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\dateposted{2025-01-21}
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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Magma degassing and its impact on the Earth's atmosphere: from magma oceans to lava lakes}
%\def\cdr@specialtitle@french{Impact atmosph\'erique du d\'egazage magmatique : des oc\'eans de magma aux lacs de lave}
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\title{Magma degassing and its impact on Earth's atmosphere: from magma
oceans to lava lakes}

\alttitle{D\'{e}gazage du magma et son impact sur l'atmosph\`{e}re
terrestre : des oc\'{e}ans de magma aux lacs de lave}

\author{\firstname{Bruno} \lastname{Scaillet}\CDRorcid{0000-0003-1561-0226}\IsCorresp}
\address{Institut des Sciences de la Terre d'Orl\'{e}ans, UMR 7327,
CNRS--Universit\'{e} d'Orl\'{e}ans--BRGM -- Universit\'{e}
d'Orl\'{e}ans, Orl\'{e}ans, France}
\email[B. Scaillet]{bscaille@cnrs-orleans.fr}

\author{\firstname{Clive} \lastname{Oppenheimer}\CDRorcid{0000-0003-4506-7260}}
\address{Department of Geography, University of Cambridge, UK}
\address{Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio
Etneo, Catania, Italy}
\email[C. Oppenheimer]{co200@cam.ac.uk}

\author{\firstname{Manuel} \lastname{Moreira}\CDRorcid{0000-0001-5845-4660}}
\addressSameAs{1}{Institut des Sciences de la Terre d'Orl\'{e}ans, UMR
7327, CNRS--Universit\'{e} d'Orl\'{e}ans--BRGM -- Universit\'{e}
d'Orl\'{e}ans, Orl\'{e}ans, France}
\email[M. Moreira]{manuel.moreira@cnrs-orleans.fr}

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\xsection{}

\xfigure{fx01}{Photo: The persistent lava lake lying at the bottom of
the crater of Ambrym volcano in Vanuatu, and the  plume of gases and
aerosol discharged by the convecting magma column sustaining the lava
lake (credit:~C.~Oppenheimer).}

\twocolumngrid

\pagebreak

\section*{Sommaire}

\begin{enumerate}[1.]
\item Avant-propos
\item Labrosse {et~al.}
\item Sanloup {et~al.}
\item Vetere {et~al.}
\item Daffos {et~al.}
\item Moussallam {et~al.}
\item Aiuppa et Moussallam
\item Scaillet {et~al.}
\end{enumerate}

Magmas contain significant quantities of volatile compounds, including
water (H$_{2}$O), carbon dioxide (CO$_{2}$), sulphur, and halogen
species, particularly at high pressures. Their separation through
degassing is a critical process in planetary differentiation and plays
a central role in shaping planetary atmospheres and climates. Key
questions remain regarding the initial volatile inventory within
planetary interiors and the mechanisms by which these volatiles are
redistributed across the various layers of a planet.

In planets like the Earth, large-scale convection serves to erase
pre-existing temperature or density gradients, often leading to partial
melting of solid rocks, such as during adiabatic upwelling. These
partial melts act as powerful sinks for most volatiles. As a result,
magmas are among the primary conduits for volatile redistribution
within planetary interiors. When magmas reach the surface, their
degassing can impact atmospheric conditions over a range of temporal
and spatial scales. This includes secular degassing, which has
gradually contributed to the \mbox{formation} of present-day
atmospheres, as well as the ephemeral effects of individual volcanic
episodes, which can modify atmospheric chemistry and turbidity with
resultant climatic change. Volcanic eruptions also have immediate local
and regional impacts on surface environments and on society. As a
result, scientists are continuously refining monitoring
strategies to improve forecasting of the timing and progression of
eruptions.\looseness=-1

This special issue highlights several ongoing efforts to understand the
behaviour of volatiles in magmatic systems.\ It opens with a
contribution from {Labrosse {et~al.}}, who explore the behaviour of a
magma ocean at the base of the mantle. The authors develop a new set of
equations to model the coupling between a crystallizing basal magma
ocean and an overlying convecting mantle, as well as its interaction
with the core. Although the role of volatiles is not directly addressed
in this study, it lays the groundwork for future investigations by
considering the case of a magma ocean deficient in volatiles (e.g.,
water or~CO$_{2}$). Their results reveal large-scale \mbox{geochemical}
heterogeneities that may help explain seismic observations of
low-velocity zones at the core-mantle boundary.

