Showing papers by "Bruno Scaillet published in 2003"

Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

Abstract: Crystallisation and mantle-interaction experiments have been performed on the 1991 Pinatubo dacite to constrain its petrogenesis. In the dacite- H2O system at 1000 MPa, magnetite and either clinopyroxene or amphibole are the liquidus phases. No garnet is observed at this pressure. Dacite-mantle interaction at 920 MPa produces massive orthopyroxene crystallisation, in addition to amphibole ± phologopite. Amphibole crystallising in dacite at 1000 MPa faithfully reproduces aluminium-rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite, suggesting a high pressure stage of dacite crystallisation under high melt H2O contents (> 10 wt%) and low temperatures (< 950C). Altogether, plagioclase, amphibole and melt inclusion compositions suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900C, when emplaced in the shallow reservoir, in which most phenocrysts precipitated until the onset of the 1991 eruptions. The strongly fractionated REE pattern demands garnet somewhere during dacite genesis, which in turn requires high pressure derivation. Partial melting of subducted oceanic crust gives melts unlike the Pinatubo dacite. Interaction of these slab melts with sub-arc peridotite is unable to produce such type of dacite, nor is a direct mantle origin conceivable on the basis of mantle-dacite interaction experimental results. Dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low FeO/MgO, high H2O, Ni and Cr contents typical of Pinatubo-like magmas. On the face of chemical and phase equilibrium constraints, the most plausible origin of the Pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous, oxidised, primitive basalt that crystallises amphibole and garnet upon cooling, as shown by recent phase equilibrium work. Dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally, following injection of H2O-poor mantle-derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts, parental to dacite magmas, that rest at mantle depths. Dacites such as the Pinatubo one may thus witness particular H2O-rich conditions of the sub-arc mantle rather than the melting of a young and hot subducting oceanic plate.

Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Abstract: Crystallization experiments on three comendites provide evidence for the genetic relationships between peralkaline rhyolites in the central Kenya rift valley. The crystallization of calcic clinopyroxene in slightly peralkaline rhyolites inhibits increase in peralkalinity by counteracting the effects of feldspar. Fractionation under high fO2 conditions produces residual liquids that are less, or only slightly more, peralkaline than the bulk composition. In contrast, crystallization under reduced conditions ( 1·4) after 25 wt % crystallization. Upon further crystallization, extreme peralkaline compositions (NK/A 2·5) are obtained, with relatively low SiO2 (66 wt %) and Al2O3 (7·4 wt %), and high FeO (10·2 wt %) and Na2O (8·4 wt %) contents. In the absence of crystallization of sodic phases such as arfvedsonite or aegirine, fractionation may yield even more extreme compositions. Pantelleritic rhyolites can be produced at temperatures below 800°C, at low fO2, high fF2, by either extreme fractional crystallization or near-solidus melting of less peralkaline, but more silicic, sources.

Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes

Abstract: Recent phase equilibrium studies, combined with analytical and petrological data, provide rigorous constraints on the pre-eruptive P-T-fH2O-fO2-fS2-fCO2 conditions of silicic to mafic arc magmas. Pre-eruptive melts show a broad negative correlations between temperature and melt H2O contents. Pre-eruptive melt S contents cluster around 100 PPM in residual rhyolitic liquids of silicic to andesitic magmas, and range up to 5000 PPM in more mafic ones. For the entire compositional spectrum, melt sulphur contens are almost independent of prevailing fO2. In contrast, they are positively correlated to fSOe, in agreement with experimental observations. Using these intensive constraints, the composition of coexisting fluid phases has been modelled through a MRK equation of state. Pre-eruptive fluids in silicic to andesitic magmas have XH20 (mole fraction of H2O) in the range 0.65-0.95. XH2O decreases as pressure increases, whereas XCO2 increases up to 0.2-0.3. Pre-eruptive fluids in hydrous mafic arc magmas, such as high-alumina basalts, generally have similar mole fractions of H2O and CO2 at mid-crustal levels, with XH2O increasing on ly for magmas stored at shallow levels in the crust (< 1kbar). The sulphur content of the fluid phase ranges from 0.12 up to 6.4 wt% in both mafic and silicic magmas. For silicic magmas coexisting with 1-5 wt% fluid, this implies that more than 90% of the melt+fluid mass of sulphur is stored in the fluid. Calculated partition coefficients of S between fluid and melt range from 17 up to 467 in silicic to andesitic magmas, tending to be lower at low fO2, although exceptions to this trend exist. For mafic compositions, the sulphur partition coefficient is constant at around 20. The composition of both melt and coexisting fluid phases under pre-eruptive conditions shows marked differences. For all compositions, pre-eruptive fluids have higher C/S and lower H/C atomic ratios than coexisting melts. Comparison betwwen volcanic gas and pre-eruptive fluidcompositions shows good agreement in the high temperature range. However, to reproduce faithfully the compositional field delineated by volcanic gases, silicic to andesitic arc magmas must be fluid-satured under pre-eruptive conditions, with fluid amounts of at least 1 wt% whereas mafic compositions require lower amounts of fluid, in the range 0.1-1 wt%. Nvertheless, volcanic gases colder than 700°C are generally too H2O-rich and S_poor to have been in equilibrium with silicic to andesitic magmas under pre-eruptive conditions, which suggests that such gases probably contain a substantial contribution from meteoric or hydrothermal water.

