Volatile Abundances in Basaltic Magmas and Their Degassing Paths Tracked by Melt Inclusions
01 Jan 2008-Reviews in Mineralogy & Geochemistry (Mineralogical Society of America)-Vol. 69, Iss: 1, pp 363-402
TL;DR: The abundances of CO2, H2O, S and halogens dissolved in basaltic magmas are strongly variable because their solubilities and ability to be fractionated in the vapor phase depend on several parameters such as pressure, temperature, melt composition and redox state as mentioned in this paper.
Abstract: The abundances of CO2, H2O, S and halogens dissolved in basaltic magmas are strongly variable because their solubilities and ability to be fractionated in the vapor phase depend on several parameters such as pressure, temperature, melt composition and redox state. Experimental and analytical studies show that CO2 is much less soluble in silicate melts compared to H2O (e.g., Javoy and Pineau 1991; Dixon et al. 1995). As much as 90% of the initial CO2 dissolved in basaltic melts may be already degassed at crustal depths, whereas H2O remains dissolved because of its higher solubility such that H2O contents of basaltic magmas at crustal depths may reach a few percents. Most subduction-related basaltic magmas are rich in H2O (up to 6–8 wt%; Sisson and Grove 1993; Roggensack et al. 1997; Newman et al. 2000; Pichavant et al. 2002; Grove et al. 2005) compared to mid-ocean ridge basalts (<1 wt%; Sobolev and Chaussidon 1996; Fischer and Marty 2005; Wallace 2005).
During magma movement towards the surface, exsolution of major volatile constituents (CO2, H2O) causes gas bubble nucleation, growth, and possible coalescence that exert a strong control on the dynamics of magma ascent and eruption (Anderson 1975; Sparks 1978; Tait et al. 1989). Gas bubbles have the ability to move faster than magma (Sparks 1978), particularly in low viscosity basaltic magmas. Bubble accumulation, coalescence and foam collapse give rise to differential transfer of gas slugs and periodic gas bursting (Strombolian activity; Jaupart and Vergniolle 1988, 1989) or periodic lava fountains (Vergniolle and Jaupart 1990; Philips and Wood 2001) depending on magma physical properties and ascent rate. It is also thought that strombolian and lava …
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TL;DR: In this article, the authors present some of the current petrological techniques that can be used for studying eruptive products and for constraining key magmatic variables such as pressure, temperature, and volatile content.
Abstract: Explosive volcanic eruptions constitute a major class of natural hazard with potentially profound economic and societal consequences. Although such eruptions cannot be prevented and only rarely may be anticipated with any degree of accuracy, better understanding of how explosive volcanoes work will lead to improved volcano monitoring and disaster mitigation. A major goal of modern volcanology is linking of surface-monitored signals from active volcanoes, such as seismicity, ground deformation and gas chemistry, to the subterranean processes that generate them. Because sub-volcanic systems cannot be accessed directly, most of what we know about these systems comes from studies of erupted products. Such studies shed light on what happens underground prior to and during eruptions, thereby providing an interpretative framework for post hoc evaluation of monitoring data. The aim of this review is to present some of the current petrological techniques that can be used for studying eruptive products and for constraining key magmatic variables such as pressure, temperature, and volatile content. We first review analytical techniques, paying particular attention to pitfalls and strategies for analyzing volcanic samples. We then examine commonly used geothermometry schemes, evaluating each by comparison with experimental data not used in the original geothermometer calibrations. As there are few mineral-based geobarometers applicable to magma storage regions, we review other methods used to determine pre-eruptive magma equilibration pressures. We then demonstrate how petrologically-constrained parameters can be compared to the contemporaneous monitoring record. These examples are drawn largely from Mount St. Helens volcano, for which there are abundant petrological and monitoring data. However, we emphasize that our approaches can be applied to any number of active volcanoes worldwide. Finally, we illustrate the application of these techniques to two different types of magmatic systems—large silicic magma chambers and small intermediate-composition magma storage regions—with particular focus on the combined evolution of melt …
301 citations
TL;DR: In this paper, it was shown that, contrary to the widely held view, H 2 O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H + through the host olivine are possible on very short time scales.
