Etnean Magmas Research Articles

Kinetic partitioning of trace cations between zoned clinopyroxene and a variably cooled-decompressed alkali basalt: Thermodynamic considerations on lattice strain and electrostatic energies of substitution

We present kinetic partitioning data for trace cations measured in zoned clinopyroxene crystals obtained from a variably cooled and decompressed olivine basalt erupted at Mt. Etna volcano in Italy. Supersaturation effects and compositional heterogeneities at the interface melt lead to the development of sector zoning, concentric zoning, and patchy zoning in clinopyroxene crystals. Apparent partition coefficients between compositionally different growth layers and adjacent melts (Di) for isovalent groups of trace elements are tested for internal consistency on the thermodynamic basis of lattice strain (ΔGstrain) and electrostatic (ΔGelec) energies of substitutions. The excess energy of partitioning (ΔGpartitioning) for trace cations in zoned crystals accounts for a kinetic incorporation control leading to large enthalpic effects through distortion of the lattice and changes in the electrostatic forces. ΔGpartitioning depends upon the complementary relationship between ΔGstrain and ΔGelec, which is the most appropriate thermodynamic description for the accommodation of rare earth elements and high field strength elements in the lattice site of zoned crystals. Polyhedral sectors, skeletal forms, and overgrowth zones have Di values settled by the number of charge-balanced and -imbalanced configurations taking place in the lattice site as a function of aluminium in tetrahedral coordination, and crystal structural changes produced by heterovalent cation substitutions. In an energetically unstable macroscopic system ruled by cooling and decompression, thermodynamic requirements for the crystallochemical control of Di encompass the attainment of local equilibrium at the crystal-melt interface via the establishment of small-volume reaction kinetics. The requisite of local interface equilibrium is however susceptible to the anisotropic growth velocity of each specific clinopyroxene surface, thereby giving reason to different energetic properties of the crystallographic site. This axiomatic control requires that transition metal cations partition also in consideration of electronic effects related to the crystal field stabilization energy. The overriding implication is that Di values for trace cations having different size, charge, and electronic configuration serve as sensitive probes of the different crystal growth mechanisms, surface incorporation sites, and arrangements of atoms at the lattice-scale. In this perspective, fractional crystallization modeling of 2011–2013 bulk rock data from lava fountains indicates that the compositional evolution of magmas erupted at Mt. Etna cannot be described by a unique equilibrium value of Di for a given clinopyroxene-melt interface. The leverage of interface kinetics is distinctively dominant along the subvolcanic plumbing system, thereby requiring that values of Di differ for structurally and compositionally distinct zones in clinopyroxene phenocrysts. To successfully interpret the trace element signature of Etnean magmas, the archetypal constancy of partition coefficient at bulk thermodynamic equilibrium must be in some measure reappraised in favor of the establishment of a local interface equilibrium upon highly dynamic crystallization and growth conditions.

Parameterization of clinopyroxene growth kinetics via crystal size distribution (CSD) analysis: Insights into the temporal scales of magma dynamics at Mt. Etna volcano

There is increasing recognition that both textural and compositional changes of clinopyroxene crystallizing from mafic alkaline magmas are the direct expression of complex dynamic processes extending over a broad range of spatial and temporal scales. Among others, supersaturation and relaxation phenomena play a key role in controlling the final crystal cargo of variably undercooled magmas erupted from active alkaline volcanoes. Following this line of reasoning, we have carried out isothermal-isobaric, decompression, and cooling rate experiments on a basalt interpreted as the parental magma of mafic alkaline eruptions at Mt. Etna volcano (Sicily, Italy). The main purpose is to reconstruct and quantify the textural changes (i.e., length of major and minor axes, surface area per unit volume, area fraction, and maximum growth rate) of clinopyroxene at variable pressures (30–300 MPa), temperatures (1050–1100 °C), volatile contents (0–5 wt% H2O and 0–0.2 wt% CO2), and equilibration times (0.25–72 h). Melt supersaturation, corresponding to a degree of undercooling variable from 14 to 125 °C, drives the formation of clinopyroxene crystals with different textures and sizes as a function of growth rate and relaxation time. By integrating experimental data and thermodynamic modeling, the transition between interface-controlled (euhedral morphologies) and diffusion-controlled (anhedral morphologies) growth regimes has been determined to occur at degrees of undercooling higher than 30 °C. The decrease of clinopyroxene growth rate with increasing the equilibration time is combined with the crystal size distribution (CSD) analysis of naturally undercooled clinopyroxene crystals erupted during 2011–2012 lava fountain episodes at Mt. Etna volcano. Results indicate that the crystallization of microlites and microphenocrysts is on the order of ~100–101 min (large undercooling, short equilibration time) and ~ 101–102 h (small undercooling, long equilibration time), respectively. This temporal information allows to better constrain the cooling-decompression paths of Etnean magmas rising and accelerating along a vertically extended, highly dynamic plumbing system. While clinopyroxene microlites develop during the fast ascent of magmas (~100–101 m s−1) within the uppermost part of the conduit or immediately before ejection from the vent, the onset of microphenocryst crystallization occurs at depth and continues within the plumbing system during the slow ascent of magmas (~10−2 m s−1) that migrate through interconnected storage regions.

