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Volcanic calderas are affected by unrest episodes usually dominated by hybrid magmatic-hydrothermal system dynamics. Unrest episodes can evolve to eruptions of variable intensity, up to Plinian. Campi Flegrei caldera (CFc) is a type-location for this kind of activity escalation. CFc provides unique opportunity to build-up a volcanological model in which geochemical, geological and geophysical data are interpreted together to understand how degassing following magma emplacement drives the caldera resurgence. Uneruptive unrest episodes reflect i) a sudden increase of the CO2 magmatic fraction following the shallow emplacement of one single volatile-rich magma batch, ii) voluminous gas separation in a nearly single-step process, and iii), on longer times scales of 10-20 years, degassing driven by crystallization and deep gas fluxing. Our volcanological model matches three decades of geochemical constraints from fumarole discharges, as well as data from melt inclusions of past CFc eruptions. Besides, magma physical properties demanded for modeled degassing conditions are in good agreement with existing geophysical data. Our results open new perspectives to the definition of unrest scenarios at highly-populated CFc.

Widespread mantle degassing is active in many areas of Earth, contributing to volcanic and seismic processes. The Italian territory is invested dramatically by deep degassing, at both volcanic and non-volcanic sites, with huge punctual as well as diffuse emissions of carbon dioxide. The pery-thyrrenian area of southern Italy is the place where these emissions are accompanied by active volcanism. A global look at melt inclusions from southern Italian volcanoes, particularly from the Campanian volcanic district, suggests that these volcanoes produce magmas under nearly common fluid buffered conditions that initiate in the litospheric mantle. Although the role and nature of components added to the mantle source is as debated as the mantle source itself, volcanic volatiles and the ensemble of petrologic evidences promote the prevailing hypothesis involving the subduction of a continental crust and/or oceanic sediments carrying both pelagic and non-pelagic fractions. The observed extended mantle degassing suggests that volcanism is an ‘occasional’ consequence that pierces the surface at some preferential degassing sites, depending on connected geological structures, including veined mantle paths, and regional stress conditions. Despite these complexities, magma fluxes can be still used to test simple hypotheses on the homogeneity of the volatile source metasomatising the local mantle.

CONTRIB. MINERAL. PETROL.

4 páginas.-- Trabajo presentado en el VII Congreso Geológico de España, Canarias 2008.-- Vulcanología.

Atti Geodinamic evolution of the Afro-Arabian rift system

A review of the recent geochemical investigations carried out on products of the Campi Flegrei volcanic field is presented, with particular attention on radiogenic (Sr, Nd, Pb) and stable (B, O) isotope data. The studied volcanic rocks span the entire period of best known Campi Flegrei volcanic field activity, since ~58 ka to the last event, the AD 1538 Monte Nuovo eruption. The isotope data, combined with mineralogical, geochemical, geochronological and volcanological data have allowed reconstructing a complex behaviour of the Campi Flegrei volcanic field magmatic feeding system. They suggest that several, isotopically-distinct magmatic components fed the activity over various time periods. In particular, over the past 15 kyrs at least three isotopically-distinct magmatic components, located either in a deep and large reservoir or in shallower and smaller chambers, have variably interacted, eventually feeding the volcanic activity. These magmatic components originated through a complex interplay of magma supply from a variably-enriched mantle source, and crustal contamination processes occurring at depth. Mingling/mixing among these isotopically-distinct magmatic components, as well as closed-system differentiation of variable hybrid magmas and interaction with cumulate mushes, were the dominant processes acting in the Campi Flegrei caldera feeding system over the past 15 kyrs. Three examples including the Agnano-Monte Spina, Astroni and Averno 2 eruptions of this period are described to illustrate the interplay among the above-mentioned different processes.

Mt. Etna, in Sicily (Italy), produces effusive and explosive eruptions from summit craters and flanks. Petrology of erupted products has proved fundamental to explore the relationship between magma dynamics within the plumbing system and eruptive styles. Data of Sr Nd isotopes are quite scarce if compared with the great number of chemical analyses carried out in products of last decades. We measure Sr Nd isotopes, major and trace elements in products emitted from 2001 to 2012, and consider also published analyses of the 1995–2001 eruptions, because carried out in the same laboratories of 2001–2012 ones. Furthermore, we take into account other published analyses extending back to the 14th century, even if performed by different facilities. Based on this dataset, magma dynamics operating at different timescales inside the volcano's plumbing system have been explored. Our analyses evidence that, from 2001–2012, three magma types are present in the volcano's plumbing system: (i) a deep and poorly evolved basaltic and K-trachybasaltic magma, enriched in radiogenic Sr and LILE, erupted during the 2001 and 2002–03 flank activity; (ii) a shallower more evolved K-trachybasaltic magma, less enriched in radiogenic Sr, feeding both summit and flank eruptions and (iii) an uncommon, and scarcely radiogenic hawaiitic magma, LILE poor, emitted exclusively from a small sector of the 2002–03 eruptive fissures. By considering a broader time scale, from 14th to 21st century, we conclude that: (i) different magmatic components are present in the deep volcano's plumbing system, at about 10–12 km b.s.l; (ii) a decoupling of geochemical and isotopic features in magmas stored at different depths is observed after the 1974 flank eruption up to date; (iii) mixing between different magmas is an efficient magmatic process to produce magma differentiation at Mt. Etna, regardless of the investigated temporal scale.

