The evolution of the magmatic system of Stromboli volcano during the Vancori period (26–13.8 ky)

Abstract

Stratigraphic changes in bulk-rock and mineral chemistry within a volcanic sequence can be used to understand the evolution of the magma storage system beneath the volcano; in particular, changing eruptive styles and major changes in magma chemistry can be linked to open- or closed-system behaviour. Changes in phenocryst mineral chemistry, often expressed as core–rim zonation patterns or the development of reaction rims, combined with thermodynamic analysis of appropriate mineral equilibria, can provide powerful insights into the timing of inputs of new, more primitive magma. The Vancori period (26–13.8 ky) of the eruptive activity of Stromboli has a relatively simple stratigraphy, comprising three sub-periods, Lower, Middle and Upper Vancori, each one beginning and ending with an explosive volcanic event, marked by breccias. Electron microprobe analyses of phenocryst and groundmass minerals (olivine, clinopyroxene, plagioclase, Fe–Ti oxides) in representative samples from the Lower, Middle and Upper Vancori phases of activity allow tracking of the evolution of the magma chamber system with time. Thermodynamic analysis of olivine–clinopyroxene mineral equilibria is used to constrain changes in temperature within the magmatic system. The main stratigraphic discontinuities within the Vancori sequence are clearly reflected by changes in bulk-rock and mineral chemistry, and evidence of textural and thermodynamic disequilibrium (calculated temperatures) in the eruptive products. On the basis of these data the Lower and Middle Vancori periods of the eruptive history of Stromboli appear to be characterised by an open magma chamber system repeatedly re-filled with basaltic magma, which was continuously being erupted. At the end of the Middle Vancori period the system became closed, corresponding to a period of dormancy of the volcano, and fractional crystallisation dominated. Following a major flank collapse of the volcanic edifice, the Upper Vancori eruptive products show the most extreme range in bulk-rock chemistry as the chamber was progressively flushed with new basaltic magma.