Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5 ka story inferred from in-situ Sr-isotope and trace-element data

On 28 December 2002, the persistent moderate explosive activity of the Stromboli volcano (Aeolian Islands, Italy) was broken by an effusive event lasting about seven months. A more violent explosion (paroxysm) occurred on 5 April 2003. Two magma types were erupted, namely a volatile-poor and highly-porphyritic magma (HP-magma) poured out as scoria or lava and a volatile-rich, phenocryst-poor magma (LP-magma) found as pumice. LP-magma differs from the HP-magma also for its slightly less evolved chemistry, the groundmass composition and the lower Sr-isotope ratios. In-situ Sr-isotope data by microdrilling, coupled with major and trace element analyses have been performed on plagioclase and clinopyroxene from seven samples collected during the 2002-2003 eruptive crisis. Micro-Sr isotope data show the presence of zoned minerals in strong isotope disequilibrium, as previously found in products erupted in 1984, 1985 and 1996 AD, with 87Sr/86Sr values generally decreasing from cores to rims of minerals. Only some outer-rims testify for equilibrium with the host groundmass. The internal mineral zones with high Sr-isotope ratios are interpreted as “antecrysts”, crystallised during the previous activity and recycled in the present-day system since the opening shoshonitic activity of the Recent Period, which occurred at about 2.5 ka ago. This result has inferences for the dynamics of the present-day plumbing system of Stromboli at intermediate pressure (about 2-3 km depth), allowing us to propose a model whereby an HP-magma reservoir is directly interconnected at the bottom with a cumulate crystal much reservoir. Efficient mixing between residing HP- and input LP-magmas can occur in this reservoir, due to more similar rheological characteristics of the two magmas than in the conduit, where crystallization is enhanced by degassing. Antecrysts (and possibly melts) re-enter in the HP-magma reservoir both from the bottom, recycled by ascending LP-magmas crossing the crystal mush, and from the top, recycled by descending degassed and dense HP-magma, residual of the periodic Strombolian explosions at the surface. The isotope variation measured in the groundmasses allows calculating the proportion of the LP-magma entering the shallow HP-magma reservoir at ~20%. From this proportion we estimate that the total volume of LP-magma input during 2002-2003 closely matches the magma volume erupted in the effusive event, suggesting a steady-state system at roughly constant volume.