fractions, suggesting REE fractionation during weathering and pedogenic processes. Physical–chemical parameters and concentrations of total and dissolved elements were determined in 44 water samples. Waters showed near-neutral or slightly alkaline pH (6.8÷8.0) and total dissolved solids mostly in the range of 2 to 24g/L, with values increasing with increasing seawater influence. Concentrations of ΣREE determined in waters filtered through 0.4μm pore size filters varied from 0.1 to 189μg/L (0.7·10−9 ÷1.3·10−6 mol/L ΣREE), with the highest values observed in soil waters. REE values were not related to salinity, nor to major ions, while were positively correlated to Al concentrations. Further filtration through 0.015μm pore size filters removed about 90% of REE compared to the 0.4μm filtered samples, i.e. the studied waters showed a small quantity of truly dissolved REE species. Speciation modeling results by Visual MINTEQ indicated that truly dissolved REE species are dominated by bi-dentate organic complexes in the soil waters rich in dissolved organic carbon (up to 74mg/L DOC); aqueous complex with the carbonate and bicarbonate ligands increased in waters having low DOC. UCC-normalized patterns of REE in the colloidal fraction of soil waters showed slight enrichments in the middle REE (MREE), likely due to preferential adsorption of the MREE onto <0.4μm particles. UCC-normalized patterns of REE truly dissolved in soil waters showed slight enrichments in the heavy REE (HREE), probably due to dominant low molecular weight DOC in the truly dissolved fraction. For evaluating the potential REE contribution from the atmosphere, 12 vegetation samples (ivy leaves) were submitted to leaching tests using acidulated water as a proxy of rain water. The REE leached from leaves were in the range of 0.5 to 10μg/kg ΣREE, suggesting a significant atmospheric contribution to the water–soil system. UCC-normalized patterns of REE leached from leaves were nearly flat, possibly indicating a dominant geogenic source of atmospheric REE. [Copyright &y& Elsevier]

Mount Etna volcano is often characterized by bilateral eruptive events, involving both the south (S) and the north east (NE) rifts. The last event occurred in 2002-2003 from October 27 to January 28. A detailed, stratigraphically time-controlled sampling of lavas and tephra of the southern eruptive fissure was performed in order to (1) track the petrological features of products during the eruption and (2) integrate the results with those previously obtained on the NE rift. Whole-rock composition and textural observations were implemented by major and minor element analyses of plagioclases in lavas and tephra from both sides of the volcano. Fractionation models constrained by mass balance (major and trace elements) and Rayleigh calculations suggest that magmas are linked by the same liquid line of descent by fractionating 9.11 % of a mineral assemblage of Cpx (52.69 %), Plg (21.41), and Ol (7.46 %). These new data allowed us to identify at least two feeding episodes through the southern fissure and infer that high-KO porphyritic magmas, emitted on both the S and NE rifts, derives by fractionation from the same parent magma. However, lavas and tephra from the southern flank were slightly more primitive. Textural and petrological study of plagioclase moreover indicates that chemical-physical conditions in the deep feeding system were similar for magmas erupting from both rifts as suggested by the presence of dissolved rounded cores in both lavas. Magmas evolved differently on the S and the NE rifts only at shallow levels. Comparison with published seismotectonic data supports the idea that the main magma feeding the eruption on October 27 ascended along the same pathway at depth and was intercepted by the fracture system of the S and NE rifts at shallow depth, between 6 and 3 km b.s.l. [ABSTRACT FROM AUTHOR]

Abstract: Plagioclase crystals from four basalt samples dredged from different portions of the Marsili seamount (Southern Tyrrhenian, Italy) provide a detailed record of the changes occurring in the surrounding magma during its ascent in the crust. The textural features and chemical zoning (i.e., An content and Fe, Mg, Sr and Ba concentrations) of the plagioclases from each sample show that aggregation of various generations of crystals occurred in the host melt prior to eruption. Plagioclases resulting from crystallisation in small sill intrusions, where local flotation of plagioclase may have occurred, are ubiquitous. Instead, large (up to 1mm) homogeneous An-rich plagioclases are rare and have only been found in two of the studied basalts, recovered from the northern and southern portions of the volcano, respectively. These crystals record crystallisation events occurring long before eruption, and probably derive from deep-seated crystalline material. At the highest An values, the Sr and Ba concentrations of the studied plagioclases also indicate the existence of two distinct mafic magmas within the lower crustal plumbing system of the Marsili volcano, one with lower Sr and Ba contents than the other. Three of the four studied basalts came from magma with low Sr and Ba, whereas the fourth, which erupted from the southern portion of the volcano, sampled magma richer in Sr and Ba. The ascent of magma caused resorption of the previously formed plagioclase crystals and regrowth of normally or inversely zoned portions, depending on the water-saturated or water-undersaturated conditions of the surrounding magma, respectively. Small shallow magmatic reservoirs may be located under the southern part of the Marsili volcano, as testified by the resorbed sodic cores (An52–57) found in the two basalt samples from this portion. [Copyright &y& Elsevier]