Sulphur behaviour and redox conditions in etnean magmas during magma differentiation and degassing

Abstract Sulphur behaviour and variations in redox conditions during magma differentiation and degassing in Mt. Etna (Italy) volcanic system have been explored by integrating the study of olivine-hosted melt inclusions (MIs) with an experimental survey of sulphur solubility in hydrous basaltic magmas. Sulphur solubility experiments were performed at conditions relevant to the Etnean plumbing system (1200 °C, 200 MPa and oxygen fugacity between NNO + 0.2 and NNO + 1.7, with NNO being the Nickel-Nickel Oxide buffer), and their results confirm the important control of oxygen fugacity (fO2) on S abundance in mafic magmas and on S partitioning between fluid and melt phases (DSfluid/melt). The observed DSfluid/melt value increases from 51 ± 4 to 146 ± 6 when fO2 decreases from NNO + 1.7 ± 0.5 to NNO + 0.3. Based on the calculated DSfluid/melt and a careful selection of previously published data, an empirical model is proposed for basaltic magmas in order to predict the variation of DSfluid/melt values upon variations in P (25–300 MPa), T (1030–1200 °C) and fO2 (between NNO-0.8 and NNO + 2.4). Olivine-hosted melt inclusions (Fo89-91) from tephra of the prehistoric (4 ka BP) sub-plinian picritic eruption, named FS (“Fall Stratified”), have been investigated for their major element compositions, volatile contents and iron speciation (expressed as Fe3+/ƩFe ratio). These primitive MIs present S content from 235 ± 77 to 3445 ± 168 ppm, while oxygen fugacity values, estimated from Fe3+/ƩFe ratios, range from NNO + 0.7 ± 0.2 to NNO + 1.6 ± 0.2. Iron speciation has also been investigated in more evolved and volatile-poorer Etnean MIs. The only primitive melt inclusion from Mt. Spagnolo eruption (4–15 ka BP) presents a S content of 1515 ± 49 ppm and an estimated fO2 of NNO + 1.4 ± 0.1. The more evolved MIs (from 2002/2003, 2006, 2008/2009 and 2013 eruptions) have S content lower than 500 ppm, and their Fe3+/ΣFe ratios result in fO2 between NNO-0.9 ± 0.1 and NNO + 0.4 ± 0.1. Redox conditions and S behaviour in Etnean magmas during degassing and fractional crystallization were modelled coupling MELTS code with our empirical DSfluid/melt model. Starting from a FS-type magma composition and upon decrease of T and P, fractional crystallization of olivine, clinopyroxene, spinel and plagioclase causes a significant fO2 decrease. The fO2 reduction, in turn, causes a decrease in sulphur solubility and an increase in DSfluid/melt, promoting S exsolution during magma ascent, which further enhances the reduction of fO2. For the evolved melt inclusions of 2002–2013 eruptions, magma differentiation may therefore have played a crucial role in decreasing redox conditions and favouring efficient S degassing. Differently, during the unusual FS eruption, only limited melt evolution is observed and S exsolution seems to have been triggered by a major pressure decrease accompanied by H2O and CO2 exsolution during fast magmatic ascent.

