Noble gas isotopic ratios from historical lavas and fumaroles at Mount Vesuvius (southern Italy): constraints for current and future volcanic activity
Introduction
The geological setting of the Tyrrhenian Sea is dominated by the collision of Africa with continental Europe. The northward-dipping subduction zone between the converging African and European plates, inferred from both geophysical and petrological data, and the nature and distribution of volcanism, is usually located in the Ionian Sea. The earliest evidence of subduction volcanism are believed to be the Aeolian Islands, which have characteristics that suggest their derivation from a depleted mantle source 1, 2, 3 associated with a sinking slab of limited lateral extent 4, 5, 6. Seismic and bathymetric data indicate that the subduction is presently taking place along the Calabrian arc and it extends to the north [5]. In contrast, the evidence for an active subduction beneath the Neapolitan and the Roman volcanic provinces is more speculative. The subducted slab has been hypothesized to be highly deformed, with a marked concavity towards the northwest, causing the Benioff zone beneath the Aeolian Islands to dip westward beneath the Neapolitan volcanic area [5]. On the other hand, a less concave slab has also been proposed with subduction ending in the central part of the Tyrrhenian Sea [6]. Intermediate and deep-focus earthquakes indicate that the lithospheric slab is still seismically active beneath the Eolian–Calabrian area 4, 5, 6, 7, 8, continuing to the southern end of the Neapolitan volcanic region [9]. The lithospheric root, marked by very few events with depths less than 150 km (earthquakes with deeper depth are not recorded in this region), seem to be much less active in the Roman–south Tuscany volcanic region [10]. Here, the hypothesis of the existence of a relic slab is supported by surface wave dispersion and P-wave residual data [11] and by the trend of long-wavelength gravity anomalies [8]. An alternative model suggests that the subduction beneath the Neapolitan and Roman volcanic provinces is related to the counterclockwise rotation of the Corsica–Sardinia block which generated an eastward volcanic area and the Aeolian islands should be related to subduction processes which occurred at different times and were related to distinct (micro) plates.
Petrological and geochemical studies on the volcanics of the eastern Tyrrhenian Sea border (Sicily to south Tuscany) suggest a subduction-related magmatism 1, 2, 3. The δ18O values have a steady south to north decrease with values as low as 6.8‰ at Vulcano Island (Aeolian Islands), through 5.8–7‰, 7.0–8.4‰ and 7.3–8.3‰ Ischia Island, Campi Flegrei caldera and Mt. Vesuvio, respectively (Neapolitan province), to 7–10‰ in the Roman area and 12–13‰ in the south Tuscany volcanic province [12]. He, Sr, Nd and Pb isotopes on mafic volcanics suggest mantle sources which suffered recent enrichment events dominated by crustal-derived materials [13]. Subduction-related processes are able to explain the serial range from calc-alkaline to ultrapotassic for the Aeolian Islands [14] and the Neapolitan volcanic province [15] and the dominance of potassic and ultrapotassic rocks in the Latium–Tuscany area 1, 2, 3.
Mt. Vesuvius is a stratovolcano few kilometres southeast of Napoli (southern Italy) (Fig. 1). It is part of an older volcanic complex, the Somma–Vesuvius volcanic complex, which has been characterized by different kinds of activities and eruptive products. The three main periods of activity (25,000–79 A.D., 79 A.D.–1631, 1631–1944) are equivalent in products and eruptive pattern. Petrological and geochemical features of the volcanic products during the period of activity since 25,000 B.P. have been extensively studied 16, 17. The eruptive products range from tephrites to lucitites and were erupted during long periods of effusive activities, accompanied by pyroclastic eruptive events. The last eruptive cycle was characterized by 37 eruptions and showed mainly a strombolian eruptive pattern. Petrology and geochemical characteristics of this cycle have been discussed extensively; see Refs. 18, 19, 20 and references therein. Since the 1944 eruption, the volcano has not shown any clear signs of renewed activity. Meanwhile, the surrounding population has increased considerably and now numbers more than 1 million. Concern among scientists monitoring the volcano has also increased. However, without an extensive knowledge of past activity, it will be difficult to provide a reasonable model to forecast future events. Our aim in studying the noble gas compositions of phenocrysts is to gather information on the different sources feeding the volcano and to find out how they have changed with time and how they may change before the next possible eruptive event. These data will be then compared with those of volcanic fluids that have been collected recently at subaerial and submarine fumaroles. The data reported here will provide base-line information on the past activity at Mt. Vesuvius, which may help forecast future volcanic events. Despite the interest and concern about the possible future of Mt. Vesuvius, few studies on volcanic fluids have been carried out on this volcano. While petrology and mineralogy features of Mt. Vesuvius's last cycle have been discussed extensively 16, 17, 18, 19, 20, only recently soil gas samples have been collected repeatedly 21, 22. Both types of studies describe the scarcity of gases coming through the soil at the surface and, probably, the lack of a deep gas flux, contrary to what is usually observed on quiescent volcanoes.
