Marco Viccaro

Compositionally zoned crystals and real-time degassing data reveal changes in magma transfer dynamics during the 2006 summit eruptive episodes of Mt. Etna

One of the major objectives of volcanology remains relating variations in surface monitoring signals to the magmatic processes at depth that cause these variations. We present a method that enables compositional and temporal information stored in zoning of minerals (olivine in this case) to be linked to observations of real-time degassing data. The integrated record may reveal details of the dynamics of gradual evolution of a plumbing system during eruption. We illustrate our approach using the 2006 summit eruptive episodes of Mt. Etna. We find that the history tracked by olivine crystals, and hence, most likely the magma pathways within the shallow plumbing system of Mt. Etna, differed considerably between the July and October eruptions. The compositional and temporal record preserved in the olivine zoning patterns reveal two mafic recharge events within months of each other (June and September 2006), and each of these magma supplies may have triggered the initiation of different eruptive cycles (July 14–24 and August 31–December 14). Correlation of these observations with gas monitoring data shows that the systematic rise of the CO2/SO2 gas values is associated with the gradual (pre- and syn-eruptive) supply of batches of gas-rich mafic magma into segments of Etna’s shallow plumbing system, where mixing with pre-existing and more evolved magma occurred.

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.

Amphibole crystallization in the Etnean feeding system: mineral chemistry and trace element partitioning between Mg-hastingsite and alkali basaltic melt

Amphiboles are rather rare in the volcanics of the whole Etnean succession and commonly are represented by kaersutites to titanian pargasites, mostly found in differentiated products. Titanian Mg-hastingsites have been found in lavas and tephra from the 2001 eruption at Mt. Etna. New major (EMPA) and trace element (LAM-ICP/MS) data on amphiboles from this eruption have been compared with reference data for kaersutites from prehistoric eruptions. The two amphibole groups significantly differ from each other in their AlIV, AlVI, K and Mg# values, which are higher in Mg-hastingsite than in kaersutite. Ti and Na are lower in Mg-hastingsite than in kaersutite. REE and trace element patterns for all the analysed Mg-hastingsite crystals are quite homogeneous. Kaersutite patterns generally conform to those of Mg-hastingsite but display higher concentrations for most of the trace elements. The exceptional occurrence, exclusively in tephra, of some crystals under equilibrium conditions with the coexisting residual glass has made it possible to calculate the partition coeffcients between amphibole and melt (Amph/melt D ) for trace elements. A new set of partition coeffcients is then provided, deriving from analyses on five amphibole/melt pairs at equilibrium. These data highlight the effects of amphibole crystallization in controlling some trace element ratios ( e.g. Th (or U)/Ta, Th/Nb, La/Nb) in residual melts of alkali basaltic systems, and suggest new hints for interpreting possible geochemical anomalies of these magmas. In addition, the comparison between the calculated Amph/melt D of Mg-hastingsite and those from literature relative to kaersutite from prehistoric eruptions shows that they are generally lower in the former than in the latter one for most of the trace elements. All of the available data provide constraints on the physical growth conditions for the 2001 Mg-hastingsite. Temperatures around 980 °C and volatile pressures in the range of 200–300 MPa have been estimated by integrating geophysical and petrological data. The highest pressure values are however larger than the lithostatic pressure alone acting on the magma reservoir (~ 6 km b.s.l.), as defined on the grounds of the hypocentres depth of the seismic events associated to the magma rise. This implies that Mg-hastingsite was probably crystallizing in a closed reservoir under overpressure conditions. Finally, microchemical data and trace element partitioning suggest that the differences between Mg-hastingsites and kaersutites in the Etnean products are mainly due to the less differentiated character of the magmas emitted after the 1971, and particularly after the 2001 eruption, compared to the compositions that characterize products of the prehistoric events. Furthermore, also a higher pressure of the system where Mg-hastingsite crystallized would account for its compositional differences respect to prehistoric kaersutite.

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.

How a complex basaltic volcanic system works: Constraints from integrating seismic, geodetic, and petrological data at Mount Etna volcano during the July-August 2014 eruption

The petrological part of this study was supported by the FIR 2014 research grant to Marco Viccaro from the University of Catania (Italy), grant number 2F119B, title of the project “Dynamics of evolution, ascent and emplacement of basic magmas: case-studies from eruptive manifestations of Eastern Sicily”.

