crystallization events at Stromboli (Italy)

Journal of Volcanology and Geothermal Research 174 (2008) 325–336

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy) Patrizia Landi a,⁎, Nicole Métrich a,b, Antonella Bertagnini a, Mauro Rosi c a b c

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, via della Faggiola, 32-56126 Pisa, Italy Laboratoire Pierre Süe, CEA-CNRS, CE-Saclay, 91191 Gif/Yvette, France Dipartimento di Scienze della Terra, Universita’ degli Studi di Pisa, via Santa Maria, 53-56126 Italy

A R T I C L E

I N F O

Article history: Received 11 May 2007 Accepted 27 February 2008 Available online 15 March 2008 Keywords: mineral dissolution magma chemistry volatiles trace elements Stromboli

A B S T R A C T 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. © 2008 Elsevier B.V. All rights reserved.

1. 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-

⁎ Corresponding author. Fax: +39 050 8311942. E-mail address: [email protected] (P. Landi). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.02.013

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 volatilerich 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

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P. Landi et al. / Journal of Volcanology and Geothermal Research 174 (2008) 325–336

Fig. 1. View of the crater terrace of Stromboli. Photo taken on 3 June 2005.

pumices nearly aphyric (b5–10%), 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, honeycoloured 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. 2. 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 of products qualitatively rank the January 9 event in an intermediate position between the normal strombolian activity and small-scale paroxysms (e.g. in August 1998– 1999). Scoria lapilli (multi-clast sample: ST500) and one 20 cm-large scoria bomb (ST501), erupted during the first explosion, were collected by one of us (MR) few days after the event in the “Fossetta”, just 40–50 m from the crater rim (Fig. 1).

3. 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

Table 1 Bulk rock composition of the scoriae erupted on 9 January 2005 compared to those of August 1998 pumice and May 2005 scoria Sample

9 Jan 2005 scoriae

Pumice

Scoria

ST500

ST501

Aug 98

May 2005

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total V Cr Co Ni Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

49.17 0.92 17.39 8.76 0.16 6.08 11.34 2.56 2.09 0.57 0.05 99.08 260 47.3 31.0 40.8 64.9 706 25.6 155 17.3 4.13 904 43.5 87.8 10.3 40.4 8.04 2.10 6.65 0.92 4.83 0.89 2.37 0.34 2.21 0.35 3.51 1.15 16.4 13.8 3.58

49.39 0.92 17.31 8.76 0.16 6.10 11.26 2.58 2.14 0.57 −0.21 98.97 257 49.9 30.5 41.0 66.3 704 25.6 157 17.4 4.35 921 44.2 89.0 10.4 40.6 8.06 2.12 6.57 0.92 4.82 0.89 2.33 0.34 2.22 0.34 3.52 1.19 16.6 14.4 3.77

48.63 0.91 17.45 8.84 0.16 6.36 11.92 2.43 1.87 0.61 −0.02 98.89 268 55.4 33.7 48.3 51.9 688 24.8 136 14.6 3.26 803 39.4 80.6 9.8 39.1 7.94 2.14 6.67 0.93 4.98 0.89 2.40 0.34 2.19 0.34 3.32 1.03 14.8 11.9 3.02

49.45 0.92 17.20 8.76 0.16 6.16 11.28 2.57 2.14 0.57 − 0.22 98.98 264 56.4 32.0 43.2 68.0 715 25.9 160 17.8 4.43 940 45.2 90.9 10.7 41.7 8.26 2.16 6.90 0.93 5.0 0.91 2.45 0.36 2.30 0.36 3.72 1.19 17.4 14.9 3.91

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Fig. 2. Compositional variations of Nb vs selected trace elements and K2O for the 9 January 2005 scoriae (open diamond), in comparison with 1999–2006 bulk scoriae (open circle) and lavas (cross). The 1998–2003 pumice compositions (open triangle) are also reported (data from Métrich et al., 2001; Landi et al., 2004; Métrich et al., 2005 and unpublished data).

CRPG — CNRS of Nancy (France). Modal analyses were performed using an optical microscope equipped with a point counter. Texture and major element composition of the minerals and glassy matrices were analyzed with a Philips XL30 scanning electron microscope equipped with EDAX DX4 at Dipartimento di Scienze della Terra of the University of Pisa, Italy. In order to avoid alkali loss (Devine et al., 1995), matrix glasses were analyzed in the scanning mode with a

window N10 × 10 μm. The relative analytical errors are, respectively, 1% for Si, Al; 2% for Fe, Mg and Ca; 5% for alkalis and Ti, and 30% for Mn, P and Cl. Matrix glasses and glass embayments were also analyzed for major elements, Cl, S, using a SX50 CAMECA electron microprobe (Service Camparis, Paris, France). The analytical conditions for glasses were 10 nA beam current, 10 µm beam size, 10–15 s counting time for major elements and 30 nA, 15 µm and 140 s on peak for Cl, S and P.