Melt inclusions track melt evolution and degassing of Etnean magmas in the last 15 ka

We present major elements compositions and volatiles contents of olivine-hosted melt inclusions from Etna volcano (Italy), which extend the existing database with the aim of interpreting the chemical variability of Etnean magmas over the last 15 ka. Olivine phenocrysts were selected from the most primitive Fall Stratified (FS) eruptive products of picritic composition (Mg# = 67–70, Fo89–91), the Mt. Spagnolo eccentric lavas (Mg# = 52–64, Fo82–88) and among the more recent 2002–2013 eruptive products (Mg# = 33–53, Fo68–83). Crystal fractionation and degassing processes were modeled at temperatures of 1050–1300 °C, pressures <500 MPa, and oxygen fugacity between 1 and 2 log units above the nickel-nickel oxide buffer, in order to interpret melt inclusions data.Melt inclusions show a great variability in major elements chemistry (e.g., 44–57 wt% SiO2, 3–16 wt% CaO, 4–13 wt% FeO, 2–12 wt% MgO, 1–6 wt% K2O), designating a continuous differentiation trend from FS toward 2013 entrapped melts, which is mostly reproduced by the fractional crystallization of olivine + spinel + clinopyroxene ± plagioclase, in order of appearance.Volatile contents in the glass inclusions are also extremely variable, with maxima up to 6 wt% H2O and 0.6 wt% CO2 in FS melt inclusions, and up to 0.43 wt% S in Mt. Spagnolo inclusions. H2O and CO2 contents in the melt inclusions suggest minimum entrapment depths of 4–19 km (below crater level) for FS inclusions and <10 km for the 2002–2013 trachybasalts.Petrological arguments coupled to the modeling of fractional crystallization and degassing processes concur to suggest that magmas from Mt. Spagnolo and the recent eruptions may be produced by differentiation from the most primitive volatile-rich FS magma along variable P-T paths, occasionally accompanied by secondary processes as crustal assimilation, mixing, and CO2 flushing. We do not exclude the occurrence of source processes at Etna, e.g., variable degrees of mantle melting and/or variable degrees of mantle contamination, already proposed by previous authors. Our data, nevertheless, suggest that the first-order features of the Etnean magmas erupted in the last 15 ka can be modeled by differentiation through fractional crystallization and degassing.

An improved clinopyroxene-based hygrometer for Etnean magmas and implications for eruption triggering mechanisms

We have refined the clinopyroxene-based hygrometer published by Armienti et al. (2013) for a better quantitative understanding of the role of H2O in the differentiation of Etnean magmas. The original calibration data set has been significantly improved by including several experimental clinopyroxene compositions that closely reproduce those found in natural Etnean products. To verify the accuracy of the model, some randomly selected experimental clinopyroxene compositions external to the calibration data set have been used as test data. Through a statistic algorithm based on the Mallows’ CP criterion, we also check that all model parameters do not cause data overfitting, or systematic error. The application of the refined hygrometer to the Mt. Etna 2011–2013 lava fountains indicates that most of the decreases in H2O content occur at P < 100 MPa, in agreement with melt inclusion data suggesting abundant H2O degassing at shallow crustal levels during magma ascent in the conduit and eruption to the surface.

Experimental constraints on the origin of pahoehoe “cicirara” lavas at Mt. Etna Volcano (Sicily, Italy)

We present results from phase equilibria experiments conducted on the most primitive pahoehoe “cicirara” trachybasaltic lava flow ever erupted at Mt. Etna Volcano. This lava is characterized by a pahoehoe morphology in spite of its high content of phenocrysts and microphenocrysts (>40 vol%) with the occurrence of centimetre-sized plagioclases (locally named cicirara for their chick-pea-like appearance). Our experiments have been performed at 400 MPa, 1100–1150 °C and using H2O and CO2 concentrations corresponding to the water-undersaturated crystallization conditions of Etnean magmas. Results show that olivine does not crystallize from the melt, whereas titanomagnetite is the liquidus phase followed by clinopyroxene or plagioclase as a function of melt–water concentration. This mineralogical feature contrasts with the petrography of pahoehoe cicirara lavas suggesting early crystallization of olivine and late formation of titanomagnetite after plagioclase and/or in close association with clinopyroxene. The lack of olivine produces MgO-rich melt compositions that do not correspond to the evolutionary behaviour of cicirara magmas. Moreover, in a restricted thermal path of 50 °C and over the effect of decreasing water concentrations, we observe abundant plagioclase and clinopyroxene crystallization leading to trace element enrichments unlikely for natural products. At the same time, the equilibrium compositions of our mineral phases are rather different from those of natural cicirara phenocrysts and microphenocrysts. The comparison between our water-undersaturated data and those from previous degassing experiments conducted on a similar Etnean trachybasaltic composition demonstrates that pahoehoe cicirara lavas originate from crystal-poor, volatile-rich magmas undergoing abundant degassing and cooling in the uppermost part of the plumbing system and at subaerial conditions where most of the crystallization occurs after the development of pahoehoe surface crusts.