Petrological and volcanological constraints on volcanic sulfur emissions to the atmosphere

Abstract: A review of current knowledge about the physics of volcanic eruptions and the experimental constraints on sulfur behavior in magmas is presented with the aim of evaluating the range of sulfur yields from major explosive eruptions based on petrological and volcanological data. The so-called petrologic method, used to evaluate syn-eruptive melt degassing, is expanded by also considering the role of a pre-eruptive gas phase. It is shown that more than 90% of the atmospheric sulfur loading may come from release of the pre-eruptive gas phase during eruptions of intermediate to silicic magmas at subduction-related volcanoes. In contrast, the role of a pre-eruptive gas phase may be much less important in more mafic magmas erupted in other tectonic settings. Bulk sulfur contents are weakly (spreading-ridges and hot spots) to non-correlated (subduction-zones) with bulk-rock composition. With the exception of some persistently active volcanoes, bulk sulfur contents generally do not exceed 0.5 wt %. The sulfur yields are positively correlated with mass of erupted magma, but the dispersion in sulfur emission for a given erupted mass increases progressively as the erupted mass decreases. The sulfur yields of single eruptive events retrieved from this improved petrologic method are shown to closely agree with independent estimates obtained from analysis of ice cores, optical-depth measurements, and remote-sensing spectroscopic techniques (TOMS and COSPEC).

Massive atmospheric sulfur loading of the AD 1600 Huaynaputina eruption and implications for petrologic sulfur estimates.

Abstract: We combine petrological, analytical, and thermodynamical data to constrain the sulfur yield of the AD 1600 Huaynaputina eruption which has been associated with the largest Earth's temperature shift in the last 600 years. The calculated amount of S (26–55 Tg), partly overlaps, but ranges to almost twice the amount estimated from ice-core data (16–32 Tg), the higher values of our estimate probably reflect that not all S released by the eruption reached the stratosphere. Our study also shows that it is possible to estimate the atmospheric sulfur loading from the volcanic products themselves, which opens the possibility to explore volcano-climate links beyond the time period covered by ice-core archives

Chemical transfer during redox exchanges between H2 and Fe-bearing silicate melts

Abstract: Kinetics and reaction paths of Fe 3+ reduction by H 2 in high-Fe and low-H 2 O silicate melts have been investigated at 800 °C. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure H 2 using rapid-heating and rapid-quench strategies. Within the first minutes of the experiments, a fast partitioning of Na occurred between the gas and the melt due to the reducing conditions. Kinetically decoupled from the Na partitioning, the progression of a front of Fe 3+ reduction within the quenched melt was observed and was identified as a diffusion-limited process. The growth of the reduced layer is accompanied by an increase in concentration of OH-groups suggesting that reduction operates through proton incorporation within the melt. As this growth rate is slightly faster than predicted from the diffusion of molecular H 2 O, a different and mobile water-derived species seems likely. One possible mechanism is the reduction of Fe 3+ by the transport of molecular H 2 . As this process is limited by the flux of H 2 , it will depend on both diffusivity and solubility of H 2 in the melt. Alternatively, migration of protons (H + ) and electronic species within the melt could control the velocity of the reduction front. The increase in concentration of the reaction-derived OH groups produces a water over saturation followed by partial dehydration of the melt. This dehydration leads to a change in the redox conditions within the gas that influences the Na partitioning between gas and melt.