Abstract: Luminescence dating was performed on three different sediments, collected in Italy. Measurements were accomplished on K-rich minerals extract, where we dispensed a small amount of grains on each aliquots. Anomalous fading was measured for each aliquot. We observed a negative correlation between the measured age and associated rate of fading, for each sample. In two samples, a few outliers were identified, where they displayed a lower measured age and fading rate. We positively identified the presence of Na-rich plagioclase minerals dominating the luminescence output of these outliers. [Copyright &y& Elsevier]

Abstract: Volcanic rock samples were systematically collected during the 2006 eruptive episodes at Mt. Etna. Major and trace elements, together with Sr–Pb isotope ratios, were then obtained on the whole rocks, for different times of emission and vent locations. On the same samples, selected plagioclase crystals were investigated through high-contrast BSE images and core-to-rim compositional profiles for An% and FeO wt.% were obtained. The major element variation through time suggests the involvement of a more basic magma, because MgO/FeOtot and CaO/Al2O3 ratios tend to increase from the beginning to the end of the eruption. This more basic magma was characterized by a peculiar geochemical and isotopic signature. This is testified by the negative trend of LREEs, some HFSEs (e.g., Nb, Zr, Y) and lead isotopes, together with increase of some incompatible element ratios (e.g., Rb/La, Zr/Nb) with respect to products erupted before. Concerning the plagioclase crystals, four main texture types were recognized at the core and rim, which indicate either near-equilibrium crystallization or disequilibrium accompanied by strong destabilization. These textures were attributed to: i) crystallization at equilibrium conditions at depth or within the open-conduit (two types of oscillatory zoning), ii) magma decompression at variable ascent rates (coarsely sieve-textures and dissolution at the core), iii) inputs of more basic magma (sieve-textured rims). All the available data, coupled with CO2/SO2 molar ratio measured in the gas plume obtained from literature, support the idea that the pulsating regime observed for the volcanic phenomena at the surface during the period July–December 2006 reflects the transient, rapid ascent of several CO2 bubble fluxes and/or CO2-rich magma batches from the deep to the shallow levels of the plumbing system. This integrated study has also led to the identification of the replenishment event during the eruption that allowed the ingress of the magma with distinct geochemical/isotopic signature. [Copyright &y& Elsevier]

Abstract: Seven rock samples were systematically collected from innermost to the outermost portion of a dike outcropping at Mt. Etna volcano. Results show that, from dike core-to-rim, plagioclase, clinopyroxene and titanomagnetite show compositional variations due to increasing cooling rate. Plagioclase is progressively enriched in An from innermost to the outermost part of the dike. Similarly, clinopyroxene components En+CaTs+CaFeTs increase, whereas Di+Hd decrease. The Usp content in titanomagnetite also systematically decrease from dike core-to-rim. Partition coefficients and thermometers based on the crystal-liquid exchange reaction indicate that, due to rapid cooling rates at the dike outer portions, early-formed crystal nuclei do not re-equilibrate with the melt. The chemistry of minerals progressively deviates from that of equilibrium; consequently, from dike core-to-rim, mineral compositions resemble those of high-temperature formation. The chemical variations of clinopyroxene and plagioclase in dike samples mirror those obtained from cooling experiments carried out on alkaline basalts. Accordingly, we used an experimental equation based on clinopyroxene compositional variation as a function of cooling rate to determine the cooling conditions experienced by the crystals during dike emplacement. The estimated cooling rates are comparable to those predicted by thermal modeling based on an explicit finite-difference scheme. [Copyright &y& Elsevier]

