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Article

Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano

by
Cristina Perinelli
1,*,
Silvio Mollo
1,2,
Mario Gaeta
1,
Serena Pia De Cristofaro
3,
Danilo Mauro Palladino
1 and
Piergiorgio Scarlato
2
1
Dipartimento di Scienze della Terra, Sapienza-Università di Roma, P.le Aldo Moro 5, 00185 Roma, Italy
2
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy
3
Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
*
Author to whom correspondence should be addressed.
Minerals 2018, 8(11), 482; https://doi.org/10.3390/min8110482
Submission received: 18 September 2018 / Revised: 17 October 2018 / Accepted: 22 October 2018 / Published: 25 October 2018
(This article belongs to the Special Issue Mineral Textural and Compositional Variations as a Tool for Understanding Magmatic Processes)
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Versions Notes

Abstract

:
Magma dynamics at Mt. Etna volcano are frequently recognized as the result of complex crystallization regimes that, at shallow crustal levels, unexpectedly change from H2O-undersaturated to H2O-saturated conditions, due to the impulsive and irregular arrival of volatile-rich magmas from mantle depths. On this basis, we have performed hydrous crystallization experiments for a quantitative understanding of the role of H2O in the differentiation of deep-seated trachybasaltic magmas at the key pressure of the Moho transition zone. For H2O = 2.1–3.2 wt %, the original trachybasaltic composition shifts towards phonotephritic magmas never erupted during the entire volcanic activity of Mt. Etna. Conversely, for H2O = 3.8–8.2 wt %, the obtained trachybasalts and basaltic trachyandesites reproduce most of the pre-historic and historic eruptions. The comparison with previous low pressure experimental data and natural compositions from Mt. Etna provides explanation for (1) the abundant release of H2O throughout the plumbing system of the volcano during impulsive ascent of deep-seated magmas; (2) the upward acceleration of magmas feeding gas-dominated, sustained explosive eruptions; (3) the physicochemical changes of gas-fluxed magmas ponding at shallow crustal levels; and (4) the huge gas emissions measured at the summit craters and flank vents which result in a persistent volcanic gas plume.

1. Introduction

The plumbing system of Mt. Etna volcano (Sicily, Italy) has a multifaceted geometry, variable in space and time and consisting of storage zones at different depths, where more or less primitive magmas containing variable H2O contents undergo degassing, fractional crystallization and mixing processes (cf. [1]). For example, the explosive activity of the volcano is ascribed to the impulsive upward migration of gas-rich magmas and/or fluxes of abundant volatiles from the depths [2,3,4]. Evidence of pulsating volatile flushing is also provided by the great volume of gases released from the summit craters and flanks of the volcano that clashes with the relatively small amount of erupted products [5]. The release of volatiles has drastic effects on the rheology of Etnean magmas and, consequently, the internal dynamics of the volcano [6]. Moreover, the large quantities of gas released may change the geochemical and isotopic compositions of near-liquidus lavas erupted at the vents and flowing onto the surface [7].
In this scenario, the vertically developed plumbing system represents a pathway for volatiles migration as supercritical fluids [4,8,9]. The impulse migration of volatiles through magmas stored beneath (or within) the volcanic edifice changes the physicochemical state of the system from H2O-undersaturated to H2O-saturated, with implications for mineral and melt compositions, degree of crystallization, magma ascent velocity and style of eruption [1,3,4,8,10,11,12,13].
Relevant insights into the highly variable volatile concentrations of magmas (H2O = 0.5–3.5 wt % and CO2 = 0.02–0.25 wt %) have been provided by data on melt inclusions entrapped at both shallow and moderate pressures (25–400 MPa) [14]. Within this pressure range, the differentiation process of Etnean liquids upon the effect of variable H2O concentrations have been explored through hydrous crystallization experiments [13,15,16,17]. However, few data are available in the literature at the key pressure of the Moho transition zone (~800 MPa or ~24 km b.s.l.; [18]) at which the solubility of H2O may be very high (>10 wt %; [19,20] and references therein), thus remarkably influencing the early crystallization of gas-rich trachybasaltic magmas erupted at Mt. Etna [12,21].
In this study, we present hydrous crystallization experiments conducted at 800 MPa, 1020–1200 °C, 2.1–10 wt % H2O, and NNO buffer with the aim of better understanding the role of H2O in the differentiation of deep-seated trachybasaltic magmas. Considering the crystallization temperatures at the Moho depth recorded by clinopyroxene phenocrysts from lava fountains and lava flows, the results of our experiments show that the irregular arrival of deep-seated, volatile-rich magmas into the uppermost portions of the plumbing system may release a huge amount of H2O with remarkable implications for the dynamics of H2O-undersaturated magmas ponding at shallow crustal levels.