Table 2 Major element average compositions of the matrix glasses of the scoriae erupted on 9 January 2005 Sample ST500

Rim glass (ol13)

Rim glass (ol3)

Matrix

Quoted anal.⁎

7

5

16

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S Cl Total

51.05 1.27 16.81 9.31 0.22 4.20 9.31 3.13 3.21 0.95 0.058 0.131 99.64

0.20 0.10 0.13 0.30 0.05 0.10 0.25 0.09 0.04 0.02 0.004 0.005

51.34 1.39 16.63 9.21 0.21 4.19 8.89 3.09 3.39 0.99 0.012 0.121 99.48

0.35 0.04 0.23 0.39 0.04 0.07 0.24 0.12 0.09 0.02 0.005 0.003

51.51 1.34 16.66 9.25 0.18 4.33 8.92 3.06 3.32 1.02 0.008 0.107 99.70

⁎Number of electron microprobe analyses. Average standard deviation is reported in italic.

Matrix

Matrix

11 0.28 0.07 0.13 0.25 0.05 0.06 0.15 0.07 0.08 0.03 0.004 0.008

51.72 1.41 16.52 9.31 0.18 4.18 8.65 3.18 3.49 1.04 0.009 0.115 99.83

Matrix

13 0.31 0.07 0.18 0.37 0.05 0.06 0.12 0.06 0.07 0.02 0.003 0.006

51.48 1.37 16.58 9.29 0.18 4.22 8.62 3.15 3.50 1.03 0.010 0.108 99.52

3 0.34 0.05 0.20 0.31 0.04 0.18 0.24 0.08 0.20 0.04 0.003 0.006

51.00 1.33 16.52 9.21 0.15 4.39 8.90 3.15 3.37 1.00 0.022 0.117 99.15

0.43 0.15 0.09 0.26 0.02 0.17 0.23 0.22 0.01 0.03 0.022 0.006

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Reproducibility and accuracy of analyses were checked for S and Cl on Alv981 (S = 1110 ± 110 ppm), and VG2 (Cl = 300 ± 35 ppm; S = 1430 ± 60 ppm) basaltic glasses (Métrich et al., 2001). The WDS analytical error is b1–3% for major elements, ≤7% for Cl and P and ≤10% for S. There is a good agreement between the EDS and WDS results, with non significant difference between the alkali concentrations. Trace element concentrations in matrix glasses were determined with laser ablation (LA)-ICP-MS at the CNR-IGG-PV (Italy). The instrument couples a 266 nm Nd:YAG laser source (Brilliant-Quantel) to a quadrupole ICP-MS (DRCe-Perkin Elmer). The laser was operated at 10 Hz adopting a spot size of 20–40 µm and a pulse energy of about 0.01–0.03 mJ. Selected masses were acquired in peak hopping mode with a dwell time of 10 ms. Each analysis consisted in the acquisition of 60 s on background and 60 s on peak. SRM Nist612 and 44Ca were used as external and internal standard, respectively. Data reduction

was carried out with the Glitter software (Van Achterbergh et al., 2001). Accuracy was tested on the BCR2 USGS reference glass and it is estimated to be better than 5% relative (for further details, see Tiepolo et al., 2003). 4. 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 proportions between the mineral phases are

Table 3 Average compositions of matrix glasses of the crystal-poor pumices and crystal-rich scoriae erupted at Stromboli Sample

Pumice Aug 98

Pumice Nov 98

1

13

48.55 0.94 18.04 7.92 0.12 5.02 12.27 2.53 1.95 0.57 0.100 0.165 98.17 0.68 0.77

50.31 1.04 17.47 8.68 0.15 5.48 11.02 2.67 2.47 0.76 0.014 0.104 100.15 0.63 0.93

Quoted anal.b

8

4

12

Li B Sc V Cr Co Ni Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

nd nd 32 280 30 nd nd nd 59 647 24 136 19 3.7 851 43 84 10.0 39 8.0 1.9 5.9 0.8 4.6 1.0 2.3 0.3 2.4 0.3 3.5 1.0 16.3 12.3 2.9

nd nd 29 277 29 nd nd nd 67 659 26 151 21 3.6 855 44 86 10.2 43 8.9 1.9 7.0 0.9 5.1 1.0 2.8 0.3 2.6 0.3 2.6 1.2 18.5 13.9 3.5

15 45 25 373 7 25 11 148 119 531 38 264 39 7.7 1476 81 155 18.2 70 13.1 2.9 10.1 1.3 8.1 1.5 3.9 0.6 3.9 0.6 6.1 2.1 26.0 27.2 6.9

Quoted anal.

a

SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O P2O5 S Cl Total CaO/Al2O3 K2O/Na2O

a

2 18 7

3 51 2 11 1 0.5 24 2 4 0.7 5 0.9 0.5 1.4 0.2 0.9 0.1 0.5 0.1 0.6 0.1 0.3 0.2 3.2 1.0 0.4

