Eugenio Nicotra

Continuous magma recharge at Mt. Etna during the 2011–2013 period controls the style of volcanic activity and compositions of erupted lavas

Volcanic rocks erupted during the January 2011 - April 2013 paroxysmal sequence at Mt. Etna volcano have been investigated through in situ microanalysis of mineral phases and whole rock geochemistry. These products have been also considered within the framework of the post-2001 record, evidencing that magmas feeding the 2011–2013 paroxysmal activity inherited deep signature comparable to that of the 2007–2009 volcanic rocks for what concerns their trace element concentration. Analysis performed on plagioclase, clinopyroxene and olivine, which are sensitive to differentiation processes, show respectively fluctuations of the An, Mg# and Fo contents during the considered period. Also major and trace elements measured on the whole rock provide evidence of the evolutionary degree variations through time. Simulations by MELTS at fixed chemical-physical parameters allowed the definition of feeding system dynamics controlling the geochemical variability of magmas during the 2011–2013 period. Specifically, compositional changes have been interpreted as due to superimposition of fractional crystallization and mixing in variable proportions with more basic magma ascending from intermediate to shallower levels of the plumbing system. Composition of the recharging end-member is compatible with that of the most basic magmas emitted during the 2007 and the early paroxysmal eruptions of 2012. Analysis of the erupted volumes of magma combined with its petrologic evolution through time support the idea that large volumes of magma are continuously intruded and stored in the intermediate plumbing system after major recharging phases in the deepest levels of it. Transient recharge from the intermediate to the shallow levels is then responsible for the paroxysmal eruptions.

Timescales of magma storage and migration recorded by olivine crystals in basalts of the March–April 2010 eruption at Eyjafjallajökull volcano, Iceland

Abstract The early eruptive phase of the 2010 eruption at the Fimmvörðuháls Pass, east of Eyjafjallajökull volcano, produced poorly evolved basalts with mildly alkaline affinity, and benmoreitic tephra were emitted during the second explosive phase from the summit vent of the volcano. In this study, textural features and chemical zoning preserved in olivine crystals of the early erupted basalts have been used to define the timescales of differentiation processes and magma ascent before the eruption. These lavas contain a mineral assemblage constituted by olivine (Fo70–88) and plagioclase (An57–83) in similar proportions with scarce clinopyroxene and opaque oxides. Olivine occurs as euhedral or embayed crystals characterized by different core compositions and zoning patterns. Three main olivine populations have been found, namely crystals with: (1) wide Fo88 cores with normal zoning toward narrow rims (P1); (2) ∼Fo81 cores with either no zoning or slight reverse zoning patterns toward the rims (P2); (3) ∼Fo77 cores with reverse zoning at the rims (P3). The olivine reverse zoning indicates that these poorly evolved magmas experienced mixing processes in addition to limited fractional crystallization at different levels of the plumbing system. Timescales of transfer dynamics before the eruption have been estimated through Fe-Mg diffusion modeling on these olivine populations. The olivine-melt equilibration through diffusion was triggered by interaction of magmas differing in their evolutionary degree. P1 and P2 crystals recorded a first event of interaction in a ∼22 km deep reservoir that took place about one month before the emission of the analyzed products. Only part of P2 crystals records reverse zoning due to interaction with more basic magma bearing P1 crystals (which consequently develop normal zoning), suggesting fast timescales of magma mixing that prevented the complete homogenization. A second mixing event, which is evident in the P3 olivines, occurred at shallower levels (5–6 km of depth) ∼15 days before the emplacement and can be considered the triggering mechanism leading to the eruption at the Fimmvörðuháls Pass. Integration of our timescales with seismic data relative to the hypocenter migration indicate rates of magma ascent throughout the deep plumbing system of ∼0.01 m/s. This study provides evidence that magmas emitted at Eyjafjallajökull volcano, and more in general at similar other volcanic systems in ocean ridge settings, can undergo complex processes during their storage and transport in the crust, chiefly due to the presence of a multilevel plumbing system.

Fluorophlogopite from Piano delle Concazze (Mt. Etna, Italy): Crystal chemistry and implications for the crystallization conditions

