Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy)

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Abstract

Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.

Introduction

Stromboli is the archetype volcano for intermittent mild-explosive events, lasting only a few seconds, caused by bursting of large gas bubbles each 10–20 min (the “normal activity”). The present activity takes place at craters located at an elevation of 750 m inside the Sciara del Fuoco, a horseshoe scar that occupies the NW sector of the island likely resulting from at least four sector collapses that occurred in the last 13 ka (Tibaldi, 2001).

The normal mild-explosive activity is occasionally punctuated by more energetic explosive events (paroxysms). These latter usually consist in multiple, quasi-contemporaneous bursts from different vents lasting from a few minutes to days. Strong detonations often accompany the impulsive emission of hundred-meters-high jets of gas, ash and incandescent materials rapidly evolving into convective plumes, up to 10 km high in the largest events (Barberi et al., 1993).

During small-scale paroxysms, dm-sized ballistic blocks and bombs are ejected within a distance of several hundred meters from the crater terrace (Fig. 1). Ash and scattered light-lapilli fallout are restricted to the upper volcano. During the large-scale paroxysms m-sized clasts fall on the volcano slopes up to the villages of Stromboli and Ginostra, located along the coast at distances of 2–3 km.

Every 10–20 years in the past two centuries the volcano has produced outpouring of lavas which flowed down the Sciara del Fuoco. The last two effusive episodes occurred from December 2002 to July 2003 and in February–April 2007, respectively.

Nearly anhydrous, crystal-rich magma sustains the present activity of Stromboli. It is typically emitted as shoshonitic–basaltic black, dense scoriae during the normal strombolian activity and lava flows during effusive episodes. Deeper HK-basaltic, nearly aphyric magma, initially containing ~ 3 wt.% of dissolved water, is emitted only during paroxysmal eruptions as golden light pumice. These magmas share similar bulk chemical compositions but strongly differ in their mineral paragenesis, crystal content and glassy matrix chemistry. Glassy matrices have the basaltic composition of their bulk pumice, whereas they are shoshonitic in scoriae (Métrich et al., 2001, Bertagnini et al., 2003, Landi et al., 2004, Francalanci et al., 2004). Owing to the high initial content of dissolved H2O, plagioclase is not stable in the volatile-rich magma, except at time of eruption, whereas it represents the main mineral phase in the degassed magma. The crystal-rich (CR) magma resides in the upper part of the feeding system and appears to derive from the deeper volatile-rich and crystal-poor (CP) magma mainly via crystallization driven by low-pressure water loss (Métrich et al., 2001). In essence, scoriae are crystal-rich (~ 50 vol.%; Landi et al., 2004) and pumices nearly aphyric (< 5–10%), most of the crystals being inherited (Bertagnini et al., 2003, Francalanci et al., 2004).

Intermediate products between pumice and scoria/lava in terms of crystal content and matrix glass compositions, were never reported before the 9 January 2005 (9 Jan) event when vesicular, honey-coloured scoriae were erupted. We provide here a detailed mineralogy and chemistry of the products emitted during the short-lived explosive event on January 2005 and compare this new data set with the previously published mineralogical and chemical features of the typical Stromboli volcanic products. We discuss the origin of the 9 Jan products that possibly derived from the CR magma via mineral dissolution at depth.

Disequilibrium textures, episodes of dissolution–crystallization and reversed and/or oscillatory zoning are ubiquitous features of minerals from both scoriae and pumices erupted at Stromboli. These phenomena have been mainly attributed to interaction between magmas differing by their volatile content (Landi et al., 2004). Particularly, plagioclase in scoriae records successive events of dissolution–crystallization, and a strong variation of its composition from An88 to An68 (Landi et al., 2004, Francalanci et al., 2004). The composition An68 corresponds to the low-temperature, low-pressure equilibrium with the residual degassed shoshonitic melt (H2O  0.5 wt.%). On the other hand, permanent degassing at the summit craters makes Stromboli a good example of open-conduit volcano where intrusive degassing and recycling of degassed and dense magma at depth represent important processes (Francis et al., 1993, Allard et al., 1994, Harris and Stevenson, 1997, Stevenson and Blake, 1998). The study of the 9 Jan products allows us to discuss in terms of chemistry and texture of minerals, such phenomena of dense magma blobs recycling in the lower portion of the shallow body. We also suggest that samples with mineralogical characteristics intermediate between pumices produced during energetic explosive events and scoriae ejected during normal Strombolian activity reflect transient magma with short life duration.

Section snippets

The 9 January 2005 explosive event

The explosive event of 9 January lasted about 45 s and consisted of two violent explosions both followed by 100–120 m high jets of gas, ash and incandescent fragments, from two different vents located in the south-western sector of the crater terrace. Bombs and lapilli thrown out by the first explosion mainly fell on the south-eastern slopes of the crater terrace. Products emitted during the second explosion were dispersed to the north-west (Cristaldi and Coltelli, 2005). The dispersal and mass

Analytical techniques

The samples ST500 and ST501 were analyzed for their major and trace element bulk compositions, petrography and mineral chemistry. Microlite-free, glassy matrices of the ST500 sample were also specifically selected for microanalysis of their major and trace elements.

Whole rock major and trace elements were analyzed by ICP-AES and ICP-MS at Service d'Analyse des Roches et des Minéraux of CRPG — CNRS of Nancy (France). Modal analyses were performed using an optical microscope equipped with a point

Chemistry and mineralogy of the 9 Jan 2005 products

The 9 Jan scoriae contain seriate plagioclases from 0.1 to 2 mm in size, clinopyroxenes up to 3.5 mm and olivines up to 2.5 mm. Modal analyses yield 30 and 41 vol.% of crystals for the samples ST500 and 501, respectively. Plagioclase is the dominant mineral phase (56–66 vol.%), followed by clinopyroxene (20–21 vol.%) and olivine (14–23 vol.%). These crystal contents are lower than that of the typical CR scoriae emitted during normal strombolian activity (47–55 vol.%), whereas the relative

Discussion

The bulk scoriae erupted on 9 January 2005 are chemically equivalent to the magmas emitted either as lava in 2003 or as scoria during the present-day mild strombolian activity. In contrast their matrix glasses vary in composition from shoshonite to SHO-basalt, their crystal content is relatively low and their minerals have variable rim textures and compositions. The euhedral crystals wetted by shoshonitic glass testify to syn-eruptive mingling between the 9 Jan magma and the CR degassed magma

Concluding remarks

The 9 Jan less than one minute lasting explosive event ejected products whose residual glasses are non homogeneous and intermediate in composition between the matrix glass of the pumices and scoriae commonly emitted at Stromboli. Specifically, matrix glasses of scoriae emitted during typical strombolian activity is statistically proved to be very homogeneous. Textural characteristics, mineral and glass chemistry coherently point to an origin of the 9 Jan magma from the CR magma via mineral

Acknowledgements

This work has been supported by the INGV-DPC program: Monitoring and research activity at Stromboli and Panarea. Thorough reviewing by B. Scaillet and an anonymous referee is greatly appreciated and has improved the paper considerably. We thank P. Pantani for graphic assistance.

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