Scoria Oct 2005

Scoria May 2005

16 0.31 0.05 0.17 0.25 0.07 0.05 0.10 0.11 0.08 0.12 0.004 0.005

6 22 4

3 75 4 19 2 0.4 22 3 1 0.8 5 1.8 0.5 1.7 0.1 0.5 0.1 0.4 0.1 0.7 0.1 1.1 0.4 1.6 1.0 0.3

52.17 1.49 16.03 9.95 0.19 3.64 7.86 3.26 3.98 1.17 0.009 0.120 99.87 0.49 1.22

WDS analyses; bLA-ICP-MS analyses. For each analysis, the standard deviation is reported in italic; nd: not determined.

16 0.76 0.07 0.23 0.27 0.05 0.08 0.13 0.15 0.07 0.03 0.005 0.004

52.39 1.60 15.81 9.96 0.21 3.41 7.47 3.40 4.22 1.21 0.006 0.121 99.81 0.47 1.24

3 9 1 20 3 2 1 104 9 18 3 19 3 0.7 73 4 8 1.0 4 1.2 0.4 0.9 0.1 1.0 0.1 0.4 0.1 0.6 0.1 0.7 0.3 2.6 1.8 0.5

13 45 24 363 8 24 11 105 114 511 37 257 38 7.6 1415 78 150 17.7 68 11.7 2.8 9.9 1.3 7.4 1.4 3.6 0.5 3.3 0.5 5.6 2.1 26.9 26.8 6.7

0.25 0.07 0.14 0.23 0.05 0.04 0.11 0.09 0.10 0.03 0.003 0.004

11 4 11 3 20 6 1 2 32 10 24 3 21 3 0.7 89 6 11 1.5 5 1.7 0.1 1.0 0.2 0.6 0.1 0.6 0.1 0.6 0.1 0.7 0.2 4.5 1.9 0.6

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Fig. 3. Comparison between the matrix glass compositions of the 9 Jan 2005 products and of CR scoriae produced during the normal strombolian activity. Representative matrix glasses from the present-day CP pumices are also reported (data from Landi et al., 2004; and unpublished data). Analyses performed with EDS techniques. Black: 9 Jan products; dark gray: 1998–2006 scoriae; light gray: scoriae from ancient eruptions; crossed: pumices from the paroxysms of Aug 1998, Nov 1998 and April 2003.

quite comparable (in vol.%: PL: 57–70; CPX: 22–34; OL: 4–12; Landi et al., 2004) . The bulk scoriae are shoshonitic-basalts (SHO-basalt hereafter) (6.1 wt.% MgO; 2.1 wt.% K2O; Table 1). They plot in the compositional domain of the scoriae and lavas usually emitted at Stromboli, although, they systematically show the lowest contents of incompatible elements observed in the crystal-rich products erupted in the past 7 years (Fig. 2). In contrast, their matrix glasses cover a broader compositional domain than that of the typical shoshonitic residual glass of scoriae produced during the normal strombolian activity (Tables 2 and 3). This feature is illustrated in Fig. 3, where are plotted not less than 500 major element analyses of glassy matrices of scoriae that were emitted between 1998 and 2006 and during ancient strombolian activity (Rosi et al., 2006; Landi et al., 2004; and unpublished data). Only 20% of the 9 Jan 2005 matrix glasses are similar to the typical shoshonitic redisual glass of Stromboli shallow magma (CaO = 7.6 ± 0.19 wt.%; K2O = 4.1 ± 0.15 wt.%; MgO = 3.5 ± 0.14 wt.%). Most of them (80%) are less evolved with CaO/Al2O3 ratio of 0.50– 0.58. They plot in the field of the SHO-basalts with rather high contents in CaO (up to 9.4 wt.%), MgO (up to 4.6 wt.%), and lower K2O content (3.1–3.8). Systematic compositional traverses highlight chemical heterogeneity at the scale of 20–30 µm (Fig. 4). However, we did not find

basaltic glasses typical of pumice as those emitted during the recent paroxysms of August and November 1998 and April 2003 (Fig. 3). LA-ICP-MS trace element analysis was systematically performed in glassy matrices of 9 Jan 2005 scoriae together with those of scoriae (9 samples) erupted between 1999 and 2006 and of the 1998 pumice

Fig. 4. Compositional traverses through the matrix glass of the 9 Jan scoriae.

