Mercury emissions and stable isotopic compositions at Vulcano Island (Italy)

Sampling and analyses methods for determining the stable isotopic compositions of Hg in an active volcanic system were tested and optimized at the volcanic complex of Vulcano (Aeolian Islands, Italy). Condensed gaseous fumarole Hg(fum) T , plume gaseous elemental Hg(g) 0 and plume particulate Hg(p) II were obtained at fumaroles F0, F5, F11, and FA. The average total Hg emissions, based on HgT/SO2 in condensed fumarolic gases and plumes, range from 2.5 to 10.1 kg y−1, in agreement with published values [Ferrara, R., Mazzolai, B., Lanzillotta, E., Nucaro, E., Pirrone, N., 2000. Volcanoes as emission sources of atmospheric mercury in the Mediterranean Basin. Sci. Total Environ. 259(1–3), 115–121; Aiuppa, A., Bagnato, E., Witt, M.L.I., Mather, T.A., Parello, F., Pyle, D.M., Martin, R.S., 2007. Real-time simultaneous detection of volcanic Hg and SO2 at La Fossa Crater, Vulcano (Aeolian Islands, Sicily). Geophys. Res. Lett. 34(L21307).]. Plume Hg(p) II increases with distance from the fumarole vent, at the expense of Hg(g) 0 and indicates significant in-plume oxidation and condensation of fumarole Hg(fum) T . Relative to the NIST SRM3133 Hg standard, the stable isotopic compositions of Hg are δ202Hg(fum) T =−0.74‰±0.18 (2SD, n=4) for condensed gaseous fumarole Hg(fum) T , δ202Hg(g) 0 =−1.74‰±0.36 (2SD, n=1) for plume gaseous elemental Hg(g) 0 at the F0 fumarole, and δ202Hg(p) II =−0.11‰±0.18 (2SD, n=4) for plume particulate Hg(p) II . The enrichment of Hg(p) II in the heavy isotopes and Hg(g) 0 in the light isotopes relative to the total condensed fumarolic Hg(fum) T gas complements the speciation data and demonstrates a gas-particle fractionation occurring after the gas expulsion inambient T° atmosphere. A first order Rayleigh equilibriumcondensation isotope fractionation model yields a fractionation factor αcond-gas of 1.00135±0.00058

Silicon isotope variations in the Earth and meteorites

A fluorhydric acid-free sample preparation method derived from Georg et al. [1] has been used to measure the natural variations of silicon isotope compositions in terrestrial (including 12 geological standard materials) and meteoritic bulk-rock samples. All measurements were done using a Neptune MC-ICPMS in medium resolution mode (m/Δm = 7000, peak-edge definition). Magnesium was used as internal standard for mass-bias drift correction. The δ30Si values are expressed relative to the NBS-28 silica standard. IRMM-17 reference material yields a δ30Si of -1.4‰ ± 0.05‰ (2SD, n=11) in agreement with previous data [2-3]. Long-term reproducibilities were obtained for BHVO-2 (δ30Si = -0.27‰ ± 0.08‰ (2SD, n=30)) and a in-house Si standard (δ30Si = -0.01‰ ± 0.07‰ (2SD, n=20)) on a 7 months time scale. Total variation of δ30Si in natural samples ranges from - 0.5‰ to -0.1‰. Comparison with δ29Si values shows that this isotopic fractionation is mass-dependent. A 0.2‰ isotopic variation occurs among terrestrial samples suggesting an enrichment in the heavier silicon isotopes as a function of magma differentiation, as initially hinted by Douthitt [4]. Terrestrial samples mean value (δ30SiEarth= -0.23‰) is heavier by about 0.24‰ in δ30Si compared to chondrites. This may be explained by silicon isotope fractionation during planetary accretion and/or differentiation

Precise determination of silicon isotopes in silicate rock reference materials by MC-ICP-MS

