Deciphering Degassing and Source Effects in Cl Isotopes in Melt Inclusions: The Possible Role of Amphibole in the Magma Source of Stromboli (Aeolian Island Arc)

Chlorine isotopes have emerged as a new geochemical tool over the past 15 years. Most of the data consist of bulk rock data, with a minority carried out in situ on melt inclusions using secondary ion mass spectrometry. More data are necessary to understand the relationship between δ³⁷Cl measured in melt inclusions and that in bulk rocks from the same volcanic center. Here we have analyzed a suite of melt inclusions entrapped in olivine Fo_63-85, as well as some from clinopyroxene crystals, from a single hand-sample from the Vancori unit of Stromboli, Aeolian Islands. The 27 selected melt inclusions have major element compositions ranging from high potassium alkali basalt to evolved shoshonite. Their δ³⁷Cl vary from −2.6 ± 0.1‰ to +1.2 ± 0.2‰, a far larger range than for Stromboli bulk rocks. In this dataset, the δ³⁷Cl variation in melt inclusions is not related to Cl degassing, or to fractional crystallization. Instead, correlations between δ³⁷Cl and S/Cl, K₂O and trace element ratios suggest mixing of two Cl endmembers with distinct δ³⁷Cl signatures. A first endmember is characterized by high potassium alkali basalt compositions, high Ba/La (∼28), high S/Cl, and high δ³⁷Cl (>1‰), confirming the influence in the mantle source of an aqueous fluid and providing a new constraint on its composition: that it derives from the breakdown of amphibole. The second endmember has a more evolved composition, high La/Yb, low S/Cl, and low δ³⁷Cl (<−2‰), identifying the influence of a solute-rich component derived from subducted sediments. The δ³⁷Cl data thus help refine the two sources initially identified from bulk rock studies and δ³⁷Cl proves to be a potential tracer for amphibole.

Introduction

Chlorine is one of the most abundant volatile elements, along with water, carbon, and sulfur, and is the most abundant halogen element. Its concentration in arc magmas can reach up to 7,500 μg/g (e.g., Aiuppa et al., 2009 for a review), whereas it is usually less than 100 μg/g in unaltered MORB samples (e.g., Jambon et al., 1995). Some unaltered MORB samples can however exhibit high Cl contents (up to > 100 μg/g) associated with high Cl/K due to assimilation of altered material (e.g., Marschall et al., 2017). Chlorine has two stable isotopes, which fractionate at low temperatures (Schauble et al., 2003). Chlorine isotopes have been used to track the deep Cl cycle as well as the behavior of Cl during degassing (e.g., Barnes et al., 2008; Barnes et al., 2009; Sharp et al., 2010; Chiaradia et al., 2014; Bouvier et al., 2019). As summarized by Barnes and Sharp (2017), terrestrial whole rocks span a relatively large range in terms of δ³⁷Cl, ranging from −3.7 to +3.0‰. The most negative values are usually attributed to sediments or sediments influenced by magma genesis, whereas the most positive values might reflect the influence of amphibole (Barnes and Cisneros, 2012). Indeed, based on theoretical and experimental results, amphibole can be slightly enriched in ³⁷Cl compared to co-existing fluids (Schauble et al., 2003; Cisneros, 2013). Barnes and Sharp (2017) pointed out that despite the theoretical fractionation at high temperature (700°C) between amphibole and a fluid at equilibrium, there might be another process, such as kinetic fractionation, shifting the δ³⁷Cl to the highest measured values (up to 1.8‰). During degassing, Cl isotopes do not usually fractionate. However, Sharp et al. (2010) demonstrated that while Cl isotope fractionation between silicate melt and gaseous HCl is limited (<0.6‰ at 500°C), the Cl isotopes could significantly fractionate toward strongly positive values at near surface pressures, due to interaction between gaseous HCl and liquid phases. Also, Fortin et al. (2017) showed that Cl isotopes could be largely fractionated during diffusion (up to 5‰), ³⁵Cl moving faster in the vapor phases, leading to a progressively ³⁷Cl-enriched degassed melt phase. Recently, δ³⁷Cl has been measured in situ in melt inclusions (Bouvier et al., accepted; Layne et al., 2009; Manzini et al., 2017; Bouvier et al., 2019). These melt inclusions preserve the least degassed Cl compositions compared to bulk rock, and the diffusion of Cl out of the melt inclusions is limited due to the large ionic radius of Cl⁻ (Le Voyer et al., 2014).