The magma source at Mount Etna volcano: Perspectives from the Hf isotope composition of historic and recent lavas

High-precision Hf isotopic data for selected historic (pre-1971 eruption) and recent (post-1971) Etnean lavas are discussed in light of their covariation with previously reported Sr–Nd–Pb isotope ratios. 176Hf/ 177Hf ranges between 0.282961 and 0.282989 in the historic lavas, and between 0.282943 and 0.282990 in the recent ones, displaying, especially among the most recently erupted products, an overall decrease. In Hf–Nd and, to a minor extent, Hf–Sr–Pb isotope spaces, Etnean lavas plot slightly below the linear array defined by MORB and OIB, between the enriched end of the FOZO and HIMU fields. This may suggest that, prior to the formation of Etnean melts, the garnet-bearing source experienced an episode of low-degree melting; this source then evolved for enough time to produce the 176Hf/ 177Hf and 143Nd/ 144Nd decoupling observed in the historic and recent lavas. In addition, the distinct εHf values of the historic and recent lavas may be accounted for by portions of the mantle with distinct Lu/Hf ratios, likely related to decreasing modal proportions of garnet in favour of metasomatic phases involved in partial melting. Integration of all the available data suggests that FOZO is the dominant component in the source of Etnean magmas, although this is not enough to solve the observed 176Hf/ 177Hf change coupled with the Sr–Nd–Pb variation through time. Binary mixing between FOZO and EM-type components demonstrates that addition of about 10% of an EM1-type component to FOZO can account for the isotopic variability of Etnean magmas, a feature more emphasized in the most recent lavas. Divergences from a dominant FOZO signature of erupted magmas at Mt. Etna may therefore be attributed to small-scale heterogeneity produced by access into the source of enriched material with an EM1-type signature. The coupled analysis of 176Hf/ 177Hf and trace element ratios, such as Sm/Hf and Th/Hf, provides evidence that this enriched infiltrating component could be metasomatizing silicate melts rather than aqueous, low-T fluids.

The role of syn-eruptive vesiculation on explosive basaltic activity at Mt. Etna, Italy

We investigated the dynamics of explosive activity at Mt. Etna between 31 August and 15 December 2006 by combining vesicle studies in the erupted products with measurements of the gas composition at the active, summit crater. The analysed scoria clasts present large, connected vesicles with complex shapes and smaller, isolated, spherical vesicles, the content of which increases in scoriae from the most explosive events. Gas geochemistry reports CO 2/SO 2 and SO 2/HCl ratios supporting a deep-derived gas phase for fire-fountain activity. By integrating results from scoria vesiculation and gas analysis we find that the highest energy episodes of Mt. Etna activity in 2006 were driven by a previously accumulated CO 2-rich gas phase but we highlight the lesser role of syn-eruptive vesicle nucleation driven by water exsolution during ascent. We conclude that syn-eruptive vesiculation is a common process in Etnean magmas that may promote a deeper conduit magma fragmentation and increase ash formation.

Nature of mantle heterogeneity and its role in the short-term geochemical and volcanological evolution of Mt. Etna (Italy)

