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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 |
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| Authors: | FRANCALANCI LORELLA; DAVIES GARETH R; LUSTENHOUWER WIM; TOMMASINI SIMONE; MASON PAUL R D; CONTICELLI SANDRO |
| Institution: | 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 |
| Abstract: | Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (1985–1986effusive event), a scoria bomb from the ‘normal’present-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An62–90) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high 87Sr/86Sr (0·70635–0·70630)or low 87Sr/86Sr (0·70614–0·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate87Sr/86Sr (0·70628–0·70613). Similarly,mineral growth zones with three groups of 87Sr/86Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high 87Sr/86Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of 87Sr/86Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli |
| Keywords: | : isotopic microsampling mineral recycling mixing Sr isotope disequilibria Stromboli |
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