Somma-Vesuvius is one of the most dangerous active Italian volcanoes, due to the explosive character of its activity and because it is surrounded by an intensely urbanized area. For mitigating the volcanic risks, it is important to define how the Somma-Vesuvius magmatic system worked during the past activity and what processes took place. A continuous coring borehole drilled at Camaldoli della Torre, along the southern slopes of Somma-Vesuvius, allowed reconstructing its volcanic and magmatic history in a previous study. In this work, the wide range of chemical (Mg# = 92–69) and isotopic (87Sr/86Sr = 0.70781–0.70681) compositions, collected on single clinopyroxene crystals separated from selected lava flow units of the Camaldoli della Torre sequence, have been integrated with the already available bulk geochemical and Sr-isotopic data. The detected chemical and isotopic signatures and their variation through time allow us to better constrain the behavior of the volcano magmatic feeding system, highlighting that mixing and/or assimilation processes occurred before a significant change in the eruptive dynamics at Somma-Vesuvius during a period of polycyclic caldera formation, starting with the Pomici di Base Plinian eruption (ca. 22 ka).

We illustrate a quantitative conductive/convective thermal model incorporating a wide range of geophysical, petrological, geological, geochemical and isotopical observations that constrain the thermal evolution and present state of the Campi Flegrei caldera (CFc) magmatic system. The proposed model has been computed on the basis of the current knowledge of: (1) the volcanic and magmatic history of the volcano over the last 44 ka, (2) its underlying crustal structure, and (3) the physical properties of the erupted magmas. 3D numerical simulations of heat conduction and convection within heterogeneous rock/magma materials with evolving heat sources and boundary conditions that simulate magma rise from a deep (≥ 8 km depth) to shallow (2–6 km) reservoirs, magma chamber formation, magma extrusion, caldera collapse, and intra-caldera hydrothermal convection, have been carried out. The evolution of the CFc magmatic system through time has been simulated through different steps related to its changes in terms of depth, location and size of magma reservoirs and their replenishment. The thermal modeling results show that both heat conduction and convection have played an important role in the CFc thermal evolution, although with different timing. The simulated present heat distribution is in agreement with the measured geothermal profiles (Agip, 1987), reproduces the thermal gradient peaks at the CFc margins in correspondence to the anomalies in surface gradients (Corrado et al., 1998), and suggests temperatures of 700 °C at depth of 4 km in the central portion of the caldera, in agreement with the estimated temperature for the brittle-ductile transition (Hill, 1992 ).

Barium diffusion chronometry applied to sanidine phenocrysts from the trachytic Agnano-Monte Spina eruption (∼4.7 ka) constrains the time between reactivation and eruption of magma batches in the Campi Flegrei caldera. Backscattered electron imaging and quantitative electron microprobe measurements on 50 sanidine phenocrysts from representative pumice samples document core-to-rim compositional zoning. We focus on compositional breaks near the crystal rims that record magma mixing processes just prior to eruption. Diffusion times were modeled at a magmatic temperature of 930 °C using profiles based on quantitative BaO point analyses, X-ray scans, and grayscale swath profiles, yielding times ≤60 years between mixing and eruption. Such short timescales are consistent with volcanological and geochronological data that indicate that at least six eruptions occurred in the Agnano-San Vito area during few centuries before the Agnano-Monte Spina eruption. Thus, the short diffusion timescales are similar to time intervals between eruptions. Therefore, the rejuvenation time of magma residing in a shallow reservoir after influx of a new magma batch that triggered the eruption, and thus pre-eruption warning times, may be as short as years to a few decades at Campi Flegrei caldera.

