期刊
JOURNAL OF PETROLOGY
卷 46, 期 10, 页码 1997-2021出版社
OXFORD UNIV PRESS
DOI: 10.1093/petrology/egi045
关键词
isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli
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 (An(62-90)) 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 Sr-87/Sr-86 (0.70635-0.70630) or low Sr-87/Sr-86 (0.70614-0.70608); the latter values are similar to the values of the outer cores. Mineral rims and glassy groundmasses generally have intermediate Sr-87/Sr-86 (0.70628-0.70613). Similarly, mineral growth zones with three groups of Sr-87/Sr-86 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 Sr-87/Sr-86 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 Sr-87/Sr-86 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.
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