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textured, patchy-zoned bytownite (An 70–90); clinopyroxene composition mainly ranges from Fs12 and Fs15 with rounded core and/or thin intermediate zones Fs5–10, whereas olivine has quite homogeneous composition Fo70–73. Their rims, with respective compositions An62–70, Fs13–14, Fo70–73, registered the low-temperature, low-pressure equilibria of the shallow, degassed CR magma (Landi et al., 2004; Francalanci et al., 2004; Landi et al., 2006). Conversely, crystals associated with SHO-basaltic glass have the typical composition and textures described above but always bring evidence of resorption event(s) with systematic overgrowths of few to tens µm on surface dissolution (Figs. 6, 7; Tables 5–7). The 20–100 µm thick rims of plagioclase show skeletal texture and patched-zoning with abundant glass inclusions and rare bubbles. Their compositions mainly range from An75 to An84, more scarcely An87 (Figs. 6a–c and 7a; Table 5). They overgrew on erosional surfaces that are marked by angular discordances which commonly cut labradoritic (An b70) layers of the inner part of the crystals (Fig. 6a, c). The olivine shows a spectrum of

(Tables 3 and 4). As observed for major elements, the 9 Jan matrices have variable trace element contents that define a continuum between an intermediate SHO-basaltic composition and typical shoshonitic residual glass (Fig. 5). Incompatible elements are positively correlated with constant ratios between such elements as Th, Nb, La. Compatible elements as Sc, Cr are quite dispersed. Trace elements confirm the compositional gap between the 9 Jan glasses and the basaltic matrices of pumice that was observed with major elements (Fig. 3). Finally, the 9 Jan matrix glasses are degassed, as expected (Cl 900– 1240 ppm, S b200 ppm). Only few analyzed olivine-hosted embayments (OL: 13–19 in Table 4, Fig. 6d) preserved rather high contents in Cl and S (up to 1370 ppm and 640 ppm, respectively; Tables 2, 4 ). Crystals wetted by residual shoshonitic glass have the same textures and compositions than phenocrysts of scoriae commonly emitted at Stromboli: plagioclase shows the typical concentric zoning consisting of alternating layers of labradorite (An60–70) and sieve-

Table 4 Representative major and trace element analyses of the matrix glasses of the 9 January 2005 scoriae Sample

ST500 OL10–34

Ol10–38

OL13–27

OL13–16a

OL13–19

OL13–22

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S Cl Total

51.24 1.37 16.09 9.73 0.22 3.80 8.04 3.41 4.01 1.05 0.013 0.107 99.05

51.73 1.33 16.25 9.06 0.19 3.86 8.25 3.35 3.65 1.13 0.010 0.104 98.93

52.24 1.33 16.69 9.58 0.17 4.16 8.57 3.04 3.51 1.08 0.009 0.105 100.47

50.98 1.10 16.76 9.15 0.22 4.16 8.91 3.05 3.25 0.97 0.052 0.132 98.73

50.75 1.33 16.85 9.09 0.21 4.17 9.48 3.08 3.20 0.93 0.055 0.122 99.25

51.34 1.26 16.70 9.76 0.31 4.40 9.70 3.02 3.19 0.93 0.058 0.137 100.79

Li B Sc V Cr Co Ni Zn Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

13 33 25 348 29 26 10 72 125 604 38 253 38 8.3 1426 78 149 16.7 68 12 2.9 8.2 1.4 6.7 1.2 3.2 0.72 3.3 0.59 6.0 1.9 28 28 6.7

σ 3 9 2 24 7 2 2 7 7 29 3 22 2 0.7 86 4 7 0.8 4 1 0.2 0.8 0.1 0.6 0.1 0.4 0.08 0.4 0.08 0.6 0.2 3 1 0.5

σ 15 38 25 357 31 29 31 60 109 633 36 225 33 6.6 1312 73 134 15.6 58 9.3 2.9 9.1 1.2 6.3 1.3 3.3 0.36 1.6 0.37 4.6 1.6 24 23 4.9

3 10 1 24 5 2 4 6 6 30 3 19 2 0.5 78 3 6 0.7 3 0.9 0.2 0.8 0.1 0.6 0.1 0.4 0.06 0.3 0.06 0.5 0.1 3 1 0.4

σ 10 27 26 294 54 28 12 61 97 596 35 209 31 7.0 1287 71 130 15.1 53 11 3.5 9.3 1.4 7.1 1.3 3.5 0.27 2.7 0.53 3.5 1.2 24 21 5.6

3 8 2 19 9 2 2 6 6 28 3 17 2 0.6 74 4 6 0.8 3 1 0.3 1 0.1 0.7 0.1 0.5 0.07 0.5 0.1 0.5 0.1 2 1 0.5

σ 15 39 24 341 9 25 18 76 95 635 30 205 31 5.9 1288 63 136 16.0 56 12.0 2.4 8.0 1.1 5.4 1.1 3.3 0.50 4.5 0.39 4.4 1.7 21 21 5.9

3 9 1 20 4 2 2 7 5 27 2 15 2 0.4 67 3 6 0.7 3 1 0.2 0.7 0.1 0.5 0.1 0.4 0.06 0.5 0.06 0.5 0.1 2 1 0.4