Abstract: A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H2O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile-rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H2O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions. Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H2O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm). [Copyright &y& Elsevier]

Abstract: In-situ cosmogenic 36Cl production rates from spallation of Ca and K determined in several previously published calibration studies differ by up to 50%. In this study we compare whole rock 36Cl exposure ages with 36Cl exposure ages evaluated in Ca-rich plagioclase in the same 10±3ka lava sample taken from Mt. Etna (Sicily, 38°N). The exposure age of the sample was determined by K–Ar and corroborated by cosmogenic 3He measurements on cogenetic pyroxene phenocrysts. Sequential dissolution experiments showed that high Cl concentrations in plagioclase grains could be reduced from 450ppm to less than 3ppm after 16% dissolution. 36Cl exposure ages calculated from the successive dissolution steps of this leached plagioclase sample are in good agreement with K–Ar and 3He age. Stepwise dissolution of whole rock grains, on the other hand, is not as effective in reducing high Cl concentrations as it is for the plagioclase. 330ppm Cl still remains after 85% dissolution. The 36Cl exposure ages derived are systematically about 30% higher than the ages calculated from the plagioclase. We could exclude contamination by atmospheric 36Cl as an explanation for this overestimate. Magmatic 36Cl was estimated by measuring a totally shielded sample, but was found to account for only an insignificant amount of 36Cl in the case of the 10ka whole rock sample. We suspect that the overestimate of the whole rock exposure age is due to the difficulty in accurately assessing all the factors which control production of 36Cl by low-energy neutron capture on 35Cl, particularly variable water content and variable snow cover. We conclude that some of the published 36Cl spallation production rates might be overestimated due to high Cl concentrations in the calibration samples. The use of rigorously pretreated mineral separates reduces Cl concentrations, allowing better estimates of the spallation production rates. In the Appendix of this paper we document in detail the equations used. These equations are also incorporated into a 36Cl calculation spreadsheet made available in the supplementary data. [Copyright &y& Elsevier]

Abstract: Crystals in volcanic rocks are sensitive records of magma chamber and conduit conditions under volcanoes. Plagioclase is an invaluable tool to identify ascent rates for calc-alkaline magmas, but may be absent in alkaline melts. In contrast, leucite is common in alkaline magmas and is potentially useful to investigate storage and ascent conditions prior to volcanic eruptions. Leucite microphenocrysts are ubiquitous within the products from all phases of the 79AD eruption of Vesuvius. Steady-state (isobaric–isothermal) and dynamic (decompression) experiments on white phonolitic pumice from the opening (EU1) and lower Plinian (EU2) phases of the eruption were performed at temperature conditions ranging from 800 to 850 °C to test the possibility that leucites within this ‘white’ magma formed during ascent. However, multiple-step decompression (MSD) experiments using a decompression rate of 0.25 MPa/s failed to crystallize leucite even at pressures well below its stability domain. On the other hand, single-step decompression (SSD) experiments from 150 MPa to 25 MPa result in leucite crystallization after a ~12 h lag period, but the skeletal habit and size distribution differ from those seen in natural pumices. Instead, euhedral leucites texturally matching those observed in 79 AD samples formed after 5 days under isobaric and isothermal (IB–IT) experimental conditions. Crystallization conditions derived from the latter experiments suggest the magma reservoir was thermally zoned with cooler EU1 (T =830–840 °C) overlying slightly hotter EU2 (T =850–925 °C) magma. Two models for natural crystallization conditions are consistent with the experimental data: either leucites formed at ~4 km depth (P ~100 MPa) in a steady storage environment inside a magma saturated with H2O-rich vapor, or, alternatively, the white magma was initially undersaturated with respect to H2O and leucites formed during a slow depressurization event prior to the eruption. Leucite crystallization seemingly adheres to the classical nucleation theory, and supports a compositional (i.e. H2O) control on surface tension. Derived leucite growth rates reach ~10−7 mm s−1 minimum, comparable to the fastest growth rates observed for plagioclase crystals in calc-alkaline magmas. [Copyright &y& Elsevier]