On 27 February 2007, at 12.49 GMT, a new eruption of Stromboli took place with the effusion of a lava flow from a fracture cutting the flank of the NE cone, which rapidly reached the sea. The eruption had been heralded by an increase in the amplitude of tremor and flank movement since at least the 14th of February. Short-term precursors were an increase in the rate of occurrence of small landslides within the “Sciara del Fuoco” scar on the North-western flank of the volcano. A new effusive vent opened at 18.30 GMT on the Sciara del Fuoco at an height of 400 m asl. The new lava emission caused the sudden termination of the summit flow and initiated a period of non-stationary lava outpouring which ended on 2 April, 2007. The eruption has been characterized by a rapid decrease in the eruption rate after the first days and subsequently by episodic pulse increases. On the 15th of March, the increase in lava outpouring, monitored by a thermal camera, heralded by 9 min the occurrence of a violent paroxysmal explosion with the formation of an impulsive eruption column and the emission of small pumices mingled with black scoriae. The pumice had a bulk composition similar to that of the lava and of the black scoriae, but with a distinct lower content of phenocrysts. A similar feature has been repeatedly observed during the major explosive paroxysms of Stromboli. Short term precursors of the paroxysm were recorded by strainmeter and tiltmeter stations. The volcano monitoring activity has been made by a joint team of researchers from the INGV sections of Catania, Napoli, Palermo and Rome, along with researchers from the Universities of Florence, Pisa, Roma Tre, and Palermo. The scientific activity was coordinated by a Synthesis Group made up by scientists responsible for the different monitoring techniques of INGV and Universities and by the volcanic experts of Commissione Nazionale Grandi Rischi of the Prime Minister Office (Civil Protection Department). The group made a daily evaluation of the state of the volcano and transmitted its recommendations to the Civil Protection Department (DPC). Several prevention measures were adopted by DPC, the main of which were the evacuation of the coast zone when strong acceleration of the Sciara del Fuoco slope motion (occurred twice) could led to a dangerous tsunami by flank collapse (as last occurred on 30 December 2002) and four days before the 15 March paroxysm when access was prohibited to the part of the volcano above 290 m asl.

New geochemical and isotopic data on volcanic rocks spanning the period ~75–50 ka BP on Ischia volcano, Italy, shed light on the evolution of the magmatic system before and after the catastrophic, caldera-forming Monte Epomeo Green Tuff (MEGT) eruption. Volcanic activity during this period was influenced by a large, composite and differentiating magmatic system, replenished several times with isotopically distinct magmas of deep provenance. Chemical and isotopic variations highlight that the pre-MEGT eruptions were fed by trachytic/phonolitic magmas from an isotopically zoned reservoir that were poorly enriched in radiogenic Sr and became progressively less radiogenic with time. Just prior to the MEGT eruption, the magmatic system was recharged by an isotopically distinct magma, relatively more enriched in radiogenic Sr with respect to the previously erupted magmas. This second magma initially fed several SubPlinian explosive eruptions and later supplied the climactic, phonolitic-to-trachytic MEGT eruption(s). Isotopic data, together with erupted volume estimations obtained for MEGT eruption(s), indicate that >5–10 km3 of this relatively enriched magma had accumulated in the Ischia plumbing system. Geochemical modelling indicates that it accumulated at shallow depths (4–6 km), over a period of ca. 20 ka. After the MEGT eruption, volcanic activity was fed by a new batch of less differentiated (trachyte-latite) magma that was slightly less enriched in radiogenic Sr. The geochemical and Sr–Nd-isotopic variations through time reflect the upward flux of isotopically distinct magma batches, variably contaminated by Hercynian crust at 8–12 km depth. The deep-sourced latitic to trachytic magmas stalled at shallow depths (4–6 km depth), differentiated to phonolite through crystal fractionation and assimilation of a feldspar-rich mush, or ascended directly to the surface and erupted.

[1] Melt inclusions in phenocrysts of the Minopoli2 shoshonite and Fondo Riccio latite eruptive products (Campi Flegrei caldera; 10.2 to 9.5 ka BP) constrain the nature of deep Phlegraean magmatic fluids, their role in volcanism and give new insights into the magmatic plumbing system. The analyzed melt inclusions show that CO2 is an abundant gaseous species, confirming the results of geochemical studies on the magmatic gas fraction involved in present-day fumarolic emissions. Volatile concentrations within the melt inclusions require gas-melt equilibria between 8 and 9 km depth, and closed-system degassing in presence of ascending CO2-rich fluids of deep provenance. Magmas later re-equilibrate at depths up to 3–2 km, i.e. above the top of the carbonate basement. These observations correlate well with independent geophysical and geochemical evidence and are useful constraints for volcanic hazards assessment at Campi Flegrei.

[1] Combining tectonics, with seismological and geochemical data, we reconstruct the deformation history of the presently narrow Calabrian slab and the path of mantle circulation during the last 10 Ma. We show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1–2 myr.