P. Allard

Spectroscopic evidence for a lava fountain driven by previously accumulated magmatic gas

Lava fountains are spectacular continuous gas jets, propelling lava fragments to heights of several hundred metres, which occasionally occur during eruptions of low-viscosity magmas. Whether they are generated by the effervescent disruption of fast-rising bubbly melt or by the separate ascent of a bubble foam layer accumulated at depth still remains a matter of debate. No field measurement has yet allowed firm discrimination between these two models. A key insight into the origin of lava fountains may be gained by measuring the chemical composition of the driving gas phase. This composition should differ markedly depending on whether the magma degassing occurs before or during eruption. Here we report the analysis of magmatic gas during a powerful (250–600 m high) lava fountain, measured with Fourier transform infrared spectroscopy on Mount Etna, Sicily. The abundances of volcanic gas species, determined from absorption spectra of lava radiation, reveal a fountain gas having higher CO2/S and S/Cl ratios than other etnean emissions, and which cannot derive from syn-eruptive bulk degassing of Etna basalt. Instead, its composition suggests violent emptying of a gas bubble layer previously accumulated at about 1.5 km depth below the erupting crater.

Endogenous magma degassing and storage at Mount Etna

Combining SO2 plume emissions from Mt. Etna in 1975–1995 with the magma S content suggests that between 3.5 and 5.9 km³ of basalt were degassed for sulfur over two decades, only 10–20% of which actually extruded. Except during extensive lava outbreak (1992), the SO2 flux was mostly supplied by endogenous degassing of non-erupted basalt, replaced at a time-averaged rate of 4.5–8 m³/s. The unerupted degassed magma cannot be accomodated by the upper plumbing system (whose maximum capacity is estimated at 0.6 km³), nor by simple intrusive growth of the volcanic pile. Being denser than undegassed melt, most of it was probably removed by gravitational convection in sub-volcanic feeders. Ultimately, part of this degassed magma may solidify within the crust, contributing to the accretion of a wide ‘plutonic‧ complex, ≥3 times larger than the volcano itself, that has grown within its sedimentary basement.

Mobility and fluxes of major, minor and trace metals during basalt weathering and groundwater transport at Mt. Etna volcano (Sicily)

Abstract The concentrations and fluxes of major, minor and trace metals were determined in 53 samples of groundwaters from around Mt Etna, in order to evaluate the conditions and extent of alkali basalt weathering by waters enriched in magma-derived CO 2 and the contribution of aqueous transport to the overall metal discharge of the volcano. We show that gaseous input of magmatic volatile metals into the Etnean aquifer is small or negligible, being limited by cooling of the rising fluids. Basalt leaching by weakly acidic, CO 2 -charged water is the overwhelming source of metals and appears to be more extensive in two sectors of the S-SW (Paterno) and E (Zafferana) volcano flanks, where out flowing groundwaters are the richest in metals and bicarbonate of magmatic origin. Thermodynamic modeling of the results allows to evaluate the relative mobility and chemical speciation of various elements during their partitioning between solid and liquid phases through the weathering process. The facts that rock-forming minerals and groundmass dissolve at different rates and secondary minerals are formed are taken into account. At Mt. Etna, poorly mobile elements (Al, Th, Fe) are preferentially retained in the solid residue of weathering, while alkalis, alkaline earth and oxo-anion-forming elements (As, Se, Sb, Mo) are more mobile and released to the aqueous system. Transition metals display an intermediate behavior and are strongly dependent on either the redox conditions (Mn, Cr, V) or solid surface-related processes (V, Zn, Cu). The fluxes of metals discharged by the volcanic aquifer of Etna range from 7.0 × 10 −3 t/a (Th) to 7.3 × 10 4 t/a (Na). They are comparable in magnitude to the summit crater plume emissions for a series of elements (Na, K, Ca, Mg, U, V, Li) with lithophile affinity, but are minor for volatile elements. Basalt weathering at Mt Etna also consumes about 2.1 × 10 5 t/a of magma-derived carbon dioxide, equivalent to ca. 7% of contemporaneous crater plume emissions. The considerable transport of some metals in Etna’s aquifer reflects a particularly high chemical erosion rate, evaluated at 2.3∗10 5 t/a, enhanced by the initial acidity of magmatic CO 2 -rich groundwater.

Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy

We report the first detailed investigation of both helium and carbon isotopes in groundwaters and gases of Mt. Etna, providing new insight into the distribution, origin and budget of magmatic gas release at this very active volcano [1]. A mantle-derived magmatic component, with ultimate3He/4He ratio of 6.9 ± 0.2 Ra and δ13C of about −4‰, is identified in both types of fluids, depending on their location and the extent of their dilution by either air (gases) or a mixture of dissolved air and organic carbon (waters). Apart from the summit zone, this magmatic component is preferentially concentrated in CO2-rich groundwaters that issue from two remote sectors of the south-southwest and eastern volcano flanks, where its proportion increases with the altitude of meteoric recharge (or the length of pathflow) of the waters. Such a pattern suggests that, in addition to possible local gas input, the groundwaters collect much of their dissolved magmatic He and C while they infiltrate and flow through one of the more elevated, gas-effusing parts of the volcanic pile, among which the south-southeast fracture zone of Etna is the best candidate. These observations provide a new framework for remote geochemical monitoring of the volcano. The3He/4He ratio of the magmatic gas end-member coincides with that of helium trapped in the He-rich olivine crystals of Etna basalts (mean range: 6.7 ± 0.4 Ra, [2,3]), pointing to its negligible dilution by radiogenic He from the crustal basement and further constraining the3He/4He ratio of the present-day Etna magma. While being lower than the typical MORB value of 8 Ra, this ratio is the highest for an active volcano in continental Europe and probably tracks a relatively radiogenic upper mantle zone that is upwelling beneath this region [4]. The estimated outputs of mantle-derived CO2 and3He from Etna account for 10% and 15%, respectively, of estimates for global subaerial volcanic emissions. This huge contribution results from continuous degassing of mostly unerupted He- and C-rich alkaline basaltic magma, which occurs principally through the central open conduits and secondarily through the flanks of the volcano. Groundwaters carry only a minor fraction (≈ 3%) of total emitted CO2 and3He.

Geophysical and geochemical modelling of the 1982–1984 unrest phenomena at Campi Flegrei caldera (southern Italy)

Abstract Based on both geophysical and geochemical data collected during the 1970–1972 and 1982–1984 volcano-seismic crises at Campi Flegrei caldera, the possible interpretative models of the events are reviewed and discussed. It is shown that models involving exclusively an overpressure in the magma chamber cannot reconcile all the data sets and that convective heat transfer to confined shallow aquifers may be an important process contributing to unrest phenomena at Campi Flegrei.

Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

-European Union, -Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica; -CNR–Gruppo Nazionale per la Vulcanologia.

Acid gas and metal emission rates during long‐lived basalt degassing at Stromboli Volcano

The discharge of acid gases and metals from Stromboli is determined from airborne and ground-based filter sampling of particulate matter in the volcanic plume, combined with COSPEC measurements of SO2 fluxes. Smaller particle sizes and high enrichment factors distinguish the most volatile elements (by order: S, Se, Br, Cl, Cd, Bi, In, As, Sb, Sn, F, Au, Pb, Cr, Cu) from those strictly (Fe, Mn, REE, Sc, Sr, Th, Ti, V) or mainly (Al, Ba, Ca, Co, K, Na, U) derived from volcanic ash. Time-averaged volatile fluxes show that Stromboli is a representative arc emittor, producing 1–2% of the global volcanic budget of sulfur, halogens and several trace metals, while 15–25% of volcanic emissions of Bi, Cd, Cs, Pb and Sn in southern Italy. Subaerial degassing of its S-Cl-rich shoshonitic magma over the last 2 ky of similar activity may have released as much copper and gold as is encountered in magma-derived high-sulfidation ore deposits.