Emplacement of the lyngdal granodiorite (SW Norway) at the brittle-ductile transition in a hot crust.

Abstract: The 1.0–0.9 Ga plutons of the HBG suite (Hornblende-Biotite Granitoids) associated with the Rogaland anorthosites belong to the Sveconorwegian post-collisional magmatism in southwestern Norway. The comparison between experimental and natural phase equilibria indicates that the Lyngdal granodiorite (HBG suite) crystallized between 4 and 2 kbar, the magma having 6 wt% H2O in melt at early stages and an fO2 in the range NNO/NNO+1 (Bogaerts et al., 2002).

Experimental constraints on pre-eruption conditions of Vesuvius phonolites.

Abstract: We are currently establishing the phase relationships of Vesuvius phonolites (white pumices of Mercato, Avellino, Pompei and Pollena eruptions) and tephrites with the aim of defining the P-T-fH2O-fO2 pre-eruption conditions of erupted magmas Here we will present the results so far obtained on the phonolites We used dry starting glasses prepared from fusing twice powdered pumice at 1 bar, to which various amounts of H2O and CO2 were added to achieve fluid saturation, loaded to Au capsules Experiments were performed at 1-3 kbar, 750-900°C, H2Omelt 1-10 wt% and NNO-NNO+1, using an IHPV fitted with a H2-membrane, with Ar-H2 mixtures as pressure media Run durations were 7-30 days, and were ended by isobaric quenches Phase relationships were established either isobarically (2 kb, 750-900°C) or isothermally (800°C, 1-3 kb) Phases crystallised are: sanidine, plagioclase, leucite, nepheline, analcime, scapolite, sodalite, biotite, amphibole, garnet, clinopyroxene, magnetite Under water-rich conditions, clinopyroxene is the liquidus phase in all compositions In Pompei and Pollena phonolites, however, it reacts out to garnet at temperatures below 800-820°C Amphibole is not stable at temperatures above 825°C at 2 kb in Avellino, Mercato and Pompei pumices, and it is always absent in Pollena at any P-T so far explored Experiments realised below 2 kb and 800°C do not crystallise amphibole in any compositions, except in Pompei at low H2Omelt Similarly, at 3 kb and 800°C, amphibole is absent in the Pompei Leucite crystallises only in Pompei and Pollena, and is present at and below 2 kb At 2 kb, garnet crystallisation is restricted to below 825°C in Pompei and below 800°C in both Mercato and Avellino pumices, whereas it is stable up to 900°C in Pollena The presence of amphibole in Mercato, Avellino and Pompei pumices constrains pre-eruption temperatures of the magmas to be below 825°C For Pompei, amphibole and leucite occurrences suggest a pressure of magma storage of 21±02 kb At this pressure, the phase assemblage of Pompei white pumice is reproduced at 815±10°C, H2Omelt of 6-65 wt% (XH2Ofluid of 08±005), at an fO2 of NNO+05±05, in agreement with melt inclusion constraints (Cioni, 2000) For both Mercato and Avellino, experiments are not yet conclusive with respect to pressure If, however, the magma reservoir that fed these two eruptions was at a similar pressure depth than that of the Pompei event then phase equilibria imply pre-eruption temperatures 08) which contrast with melt inclusion data for Avellino (31±07 wt%, Signorelli et al, 1999) Additional experiments will test these estimates For Pollena, the presence of amphibole in the pumice shows that there too additional experiments are needed Ignoring this problem, phase equilibria suggest temperatures higher than 800°C, H2Omelt < 6 wt% (XH2Ofluid<08) and pressure below 3 kb Cioni, R, 2000, Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy), Contrib Miner Petrol, 140, 40-54 Signorelli, S, Vaggelli, G and Romano, C, 1999, Pre-eruptive volatile (H2O, F, Cl and S) contents of phonolitic magmas feeding the 3550-year old Avellino eruption from Vesuvius, southern Italy J Volc Geotherm Res, 93, 237-256

