Are Volcanic Gases Serial Killers?

International audienceVolatiles released by volcanic eruptions are often cited as a possible cause of major environmental changes. On a decadal time scale, at least, the connection between volcanic eruptions and climate was firmly established after the 1991 eruption of Mount Pinatubo, Philippines, whose climate aftermaths have been extensively documented and modeled (1). The remaining debate concerns the effect of magmatic volatiles on long-term climate trends (2). On page 1654 of this issue, Self et al. (3) fill in the picture of what gases have been released by volcanoes, and how much, during the so-called flood events. Such events are the most important volcanic eruptions that occurred on Earth. They are produced by mantle upwelling and its partial melting, resulting in massive basalt (a magma poor in silica) outpouring with volumes often exceeding 1 million km3

Physical conditions of primitive tephritic magmas from vesuvius : first experimental results.

Volatile-rich tephritic melts represent the most primitive compositions at Vesuvius. Recent eruptions (1906, 1944) record the multiple injections of such compositions at shallow levels, and their crystallization and mixing. Plinian magma chambers (eg, the 79 AD Pompei eruption) grow from the periodic supply of K-rich mafic melts

Fluorite stability in silicic magmas.

Recent experimental evidence is used to assess the conditions under which fluorite forms an early crystallising phase in silicic magmas. Fluorite solubility primarily depends on the (Na + K)/Al balance in the coexisting silicic melt, reaching a minimum in metaluminous melts. It can display reaction relationships with topaz and titanite, depending on changes in melt composition during crystallisation. An empirical model of fluorite stability in Ca-poor peralkaline rhyolite melts is derived and applied to selected rocks: $${\text{F}}_{{{\text{melt}}}} {\left( {{\text{wt}}\% } \right)} = {\left[ {{\left( {{\text{Na}} + {\text{K}}/{\text{Al}}} \right)}{\left\{ {2.1110^{{ - 3}} {\text{T}}{\left( {^\circ C} \right)} + 1.5778} \right\}}} \right]} - 2.789$$ At the F contents preserved in most silicic rocks, fluorite should normally appear late in the crystallisation sequence, in agreement with petrographic observations. During fluid-absent crustal anatexis, fluorite should melt at a relatively early stage and restitic fluorite is unlikely to persist during prolonged melting. Fluorite may, however, exert a decisive control on the alkali/alumina balance of sub-aluminous anatectic melts and it can affect the liquid line of descent of silicic magmas once extracted from source

Oceanic slab melting and mantle metasomatism

Modern plate tectonic brings down oceanic crust along subduction zones where it either dehydrates or melts. Those hydrous fluids or melts migrate into the overlying mantle wedge trigerring its melting which produces arc magmas and thus additional continental crust. Nowadays, melting seems to be restricted to cases of young (<50 Ma) subducted plates. Slab melts are silicic and strongly sodic (trondhjemitic). They are produced at low temperatures (<1000°C) and under water excess conditions. Their interaction with mantle peridotite produces hydrous metasomatic phases such as amphibole and phlogopite that can be more or less sodium rich. Upon interaction the slab melt becomes less silicic (dacitic to andesitic), and Mg, Ni and Cr richer. Virtually all exposed slab melts display geochemical evidence of ingestion of mantle material. Modern slab melts are thus unlike Archean Trondhjemite–Tonalite–Granodiorite rocks (TTG), which suggests that both types of magmas were generated via different petrogenetic pathways which may imply an Archean tectonic model of crust production different from that of the present-day, subduction-related, one

Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift

The phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni–NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5–6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent–daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO⁎ contents above 12 wt.% and Na2O contents approaching 10 wt.%. Pantellerites are therefore the true near-minimum melt compositions of alkaline oversaturated magma series

Experimental constraints on the origin and evolution of the Bishop Tuff.

