Grain scale processes recorded by oxygen isotopes in olivine-hosted melt inclusions from two MORB samples

International audienceAlthough olivine-hosted melt inclusions from mid ocean ridge basalts (MORB) are commonly used as a proxy formantle composition, these melt inclusions generally show larger elemental and isotopic compositional variationthan their host lavas and the origin of these heterogeneities remains disputed. Here we present oxygen isotopedata from melt inclusions hosted in olivine from two samples from the Mid-Atlantic ridge. Melt inclusions fromdifferent crystals within the same sample show>2.5‰ δ18O variation within each sample, which is nearly eighttimes the analytical error of 0.3‰ (2 standard deviations) and five times the δ18O range in unaltered MORB.Measured δ18O in melt inclusions do not correlate with common magmatic tracers, and δ18O measured in thehost olivines suggest a maximum of 1‰ δ18O source heterogeneity. Less than half of the melt inclusions fromeach sample are in equilibrium with their host crystals; the remaining melt inclusions have either lower or higherolivine-melt oxygen isotope partition coefficients compared to the theoretical equilibrium values. Here wediscuss several potential processes that could contribute to these observations, but none satisfactorily explain theolivine-melt inclusion oxygen disequilibrium that we observe in these samples. Nevertheless, it seems clear thatthe variability of δ18O in melt inclusion from two MORB samples do not record only common magmatic process(es), but rather a localized grain scale process. Any δ18O variation in melt inclusions should thus be interpretedwith caution

A reassessment of the sulfur, chlorine and fluorine atmospheric loading during the 1815 Tambora eruption

International audienceThe 1815 eruption of Mount Tambora (Sumbawa Island, Indonesia), largest known explosive eruption in recorded history, was cataclysmic. It was responsible for a strong short-term global atmospheric cooling the following year, known as "the year without a summer". To evaluate the climatic impact, an accurate quantification of volatile elements degassed during this eruption is crucial. In this study, we re- evaluate the atmospheric release of sulfur, chlorine and fluorine during the 1815 eruption using the petrological approach based on plagioclase-hosted melt inclusions. The pre-eruptive (melt inclusions) and post-eruptive (matrix glass) volatile element concentrations of the magma are measured by electron microprobe. We discuss three different outgassing scenarios and conclude that 147 ± 17 Tg of SO2, 49 ± 5 Tg of Cl and 20 ± 2 Tg of F were degassed during the eruption, considering closed system ascent and degassing. The SO2 results take into account the dissolution of sulfides which are present in melt inclusions and plagioclase crystals but not in matrix glasses. Our new estimates are higher than previous estimations from petrological methods or derived from ice cores but are consistent with atmospheric optical depth observations from 1816. The 1815 eruption of Tambora ranks in first place in terms of volcanic SO2 emission in the last 2000 years, higher than the 1257 Samalas eruption (Lombok Island, Indonesia) if equal methodologies are applied. These estimates remain nonetheless minima as they do not account for the possible additional contribution of a pre-existing gas phase in the magma reservoir

Fluorine

International audienc

Two types of slab components under Ecuadorian volcanoes supported by primitive olivine-hosted melt inclusion study

International audienceThe origin of several geochemical parameters in continental arc lavas (e.g., La/Yb, Ba/Th) is controversial as to whether it is imparted by the slab or acquired in the crust. In Ecuador, where volcanoes are built over a thick crust (∼50 km), this problem is grounded in the lack of primitive rocks. Here, we use melt inclusions hosted in Fo80-90 olivines to decipher the slab component signatures that metasomatises the sub-arc mantle. We report major, trace, and volatile elements analyzed in experimentally heated melt inclusions, which are from primitive rocks of Cotacachi, Cubilche, Cono de la Virgen, Conos de Licto, and Sangay volcanoes located in the north and south of the Ecuadorian arc. Based on trace element data and geochemical modeling, we recognize two types of slab components: one is indicative of aqueous fluids (e.g., high Ba/La, Pb/Ce, B/Nb), and the other is indicative of hydrous siliceous melts (e.g., high La/Nb, Th/Nb). The aqueous fluid signature is recognized in all volcanoes (except for Cono de la Virgen), and their F/Cl are distributed around 0.43 ± 0.07, independent of the distance to the Benioff zone. We propose a model where hydrous siliceous melts result from the subduction of a young oceanic crust north of the Grijalva fracture zone. Additionally, we show that the mantle under Licto and Sangay volcanoes is enriched prior to metasomatism by slab fluids. This study shows that melt inclusions give valuable insights into the composition of primitive melts that are rarely accessible as whole rocks

Kinetics of Sulfur Isotope Fractionation from experiments in Melt and Sulfide using a dynamic 1-atm Gas-mixing Furnace

