Revisiting the last major eruptions at Stromboli volcano: inferences on the role of volatiles during magma storage and decompression

Abstract

AbstractStromboli is a unique open conduit volcano and a natural laboratory for investigating how volatiles migrate and concentrate under dynamic conditions. Fluid phases are involved in magma decompression and pressurization, modulate Strombolian activity and govern magma rise and fragmentation processes. Here, we have revisited the available data on the last two major eruptions at Stromboli volcano and concentrated our analysis on the 2007 eruption. First, we analysed petrological-geochemical data to assess equilibrium conditions by using standard thermobarometry; we then used a grid of selected reactions which involve solid-melt-fluid equilibria to better constrain the P–T regimes that adequately describe our system. Primitive hydrous basaltic melts, reported in literature and preserved as melt inclusions in olivine (with 2.3–3.8 wt% of H2O and 890–1590 ppm CO2), are in equilibrium with forsteritic olivine and a diopsidic clinopyroxene at average pressures of 260 (±47) MPa for temperatures approaching 1170 (±17) °C and calculated (mole fraction of CO2 within the melt) in the range 0.60–0.76. Ca-rich or ultracalcic melts are regarded as the result of decompression along a steep adiabatic and/or isothermal curve. During this process the magma will cross-cut the stability field of diopside and enter the liquidus field. The earlier crystallized diopside is destabilized and reacts with the coexisting liquid phase leading to the formation of ultracalcic melts. Ejected golden pumices (with 2–3 wt% H2O) are in equilibrium with Ca-pyroxene, forsteritic olivine and anorthitic plagioclase at 150–220 MPa and temperatures of 1120–1150 °C. Evolved melt inclusions (substantially degassed) in less magnesian olivine (c. Fo70) of the scorias show average equilibration pressures of 78 (±20) MPa and temperatures of 1138 (±14) °C. In summary, the higher P–T regimes associated with the origin of primitive melt inclusions are representative of the base of the chamber, where the ferromagnesian phases may crystallize and cumulate. The magma with a bulk composition typical of the pumices is stored in the middle and main part of the chamber (likely its axial sector) and these materials are erupted during paroxysmal and, more rarely, major explosions. Finally, more evolved melt inclusions found in the olivine of the scorias are indicative of crystallization within the conduit or its root zone connected to the upper part of the chamber.Pure extensional regimes and recent geophysical data suggest the existence of a prolate ellipsoidal magma chamber below Stromboli. To constrain its volume we estimated the magma volumes associated with SO2 degassing (during the 2007 major eruption) by applying a refined petrological model that allowed us to estimate the magma fluxes in the subvolcanic region (i.e. the magma flux entering the degassing zone). The long-term trend of this magma flux follows an overall exponential decay, typical of pressurized magmatic systems, and indicates that magma rise was accompanied and followed by slow decompression. This trend was shown to be consistent with release of elastic strain accumulated either by pressurization of the rocks surrounding the magma reservoir, by pressurization of the magma itself or both. By analysing the reservoir elastic response during magma decompression, we found that the current Stromboli magma chamber volume may be adequately constrained to 1–2 km3.