There are several factors that make Stromboli a good study-volcano for deciphering the different slab inputs into the mantle source of the magma. In addition to the high diversity of geochemical compositions of the erupted products (e.g., Francalanci et al., 1988; Francalanci et al., 1989; Francalanci et al., 2007) and its persistent activity, the subducting Ionian plate (slab) has a high-angle dip (about 70°), and this slab is reported to be almost entirely recycled into the mantle (e.g., Gvirtzman and Nur, 1999; Meletti et al., 2000; Gvirtzman and Nur, 2001; Pontevivo and Panza, 2006). Bulk rock studies of lavas from Stromboli reported that the geochemical characteristics of the erupted magmas revealed a contribution from both altered oceanic basalt and sediments from the subducted slab (e.g., Francalanci et al., 2007). Stromboli melt inclusion studies based on major and trace elements and isotopes have confirmed the contribution of these two slab components (e.g., Rose-Koga et al., 2012), and in some cases have added constraints as to the nature of the contribution (e.g., Schiavi et al., 2012). More recent studies, using the relatively new chlorine isotope systematics, have shown that Stromboli melt inclusions have lower δ³⁷Cl (−3.2 to −1.4‰, n = 5; Manzini et al., 2017) compared to bulk rocks from the same island (−1.0 to +0.7‰, n = 7; Liotta et al., 2017). Bouvier et al. (2019) suggested that melt inclusions preserve the δ³⁷Cl signature of an undegassed melt, while the bulk rocks might have been affected by fractionation during shallow degassing. This was supported by gas measurements showing negative δ³⁷Cl values (down to −2.2‰; Liotta et al., 2017). In order to test this hypothesis here, we measured δ³⁷Cl in melt inclusions from one single hand-sample from Stromboli (Vancori unit 26,000–13,000 years; e.g., Francalanci et al., 2013), in olivines covering a wide range of forsterite (Fo) contents and in a few pyroxenes. Thus, the melt inclusions we analyzed represent the progressive chemical change of the magma (by crystallization and degassing) in which they crystallized, and we can identify (and quantify) the magmatic processes that cause the δ³⁷Cl variations.

We found that the melt inclusions recorded a greater variation in δ³⁷Cl than previously reported by Manzini et al. (2017), almost as large as that of the δ³⁷Cl measured in Stromboli gas samples, and far greater than that reported for bulk rocks from Stromboli (Liotta et al., 2017). Combined with major and trace elements, our results confirm that Cl isotopes are not significantly fractionated during magma crystallization or during magma degassing. The δ³⁷Cl results suggest that the Cl addition beneath Stromboli originates from the subducted sediments and that δ³⁷Cl is a tracer of amphibole breakdown. δ³⁷Cl could prove to be a useful geochemical tool to trace the potentially widespread “cryptic amphibole fractionation” which occurs during the differentiation of arc magmas (Davidson et al., 2007).

Geological Context

Stromboli is located at the eastern end of the Aeolian arc (southern Italy), generated by the active subduction of the African plate beneath the European plate (e.g., Barberi et al., 1974; Gasparini et al., 1982; Ellam et al., 1989). Geophysical data indicate the presence of a slab (about 200 km wide) dipping at a high angle of ∼50–70° to the northwest below the Aeolian arc. Volcanic activity on Stromboli can be divided into four main periods over the last 100 ka (e.g., Francalanci et al., 2013). These eruptions have led to a great diversity of volcanic products which can be classified into four series based on their SiO₂ and K₂O content: calc-alkaline, high-potassium calco alkaline, shoshonitic, and potassic (e.g., Francalanci et al., 2013).

The increase in K₂O from calc-alkaline to shoshonitic to potassic magmas is associated with a decrease in ¹⁴³Nd/¹⁴⁴Nd and an increase in ⁸⁷Sr/⁸⁶Sr ratios (e.g., Francalanci et al., 2007). Similarly, incompatible trace elements tend to increase from calc-alkaline to potassic (Francalanci et al., 2007). This wide variability of geochemical signatures encountered in the magmas of Stromboli is mostly acquired in the mantle wedge, with limited assimilation of continental crust (e.g.,Francalanci et al., 1988; Ellam et al., 1989; Francalanci et al., 1989; Ellam and Harmon, 1990; Francalanci et al., 1993; De Astis et al., 2000; Francalanci et al., 2004; Francalanci et al., 2007; Tommasini et al., 2007). The processes responsible for this geochemical heterogeneity of the mantle wedge beneath Stromboli are attributed to complex melting and/or dehydration mechanisms in the Ionian slab (e.g., Schiavi et al., 2012).

The plumbing system of Stromboli is thought to have at least two (e.g., Vaggelli et al., 2003) or three (Francalanci et al., 2005) melt reservoirs. Present day activity is characterized by persistent moderate explosions (Strombolian activity), emitting highly porphyric (HP) scoriae and lavas, whereas during paroxysmal eruptions pumices with low phenocryst contents (LP) are emitted (Francalanci et al., 2004). LP magmas are suggested to originate from a deep reservoir (∼10–11 km, e.g., Bertagnini et al., 2003; Vaggelli et al., 2003; Pino et al., 2011), with HP melts deriving from LP through degassing, crystallization, and mixing in a shallower reservoir (∼1–3 km; e.g., Vaggelli et al., 2003; Burton et al., 2007; Métrich et al., 2010).

