Nicole Métrich

Sulfur abundance and its speciation in oxidized alkaline melts

Abstract The sulfur concentrations and the relative proportions of S2− and S6+ were measured by electron microprobe in a series of melt inclusions trapped in phenocrysts from different subduction-related and within-plate volcanoes. The melt inclusions correspond to potassic and shoshonitic primary melts to tholeiitic and hawaiitic primitive melts. In the tholeiitic and the transitional basaltic melt inclusions, sulfur is mainly present as S2− ( S 6+ S total = 0.03 to 0.08 ), and varies from 0.13 to 0.18 wt%. The occurrence of immiscible sulfides attests to their saturation. In shoshonitic and potassic primary melts, sulfur (S = 0.12 to 0.32 wt%) is dissolved as both S2− and S6+ ( S 6+ S total = 0.3 to 0.7 ). Their oxygen fugacity, estimated from the S 6+ S total ratios, ranges from NiNiO to NiNiO + 1 log unit. Hawaiitic melts may also dissolve up to 0.3 wt% sulfur possibly because of their oxidation state close to NiNiO, as illustrated by samples from Mt. Etna, Italy. Variations of sulfur, at constant S 6+ S total ratio and temperature in both the potassic and shoshonitic primary melts, indicate that these melts are undersaturated, with respect to a S-rich condensed phase, in agreement with their relatively high oxidation state. It strongly suggests that sulfur behavior in relatively oxidized primary alkaline melts is controlled by the mantle source melting conditions.

Volcanic degassing of bromine and iodine: experimental fluid/melt partitioning data and applications to stratospheric chemistry

Abstract In order to understand the degassing behavior of heavy halogens in volcanic processes, we experimentally studied the distribution of Cl, Br, and I between albite melt and hydrous fluids containing 0.01–2 wt% of NaCl, NaBr, or NaI, respectively. All experiments were carried out at 2 kbar and 900°C in rapid-quench cold-seal autoclaves with a run duration of 7 days. The major element compositions and Cl contents of the glassy run products were measured by electron microprobe. Bromine and iodine were measured by proton-induced X-ray emission. Fluid compositions were obtained by mass balance. All halogens investigated were found to partition strongly into the fluid phase. Over the range of concentrations studied, the halogen contents in the melt are proportional to the concentrations in the fluid. The fluid/melt partition coefficients, Df/m, are 8.1±0.2 for Cl, 17.5±0.6 for Br, and 104±7 for I. The logarithm of Df/m is linearly correlated with the ionic radius of the halogenide ion. On the basis of our experimental data, we estimate the amount of bromine injected into the stratosphere by major volcanic explosions. For the 1991 Mount Pinatubo eruption, we obtain Br yields of 11–25 kt as minimum estimates. These numbers are comparable to the total annual influx of bromine into the stratosphere from all other natural and anthropogenic sources (about 100 kt/year). Since bromine is much more efficient in destroying stratospheric ozone than chlorine, it could at least be partially responsible for the massive ozone depletion observed after the 1991 Mount Pinatubo eruption.

High-temperature experiments on silicate melt inclusions in olivine at 1 atm: inference on temperatures of homogenization and H2O concentrations

Abstract A series of heating stage experiments, at ambient atmosphere and high temperature, were conducted in order to detail the change in the homogenization temperature and the behavior of H 2 O of melt inclusions in olivine phenocrysts (Fo 82–89 ). The samples were collected at different volcanic sites (Piton de la Fournaise [PdF], Reunion Island; Stromboli, Aeolian Islands; FAMOUS zone) expected to have a range of magmatic water contents. The melt inclusions vary in composition from basaltic to shoshonitic, with H 2 O content from 0.14 to 2.9 wt.%. Temperatures of homogenization ( T h ) of melt inclusions systematically increase with time during heating experiments, regardless of their major element composition and their H 2 O. It is proposed that T h changes with time in response to the deformation of the host crystal and change in the volume of the cavity. The FTIR spectra successively acquired on the same inclusion repeatedly heated at constant temperature clearly demonstrated that the relative absorbance (Abs. n /Abs. 0 ) measured at 3535 cm −1 corresponding to hydroxyl group and molecular H 2 O decreases with time. The H 2 O-rich melt inclusions may have lost from 20% to 80% H 2 O after the first homogenization and almost total dehydration of melt inclusions may occur within few hours or less, at 1 atm. Water loss driven out off olivine-hosted melt inclusions possibly exists to some extent in natural large dunitic bodies or in mantle xenoliths.