σ 11 26 25 315 19 26 15 85 99 613 32 200 30 7.0 1251 63 131 14.4 55 9.8 2.5 7.8 1.1 6.2 1.2 3.1 0.51 3.2 0.52 4.1 1.6 25 20 6.0

2 6 1 19 4 2 2 7 5 26 2 15 2 0.5 65 3 6 0.6 3 0.8 0.2 0.6 0.1 0.5 0.1 0.3 00.6 0.4 0.06 0.4 0.1 2 1 0.4

σ 13 15 26 340 16 26 20 70 104 656 33 214 32 6.2 1291 69 129 15.1 60 8.7 2.9 8.7 1.1 6.6 1.2 2.8 0.40 2.8 0.36 4.4 1.2 24 21 5.6

2 4 1 20 4 2 2 6 5 28 2 16 2 0.4 68 3 6 0.6 3 0.7 0.2 0.7 0.1 0.5 0.1 0.3 0.05 0.3 0.05 0.4 0.1 2 1 0.4

Each analysis represents individual spot data. σ: analytical error for trace elements (see text). Errors on major elements and volatile components are reported in “Analytical techniques”.

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Fig. 5. Compositional variations of Nb versus selected trace elements in matrix glasses of the products from 9 Jan 2005 explosion (filled diamonds) in comparison with the matrix glass average compositions of the scoriae emitted between 1999 and 2006 (filled circle) and of pumices from the August 1998 and November 1998 paroxysms (filled triangle) and the bulk rock compositions of the 1998–2003 pumices (open triangle). The linear regression of glass compositions is also presented.

rim widths and textures from few μm thick rims Fo74–75 to 60 μm thick, MgO richer rims (Figs. 6d, e and 7b; Table 6). The latter are characterized by patched-zoning (Fo75 up to Fo80), abundant glassy inclusions and rare rounded gas bubble remnants (Fig. 6f). Frequent dissolution-related gulfs precede the rim growth (Fig. 6d). Clinopyroxenes have rounded shapes and few crystals have b5 µm thick rims Fs8–9 (Fig. 7c; Table 7). As a whole, these disequilibrium features (rounded shapes of the inner crystals, erosional surfaces and dissolution channels) evidence a dissolution event that affected the mineral phases of the shallow magma body, prior to the final crystallization event. Table 5 Representative compositions of plagioclase in 9 Jan 2005 scoriae Sample

pl2.6

pl6.3

pl4.3

pl5-6.5

pl5-1.1

pl4-4

pl12-1

ST500

rim

rim

rim

rim

rim

rim

rim

SiO2 Al2O3 Fe2O3 CaO Na2O K2O An mol% Ab mol% Or mol% Rim glass⁎

49.6 31.1 0.4 14.3 3.2 0.5 69.1 27.8 3.1 shoBAS

48.5 31.9 0.3 15.2 2.4 0.5 75.2 21.8 3.0 shoBAS

47.6 33.2 0.0 16.1 1.9 0.3 80.6 17.5 1.9 shoBAS

46.8 33.8 0.0 16.8 1.8 0.2 82.6 16.3 1.1 shoBAS

46.6 33.7 0.2 17.0 1.7 0.1 84.4 14.8 0.8 shoBAS

51.3 30.7 0.9 13.2 3.4 0.6 65.7 31.0 3.3 SHO

50.8 30.8 0.8 13.6 3.3 0.6 66.8 29.5 3.7 SHO

⁎Rim glass composition: shoBAS: shoshonitic basalt; SHO: shoshonite.

5. 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 capping the system. Conversely, crystals that are surrounded by less evolved, SHO-basaltic melt (CaO N8 wt.%) differ from the others by having recorded a late episode of dissolution just prior to the ultimate stage of crystallization and

Table 6 Representative compositions of olivine in 9 Jan 2005 scoriae Sample

ol16-1

ol10-1

ol13-1

ol13-1

ol1-1

ol2

ST500

Rim

rim

rim

core

rim

rim

SiO2 FeO MnO MgO CaO Fo mol% Rim glass⁎

38.6 22.7 0.5 37.8 0.4 74.9 shoBAS

39.3 19.6 0.6 40.1 0.4 78.5 shoBAS

39.1 18.5 0.3 41.8 0.3 80.1 shoBAS

37.8 25.3 0.5 36.0 0.4 71.8 shoBAS

37.7 25.9 0.6 35.5 0.3 71.0 SHO

37.9 25.0 0.4 36.4 0.3 72.2 SHO

⁎Rim glass composition: shoBAS: shoshonitic basalt; SHO: shoshonite.