Together with the analysis of phenocrysts, this study also takes into account helium ratios from fluids collected from a fumarolic field located in the inner part of the cone, from submarine fumaroles located in the vicinity of the port of Torre del Greco (at the foot of Mt. Vesuvius) and from fumaroles and bubbling sources, with an increasing distance from the central cone and located near the shore line (Fig. 1). To date, all fluids collected in the Neapolitan volcanic province, Campi Flegrei, Ischia and Mt. Vesuvius, show anomalous low 3He/4He ratios, which suggest two interpretations 23, 24, 25: (1) they are related to an enriched mantle feeding in all Neapolitan volcanic region, responsible for the peculiar 87Sr/86Sr (0.7072–0.7079), 143Nd/144Nd (0.51242–0.51260) ratios (Fig. 2) 17, 26 and δ18O (7.2–8.2‰ [12] found at Mt. Vesuvius, or (2) they are due simply to isolated, highly degassed reservoirs, depleted in 3He and not recharged by new juvenile fluids.
Section snippets
Sampling and analysis
Noble gas analyses were carried out at the Institute for Study of the Earth's Interior, Okayama University using a VG5400 noble gas mass spectrometer. The MS is a sector type, its resolution was adjusted to 600. The data acquisition was operated by computer. For the noble gas analysis clynopyroxene and olivine crystals ranging from <1 mm to 5 mm in diameter were hand-picked from the crushed lavas and then inspected to eliminate adhering rock matrix and alteration products. Phenocryst samples
Helium ratio
Mantle He is considered to be enriched in its primordial component, the 3He that is continuously released from the mantle. The 3He/4He ratio in the upper mantle is thought to be uniform, eight to nine times the atmospheric He (Ra) [29]. The He isotopic ratio, as for all other trace elements and gases, in subduction zone magmas is similar, showing a value of R/Ra between 7.0 and 8.0, suggesting a slight radiogenic 4He contamination [29]. On the contrary, He isotopic ratios which were much higher
Helium isotope ratios from fumarolic fluids
Low flux fumaroles are currently present in the inner part of the Mt. Vesuvius crater (Fig. 1). A submarine fumarole field with temperatures similar to sea water (20°±2°C) also exists near the port of Torre del Greco. Low temperature fumaroles in the vicinity of the cities of Pompei, Torre Annunziata and Castellammare di Stabia have also been collected. Helium data are shown in Table 3.
The fumarolic field inside the crater has only been sampled a few times because of its difficult access (Fig. 8
Conclusions
The helium ratio from phenocrysts from Mt. Vesuvius lava shows a decreasing trend from 1631 to 1794, and more consistent values prior to the 1944 eruption. The helium trend is explained by the continuous degassing, during the last eruptive cycle, of an isolated, deep magma reservoir which was not recharged during this event. The primitive He isotopic ratio of the Mt. Vesuvius source region, the European Sub-Continental Mantle, is suggested to be between 6.0 and 6.5 [35] lower than typical
Acknowledgements
The authors wish to thank Professor Benedetto De Vivo for having kindly provided from his own collection the lava samples used in this work. Dr David Zinbelman corrected and kindly revised the English of the manuscript. The Japanese–German Berlin Center is thanked for having supported D.T. with a one-year fellowship at the Institute for Study of the Earth Interior, Okayama University. The staff of the I.S.E.I. is thanked warmly for the help they have given during the stay of D.T. Thanks to Dr M
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