Francesco Parello

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.

Quantification of deep CO2 fluxes from Central Italy. Examples of carbon balance for regional aquifers and of soil diffuse degassing

Abstract In Central Italy non-volcanic CO 2 is discharged by focused degassing (strong diffuse emission and vents) and by high-CO 2 groundwater. 3 He / 4 He data and the carbon isotopic composition of CO 2 are compatible with derivation from mantle degassing and/or metamorphic decarbonation. The gases produced at depth accumulate in permeable reservoirs composed of Mesozoic carbonates. When total pressure (roughly corresponding to p CO 2 ) of the reservoir fluid exceeds hydrostatic pressure, a free gas phase forms gas reservoirs within the permeable host rocks from which gases may escape toward the surface. This process generates both the focused vents and the CO 2 -rich springs which characterise the study area. The storage and expulsion of CO 2 is controlled by fractures and faults and/or structural highs of permeable carbonate formations. Influx of deep CO 2 into the overlying groundwater yields a widespread elevated p CO 2 anomaly in the Tyrrhenian Central Italy aquifers. These aquifers release CO 2 to the atmosphere when groundwater is discharged at the surface from springs. The groundwater degassing flux is estimated from the carbon balance of regional aquifers computed by coupling aquifer geochemistry with isotopic and hydrogeological data. The resulting production rate of deep CO 2 ranges from 4×10 5 to 9×10 6 mol y −1 km −2 . In concert with the regional geologic setting, the deep CO 2 production rate increases westward. In the aquifers with anomalously high p CO 2 , the average CO 2 influx rate of the anomalous areas is several times higher than the value derived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Caldeira, K., 1995. Convective hydrothermal CO 2 emission from high heat flow regions. Chem. Geol., 121 (1995) 285–293.] as baseline for CO 2 emission from areas of high heat flow. The flux of CO 2 lost to the atmosphere from water emitted from springs is of the same order of magnitude as the influx of deep CO 2 into the aquifer.

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.

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.

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.

Geochemical survey of Levante Bay, Vulcano Island (Italy), a natural laboratory for the study of ocean acidification

Shallow submarine gas vents in Levante Bay, Vulcano Island (Italy), emit around 3.6t CO2 per day providing a natural laboratory for the study of biogeochemical processes related to seabed CO2 leaks and ocean acidification. The main physico-chemical parameters (T, pH and Eh) were measured at more than 70 stations with 40 seawater samples were collected for chemical analyses. The main gas vent area had high concentrations of dissolved hydrothermal gases, low pH and negative redox values all of which returned to normal seawater values at distances of about 400m from the main vents. Much of the bay around the vents is corrosive to calcium carbonate; the north shore has a gradient in seawater carbonate chemistry that is well suited to studies of the effects of long-term increases in CO2 levels. This shoreline lacks toxic compounds (such as H2S) and has a gradient in carbonate saturation states.

18O exchange between steam and carbon dioxide in volcanic and hydrothermal gases: implications for the source of water

Abstract The oxygen isotopic compositions of H 2 O and CO 2 from 31 fumaroles from different volcanoes (Vesuvio, Campi Flegrei, Vulcano and Etna in Italy; Soufriere of Guadeloupe in Lesser Antilles) prove that 18 O exchange between CO 2 and steam is fast enough in natural gas phases to allow rapid isotopic reequilibration within a wide temperature range (100–1000°C). The isotopic composition of H 2 O discharged by volcanic–hydrothermal fumaroles is affected by isotopic reequilibration along with cooling of the gas phase and this secondary process must be taken into account when interpreting the origin of the water from isotopic data. The use of corrected δ 18 O values computed considering the overall oxygen isotopic composition of the H 2 O–CO 2 system is therefore compulsory to overcome interpretative biases. As an example, we apply this method to 100–600°C fumaroles at Vulcano crater, whose isotopic composition is reinterpreted considering the effects of 18 O exchange between CO 2 and H 2 O. In contrast to most previous interpretations, the corrected isotopic values for H 2 O clearly show that the Vulcano system is fed by a two-component mixture of magmatic water with a typical “andesitic” composition and either 18 O-shifted seawater or magmatic steam partly condensed within the volcanic ground, with minor contribution of local meteoric water. The possible additional effects of isotopic reequilibration involving other O- and H-bearing gas species in volcanic fluids, such as SO 2 , H 2 S, HCl, HF, and H 2 , have been also considered. The influence of SO 2 upon the isotopic composition of H 2 O is subordinate with respect to that of CO 2 , whereas significant shifts in deuterium composition are expected from hydrogen isotopic exchanges in the H 2 O–H 2 S–HCl–HF–H 2 gas system.

Pre‐ and syn‐eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies

Soufriere Hills fumaroles contained magma-derived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal composition. Chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at Galways Soufriere, the most active fumarolic field. Hydrothermal buffering of the fumaroles has been favoured by their remote location (1–2 km) from the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater. High temperature (720°C) gas collected from the extruding lava dome in Feb. 1996 was chemically and isotopically representative of the magmatic gas stream. Its composition allows assessment of average eruptive fluxes of H2O, CO2 and HCl which require the degassing of only 2.5–3 times more magma than erupted.

Geochemical mapping of magmatic gas–water–rock interactions in the aquifer of Mount Etna volcano

Abstract Systematic analysis of major and minor elements in groundwaters from springs and wells on the slopes of Mt. Etna in 1995–1998 provides a detailed geochemical mapping of the aquifer of the volcano and of the interactions between magmatic gas, water bodies and their host rocks. Strong spatial correlations between the largest anomalies in pCO2 (pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas (CO2) and consequent basalt leaching by acidified waters of the shallow (meteoric) Etnean aquifer. Most groundwaters displaying this magmatic-type interaction discharge within active faulted zones on the S–SW and E lower flanks of the volcanic pile, but also in a newly recognised area on the northern flank, possibly tracking a main N–S volcano-tectonic structure. In the same time, the spatial distribution of T°C, TDS, Na, Li, Cl and B allows us to identify the existence of a deeper thermal brine with high salinity, high content of B, Cl and gases (CO2, H2S, CH4) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at the periphery of the volcano near the Village of Paterno, where it gives rise to mud volcanoes called “Salinelle di Paterno”. However, the contribution of similar brines to shallower groundwaters is also detected in other sectors to the W (Bronte, Maletto), SW (Adrano) and SE (Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also closely associated with hydrocarbon fields all around the volcano and its rise, generally masked by the high outflow of the shallow aquifer, may be driven by the ascent of mixed sedimentary–magmatic gases through the main faults cutting the sedimentary basement.

Interaction between the deep fluids and the shallow groundwaters on Vulcano island (Italy)

The aim of this work is to study the interactions processes between the fluids of deep origin and the shallow groundwaters of the Vulcano Porto area. During 1995, 13 well waters were sampled three times (May, July and November) and analysed for major and some minor elements (B, Br and NH4) and for dissolved gases. The close relationship of these waters with the deep magmatic source is highlighted by the composition of the dissolved gases. Furthermore, the areal distribution of dissolved species is controlled mainly by the gas fluxes from depth and by the presence of a deeper thermal aquifer. The distribution of major anomalies in the parameters measured in the groundwaters, in fact, overlaps the soil gas fluxes distribution that are related to the existence of preferential upflow routes of fumarolic fluids through weak structural lines in the volcanic edifice.