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Table 7 Representative compositions of clinopyroxene in 9 Jan 2005 scoriae Sample

px15-1

px15-4

px1-1

px2-1

ST500

rim

rim

rim

rim

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O Cr2O3 Wo mol% En mol% Fs mol% Mg / (Mg + Fe)⁎ Rim glass⁎⁎

51.8 0.6 3.4 5.7 0.1 16.2 22.0 0.0 0.2 44.8 46.0 9.2 0.83 shoBAS

51.8 0.7 3.4 5.5 0.1 16.2 22.3 0.0 0.0 45.3 45.8 8.9 0.84 shoBAS

50.3 1.1 3.6 8.9 0.2 14.4 21.3 0.2 0.0 44.0 41.4 14.6 0.74 SHO

51.5 0.9 3.0 8.1 0.1 15.3 21.0 0.0 0.1 43.1 43.6 13.3 0.77 SHO

⁎Fe total expressed as Fe2+; ⁎⁎shoBAS: shoshonitic basalt; SHO: shoshonite.

eruption. We propose that the 9 Jan products illustrate a peculiar event of the CR magma evolution. 5.1. Possible processes to explain the origin of the 9 January 2005 magma As previously reported, scoria and pumice that are presently erupted at Stromboli represent CR and CP magmas with comparable bulk rock compositions but strongly contrasting crystal amounts. The latter feature is believed to be controlled by the dissolved volatile content and temperature gradient (Métrich et al., 2001 and references therein). However, a simple crystallization process driven by water loss cannot account for the mineralogy and the trace element geochemistry of the 9 Jan scoriae as discussed below. Firstly, we assumed crystal fractionation process. We used the Rayleigh law: C / C0 = f (D − 1), where D is the bulk partition coefficient of incompatible elements between solid and melt (D N8 wt.%) would derive from the bulk rock by 30 to 45 wt.% of solid removal. In each case the proportion of solid that is involved is inconsistent with the mineral chemistry of the 9 Jan scoriae. Actually, mineral crystallization from the less evolved SHO-basaltic glass is only testified by the late growth of b50 μm rims. Second, we explored the effect of mixing process. Magma mixing is a likely process because (i) the shallow body is refilled by volatile-rich melt, and (ii) fast injection of pumice-like melt at shallow levels ensues paroxysms (e.g. 5 April 2003), where a magma blob and its gas bubbles rise fastly across the shallow magma body. Such mixing event was hypothesized by Landi et al. (2006) in the case of the December 2002–July 2003 lavas. In the latter case, the relative proportion of mixing between the ascending basaltic magma and the shallow, degassed magma (including its ~54 wt.% of crystals) was assessed to be 0.2:0.8 (Landi et al., 2006). Because of the chemical similarity between the 9 Jan products and the 2003 lavas we used the same proportions of mixing. Following this procedure the resulting magma would have 43 wt.% of crystals, a proportion higher than that observed in the sample ST500 (30 vol.% corresponding to ~33 wt.%). Moreover, such a mixing process does not fit with trace and major element compositions of the 9 Jan glasses (Fig. 8a, d). Third, we examined the effect of mineral dissolution on the liquid composition using MELTS algorithm (Ghiorso and Sack, 1995). We simulated the assimilation of a solid – with the mineral compositions and modal proportions of the typical scoria and lava – by a shoshonitic melt. The major element composition of the residual shoshonitic melt of the May 2005 sample that has been considered is reported in Table 3. The solid is composed of 60–66 wt.% plagioclase (An70), 23– 28 wt.% clinopyroxene (Fs13.5) and 11–12 wt.% olivine (Fo72). The temperature was chosen between 1115 °C and 1145 °C ± 10 as