The Bishop Tuff has benefited from extensive field, petrological and geochemical studies for more than 50 years to the point of becoming a classical example of a zoned magma chamber in many geological textbooks. The mechanism(s) leading to the development of geochemical zoning in such magmas are still vigorously debated, however. Fractionation mechanisms invoked so far call upon some sort of separation between early formed phenocrysts and liquid (Wallace et al., 1999; Anderson et al., 2000), mixing between various end-members, or on the establishment of chemical gradients within the liquid resulting from thermal gradients in the magma body (Hildreth, 1981). Early work rejected the possibility of fractionation being driven by crystal settling (Hildreth, 1981), but recent melt inclusion studies have resurrected some kind of crystal-liquid separation (Anderson et al., 2000). Interest in the petrogenesis of large silicic magma chambers revived in the early nineties when detailed isotopic work concluded that phenocryst crystallisation in rhyolitic magma chambers might precede by several hundred thousand years the time of eruption (Halliday et al., 1989), implying maintaining largely liquid for protracted periods huge amounts of relatively cold magma in upper crust. This model was questioned, mainly on physical grounds, on the basis that the thermal regime even of large silicic bodies would be unable to ensure magmatic lifetimes in excess of 100 kyr (Sparks et al., 1990), unless the heat supplied by putative underlying basalt strictly balances that resulting from conductive cooling atop the silicic magma body. Subsequent isotopic works have either supported the hypothesis of enhanced longevity (e.g., van den Bogaard and Schirnick, 1995; Reid et al., 1997; Davies and Halliday, 1998) or criticized it (Reid and Coath, 2000)

Experimental Constraints on Sulphur Behaviour in Subduction Zones: Implications for TTG and Adakite Production and the Global Sulphur Cycle since the Archean

International audienceWe performed crystallization experiments at 2-3 GPa at 700-950°C on basaltic and pelitic lithologies with added water and sulphur to constrain the factors controlling sulphur behaviour in subduction zones and how it may have varied through geological time. The resulting hydrous silicic melts have up to 20 times more dissolved sulphur (up to 1 wt %) than at 0*2-0*4 GPa, when moderately oxidized conditions prevail. Such high solubilities appear to result from the combined effects of enhanced solubility of water in high-pressure silicate melts (10-20 wt % H2O), which acts to decrease silica activity, and oxidizing conditions. The results confirm previous findings that high sulphur contents in silicate melts do not necessarily require iron-rich compositions, suggesting instead that sulphur-water complexes play a fundamental role in sulphur dissolution mechanisms in iron-poor silicic melts, in agreement with recent spectroscopic data. The experimental melts reproduce Phanerozoic slab-derived magmas, in particular their distinct Ca- and Mg-rich composition. The results also show that sulphur increases the degree of melting of basalt lithologies. Hence, we suggest that subducted slabs will preferentially melt where sulphur is present in abundance and that the variability in arc magma sulphur output reflects, in part, the vagaries of sulphur distribution in the slab source. In contrast, comparison with the composition of Archean felsic rocks suggests that, in the early Earth, much less sulphur was present in subducted slabs, in agreement with a number of independent lines of evidence showing that the Archean ocean, hence the hydrothermally altered subducted Archean oceanic crust, was considerably poorer in sulphur than at present. Volcanic degassing of sulphur was thus probably much weaker during the Archean than in Proterozoic-Phanerozoic times

Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

International audienceCrystallization (dacite) and interaction (dacite–peridotite) experiments have been performed on the 1991 Pinatubo dacite (Luzon Island, Philippines) to constrain its petrogenesis. In the dacite–H2O system at 960 MPa, magnetite and either clinopyroxene (low H2O) or amphibole (high H2O) are the liquidus phases. No garnet is observed at this pressure. Dacite– peridotite interaction at 920 MPa produces massive orthopyroxene crystallization, in addition to amphibole ± phlogopite. Amphibole crystallizing in dacite at 960 MPa has the same composition as the aluminium-rich hornblende preserved in the cores of amphibole phenocrysts in the 1991 dacite, suggesting a high-pressure stage of dacite crystallization with high melt H2O contents (>10 wt %) at relatively low temperature (<950°C). The compositions of plagioclase, amphibole and melt inclusion suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900°C, when emplaced into the shallow magma reservoir in which most phenocrysts precipitated before the onset of the 1991 eruption. The LREE-enriched REE pattern of the whole-rock dacite demands garnet somewhere during its petrogenesis, which in turn suggests high-pressure derivation. Partial melting of subducted oceanic crust yields melts unlike the Pinatubo dacite. Interaction of these slab melts with sub-arc peridotite is unable to produce a Pinatubo type of dacite, nor is a direct mantle origin conceivable on the basis of our peridotite–dacite interaction experimental results. Dehydration melting of underplated basalts requires unrealistically high temperatures and does not yield dacite with the low FeO/MgO, and high H2O, Ni and Cr contents typical of the Pinatubo dacite. The most plausible origin of the Pinatubo dacite is via high-pressure fractionation of a hydrous, oxidized, primitive basalt that crystallized amphibole and garnet upon cooling. Dacite melts produced in this way were directly expelled from the uppermost mantle or lower crust to shallow-level reservoirs from which they erupted occasionally. Magmas such as the Pinatubo dacite may provide evidence for the existence of particularly H2O-rich conditions in the sub-arc mantle wedge rather than the melting of the young, hot subducting oceanic plate

