Intra-grain Sr isotope evidence for crystal recycling and multiple magma reservoirs in the recent activity of Stromboli volcano, Southern Italy

Abstract

Over the last several hundred years, Stromboli has been characterized by steady-state Strombolian activity. The volcanic products are dominated by degassed and highly porphyritic (HP-magma) black scoria bombs, lapilli and lava flows of basaltic shoshonitic composition. Periodically (about one to three events per year), more energetic explosive eruptions also eject light coloured volatile-rich pumices with low phenocryst content (LP-magma) that have more mafic compositions than the HP-magma. An in situ major and trace element and Sr isotope microanalysis study is presented on four samples chosen to characterize the different modes of activity at Stromboli: a lava flow (1985-1986 effusive event), a scoria bomb from the 'normal' present-day activity of Stromboli (April 1984), and a scoria and coeval pumice sample from a recent more explosive eruption (September 1996). Plagioclase (An62-90) and clinopyroxene (Mg-number between 0·69 and 0·91) phenocrysts in all samples record marked major element variations. Large and comparable Sr isotope variations have been detected in plagioclase and clinopyroxene. HP-magma crystals have resorbed cores, with either high 87Sr/86Sr (0·70635-0·70630) or low 87Sr/86Sr (0·70614-0·70608); the latter values are similar to the values of the outer cores. Mineral rims and glassy groundmasses generally have intermediate 87Sr/ 86Sr (0·70628-0·70613). Similarly, mineral growth zones with three groups of 87Sr/86Sr values characterize minerals from the LP-pumice, with the lowest values present in mineral rims and groundmass glass. These results define a mixing process between HP- and LP-magmas, plus crystallization of clinopyroxene, plagioclase and olivine, occurring in a shallow magma reservoir that feeds the present-day magmatic activity of Stromboli. An important observation is the presence of a third component (high 87Sr/86Sr in mineral cores) considered to represent a pre-ad 1900 cumulus crystal mush reservoir situated just below the shallow magma chamber. These cumulus phases are incorporated by the LP-magma arriving from depth and transported into the shallow reservoir. A rapid decrease of 87Sr/86Sr in the replenishing LP-magma immediately prior to eruption of the ad 1985 lava flow is associated with an increased volume of LP-magma in the shallow magma chamber. The HP-magma in the shallow reservoir is not fully degassed when it interacts with the LP-magma, making efficient mixing possible that ultimately produces a well overturned homogeneous magma. Further degassing and crystallization occur at shallower levels as the HP-magma moves through a conduit to the surface. © The Author 2005. Published by Oxford University Press. Al rights reserved.