Alessandro Sbrana

Structural evolution of Campi Flegrei caldera in light of volcanological and geophysical data

Abstract The structural evolution of Campi Flegrei (CF) has been investigated by combining on land and offshore gravimetric and aeromagnetic data, with surface geological and volcanological evidences and subsurface drilling information. A relatively small (4–5 km across) collapsed circular structure, produced by eruption along a ring fracture, has been identified for the first time in the Agnano-Astroni-Solfatara (AAS) sector. Data permitted the definition of the geometry of large CF caldera, which is 13 km across and includes an inner more collapsed zone, 11–12 km across with a total sinking of about 1,600 m. Its volume (80 km 3 ) is consistent with that estimated for the Campanian Ignimbrite erupted 35,000 a B.P.—A further collapse occurred after the eruption of the Neapolitan Yellow Tuffs (10 km 3 ; about 12,000 a B.P.) and finally the AAS structure was formed in the last phase of activity, about 4,000 a B.P.—The CF volcanic and structural evolution is therefore characterized by the repetition of a similar eruption mechanism, which includes the formation of a ring fracture, magma uprise and eruption from several points along the fracture and collapse of the central sector into the emptied magma chamber. The decrease with time of the dimensions of the resulting structure (diameter of the collapsed area; total sinking) reflects the progressively decreasing volume of the magma available in the chamber.

Magmatic and phreatomagmatic phases in explosive eruptions of Vesuvius as deduced by grain-size and component analysis of the pyroclastic deposits

Abstract Grain-size and component analyses have been carried out on pyroclastic deposits of three well-known explosive eruptions of Vesuvius: those of 79 A.D., 1631 and 1906. These eruptions cover a wide energy spectrum, from Plinian to Strombolian, and each includes a transition from a magmatic to a well-documented and clearly distinguishable phreatomagmatic phase of activity. The chronicles of the eruptions are revisited in light of the granulometric and component data obtained from the study of the deposits and interpretation of the dynamics of the main eruptive episodes is provided for each eruption. The phreatomagmatic products (originated from explosive interaction of magma with groundwater) have characteristic grain-size and component distribution patterns distinct from associated purely magmatic pyroclastic products: in any eruption phreatomagmatic products exhibit: (a) a marked increase in both the lithic/juvenile and the crystal/total juvenile ratios, and (b) a preferential fragmentation of the juvenile fraction. Moreover, eruption energy, degree of evolution of erupted magma, lithic content and depth of provenance are all clearly correlated. The abundance of lithics and crystals in phreatomagmatic deposits can be ascribed to (a) preferential fragmentation of the aquifer-hosting rocks due to explosive vaporization of ground water; (b) vent flaring and craterization possibly related to overpressure conditions following the entrance of large quantities of steam in the conduit; (c) indirect enrichment in both crystal and lithic fractions by removal of juvenile fines from the eruptive cloud. In each eruption some of the deposits with “phreatomagmatic” component distribution patterns also exhibit evidence of steam condensation (accretionary lapilli, vesiculated tuff, mud coating, soft deformations) and some do not: these are therefore to be considered sufficient but not necessary diagnostic features for a hydromagmatic origin of the deposit (“wet” in the sense of Sheridan and Wohletz, 1983). “Dry” phreatomagmatic products correspond to higher energy conditions allowing production and maintenance of overheated steam. The magma/water ratio in terms of heat exchange surface (i.e. the degree of primary fragmentation of the magma) is certainly the main factor controlling the efficiency and the energy balance of the interaction. A series of observations strongly suggest that a primarily fragmented magma is a basic requisite for the explosive magma/water interaction: (a) occurrence of short phreatomagmatic episodes interrupting lava fountains activity (1906 eruption) in connection with drawdown of the magma column in the conduit; (b) parallel increase in depth of magma fragmentation level and magma/water interaction level (as deduced by the nature of lithics), and (c) preferential occurrence of phreatomagmatic activity at the end of explosive eruptions (when juvenile gas pressure declines and water from the confining aquifer more easily enters the conduit).