Low Pressure Crystallization Paths of H2O-Saturated Basaltic-Hawaiitic Melts from Mt Etna: Implications for Open-System Degassing of Basaltic Volcanoes

Melt inclusions indicate that the basaltic-hawaiitic magmas from Mt Etna (Sicily) contained up to 2.3 wt% H2O dissolved in the melt, prior to eruption. The effect of H2O degassing during magma ascent has been experimentally constrained between 1135 and 1009°C, for PH2O = Ptotal varying from 800 to 270 bars. The starting material was a primitive hawaiitic lava sample (MgO = 7.1 wt%) representative of the less evolved lava emitted at Mt Etna. Experiments were conducted in TZM pressure vessels, with Ag70Pd30 capsules in order to minimize the FeO loss. At temperatures of 1135–1090°C, PH2O = 800 bars, with NiNiO and FMQ buffers, olivine (Fo83-80) is the liquidus phase in equilibrium with a residual hawaiitic melt (Mg# 0.60–0.57; CaO/Al2O3 = 0.82). Salitic pyroxene begins to crystallize at 1075°C, plagioclase at 1025°C and at 1009°C, the magma is 33.5% crystallized with olivine, Ca-rich pyroxene, and plagioclase (16:60:24). At PH2O = 270 bars, FMQ buffer, olivine (Fo79.3-80) and salitic pyroxene are the main liquidus phases between 1100 and 1090°C. They are in equilibrium with hawaiitic melts (Mg# 0.51; CaO/Al2O3 = 0.73). At 1070°C, the experimental charges are highly crystallized (≈51.5%) with olivine (Fo70), salite, and plagioclase (An78.4-76.3) in 14:44:42 relative proportions. Increasing the water content of hawaiitic-basaltic magmas expands the stability field of the olivine (relative to the other phases), lowers the crystallization temperatures of pyroxene and plagioclase, and results in the development of a more An-rich plagioclase. When compared to the natural samples, these results support a model of early and moderate crystallization of olivine from Etnean primitive hawaiitic magmas, containing close to 2.5 wt% H2O, with NNO oxygen buffering conditions, at low pressure (PH2O =Ptotal). We propose a model of crystallization driven by decompression and water degassing during the emplacement of magma in the volcanic pile itself and possibly concomitant with the opening of fractures.

Mafic magma batches at Vesuvius: a glass inclusion approach to the modalities of feeding stratovolcanoes

Glass inclusions in olivine and diopside phenocrysts from pyroclasts of various eruptions of Vesuvius are representative of the magmas that supplied the volcano in the last 4–5000 years. During this interval the volcano alternated between open conduit activity (e.g. 1944 and 1906 eruptions) with long pauses interupted by Plinian and sub-Plinian eruptions (e.g. 3360 B.P. “Avellino”, A.D. 79 “Pompei”, A.D. 472 “Pollena”). The eruptive behaviour was conditioned in all cases by the presence of shallow reservoirs: two cases are distinguished: (1) small and very shallow, 1906-type; (2) large and deeper Plinian-sub-Plinian magma chamber. Lapilli of 1906 lava fountains contain olivine (Fo89.5–90.4) including Cr-spinel [Cr/(Cr+Al)] (Cr#>75) and volatile-K-rich tephritic glasses, which represent the first recognized Vesuvius primary magmas. Mg-poorer olivine (Fo83–89) also occurs in 1906 and 1944 products; it formed within the shallow reservoir, together with pyroxene and leucite, between 1200 and 1130°C, from K-tephritic melts (MgO=6–8 wt%). The Plinian and sub-Plinian pumices contain diopside, phlogopite and minor olivine (Fo85–87) representing adcumulates wrenched from the chamber walls. Glass inclusions in diopside (and some olivine) range from K-basalt to K-tephrite (MgO=6–8 wt%), with homogenization temperature of 1130–1170°C. They have been regarded as representative of the magmas supplying the Plinian-sub-Plinian chamber(s). The Avellino glass inclusions have K-basaltic compositions, contrasting with the mostly K-tephritic Pompei and Pollena inclusions. They display lower C1 and P contents with respect to the younger tephritic melts, and these variations should reflect primary features of the mantle-derived magmas. The primary and the near-primary Vesuvius magmas, as illustrated by melt inclusions, emphasize high K, P and volatile (H2O, Cl, F, S) contents, with high K2O/H2O (2–2.5), Cl/F (2.5) and Cl/S (2–3) ratios, consistent with a metasomatized mantle source, and distinguishing the Vesuvius potassic primary magmas from those of the northern part of the Roman Province.