A HF-free sample preparation method was used to purify silicon in twelve geological RMs. Silicon isotope compositions were determined using a Neptune instrument multi-collector-ICP-MS in high-resolution mode, which allowed separation of the silicon isotope plateaus from their interferences. A 1 mu g g-1 Mg spike was added to each sample and standard solution for online mass bias drift correction. delta 30Si and delta 29Si values are expressed in per mil (parts per thousand), relative to the NIST SRM 8546 (NBS-28) international isotopic RM. The total variation of delta 30Si in the geological reference samples analysed in this study ranged from -0.13 parts per thousand to -0.29 parts per thousand. Comparison with delta 29Si values shows that these isotopic fractionations were mass dependent. IRMM-17 yielded a delta 30Si value of -1.41 +/- 0.07 parts per thousand (2s, n = 12) in agreement with previous data. The long-term reproducibility for natural samples obtained on BHVO-2 yielded delta 30Si = -0.27 +/- 0.08 parts per thousand (2s, n = 42) on a 12 month time scale. An in-house Si reference sample was produced to check for the long-term reproducibility of a mono-elemental sample solution; this yielded a comparable uncertainty of +/- 0.07 parts per thousand (2s, n = 24) over 5 months.Une methode de digestion ne requerant pas l'emploi d'acide fluorhydrique (HF) a ete developpee pour purifier le silicium de douze materiaux de reference geologiques. Les signatures isotopiques du silicium ont ete determinees a l'aide d'un spectrometre de masse multi collecteurs a torche a plasma (MC-ICP-MS) Neptune en haute resolution, permettant de separer les plateaux des isotopes du silicium de leurs interferences. Les variations temporelles du biais de masse ont ete corrigees par standardisation interne, via l'addition de 1 mu g g-1 de magnesium dans les echantillons et les standards. Les valeurs delta, exprimees en pour mille (delta 30Si et delta 29Si), ont ete determinees relativement au materiel de reference isotopique international de silicium NIST SRM 8546 (NBS-28). La variation totale du delta 30Si pour les materiaux de reference geologiques mesures dans cette etude s'echelonne de -0.13 parts per thousand a -0.29 parts per thousand. La comparaison avec delta 29Si montre un fractionnement isotopique dependant de la masse. Nous obtenons pour IRMM-17 un delta 30Si = -1.41 +/- 0.07 parts per thousand (2s, n = 12), en accord avec les valeurs proposees anterieurement. La reproductibilite externe sur le long terme pour les echantillons naturels a ete determinee par la mesure repetee de BHVO-2 (n = 42) sur une echelle de temps de 12 mois, donnant une valeur delta 30Si = -0.27 parts per thousand et une incertitude de +/- 0.08 parts per thousand (2s). Un materiel de reference maison (IHRS) a ete produit pour evaluer la reproductibilite sur le long terme d'une solution mono elementaire. Il donne une incertitude comparable de +/- 0.07 parts per thousand (2s, n = 24) sur 5 mois

An Earth-Moon silicon isotope model to track silicic magma origins

A comparison between terrestrial and lunar igneous rocks reveals that Si isotope compositions become slightly, though significantly enriched in heavy isotopes from basalts to granites and anorthosites as a function of the rock SiO2 concentration and/or tectosilicate content. This is interpreted as the result of a global igneous differentiation process that leads to an increased amount of tectosilicates in the rocks. This relationship of increasing degree of melt polymerization with increasingly heavy silicon isotope composition is particularly apparent in lunar rocks. The terrestrial trend, however, is more scattered. Given the sensitivity of Si isotopes to water-rock interactions, it is likely that the more erratic terrestrial trend reveals the involvement of water that does not occur on the Moon. Hence, Si isotopes appear to reflect the occurrence of low temperature water-rock interaction products, like clay minerals, in the source of peraluminous leucogranites. Conversely, the heavy silicon isotope composition of some andesites possibly trace the input of fluids involved in subduction zones and/or interaction of the oceanic crust with isotopically heavy seawater before subduction

Mercury emissions and stable isotopic compositions at Vulcano Island (Italy)

Sampling and analyses methods for determining the stable isotopic compositions of Hg in an active volcanic system were tested and optimized at the volcanic complex of Vulcano (Aeolian Islands, Italy). Condensed gaseous fumarole Hg(fum) T , plume gaseous elemental Hg(g) 0 and plume particulate Hg(p) II were obtained at fumaroles F0, F5, F11, and FA. The average total Hg emissions, based on HgT/SO2 in condensed fumarolic gases and plumes, range from 2.5 to 10.1 kg y−1, in agreement with published values [Ferrara, R., Mazzolai, B., Lanzillotta, E., Nucaro, E., Pirrone, N., 2000. Volcanoes as emission sources of atmospheric mercury in the Mediterranean Basin. Sci. Total Environ. 259(1–3), 115–121; Aiuppa, A., Bagnato, E., Witt, M.L.I., Mather, T.A., Parello, F., Pyle, D.M., Martin, R.S., 2007. Real-time simultaneous detection of volcanic Hg and SO2 at La Fossa Crater, Vulcano (Aeolian Islands, Sicily). Geophys. Res. Lett. 34(L21307).]. Plume Hg(p) II increases with distance from the fumarole vent, at the expense of Hg(g) 0 and indicates significant in-plume oxidation and condensation of fumarole Hg(fum) T . Relative to the NIST SRM3133 Hg standard, the stable isotopic compositions of Hg are δ202Hg(fum) T =−0.74‰±0.18 (2SD, n=4) for condensed gaseous fumarole Hg(fum) T , δ202Hg(g) 0 =−1.74‰±0.36 (2SD, n=1) for plume gaseous elemental Hg(g) 0 at the F0 fumarole, and δ202Hg(p) II =−0.11‰±0.18 (2SD, n=4) for plume particulate Hg(p) II . The enrichment of Hg(p) II in the heavy isotopes and Hg(g) 0 in the light isotopes relative to the total condensed fumarolic Hg(fum) T gas complements the speciation data and demonstrates a gas-particle fractionation occurring after the gas expulsion inambient T° atmosphere. A first order Rayleigh equilibriumcondensation isotope fractionation model yields a fractionation factor αcond-gas of 1.00135±0.00058.Published236-2431.2. TTC - Sorveglianza geochimica delle aree vulcaniche attiveJCR Journalope

