Forni, Francesca

The peninsular and insular Italy are punctuated by Quaternary volcanoes and their rocks constitute an important aliquot of the Italian Quaternary sedimentary successions. Also away from volcanoes itself, volcanic ash layers are a common and frequent feature of the Quaternary records, which provide us with potential relevant stratigraphic and chronological markers at service of a wide array of the Quaternary science issues. In this paper, a broad representation of the Italian volcanological community has joined to provide an updated comprehensive state of art of the Italian Quaternary volcanism. The eruptive history, style and dynamics and, in some cases, the hazard assessment of about thirty Quaternary volcanoes, from the northernmost Mt. Amiata, in Tuscany, to the southernmost Pantelleria, in Sicily Channel, are here reviewed in the light of the substantial improving of the conceptual models, methodological approaches and the overall knowledge made in the last decades in the volcanological field study. We hope that the prest review can represent an useful and agile document summarising the knowledege on the Italian volcanism at the service of the Quaternary community operating in central Mediterranean area.

Somma-Vesuvius is one of the most iconic active volcanoes with historic and archeological records of numerous hazardous eruptions. Petrologic studies of eruptive products provide insights into the evolution of the magma reservoir before eruption. Here, we quantify the duration of shallow crustal storage and document the evolution of phonolitic magmas before major eruptions of Somma-Vesuvius. Garnet uranium-thorium petrochronology suggests progressively shorter pre-eruption residence times throughout the lifetime of the volcano. Residence times mirror the repose intervals between eruptions, implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution, thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longer-lived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that long-lived deep-seated reservoirs and transient upper crustal magma chambers are common features of subvolcanic plumbing systems.

This Special Issue of Minerals collects seven different scientific contributions highlighting how magma chamber processes and eruption dynamics studied either in the laboratory or in nature may ultimately control the evolutionary histories and geochemical complexities of igneous rocks. The main purpose is to report novel results bearing on the most important mechanisms that govern the textural and compositional changes of minerals and their host rocks. Different scenarios and case studies are presented here regarding the polybaric–polythermal solidification of magmas under either closed or open conditions. Results from these scientific contributions have the potential to elucidate those physicochemical parameters which play key roles in crystallizing systems, when degassing, magma mixing, and magma–fluid–crust interaction phenomena take place over different temporal and spatial scales. Rare-earth element (REE) modeling based on bulk rock analyses of crustal and mantle xenoliths from alkaline lavas and pyroclastic rocks is used to highlight magma–crust interaction processes between MORB oceanic crust and overlying sediments located beneath the Cape Verde oceanic plateau [1]. Clinopyroxene-based thermobarometric calculations indicate crystallization from magmas stored within the oceanic lithospheric mantle. During ascent towards the surface, these magmas encountered overlying sedimentary rocks and, occasionally, incorporated xenoliths, recording variable degrees of crustal contamination [1]. By integrating geochronological (U-Pb and 40Ar/39Ar) and isotope (Sr-Nd-Pb-Hf) data, a geotectonic model is proposed for Mesozoic magmatic rocks from the South China Sea by involving subduction of an early Neo-Tethyan lithospheric domain [2]. The geochronological and geochemical signatures of adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount outline contemporaneous formation of these products during northward subduction of the proto-South China Sea along the SE Asian continental margin in the Early Cretaceous [2]. A singularity analysis (frequency anomalies) is also applied to U–Pb ages of zircons from the Great Xing’an Range to characterize the causative relationship between age abnormality and Pacific Plate subduction [3]. Data interpretation is consistent with independent geological and geochemical constraints, thus emphasizing the importance of singularity analysis for quantitative characterization of the time scales involved in long-term volcanic activity and past tectonic regimes [3]. The Bushveld Complex (South Africa) is one of the largest igneous complexes on Earth and is also a major deposit of economically viable mineralization. The Merensky Unit, within the Bushveld Complex, indicates origin by magma mixing phenomena resulting from multiple replenishment events [4]. The final solidification process is governed by variable degrees of undercooling, with a role also played by crystal compaction and migration of interstitial fluids. In this perspective, the Merensky Unit is not a cyclic unit sensu strictu as the product of closed-system fractional crystallization but rather the Bushveld magma chamber was characterized by open-system processes leading to formation of the Merensky Unit [4]. The Zhesang gold deposit (southeastern Yunnan, China) is another important mining area, located in the Dian-Qian-Gui region. Calcite is a common gangue mineral of the Zhesang deposit, and its origin is related to both ore-stage or post-ore-stage processes [5]. REE and C-O isotopic characteristics of calcite suggest that fluids feeding the growth of ore-stage crystals were derived from a mixture of crustal fluids by meteoric water-leaching wall rocks and a small amount of basic magmatic fluids. On the other hand, the formation of post-ore calcite crystals was driven by fluid–rock interaction phenomena between meteoric water and marine carbonates [5]. Mt. Etna (Sicily, Italy) is the largest volcano in Europe and one of the most active and most intensely monitored on Earth. Hydrous crystallization experiments conducted on trachybasaltic compositions document the intriguing role of H2O in the differentiation of magmas ascending throughout the entire plumbing system [6]. The crystallization regime at shallow crustal levels suddenly shifts from H2O-undersaturated to H2O-saturated conditions due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. Abundant release of H2O leads to upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions, as well as to the voluminous gas emissions measured at summit craters and flank vents [6]. Furthermore, kinetically controlled cation redistributions in phenocrysts from sill-like magmatic intrusions at Mt. Etna testify to dynamic crystallization conditions that are principally controlled by degassing-induced undercooling paths [7]. Early formation of clinopyroxene at depth was the main controlling factor for the REE signature of the sill, whereas subsequent H2O loss at lower pressures enlarged the stability field of plagioclase, causing trace element enrichments at pre- and syn-eruptive conditions [7].