Abstract: Plagioclase phenocrysts in dacites from the high-K calc-alkaline Capraia Volcano were investigated for major, trace element and Sr isotope variations in order to gain better insight into the proposed open-system behaviour of the volcano. Repeated dissolution zones in plagioclases from the early-erupted Monte Castello (MC) and later Monte Rucitello (MR) dacitic units are associated with a sharp increase in An content and variation in isotopic compositions (87Sr/86Sr=0.70872–0.71004), which argue for repeated magma mixing. In particular, petrographic and mineralogical evidence suggest that recharge with hot, basaltic magma occurred repeatedly during the lifetime of the Capraia volcanic system. We attempted to correlate shared crystallization and resorption histories using chemical (mol% An, Fe) and Sr isotopic zoning in plagioclase. In this correlation framework, the observed mismatch between chemical and isotopic data in some phenocrysts is interpreted to reflect either growth of phenocrysts in a separate magma chamber, or compositional zoning within a single magma chamber. A minor role of crustal contamination is inferred. Instead, the intra-crystal variations in isotopic composition in plagioclase reflect the existence of distinct components that may have thoroughly mixed at the end of the MC plagioclase growth history. The dominant radiogenic component is similar to lamproites occurring in Tuscany. Recharge with this component occurred throughout the genesis of the MR dacites, but was restricted to the early growth history of the MC dacites. The MR plagioclases are inferred to have crystallized from separate, more differentiated batches of magma that were likely to have evolved at the top of a zoned magma reservoir. Decrease in the An content after most dissolution zones in all phenocrysts, and low temperature estimates from zircon saturation thermometry (<800 °C) also emphasize the importance of crystal fractionation during the evolution of the volcano, in agreement with the occurrence of a significant amount of rhyolitic material. Thus, it is inferred that a rather large magma chamber may have existed during the first cycle of eruptive activity. [Copyright &y& Elsevier]

The 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 ΔNNO 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 ΔNNO. 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 ΔNNO = +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. [ABSTRACT FROM AUTHOR]

We report the occurrence of unusual, high-magnesium (Fo96) olivine phenocrysts in a basaltic lava and an ejected lithic block from the Upper Vancori period (∼13 ka) and the recent activity (2002–2003) of Stromboli volcano, Italy. The samples that contain this distinctive mineral chemistry are a shoshonitic basalt and a basaltic andesite with anomalous bulk-rock chemical characteristics in which the iron is highly oxidized (6–8 wt % Fe2O3 and <1 wt % FeO). In other respects these samples are similar to the majority of Stromboli basalts, characterized by the coexistence of olivine, clinopyroxene, plagioclase and Fe–Ti oxides as phenocrysts, and clinopyroxene, plagioclase and Fe–Ti oxides in the groundmass. In the high-magnesium olivine samples, Fe–Ti oxides (pseudobrookite) typically occur as symplectitic intergrowths with the olivine phenocrysts, indicating simultaneous growth of the two phases. We propose, as a paragenetic model, that the Fo96 olivine phenocrysts crystallized from a highly oxidized basaltic magma in which most of the iron was in the ferric state; hence, only magnesium was available to form olivine. The highly oxidized state of the magma reflects sudden degassing of volatile phases associated with instantaneous, irreversible, transient degassing of the magma chamber; this is postulated to occur during periods of sudden decompression induced by fracturing of the volcanic edifice associated with paroxysmic activity and edifice collapse. [ABSTRACT FROM AUTHOR]

Oscillatory zoning in plagioclase crystals from different mixed lava flows cropping out on the Island of Capraia (Italy) has been investigated. An-calibrated profiles from back-scattered-electron images are used as compositional time series of oscillatory zoning. They are analyzed by a qualitative visual method that allows to appreciate at first sight the global structure of the time series and by a quantitative method to reconstruct the attractors associated to the oscillatory patterns, and to calculate the fractal dimension of attractors. Results show that attractors have fractional dimensions, indicating that series are chaotic. In addition, it is evidenced that there is a wide variation in oscillatory zoning in crystals from the same lava flow and that plagioclase populations from the different lava flows differ in the shape of frequency histograms of attractor dimension. The development of oscillatory zoning is simulated by considering a chemically inhomogeneous magmatic mass governed by chaotic flow fields, coupled with chemical diffusion, in which plagioclase crystals grow according to the availability of nutrients in their neighborhoods. Results show that in such a dynamical system plagioclase crystals develop chaotic zoning patterns analogous to those observed in natural plagioclases. This approach allows us to explain the differences observed in plagioclase crystals from the same lava flow and the differences in the shape of frequency histograms of attractor dimensions in the four lava flows. [ABSTRACT FROM AUTHOR]

