U-series isotopes in the present-day activity of Stromboli Volcano: constrains on the plumbing system

U-series isotopes represent a unique opportunity to study volcanic systems since they provide constraints on mantle melting and on timing of shallow magmatic processes. Particularly, parent-daughter isotopes pairs with different half-lives (e.g. 230Th-238U; 226Ra-230Th) are able to unravel processes at different timescales. Stromboli is a perfect case study for U-series investigations due to its continuous and regular activity, which allow studying variations of the feeding system with an almost continuous time-resolution. Moreover, previous multidisciplinary studies on the plumbing system provide well established constraints to explore the full potentialities of U-series isotopes in volcanic systems. Two different types of magma characterize the present-day activity of Stromboli. The LP magma, with low phenocryst content is erupted only during major explosions and paroxysms. It represents deep fresh magma input into the shallow magma chamber. The HP magma occurs during normal strombolian activity and lava flows. It shows low phenocryst content and, in general, it can be considered to derive from the LP magma through differentiation processes in the shallow magma chamber. We analyzed the parent-daughter pairs 226Ra-230Th, 230Th-238U, as well as the activity ratios (230Th/232Th) and (238U/232Th) in HP and LP magmas ejected in the last 20 years. Few older samples from the last century were also analyzed for comparison. The two magmas show similar but distinct composition for U-series isotopes. (238U/232Th) is slightly higher in HP (0.800-0.808) than in LP magma (0.785-0.795). (230Th/232Th) is the same in the two groups whereas (226Ra/230Th) is much lower in HP magmas (2.23-2.46 versus 2.51-2.66). Variations in U/Th can’t be explained by fractionation and/or assimilation of mineral phases since they must involve a component enriched in U and Th. This material represents the residue of an old magma recycled within the present-day shallow chamber. Most likely, this component shares a common origin and age (i.e. <2 ka) with the old crystals (antecrysts) described by previous works [e.g. Francalanci et al. 2012]. This process is also confirmed by the lower excess of (226Ra/230Th) since the disequilibrium decreases over time due to the aging. However, the correlation of (226Ra/230Th) with Ba/Th implies plagioclase fractionation in the HP magma. Such process must be extremely quick and in strong disequilibrium with the liquid in order to describe the similar partitioning of Ra and Ba into the plagioclase. Thus, the crystallization and fractionation of plagioclase probably occur during the final ascent of the magma towards the surface.