Renato Cristofolini

Feeding Mechanism of Eruptive Activity at Mt. Etna Based on Seismological and Petrological Data

Seismic and petrological data are analyzed with reference to the eruptive phenomena at Mt. Etna from 1974–1984. Seismological data show that most earthquakes occur within the upper 16 km of the crust and very few below 30 km. Focal solutions for events at depth between 7 and 16 km show evidence of both normal and thrust faults, due to an unstable stress field, whereas at shallower levels only normal mechanisms are observed. In most cases, enhanced seismic activity preceded the beginning of adventive eruptions, but neither upward migration nor clear concentration of epicenters near the eruptive area (except for the shallowest events; h < 1 km) was observed.

An approach to problems on energy sources at Mount Etna based on seismological and volcanological data

Based on seismograms from the most continuously operating station of the seismic network at Mt. Etna (Monte Vetore, 1665 m a.s.l.), the energy release patterns through time have been obtained for volcanic tremors and earthquakes which occurred between 1978–1982. Both energies range between 1011∓1013 J/a, but their release patterns are not strictly correlated to each other. Considering only the eruptions which occurred during the same time span, the amount of thermal energy released and the associated potential energy needed to raise the magma to the surface from a 2-km-deep hydrostatic equilibrium level were estimated to be about 1017 and 1015 J/a, respectively. The computed energies for earthquakes and tremors are at least one order of magnitude less than what is needed to match a model of magma transport based solely on the jerky propagation of melt-filled cracks. The energy needed to raise the magma to the surface could be supplied by expanding gases in the upper levels of the magma column and/or by tectonic stresses acting on shallow batches of magma.

Petrological notes on the 1983 lavas at Mount Etna, Sicily, with reference to their REE and Sr Nd isotope composition*

The 1983 hawaiite of Mount Etna was sampled and analyzed for groundmass and mineral compositions, rare-earth-element concentrations and Sr-Nd isotope ratios.Microprobe data for coexisting mineral phases and glass show crystallization temperatures of around 1100° C from a rather differentiated hawaiite magma at rather highfO2 (10−8 at 1100° C), well above the QFM buffer.The hawaiites are characterized by a marked enrichment in the light REE similar to other alkaline magmas: the (Ce)N/(Yb)N is greater than 10, a feature these hawaiites have in common with alkaline magmas erupted earlier on Mount Etna. Since the Ce/Yb ratio cannot be affected by fractionation of clinopyroxene and plagioclase, it is taken as an accurate reflection of the LREE-enriched nature of the hawaiites. From this point of view, the Etnean hawaiites are identical to within-plate alkaline magmas erupted on the Hawaiian islands.This similarity extends to the Nd-Sr isotope features. Two hawaiites have87Sr/86Sr=0.70346 and 0.70352 and143Nd/144Nd=0.51286 and 0.51284. These data indicate a source similar to oceanic-island basalts, a source depleted in Rb/Sr and Nd/Sm for some period of time. The Sr isotope data are identical to that previously reported for Mount Etna.Extraction of hawaiites from depleted source regions requires either recent enrichment events, mixing of asthenospheric and lithospheric melts, or variable degrees of melting. At present, the data do not allow a clear decision.The peculiar tectonic setting of Mount Etna, between the relatively undeformed African foreland and the active Aeolian volcanic islands, may suggest contributions to the source region from subduction and within-plate processes. Etnean lavas have a geochemical imprint of subduction-related enrichment processes, and they also share petrological and chemical features identical to oceanic-island basalts whose source region has been affected by within-palte enrichment processes.