In a separate study, {Sanloup {et~al.}}\ investigate the behaviour of
iodine in basaltic magmas under mantle pressures, particularly its
speciation, using in situ X-ray measurements at high pressures and
temperatures. Although iodine is a heavy halogen present in trace
amounts in magmas (typically less than 1~ppm), it plays important roles
in atmospheric chemistry, particularly in ozone depletion during
explosive eruptions. The authors challenge previous assumptions by
showing that, at high pressures, iodine binds to oxygen in the melt
rather than to network-modifying cations (as is the case with
chlorine). This finding suggests that oxidized iodine species may have
been overlooked in past studies and could play a role in the degassing
of water-poor magmas. This work highlights the need for further
research into the speciation of iodine and other halogens in natural
silicate melts.

{Vetere {et~al.}}\ explore the effects of varying H$_{2}$O and CO$_{2}$
contents on phase equilibria in a shoshonite basaltic magma sampled
from Vulcano, an active volcano in the Aeolian Arc. Phase equilibria
experiments are a powerful tool for constraining the effects of
volatiles on magma behaviour and identify the storage conditions of
magmas beneath active volcanoes. While H$_{2}$O is well-known to
depress the melting temperature of silicates, CO$_{2}$ was
traditionally thought to have little such effect due to its lower
solubility in silicate melts. However, {Vetere {et~al.}}\ show that this
is not the case for alkali-rich and water-poor melts, where high
CO$_{2}$ concentrations decrease crystal fraction, potentially
reflecting speciation effects linked to alkali elements. This finding
underscores the need for further investigation into the effects of
volatiles on such melts.

{Daffos {et~al.}}\ examine how crystal content and strain influence the
degassing behaviour of magmas during ascent in volcanic conduits. The
loss or retention of volatiles during ascent dictates eruptive style at
the surface.\ Magmas that retain most of their volatiles until
approaching the surface tend to produce explosive eruptions,
particularly in arc settings where magmas are rich in water.
Conversely, any factor that promotes volatile loss during ascent will
act to reduce explosivity, often leading to lava or dome-forming
eruptions. {Daffos {et~al.}}\ demonstrate, through high-pressure
experiments with bubble- and crystal-bearing magmas, that even moderate
deformation at relatively low crystal fractions significantly enhances
bubble connectivity, thereby facilitating gas loss during magma ascent.
This has implications for understanding the dynamics of explosive
eruptions, such as the infamous 1902 eruption of Montagne Pel\'{e}e.

{Moussallam {et~al.}}\ revisit geochemical gas data from several
volcanic regions to identify signals of changes in deep magmatic
processes, with a focus on temperature and oxygen fugacity
($f\text{O}_{2}$).\ Gas monitoring is a key method for forecasting
volcanic activity, as the composition and flux of gases escaping from
magmatic reservoirs can indicate magma migration, recharge,
hydrothermal interaction and pressurization.\ Using data from three
\mbox{well-monitored} active volcanoes in Japan, the authors show that
variations in temperature and $f\text{O}_{2}$ can occur during the
course of an eruption, with changes also observable through other
petrological proxies. They propose that thermodynamic processing of
routinely collected gas compositions could yield real-time insights
into ongoing magmatic processes, thereby supporting hazard assessment.

In another study, {Aiuppa and Moussallam} examine global volcanic gas
data to estimate fluxes of hydrogen (H$_{2}$) and hydrogen sulphide
(H$_{2}$S). These gases play a crucial role in the Earth's atmosphere,
contributing to the balance of oxidized and reduced species, yet their
global fluxes remain poorly constrained. The low concentrations of
these gases and their rapid oxidation in the atmosphere make them
difficult to measure accurately. The authors discuss the factors
affecting H$_{2}$ and H$_{2}$S concentrations in volcanic gases, such
as cooling and rock interactions, and provide new insights into their
global\break fluxes.

Finally, {Scaillet {et~al.}}\ aim to quantify the sulphur yield from
eruptions of alkali-rich and evolved magmas, such as phonolites and
trachytes. These magmas have been associated with several major
eruptions, including the 1815 eruption of Tambora, which had a
significant impact on global climate and is \mbox{associated} with
``the year without a summer'' of 1816. While the atmospheric and
climatic impact of the Tambora eruption has been widely studied, the
potential climatic effects of other such events, like the 13~kyr calBP
Laacher See eruption, remain uncertain. Scaillet {et~al.}\ use
thermodynamic calculations to show that the sulphur yield from
alkali-rich magmas is not higher than that of their calc-alkaline
counterparts, which dominate explosive eruptions. They suggest that the
Laacher See eruption likely did not have a strong global climate
impact, highlighting the importance of accurately assessing sulphur
content under pre-eruptive conditions on a case-by-case basis.

\end{document}