Clues for investigating on the origin and short-term geochemical changes of erupted magmas at Mt. Etna have arisen from an extensive sampling of historic (pre-1971) and recent (post-1971) volcanic rocks, mostly carried out on the southern flank of the volcano. Major and trace element data, obtained by WD-XRF and ICP-MS respectively, display rather similar abundances in both the historic and recent products; only K 2O (mean 1.43 and 1.93 wt.% respectively) and Rb (mean 32 and 50 ppm respectively) exhibit a significant time-correlated increase, whereas Th (mean 10.1 and 9.4 ppm respectively) gradually decreases. Radiogenic isotope ratios, obtained by TIMS, increase through time for 87Sr/ 86Sr (from 0.703343 to 0.703624), whereas for Pb (i.e., 206Pb/ 204Pb from 19.93 to 19.73) and 143Nd/ 144Nd (from 0.512908 to 0.512852) the ratios gradually decrease. Finally, δ 18O values, obtained by dual inlet mass spectrometry, slightly decrease from the historic to recent products (mean + 6.6‰). In order to interpret the observed trends, the role of magma evolution processes in the shallow feeding system was first tested by numerical modeling. Results show that both fractional crystallization and crustal assimilation cannot fully account for the observed variations of some trace elements (particularly Rb and Th) as well as of 87Sr/ 86Sr and δ 18O. On the other hand, partial melting modeling, performed with pre-1971 lava compositions equilibrated to those of mantle melts, supports the occurrence of accessory hydrous phases, such as amphibole and phlogopite, in the source of Etnean magmas. Partial melting of a source characterized by increased amounts of hydrous phases appears consistent with the peculiar geochemical features of the post-1971 magmas. Our findings imply that metasomatic influxes could have heterogeneously affected the pristine mantle, and that the degree of partial melting of the volatile-enriched portions increased. In accordance with trace element and isotope ratios, the model proposed for the genesis and evolution of magmas takes into account melting of a depleted mantle re-fertilized (hundreds of Ma ago) by plume-related silicate melts/fluids, inheriting a signature likely derived from ancient hydrothermally altered oceanic crust stored for long times in the mantle. A limited mantle upwelling, due to the complex recent geodynamics of Southern Italy, could have easily triggered partial melting of this heterogeneously metasomatized mantle, which produced magmas differing in their composition depending on the involved mantle domain. Since the increased extent of partial melting results in a larger melt production, our data are also consistent with the observed volcanic behavior, and could account for the increased frequency of eruptive events and emission rates of volatile-rich magmas observed since 1971, and especially after the 2001 eruption.

Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy)

A set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline magmas feeding Mt. Etna volcanic system. The obtained data set forms the first attempt to characterise the solid/melt trace-element partitioning for Etnean magmas, and can be usefully applied in other sites of alkaline volcanism. The partition coefficients are here used to define the scale of incompatibility of 29 trace elements and to asses the extent of differentiation processes and the prevailing oxygen fugacity of Ellittico magmas.

CO 2 output and δ 13 C(CO 2 ) from Mount Etna as indicators of degassing of shallow asthenosphere

An estimated average CO2 output from Etna's summit craters in the range of 13±3 Mt/a has recently been determined from the measured SO2 output and measured CO2/SO2 molar ratios. To this amount the CO2 output emitted diffusely from the soil (≈ 1 Mt/a) and the amount of CO2 dissolved in Etna's aquifers (≈ 0.25 Mt/a) must be added. Data on the solubility of CO2 in Etnean magmas at high temperature and pressure allow the volume of magma involved in the release of such an amount of this gas to be estimated. This volume of magma (≈ 0.7 km3/a) is approximately 20 times greater than the volume of magma erupted annually during the period 1971–1995. On the basis of C-isotopic data of CO2 collected in the Etna area and of new hypotheses on the source of Mediterranean magmas, significant contributions of CO2 from non-magmatic sources to the total output from Etna are unlikely. Such large outputs of CO2 and also of SO2 from Etna could be due to an anomalously shallow asthenosphere beneath the volcano that allows a continuous escape of gases toward the surface, even without migration of magma.

The influence of loading by lavas on the siting of volcanic eruption vents on Mt Etna

Until now it has been considered that the positioning of magma conduits, dykes and eruption sites on Mt Etna is controlled by stresses set up by regional tectonism. It is also likely that gravitational stresses have some influence on the subsurface path taken by Etnean magmas prior to eruption. An additional effect discussed here is based mainly on convincing field evidence gathered over a number of years. It is proposed that fracturing of the surface and changes in the stress pattern due to loading by recent lava flows may strongly influence the siting of eruptions. Detailed field observations are presented, showing that cracks and fumaroles have consistently appeared round the edges of recent lava flows, and that these cracks are sometimes later used by erupting lavas. Eruption statistics indicate that since 1970, 50% of new eruptive vents have appeared within 80 m of a recent flow edge, and field measurements show that downward flexure is still occurring at the edges of lava flows more than sixteen years old. Simple theoretical modelling indicates that this effect is strongest at the surface, with gravitational and regional tectonic control of eruption sites dominating at progressively deeper levels.