2. Starting Materials and Methods

The starting materials used in this study are two trachybasalts (i.e., HE and ME hawaiitic products) from Mt. Etna showing virtually identical compositions: on anhydrous basis, HE and ME have 47.78 and 48.27 wt % SiO2, and 5.87 and 5.79 wt % Na2O + K2O, respectively (Table 1). HE is a lava flow erupted from the western flank of the Valle del Bove during the 1991–1993 effusive activity [22]. ME is a lava flow erupted from the South-East Crater during the 2011–2013 paroxysmal activity comprising both Strombolian explosions and lava effusions [23,24]. These two compositions are, indeed, the most representative of the recent activity at Mt. Etna volcano.
Both experiments and analyses were conducted at the Department of Earth Sciences, Sapienza-University of Rome, Italy. The starting glass was produced by melting the powdered sample at 1350 °C and atmospheric pressure for 1 h. Some glass chips were then analyzed by scanning electron microscopy, and no crystalline phases were detected. Phase equilibrium crystallization experiments were performed at 800 MPa in a half-inch, end-loaded piston cylinder apparatus. The starting material, composed of the powdered glass and deionized H2O, was loaded into a Au75Pd25 capsule. Variable amounts of H2O were added to the charges in order to obtain melt-H2O concentrations in the range between 2.1 and 10 wt % (Table 2), as determined by the “difference from 100” method (i.e., based on the difference to 100% of the total obtained by microprobe analyses of glasses, with accuracy of ~0.5 wt % H2O; [25].
According to H2O solubility data ([19] see discussion below) and the absence of gas bubbles into the quenched charges, the experiments were H2O-undersaturated, thus reproducing the crystallization conditions observed for Etnean magmas residing at depth [26]. The assembly consisted of fluorite-graphite-magnesia. The charge was heated directly to the target temperature (i.e., between 1020 and 1200 °C) that was kept constant for 8 h (Table 2). The temperature was controlled by a factory calibrated thermocouple (W95Re5-W74Re26; type D) with accuracy of ± 3 °C. The experiment was quenched through a cooling rate of approximately 150 °C/s and then the run product was mounted in an epoxy disk to expose a polished surface.
All the runs were self-buffered; we attempted to estimate fO2 through the equation of [27] using the liquid Fe3+/Fetot mole ratios from olivine-melt Fe-Mg exchange calculated according to [28]. This procedure yielded fO2 values of −0.3 to +0.7 NNO, in agreement with those estimated for similar furnace assemblages [29,30].
The chemical analyses were performed at the CNR-Istituto di Geologia Ambientale e Geoingegneria (Rome, Italy) using a Cameca SX50 electron microprobe (CAMECA, Gennevilliers, France) equipped with five wavelength-dispersive spectrometers using 15 kV accelerating voltage, 15 nA beam current for minerals and 2 nA for glass to obviate the alkali loss during the analyses, particularly for hydrous glasses. Moreover, sodium and potassium were analyzed first to prevent alkali migration effects. The beam diameter was of 10 μm, and the counting time was 20 s. The following standards were used: wollastonite (Si and Ca), corundum (Al), diopside (Mg), andradite (Fe), rutile (Ti), orthoclase (K), jadeite (Na), apatite (P) and metal (Mn).

3. Results

3.1. Phase Relations

Modal phase proportions (wt %) were derived by mass balance calculations [31], yielding acceptable residual sum of squares (Σr2 < 0.32; see Table 2). For a better comparison of these estimates with vol % data from lava flows, the volume proportions were recalculated using average densities of olivine (3.3 g/cm3) clinopyroxene (3.4 g/cm3), plagioclase (2.6 g/cm3), titanomagnetite (5.1 g/cm3) and melt (2.6–2.7 g/cm3 from the model of [32]). According to [17], both wt % and vol % estimates provided almost identical values (Table 2).
Figure 1 shows that the phase assemblage is characterized by the ubiquitous occurrence of clinopyroxene and titanomagnetite at near-liquidus conditions, followed by the crystallization of plagioclase and olivine at lower temperatures and/or H2O contents. In highly crystallized charges, plagioclase is the most abundant mineral phase but its stability field progressively decreases with increasing H2O. The crystallization of clinopyroxene and plagioclase increases by 50 vol % with decreasing temperature (Table 2). In particular, the plagioclase/clinopyroxene ratio is 0.7–1.2 at T ≥ 1125 °C, but abruptly increases to 1.5–2.2 at T ≤ 1090 °C due to the predominant crystallization of plagioclase (Table 2). It is interesting to note that olivine crystallization occurs only at T ≤ 1150 °C and is subordinated to that of other mineral phases.

3.2. Phase Compositions

Major element concentrations of both experimental glasses and minerals are reported in Table 3. In the total alkali versus silica diagram (Figure 2), the original trachybasaltic melt obtained at H2O = 2.1–3.2 wt % evolves towards basaltic trachyandesitic and phonotephritic compositions, as well as the Mg-number (i.e., Mg# = atomic Mg/(Mg + Fe2+) × 100) decreases from Mg#42 to Mg#37. At H2O = 3.8–8.2 wt %, the starting melt differentiates from basaltic trachyandesite to trachyandesite to tephriphonolite with Mg#33–43. The residual melts show a significant increase of SiO2 content (from 48.89 to 53.39 wt %) associated with a narrow increase of alkali (from 6.49 to 9.57 wt %) that, in turn, is weakly observed at H2O = 2.1–3.2 wt % (Figure 2).
The amount of diopside (Di) and hedenbergite (Hd) in clinopyroxene generally decreases with increasing temperature and H2O (Table 3). At H2O = 2.1–3.2 wt %, the ferri-calcium Tschermak molecule (CaFeTs) slightly decreases with decreasing temperature (Figure 3a). Conversely, at H2O = 3.8–8.2 wt %, both Aliv and Fe3+ are preferentially incorporated into clinopyroxene crystal lattice, leading to higher CaFeTs contents [33].
Overall, the anorthite (An) molecule in plagioclase shows a rough decrease (from An75 to An63) with decreasing temperature (Figure 3b). However, An in plagioclase is also influenced by the initial H2O content, according to the dependence of CaO-NaO exchange on hydroxyl groups dissolved in the melt [34].
The ulvospinel (Usp) content in titanomagnetite progressively increases along an almost linear trend resulting from the combined effect of both temperature and H2O (Figure 3c). This compositional change is also associated with lower Al and Mg concentrations into the titanomagnetite crystal lattice, due to their preferential incorporation in clinopyroxene and plagioclase (Table 3).
At H2O = 2.3–3.8 wt %, the amount of forsterite (Fo) in olivine decreases from 74 to 65 with decreasing temperature, whereas scattered Fo values are observed at the H2O = 2.1 and 5 (Figure 3d). As documented for plagioclase, hydroxyl groups dissolved in the melt preferentially complex with Mg2+ rather than Fe2+, thus reducing the activity of MgO relative to FeO in olivine, and viceversa [34].