Material and Methods

Material

The sample studied is ST2, the same one that was used for four out of five of Manzini et al. (2017)’s melt inclusion data. This sample comes from a basaltic lapilli deposit of the Vancori unit (Figure 1). Olivine grains and a few pyroxene grains were handpicked to select crystals that had an exposed surface of glassy melt inclusion, of more than 30 μm in size and free of cracks. Almost half of the melt inclusions contained a shrinkage bubble (Figure 2).

FIGURE 1

FIGURE 1. Map of Stromboli. The different lava units are shown in different colors (modified after Francalanci et al., 2013). The yellow star represents the site of sample ST2, used for this study.

FIGURE 2

FIGURE 2. Transmitted light images of different melt inclusions. (A) Melt inclusion in olivine N1, with no bubble inside. (B) Several melt inclusions, all containing a bubble, in olivine I5.

Methods

Electron Microprobe

Major elements in melt inclusions and olivines were measured by electron microprobe using a Cameca SXFive-TACTIS at the Laboratoire Magmas et Volcans (Clermont-Ferrand, France). Melt inclusions were analyzed using a defocused 10 μm beam, at 8 nA and 15 kV, whereas olivine grains were measured with a focused beam at 15 nA and 15kV, following the procedure described in Rose-Koga et al. (2021). Sulfur and Cl were also determined in melt inclusions using a 10-μm beam at 40 nA, 15 kV. Glass VG2 (Jarosewich et al., 1980) was also measured during the session to check the accuracy of the measurements. The errors on the measurement for each element are given in 1σ for host crystals and 3σ for melt inclusions: typically <1% SiO₂, Al₂O₃, FeO, MgO, CaO, K₂O, <4% TiO₂, <2% Na₂O, <20% MnO, P₂O₅, and <10% Cl, S. The analytical conditions are summarized in the Supplementary Table S1.

SIMS

Chlorine isotopes were measured in melt inclusions using a CAMECA 1280HR at the University of Lausanne, following the protocol of Manzini et al. (2017). In brief, a Cs⁺ source was used to bombard the sample surface with an acceleration voltage of 10 kV and a primary intensity of 3 nA. An electron gun was used to compensate the build-up of charge on the sample surface. ³⁵Cl⁻ and ³⁷Cl⁻ were collected simultaneously in two faraday cups equipped with 10¹¹Ω resistors. Each measurement started with 150 s pre-sputtering using a 25-μm raster and automated centering in field and contrast apertures, followed by 40 cycles of 4 s of acquisition. This led to an average measurement uncertainty of ∼0.15‰ (2 standard errors, 2SE). Mass was stabilized using nuclear magnetic resonance. Calibration of the mass instrumental fractionation was done using Manzini et al. (2017) reference material (Figure 3). Reproducibility of the standards was greater than 0.35‰ (2 standard deviations, 2SD), except for the less homogeneous reference material RMR (Godon et al., 2004; Manzini et al., 2017) which usually reproduced at 1‰ (2SD). Reference material UNIL-B4 and UNIL-B6 were also on ST2 mounts. Repeated measurements of these reference materials during the 6-h session showed a reproducibility of 0.27 and 0.20‰ (2SD), respectively (see Supplementary Material S2). Accuracy of the measurements, defined here as the deviation of the reference materials from the calibration line, is 0.26‰ on average, but could reach up to 0.5‰ for PR2 reference material (Godon et al., 2004; Manzini et al., 2017).

FIGURE 3

FIGURE 3. Instrumental mass fractionation plotted against the equation defined by Manzini et al. (2017). The different reference materials used are the same as in Manzini et al. (2017) and all have more than 1,000 μg/g Cl.

LA-ICP-MS

Most melt inclusions were also analyzed for trace elements. A few were either too thin or too small to extract reliable LA-ICP-MS data.

Trace element abundances of the melt inclusions were analyzed by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Laboratoire Magmas et Volcans, Clermont-Ferrand, France using a 193 nm Excimer Resonetics M-50E laser with an Agilent 7500 ICP–MS. Analysis followed routine in-house procedures outlined in previous studies (e.g., Le Voyer et al., 2010; Rose-Koga et al., 2012). Briefly, a pulse energy of about 5 mJ was used with a spot diameter of between 20 and 33 μm (depending on melt inclusion size), and a laser pulse frequency of 2 Hz, to keep a fluence at sample surface of about 2.70 J/cm⁻². The following masses were analyzed for the target elements: ⁶Li, ⁷Li, ⁴³Ca, ⁴⁴Ca, ⁴⁵Sc, ⁴⁷Ti, ⁵¹V, ⁵⁹Co, ⁶⁰Ni, ⁶³Cu, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁶³Dy, ¹⁶⁶Er, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta,²⁰⁸Pb, ²³²Th, ²³⁸U. The background was measured for about 30 s before ablation and analysis time was approximately 100 s. This technique uses CaO (measured by electron microprobe) as an internal standard during data reduction with the GLITTER software (www.es.mq.edu.au/GEMOC). Systematic analysis of two glass reference materials (BCR2-G and NIST 612; see Supplementary Material) before, after, and during the session were used to determine the reproducibility and accuracy of the analysis. The average 1σ error of the mean on the samples was less than 10% for all trace elements, except for Lu (<20%) and B (<40%).