The experimental determination of equilibrium Si isotope fractionation factors among H4SiO4 degrees, H3SiO4- and amorphous silica (SiO2 center dot 0.32 H2O) at 25 and 75 degrees C using the three-isotope method

The accurate interpretation of Si isotope signatures in natural systems requires knowledge of the equilibrium isotope fractionation between Si-bearing solids and the dominant Si-bearing aqueous species. Aqueous silicon speciation is dominated by silicic acid (H4SiO4o) in most natural aqueous fluids at pH 9), equilibrium Si isotope fractionation factors between solid and aqueous solution are higher, at 1.63 ± 0.23‰ at 25 °C, and 1.06 ± 0.13‰ at 75 °C. Taking account of the distribution of the aqueous Si species, equilibrium Si isotope fractionation factors between H3SiO4− and H4SiO4o of −2.34 ± 0.13‰ and −2.21 ± 0.05‰ at 25 and 75 °C, respectively, were determined. The distinct equilibrium isotope fractionation factors of H3SiO4− and H4SiO4o, and its variation with temperature can be used to establish paleo-pH and temperature proxies. The application of the three-isotope method also provides insight into the rates of isotopic exchange. For the solid grain size used (∼20 nm), these rates match closely the measured bulk dissolution rates for amorphous silica for most of the isotope exchange process, suggesting the dominant and rate controlling isotope exchange mechanism in the experiments is detachment and reattachment of material at the amorphous silica surface

Origin of silica in rice plants and contribution of diatom Earth fertilization : insights from isotopic Si mass balance in a paddy field

International audienceBackground and aims: The benefits of Si for crops is well evidenced but the biogeochemical cycle of Si in agriculture remains poorly documented. This study aims at identifying and quantifying the Si sources (primary and secondary soil minerals, amorphous silica, irrigation, Si-fertilizer) to rice plants.Method: Field experiments were carried out with and without application of diatomaceous earth (DE) under rice and bare conditions to determine the water and dissolved mass balance in paddy fields (Karnataka, Southern India). The fate of the Si brought by irrigation (DSi) (uptake by rice, uptake by diatoms, adsorption) was assessed through a solute mass balance combined with silicon isotopic signatures.Results: Above the ground-surface, about one third of the DSi flux brought by borewell irrigation (545 mmol Si.m−2) to bare plots and half of DSi in rice plots were removed from solution within minutes or hours following irrigation. Such rate is consistent with the rate of DSi adsorption onto Fe-oxyhydroxides but not with diatom blooms. In rice and rice + DE experiments, the isotopic fractionation factor (30ε) between bore well and stagnant water compositions is close to −1 ‰, i.e. the isotopic fractionation factor known for rice, indicating that above-ground DSi removal would be dominated by plant uptake upon adsorption. Within the soil layer, pore water DSi decreases much faster in rice experiments than in bare ones, demonstrating the efficiency of DSi rice uptake upon adsorption. Total irrigation-DSi to plant-Si would then represent 24 to 36% in rice experiments (over 1460 ± 270 mmol Si m−2 in biomass) and 15 to 23% in rice + DE ones (over 2250 ± 180 mmol Si m−2). The δ30Si signature of whole plants was significantly different in the rice + DE plot analyzed, 0.99 ± 0.07 ‰, than in the rice one, 1.29 ± 0.07 ‰. According to these δ30Si signatures, the main Si source from the soil would be the amorphous silica pool (ASi). A slight contribution of DE to the rice plant could be detected from the Si isotopic signature of rice.Conclusions: The δ30Si signatures of the various soil-plant compartments, when associated to Si mass balance at soil-plant scale, constitute a reliable proxy of the Si sources in paddy fields. The solute Si balance is controlled by rice uptake in rice plots and by adsorption in bare ones. The main Si sources for the rice plants were soil ASi, irrigation Si and to a lesser extent Si fertilizer when it was applied