Textures of Peritectic Crystals as Guides to Reactive Minerals in Magmatic Systems: New Insights from Melting Experiments

International audiencePeritectic crystals in igneous rocks may be derived from either the source or country rocks, or may have formed by reactive assimilation of source-inherited solids, primary magmatic minerals during self- or magma mixing, or country-rock xenoliths or xenocrysts. Identifying such peritectic crystals is important for constraining the components and textures of igneous rocks and the underlying processes of magmatic evolution. In this study we demonstrate that peritectic olivine formed in melting experiments crystallizes as clusters of euhedral to subhedral crystals. Olivine replacing orthopyroxene, amphibole, and phlogopite forms crystal clusters with distinct crystal to melt ratios, 2D surface area, grain boundary segmentation, and inclusion relations. In our experiments the textures of peritectic crystals are primarily controlled by the stability temperature and decomposition rate of reactive minerals. High-temperature minerals such as orthopyroxene slowly decompose to form high-density clusters of large crystals with long grain boundary segments. The SiO2-rich peritectic melt produced favours formation of melt inclusions. Low-temperature minerals such as amphibole and phlogopite rapidly decompose to form low-density clusters of small crystals with short grain boundary segments. The relatively SiO2-poor peritectic melt produced results in the formation of fewer melt inclusions. Host melt composition has a minor effect on the textures of peritectic olivine formed in the melting experiments of this study and previous contamination experiments, but affects the assemblages of the peritectic crystal clusters. Cluster density and 2D surface area of peritectic olivine tend to decrease, whereas grain boundary segment length increases with increasing experimental temperature and H2O content. Using textural criteria that distinguish olivine formed after different minerals in our melting experiments, we hypothesize that two olivine populations from a basaltic-andesitic lava flow of the Tatara-San Pedro volcanic complex, Chile, may be peritectic crystals formed after amphibole and orthopyroxene. Both amphibole and orthopyroxene are common in xenoliths preserved in some Tatara-San Pedro lava flows. One notable difference between the experimental and natural olivine crystals is that the natural olivine crystals have 2D surface areas and 2D grain boundary segments up to ∼1000 and ∼100 times larger, respectively, than those produced in our experiments. We propose that this size difference is primarily controlled by comparatively slow heating and decomposition of reactive crystals and textural coarsening of peritectic crystals during prolonged magma residence in the natural system

Experimental determination of activities of FeO and Fe2O3 components in hydrous silicic melts under oxidizing conditions.

PhD Financed by the Région Centre and by the EC TMR network "Hydrous Silicate Melts"The critical role of iron on crystal-silicate liquid relationships and melt differentiation is mainly controlled by the redox conditions prevailing in magmas, but the presently available database merely constrains the thermodynamic properties of iron-bearing components in strongly reduced and anhydrous molten silicate where iron is in the ferrous form. This paper provides new standard states for pure ferrous (FeOliq) and ferric (Fe2O3liq) molten iron oxides and extends the experimental database towards oxidizing and water-bearing domains. Iron-iridium, iron-platinum alloys, magnetite or hematite were equilibrated with synthetic silicic liquids at high temperature and high pressure under controlled oxygen fugacity (fO2) to determine activity-composition relationships for FeOliq and Fe2O3liq. Between 1000 and 1300°C, the fO2 ranges from that in air to 3-log units below that of the nickel-nickel oxide buffer (NNO). Experiments were performed on both anhydrous and hydrous melts containing up to 6-wt.% water. Incorporation of water under reducing conditions increases the activity coefficient of FeOliq but has an opposite effect on Fe2O3liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe2O3liq. Under oxidizing conditions, water has a negligible effect on both activities of FeOliq and Fe2O3liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeOliq and Fe2O3liq, which increase significantly with increasing fO2. The present results combined with the previous work provide a specific database on the energetics of iron in silicate melts that cover most of the condition prevailing in natural magmas