BE ISOTOPE AND B-BE INVESTIGATIONS OF THE HISTORIC ERUPTIONS OF MT VESUVIUS

Cosmogenic and radioactive Be-10, stable Be-9 and B concentrations have been determined for four alkaline lavas from the recent cycle of historical activity at Mt. Vesuvius, in the Central Campania Province (Italy). The goal of this study was to use the Be isotope and Be-B systematics of these lavas in a manner analogous to that used in regions unequivocally related to active subduction, in hopes of being able to document a subduction origin for the Vesuvian lavas. Four lavas measured to date have low Be-10 concentrations as well as low B/Be ratios. While the low Be-10 concentrations could reflect subduction and incorporation of old sediments and/or only contributions from the basaltic part of the subducting plate, the combination of low Be-10 and low B/Be ratios must be interpreted as the absence of a subduction signature in these lavas. Unfortunately, the absence of a subduction signature in the lavas cannot be used unequivocally to argue against recent subduction in the Central Campania region. Subduction of an unusually young, hot slab which has lost its B through prograde metamorphic reactions at shallow levels could explain the absence of a subduction signature, as could extensive crustal contamination. In addition, recent studies in Java show that alkaline lavas in this region of active subduction never show a subduction signature, even when erupting in close proximity to calc-alkaline and tholeiitic lavas which do; by analogy, the Vesuvian lavas studied may have been generated in a part of the sub-are mantle which either does not experience or does not preserve the chemical signature of subduction modification. The present data set does not allow us to make definitive statements about the role of subduction in the origin of the Central Campania lavas.