Assessing pyroclastic fall hazard through field data and numerical simulations: Example from Vesuvius

Received 12 April 2001; revised 2 April 2002; accepted 7 May 2002; published 1 Feburary 2003. [1] A general methodology of pyroclastic fall hazard assessment is proposed on the basis of integrated results of field studies and numerical simulations. These approaches result in two different methods of assessing hazard: (1) the ‘‘field frequency,’’ based on the thickness and distribution of past deposits and (2) the ‘‘simulated probability,’’ based on the numerical modeling of tephra transport and fallout. The proposed methodology mostly applies to volcanoes that, by showing a clear correlation between the repose time and the magnitude of the following eruptions, allows the definition of a reference ‘‘maximum expected event’’ (MEE). The application to Vesuvius is shown in detail. Using the field frequency method, stratigraphic data of 24 explosive events in the 3–6 volcanic explosivity index range in the last 18,000 years of activity are extrapolated to a regular grid in order to obtain the frequency of exceedance in the past of a certain threshold value of mass loading (100, 200, 300, and 400 kg/m 2 ). Using the simulated probability method, the mass loading related to the MEE is calculated based on the expected erupted mass (5 10 11 kg), the wind velocity profiles recorded during 14 years, and various column heights and grain-size populations. The role of these factors was parametrically studied performing 160,000 simulations, and the probability that mass loading exceeded the chosen threshold at each node was evaluated. As a general rule, the field frequency method results are more reliable in proximal regions, provided that an accurate database of field measurements is available. On the other hand, the simulated probability method better describes events in middle distal areas, provided that the MEE magnitude can be reliably assumed. In the Vesuvius case, the integration of the two methods results in a new fallout hazard map, here presented for a mass loading value of 200 kg/m 2 . INDEX TERMS: 8404 Volcanology: Ash deposits; 3210 Mathematical Geophysics: Modeling; 1035 Geochemistry: Geochronology; 5480 Planetology: Solid Surface Planets: Volcanism (8450); 9335 Information Related to Geographic Region: Europe; KEYWORDS: Vesuvius, ash fallout, explosive eruptions, volcanic hazard, numerical modeling, hazard mapping

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

Abstract The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and hauyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87 Sr/ 86 Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvospinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

The deep structure of the Eolian arc (Filicudi-Panarea-Vulcano sector) in light of gravity, magnetic and volcanological data

Abstract The magnetic and gravimetric methods of investigation have been applied to the central sector of the Eolian arc, combined with geovolcanological data, for the purpose of defining the structural framework in which the volcanism developed. The processing of the magnetic and gravimetric data involved 3D modeling of the volcanic bodies and the application of a “depth separation filter”, in order to subtract from the measured field the effect caused by the subaerial and submarine volcanic edifices and by all surface inhomogeneities. Magnetic and gravimetric 2.5D modeling on selected profiles has also been done, in a joint interpretation of the magnetic and gravimetric data, constrained by volcanological knowledge. The interpretation of the final maps and profiles has demonstrated the existence of extensional tectonics, resulting in the development of horst and graben. Volcanism occurs in the graben areas. In particular, the Salina-Lipari-Vulcano magmatic axis has developed in a very deep articulated graben of lithospheric importance and completely filled by volcanic deposits and intrusive bodies. The origin of this structure and of the associated igneous activity is suggested to be related to trans-tensional tectonics along a Plio-Pleistocene regional strike-slip fault system.

Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding stratovolcanoes

Glass inclusions in olivine and diopside phenocrysts from pyroclasts of various eruptions of Vesuvius are representative of the magmas that supplied the volcano in the last 4–5000 years. During this interval the volcano alternated between open conduit activity (e.g. 1944 and 1906 eruptions) with long pauses interupted by Plinian and sub-Plinian eruptions (e.g. 3360 B.P. “Avellino”, A.D. 79 “Pompei”, A.D. 472 “Pollena”). The eruptive behaviour was conditioned in all cases by the presence of shallow reservoirs: two cases are distinguished: (1) small and very shallow, 1906-type; (2) large and deeper Plinian-sub-Plinian magma chamber. Lapilli of 1906 lava fountains contain olivine (Fo89.5–90.4) including Cr-spinel [Cr/(Cr+Al)] (Cr#>75) and volatile-K-rich tephritic glasses, which represent the first recognized Vesuvius primary magmas. Mg-poorer olivine (Fo83–89) also occurs in 1906 and 1944 products; it formed within the shallow reservoir, together with pyroxene and leucite, between 1200 and 1130°C, from K-tephritic melts (MgO=6–8 wt%). The Plinian and sub-Plinian pumices contain diopside, phlogopite and minor olivine (Fo85–87) representing adcumulates wrenched from the chamber walls. Glass inclusions in diopside (and some olivine) range from K-basalt to K-tephrite (MgO=6–8 wt%), with homogenization temperature of 1130–1170°C. They have been regarded as representative of the magmas supplying the Plinian-sub-Plinian chamber(s). The Avellino glass inclusions have K-basaltic compositions, contrasting with the mostly K-tephritic Pompei and Pollena inclusions. They display lower C1 and P contents with respect to the younger tephritic melts, and these variations should reflect primary features of the mantle-derived magmas. The primary and the near-primary Vesuvius magmas, as illustrated by melt inclusions, emphasize high K, P and volatile (H2O, Cl, F, S) contents, with high K2O/H2O (2–2.5), Cl/F (2.5) and Cl/S (2–3) ratios, consistent with a metasomatized mantle source, and distinguishing the Vesuvius potassic primary magmas from those of the northern part of the Roman Province.

Assessment of a shallow magmatic system: the 1888–90 eruption, Vulcano Island, Italy

The magmatic system feeding the last eruption of the volcano La Fossa, Vulcano Island, Italy was studied. The petrogenetic mechanisms controlling the differentiation of erupted rocks were investigated through petrography, mineral chemistry, major, trace and rare earth element and Sr, Nd and Pb isotopic geochemistry. In addition, melt inclusion and fluid inclusion data were collected on both juvenile material and xenolithic partially melted metamorphic clasts to quantify the P-T conditions of the magma chamber feeding the eruption. A regular and continuous chemical zoning has been highlighted: rhyolites are the first erupted products, followed by trachytes and latites, whereas rhyolitic compositions were also found in the upper part of the sequence. The chemical and isotopic composition of the rhyolites indicates that they originated by fractional crystallization from latitic magmas plus the assimilation of crustal material; the trachytes represent hybrid magmas resulting from the mixing of latites and rhyolites, contaminated in the shallow magmatic system. The erupted products, primarily compositionally zoned from latites to rhyolites, are heterogeneous due to syn-eruptive mingling. The occurrence of magmacrust interaction processes, evidenced by isotopic variations (87Sr/86Sr=0.70474±3 to 0.70511±3; 143Nd/144Nd=0.512550±6 to 0.512614±8; 206Pb/204Pb=19.318–19.489; 207Pb/204Pb=15.642–15.782; 208Pb/204Pb=39.175–39.613), is confirmed by the presence of partially melted metamorphic xenoliths, with 87Sr/86Sr=0.71633±6 to 0.72505±2 and 143Nd/144Nd=0.51229±7, in rhyolites and trachytes. AFC calculations indicate a few percentage contribution of crustal material to the differentiating magmas. Thermometric measurements on melt inclusions indicate that the crystallization temperatures of the latites and trachytes were in the range of 1050–1100° C, whereas the temperature of the rhyolites appears to have been around 1000°C at the time of the eruption. Compositional data on melt inclusions reveal that the magmas involved in the eruption contained about 1–1.5 wt.% dissolved H2O in pre-eruptive conditions. Secondary fluid inclusions found in metamorphic xenoliths give low equilibration pressure data (30–60 MPa), giving the location of the higher portions of the chamber at around 1500–2000 m of depth.