Trachytes from the Euganean Hills District (Italy) contain metapelitic xenoliths that have been pyrometamorphosed during incorporation in the melt. In xenoliths containing sillimanite crystallized during a previous regional HT/LP metamorphism, fibrolite systematically nucleates at the grain boundaries of sillimanite prisms and within plagioclase crystals. Ternary feldspar thermometry shows that plagioclase in contact with sillimanite plots along the 750°C solvus that reflects near-equilibrium conditions of regional metamorphism. Plagioclase containing fibrolite plots closer towards the 950°C solvus, reflecting the tendency of plagioclase to re-equilibrate at high temperature during pyrometamorphism by a fibrolite-forming reaction: K-feldspar (1) + plagioclase (1)
→K-feldspar (2) + plagioclase(2) + fibrolite. In sillimanite-free xenoliths, fibrolite did not form during pyrometamorphism, because these xenoliths do not contain plagioclase. In these xenoliths, andalusite prisms are replaced by oriented fibrous corundum needles and K-feldspar. The petrographic evidence suggests that when reaction rate is high as a result of reaction temperature overstepping, sillimanite grows as fibrolite because the surface energy of {110} faces is low and their growth rate is rapid. The same explanation may hold also for the growth of acicular corundum pseudomorphing andalusite prisms in sillimanite-free xenoliths. [ABSTRACT FROM AUTHOR]

Summary ¶Fine- to coarse-grained plutonic nodules within the Petrazza pyroclastics (Paleo-Stromboli I period) consist of gabbroic rocks with variable amounts of interstitial material. They are characterised by cumulate textures and low pressure modal mineralogy formed by plagioclase (An96–87)+clinopyroxene (Mg-v 82–94)+olivine (Fo83–74)±amphibole±opaque minerals; the interstitial material consists of newly crystallised microlites (quenching) of plagioclase (An73–55)+amphibole+clinopyroxene±olivine±biotite±opaques and highly variable amounts of residual glasses that range in composition from shoshonite and high-K basaltic andesite to high-K andesite and latite. The interstitial material has a relatively high but variable degree of vesicularity. The whole rock incompatible element abundances are lower than – but the patterns are typical of – in subduction related magmas and the incompatible trace-elements are well correlated with the amount of the interstitial material. The Sr, Pb and Nd isotopic ratios resemble those of the extrusive rocks of Stromboli older series and the mineral chemistry of the gabbros is similar to that of the HKCA Paleo-Stromboli lavas. Modal mineralogy, mineral chemistry and chemical-isotopic whole rock compositions suggest that the cumulus portions of the gabbroic nodules crystallised from basaltic magmas compositionally compatible with those erupted by Stromboli volcano. The interstitial material does not represent the residual liquid after in situ crystallisation of the gabbros; it is also distinct from the juvenile host andesite magma. Textural evidence, Fe–Mg mineral/liquid partioning and mass balance calculations indicate that the interstitial material (quench crystals and vesicular glass) derived from infiltrated hydrous basaltic liquid undercooling and vesiculation of which occurred during the eruption of the Petrazza pyroclastics. [ABSTRACT FROM AUTHOR]

K-feldspar–plagioclase–quartz mineral textures as well as biotite and hornblende compositions are compared for suites of metamorphosed mafic rocks from two widely separated traverses. A portion of either traverse has experienced a high-grade dehydration event transforming it from an H2O-rich, hornblende-bearing zone to an H2O-poor, hornblende-free, orthopyroxene-bearing, ‘granulite facies’ zone at 700–800°C and 7–8 kbar. In the Kigluaik Mountains, Seward Peninsula, Alaska, dehydration took place over an 85 cm thick layer of metatonalite in contact with a marble during regional metamorphism and involved a CO2-rich fluid, whereas for the Val Strona di Omegna traverse, Ivrea–Verbano Zone, northern Italy, dehydration took place over a 3–4 km thick sequence of metabasites interlayered with metapelites in a contact metamorphic event involving basaltic magmas intruded at the base of the sequence. Orthopyroxene-bearing samples from both dehydration zones show micro-veins of K-feldspar along quartz and plagioclase grain boundaries as well as replacement antiperthite in plagioclase. K came primarily from the breakdown of hornblende + quartz to orthopyroxene ± clinopyroxene, feldspar and fluid. Biotite either was stabilized or formed in the dehydration zones and is enriched in Ti, Mg, F and Cl relative to biotite in the amphibolite facies zone. [ABSTRACT FROM PUBLISHER]