3.3. Achievement of Equilibrium

In order to test the achievement of equilibrium crystallization in the experimental charges, different textural and compositional features have been considered: (1) the euhedral shape of crystals, (2) the homogeneous distribution of mineral phases in the glass, (3) the lack of quench crystals, (4) the smooth variation of phase compositions with experimental conditions, (5) the small sum of residuals (generally ≤0.5; Table 2) from mass-balance calculations, and (6) crystal-melt exchange coefficients (Kd) for clinopyroxene and plagioclase comparable to those from previous equilibrium studies. Concerning the clinopyroxene phase, we have used the temperature-sensitive cpx-meltKdFe-Mg model derived by Putirka [35] and based on deviations in observed and calculated Fe-Mg cation partitioning between crystals and coexisting melts. cpx-meltKdFe-Mg values of clinopyroxene-melt pairs from this study, calculated with the total Fe as Fe2+ according to Putirka et al. [36], vary between 0.26 and 0.27 (Table 2), in agreement with the equilibrium range of 0.27 ± 0.03 reported in literature (e.g., [36]). As a further test for clinopyroxene, we have also adopted the model of Mollo et al. [37], calibrated through the difference between diopside + hedenbergite components (ΔDiHd) predicted for clinopyroxene via regression analysis of clinopyroxene-melt pairs in equilibrium conditions, and those measured in the analyzed crystals (Table 2). ΔDiHd values derived for our experimental compositions are remarkably low (0.03–0.08) and close to zero, as expected for thermodynamically equilibrated phases (e.g., [38]).
The equilibrium crystallization of plagioclase has been tested through the Ab-An (albite-anorthite) exchange reaction proposed by Putirka [35]. According to this model, the equilibrium constant is constrained within two temperature-dependent intervals of plg-meltKdAb-An = 0.10 ± 0.05 at T < 1050 °C and plg-meltKdAb-An = 0.25 ± 0.11 at T > 1050 °C. Our estimates show that plg-meltKdAb-An yields values between 0.15 and 0.21, suggesting equilibrium crystallization for experimental plagioclase crystals formed at T > 1050 °C.

4. Discussion

4.1. H2O-Undersaturated Versus H2O-Saturated Crystallization Regimes

Previous experiments conducted at 200–400 MPa on Etnean magmas have demonstrated that olivine does not crystallize under H2O-undersaturated conditions [12,17,39], whereas at PH2O = Ptotal its stability field expands at the expense of other minerals, and olivine appears as liquidus phase [16]. A similar result has been also documented by the experimental study of Di Carlo et al. [40] for a K-basalt from Stromboli, where clinopyroxene dominates the phase assemblage at 400 and 300 MPa. Clinopyroxene and olivine co-saturate the melt at 200 MPa and olivine becomes the liquidus phase at 100 MPa and H2O > 3.5 wt % [40]. Our experiments confirm that olivine is a late crystallizing phase from H2O-undersaturated magmas equilibrated at high pressure. This agrees with the petrological observation of Tanguy et al. [41] that clinopyroxene oversteps olivine crystallization at the Moho depth (28 km, ~800 MPa; [18]), for most of the trachybasaltic lava flows erupted at Mt. Etna. Moreover, thermobarometric estimates demonstrate that the initial stages of crystallization of Etnean trachybasalts take place near the Moho boundary [21], pointing to a deep feeder level of the plumbing system. We are aware of the critical role of CO2 on the activity of H2O in the melt and degassing/crystallization processes at Mt. Etna volcano, especially at high pressure, where the solubility of CO2 may be significant (e.g., up to ~0.7 wt % at 800 MPa; [19]). However, previous experiments on Etnean trachybasalts show that phase relations and compositions are weakly affected by CO2 concentration in the melt, successfully reproducing the textures and compositions of natural products under CO2-absent experimental conditions. For example, Vetere et al. [17] have performed experiments on Etnean trachybasalts equilibrated at 400 MPa, 1100–1150 °C and NNO + 3.7 buffering conditions, as well as in presence of volatile components of 1–3 wt % H2O and 0–0.3 wt % CO2. Results from these experiments indicate that the role of CO2 in controlling the final composition of magmas is negligible with respect to that played by T and H2O at degrees of crystal fractionation in the range of 6–40 wt %. More specifically, in terms of major and rare earth elements, the Etnean compositions are well reproduced in laboratory irrespective of the C–O–H species in the melt phase. Logically, an increasing concentration of CO2 in the melt reduces the solubility of H2O, thus modulating the degree of crystal fractionation, as it is obtained by lowering the amount of H2O added in the experimental charge.
In Figure 4, the compositions of pre-historic, 1963–1995, and 2001–2006 lava flows at Mt. Etna are compared with those experimentally derived at high (P = 800 MPa), moderate (P = 200–400 MPa) and low (P = 27–120 MPa) pressures (Table S1, Supplementary Materials). In H2O-undersaturated experiments conducted at both high and moderate pressures, plagioclase crystallization is favored at low H2O content, leading to a relatively high plagioclase/clinopyroxene ratio (Plg/Cpx = 0.7–2.2). As a consequence, the CaO/K2O ratio in the melt decreases significantly at the early stage of crystallization, reproducing the bulk rock compositions of 2001–2006 eruptions, as well as part of the evolutionary trend of the less differentiated historic eruptions. In contrast, the high H2O content in the low pressure experiments suppresses the crystallization of plagioclase along most of the magma differentiation path. Plagioclase starts to crystallize at low temperature (1075 °C) and the plagioclase/clinopyroxene ratio remains relatively low (Plg/Cpx = 0.3–0.9). Thus, the experimental melts exhibit high CaO/K2O ratios that capture the bulk rock compositions of 1763–1995 eruptions.
According to Figure 4, it is apparent that the differentiation of Etnean magmas is controlled by the stability of plagioclase that, in turn, depends on the amount of H2O dissolved in the melt. At very shallow crustal levels, the core-to-rim compositional variation of plagioclase phenocrysts is driven by the exsolution of ~3 wt % H2O during magma decompression from 85 to 5 MPa [12]. Conversely, at P > 100 MPa, disequilibrium textures of plagioclase testify to mixing induced by recharge of volatile-rich magmas or injection by volatile flushing into the system [9,24]. This applies especially to the onset of lava fountain eruptions that are triggered by the irregular arrival of primitive, volatile-rich magma batches that mixed with more evolved, H2O-undersaturated trachybasalts residing in the upper conduit region of the volcano [1,23].
In order to explain the most differentiated trachyandesitic lavas erupted during the pre-historic activity of Mt. Etna, we have modelled the evolution of the HE trachybasalt using the MELTS algorithm of Ghiorso and Sack [42] assuming a fractional crystallization process. The best fitting differentiation path is obtained at 1040–1160 °C, 150 MPa, 1.5 wt % H2O and NNO buffer, when the system is H2O-undersaturated, but the H2O concentration increases from 1.5 to 3.2 wt % during crystallization (Table S2, Supplementary Materials). Under such circumstances, the plagioclase/clinopyroxene ratio is intermediate (Plg/Cpx = 0.3–1.8) to that experimentally derived at low and high pressure conditions (Figure 4), thus reproducing the bulk rock trachyandesitic compositions of natural products. This finding confirms the differentiation model proposed by [41], where pre-historic trachybasaltic magmas underwent occasional pulses of low-pressure plagioclase fractionation causing the eruption of more evolved trachyandesitic lava flows. Importantly, high-pressure experiments from this study point out that, at H2O = 2.1–3.2 wt %, the degree of crystallization is generally very high (~40–50 vol %), so that deep-seated trachybasaltic magmas shift towards phonotephritic compositions never erupted during the entire volcanic activity of Mt. Etna.