Rare-earth element (REE) behaviour in the alteration facies of the active magmatic-hydrothermal system of Vulcano (Aeolian Islands, Italy)

Abstract This work reports a study on the rare-earth element (REE) behaviour in the high-sulfidation-type superficial alteration and in the phyllic and propylitic alteration in the subsurface of the island of Vulcano. The results of this study demonstrate notable differences in the REE behaviour in the different alteration facies. In silicic alteration, all REE are equally strongly depleted; in advanced argillic alteration, LREE are quite immobile whereas HREE are strongly depleted; in intermediate argillic, phyllic and propylitic alteration, REE patterns are almost unchanged compared with their fresh rock equivalents. These features indicate that the behaviour of REE in hydrothermal alteration facies at La Fossa is mainly controlled by pH, availability of complexing ions in the fluid (mainly SO42− and, in the silicic facies, F−) and the presence of secondary minerals able to host REE in their structures. The origin of the acid fluids is related to the hypogenic introduction of prevalently magmatic acid gases produced by the degassing of the shallow magmatic system of La Fossa volcano. In the active high-temperature fumarolic field of La Fossa, REE behaviour illustrates the transition from a dry vapor system, corresponding to the high-temperature fumaroles, to a wet condensing system in the surrounding area. The results of this research, carried out on an active high-sulfidation system, provide a useful tool for the study of high-sulfidation epithermal ore deposits and particularly help to understand the alteration processes during the main stage of hypogene wall rock alteration.

Magma chamber of the Campi Flegrei supervolcano at the time of eruption of the Campanian Ignimbrite

A supereruption that occurred in the Campi Flegrei area, Italy, ca. 39 ka had regional- and global-scale environmental impacts and deposited the Campanian Ignimbrite (CI). We attempt to shed light on critical aspects of the eruption (depth of magma chamber, intensive pre-eruptive magma conditions) and the large-volume magma plumbing system on the basis of information derived from analyzing melt inclusion (MI) data. To achieve these aims, we provide new measurements of homogenization temperatures and values of dissolved H2O within phenocryst-hosted MIs from pumices erupted during different phases of the CI eruption. The MI data indicate that a relatively homogeneous overheated trachytic magma resided within a relatively deep magma chamber. Dissolved water contents in MIs indicate that prior to the eruption the magma chamber underwent radical changes related to differential upward movement of magma. Decompression of the rising trachytic magma caused a decrease in water solubility and crystallization, and trachytic bodies were emplaced at very shallow depths. The proposed eruptive model links portions of the main magma chamber and apophyses with specific eruptive units.

The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach ☆

The present-day tectonic setting of the Tyrrhenian Sea is dominated by the eastward migration of the Tyrrhenian^ Appenines subduction system and the existence of a contemporaneous and parallel extensional^compressional regime. This complex setting leads to the occurrence of a wide spectrum of magma-types in the South Italy volcanoes. Here, major and trace-element data for primitive melt inclusions preserved in olivine phenocrysts have been obtained in order to add constraints on the origin of the calc-alkaline magmas from the Aeolian arc (Stromboli and Vulcano islands), the potassic magmas from the Campania Province (Vesuvius and Phlegraean Fields) and the Na-alkaline magmas from Ustica Island. The approach used to determine the possible mantle sources of the trapped melts for each population of melt inclusions is based on the determination of the trace-element incompatibility sequence taken as the relative order of increasing bulk partition coefficients, which depends on the mineralogy of the source and gives direct information about minerals residual at the time of melting. Compositional similarities between the melt inclusions and their host lavas suggest that shallow-level magma contamination did not contribute significantly to the geochemical characteristics of the magma-types encountered in the region. Results of the trace-element modelling indicate that the melt inclusions from the Aeolian Islands and Campania Province volcanoes originate from mantle sources strongly affected by subduction-related metasomatic processes. Trace-element relationships of melt inclusions from Vulcano and Stromboli reflect melting of peridotitic sources that have been enriched by a slab-derived, aqueous fluid formed during dehydratation of K-free phases at shallow to intermediate depths. The negative high-field strength elements (HFSE) anomalies of these inclusions were generated in the absence of any residual phase in which HFSE might be compatible. In addition, their major element characteristics require the involvement of a clinopyroxene-rich source component in their genesis. With regard to the Vesuvius and Phlegraean Fields melt inclusions, their calculated incompatibility sequences point to a common phlogopite-bearing mantle source likely to be the result of interaction and hybridisation reactions with K2O and H2O-rich phases released from the slab at larger depths. Finally, trace