Linking mantle melting and eruption rates at Stromboli volcano: a U-series perspective

Over the last several hundred years, the steady-state activity of Stromboli volcano has been characterised by persistent mild explosive eruptions, ejecting black scoria bombs. Periodically, lava flows and paroxysms, ejecting also light-coloured pumices, interrupt the ‘normal’ activity. A degassed and highly porphyritic basaltic-shoshonitic magma (HP-magma), is erupted by the normal activity and lava flows, whereas a slightly more mafic and volatile-rich magma with low phenocryst content (LP-magma) is erupted as pumices only by the paroxysms. Whilst the former is considered as deriving from a shallow level reservoir, the latter has a deeper origin and might resemble the primitive melts generated from the mantle source. We present new U-series disequilibria measurements on a suite of HP and LP magmas from the present-day activity of Stromboli volcano. Small but significant differences are observed in (238U/232Th), (230Th/238U) and (226Ra/230Th) between HP and LP magmas, testifying and further constraining the complex processes occuring in the shallow level magma chamber (e.g. crystal fractionation, mixing, recycling of material from previous activity). On the contrary, the variation of (230Th/232Th) seems to be little affected by the shallow level processes and can be used to constrain the melting regimes beneath the volcano. This is particularly true for the LP magmas that should reflect the composition of the primitive magma. Indeed, the LP data show an impressive time-related constant variation of (230Th/232Th) during the last 15 years that is interpreted as an increase in the melting rate of the mantle source. More importantly this variation can be directly related with the increase in eruption rate that is suggested by both observation (increase of major explosive events and lava flows) and estimates based on the volcano morphology and erupted volumes. This suggests that U-series, and (230Th/232Th) in particular, can represent an important tool to estimate the rate of magma production and then possibly forecast the eruption rates in basaltic volcanoes such as Stromboli. The variation in (230Th/232Th) is also discussed with the aim of estimating the turnover times of both the HP and LP systems