4.2. H2O Release upon Magma Ascent during 2001 and 2006 Eruptions

Through the thermodynamically based equation of state proposed by Duan [19], we have estimated the solubility of H2O and CO2 in Etnean magmas. One of the advantages of this model consists of its applicability to high pressure (>500 MPa) and high temperature conditions that are not adequately reproduced by previous equations (see [20] for a review). Our calculations referred to 100–800 MPa and 1200 °C using the HE trachybasalt as input composition (Table 1); notably, replicated calculations at 1100–1150 °C with more or less differentiated trachybasalts (MgO = 4–6 wt % as for 2001–2012 Etnean lavas; [12]) provided changes in estimate (6.5%) within the accuracy of the model for the prediction of H2O (7.2%) and CO2 (9.7%). Results are plotted in Figure 5a together with the volatile contents derived by melt inclusion data from 2001 and 2006 lava flows [3,14,43]. Generally, the molar fraction of H2O in the gas phase ranges from 20 to 40 mol % as documented by Collins et al. [3] and, at ~400 MPa, the maximum concentrations of H2O and CO2 are 3.5 wt % and 0.3 wt %, respectively (Figure 5a). The amount of volatiles dissolved in the 2001 magma is also higher than that measured for the 2006 eruption. Indeed, the 2001 lava flow was fed by a volatile-rich primitive basalt rising along a closed-system degassing path [14,43], whereas the 2006 eruption involved the slow effusion of a gas-poor magma ponding within the volcanic edifice [3,44]. An important outcome from our calculations is that, at 800 MPa, the solubility of H2O can be very high, ranging from 6 to 8.5 wt % (Figure 5a). Therefore, trachybasaltic magmas stored at the Moho depth may be source of sustained H2O supply during upward migration throughout the plumbing system of the volcano.
Using a specific algorithm based on the equilibrium partitioning between mineral and melt, Armienti et al. [21] estimated the near-liquidus temperature of Etnean trachybasaltic magmas at the early stage of clinopyroxene crystallization. At ~800 MPa, the authors found that clinopyroxene saturation occurred at 1120–1200 °C and 1080–1150 °C for the 2001 and 2006 composition, respectively (Figure 5b). Looking at our experimental data, clinopyroxene is in equilibrium with a trachybasaltic magma when the degree of crystallization is ≤13 vol % (Table 2), otherwise the melt composition would readily evolve towards either basaltic trachyandesite or phonotephrite as a function of H2O (Figure 2). The trachybasaltic melts experimentally-derived in this study confirm that the temperatures estimated by Armienti et al. [21] are close to the near-liquidus region of the 2001 and 2006 eruptions. Figure 1 shows that, as the temperature decreases, the near-liquidus surface is controlled by increasing amounts of H2O. According to this experimental curve, we found that the original H2O concentrations in equilibrium with 2001 and 2006 magmas were 3–6 wt %. Coherently, these H2O contents closely match with the hygrometric estimates of 3–6 and 5.5–7 wt % for 2001 and 2006 eruptions, respectively, reported by Armienti et al. [21] (Figure 5b). With respect to the degassed melt inclusions found at the storage level of ~200 MPa (see also [1]), our experimental data provide quantification for the overall release of H2O (1.5–4 wt %) that could be expected during impulsive ascent of deep-seated trachybasaltic magmas at Mt. Etna [4,45,46]. A migration of volatiles (particularly H2O) transported as supercritical fluids throughout the Etnean plumbing system is also consistent with the early “volatile-induced differentiation model” proposed by Ferlito and Lanzafame [8] and further reappraised by Ferlito [47] to explain the potassium enrichment observed in post-1971 lava flows and the transition from a low-density H2O-melt solution to a high-density melt phase erupted concomitant to a sustained gas flux. Therefore, the huge flux of magmatic gases from depth provides explanation for (1) the low volume of magma erupted in recent years with respect to the great amount of gases measured in the volcanic plume [5,47], (2) the compositions of unusually explosive eruptions fed by primitive, volatile-rich magmas [48], and (3) the textural and compositional variations of plagioclases related to fluid-induced dissolution reactions [9]. In this context, Figure 6 shows that, at 800 MPa and 1070–1090 °C, the basaltic trachyandesitic magma is saturated with clinopyroxene when the amount of H2O is 5.2–8 wt %. Conversely, at 200–400 MPa and 1050–1070 °C, the H2O content decreases to 2.5–4 wt %, implying that at least 50% of H2O is released as a free fluid phase. Both melt inclusion data [3,43] and clinopyroxene-based hygrometric estimates [39,49] confirm that H2O degassing starts at ~400 MPa, with most of the H2O loss occurring at P < 100 MPa.