Results

Olivine hosts have a forsterite content (Fo = 100 × Mg/(Fe + Mg)) ranging from 66.8 to 84.6. Melt inclusions hosted in these olivines were corrected for post-entrapment crystallization using Petrolog software (Danyushevsky and Plechov, 2011). Following the guideline for melt inclusion studies (Rose-Koga et al., 2021), we specify that in Petrolog we used the olivine model of Ford et al. (1983), as well as the density model of Lange and Carmichael (1987). The initial oxidation state model used is the model of Kress and Carmichael (1988), and we fixed Fe³⁺/(Fe²⁺+Fe³⁺) at 0.2 as determined for the Stromboli lavas (Metrich and Clocchiatti, 1996). Melt inclusions displayed a large range of major element compositions, with SiO₂ ranging from 48.3 to 62.0 wt% and K₂O from 1.3 to 5.0 wt% (Figure 4A), with no systematics between olivine-hosted and pyroxene-hosted melt inclusions. In a TAS diagram (Le Bas et al., 1986), the melt inclusions show broad compositions ranging from basalt to trachyandesite (Na₂O + K₂O ∼3.6–8.6 wt%, Figure 4B). Major element concentrations generally show similar compositions to bulk rocks (shaded field) with a few points at less evolved values (SiO₂ <50 wt% and Na₂O + K₂O < 4 wt%) corresponding to Fo > 80 olivines. The major element compositions of the melt inclusions are comparable to other melt inclusion studies on Vancori deposits (e.g., Rose-Koga et al., 2012; Manzini et al., 2017), and also comparable to melt inclusion compositions of Stromboli in general (Métrich et al., 2001; Bertagnini et al., 2003; Vaggelli et al., 2003; Schiano et al., 2004; Aiuppa et al., 2010; Métrich et al., 2010).

FIGURE 4

FIGURE 4. Silica content plotted against (A) potassium content and (B) K₂O + Na₂O (wt%) for ST2 melt inclusions. Manzini et al. (2017) data from ST2 are also plotted for comparison (empty triangles, [1]), as well as the Stromboli bulk data for which δ³⁷Cl have been calculated ([2]: Liotta et al., 2017; crosses). The shaded field represents whole rocks (georock database) and blue dots ([3]) represent melt inclusions from other Stromboli samples (e.g., Métrich et al., 2001; Bertagnini et al., 2003; Métrich et al., 2010; Rose-Koga et al., 2012; Schiavi et al., 2012). Major elements of melt inclusions entrapped in olivine have been corrected for post-entrapment crystallization.

Chlorine varies from 1,108 to 3,320 μg/g over the entire range of K₂O (from 1 to 5 wt%) without any significant trend (Figure 5A). In contrast, a clear negative correlation exists between S (varying between 49.7 and 1,324 μg/g) and K₂O (Figure 5B). The measured matrix glass shows lower Cl concentration (1,199 μg/g) than in the melt inclusions and lower S concentrations (77 μg/g). The highest S content measured in ST2 melt inclusions (1,324 μg/g) is lower than the highest S content reported for Stromboli melt inclusions (up to 2,620 μg/g; Bertagnini et al., 2003). Although H₂O was not measured, we can infer that H₂O concentration in our melt inclusions is low since the total of the major element oxides reaches 100% in many cases (Supplementary Material S3). Values from the literature for the amount of H₂O dissolved in Stromboli magma are ∼2.7–3.5 wt% prior to outgassing, especially for the present day LP melts, coming from a deep reservoir (Métrich et al., 2010). Therefore, in this study most of the H₂O must have been lost from the melt inclusions (e.g., by H⁺ diffusion; Gaetani et al., 2012) or by degassing, as for the present day HP melts (Métrich et al., 2010).

FIGURE 5

FIGURE 5. (A) Chlorine and (B) sulfur contents compared to K₂O wt% in ST2 melt inclusions. Symbols are the same as in Figure 4. (A) No relationship between Cl and K₂O is observed. Potassic melt inclusions have lower Cl content than shoshonitic and high potassium calco alkaline ones. All melt inclusions have higher Cl content than bulk rocks. (B) High potassium calco alkaline melt inclusions have higher S content than shoshonitic and potassic melt inclusions, which have lost S due to degassing. High potassium calco alkaline melt inclusions have moderate S content compared to Stromboli melt inclusions from the literature, having up to 2,600 μg/g S.

Trace element patterns display the typical features of arc settings, with higher ratios of fluid-mobile elements (B, Pb, U, and LILE) to less fluid-mobile elements (REE, Th, HFSE) than those in MORB (Figure 6). Chlorine isotopes range from −2.6 ± 0.1‰ (2 s.e.) to +1.3 ± 0.2‰ (2 s.e.). This range reproduces and extends the range of the five Stromboli values in Manzini et al. (2017), which came from the same sample as that used here. The δ³⁷Cl weighted average of all melt inclusions from this study and the four melt inclusions from ST2 from Manzini et al. (2017) is −1.15‰, lower than the δ³⁷Cl weighted average of the bulk rock values (0‰) from Liotta et al. (2017). All results can be found in Supplementary Material S3.