Transfer of sulfur in subduction settings: an example from Batan Island (Luzon volcanic arc, Philippines)

Sulfur abundances have been determined in silicic and basaltic melt inclusions in olivines from harzburgitic xenoliths and a basaltic lava sample, all from Batan Island. In mantle xenoliths, olivines (Fo80–91) are present as neoblasts or in finely recrystallized patches. The most magnesian olivines (Fo89.7–91, CaO 85, CaO = 0.25 wt%) and their melt inclusions (CaO/Al2O3 from 0.8 to 1.15) have recorded early stages of crystallization. The sulfur concentrations for these calc-alkaline basaltic melts are estimated between 1720 and 3200 ppm, with a mean value at 2550 ppm (1σ=390) and S/Cl ratio at nearly 1. This is in agreement with the idea that arc basaltic melts may contain high concentrations of sulfur (S > 2000 ppm), at 1200°C. However, the heterogeneous distribution of S and its partitioning between silicate melts, H2O-rich vapor and S-bearing solid phases as illustrated by the Batan mantle xenoliths would result in highly variable sulfur concentrations in island arc basaltic magmas, mostly controlled by fO2 and fS2.

The 1989–1990 activity of Etna magma mingling and ascent of H2OClSrich basaltic magma. Evidence from melt inclusions

Abstract The melt inclusions in olivine phenocrysts from basaltic tephra erupted by Etna volcano during the 1989–1990 eruptive period were investigated for major elements, Cl and S by electron microprobe, fluorine by PIGME (Particule Induced Gamma-ray Emission) and H 2 O by SIMS. The results indicate the ascent of basaltic magmas, with different K 2 O concentrations possibly related to different feeding-dykes. “LK” basaltic melt inclusions, rich in Cl, S and H 2 O, are trapped in olivine (Fo 84 ) from the 1990 tephra and represent the early crystallization stage. Their composition in major elements is quite similar to those of most of the historic and even pre-etnean alkali-basalts, but significantly lower in K 2 O compared with the 1989–1990 lavas. Only scarce “HK” basaltic melt inclusions were observed in the outer rim (Fo 79.6 ) of reversely-zoned olivine phenocrysts and correspond to late crystallization-stage, at low pressure. High sulfur concentration ( S ≈ 0.3 wt.%) is a general characteristic of the etnean basaltic magmas, with H 2 O/S ratio as high as 7. The volatile abundances of the Etna primitive basalts were computed at 1.45 wt.% for H 2 O, 0.17 wt.% for Cl and 0.047 wt.% for F, with a high Cl/F ratio close to 3.6. The 1989 lava samples with Cl at 660 ± 90 ppm and F at 770 ± 30 ppm have suffered a significant Cl loss. The possible behaviour of the volatile phase as CO 2 , H 2 O even Cl is discussed, with H 2 O and chlorine late degassing and an active role of H 2 O during the lava fountaining activity.

A melt and fluid inclusion study of the gas phase at Piton de la Fournaise volcano (Réunion Island)

Abstract We have studied carbon, water, sulphur and chlorine contents in melt and fluid inclusions trapped in olivines from diverse feeding systems of Piton de la Fournaise volcano (Reunion Island, Indian Ocean). As a whole, water concentrations (0.59–1.10 wt%) are intermediate between those of tholeiites and alkali basalts. Sulphur concentrations (400–1700 ppm) are comparable to those observed in MORB whereas chlorine concentrations (200–300 ppm) are typical of alkali basalts. Carbon contents range from a few to 520 ppm C. We also observe sulphide globules that indicate saturation of the melts with sulphur compounds. The combined study of dissolved carbon contents in melt inclusions and of CO 2 -rich fluid inclusions indicates that the trapped melts may be saturated with a fluid phase from at least 500 MPa total pressure to the surface. The CO 2 and H 2 O contents in the trapped melts, their major element compositions and the composition of the host olivines reveal a complex crystallisation and entrapment history. In general, we observe that the less differentiated melt inclusions appear to have been trapped at greater depth. Magmas fed to both the Summit Crater and the southeast rift zone must be stored at shallow levels before eruption to account for the observed distributions of water and carbon concentrations and their low values. These indicate that a large proportion of the volatiles that are present at greater depths are lost before venting. The North-West feeding system corresponds to magma that was much less degassed before venting. The range of concentrations of H 2 O and CO 2 found in samples of a single eruption cannot be reconciled with continuous degassing of a single magma batch. We therefore propose that most olivines occurring in the lavas have trapped melt batches at different stages of their volatile evolution and/or that olivines included melts of differing mantle provenances. This also implies that melt entrapment occurred at different depths. Carbon and water evolution in parental magmas is calculated to be from 1270 to 2770 ppm C and from 0.7 to 1.1 wt% water, respectively. The volatile contents of parental magmas allow computation of the following volatile fluxes for the average magma production rate of 0.1 m 3 /s observed in the last 50 years: CO 2 , 88×10 3 tons a −1 ; S, 123×10 2 tons a −1 ; Cl, 2.6×10 2 tons a −1 , and (magmatic) H 2 O, 62×10 3 tons a −1 . With literature values of the degree of melting, water and carbon contents in the mantle source would be between 350 and 1100 ppm and between 63 and 277 ppm C, respectively. Because no permanent emanations nor strong degassing during eruption is observed at the surface, it is thought that most gases are intercept at an interface in the building system.

