Geochemical heterogeneities and dynamics of magmas within the plumbing system of a persistently active volcano: evidence from Stromboli

Abstract

We report here the most complete dataset for major and trace elements, as well as Sr isotopic compositions, of magmas erupted by Stromboli since the onset of present-day activity 1,800 years ago. Our data relate to both porphyritic scoria and lava originating in the uppermost parts of the feeding system, plus crystal-poor pumice produced by paroxysmal explosive eruption of deep-seated, fast ascending, magma. The geochemical variations recorded by Stromboli's products allow us to identify changes in magma dynamics affecting the entire plumbing system. Deep-seated magmas vary in composition between two end-members having different key ratios in strongly incompatible trace elements and Sr isotopes. These features may be ascribed to mantle source processes (fluid/melt enrichment, variable degrees of melting) and occasional contamination by deep, mafic, cumulates. Temporal trends reveal three phases during which magmas with distinct geochemical signatures were erupted. The first phase occurred between the third and fourteenth centuries AD and was characterised by the eruption of evolved magmas sharing geochemical and Sr isotopic compositions similar to those of earlier periods of activity (<12 ka-Neostromboli and San Bartolo). The second phase, which began in the sixteenth century and lasted until the first half of the twentieth century, produced more primitive, less radiogenic, magmas with the lowest Ba/La and Rb/ Th ratios of our dataset. The last phase is ongoing and is marked by a magma having the lowest Sr isotopic composition and highest Rb/Th ratio of the dataset. While this new magma can be clearly identified in the pumice erupted during the last two paroxysmal eruptions of 2003 and 2007, shallow degassed magma extruded during this time span records significant geochemical and isotopic heterogeneities. We thus suggest that the shallow reservoir has been only partially homogenised by this new magma influx. We conclude that compositional variations within the shallow magma system of a persistently active volcano provide only a biassed signal of ongoing geochemical changes induced by deep magma refilling. We argue that source changes can only be identified by interpreting the geochemistry of pumice, because it reliably represents magma transferred directly from deep portions of the plumbing system to the surface.

Paroxysmal activity at Stromboli: lessons from the past

“…Many juvenile products are the result of intense, syneruptive mingling between LP and HP magma. The LP magma is a volatile-rich (H 2 O=1.8-3.4 wt.%; CO 2 =890-1,890 ppm; S=1,660-2,250 ppm; Cl=1,660-2,030) HK basalt (SiO 2 =48-49.5 wt.%; K 2 O=1.6-2.2 wt.%; CaO/ Al 2 O 3 =0.68-0.81), containing <5 vol.% of <1 mm diopside and homogeneous skeletal or euhedral olivine Pompilio et al 2010;Métrich et al 2010) and originating at a depth of 7-10 km.…”

Revisiting the last major eruptions at Stromboli volcano: inferences on the role of volatiles during magma storage and decompression

“…show the glass composition of the 2002-2003 and 2007 erupted materials (data reported by Landi et al, 2006;2009) ; b) pumices erupted during paroxysmal and major explosions; melt inclusions are those reported by Métrich et al (2001;2010) Bertagnini et al (2003. The grey field shows the bulk composition of the hystorical pumices reported by Pompilio et al (2012). Table A1).…”

“…Recent San Bartolo lavas, erupted during roman times (Arrighi et al, 2004;Speranza et al, 2008), are high-K calc-alkaline basalts with abundant mafic and ultramafic mantle xenoliths, ascribed to the roots-zone of Stromboli plumbing system (Laiolo & Cigolini 2006). Current lavas and tephra are high-K calc-alkaline basalts and shoshonites (cf., Francalanci et al, 2004;Landi et al, 2009;Pompilio et al, 2012).…”

“…LVZ: low velocity zone; b) simplified geological map of Stromboli with major collapses and faults (modified after Hornig-Kjarsgaard et al, 1993;Tibaldi et al, 2009). (these data were summarized by Francalanci et al, 2004;Pompilio et al, 2012). The dark grey field…”

Stromboli – the best place to actively learn and understand the behavior of an active volcano and its processes

“…Violent explosions eject the juvenile scoriaceous fragments and highly vesicular fragments called 'golden pumice' (up to 90% vol. of voids), which represent gas-rich, low-porphyricity pumices (high-K basalts) with crystals of clinopyroxene, olivine and plagioclase (up to 30 vol%) (e.g., Metrich et al, 2010;Allard, 2010;Pichavant et al, 2011;Pompilio et al, 2012). …”