Geochemistry of some volcanic rocks from south-eastern sicily: Rare earth and other trace element distribution

Some trace element data for volcanic rocks found at different levels, from Tertiary to Holocene, in south-eastern Sicily (Iblean Plateau and Mt. Etna) are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types.Four groups of volcanic rocks have been recognized on the basis of their major element chemistry: 1) low-K tholeiites, 2) associated alkali basalts to nephelinites of the Iblean Plateau (Upper Pliocene to Lower Pleistocene), 3) the basal subalkaline lavas of Mt. Etna, and 4) the alkalic suite rocks that make up the bulk of the volcano.The distribution of Rb, Sr, Ni, Cr, Co, Cu, REE, Th and Sc suggests:i)an origin of the Iblean magmas by a different degree of partial melting of a Rb-poor and possibly slightly hetereogeneous mantle;ii)quite distinct source compositions for the Etnean magmas, relative to those of the Iblean area, on the basis of their Rb and Sr contents;iii)an origin of the alkalic rocks of Mt. Etna from independently generated magma(s) rather than by crystal fractionation of the Etnean subalkaline magmas or of a magma having the geochemical features of the Iblean alkali basalts; evidence for this is given by the distribution features of the incompatible elements showing an origin for these rocks from compositionally different parent magmas and/or an evolution under widely variable environmental conditions;iv)the primary character for the chemical differences observed in some of the Etnean subalkaline rocks that can be accounted for by different physico-chemical conditions at their source rather than by crystal fractionation processes.

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.

Rare-earth and other trace-element distributions in the calc-alkaline volcanic rocks from deep boreholes in the Phlegrean Fields, Campania (south Italy)

Abstract Lavas and other volcanic products have been found in boreholes at depths of 300–1900 m, in Campania. They underlie the volcanics of the potassic and highly potassic sequence of this region, and show geochemical and mineralogic features similar to those of the K-rich calc-alkaline volcanics. Five of these rocks, ranging from basalts to basaltic andesites and andesites, have been analyzed for REE and other trace elements; only REE have been determined in six other more altered rocks. The geochemical data for these rocks agree with their petrology, and are similar to those of the calc-alkaline volcanics found along continental margins, and also of some products of the active volcanic area in the Eolian Islands. The petrography and the major- and trace-element chemistry of these rocks show that they reached different fractionation stages (SiO2 = 48–60%; SI = 33−20; Cr = 80−18 ppm; Ce = 15–44 ppm), but not along one definite line of descent, the rocks having probably been produced from slightly different parent magmas.

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.

Investigation of submicron volcanic aerosol-particles by photoelectron emission

Abstract Volcanic aerosol emissions have been studied for the first time by in situ photoelectric charging. Explorative studies on Mt Etna reveal large concentrations of particles below 1000 nm with the spectrum peaking in the size range of 100–1000 nm diameter. Although a large fraction of the particles is already charged upon emission, the net electrical charge carried by the aerosol turned out to be close to zero. Particles with high photoelectric yield vary greatly in their relative abundance and seem to occur mainly at active points of the volcano.

Magmatic crystallisation of Cr-Al diopside and Al-Fe3+ diopside from the ancient alkaline basalts (Mt. Etna, Sicily)

The petrography of basalts from Ancient Alkaline Centres of Mt. Etna (Sicily) shows several disequilibrium textures, particularly in pyroxene and plagioclase. As regards the former, this disequilibrium is shown by complex zoning: Cr-Al diopside (Di) composition for the core and Al-Fe 3+ Di for the rim, with different thicknesses. More rarely, homogeneous Al-Fe 3+ Di phenocrysts are found. MELT calculations indicate that the only pyroxene to be in equilibrium with the bulk rock composition is Al-Fe 3+ Di, in agreement with petrographic observations. A crystal-chemical study performed on zoned and unzoned phenocrystic pyroxene suggests similar conditions of crystallisation pressure, and a magmatic origin for both compositions. The clinopyroxene geobarometer of Nimis (1999) was applied and maximum values around 0.5 (± 0.2) GPa were obtained. A similar crystallisation pressure is suggested both by link-band deformation of olivine, and olivine-hosted CO 2 inclusions in one of the studied basalts. Excluding variations of crystallisation pressure as responsible for the disequilibrium between the two pyroxenes, this may be ascribed to time-related changes in the geochemical character of the melt. Clinopyroxene crystal chemistry, host-rock petrology, and fluid-inclusion data are also consistent with seismic data suggesting a complex reservoir system feeding Etnean activity at depths around 15–20 km.

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.