Understanding the mechanisms that control the accumulation of large silicic magma bodies in the upper crust is key to determining the potential of volcanoes to form caldera-forming eruptions. Located in one of the most populated regions on Earth, Camp Flegrei is an active and restless volcano that has produced two cataclysmic caldera-forming eruptions and numerous smaller eruptive events over the past 60,000 years. Here, we combine the results of an extensive petrological survey with a thermomechanical model to investigate how the magmatic system shifts from frequent, small eruptions to large caldera-forming events. Our data reveal that the most recent eruption of Monte Nuovo is characterized by highly differentiated magmas akin to those that fed the pre-caldera activity and the initial phases of the caldera-forming eruptions. We suggest that this eruption is an expression of a state shift in magma storage conditions, whereby substantial amounts of volatiles start to exsolve in the shallow reservoir. The presence of an exsolved gas phase has fundamental consequences for the physical properties of the reservoir and may indicate that a large magma body is currently accumulating underneath Campi Flegrei.

Understanding the mechanisms responsible for the generation of chemical gradients in high-volume ignimbrites is key to retrieve information on the processes that control the maturation and eruption of large silicic magmatic reservoirs. Over the last 60 ky, two large ignimbrites showing remarkable zoning were emplaced during caldera-forming eruptions at Campi Flegrei (i.e., Campanian Ignimbrite, CI, ~ 39 ka and Neapolitan Yellow Tuff, NYT, ~ 15 ka). While the CI displays linear compositional, thermal and crystallinity gradients, the NYT is a more complex ignimbrite characterized by crystal-poor magmas ranging in composition from trachy-andesites to phonolites. By combining major and trace element compositions of matrix glasses and mineral phases from juvenile clasts located at different stratigraphic heights along the NYT pyroclastic sequence, we interpret such compositional gradients as the result of mixing/mingling between three different magmas: (1) a resident evolved magma showing geochemical characteristics of a melt extracted from a cumulate mush dominated by clinopyroxene, plagioclase and oxides with minor sanidine and biotite; (2) a hotter and more mafic magma from recharge providing high-An plagioclase and high-Mg clinopyroxene crystals and (3) a compositionally intermediate magma derived from remelting of low temperature mineral phases (i.e., sanidine and biotite) within the cumulate crystal mush. We suggest that the presence of a refractory crystal mush, as documented by the occurrence of abundant crystal clots containing clinopyroxene, plagioclase and oxides, is the main reason for the lack of erupted crystal-rich material in the NYT. A comparison between the NYT and the CI, characterized by both crystal-poor extracted melts and crystal-rich magmas representing remobilized portions of a “mature” (i.e., sanidine dominated) cumulate residue, allows evaluation of the capability of crystal mushes of becoming eruptible upon recharge.