Experimental investigations on the role of sulfur in magmas

Abstract: Sulfur (S) is the third volatile element in importance in Earth's magmas, after water and carbon dioxide. Recent work has shown that sulfur abundance (melt+gas) in typical arc magmas commonly exceeds 0.1 wt%, with maxima in the range 0.5-1 wt% [Scaillet et al., 2003]. In this presentation we will review recent advances with respect to the solubility and partitioning of S in silicate melts and give a general outline of on going, as well as next, research priorities. The solubility of S in metaluminous rhyolitic and phonolitic melts has been extensively investigated by Clemente et al. [2003] and Moncrieff et al. [in prep], respectively. Experiments have been performed at 800- 1000°C, 1-4 kb, and fO2 ranging from NNO-2 up to NNO+4. Both studies have shown that S solubility is sensitive to T, fO2 and fS2 variations, pressure control being less important. At any given fO2, the higher the fS2, the higher the S solubility. Rhyolite and phonolite melts follow the same pattern. The modeling of S solubilities has been attempted using either empirical or thermodynamical approaches. Empirical equations are available for either a given melt composition (rhyolite) or are valide over a large compositional spectrum (rhyolite-basalt). The thermodynamic modelling of S solubilities can be done considering that the total dissolved S results from the addition of H2S and SO2 species dissolution reactions, whose relative abundances depend on the prevailing fO2. This model has been calibrated only on rhyolite compositions. The S2-/S6+ proportions predicted by this model are in good agreement with experimental observations. The partition coefficient of S between melt and gas has been determined mostly in silicic magmas [Scaillet et al., 1998; Keppler, 1999], for which there appears to be a strong control of fO2. Thermodynamic calculations predict that the partition coefficient between gas and melt should decrease with melt silica content, from ca 1000 in rhyolite, to 100 in andesite, to 10 in basaltic melts [Scaillet and Pichavant, 2003]. Recent experimental work has explored the S behaviour in peralkaline rhyolites, which appears to dissolve up to 20 times more sulfur than their metaluminous counterpart [Scaillet and Macdonald, in prep]. Despite its relative low concentrations, S affects the stability of phases such as pyroxenes, amphiboles and biotite. In moderately oxidized dacite magmas, the addition of a few thousands ppm of S enhances the thermal stability of biotite by as much as 60°C [Costa et al., submitted]. In strongly oxidized dacites, the incorporation of S leads to the breakdown of horblende at the expense of orthopyroxene [Scaillet and Evans, in prep]. These effects on phase relations suggest the existence of various sulfur complexes in the silicate melt. We are currently concentrating our efforts on (1) calibrating existing solubility models on mafic compositions, (2) determining the S partitioning between melt and gas in mafic melts. We suggest that additional investigations should be devoted to (1) investigating the effects of S on silicate melt transport properties (density, diffusivity and viscosity), (2) determining the nature and proportion of S species dissolved in silicate melts so as to build more realistic thermodynamic models. References Clemente, B., Scaillet, B. and Pichavant, B. (2003). The solubility of sulphur in rhyolitic melts. Journal of Petrology, in press. Keppler, H. (1999). Experimental evidence for the source of excess sulfur in explosive volcanic eruptions. Science, 284, 1652-1654. Scaillet, B., Clemente, B., Evans, B. and Pichavant, M. (1998). Redox control of sulfur degassing in silicic magmas. Journal of Geophysical Research, 103, 23937-23949. Scaillet, B. and Pichavant, M. (2003). Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes. Geol. Soc. Spec. Pub., in press. Scaillet, B., Luhr, J. and Carroll, M. (2003). Petrological and volcanological constraints on volcanic sulfur emissions to the atmosphere. In Volcanoes and the Earth atmosphere, A. Robock and C. Oppenheimer (eds.), AGU, in press.

Experimental Phase Equilibria: Data and Models

Abstract: Equilibria between silicate melts, crystalline and vapour phases essentially control major and trace element compositions of natural magmas and play an essential role in partial melting as well as in differentiation processes. The development of models of phase equilibria is a prerequisite for assessing the dynamics of magmatic processes, such as magma storage in a reservoir or magma ascent in a conduit. The interest of experimental phase equilibria is essentially twofold. (1) Experimental phase equilibrium results can put precise constraints on the magmatic conditions, either in the magma storage region or in the conduit. Knowledge of pre-eruptive parameters (P, T, H2O in melt, fO2, ...) is necessary for eruption models and consequently for the evaluation of volcanic risk. (2) Experimental phase equilibria constitute our main source of information for the calibration of the mixing properties of multicomponent silicate melts, and for the construction of thermodynamic models which are our tomorrow's tools for the simulation of magmatic processes.