FIGURE 6

FIGURE 6. Spidergrams of ST2 melt inclusions. Symbols are the same as in Figure 4 and represent the different groups of melt inclusions. The different groups cannot be differentiated based on their trace element compositions. Trace elements have been normalized to DMM (Ryan and Langmuir, 1987; Workman and Hart, 2005). ST2 melt inclusions are enriched in mobile elements compared to MORB (Hofmann, 1988).

Discussion

Chlorine Isotope Variation of Melt Inclusions and Magma Evolution

The 27 melt inclusions and the matrix glass from ST2 studied here span a very large range of major elements, almost as large as the range of erupted lavas on Stromboli (e.g., review from Bertagnini et al., 2008). The melt inclusions can be divided into three groups based on their SiO₂ and K₂O contents, which correspond to the different series shown by the lavas erupted at Stromboli (Peccerillo and Taylor, 1976; Francalanci et al., 2013): high-potassium calco alkaline, shoshonitic, and potassic (Figure 4). The olivine hosts of high-potassium calco alkaline melt inclusions have the widest range of Fo content and extend toward the highest Fo content (66.9–84.6, average: 77.5) compared to olivine hosts of potassic and shoshonitic melt inclusions (Fo: 71.5–73.2, average: 72.0, and 66.8–79.0: average: 69.6, respectively). However, there is no clear relationship between the SiO₂ (or K₂O) of the melt inclusions and the Fo content of the host olivine. This suggests that the analyzed crystals from the three series might have formed in different reservoirs, at different stages of their magmatic evolution. The presence of at least three different reservoirs, located at different depths within Stromboli’s plumbing system has already been proposed, based on the Sr isotope analysis of whole rock and plagioclases, and on in situ geochemical analysis of these plagioclases (Francalanci et al., 2005). One of the reservoirs is sited at a depth of ∼10–11 km (e.g., Bertagnini et al., 2003; Pino et al., 2011) and contains a small amount of phenocrysts (generating LP pumices), an intermediate reservoir consists of an old cumulate mush, and the third reservoir, possibly at 1–3 km (e.g., Burton et al., 2007), in which the HP melts originate, is enriched in phenocrysts.

The lack of correlation between Cl and K₂O (Figure 5A) suggests different sources of Cl for this sample, given that both are incompatible elements during crystallization and should thus show a positive correlation if derived from a single source. There is no correlation between SiO₂ and δ³⁷Cl (Figure 7A), but no significant δ³⁷Cl variation within each series was expected as Cl isotopes do not fractionate at magmatic temperatures in the absence of kinetic diffusion (Schauble et al., 2003; Fortin et al., 2017). However, a clear negative correlation between δ³⁷Cl and K₂O content is observed in the melt inclusions (Figure 7B). The correlation cannot be related to analytical bias, despite K₂O being a component of the SIMS data reduction (Figure 3). Indeed, UNIL-B4, one of the glass reference materials used for the calibration, has a major element composition very close to those of the Stromboli melt inclusions, especially the shoshonitic and KS melt inclusions, with 56 wt% SiO₂ and 4.34 wt% K₂O (Manzini et al., 2017). Instead, the difference in δ³⁷Cl between the high-potassium calco alkaline and shoshonitic-potassic melt inclusions is inferred to reflect different Cl sources of the mantle. Two endmembers (EDM1 and EDM2) can be identified: EDM1 with high δ³⁷Cl and low K₂O, and EDM2 with low δ³⁷Cl and high K₂O (Figure 7B). The nature and origin of these endmembers are discussed in the last section of the discussion.

FIGURE 7

FIGURE 7. Chlorine isotopes plotted against SiO₂ (A) and K₂O (B) content of melt inclusions. For comparison, average MORB composition is plotted (dark star: Hofmann, 1988; Sharp et al., 2010). Only high potassium calco alkaline melt inclusions have similar composition to bulk rocks on these two plots. A positive correlation can be seen between δ³⁷Cl and K₂O, whereas there is no correlation with SiO₂. Two main Cl endmembers (EDM1 and EDM2) are plotted and discussed in the text.

