The volcanic activity of Stromboli in the 1906–1998 AD period: mineralogical, geochemical and isotope data relevant to the understanding of the plumbing system

Abstract

Volcanic activity of Stromboli in the last 100 years was characterised by regular Strombolian eruptions with some occurrences of major explosions and paroxysms at the summit crater and lava flows down into the Sciara del Fuoco. Two types of juvenile fragments, shoshonitic to high-K basalts in composition, are peculiarly outpoured during major explosions: black scoriae, similar to those erupted by the normal Strombolian activity, representing a highly porphyritic (45–60 vol%), volatile-poor magma (HP magma) and a small volume of light pumice, representing a low-porphyritic (<10 vol%), volatile-rich magma (LP magma). Lava flows are constituted by the HP magma. The LP magma has a less evolved composition, lower incompatible trace element contents and Sr isotope ratios (0.70610) than the HP magma. Mineral phases in equilibrium with the LP and HP magmas have distinct compositions (Fo% in olivine: 80–85 and around 70, Mg# in clinopyroxene: 0.83–0.91 and around 0.75, An% in plagioclase: 80–90 and around 65, respectively), in spite of syn-eruptive mingling, and disequilibrium processes lead to large compositional ranges in minerals of both LP and HP magmas. Olivine and clinopyroxene of pumice were equilibrated at higher temperatures (and probably higher pressures) than the same crystals of scoriae. All these data provide evidence that the two magmas are characterised by sharply distinct physico-chemical conditions. It has also been pointed out that the plumbing system is under overall steady-state conditions. The HP magma resides at shallow level and evolves in a continuously erupting, crystallising (olivine+clinopyroxene+plagioclase) and replenished reservoir, which is fed by the LP magma. The LP magma derives from a deeper reservoir in which it undergoes limited crystallisation of femic phases. Plagioclase begins to crystallise only at lower depth. Significant geochemical, mineralogical and isotope variations with time in scoriae and lavas of the 20th century are observed. The MgO, V, Ni and Cr contents of magmas slightly increase from 1906 to 1930, then decrease from 1965 towards the present. Mineral chemistry data also vary accordingly. An increase of incompatible trace element contents after 1930 and a decrease of Sr isotope ratios after 1980–85 (from 0.70626 to 0.70616) have also been pointed out. These variations are thought to be due to changes in the crystallisation/replenishment equilibrium of the magma reservoir or to compositional modifications of the feeding magma. Some correlations between the main compositional variations and the type of eruptive activity also seem to be present. The occurrence of relatively frequent paroxysms during the first part of the 20th century, associated with the observed decrease of magma evolution, seems to indicate that more energetic explosions are associated with the rise of a higher volume of refreshing magma.