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.

Ultrafast syn-eruptive degassing and ascent trigger high-energy basic eruptions

Lithium gradients in plagioclase are capable of recording extremely short-lived processes associated with gas loss from magmas prior to extrusion at the surface. We present SIMS profiles of the 7Li/30Si ion ratio in plagioclase crystals from products of the paroxysmal sequence that occurred in the period 2011–2013 at Mt. Etna (Italy) in an attempt to constrain the final ascent and degassing processes leading to these powerful eruptions involving basic magma. The observed Li concentrations reflect cycles of Li addition to the melt through gas flushing, and a syn-eruptive stage of magma degassing driven by decompression that finally produce significant Li depletion from the melt. Modeling the decreases in Li concentration in plagioclase by diffusion allowed determination of magma ascent timescales that are on the order of minutes or less. Knowledge of the storage depth beneath the volcano has led to the quantification of a mean magma ascent velocity of ~43 m/s for paroxysmal eruptions at Etna. The importance of these results relies on the application of methods, recently used exclusively for closed-system volcanoes producing violent eruptions, to open-conduit systems that have generally quiet eruptive periods of activity sometimes interrupted by sudden re-awakening and the production of anomalously energetic eruptions.

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.

A Branched Magma Feeder System during the 1669 Eruption of Mt Etna: Evidence from a Time-integrated Study of Zoned Olivine Phenocryst Populations

The 1669 eruption of Mt Etna was one of the most voluminous and devastating of its flank eruptions in historical times. Despite a large body of relevant research, knowledge of the timing and duration of magma transfer and magma recharge through the internal plumbing system preceding and during the eruption is still limited. To address that lack of knowledge, we apply a three-way integrated method, linking systems analysis of crystals, a time-integrated study of zoned olivine populations, and a forward-modelling approach using thermodynamic calculations. Analysis of 202 olivine crystals erupted during the initial (pre-March 20, i.e. SET1) and the final (post-March 20;i.e. SET2 and MtRs) stages of the eruption reveals the existence of three magmatic environments (MEs) in which the majority of the olivine cores [M-1 (= Fo(75-78))] and rims [i.e. M-5 (= Fo(51-59)) and M-3 (= Fo(65-69))] formed. Application of the rhyolite-MELTS software allowed us to constrain the key intensive variables associated with these MEs. We find that temperature, water content and oxidation state vary between these MEs. Application of diffusion modelling to the zoned olivine crystals allowed us to reconstruct the timing and chronology of melt and crystal transfer prior to and during the 1669 flank eruption. We find that, following the formation of the olivine cores [M-1 (= Fo(75-78))], the reservoir M-1 was intruded by batches of more evolved, degassed and possibly aphyric M-5-type magma, commencing 1 center dot 5 years prior to eruptive activity. This is the origin of the SET1 olivine rims (i.e. Fo(51-59)). In the months prior to eruption, timescale data show that recharge activity along the newly established pathway M-1-M-5 increased notably. Starting in November 1668, only a few weeks after the first intrusive episode into the M-1 reservoir, a second pulse of magma injections (M-3-type magma) occurred and a new pathway M-1-M-3 opened;this is how the SET2 olivine rims (i.e. Fo(65-69)) formed. For several weeks a bifurcated transport system with two dominant magma pathways developed along M-1-M-5 and M-1-M-3 dyke injections. Accompanied by vigorous seismicity, in the days immediately before eruption the local magma transfer dynamics changed and the M-1-M-5 recharge activity slowed down, as shown by a relative lack of crystals recording shorter timescales. M-1-M-3 recharge, however, remained high and persisted following the eruption onset on March 11, during which the SET1 lavas were drained. We propose that the change of the local magma transfer dynamics might be linked to changes in the local stress field brought on during eruption. This may potentially have been due to repeated dyke injections into Etnas shallow plumbing system disrupting the early M-1-M-5 pathway and at the same time stabilizing the M-1-M-3 route as a dominant feeder. This transfer of system feeding would reproduce the observed syn-eruptive recharge and mixing in the weeks following eruption onset, culminating in the eruption of the later SET2 lavas.