International audienceSulfur isotope fractionation plays a crucial role in understanding the evolution of sulfur concentration and isotopes in natural magmatic systems. This is because sulfur has multiple oxidation states varying from -2 to +6, a multitude of reactions, not limited to precipitation, dissolution, phase separation occurs in these systems. It is during these reactions, in a dynamic environment, that sulfur could fractionate. Understanding this fractionation can help us estimate critical parameters related to degassing processes, such as magma ascent rate, residence time in the reservoir. We conducted experiments in dynamic conditions to determine sulfur isotope fractionation factors in a magma containing sulfides, in a 1-atm gas-mixing furnace using the gas mixture CO-CO2-SO2. Disequilibrium conditions in the furnace can be modelled to simulate volcanic degassing, as the maximum rate of the change of fO2 within the furnace is 0.013 log unit s-1. Sets of time-series experiments have shown that the system reaches equilibrium, both in terms of major element and sulfur isotope composition within 8 hours. Once at equilibrium, dynamic conditions were imposed, by varying fO2 & fS2 from the initial to the final conditions. From initial tests at 1300 °C, changing ~1.2 log unit over 4 hours (log fO2= -10.3 to -11.1, log fS2= -1.2 to -2.5). At the start the Δ34Smelt-sulfide is 0.8 ± 0.8 ‰ and after 4h we measured a Δ34Smelt-sulfide of -0.1 ± 0.5 ‰ which is in agreement with the expected equilibrium value of -0.4 ± 0.3 ‰, but the kinetic effect of isotopic fractionation remains hidden by the poor analytical precision. We will further test and present the results from multiple series of dynamic experiments at 1300 °C, running from a reducing (log fO2, fS2: -11.1, -2.5) to an oxidizing condition (log fO2, fS2: -8.5, -1.4) each lasting a duration between 5 minutes and 1 hour after equilibration. The equilibrium isotopic fractionation values for these 2 conditions are Δ34Smelt-sulfide of - 0.1 ± 0.5 ‰ & 8.2 ± 0.6 ‰. We expect to see a more significant and measurable change in Δ34Smelt-sulfide, showing how sulfur fractionates during rapid natural degassing processes and the kinetic processes at play

Kinetics of Sulfur Isotope Fractionation from experiments in Melt and Sulfide using a dynamic 1-atm Gas-mixing Furnace

International audienceSulfur isotope fractionation plays a crucial role in understanding the evolution of sulfur concentration and isotopes in natural magmatic systems. This is because sulfur has multiple oxidation states varying from -2 to +6, a multitude of reactions, not limited to precipitation, dissolution, phase separation occurs in these systems. It is during these reactions, in a dynamic environment, that sulfur could fractionate. Understanding this fractionation can help us estimate critical parameters related to degassing processes, such as magma ascent rate, residence time in the reservoir. We conducted experiments in dynamic conditions to determine sulfur isotope fractionation factors in a magma containing sulfides, in a 1-atm gas-mixing furnace using the gas mixture CO-CO2-SO2. Disequilibrium conditions in the furnace can be modelled to simulate volcanic degassing, as the maximum rate of the change of fO2 within the furnace is 0.013 log unit s-1. Sets of time-series experiments have shown that the system reaches equilibrium, both in terms of major element and sulfur isotope composition within 8 hours. Once at equilibrium, dynamic conditions were imposed, by varying fO2 & fS2 from the initial to the final conditions. From initial tests at 1300 °C, changing ~1.2 log unit over 4 hours (log fO2= -10.3 to -11.1, log fS2= -1.2 to -2.5). At the start the Δ34Smelt-sulfide is 0.8 ± 0.8 ‰ and after 4h we measured a Δ34Smelt-sulfide of -0.1 ± 0.5 ‰ which is in agreement with the expected equilibrium value of -0.4 ± 0.3 ‰, but the kinetic effect of isotopic fractionation remains hidden by the poor analytical precision. We will further test and present the results from multiple series of dynamic experiments at 1300 °C, running from a reducing (log fO2, fS2: -11.1, -2.5) to an oxidizing condition (log fO2, fS2: -8.5, -1.4) each lasting a duration between 5 minutes and 1 hour after equilibration. The equilibrium isotopic fractionation values for these 2 conditions are Δ34Smelt-sulfide of - 0.1 ± 0.5 ‰ & 8.2 ± 0.6 ‰. We expect to see a more significant and measurable change in Δ34Smelt-sulfide, showing how sulfur fractionates during rapid natural degassing processes and the kinetic processes at play

Kinetics of Sulfur Isotope Fractionation from experiments in Melt and Sulfide using a dynamic 1-atm Gas-mixing Furnace