Anatectic granites represent the products of crustal maturation by partial melting, therefore, studies on their origin are crucial to understanding how continental crust forms and evolves. This contribution compares composition of melts obtained from three tools allowing the investigation of anatectic granites in their source region: experiments, phase equilibria modelling and nanogranitoids. Benchmark experimental runs are selected along with nanogranitoids from the Ivrea–Verbano Zone (IVZ; NW Italy) and compositions of melt are then calculated using the same bulk rock compositions considered in experimental and natural case studies. The possible effects of melt loss, diverse P–T paths and variable equilibration volumes on melt composition are modeled for IVZ rocks. Results are then compared with those of previous comparative studies to verify the presence of systematic compositional divergences and similarities. Collectively, all data reflect pressure and temperature conditions (≈700–950 °C and 0.3–1.5 GPa) typical of the suprasolidus orogenic crust, with source regions ranging from metapelite, metagreywacke to metagranite. Contents of FeO, MgO, K 2 O and, to a lesser extent, CaO in the melt show the largest divergences. FeO, MgO and CaO tend to be enriched in experimental melts and nanogranitoids, whereas K 2 O is mostly overestimated in calculated melts. After a consideration of the common pitfalls for each approach we discuss the potential causes for the main geochemical discrepancies. The most extreme deviations of FeO and MgO, often observed in low-T (≤ 750 °C) melts, seem to be related to the selected melt models, which have been mostly calibrated on the basis of high-T (≥800 °C) experiments. Some experimental melts tend to be strongly depleted in K 2 O due to the metastable persistence of micas at much higher temperatures in laboratory runs, whereas the compositions of nanogranitoids in some investigated rocks may be partially controlled by diffusion in the melt. Disequilibrium melting of plagioclase may explain the CaO enrichment observed in some experimental and natural melts. When local equilibration volumes are considered in the calculations, a better match is observed for some oxides (e.g. FeO and MgO) between nanogranitoids and their calculated counterparts, even though other significant divergences remain unaffected. The combination of diverse petrologic tools, with proper recognition of their shortcomings, represent the best approach to shed light into the complexities of melting processes in nature which control the composition of anatectic granites at the source. • Experiment-model melt and nanogranitoid-model melt comparisons were made. • Contents of FeO, MgO, K 2 O and CaO in the melt show the largest divergences. • Deviations of FeO and MgO at ≤750 °C seem to be related to the selected melt models. • Experimental melts strongly depleted in K 2 O due to the metastable persistence of micas. • Nanogranitoid composition may be partially controlled by diffusion in the melt. [ABSTRACT FROM AUTHOR]

In the Tethyan realm, leucocratic rocks were recognized as dikes and layers outcropping in the ophiolitic rocks of the Western Alps, in Corsica, and in the Northern Apennines. Several authors have suggested that the origin of leucocratic rocks is associated with partial melting of cumulate gabbro. Major and trace elements composition and paragenesis provided information about the leucocratic rocks genetic processes. This research aims at disclosing, for the first time, the petrographical and geochemical features of Timpa delle Murge leucocratic rocks, Pollino Massif (southern Italy), in order to discuss their origin and geodynamic significance through a comparison with other Tethyan leucocratic rocks. These rocks are characterized by high amounts of silica with moderate alumina and iron-magnesium contents showing higher potassium contents than plagiogranites, due to plagioclase alteration to sericite. Plagioclase fractionation reflects negative Eu anomalies indicating its derivation from gabbroic crystal mushes. The chondrite normalized REEs patterns suggest the participation of partial melts derived from a metasomatized mantle in a subduction environment. The results reveal some similarities in composition with other Tethyan leucocratic rocks, especially those concerning Corsica and the Northern Alps. These new data provide further clues on the origin of these leucocratic rocks and the Tethyan area geodynamic evolution. [ABSTRACT FROM AUTHOR]