Abstract Fluorine is an important proxy for magmatic differentiation processes in the shallow parts of volcanic plumbing systems. Fluorphlogopite is one of the more important fluorine carriers in magmatic rocks. In the present study, a full crystal chemical investigation of fluorophlogopite 1M from Piano delle Concazze, Mt. Etna volcano, Italy, is carried out. The fluorophlogopite occurs in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive center; ~15 ka BP). It is primarily associated with fluorapatite covered with amorphous SiO2 and crystallized during syn/post-eruption pneumatolytic stages. The mica sample studied here is among the most Fe- and Ti-rich fluorophlogopite found in nature. EPMA data yielded the following mean chemical formula for this mineral (K0.83Na0.13)(Fe2+0.44Fe3+0.09Mg2.18Al0.05Ti0.23Mn0.01)(Al0.92Si3.08)O10.64(Cl0.01F1.35). Structure refinements on four fluorophlogopite crystals, performed in space group C2/m, converged at R = 0.03-0.04, with cell parameters in the ranges a = 5.323-5.324, b = 9.219-9.222, c = 10.116- 10.119 Å, β = 100.1-100.3°. Major substitutions are OH- ↔ F-,M3+-oxy (VIM2++OH- ↔ VIM3++O2-) and Ti-oxy substitution: VIM2++2(OH)- ↔ VITi4++2O2-. The fluorophlogopite from Piano delle Concazze exhibits the shortest c-parameter with respect to other fluorophlogopites found in nature. The short c parameter is essentially due to the absence of the hydroxyl group in favor of F- and especially of O2- and to the thus increased attractive interaction between the interlayer cation and the anion content (F-, O2-) located at the O4 site. A comparison with other natural fluorophlogopites (namely from Biancavilla, Etna and Presidente Olegario, Brazil) show intermediate crystal-chemical features for the Piano delle Concazze fluorophlogopite. Particularly at Etna, differences in the chemical composition of the crystallized fluorophlogopites could be related to the various extent of enrichments by transfer of a gas phase achieved in specific parts of the volcanic plumbing system.

Halogen-dominant mineralization at Mt. Calvario dome (Mt. Etna) as a response of volatile flushing into the magma plumbing system

The exceptional occurrence of fluorine-rich mineral phases in the benmoreitic lava dome of Mt. Calvario (south-western flank of Mt. Etna) has given the opportunity to understand the genetic process allowing their crystallization. Both primary and secondary mineral associations were found, namely: plagioclase, clinopyroxene, olivine, fluorapatite and iron oxides as primary assemblage, whereas fluoro-edenite and fluorophlogopite, ferroan-enstatite, hematite, pseudobrookite and tridymite as secondary mineralization. In addition to some major and trace elements (e.g., Fe, Ti, Na, K, P, Ba, Rb, Sm, Zr), particularly fluorine and chlorine concentrations of the whole rock are significantly higher than other Etnean prehistoric benmoreites, and cannot be accounted for common differentiation processes in the feeding system. The selective enrichment in some elements has been here attributed to volatile flushing occurring in the plumbing system, with fluid/melt ratio of ~0.65:1. The resulting high amount of fluorine, coupled with its high solubility even at low pressure for benmoreitic melts, finally led to nucleation and growth of F-rich mineral phases during syn- and post-eruptive conditions.

Influx of volatiles into shallow reservoirs at Mt. Etna volcano (Italy) responsible for halogen-rich magmas

The study of F-rich mineral phases, namely fluorophlogopite and fluorapatite, occured in a benmoreitic lava from prehistoric volcanic activity at Mt. Etna (post-caldera forming phase of the “Ellittico” eruptive centre; ~15 ka BP) allowed us to define the physical and chemical crystallization conditions of such minerals. Textural evidence suggests a late-stage crystallization of the F-rich minerals, since fluorapatite is exclusively found in the groundmass and fluorophlogopite within lava vesicles. Furthermore, a colourless SiO 2 -rich amorphous phase, characterized by multi-stage deposition, has overgrown the fluorophlogopite crystals. Comparison with simulations of crystal fractionation demonstrates that the benmoreitic lava characterized by the occurrence of F-dominant minerals is anomalously enriched in some major and trace elements ( e.g. , Ti, Fe, K, Ba and, to a minor extent, Rb and REEs). Even the modelling of crustal contamination, possibly caused by assimilation of the sedimentary basement underlying the volcano edifice, is poorly consistent with the geochemical features of the considered benmoreite. Chlorine and fluorine concentrations estimated for this lava sample are 0.20 and 0.34 wt% respectively, which are significantly higher than those of other Etnean prehistoric mugearites and benmoreites. The selective enrichment in major and trace elements, and particularly in halogens, has been therefore related to other rarely recognized differentiation processes acting in the feeding system. Specifically, volatile-induced differentiation, ruled by elemental transfer (as metal-halogen complexes) in a volatile phase, is able to account for the observed geochemical variations. Such a volatile influx might be released by more primitive, deeper and volatile-rich magmas while rising up towards shallower levels of the feeding system. Considering the solubility of fluorine in silicatic systems at low pressure higher than that of chlorine, we suggest that fluorapatite and fluorophlogopite were likely grown during syn- or post-eruption pneumatolytic stages, probably after open-system degassing when a gas phase characterized by a high Cl/F ratio was released. The paramount role played by volatiles is also consistent with the occurrence of SiO 2 -rich amorphous concretions surrounding the fluorophlogopite crystals. Indeed, large amounts of SiF 4 in the gas phase can sublimate under cooling conditions into Si-rich amorphous concretions. On the grounds of our findings, the process here described could have significant implications to explain unexpected eruptive behaviours at Mt. Etna, such as highly explosive dynamics of extrusion or the rather low viscosity of highly evolved lavas.

