Journal of Petrology Advance Access originally published online on June 13, 2005
Journal of Petrology 2005 46(10):1997-2021; doi:10.1093/petrology/egi045
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Intra-Grain Sr Isotope Evidence for Crystal Recycling and Multiple Magma Reservoirs in the Recent Activity of Stromboli Volcano, Southern Italy
1 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DEGLI STUDI DI FIRENZE, VIA LA PIRA 4, I-50121, FLORENCE, ITALY
2 ISTITUTO DI GEOSCIENZE E GEORISORSE, CNR, SEZIONE DI FIRENZE, VIA LA PIRA, 4, I-50121, FLORENCE, ITALY
3 DEPARTMENT OF EARTH AND LIFE SCIENCES, VRIJE UNIVERSITEIT, DE BOELELAAN 1085, 1081 HV AMSTERDAM, NETHERLANDS
4 DEPARTMENT OF EARTH SCIENCES, UTRECHT UNIVERSITY, BUDAPESTLAAN 4, 3584 CD UTRECHT, NETHERLANDS
RECEIVED JUNE 2, 2004; ACCEPTED MARCH 30, 2005
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.
KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli
* Corresponding author. Telephone: +39 055 2757502. Fax: +39 055 2756242. E-mail: lorella.francalanci{at}unifi.it
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