From Deep to Shallow: Increasing Melting Rates, Short Residence Times and Recycling of Crystal Mush-Derived Melts Revealed by U-series Disequilibria at Stromboli Volcano

The presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts, representing the crystalline portion of old magmas recycled in the juvenile material, but very little is known about the fate of interstitial liquid hosted within the crystal-rich mush (i.e. antemelt). U-series disequilibria measured in magmas erupted in the past 18 years at Stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano. Despite almost constant major and trace element composition, significant variations in isotope ratios are observed. (230Th/232Th) decreases with time, whilst (238U/232Th), (226Ra/230Th) and 87Sr/86Sr are different in the two types of magma erupted. Magma with low phenocryst content (lp) is erupted as pumices during paroxysm and is thought to belong to a deep reservoir. Highly porphyritic magma (hp) is erupted during the normal “strombolian” activity as scoria and during the effusive events as lavas, and it is considered to derive from the former one within a shallow reservoir through degassing-driven crystallization, mixing and incorporation of antecrysts. The distinct (238U/232Th) of lp and hp magma requires the involvement of a component with high 87Sr/86Sr and (238U/232Th) deriving from older magmas erupted earlier in the volcano history (up to 2.5 ka). The incompatibility of U and Th in major mineral phases limits the possible effect of antecrysts, hence requiring the involvement of a U- and Th-rich antemelt. The decrease of 226Ra-excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old ante melt. The variation with time of (230Th/232Th) within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to < 55 years (inferred reservoir volume < 0.5 km3) and 2-10 years (inferred reservoir volume 0.02-0.09 km3), respectively. Our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so called steady state volcano, such as Stromboli. We show that, while most of the magma is erupted within few years, a portion of it, made of both crystals (antecrysts) and residual liquid (antemelt), can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir. The presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes, hence favouring their remobilisation. In addition the impressively smooth time-related variation of (230Th/232Th) in lp products during from 1997 to 2007 may provide a link between processes occurring at depth and at the surface. Mantle melting models show that the decrease in (230Th/232Th) reflects an increase in the melting rate of the mantle source beneath Stromboli. Notably, this variation can be directly related with the increase in eruption rate estimated from the volcano morphology and erupted volumes, but also from the observed increase of major explosive events and lava flows, that continues up to the present days with the latest effusive event of August 2014. We suggests that U-series disequilibria, and (230Th/232Th) in particular, may represent an important tool to estimate the rate of magma production and then possibly forecast the eruption rates in basaltic volcanoes such as Stromboli