Comment on “Complex magma dynamics at Mount Etna revealed by seismic, thermal, and volcanological data” by B. Behncke, S. Falsaperla, and E. Pecora

[1] Behncke et al. [2009] deal with three paroxysms which occurred at the Southeast Crater (SEC) of Mount Etna on 16, 19, and 24 November 2006. In particular, the first of these was an exceptional eruptive episode. During this event, a small but significant pyroclastic flow took place at the southeastern base of the SEC. Erroneous assumptions and omissions of evidences affect this paper in particular with regards to the interpretation of the 16 November paroxysm. We believe that, if correctly presented, the assumptions contained in the work would completely reverse the conclusions. [2] In their paper, Behncke et al. integrate volcanological observations and seismic and thermal data to prove that the short (<2 min), violent outburst, which occurred at the southeastern base of the SEC at 1425 UT and gave rise to the small pyroclastic flow, occurred when the lava flow, which has been coming out for several hours from the summit of the SEC, suddenly interacted with water-saturated sediments and interlayered snow. This idea constituted the basis of the recently published papers by Behncke et al. [2008] and Behncke [2009]. This occurrence has posed some intriguing questions concerning the conditions that can give origin to such episodes in basaltic volcanoes, usually characterized by the quiet emission of lava flows and Strombolian activity. As it is often the case when dealing with natural phenomena, other interpretations are possible. In particular, a different interpretation was proposed by Ferlito et al. [2007, also Relationship between the flank sliding of the South East Crater (Mt. Etna, Italy) and the paroxysmal event of November 16, 2006, submitted to Bulletin of Volcanology, 2009], who envisage the outburst as associated with a rapid and superficial fracturing (<250m in depth) occurred at the base (3050m above sea level (asl)) of the SEC due to the overpressure of a small, gas-rich batch of magma. [3] One of the fundamental points emphasized by Behncke et al. [2009] regarding the paroxysm of 16 November is the absolute lack of eruptive fracturing associated with the outburst. The evidence used to support this view is the absence of specific seismic signals associated with fracture opening, even considering that at the moment of the paroxysm the seismic network was integrated with four stations close to the Etnean summit craters (EPLC, ECPN, EPDN, EBEL). In particular, they write (paragraph 54) that ‘‘the lack of any peculiar trace in the seismic record along with the observation that the opening of eruptive fractures at Etna is usually associated with changes in seismic activity [e.g., Patane et al., 2004] leads us to support the hypothesis that the mechanism at the origin of the PDCs was extremely shallow and rootless, as proposed by Behncke et al. [2008].’’ But this sentence is quite dissimilar to that expressed by Patane et al. [2004], reiterated also in the conclusive remarks of Patane et al. [2005, paragraph 12], who conversely stated that ‘‘a repeatedly observed feature at Mt. Etna is that the summit eruptions are not generally preceded by significant variations in the pattern of deformation and volcano-tectonic seismicity [Patane et al., 2004].’’ A conspicuous literature exists with regard to this matter [cf. Cosentino et al., 1989; Gresta and Patane, 1987; Cardaci et al., 1993; Ferrucci and Patane, 1993; Patane et al., 2003]. The lack of specific seismic signals recorded during fracture opening has lately been confirmed at the beginning of the 2004–2005 eruption. In this regard, Di Grazia et al. [2006, paragraph 1] write that ‘‘neither earthquake seismicity heralded or accompanied the opening of the fracture field from which the lava poured out, nor volcanic tremor changed in amplitude and frequency content at the onset of the effusive activity.’’ The eruptive event of 2004–2005, which, according to Di Grazia et al. [2006], did not cause any particular seismicity, was not a <2 min outburst of lava but a 6 month long eruption that emitted 60 10 m of lava [Neri and Acocella, 2006] and had a complex feeding history, involving also the occurrence of shallow magma mixing [Corsaro et al., 2009]. These observations should suffice to consider with more caution the use and interpretation of the seismic signature or its lack as associated to fracture opening at Mount Etna. Furthermore, the volcanic tremor level recorded at all the 4 summit stations at the moment of the outburst was extremely high [see Behncke et al., 2009, Figure 10]. Under these conditions it is unconvincing to affirm the absence of the variation of seismic signal for low magnitude events. Consider that the fracture opened along an E-W direction at 3050 m asl and intersected magma that was just below the surface (the erupting vents at the moment of the outburst JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B12204, doi:10.1029/2009JB006511, 2009 Click Here for Full Article