Influence of Magma Degassing on Chlorine Isotope Composition of Melt Inclusions

In this study, the decrease in S/Cl with increasing K₂O is mainly due to a combination of S degassing and to the increase in Cl concentration (Figures 5, 8). Sulfur solubility in basalts is a function of the relative proportions of sulfate and sulfide dissolved in the melt (e.g., Carroll and Webster, 1994; Baker and Moretti, 2011). In addition, S partitions preferentially into the vapor phase (e.g., Scaillet and Pichavant, 2005), whereas Cl is more soluble in melt and is incompatible in anhydrous minerals (e.g., Webster and De Vivo, 2002; Joachim et al., 2017). This results in a progressive increase in Cl during magmatic differentiation, which is observed in melt inclusions from high potassium calco alkaline and shoshonitic (Figure 8B). There is no clear trend for the potassic series due to the small number of melt inclusions analyzed (Figure 8). We can distinguish melt inclusions in the shoshonitic series which have low S/Cl and K₂O > 3wt%, from melt inclusions in the high potassium calco alkaline series which have slightly higher S/Cl and low K₂O < 3wt%. The low S/Cl, high K₂O group has been interpreted as resulting from more extensive exsolution of S at deeper depth than Cl (≈150 MPa for S; Spilliaert et al., 2006), associated with a slower rate of magma rise in the case of Etna melt inclusions (Spilliaert et al., 2006). For Stromboli it has been proposed that it results from the redox change of sulfur in the shoshonitic melt (Métrich et al., 2010).

FIGURE 8

FIGURE 8. Silica content of melt inclusions plotted against (A) S/Cl and (B) Cl content. Melt inclusions record S degassing, as reflected by the decrease in S/Cl with increasing SiO₂, while Cl increases slightly. Potassic melt inclusions have the lowest Cl content, but all melt inclusions are enriched in Cl compared to Stromboli bulk rocks.

High potassium calco alkaline melt inclusions are the least degassed in terms of S (Figure 8A) but are degassed with respect to H₂O based on the totals of the electron probe measurements. However, H₂O loss occurs at a shallower depth than S degassing (Spilliaert et al., 2006) so melt inclusions from high potassium calco alkaline could still represent deeper melts. In terms of Cl content, potassic melt inclusions are the most depleted (Figure 8B) but are richer than the bulk rocks. The similar SiO₂ content and lower Cl content of potassic melt inclusions relative to high potassium calco alkaline melt inclusions (Figure 8B) could suggest that the potassic melt inclusions are degassed products of high potassium calco alkaline melts. However, the relationship between δ³⁷Cl and K₂O (Figure 7B) suggests that there might be two different sources of Cl for these two series.

Based on theoretical fractionation factors, at 600°C, 90% removal of Cl due to HCl degassing would only lower the δ³⁷Cl in the melt by only 0.5‰ (Schauble et al., 2003; Ranta et al., 2021). Since δ³⁷Cl is almost unaffected by equilibrium degassing (e.g., Sharp et al., 2010), there should be no correlation between δ³⁷Cl and Cl content. Similarly, Cl isotopic fractionation between melt and NaCl, RbCl, or KCl gases or liquids is expected to be smaller than for HCl (Schauble et al., 2003; Sharp et al., 2010). However, in the case of Cl kinetic fractionation during degassing (or, more generally, Cl removal from the melt by CO₂ fluxing or during brine formation), ³⁵Cl moves faster, leading to a degassed melt with higher δ³⁷Cl relative to an undegassed melt. Shoshonitic and potassic melt inclusions do not show any relationship between Cl and δ³⁷Cl (Figure 9A), whereas there is a slight negative relationship for high potassium calco alkaline melt inclusions, which may represent Cl isotopic fractionation by kinetic diffusion during degassing (or Cl removal). Since potassic melt inclusions have lower contents of both δ³⁷Cl and Cl, this is another argument against potassic melts being degassed from high potassium calco alkaline melts and reinforces the hypothesis that there are two different sources of Cl for the two series.

FIGURE 9

FIGURE 9. Chlorine isotopes compared to (A) Cl content in melt inclusions and (B) S/Cl in melt inclusions and gases ([2]: Liotta et al., 2017). (A) For comparison, bulk rocks from Stromboli ([2]: Liotta et al., 2017) are plotted, as well as the olivine-hosted melt inclusions from [1]: Manzini et al. (2017). Melt inclusions are all enriched in Cl compared to bulk rocks. Overall, no relationship between Cl and δ³⁷Cl can be seen. (B) Melt inclusions have similar δ³⁷Cl to Stromboli gases. As for the gases, a positive correlation can be seen between δ³⁷Cl and S/Cl, which cannot be linked to S or Cl degassing.

