Arsenic in fumarolic gases of Vulcano (Aeolian Islands, Italy) from 1978 to 1993: Geochemical evidence from multivariate analysis

抄録

A program of geochemical surveillance of the volcanic activity at Vulcano island (southern Italy) has allowed to collect a large set of observations, and the main chemical composition of volcanic gases is consequently known for the period from 1978 to 1993. This systematic monitoring with time of fumarolic activity at Vulcano has represented a good opportunity to investigate variations of arsenic concentrations in the condensates of fumarolic gases, and to correlate them both with temperature and chemical composition of the gases. Previous studies on arsenic behaviour in fumarolic gases for a similar long period are not reported in literature so far. Multivariate methodologies are a correct tool in order to investigate the simultaneous relationships between a large number of variables and to evaluate their changes with time. Since the chemical composition of volcanic gases is expressed by closed (or compositional) variables, the log-ratio approach has been adopted to overcome the induced correlations. By considering the results obtained by the application of Principal Component Analysis, the following considerations can be formulated: 1) arsenic behaviour appears to be mainly influenced by the presence of intense hydrothermal circulation as represented by the multivariate association of the element with variables as T, HF/H₂O, NH₃/H₂O and H₂S/H₂O ratios; 2) the hydrothermal circulation, characterised by a continuous increase with time, appears as the main feature of the volcanic system; in this framework, the seismic event occurred in 1984 may have contributed to facilitate the fluid circulation; 3) no obvious relationships appear to link the increase of gas temperature with typical magmatic contributions as expressed by CO₂/H₂O and SO₂/H₂O ratios. The results indicate that the correlations between arsenic and the other physical-chemical parameters characterising the volcanic system are useful to constrain the interpretations on its geochemical evolution with time. This is particularly true if the system is dominated by hydrothermal processes as those of Vulcano island, where arsenic undergoes intensive remobilization.