Abstract: The solubility of H 2 O in silicate melt drops substantially with decreasing pressure, so that a magma initially containing several weight percent H 2 O in a crustal magma reservoir is left with only a few thousand parts per million following ascent and eruption at the Earth’s surface. This rapid release of volatiles makes determining the pre-eruptive H 2 O contents of magmas very difficult. Olivine-hosted melt inclusions are thought to retain their H 2 O because they are protected from decompression by the strength of the host crystal, and pre-eruptive concentrations obtained from melt inclusions have been used to both estimate the amount of H 2 O in the upper mantle and investigate its role in the melt generation process. The greatest uncertainty involved in constraining upper mantle conditions from melt inclusions is the potential for rapid diffusive loss or gain of H + (protons) through the host olivine. Here we present results from hydration and dehydration experiments that demonstrate that, contrary to the widely held view, H 2 O loss or gain in melt inclusions is not limited by redox reactions and significant fluxes of H + through the host olivine are possible on very short time scales. We also show that the Fe 3+ /ΣFe of an olivine-hosted melt inclusion maintains equilibrium with the external environment via diffusion of point defects through the host olivine. Our results demonstrate that, while pre-eruptive H 2 O and Fe 3+ /ΣFe can be reliably estimated, olivine-hosted melt inclusions do not necessarily retain a record of the H 2 O and O 2 fugacity conditions at which they formed. High-H 2 O melt inclusions are particularly susceptible to diffusive dehydration, and therefore are not reliable proxies for the state of the upper mantle.
282 citations
TL;DR: In this paper, the authors integrate microanalyses of ore minerals, experimental data that describe metal partitioning, and published age and isotopic data to suggest that the Carlin-type gold deposits in Nevada are sourced from magma.
Abstract: The Eocene epoch in the Great Basin of western North America was a period of profuse magmatism and hydrothermal activity During that period, the Carlin-type gold deposits in Nevada were produced, Earth’s second largest concentration of gold after deposits in South Africa The characteristics of the Carlin-type deposits have been documented, but a widely acceptable explanation for their genesis is outstanding Here we integrate microanalyses of ore minerals, experimental data that describe metal partitioning, and published age and isotopic data, to suggest that the gold is sourced from magma We relate gold deposition to a change from shallow subduction to renewed magmatism and the onset of extension We propose that upwelling asthenosphere impinged on a strongly modified subcontinental lithospheric mantle, generating magmas that released gold-bearing fluids at depths of 10 to 12 km The rising aqueous fluids with elevated hydrogen sulphide concentrations and a high ratio of gold to copper underwent phase changes and mixed with meteoric water Within a few kilometres of the surface, the fluids dissolved and sulphidized carbonate wall rocks, leading to deposition of gold-bearing pyrite We conclude that the large number and size of Carlin-type deposits in Nevada is the result of an unusual convergence of a specific geologic setting, together with a tectonic trigger that led to extremely efficient transport and deposition of gold
259 citations
TL;DR: The major magmatic volatile components (H2O, CO2, S, Cl, and F) play an important role in the formation, evolution, and eruption of magma as mentioned in this paper.
Abstract: The major magmatic volatile components—H2O, CO2, S, Cl, and F— play an important role in the formation, evolution, and eruption of magma. Knowledge of magmatic concentrations and fluxes of these volatiles is thus important for understanding explosive eruptive behavior of volcanoes, recycling of volatiles in subduction zones, formation of magmatic-hydrothermal ore deposits, fluxes of volcanic gases to Earth’s atmosphere, and potential climatic impacts of large volcanic eruptions. Over the past 30 years, new analytical techniques for measuring volatiles in melt inclusions and glasses from volcanic rocks and new developments in remote sensing technology used for quantifying volcanic emissions have led to major advances in our understanding of volatiles in magmatic systems and their fluxes from Earth’s mantle to the crust and hydrosphere.
Sulfur plays a particularly important role in many of the processes noted above because it affects partitioning of metals into sulfide phases or vapor in magmas during crustal storage, and when released to the atmosphere, it forms sulfuric acid aerosol droplets that catalyze ozone destruction, influences other aspects of atmospheric chemistry, and blocks incoming solar radiation. In addition, S may play a role in causing oxidation of the mantle wedge above subduction zones (Kelley and Cottrell 2009). In silicate melts, the solubility behavior, activity-composition relations, and vapor-melt partitioning of S are complex due to multiple valence states and species (S2−, S6+ in melt; H2S, S2, SO2, SO3 in vapor) and the occurrence of non-volatile S-rich phases (immiscible Fe-S-O liquid, pyrrhotite, monosulfide and intermediate solid solutions, anhydrite).
Sulfur dioxide (SO2) is the easiest of the main magmatic volatiles to measure in volcanic plumes using ground- and satellite-based remote sensing techniques because of its relatively high concentration in volcanic plumes relative to background values. More …
237 citations
TL;DR: Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems as mentioned in this paper.