The potential effect of degassing on the Cl isotope values of bulk rocks, as suggested by Manzini et al. (2017), can be indirectly investigated by comparing the δ³⁷Cl of melt inclusions from ST2 and those of bulk samples from different locations in Stromboli. Almost half of the melt inclusion data has lower δ³⁷Cl than those of the bulk rocks (Figure 9A). Interestingly, high potassium calco alkaline melt inclusions have similar δ³⁷Cl and major elements to the bulk rocks (Figures 4, 7), which would indicate that no isotopic fractionation had taken place during magmatic degassing of this melt batch. On the contrary, shoshonitic and potassic melt inclusions have lower δ³⁷Cl compared to the bulk rock δ³⁷Cl (Figures 7, 9A). Only one bulk rock has similar major elements to the potassic melt inclusions, with a δ³⁷Cl of −0.5 ± 0.4‰ (Liotta et al., 2017; Figure 4), which is statistically higher than the δ³⁷Cl range for potassic melt inclusions (−1.9 to −2.6 ± 0.2‰). This difference could theoretically be explained by kinetic fractionation during degassing. In that case, potassic melt inclusions, which have lower Cl content, should have a smaller difference from the bulk rocks compared to shoshonitic melt inclusions, as the parental melts of these melt inclusions would suffer a larger Cl loss. Indeed, the greater the Cl loss, the greater the δ³⁷Cl difference between the bulk rock and melt inclusions in the event of kinematic Cl isotope fractionation. In fact, potassic melt inclusions have a slightly bigger difference with respect to the δ³⁷Cl bulk rock compared to shoshonitic melt inclusions. A comparison between δ³⁷Cl and S/Cl (Figure 9B), recording S degassing, shows that ST2 melt inclusions demonstrate a positive relationship between these two geochemical indicators. However, S degassing alone should not fractionate δ³⁷Cl. Even if this were to be the case, it would result in an increase in δ³⁷Cl with decreasing S/Cl due to faster ³⁵Cl diffusion (Fortin et al., 2017), whereas in fact the opposite is observed for ST2 melt inclusions. Therefore, these different observations do not support Cl kinetic fractionation during degassing to explain the differences in δ³⁷Cl between ST2 melt inclusions, especially those from potassic and shoshonitic series, relative to bulk rock values for Stromboli. The low δ³⁷Cl in melt inclusions might thus represent a component that is not clearly expressed in the bulk rocks. Indeed, the weighted average δ³⁷Cl of the melt inclusions (−1.15‰), which should represent the bulk δ³⁷Cl of the ST2 sample, is lower than the average of the seven bulk rock values (Liotta et al., 2017), but is similar to that of the lowest bulk rock value (−1.0 ± 0.4‰). Remarkably, Liotta et al. (2017) also observed a positive relationship between S/Cl and δ³⁷Cl in Stromboli gases, related to the volcanic activity.

Magma Sources of Chlorine Beneath Stromboli

The correlation between δ³⁷Cl and K₂O, as well as with S/Cl, in ST2 melt inclusions (Figures 7B, 9B) are most likely due to mixing between two endmembers. We define two possible Cl endmembers: EDM1 with a higher δ³⁷Cl (∼+1.2‰), lower K₂O (∼1wt%), and higher S/Cl (∼0.6), and EDM2 with a low δ³⁷Cl ∼ −2.6‰, high K₂O ∼5wt%, and low S/Cl < 0.1. These endmembers do not have a specific SiO₂ content (Figure 7A), with EDM2, represented by the potassic and shoshonitic melt inclusions, having a variable SiO₂ (from 51 to 60 wt%). This means that a primitive melt batch with an imprint of EDM1 or EDM2 will conserve its δ³⁷Cl signature during magmatic differentiation and degassing.

Chlorine sources are investigated using Cl isotopes combined with trace element ratios. For example, the Ba/La ratio is a tracer of aqueous fluids, as Ba is a fluid-mobile element and La is a fluid-immobile element. Similarly, the La/Yb ratio traces variations in the degree of melting and, more importantly, the possible input of a melt or supercritical liquid component to the mantle source, as both La and Yb are fluid-immobile elements. On a plot of Ba/La against δ³⁷Cl (Figure 10A), melt inclusions display a slight positive correlation, with higher Ba/La for EDM1 (∼29) compared to EDM2 (∼22). Both endmembers have higher Ba/La than MORB (e.g., Hofmann, 1988), reflecting the addition of an aqueous fluid to the mantle wedge, with EDM1 being more enriched in fluid-mobile elements. Similarly, the negative trend between δ³⁷Cl and La/Yb (Figure 10B) suggests that the most negative δ³⁷Cl values, representing EDM2, reflect the influence of a melt-like component or of a supercritical fluid. Their significantly different δ³⁷Cl compositions point strongly to different lithologies.

FIGURE 10

FIGURE 10. Chlorine isotopes plotted against Ba/La (A) and La/Yb (B) ratios in melt inclusions. EDM1 and high potassium calco alkaline melt inclusions have higher Ba/La compared to EDM2, shoshonitic, and potassic melt inclusions, whereas EDM2 has a higher La/Yb ratio compared to EDM1. EDM1 is thus more enriched in fluid-mobile elements relative to EDM2.