previously estimated, respectively, for the CR and CP magmas (Métrich et al., 2001). Simulations were achieved assuming anhydrous melt and different dissolved H2O concentrations of 0.8, 1.2, 1.5 wt.%, redox conditions at Ni–NiO and a fixed pressure of 100 MPa. The total pressure should not affect the results since the most important parameter on phase equilibria and plagioclase crystallization is the partial pressure of water for the same temperature (Di Carlo et al., 2006). Dissolution of between 12 and 20 wt.% of solid in a hydrous shoshonitc melt (H2O = 1.2–1.5 wt.%) accounts for the compositional range of the 9 Jan glassy matrix (Fig. 8a,b). As a whole, MELTS simulations predict no mineral dissolution at low water content (b0.8 wt.%), even at temperature of 1130–1140 °C. We verified that mass balance calculations using trace elements indicate similar extents of dissolution (10–25 wt.%) of the same solid into the shoshonitic melt (Fig. 8c, d). In summary, the compositional variability of the 9 Jan glassy matrices is well reproduced for both major and trace elements, as well as the effect of plagioclase dissolution on the Sr behaviour (Fig. 8d). Mineral dissolution under hydrous conditions is thus an alternative process to explain the textural and chemical particularities of the 9 Jan magma. Mixing with volatile-rich magma likely played a minor role given that there is no mineralogical or chemical testimony of input of pumice-like magma containing 2.8–3.3 wt.% of dissolved water. The skeletal textures of the crystal outer rims testify to a rapid growth rate induced by high undercooling, likely N100 °C (Lofgren, 1974). The relatively low thermal gradient ~ 30 °C measured in the Stromboli magmas suggests that these textures are mainly controlled by water loss due to decompression during magma rise (Landi et al., 2004). 5.2. Mineral dissolution The mineral dissolution features observed in the 9 Jan samples possibly derive from the downwards motion of the degassed magma and its re-hydration and subsequent re-equilibration at higher PH2O. Crystals settling in the lower part of the shallow magma body can hardly be reconciled with the physical characteristics of CR magma. However, we calculated the mineral settling velocity (v) using Stokes law: v ¼ 2R2 ðdc  dm Þg=9g where R is the radius of the crystal, g the gravity acceleration, dc and dm the density of the melt and crystals, respectively; and η the melt viscosity. The melt density and viscosity were calculated to be 2.6 g/ cm3 and 1900 poise, respectively, according to Lange (1997) and Shaw (1972). The respective density of plagioclase, olivine and pyroxene was assessed to be 2.7, 3.5, and 3.6 g/cm3. Interactions between crystals and flux of volatiles are not considered in the model, so the calculated velocity has to be considered overestimated in the vastly dynamic system of Stromboli. The crystal settling rates are low for plagioclase (b0.4 m/year for 2 mm sized crystals) and mafic phases (b20 m/year for 4 mm sized crystals). Hence, sinking of degassed magma in mass is the expected process, as previously proposed at Stromboli (Francis et al., 1993; Allard et al., 1994; Harris and Stevenson 1997; Stevenson and Blake, 1998) and other volcanoes (e.g. Kazahaya et al., 1994). The temperature and the total (dissolved and exsolved) water content are the main parameters that control mineral dissolution. The thermal gradient is ~30 °C as deduced from previous temperature estimates obtained for the degassed (1115 °C) and undegassed (1145 °C) magmas (Landi et al., 2004). According to MELTS calculations such temperature gradient cannot account for mineral dissolution in anhydrous melt. Moreover, the late stage recorded by phase equilibrium (on average: An78–80–Fo80–Fs8) and the composition of the associated glasses of the 9 Jan scoriae are experimentally

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Fig. 6. Back-scattered electron images of plagioclase and olivine phenocrysts from the 9 Jan 2005 products, rimmed by shoshonitic–basaltic glass. a, b, c: plagioclase phenocrysts showing bytownitic b100 μm thick rims (An78–87), with skeletal textures growing on dissolution surfaces which cut labradoritic layers (An66–70); d: olivine phenocryst Fo72 with large embayments, surrounded by a thin rim with less evolved composition (Fo80); e: euhedral olivine with a rim 5–10 μm thick Fo80; f: example of skeletal texture typical of the less evolved rims of the olivine.

reproduced at 1100 °C, for a dissolved water content of ~1.2–1.7 wt.% (Di Carlo et al., 2006). It strongly suggests that mineral dissolution occurs when the magma water content is ≥1.2 wt.%. We thus explored the mechanisms able to bring water to the system in order to induce pre-existing mineral dissolution and new phase crystallization. Our previous data on melt inclusions indicate that crystal-poor, HK-basaltic magma at Stromboli contains 2.8–3.3 wt.% of dissolved H2O (3.1 ± 0.2 wt.% on average) and the up to 1550 ppm of CO2 (Métrich et al., 2001; Bertagnini et al., 2003, unpublished data). Burton et al. (2007a,b) proposed that the HK-basaltic magma rises under

closed system conditions until the magma vesicularity achieves 30 to 50%, a threshold at which the system becomes permeable (Blower, 2001) promoting gas differential transfer. Following this line of reasoning, we simulated closed system magma ascent starting from 320 MPa where the magma already coexists with 2.4 wt.% exsolved, CO2-dominated gas phase, according to Burton et al. (2007a,b). We used Volatilecalc (Newman and Lowenstern, 2002), assuming 48 wt.% SiO2 as proposed by Métrich et al. (2005) in order to fit as much as possibly the composition of Stromboli magma and its CO2 solubility, since no experimental solubility data are available, yet. As expected,