The lithosphere–asthenosphere boundary (LAB) is a zone where thermal, mechanical, and material interactions take place between the conductive mantle and the underlying convective mantle, and it plays an important role in plate tectonics. In this paper we focus on the thermal aspects of the LAB zone, based on a petrological study of a large peridotite complex that experienced exhumation in the solid state, for the most part. The complex of interest is the Lanzo Massif in the western Alps, northern Italy, where we were able to clarify its thermal and exhumation history and estimate the original thermal structure before exhumation. We examined plagioclase-bearing lherzolites collected from 16 localities, covering the entire massif. All the constituent minerals show compositional heterogeneities on the grain scale. The patterns of Ca, Al, and Cr zoning in pyroxene, the fluorescence-corrected Ca zoning of olivine adjacent to pyroxenes, the Cr and Al zoning of spinel adjacent to plagioclase, and the Ca and Na zoning of plagioclase suggest that an early, nearly isothermal decompression of the Lanzo Massif partly crossing the dry solidus was followed by monotonous cooling through the plagioclase-facies peridotite field. Various deformation microstructures allowed us to specify the timing of the deformation in the framework of the decompression history by carefully observing their relationships with the compositional zoning of the minerals. We show that the deformation took place mainly when effective cooling had started following a period of nearly isothermal exhumation. By applying several geothermometers and evaluating the compositional zoning of the minerals, we were able to quantify the spatial variations in the thermal and decompression history of the Lanzo Massif and constrain the timescales of decompression and cooling. All the estimated temperatures decrease from the southern body towards the northern body. The grain-scale patterns of zoning indicate that the temperatures recorded by the cores of orthopyroxene (1000–1200 °C) indicate a long period of residence in the mantle, whereas those recorded by the rims of pyroxenes and the cores and rims of olivine (600–1100 °C) represent closure temperatures at various times during the decompression. All the closure temperatures decrease from south to north, while the temperature differences between the cores and rims of orthopyroxene increase. This suggests that the cooler and probably shallower northern body cooled at a relatively slow rate than the hotter and probably deeper southern body. The decrease in temperature of ~60 K from south to north, calculated from orthopyroxene cores, may represent the geotherm near the LAB zone. A thermal gradient of ~10 K/km is indicated, which is significantly greater than that estimated for deep subcontinental lithosphere in a steady thermal state. Such a high geotherm might have been caused by thermal perturbation from the underlying hotter asthenospheric mantle. • The Lanzo massif was studied to estimate thermal structure near the ancient LAB. • Chemical zonings in all mineral grains indicate simple cooling of the massif. • Both closure and primary T decrease from south to north in the massif. • The variation of closure T indicate decreasing cooling rate from south to north. • The high primary thermal gradient is due to transient heating from asthenosphere. [ABSTRACT FROM AUTHOR]

Late-Variscan granitoid rocks of trondhjemitic and granitic composition, intruded in migmatitic paragneisses in the north-eastern Peloritani Mountains (southern Italy) at ~310 Ma and ~300 Ma, respectively, exhibit a range of deformation microstructures developed under a shear regime at decreasing temperatures. Non-coaxial deformation is documented by sigmoidal feldspar porphyroclasts, mica fish, and asymmetric boudins affecting tiny andalusite crystals. Late-Variscan shearing during granitoid cooling is constrained by largely represented chessboard patterns in quartz and, especially, submagmatic fractures in plagioclase, indicating deformation at high-temperature conditions (T > 650 °C), in the presence of melt. Submagmatic deformation was extensively superseded by deformation at lower temperatures. Examples of solid state-high temperature deformation-related microstructures (T > 450 °C) include feldspar bulging, quartz grain boundary migration, and subgrain rotation recrystallization. Finally, low temperature subsolidus microstructures (T < 450 °C) consist of quartz bulging, mica kinks, and feldspar twinning and bending. A complete sequence of deformation, operating from submagmatic to low-temperature subsolidus conditions is recorded in both the older and younger granitoids, suggesting a duration of ~20 Ma for shear zone activity during post-collisional exhumation of the Variscan middle crust in southernmost Italy. [ABSTRACT FROM AUTHOR]