Pre-eruptive volatile (H2O, F, Cl and S) contents of phonolitic magmas feeding the 3550-year old Avellino eruption from Vesuvius, southern Italy

The behaviour of H2O, F, Cl and S prior to and during the Plinian phase of the 3550-year old Avellino eruption was determined by the study of unheated primary glass inclusions and matrix glasses. The fall deposit can be divid ed into two sub-units on the basis of an abrupt change in colour: a phonoli tic white layer at the base and a tephriphonolitic banded grey pumice layer at the top. The original tephriphonolitic magma is always found mixed with the phonolitic magma. Primary glass inclusions were only found in feldspar , amphibole and garnet associated with the white pumice. Glass inclusions i n the products of the Avellino Plinian eruption contain a mean of 3.10 +/- 0.74 wt.% H2O, 0.95 +/- 0.15 wt.% F, 0.59 +/- 0.02 wt.% Cl, and 560 +/- 55 ppm S. The concentration of CO2 was below the detection Limit. Coexisting m atrix glasses contain similar amounts of halogens but significantly less wa ter (similar to 0.5 wt.%) and sulphur (similar to 100 ppm), suggesting that during the eruption the halogens did not behave as volatiles. Owing to the lack of primary glass inclusions and its hybrid nature, similar conclusion s can only be hypothesised for the tephriphonolitic magma on the basis of v olatile element distribution in the matrix glasses. Taking into account bot h our data and data from parental mafic melt inclusions by a previous study by P. Marianelli and co-workers, it has been possible to reconstruct the b ehaviour of volatiles in the Avellino magmatic reservoir in two stages: (1) variation of volatiles during crystal fractionation; and (2) exsolution of an "excess" gas phase when volatiles reach their solubility limit in silic ate melts. The agreement between our data and the solubility limit of chlor ine, sulphur and probably water in phonolitic melts at the postulated press ure and temperature (i.e., 100 MPa and 850 degrees C), suggests that such a phase exists. Compared to the other volatiles, fluorine does not exhibit a behaviour (i.e., flat distribution) consistent with the attainment of its solubility limit in phonolites. Using the petrologic method, the minimum ou tput of volatiles during the Plinian phase of the Avellino eruption was hyp othesised to be about 2 x 10(6) tons of H2SO4 and 25 x 10(6) tons of H2O. D ue to both the presence of an "excess" vapour phase prior to eruption and t he assumptions inherent in the petrologic method, the total calculated amou nts should be considered as very conservative estimates of volatile outputs during the eruption. (C) 1999 Elsevier Science B.V. All rights reserved.