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sure where the magma vesicularity is high enough to allow bubble segregation from the magma and independent gas transfer (Burton et al., 2007a,b). Moreover, viscosity contrasts at such transition zone could facilitate the formation of plumes of the deeper basaltic magma blobs with highly vesicular heads as previously proposed (Métrich et al., 2001). Hence interactions between segregated water-bearing gas bubbles with recycled degassed magma and its subsequent reequilibration is a likely process to explain mineral dissolution as observed in the 9 Jan 2005 sample. At a total fluid pressure of nearly 50 MPa, recycled dense magma blobs containing ≤0.5 wt.% of water can be locally re-equilibrated. A dissolved water content of 1.5 wt.% is consistent with the phase equilibria and the late stage mineral paragenesis. In order to achieve water concentration ≥1.2–1.5 wt.% via magma mixing between degassed (b0.5 wt.%) and undegassed (H2O b 3 wt.%) magmas, the relative proportions of the components should be 0.5:0.5. The latter values are inconsistent with the bulk chemistry and the mineralogy of the products and imply that the gas phase plays a chief role in the re-hydration of the degassed magma and resorption process. The contrasting behaviour between Cl and S reinforces our hypothesis of interaction between the sinking degassed magma with a gas phase, even though a small fraction of melt is likely entrained by the gas bubbles. Actually, few glass embayments (open gulfs) analyzed in olivine show rather high sulfur content (up to 600 ppm) but relatively low Cl abundance (1100 ppm) equivalent to that of the degassed residual glassy matrices of scoriae. Addition of sulfur to degassed melt is possibly accounted for by the deeper-derived gas phase and/or the recycling of sulfide globule already observed in melt inclusions (Fo75–72) testifying dissolution/crystallization event (unpublished data). A pressure of 50 MPa (limit condition) corresponds to a lithostatic depth of ~ 1800 m below the crater terrace, taking an average rock density of 2750 kg m− 3. Accordingly, the 9 Jan 2005 explosive event was caused by the rise of a magma pocket from a depth ≤1.8 km, without clearly evidence of deeper-derived magma injection. The small-scale heterogeneity observed in the 9 Jan glassy matrix and the rapid crystal growth during the final stage of crystallization strongly support the idea that such magma is most likely transient and cannot survive a long span of time. Actually plagioclase (~An75–80) in equilibrium with melts having ~1.5 wt.% of dissolved H2O commonly occurs as nuclei or intermediate layers in phenocrysts of the CR scoriae. Their rapid growth textures always testify to transitory states which switch towards late stage equilibrium (Landi et al., 2004). The ubiquity of dissolution–crystallization episodes recorded in mineral textures and chemistry, as particularly well illustrated in plagioclase, were interpreted as resulting from the dominant effect of the water in addition to a minor role of the temperature (Landi et al., 2004). Mineral dissolution is counterbalanced by crystallization of magmas having comparable bulk chemistry, however the respective importance of mineral dissolution, magma mixing and crystallization is rather difficult to assess in such a system. The key point that arises here is that re-equilibration of recycled degassed magma under variable partial pressures of water is regarded as transient situation. It also supports the non existence of a long-standing, chemically zoned shallow plumbing system at Stromboli. 6. Concluding remarks Fig. 7. Plagioclase (a), olivine (b) and clinopyroxene (c) compositions, expressed in terms of An mol%, Fo mol% and Fs mol%, respectively, in 9 Jan 2005 samples (black) and in scoriae and lavas erupted within the past 10 years (gray) at Stromboli. Note that the rims with skeletal texture (see Fig. 6) have less evolved composition (An75–87, Fo74–80 and Fs8–9).

the simulated H2Oexsolved/H2Odissolved ratio increases from 0.6 to 1.2 as the total pressure decreases from 100 to 50 MPa, whereas the abundance of exsolved water varies from 1.5 to 2.2 wt.% (Fig. 9). Hence, water is already available in the gas phase in this range of pres-

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 dissolution rather than simple crystallization. We propose that dissolution processes are mainly induced by

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335

Fig. 8. Dissolution trends simulated using MELTS code for major elements (a, b) and mass balance calculations for trace elements (c, d). We used, respectively, the major and trace element compositions representative of the shoshonitic glassy matrix and of mineral phases of the scoriae. MELTS calculations are performed at PTOT = 100 MPa, assuming H2O = 1.2 and 1.5 wt.%, T = 1120–1140 °C (plain lines in Fig. 8a and b). The numbers in the graphs indicate the percentage (wt.%) of dissolved solid. For symbols see Figs. 2 and 5.

“re-hydration” of the sinking shallow degassed magma and its reequilibration at higher water pressure. Indeed, the 9 Jan products mirror a transient situation with short-time duration, related to the sinking into the lower part of the magmatic system of Stromboli, crystal-rich body followed by a rapid ascent to the surface immedi-

ately before the explosion. Because dissolution–crystallization processes are common features observed in the degassed CR products, it appears likely that magma convection and mineral dissolution induced by re-equilibration of the sinking shallow magma at higher PH2O represents a common process operating in the shallow magma system of Stromboli. The corresponding solid products should be associated with somehow energetic strombolian activity. 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. References

Fig. 9. Evolution of the water concentrations in the melt and the coexisting gas phase and of their ratio upon decompression. Simulations are made using Volcatilecalc (Newman and Lowenstern, 2002), assuming basaltic undegassed melt ascent under closed system degassing, according to Burton et al. (2007a,b) (see text for details). a: H2Oexsolved/H2Odissolved ratio; b: exsolved H2O; c: dissolved H2O.

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