Paolo Fulignati

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.

Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius

Foid-bearing syenites and endoskarn xenoliths of the A.D. 472 Vesuvius eruption represent the magma chamber-carbonate wall-rock interface. Melt inclusions hosted in crystals from these rocks offer a rare opportunity to depict the formation and the composition of metasomatic skarn-forming fluids at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. Four principal types of melt inclusions represent highly differentiated phonolite (type 1), hydrosaline melt (type 3), unmixed silicate-salt melts (type 2), and a complex chloride-carbonate melt with minor sulfates (type 4). The high-temperature (700-800oC) magmatic-derived hydrosaline melt is considered to be the main metasomatic agent for the skarn-forming reactions. The interaction between this melt (fluid) and carbonate wall rocks produces a Na-K-Ca carbonate-chloride melt that shows immiscibility between carbonate and chloride constituents at ~700oC in 1 atm experiments. This unmixing can be viewed as a possible mechanism for the origin of carbonatites associated with intrusion-related skarn systems.

The Pantelleria caldera geothermal system: Data from the hydrothermal minerals

Abstract This paper proposes, on the basis of petrographic and mineralogic data on cutting and cores from two deep wells (“Pantelleria 1” and “Pantelleria 2”), the first model of the active hydrothermal system of the island of Pantelleria. Phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes. The results of these studies emphasize differences between the intracaldera and pericaldera areas of the island. Within the 45 ka caldera there is a high-temperature (240–260 °C at 600–800 m depth) active hydrothermal system with five zones of characteristic alteration minerals with increasing depth. Rocks are unaltered to a depth of 200 m, contain smectite and mixed-layer chlorite-smectite (C/S) between 200 and 380 m, chlorite, illite, chalcedony and quartz from 380 to 500 m, albite, adularia and saponite from 500 to 680 m, mixed-layer biotite-vermiculite from 680 m to the depth drilled (1100). Outside the caldera, but near the rim, a low-temperature and low-permeability (

The skarn shell of the 1944 Vesuvius magma chamber. Genesis and P-T-X conditions from melt and fluid inclusion data

Skarn rocks are a component of tephra ejected during the 1944 eruption of Vesuvius. Three different types were recognized: 1. melilite-, 2. phlogopite- and 3. periclase-bearing. The presence of well preserved melt and fluid inclusions and fresh interstitial glass, allowed the reconstruction of the processes occurring during the formation of skarn rocks and the establishment of the P-T-X conditions within the magma chamber wall rocks. Skarn formed at temperatures of about 1000 and 800°C in Type-1 and Type-2 respectively, and at pressures of about 100 MPa. These rocks record in-situ endoskarn genesis at the interface between magma and carbonate rocks in which bimetasomatic diffusion of constituents down chemical potential gradients between magma and carbonate rocks (reciprocal diffusion metasomatism) takes place. Skarn rocks constitute a transition zone between the carbonate country rocks and magma chamber, where the well-defined reaction zones are due to melts infiltrating porous decarbonating rocks. Magmatic melts are modified by addition of Ca and Mg. These melts metasomatize the carbonates inducing skarn reactions. The modified melts during their differentiation may exsolve hypersaline fluids. Different metasomatising agents (melt, melt + hypersaline fluids and possibly hypersaline fluids) produce the different types of skarn facies that are transitional to thermometamorphic marbles.

Evidences for disruption of a crystallizing front in a magma chamber during caldera collapse: An example from the Breccia Museo unit (Campanian Ignimbrite eruption, Italy)

Abstract This work is focused on juvenile components and some cognate xenoliths of the Breccia Museo (BM) unit. The BM is a coarse-grained proximal unit of the caldera-forming phase of the Ignimbrite Campana (IC) eruption, southern Italy. The BM products show some peculiar characteristics that distinguish them from the other IC deposits. In particular, different types of pumice fragments constitute the juvenile fraction and their crystal contents are remarkably higher than the other IC units. Slightly porphyritic and highly porphyritic trachytic to phonolitic pumices were distinguished in each sample and investigated separately for mineralogy, matrix glass composition, melt and fluid inclusion studies. Most feldspar crystals may have formed at the margins of the magma chamber and the crystal content of both types of pumice fragments can be ascribed to variable entrainment of these crystals (from the solidification front) by the melt. Variably porphyritic (

Towards a reconstruction of the magmatic feeding system of the 1944 eruption of Mt Vesuvius

Abstract Geochemistry and mineralogy of both juvenile clasts and xenoliths ejected during the 1944 eruption of Mt Vesuvius provide major contraints on the magmatic feeding system. Melt inclusions in phenocrysts of juvenile scoriae highlight that the magmas feeding the eruption underwent differentiation at different pressures. A K-tephritic volatile-rich melt evolved to reach K-phonotephritic composition at pressures higher than 300 MPa before being fed into a very shallow reservoir (P

Exhumation of an active magmatic–hydrothermal system in a resurgent caldera environment: the example of Ischia (Italy)

Abstract: Ischia is a rare case of a well-exposed caldera system that has experienced rapid recent resurgence, which can be used to dissect the anatomy of an otherwise inaccessible active hydrothermal system developed in a caldera-forming environment. Integrated analyses of melt and fluid inclusions, mineralogy and stable isotopic compositions of pumices, tuffs and syenitic xenoliths of the Ischia volcanic system provide strong physico-chemical constraints on the shallow magmatic reservoir and the hydrothermal system. The hydrothermal system has been exposed by the rapid uplift of the Mt. Epomeo resurgent block. The engine of the hydrothermal system of Ischia can be identified in the shallow magmatic system (at around 2 km depth) that hosts hot (c. 1000 °C) trachytic magma. The hydrothermal system developed principally within thick intracaldera ignimbrite deposits, and extended to a depth of at least 1 km, defining a series of facies characterized by associations of alteration mineral assemblages typical of a seawater-dominated high-temperature geothermal system. Supplementary material: Sample descriptions and analytical methods are available at http://www.geolsoc.org.uk/SUP18371.