5. Conclusions

High-pressure crystallization experiments carried out on a trachybasaltic composition, under H2O-undersaturated and H2O-saturated conditions, shed new light on the role of H2O on the high-pressure crystallization path of magma at Mt. Etna. We found that the crystallization sequence, phase stability, textural and compositional evolution of natural minerals are directly correlated to the transition from H2O-undersaturated to H2O-saturated crystallization regimes. Olivine is a late crystallizing phase from H2O-undersaturated magmas equilibrated at high pressure. In contrast, the clinopyroxene saturation surface is encountered at 800 MPa, when the solubility of H2O can be very high, ranging from 6 to 8.5 wt %. During upward migration throughout the plumbing system of the volcano, the overall release of H2O accompanying clinopyroxene crystallization is constrained between 1.5 and 4 wt %. Therefore, the impulsive ascent of deep-seated trachybasaltic magmas may release a great amount of H2O. Most of the excess of H2O may contribute to the gas emissions from the summit craters and volcano flanks feeding the persistent gas plume at Mt. Etna. However, high-pressure experiments conducted at H2O-undersaturated conditions cannot account for the distinctly non-equilibrium processes related to the large gas flux that may control magma compositions and trigger eruptions at Mt. Etna volcano. Therefore, further laboratory investigations on decompression and volatile degassing mechanisms may be of help to elucidate magma dynamics related to intense supply of a gas phase (mostly H2O) in the uppermost section of the feeding system.

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/8/11/482/s1: Table S1. Natural and experimental compositions of Etnean melts; Table S2. MELTS simulations conducted using the HE trachybasalt as starting composition.

Author Contributions

Conceptualization, C.P., S.M. and M.G.; formal analysis, C.P. and S.P.D.C.; investigation, C.P., S.M., M.G. and S.P.D.C.; data curation, C.P.; S.M. and S.P.D.C.; writing—original draft reparation, C.P., S.M., M.G., S.P.D.C., D.M.P., P.S.; writing—review and editing, C.P., S.M., M.G., S.P.D.C., D.M.P., P.S.; visualization, C.P., S.M., M.G., S.P.D.C., D.M.P., P.S.; funding acquisition, M.G., P.S.

Funding

This research received no external funding.

