Why do mafic arc magmas contain ∼4wt% water on average?

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 …

Volatile Abundances in Basaltic Magmas and Their Degassing Paths Tracked by Melt Inclusions

[1] Two unusual, highly explosive flank eruptions succeeded on Mount Etna in July August 2001 and in October 2002 to January 2003, raising the possibility of changing magmatic conditions. Here we decipher the origin and mechanisms of the second eruption from the composition and volatile (H2O, CO2, S, Cl) content of olivine-hosted melt inclusions in explosive products from its south flank vents. Our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive, volatile-rich (≥4 wt %) basaltic magma that rose from ≥10 km depth below sea level (bsl) and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading. This magma, the most primitive for 240 years, probably represents the alkali-rich parental end-member responsible for Etna lavas' evolution since the early 1970s. Few of it was directly extruded at the eruption onset, but its input likely pressurized the shallow plumbing system several weeks before the eruption. This latter was subsequently fed by the extrusion and degassing of larger amounts of the same, but slightly more evolved, magma that were ponding at 6–4 km bsl, in agreement with seismic data and with the lack of preeruptive SO2 accumulation above the initial depth of sulphur exsolution (∼3 km bsl). We find that while ponding, this magma was flushed and dehydrated by a CO2-rich gas phase of deeper derivation, a process that may commonly affect the plumbing system of Etna and other alkali basaltic volcanoes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JB003934

Melt inclusion record of the conditions of ascent, degassing, and extrusion of volatile-rich alkali basalt during the powerful 2002 flank eruption of Mount Etna (Italy)

Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO2 concentrations (Berner 1991). Atmospheric CO2 concentrations are therefore stabilized over long timescales by this feedback mechanism (Zeebe and Caldeira 2008). This process may have played a role (Feulner et al. 2012) in stabilizing temperatures on Earth while solar radiation steadily increased due to stellar evolution (Bahcall et al. 2001). In this context the role of CO2 degassing from the Earth is clearly fundamental to the stability of the climate, and therefore to life on Earth. Notwithstanding this importance, the flux of CO2 from the Earth is poorly constrained. The uncertainty in our knowledge of this critical input into the geological carbon cycle led Berner and Lagasa (1989) to state that it is the most vexing problem facing us in understanding that cycle.

Notwithstanding the uncertainties in our understanding of CO2 degassing from Earth, it is clear that these natural emissions were recently dwarfed by anthropogenic emissions, which have rapidly increased since industrialization began on a large scale in the 18th century, leading to a rapid increase in atmospheric CO2 concentrations. While atmospheric CO2 concentrations have varied between 190–280 ppm for the last 400,000 years (Zeebe and Caldeira 2008), human activity has produced a remarkable increase …

/pdf/deep-carbon-emissions-from-volcanoes-17q01rpsfk.pdf

Deep Carbon Emissions from Volcanoes

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 …

N. Spilliaert

Papers

Melt inclusion record of the conditions of ascent, degassing, and extrusion of volatile-rich alkali basalt during the powerful 2002 flank eruption of Mount Etna (Italy)

2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades

First Ion Microprobe Determination Of Water And Sulfur Isotopic Ratios In Melt Inclusions Of Olivines At Mount Etna