The 1783 Lakagigar eruption in Iceland: geochemistry, CO2 and sulfur degassing

About 12.3 km3 of basaltic magma were erupted from the Lakagigar fissure in Iceland in 1783, which may have been derived from the high-level reservoir of Grimsvotn central volcano, by lateral flow within the rifted crust. We have studied the petrology of quenched, glassy tephra from sections through pyroclastic cones along the fissure. The chemical composition of matrix glass of the 1783 tephra is heterogeneous and ranges from olivine tholeiite to Fe−Ti rich basalt, but the most common magma erupted is quartz tholeiite (Mg#43.6 to 37.2). The tephra are characterized by low crystal content (5 to 9 vol%). Glass inclusions trapped in plagioclase and Fo86 to Fo75 olivine phenocrysts show a large range of compositions, from primitive olivine tholeiite (Mg#64.3), quartz tholeiite (Mg#43–37), to Fe−Ti basalts (Mg#33.5) which represent the most differentiated liquids and are trapped as rare melt inclusions in clinopyroxene. Both matrix glass and melt inclusion data indicate a chemically heterogeneous magma reservoir, with quartz tholeiite dominant. LREE-depleted olivine-tholeiite melt-inclusions in Mg-rich olivine and anorthitic-plagioclase phenocrysts may represent primitive magma batches ascending into the reservoir at the time of the eruption. Vesicularity of matrix glasses correlates with differentiation, ranging from 10 to 60 vol.% in evolved quartz-tholeiite glasses, whereas olivine-tholeiite glasses contain less than 10 vol.% vesicles. FTIR analyses of olivine-tholeiite melt-inclusions indicate concentrations of 0.47 wt% H2O and 430 to 510 ppm for CO2. Chlorine in glass inclusions and matrix glasses increases from 50 ppm in primitive tholeiite to 230 ppm in Fe−Ti basalts, without clear evidence of degassing. Melt inclusion analyses show that sulfur varies from 915 ppm to 1970 ppm, as total FeO* increases from 9 to 13.5 wt%. Sulfur degassing correlates both with vesicularity and magma composition. Thus sulfur in matrix glasses decreases from 1490 ppm to 500 ppm, as Mg # decreases from 47 to 37 and vesicularity of the magma strongly increases. These results indicate loss of at least 75% of sulfur during the eruption. The correlation of low sulfur content in matrix glasses with high vesicularity is regarded as evidence of the control of a major exsolving volatile phase on the degassing efficiency of the magma. Our model is consistent a quasi-permanent CO2 flux through the shallow-level magmatic reservoir of Grimsvotn. Following magma withdrawal from the reservoir and during eruption from the Lakagigar fissure, sulfur degassing was controlled by inherent CO2-induced vesicularity of the magma.

Potassic primary melts of Vulsini (Roman Province): evidence from mineralogy and melt inclusions

The origin and the relationships between the high potassic (HKS) and potassic (KS) suites of the Roman Comagmatic Province and the nature of their primary magmas have been intensively debated over the past 35 years. We have addressed these problems by a study of mineralogy (olivine Fo92-87, Cr-spinel and diopside) and melt inclusions in olivine phenocrysts from a scoria sample of Montefiascone (Vulsini area). This rock is considered as one of the most primitive (MgO=13.5 wt%, NiO=340 ppm; Cr=1275 ppm) in the northern part of the Roman Comagmatic Province. The compositions of both the olivine and their melt inclusions are controlled by two main processes. In the case of the olivine Fo<90.5, fractional crystallization (olivine + diopside + minor spinel) was the principal mechanism of the magma evolution. The olivine (Fo92-90.5) and the Cr-spinel (Cr#=100. Cr/(Cr+Al)=63-73) represent a near-primary liquidus assemblage and indicate the mantle origin of their parental magmas. The compositions of melt inclusions in these olivine phenocrysts correspond to those of poorly fractionated H2O-rich (≈ 1 wt%) primary melts (MgO=8.4-9.7 wt%,FeOtotal=6-7.5 wt%). They evidence a wide compositional range (in wt%: SiO2=46.5-50, K2O=5.3-2.8, P2O5=0.4-0.2, S=0.26-0.12; Cl=0.05-0.03, and CaO/Al2O3= 0.8-1.15), with negative correlations between SiO2 and K2O, Al2O3 and CaO, as well as positive correlations between K2O, and P2O5, S, Cl, with nearly constant ratios between these elements. These results are discussed in terms of segregation of various mantle-derived melts. The high and constant Mg# [100.Mg/(Mg+Fe2+)] 73-75 of studied melts and their variable Si, K, P, Ca, Al, S contents could be explained by the melting of a refractory lithospheric mantle source, heterogeneously enriched in phlogopite and clinopyroxene (veined mantle source).