Acknowledgments

We kindly thank M. Serracino for assistance during electron microprobe analysis. Two anonymous reviewers provided instructive and helpful reviews that greatly contributed to the quality of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 08 00482 g001 550
Figure 1. Temperature vs. H2O diagram showing the stability of mineral phases in our experimental charges. As the temperature decreases, the near-liquidus surface is gained by increasing amounts of H2O that lower the degree of crystallization and buffer the melt composition to trachybasalt. Cpx, clinopyroxene. Timt, titanomagnetite. Plg, plagioclase. Ol, olivine.
Figure 1. Temperature vs. H2O diagram showing the stability of mineral phases in our experimental charges. As the temperature decreases, the near-liquidus surface is gained by increasing amounts of H2O that lower the degree of crystallization and buffer the melt composition to trachybasalt. Cpx, clinopyroxene. Timt, titanomagnetite. Plg, plagioclase. Ol, olivine.
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Figure 2. Total alkali vs. silica diagram showing the composition of residual melts from our experiments. Two distinct trends are depicted as a function of the H2O content. For H2O = 2.1–3.2 wt %, the original trachybasaltic liquid evolves towards basaltic trachyandesitic and phonotephritic compositions. For H2O = 3.8–8.2 wt %, the starting melt differentiates to basaltic trachyandesitic to trachyandesitic to tephriphonolitic compositions.
Figure 2. Total alkali vs. silica diagram showing the composition of residual melts from our experiments. Two distinct trends are depicted as a function of the H2O content. For H2O = 2.1–3.2 wt %, the original trachybasaltic liquid evolves towards basaltic trachyandesitic and phonotephritic compositions. For H2O = 3.8–8.2 wt %, the starting melt differentiates to basaltic trachyandesitic to trachyandesitic to tephriphonolitic compositions.
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Figure 3. Mineral compositional variations as a function of temperature and melt-H2O content. At H2O = 2.1–3.2 wt %, the ferri-calcium Tschermak molecule (CaFeTs) slightly decreases with decreasing temperature, whereas at H2O = 3.8–8.2 wt %, both Aliv and Fe3+ are preferentially incorporated into clinopyroxene crystal lattice, leading to higher CaFeTs contents (a). The anorthite (An) molecule in plagioclase generally decreases with decreasing temperature with the exception of compositions derived upon the effect of H2O = 2.1 and 5 wt % (b). The ulvospinel (Usp) content in titanomagnetite progressively increases along an almost linear trend resulting from the combined effect of both temperature and H2O (c). At H2O = 2.3–3.8 wt %, the amount of forsterite (Fo) in olivine decreases with decreasing temperature, whereas scattered Fo values are observed at H2O = 2.1 and 5 wt % (d).
Figure 3. Mineral compositional variations as a function of temperature and melt-H2O content. At H2O = 2.1–3.2 wt %, the ferri-calcium Tschermak molecule (CaFeTs) slightly decreases with decreasing temperature, whereas at H2O = 3.8–8.2 wt %, both Aliv and Fe3+ are preferentially incorporated into clinopyroxene crystal lattice, leading to higher CaFeTs contents (a). The anorthite (An) molecule in plagioclase generally decreases with decreasing temperature with the exception of compositions derived upon the effect of H2O = 2.1 and 5 wt % (b). The ulvospinel (Usp) content in titanomagnetite progressively increases along an almost linear trend resulting from the combined effect of both temperature and H2O (c). At H2O = 2.3–3.8 wt %, the amount of forsterite (Fo) in olivine decreases with decreasing temperature, whereas scattered Fo values are observed at H2O = 2.1 and 5 wt % (d).
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Figure 4. The compositions of pre-historic, 1963–1995, and 2001–2006 lava flows at Mt. Etna (data are from the GEOROC database—http://georoc.mpch-mainz.gwdg.de/georoc/Start.asp) are compared with those experimentally derived at high (P = 800 MPa), moderate (P = 200–400 MPa) and low (P = 27–120 MPa) pressures, as well as MELTS [42] simulations on HE trachybasalt equilibrated at 1000–1160 °C, 150 MPa, 1.5 wt % H2O and NNO buffer (see dashed line). The Plg/Cpx ratio changes upon the effect of H2O-saturated and H2O-undersaturated conditions, controlling the CaO/K2O ratio of the melt. Plg, plagioclase. Cpx, clinopyroxene.
Figure 4. The compositions of pre-historic, 1963–1995, and 2001–2006 lava flows at Mt. Etna (data are from the GEOROC database—http://georoc.mpch-mainz.gwdg.de/georoc/Start.asp) are compared with those experimentally derived at high (P = 800 MPa), moderate (P = 200–400 MPa) and low (P = 27–120 MPa) pressures, as well as MELTS [42] simulations on HE trachybasalt equilibrated at 1000–1160 °C, 150 MPa, 1.5 wt % H2O and NNO buffer (see dashed line). The Plg/Cpx ratio changes upon the effect of H2O-saturated and H2O-undersaturated conditions, controlling the CaO/K2O ratio of the melt. Plg, plagioclase. Cpx, clinopyroxene.