The volcanic activity that built up the Monte dei Porri stratocone (Salina Island) was reconstructed using new stratigraphic data, which allowed seven eruption units to be distinguished. Alternating Strombolian/Vulcanian to sub-Plinian/Plinian explosive and effusive activity emplaced fall and pyroclastic density current deposits and lava flows that formed the volcanic cone. The minimum erupted bulk volumes were assessed at 100 × 10 6 m 3 each for EU1, EU2, EU3 and EU6, while that of EU4 is ca. 200 × 10 6 m 3 . Rough estimation of EU7 volume yields values around 150 × 10 6 m 3 . The calculation of volume was not possible for the EU5 deposits. The magmas that fed the different eruption units of the Monte dei Porri succession range in composition from basalt to andesite, with the exception of dacites erupted in the initial phase of activity. SEM image analyses on coarse ash from the different pyroclastic units suggest that hydromagmatic fragmentation cannot be the cause of the large variations in explosivity observed throughout the stratigraphic succession. Based on the lithic component of pyroclastic deposits and xenolith contents of lava flows, the plumbing system that fed the different eruption units of Monte dei Porri was split into a deep magma storage level (15–20 km) and shallower magma batches (3–5 km). Our calculations indicate that the volumes of erupted material can account for magmatic triggering (injection of new magma) of eruptive units from the shallower feeding system, but they are not sufficient for suggesting magmatic initiation of the eruption units from the deeper feeding system. It is therefore assumed that the eruptions from the deep magma reservoir necessitate a favourable lithostatic stress, likely calling for a reduction of the local tectonic forces. A qualitative model explaining the eruptive style transitions among and within the different eruption units is presented, taking into account the relation between magmatic overpressure and lithostatic stress.

Caldera-forming eruptions, during which large volumes of magma are explosively evacuated into the atmosphere from shallow crustal reservoirs, are one of the most hazardous natural events on Earth. The Campanian Ignimbrite (CI; Campi Flegrei, Italy) represents a classical example of such events, producing a voluminous pyroclastic sequence of trachytic to phonolitic magma that covered several thousands of squared kilometers in the south-central Italy around 39 ka ago. The CI deposits are known for their remarkable geochemical gradients, attributed to eruption from a vertically zoned magma chamber. We investigate the relationships between such chemical zoning and the crystallinity variations observed within the CI pyroclastic sequence by combining bulk-rock data with detailed analyses of crystals and matrix glass from well-characterized stratigraphic units. Using geothermometers and hygrometers specifically calibrated for alkaline magmas, we reconstruct the reservoir storage conditions, revealing the presence of gradients in temperature and magma water content. In particular, we observe a decrease in crystallinity and temperature and an increase in magma evolution and water content from the bottom to the top of the magma chamber. We interpret these features as the result of protracted fractional crystallization leading to the formation of a cumulate crystal mush at the base of the eruptible reservoir, from which highly evolved, crystal-poor, water-rich and relatively cold melts were separated. The extracted melts, forming a buoyant, easily eruptible cap at the top of the magma chamber, fed the initial phases of the eruption, until caldera collapse and eruption of the deeper more crystalline part of the system. This late-erupted, crystal-rich material represents remobilized portions of the cumulate crystal mush, partly melted following hotter recharge. Our interpretation is supported by: 1) the positive bulk-rock Eu anomalies and the high Ba and Sr contents observed in the crystal-rich units, implying feldspar accumulation; 2) the positive Eu anomalies in the matrix glass of the crystal-rich units, testifying to the presence of liquid derived from partial melting of low temperature mineral phases within the crystal mush (mostly feldspars); 3) the Ba and Sr-rich rims in the feldspars and positive Eu anomalies in clinopyroxene rims, suggesting late rim growth from a locally enriched melt following cumulate mush remelting and 4) the occurrence of An-rich plagioclase, relict from a more mafic recharge, which acted as a heat source. Our model reconciles many observations made over the years on zoned deposits of such high-magnitude explosive eruptions, and provides a framework to understand magma chamber processes leading up to cataclysmic events.