Soil gas emanations as precursory indicators of volcanic eruptions

Field measurements conducted on several active volcanoes in Italy, the Lesser Antilles, and Indonesia demonstrate the common Occurrence of diffuse soil gas emanations from the volcanic piles, at distances from active craters or fumarolic zones. These emanations consist essentially of carbon dioxide and rare gases and their genetic link with crater fumaroles and/or magma degassing at depth can be verified both chemically and isotopically. We emphasize here the potential use of these fluids for continuous volcano monitoring and eruption forecasting.

Isotopic study of the origin of sulfur and carbon in Solfatara fumaroles, Campi Flegrei caldera

Abstract Isotopic study of the origin of sulfur and carbon in the hottest (Solfatara) fumaroles of Campi Flegrei caldera, Southern Italy, was carried out on gas samples collected between 1983 and 1988, i.e. during and after the 1982–1984 seismo-volcanic crisis. The results for sulfur (H2S), the first ever reported on these gases, indicate a mean ∂ 34 S of −0.3±0.3‰ (range: −0.7 to +0.1‰ ) versus Canyon Diablo Troilite standard, consistent with an igneous derivation of this element, from either active magma degassing or/and leaching of reduced sulfur-bearing minerals in the volcanic layers. The lack of peculiar ∂34S variation during and after the crisis suggests that the chemical variation of H2S and S/C ratio in the fumaroles (increase and then decrease by a factor 3) were not due to a changing origin of sulfur. The mean ∂13C of carbon (CO2) over the period of survey, −1.6±0.2‰ (range: −1.9 to −1.3‰ ) versus PDB standard, is similar to the values obtained before the crisis (since 1970). Such an isotopic constancy requires a large and stable source of carbon feeding the fumaroles. The measured ∂13C values are much higher than those typical of primary mantle-magmatic carbon ( −6±2‰ ) and plot within the ∂13C range for marine carbonates ( 0±2‰ ). Such high values may reflect either (a) 13C-fractionation during degassing of CO2 from the underlying (⩽5 km depth) magma chamber or (b) the contribution of heavy CO2 of sedimentary origin, derived from either thermometamorphism of Mesozoic limestone series embedding the magma chamber or, possibly, past contamination of the local mantle by subducted sediments. Various arguments, among which volcanological evidence of an isolated and cooling magma reservoir (which would have been extensively degassed and, so, depleted in 13C along with time), the low 3He/4He ratios and the broad 13C-enrichment of volcanic fluids in the region, and geochemical evidence of crust-magma fluid interactions, suggest that a considerable fraction (⩾60%) of CO2 in Solfatara fumaroles derives from carbonate sediments in the basement. The contribution of magma-derived CO2 may be higher within the central part of the caldera (including Solfatara crater) than toward its western margin, where fumarolic and geothermal well gases exhibit lower 3He/4He ratios and still higher ∂13C values. Such a geochemical pattern is consistent with the central distribution of ground deformation and seismicity during the 1982–1984 crisis and with the idea of a residual magma body, confined beneath the central part of the structure. Alternatively, higher ∂13C and lower 3He/4He ratios toward the western margin may result from dilution of Solfatara-type gas during progressively deeper water boiling. Finally, accepting that Solfatara CO2 derives from simple crustal mixing between magmatic and sedimentary carbon, its constant isotopic composition (together with the constant He isotope ratio) would restrict the possibility of magma intrusion and/or higher magmatic gas input as mechanisms responsible for the 1982–1984 events. However, this conclusion would no more hold true if the magma itself, or even its mantle source, were previously contaminated by crustal carbon.

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (∼100°C) aquifer of mixed marine–meteoric water. The measured 3He/4He ratios and δ13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO2 ratio, constrain a 3He/4He ratio of 7.3±0.1 Ra and a δ13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/4He ratio but higher than MORB CO2/3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.