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Figure 5. H2O vs. CO2 solubility diagram derived through the model of Duan [19] (a). Calculations were referred to 100–800 MPa and 1200 °C using the HE trachybasalt as input composition. The volatile contents derived by melt inclusion data from 2001 and 2006 lava flows [3,14,43] are also plotted in figure. P-T-H2O diagram derived for the stability of clinopyroxene phenocrysts from 2001 and 2006 lava flows (b). The saturation pressure (~800 MPa) and temperature (1120–1200 °C and 1080–1150 °C for the 2001 and 2006 clinopyroxenes, respectively) were estimated by Armienti et al. [21]. Our experimental data, indicate that the original amounts of H2O in equilibrium with 2001 and 2006 magmas were 3–6 and 5.5–7 wt %, respectively. Error bars come from thermometric and hygrometric estimates of Armienti et al. [21].
Figure 5. H2O vs. CO2 solubility diagram derived through the model of Duan [19] (a). Calculations were referred to 100–800 MPa and 1200 °C using the HE trachybasalt as input composition. The volatile contents derived by melt inclusion data from 2001 and 2006 lava flows [3,14,43] are also plotted in figure. P-T-H2O diagram derived for the stability of clinopyroxene phenocrysts from 2001 and 2006 lava flows (b). The saturation pressure (~800 MPa) and temperature (1120–1200 °C and 1080–1150 °C for the 2001 and 2006 clinopyroxenes, respectively) were estimated by Armienti et al. [21]. Our experimental data, indicate that the original amounts of H2O in equilibrium with 2001 and 2006 magmas were 3–6 and 5.5–7 wt %, respectively. Error bars come from thermometric and hygrometric estimates of Armienti et al. [21].
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Figure 6. Temperature vs. H2O diagram derived using data from hydrous crystallization experiments conducted at high (800 MPa from this study) and moderate (200–400 MPa from literature) pressures. Considering the natural P-T constrains of 2011–2013 lava fountains, the amount of H2O in equilibrium with clinopyroxene phenocrysts has been reconstructed. At 800 MPa and 1070–1090 °C, the basaltic trachyandesitic magma is saturated with clinopyroxene when the amount of H2O is 5.2–8 wt %. Conversely, at 200–400 MPa and 1050–1070 °C, the H2O content decreases to 2.5–4 wt %.
Figure 6. Temperature vs. H2O diagram derived using data from hydrous crystallization experiments conducted at high (800 MPa from this study) and moderate (200–400 MPa from literature) pressures. Considering the natural P-T constrains of 2011–2013 lava fountains, the amount of H2O in equilibrium with clinopyroxene phenocrysts has been reconstructed. At 800 MPa and 1070–1090 °C, the basaltic trachyandesitic magma is saturated with clinopyroxene when the amount of H2O is 5.2–8 wt %. Conversely, at 200–400 MPa and 1050–1070 °C, the H2O content decreases to 2.5–4 wt %.
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Table
Table 1. Composition of HE and ME starting materials. SD is the standard deviation.
Table 1. Composition of HE and ME starting materials. SD is the standard deviation.
#OxideHESD (5)MESD (5)
Experimental melt compositions
SiO247.780.3948.270.43
TiO21.720.021.690.03
Al2O317.100.1917.320.16
FeOtot10.180.1211.070.11
MnO0.170.010.170.03
MgO5.550.074.940.09
CaO10.400.129.940.10
Na2O3.840.053.730.07
K2O2.030.032.060.05
P2O50.550.010.570.03
Tot99.32 99.76
Notes: FeOtot is the total iron given as FeO.
Table
Table 2. Experimental conditions, mass balance calculations, and tests for equilibrium crystallization conditions.
Table 2. Experimental conditions, mass balance calculations, and tests for equilibrium crystallization conditions.
RunTtH2OGlCpxPlgTimtOlΣr2cpx-meltKdFe-MgΔDiHdplg-meltKdAb-Anol-meltKdFe-Mg
(#)(°C)(h)(wt %)(wt %–vol %) (Putirka et al., 2008)(Mollo et al., 2013b)(Putirka et al., 2008)Roeder and Emslie (1970)
HE-1120082.7100–100----0.01----
HE-2112582.159–5723–2415–162–21–10.250.260.030.140.42
HE-3105088.285–879–8-6–5-0.190.270.05--
HE-41020810100–100----0.01----
ME-1117583.287–886–56–61–1-0.320.270.020.21-
ME-2115082.364–6711–1018–164–43–30.190.270.040.180.35
ME-3115086.2100–100----0.01----
ME-4112582.469–7313–1215–141–12–20.210.270.080.140.54
ME-5109084.148–5317–1526–245–44–40.310.270.040.170.26
ME-6107083.848–5318–1628–261–15–40.260.270.010.150.49
ME-7106585.056–6011–1024–226–53–30.240.270.070.140.55
Table
Table 3. Experimental compositions of residual melts, clinopyroxenes, plagioclases, titanomagnetites, and olivines. SD is the standard deviation.
Table 3. Experimental compositions of residual melts, clinopyroxenes, plagioclases, titanomagnetites, and olivines. SD is the standard deviation.
#HE-2SD(4)HE-3SD(4)ME-1SD(4)ME-2SD(4)ME-4SD(4)ME-5SD(4)ME-6SD(4)ME-7SD(4)
Experimental melt compositions
SiO250.960.4051.710.3849.880.4550.860.4350.690.4553.510.4253.210.4553.490.41
TiO22.000.021.490.021.750.022.000.031.920.031.890.021.860.032.050.02
Al2O318.160.2019.750.2017.630.1517.320.1217.870.1618.610.2018.780.1315.970.17
