Behaviour of chlorine prior and during the 79 AD Plinian eruption of Vesuvius (southern Italy) as inferred from the present distribution in glassy mesostases and whole-pumices

This study deals with the distribution of chlorine in glassy mesostases and whole-pumices from the 79 A.D. Plinian eruption (Somma-Vesuvius volcanic c omplex. Italy). This explosive event produced a prominent Plinian fall depo sit followed by flows and surges. The fall deposit can be divided into two sub-units on the basis of an abrupt change in colour at approximately mid-l evel: a phonolitic white pumice layer at the base and a tephriphonolitic gr ey pumice layer at the top. Due to its hybrid nature (a mixture of k-tephri tic, tephriphonolitic and phonolitic magmas), information on chlorine behav iour in tephriphonolitic magma has been inferred only by means of mass bala nce calculations. In the white pumices, chlorine concentrations show consta nt whole-pumice values, whereas glassy mesostases display significant compo sitional variations. These variations have been linked to the cryptocrystal lisation of leucite in glassy mesostases, which affected the original melt compositions just before and during the eruptive event. In this framework, whole-pumice appears to better represent the pre-eruptive melt compositions . Using chlorine concentration in whole-umices, a three-stage model of chlo rine behaviour prior and during the eruptive event is predicted. (i) free v ariation during Rayleigh fractionation, according to a system with variance greater than zero; (ii) exsolution of volatile chlorine compounds (e.g., m etal chlorides), when chlorine reaches its solubility limit in silicate mel t, in coexisting hyper-saline and in dilute immiscible fluids; the variance of the system is zero at a given temperature and pressure; (iii) residual syn-eruptive variable enrichment of chlorine in the melt due to cryptocryst allization of leucite, suggesting a very minor loss of chlorine in the gas phase during the eruption. Although chlorine does not behave as a volatile element during the eruption, it is present in the volcanic plume. This is d ue to the postulated 'excess' fluid phase containing chlorine that formed i n the magma chamber prior to the eruption. The homogeneous distribution of chlorine in whole pumices, in contrast with a well-established chemical and isotopic layering in Vesuvian magmas prior to Pompei eruption, suggests th at the trace element zonation is not directly linked to chlorine distributi on in silicic melts. (C) 1999 Elsevier Science B.V. ALI rights reserved.