The positive δ³⁷Cl of EDM1 could reflect the breakdown of amphiboles from the upper slab lithologies. A recent study on electrical conductivity shows amphibole to be one of the principal hosts of Cl in the slab (Manthilake et al., 2021). Amphibole as residual phase during slab dehydration in low temperature subduction zones has been invoked to explain extremely low Cl and F in melt inclusions (Iwate; Rose-Koga et al., 2014) because it holds on to these halogens and it explains why they are subsequently less abundant in the melt. Here at Stromboli, the steep dip of the Ionian slab (≈70°; e.g., Tommasini et al., 2007), along with the thermal and rheological model, indicates a very cold subduction zone (Carminati et al., 2005; Pasquale et al., 2005). This means that dehydration and melting reactions are possible at greater depths along this slab than in many other volcanic arc systems. So, we hypothesize that in this particular setting, the slab amphibole-bearing lithologies break down and the ensuing fluids that metasomatized the mantle wedge transmit the amphibole δ³⁷Cl signature. Indeed, hydrothermal amphibole veins from oceanic crust have positive values (up to +1.7‰; Barnes and Cisneros, 2012). Melting of amphibole-bearing altered oceanic crust (AOC) has already been invoked to explain the positive δ³⁷Cl values found in the arc lavas from Ecuador (Chiaradia et al., 2014). Also, Pb isotopes on single crystals of amphibole from an arc lava (Guagua Pichincha, Ecuador; Ancellin et al., 2019) showed that the most primitive amphibole had low ²⁰⁶Pb/²⁰⁴Pb, in the order of 18.82–18.88, overlapping with the range of Pb isotopes in the Stromboli melt inclusions (in situ measurements:18.76–18.83; Rose-Koga et al., 2012). Therefore, the breakdown of amphibole in amphibole-bearing AOC could pass the identifying features into the fluid agent from the slab that are required to explain the Cl isotope and Pb isotope signatures of Stromboli melt inclusions. EDM1 is represented here by the high potassium calco alkaline melt inclusions (Figures 7, 10).

EDM2 has negative δ³⁷Cl values and is represented by the most extreme potassic and shoshonitic melt inclusions. Negative δ³⁷Cl values in arc lavas are usually linked to the influence of seafloor sediment (e.g., Barnes and Sharp, 2017). Thus, the high potassium calco alkaline melt inclusions seem to have recorded the strongest influence of the aqueous fluid originating from the subducting crust whereas the shoshonitic and potassic melt inclusions record the signature of a fluid issued from the subducted sediments enriched in immobile elements. This is in full agreement with Tommasini et al. (2007), who proposed the involvement of an aqueous fluid from the subducting crust and a supercritical liquid from the subducting sediments ± crust for the magma genesis of Stromboli magmas, based on the trace elements and radiogenic isotopes (U, Th, Sr, Nd, Pb) of the whole rocks.

Melt inclusions thus allow two different Cl components from the mantle wedge beneath Stromboli to be highlighted, with each component most probably also playing a role in the different major element signatures of the magmas, as the alkali enrichment of shoshonitic and potassic melts could be related to the addition of fluid derived from sediments. The two Cl components were not discernible in the bulk rock contents, probably due to mixing of different melt batches at a shallow depth prior to eruption, which would have smoothed and averaged the δ³⁷Cl signal. As the gases have a similar range of δ³⁷Cl to the melt inclusions, and a similar relationship for S/Cl (Figure 9B), it is possible that the gases at Stromboli actually reflect the degassing of different melt pockets/compositions.

Conclusion

The new melt inclusion data from a single hand sample (ST2) span a large range of major element compositions. Based on their major elements, three series are identified, which correspond to three series found in the Stromboli lavas (high potassium calco alkaline, shoshonitic, and potassic). Chlorine isotopes extend the previous melt inclusion data for Stromboli toward more positive values. The δ³⁷Cl vary from −2.6 to +1.2‰, with an average slightly lower than that of the bulk rocks from Stromboli. This large isotopic variation cannot be related to degassing or fractional crystallization. Instead, correlation with K₂O and S/Cl suggest mixing between at least two different sources of Cl. Correlation between δ³⁷Cl and trace elements points toward a first endmember, enriched in fluid mobile elements and with positive δ³⁷Cl, reflecting amphibole breakdown from the AOC. A second endmember, enriched in fluid immobile elements and with negative δ³⁷Cl, records the influence of subducted sediment melts (or supercritical fluids). These Cl sources have different influences on the melt inclusions from the three series: high potassium calco alkaline melt inclusions are most influenced by the aqueous fluid, whereas shoshonitic and potassic melt inclusions have stronger sediment melt signatures. δ³⁷Cl could be a promising tracer for amphibole in the slab fluxes, and could also be a valuable tool to track the role of amphibole in the context of arc magma genesis and differentiation (e.g., Médard et al., 2006; Davidson et al., 2007).

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

A-SB designed the study and did the SIMS measurements, data reduction, and writing. ER-K did sample preparation, LA-ICP-MS measurements, data reduction, and writing. AC prepared the samples, did EMPA measurements, and created some of the figures.

Funding

This work was partially funded by the French Government Laboratory of Excellence initiative ANR-10-LABX-0006. This is Laboratory of Excellence ClerVolc contribution #522.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor HM declared a past co-authorship with two of the authors ASB and EFRK.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

The authors acknowledge Mélina Manzini for sampling ST2 during a field trip in 2014. We are thankful to Jean-Luc Devidal for his guidance and advice during EMPA and LA-ICP-MS sessions. Thank you to Fran Van Wyk de Vries for proofreading the final version as well as thorough corrections. We are also grateful for the constructive review of RM, the comments of IEMS, and the manuscript handling by the editor HM.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2021.793259/full#supplementary-material

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