Noble gas isotopic ratios from historical lavas and fumaroles at Mount Vesuvius (southern Italy): constraints for current and future volcanic activity

Helium, neon and argon isotope ratios have been analysed from phenocrysts o f eleven lava samples belonging to the last eruptive cycle of Mount Vesuviu s (1631 until 1944). The phenocrysts separates include pyroxene (N = 10) an d olivine (N = 1). All phenocryst samples show similarly low gas contents ( He, Ne and Ar similar to 10(-10) cm3/g). He-3/He-4 ratios, 5.3-2.11 R-a, ar e generally low if compared to those typical of the MORE and those of the E uropean Subcontinental Mantle (ESCM), respectively R/R-a 8.5 +/- 1 and 6.0- 6.5, A decreasing trend is found from 1631 to 1796, while a more homogeneou s set of data is obtained for more recent eruptions, as evidenced by an ave rage R/R-a value of 2.85. Neon ratios (Ne-21/Ne-22 and Ne-20/Ne-22) strongl y differ from those typically found on volcanoes and suggest that a crustal component has been added in the source region to Mt. Vesuvius magmas. Argo n ratios (Ar-40/Ar-36 and As-38/Ar-36) have values similar to the atmospher e and are well correlated. The low Ar-40/Ar-36 ratio (max. 302) is, however , in the range of the Ar-40/Ar-36 ratios obtained from several lava samples at other Italian volcanoes and might be considered to have a deep origin. Two hypothesis have been discussed: (1) a deep argon-like-air source, due t o subduction of air-rich sediments and/or (2) a preferential loss of Ar, in comparison to lighter noble gases, from silicic melts. Helium isotopic ana lysis of gas samples recently collected from crater and submarine fumaroles are similar to those of lavas belonging to the final part of this eruptive cycle. This result supports the idea that no new juvenile fluids from the source region have been injected into the magmatic reservoir during the 163 1-1944 eruptive cycle and, more importantly, until 1993. Both sets of data help to understand the genesis of these fluids and to constrain the current activity of the volcano. (C) 1998 Elsevier Science B.V. All rights reserve d.