Formation of rock coatings induced by the acid fumarole plume of the passively degassing volcano of La Fossa (Vulcano Island, Italy)

At La Fossa volcano (Vulcano island) and in the surrounding environment, rock coatings (RC) develop owing to exposure of the ground surface to a volcanic acid plume. This plume is produced by the passive degassing of La Fossa active cone. Proximal and distal (with respect to the high temperature fumarolic field) RC have been identified which differ in mineralogy, texture and chemical composition. Proximal RC are characterised by high silica content, massive texture and strong leaching at the expense of the pyroclastic material. Distal RC are characterised by the presence of silica, jarosite and minor goethite, laminated texture and less developed evidence of leaching. The RC are produced by direct interaction between the acid fumarolic aerosol and rocks and by reaction of dews and rains, acidified by the absorption of acid gases (SO2, HCl, HF) of the fumarole plume, with volcanic material. Proximal RC are produced at conditions of pH<2, distal RC form at pH 2–3, based on the presence of jarosite, although the occurrence of goethite in some samples suggests higher values of pH. The influence of the volcanic plume in the development of these RC is indicated also by the coincidence of the RC areal distribution with the distribution of the volcanic plume, controlled by the local wind field. This is furthermore supported by the presence in RC of chloride and/or sulphur bearing compounds, enriched in volatile elements and metals, commonly carried by volcanic aerosols.

Deep hydrothermal circulation in a granite intrusion beneath Larderello geothermal area (Italy): constraints from mineralogy, fluid inclusions and stable isotopes

Abstract A shallow (∼2 km) granite body has been intersected by the MONT-4 well in new producing areas of the Larderello geothermal field. The granite at 2.2–2.5 km and below 3 km underwent pervasive propylitic alteration that is dominated by the assemblage chlorite+adularia+quartz+illite+calcite+albite±epidote±sphene. Fluid inclusion studies indicate two stages of alteration. The first one is characterised by high-temperature fluids that, at 2218 m, were trapped under boiling conditions at about 340°C and 15 MPa, whereas at 3520 m evidence of boiling is not revealed and the temperature of the fluid was calculated to be about 350°C. The second alteration stage shows a cooling of the system, particularly evident in the shallower portion of the intrusion (∼235°C at 2218 m and ∼315°C at 3520 m). The temperature of homogenisation for subsequent cooler inclusions approximates present-day down-hole temperatures, and may be considered as representative of the fluid actually circulating in the granite in recent times. Calculated δ 18 O values of 0±1‰, for fluids in equilibrium at 340°C (2218 m) and 350°C (3520 m) with feldspar, epidote and chlorite, and δD values of −51 to −61‰, for fluids in equilibrium with chlorite, together with directly measured δD values of −33 to −61‰ for fluid inclusions in quartz, indicate derivation from meteoric waters. The original meteoric signature of the fluids has been altered through water–rock interaction, but remains clearly distinguished from a primary magmatic signature, for which no isotopic evidence was found. Active fracturing and the high structural relief of granite emplacement promoted hydrothermal circulation in the granitic body drilled by MONT-4 well, suggesting that such characteristics may be a pre-requisite for such circulation at Larderello.

Glass-bearing felsic nodules from the crystallizing sidewalls of the 1944 Vesuvius magma chamber

Abstract In the 1944 Vesuvius eruption, the shallow magma chamber was disrupted during the highly energetic explosive phases. Abundant cognate xenoliths such as subvolcanic fergusites and cumulates, hornfels, skarns and rare marbles occur in tephra deposits. Mineral chemistry, melt inclusions in minerals and glassy matrix compositions show that fergusites (highly crystalline rocks made of leucite, clinopyroxene, plagioclase, olivine, apatite, oxides and glass) do not correspond to melt compositions but result from combined sidewall accumulation of crystals, formed from K-tephriphonolitic magma resident in the chamber, and in situ crystallization of the intercumulus melt. Very low H2O contents in the intercumulus glass are revealed by FTIR and apatite composition. Whole rock compositions are essentially determined by the bulk mineral assemblages. Glass-bearing fergusites constitute the outer shell of the magma chamber consisting of a highly viscous crystal mush with a melt content in the range 20−50 wt.%. The leucite/(clinopyroxene+olivine) modal ratio, varies with the extraction order of magmas from the chamber, decreasing upwards in the stratigraphic sequence. This reflects a vertical mineralogical zonation of the crystal mush. These data contribute to the interpretation of the subvolcanic low-pressure crystallization processes at the magma chamber sidewalls affecting alkaline potassic magmas.