The partitioning of trace elements between crystals and melts provides an important petrogenetic tool for understanding magmatic processes. We present trace element partition coefficients measured between clinopyroxene phenocrysts and trachy-phonolitic magmas at the Campi Flegrei (Italy), whose late Quaternary volcanism has been characterized by two major caldera-forming events (Campanian Ignimbrite at similar to 39 ka, and Neapolitan Yellow Tuff at similar to 15 ka). Our data indicate that the increase of trivalent rare earth elements and yttrium into the crystal lattice M2 site is facilitated by the charge-balancing substitution of Si4+ with Al3+ on the tetrahedral site. Higher concentrations of tetravalent and pentavalent high field strength elements on the M1 site are also measured when the average charge on this site is increased by the substitution of divalent cations by Al-vi. In contrast, due to these charge balance requirements, divalent transitional elements become less compatible within the crystal lattice. On the basis of the lattice strain theory, we document that the incorporation of rare earth elements and yttrium in clinopyroxene is influenced by both compositional and physical parameters. Data from this study allow to update existing partitioning equations for rare earth elements in order to construct a self-consistent model for trachy-phonolitic magmas based on the lattice strain theory. The application of this model to natural products from the Campanian Ignimbrite, the largest caldera-forming eruption at the Campi Flegrei, reveals that the complex rare earth element pattern recorded by the eruptive products can be successfully described by the stepwise fractional crystallization of clinopyroxene and feldspar where the clinopyroxene-melt partition coefficient changes progressively as a function of the physicochemical conditions of the system. (c) 2016 Elsevier B.V. All rights reserved.

We present a K-feldspar-liquid hygrometer specific to alkaline differentiated magmas that is calibrated through the regression analysis of sanidine and anorthoclase crystals coexisting with trachyte and phonolite melts. Partial-regression leverage plots were used to determine the minimum number of regression parameters that closely describe the variance of the dataset. The derived model was tested using K-feldspar-liquid pairs not included into the calibration dataset in order to address issues of systematic errors. When K-feldspar and plagioclase crystals coprecipitate from the same alkaline liquid under identical P-T-X-fO(2)-H2O conditions, the ability prediction of the new hygrometer is comparable to that of previous plagioclase-liquid models. To minimize the error of H2O estimate caused by the inadvertent use of disequilibrium data in natural samples, we have also calibrated a new test for equilibrium based on Or-Ab exchange between K-feldspar and coexisting melt. As an immediate application for both equilibrium and hygrometer models, we used as input data K-feldsparliquid pairs from alkaline explosive eruptions at the Phlegrean Fields. The estimates of H2O dissolved in natural trachyte and phonolite magmas closely match those determined by melt inclusion analysis and H2O solubility modeling. This leads to the conclusion that our new models can significantly contribute to a better quantitative characterization of the H2O content in differentiated alkaline magmas feeding large-volume explosive eruptions. (C) 2014 Elsevier B.V. All rights reserved.

We report results from mineralogical, geochemical and isotopic analyses of the three youngest pyroclastic products (ca. 86 ky) belonging to the Sabatini Volcanic District (Roman Province, central Italy). By means of thermometers, hygrometers and oxygen barometers, we have estimated that the crystallization temperature of magma progressively decreases over time (910–740 °C),whereas the amount ofwater dissolved in the melt and fO2 progressively increases as compositions of magmas become more differentiated (4.5–6.4 wt.% H2O and 0.4–2.6 ΔQFM buffer, respectively). Thermodynamic simulations of phase equilibria indicate that geochemical trends in mafic magmas (MgO N 4 wt.%) can be reproduced by abundant fractionation of olivine and clinopyroxene (~50 wt.% crystallization), while the trends of more evolved magmas (MgO ≤ 4 wt.%) originated by fractional crystallization of plagioclase and sanidine (~45 wt.% crystallization). The behavior of trace elements highlights that magmatic differentiation is controlled by polybaric differentiation that includes: (1) prolonged fractionation of mafic, anhydrous minerals from a primitive, H2O-poor magma at depth and (2) extraction of a more evolved, H2O-rich magma that crystallizes abundant felsic and subordinated hydrous minerals at shallow crustal levels. Assimilation and fractional crystallization modeling also reveal that magmas interacted with the carbonate rocks of the subvolcanic basement. The effect of carbonate assimilation accounts for both trace element and Sr–Nd isotopic variations inmagmas, suggesting amaximumdegree of carbonate assimilation of less than 5 wt.%.