International audienceSulfur isotope fractionation plays a crucial role in understanding the evolution of sulfur concentration and isotopes in natural magmatic systems. This is because sulfur has multiple oxidation states varying from -2 to +6, a multitude of reactions, not limited to precipitation, dissolution, phase separation occurs in these systems. It is during these reactions, in a dynamic environment, that sulfur could fractionate. Understanding this fractionation can help us estimate critical parameters related to degassing processes, such as magma ascent rate, residence time in the reservoir. We conducted experiments in dynamic conditions to determine sulfur isotope fractionation factors in a magma containing sulfides, in a 1-atm gas-mixing furnace using the gas mixture CO-CO2-SO2. Disequilibrium conditions in the furnace can be modelled to simulate volcanic degassing, as the maximum rate of the change of fO2 within the furnace is 0.013 log unit s-1. Sets of time-series experiments have shown that the system reaches equilibrium, both in terms of major element and sulfur isotope composition within 8 hours. Once at equilibrium, dynamic conditions were imposed, by varying fO2 & fS2 from the initial to the final conditions. From initial tests at 1300 °C, changing ~1.2 log unit over 4 hours (log fO2= -10.3 to -11.1, log fS2= -1.2 to -2.5). At the start the Δ34Smelt-sulfide is 0.8 ± 0.8 ‰ and after 4h we measured a Δ34Smelt-sulfide of -0.1 ± 0.5 ‰ which is in agreement with the expected equilibrium value of -0.4 ± 0.3 ‰, but the kinetic effect of isotopic fractionation remains hidden by the poor analytical precision. We will further test and present the results from multiple series of dynamic experiments at 1300 °C, running from a reducing (log fO2, fS2: -11.1, -2.5) to an oxidizing condition (log fO2, fS2: -8.5, -1.4) each lasting a duration between 5 minutes and 1 hour after equilibration. The equilibrium isotopic fractionation values for these 2 conditions are Δ34Smelt-sulfide of - 0.1 ± 0.5 ‰ & 8.2 ± 0.6 ‰. We expect to see a more significant and measurable change in Δ34Smelt-sulfide, showing how sulfur fractionates during rapid natural degassing processes and the kinetic processes at play

Fluid inclusions of ophicarbonates in Oman and Western Alps ophiolite and carbonation experiments of mantle minerals

International audienceCarbonated serpentinites, ophicarbonates, are exposed in ophiolites which can be remnants of oceanic lithosphere. Multiple generations of fluids for carbonation are recorded in fluid inclusions of ophicarbonates in the Oman and the Chenailet and the Lago Nero ophiolites on the Franco-Italian border in the Western Alps. The Oman ophiolite records oceanic lithosphere in fast-spreading ridges and the Chenailet and the Lago Nero ophiolites in slow-spreading ridges. The fluid inclusions of ophicarbonates are studied with Raman microscopy and microthermometry, revealing saline fluid inclusions commonly found in the carbonates. Their homogenization temperature ranges from 100 to 220 °C, suggesting they are hydrothermal origins. Three types of fluid inclusions are observed in the ophicarbonates studied in the Oman and the Western Alps: (1) fluid inclusions show a wide range of salinities from 0-11 wt. % NaCl eq., suggesting a mixing origin of brine and steam separated by seawater boiling in the ophicarbonate of the Chenaillet and Lago Nero ophiolites, (2) fluid inclusions with an average of 5 wt. % NaCl eq., a little higher than seawater, which may circulate beyond the Moho and form veins into the oceanic mantle harzburgite of the Oman ophiolite, (3) meteoric water during obduction processes in the Oman ophiolite with low salinities of 0–1 wt.% NaCl eq. during the obduction processes of the Oman. The role of slab-derived fluids in ophicarbonates can be understood by studying meta-ophicarbonates in the Queyras (France) and Monviso (Italy) regions of the Western Alps. Carbonate ions are more soluble in saline fluids than in fresh water. It is reasonable to observe the saline fluids in carbonate minerals, though the fluid inclusions with low salinities as seen in the third type mentioned above are found. Not only salinity, but pH may have great effects on the carbonate solubility especially in shallow levels. To understand the conditions for carbonation of these serpentinites, experiments have been conducted in which mantle rocks have been reacted with H2O-CO2 fluids using hydrothermal apparatus under 300–400 °C at 180 MPa with NNO oxygen buffer. The starting materials (olivine, antigorite with/without diopside) are used. In addition to magnesite and talc, quartz is also found. Experiments with magnesium end components are largely consistent with previous work in the MgO-SiO2-H2O-CO2 system [Johannes (1969) American Journal of Science, 267, 1083-1104]. Various reactions have been observed in the olivine-diopside and serpentine-diopside systems. The experimental results show that the carbonation of serpentine is accompanied by a dehydration reaction. Under all experimental conditions, the volume of the solid phase increases during reactions. The addition of a carbon dioxide-containing fluids to serpentine causes both dehydration and an increase in solid volume. Carbonate veins are commonly found in serpentinite in nature. In most cases, calcite veins are formed, but in the present experiments, only magnesite or dolomite is formed, at least at temperatures above 300°C. We conclude that natural carbonation reaction in ophicarbonates occur at low temperatures below 300°C or the fluids carry Ca, which may come from sedimentary or mafic rocks