Abstract: Melt inclusions are small parcels of melt trapped in crystals within magmatic systems, and are analogous to fluid inclusions formed by trapping of hydrothermal and other fluids during mineral growth in fluid-mineral systems (Sorby 1858; Roedder 1979, 1984). After trapping, melt inclusions are potentially isolated from external melt and thus provide a way to investigate melts trapped during magmatic evolution—driven by processes such as crystal-liquid separation, vapor saturation and degassing, magma mixing and assimilation—which can dramatically alter the compositions of the eventual erupted (or intruded) magmatic end products. Melt inclusions are a powerful tool for the study of basaltic magma systems and their mantle source regions, and are widely used to study the origin and evolution of mantle-derived magmas. Melt inclusions have specific uses in the study of volatile elements (see chapters by Metrich and Wallace 2008, Moore 2008, and Blundy and Cashman 2008), but also provide unique information about the range of melt compositions present within basaltic magmatic systems, and how these reflect mantle sources and the processes that occur during melt generation, evolution, transport and eruption. This review outlines techniques used to obtain chemical and other information from melt inclusions, discusses the processes which lead to melt inclusion trapping in phenocryst minerals, examines the possible means by which melt inclusion compositions might be fractionated during trapping or during subsequent re-equilibration with the host mineral or external melt, and discusses some implications of melt inclusion compositions for the nature of basaltic melt generation and transport systems.
This review is largely restricted in scope to studies of volcanic rocks of basaltic and related composition. This refers to rocks erupted as lavas or tephra with broadly basaltic compositions: SiO2 ~45–52 wt%, relatively high MgO and FeO, and typically containing one or more of the following minerals …
229 citations
References
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TL;DR: In this paper, the authors summarize knowledge of the behavior of elements in the subduction system and highlight the physical and chemical processes that have been invoked as being important in controlling the composition of volcanic arc magmas.
Abstract: Volcanic arc magmas can be defined tectonically as magmas erupting from
volcanic edifices above subducting oceanic lithosphere. They form a coherent
magma type, characterized compositionally by their enrichment in large ion
lithophile (LlL) elements relative to high field strength (HFS) elements. In
terms of process, the predominant view is that the vast majority of volcanic arc
magmas originate by melting of the underlying mantle wedge, which contains
a component of aqueous fluid and/or melt derived from the subducting plate.
Recently, opinions have converged over the key aspects of the physical model
for magma generation above subduction zones (Davies & Stevenson 1992),
namely:
1. that the mantle wedge experiences subduction-induced corner flow (e.g.
Spiegelman & MacKenzie 1987);
2. that the subduction component reaches the fusible part of the mantle wedge
by the three-stage process of (i) metasomatism of mantle lithosphere, followed
by (ii) aqueous fluid release due to breakdown of hydrous minerals at
depth (e.g. Wyllie 1983, Tatsumi et al 1983) and (iii) aqueous fluid migration,
followed by hydrous melt migration, to the site of melting;
3. that slab-induced flow may be locally reversed beneath the arc itself, allowing
mantle decompression to contribute to melt generation (e.g. Ida 1983).
The simplified model in Figure 1 highlights the physical and chemical processes
that have been invoked as being important in controlling the composition of
volcanic arc magmas. Magma compositions (coupled with experimental data
on element behavior) can help us gain further understanding of these physical
and chemical processes. In this review, we first summarize knowledge of the
behavior of elements in the subduction system. We then focus on compositional
evidence for the processes illustrated in Figure 1, which we group as follows:
1. derivation of the subduction component,
2. transport of the subduction component to the melting column,
3. depletion and enrichment of the mantle wedge, and
4. processes in the melting column.
2,374 citations
TL;DR: Asimow et al. as mentioned in this paper derived an estimate for the chemical composition of the depleted MORB mantle (DMM), the source reservoir to mid-ocean ridge basalts (MORBs), which represents at least 30% the mass of the whole silicate Earth.
2,340 citations
TL;DR: Measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth and temperatures of 700–1,200 °C constrain the recycling rates of key elements in subduction-zone arc volcanism.
Abstract: Fluids and melts liberated from subducting oceanic crust recycle lithophile elements back into the mantle wedge, facilitate melting and ultimately lead to prolific subduction-zone arc volcanism1,2 The nature and composition of the mobile phases generated in the subducting slab at high pressures have, however, remained largely unknown3,4,5,6,7 Here we report direct LA-ICPMS measurements of the composition of fluids and melts equilibrated with a basaltic eclogite at pressures equivalent to depths in the Earth of 120–180 km and temperatures of 700–1,200 °C The resultant liquid/mineral partition coefficients constrain the recycling rates of key elements The dichotomy of dehydration versus melting at 120 km depth is expressed through contrasting behaviour of many trace elements (U/Th, Sr, Ba, Be and the light rare-earth elements) At pressures equivalent to 180 km depth, however, a supercritical liquid with melt-like solubilities for the investigated trace elements is observed, even at low temperatures This mobilizes most of the key trace elements (except the heavy rare-earth elements, Y and Sc) and thus limits fluid-phase transfer of geochemical signatures in subduction zones to pressures less than 6 GPa
1,131 citations