Chapter 12 Influence of hydrothermal processes on geochemical variations between 79 AD and 1944 AD Vesuvius eruptions

Abstract

The authors discuss the compositional differences in homogenized silicate melt inclusions in olivine and clinopy-roxene from cumulate nodules entrained in 79 AD plinian and 1944 AD interplinian eruptions. Such compositions are compared with those of melt inclusions in thessame crystals in the volcanics and with bulk rock compositions in order to examine differences between the “closed system” represented by silicate melt inclusions and the “open system” represented by bulk rocks. Homogenized melt inclusions (HMI) in olivine and clinopyroxene from cumulate nodules collected in 79 AD volcanics, show higher P 2 O 5 /K 2 O, Na 2 O/K 2 O, Cl/K 2 O and S/Cl values compared with HMI in olivine from nodules collected in 1944 AD volcanics. In addition, the marked geochemical differences as P 2 O 5 /K 2 O, Na 2 O/K 2 O, Cl/K 2 O and S/Cl variations between 79 AD and 1944 AD melt inclusions and 79 AD and 1944 AD bulk rocks have been interpreted as being strongly influenced by hydrothermal processes active before and during explosive eruptions. During the 1944 AD interplinian activity at Vesuvus, magmas continuously ascended through the system, undergoing simultaneous degassing and fraction. The loosely cemented conduit within the volcano did not allow significant build up of volatile pressure, leading to frequent, non-violent interplinian eruptions. The 79 AD plinian explosive eruption occurred after a long repose time at closed-conduit condition. The conduit became closed as a result of the cooling of ascending magma at the end of a previous interplinian eruption.