FeO8.350.106.080.069.590.149.500.099.570.107.400.077.080.078.350.12
MnO0.160.020.200.030.140.000.200.020.220.010.210.030.240.010.200.01
MgO3.290.064.230.074.640.093.790.043.910.052.450.052.800.034.210.08
CaO7.030.069.650.119.390.077.410.077.050.075.760.045.600.057.360.07
Na2O5.760.104.340.054.260.045.130.064.870.064.990.065.380.064.640.08
K2O3.310.072.230.032.220.033.120.053.200.084.320.064.160.063.020.07
P2O50.980.020.330.020.490.010.670.030.700.030.860.040.890.020.720.03
Total97.91 91.82 96.82 97.68 97.60 95.90 96.19 95.02
#HE-2SD(3)HE-3SD(3)ME-1SD(3)ME-2SD(3)ME-4SD(3)ME-5SD(3)ME-6SD(3)ME-7SD(3)
Experimental clinopyroxene compositions
SiO247.860.3948.950.4048.950.4048.560.3949.560.4049.350.4049.290.4048.730.39
TiO21.850.020.880.011.510.021.540.021.360.021.550.021.260.021.520.02
Al2O36.220.125.340.104.980.095.340.104.560.094.520.094.630.094.990.09
FeO8.590.106.680.088.320.108.550.108.240.108.660.107.670.098.470.10
MnO0.190.020.160.010.190.020.160.010.210.020.380.030.200.020.200.02
MgO12.250.1514.860.1812.790.1512.570.1513.150.1612.640.1513.490.1612.740.15
CaO22.400.2722.460.2722.770.2722.540.2721.790.2622.130.2722.890.2722.680.27
Na2O0.420.030.600.040.470.030.480.030.530.030.580.030.730.040.440.03
Total99.77 99.93 99.99 99.74 99.39 99.81 100.17 99.76
Cations on the basis of 6 oxygens
Si1.789 1.796 1.822 1.813 1.853 1.843 1.631 1.819
Ti0.211 0.204 0.178 0.187 0.147 0.157 0.169 0.181
AlIV0.063 0.027 0.040 0.048 0.053 0.042 0.032 0.038
AlVI0.052 0.024 0.042 0.043 0.038 0.043 0.035 0.043
Fe3+0.076 0.172 0.089 0.089 0.056 0.072 0.121 0.092
Fe2+0.192 0.033 0.170 0.178 0.201 0.199 0.115 0.173
Mn0.682 0.813 0.710 0.699 0.733 0.703 0.738 0.709
Mg0.006 0.005 0.006 0.005 0.007 0.012 0.006 0.006
Ca0.897 0.883 0.908 0.902 0.873 0.886 0.900 0.907
Na0.030 0.042 0.034 0.035 0.038 0.042 0.052 0.032
Di0.58 0.67 0.62 0.60 0.60 0.60 0.65 0.62
Hd0.16 0.03 0.15 0.15 0.16 0.17 0.10 0.15
En0.05 0.07 0.04 0.05 0.07 0.05 0.04 0.05
Fs0.02 0.00 0.01 0.01 0.02 0.01 0.01 0.01
Jd0.03 0.03 0.04 0.04 0.04 0.04 0.03 0.03
CaTs0.07 0.08 0.05 0.06 0.04 0.04 0.05 0.05
CaFeTs0.04 0.09 0.04 0.04 0.03 0.04 0.06 0.05
#HE-2SD(3)ME-1SD(3)ME-2SD(3)ME-4SD(3)ME-5SD(3)ME-6SD(3)ME-7SD(3)
Experimental plagioclase compositions
SiO246.720.3848.910.4052.400.4252.100.4252.300.4250.210.4151.270.42
TiO20.120.010.050.000.090.010.080.010.100.010.070.010.110.01
Al2O332.310.3631.350.3428.990.3229.920.3328.750.3230.420.3329.990.33
FeO0.960.040.800.030.810.030.850.041.100.050.980.040.940.04
MgO0.100.010.090.010.090.010.090.010.160.010.090.010.100.01
CaO16.610.2015.460.1913.010.1613.620.1613.070.1614.080.1713.740.16
Na2O2.980.132.670.113.990.173.510.153.810.163.810.163.430.14
K2O0.160.010.210.010.440.030.340.020.450.030.310.020.380.02
Total99.96 99.53 99.82 100.51 99.74 99.96 99.97
Cations on the basis of 8 oxygens
Si2.164 2.253 2.391 2.361 2.390 2.301 2.341
Al1.764 1.702 1.560 1.598 1.549 1.644 1.614
Fe0.037 0.031 0.031 0.032 0.042 0.037 0.036
Mg0.001 0.001 0.001 0.001 0.001 0.001 0.001
Ca0.824 0.763 0.636 0.661 0.640 0.692 0.672
Na0.268 0.238 0.353 0.308 0.338 0.339 0.304
K0.010 0.012 0.025 0.019 0.026 0.018 0.022
An75 75 63 67 64 66 67
Ab24 24 35 31 34 32 30
Or1 1 2 2 3 2 2
#HE-2SD(3)HE-3SD(3)ME-5SD(3)
Experimental titanomagnetite compositions
TiO210.150.1110.140.1210.030.12
Al2O36.320.325.280.265.430.27
FeO73.010.6671.450.7974.610.60
MnO0.380.050.620.070.420.05
MgO4.190.251.530.091.890.11
Total94.05 89.01 92.38
Cations on the basis of 3 oxygens
Ti0.275 0.297 0.283
Al0.269 0.243 0.240
Fe3+0.000 0.000 0.000
Fe2+2.203 2.330 2.340
Mn0.012 0.020 0.013
Mg0.225 0.089 0.106
Usp32 37 34
#HE-2SD(3)ME-2SD(3)ME-4SD(3)ME-5SD(3)ME-6SD(3)ME-7SD(3)
Experimental olivine compositions
SiO237.280.3038.340.3138.570.3138.070.3137.280.3038.070.31
FeO30.130.3622.810.2724.790.3023.330.2830.130.3623.330.28
MnO0.740.040.660.030.640.030.590.030.740.040.590.03
MgO31.080.3436.570.4035.280.3936.470.4031.080.3436.470.40
CaO0.430.030.440.040.440.020.390.030.430.020.390.03
Tot99.66 98.82 99.72 98.85 99.66 98.85
Cations on the basis of 4 oxygens
Si1.013 1.014 1.018 1.009 1.013 1.009
Fe0.685 0.504 0.547 0.517 0.685 0.517
Mn0.017 0.015 0.014 0.013 0.017 0.013
Mg1.259 1.441 1.389 1.441 1.259 1.441
Ca0.013 0.012 0.012 0.011 0.013 0.011
Fo65 74 72 74 65 74

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MDPI and ACS Style

Perinelli, C.; Mollo, S.; Gaeta, M.; De Cristofaro, S.P.; Palladino, D.M.; Scarlato, P. Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano. Minerals 2018, 8, 482. https://doi.org/10.3390/min8110482

AMA Style

Perinelli C, Mollo S, Gaeta M, De Cristofaro SP, Palladino DM, Scarlato P. Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano. Minerals. 2018; 8(11):482. https://doi.org/10.3390/min8110482

Chicago/Turabian Style

Perinelli, Cristina, Silvio Mollo, Mario Gaeta, Serena Pia De Cristofaro, Danilo Mauro Palladino, and Piergiorgio Scarlato. 2018. "Impulsive Supply of Volatile-Rich Magmas in the Shallow Plumbing System of Mt. Etna Volcano" Minerals 8, no. 11: 482. https://doi.org/10.3390/min8110482

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