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\DOI{10.5802/crgeos.279}
\datereceived{2024-07-25}
\daterevised{2024-10-23}
\dateaccepted{2024-11-04}
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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}
%\makeatother

\title{Iodine speciation in basaltic melts at depth}

\alttitle{Sp\'{e}ciation de l'iode dans les fusions basaltiques en profondeur}

\author{\firstname{Chryst\`ele} \lastname{Sanloup}\IsCorresp\CDRorcid{0000-0003-2412-6073}}
\address{Institut de min\'eralogie, physique des mat\'eriaux et
cosmochimie, Sorbonne Universit\'e, CNRS, 4 Place Jussieu, France}
\address{Institut Universitaire de France (IUF), Paris, France}
\email[C. Sanloup]{chrystele.sanloup@sorbonne-universite.fr}

\author{\firstname{Cl\'emence}  \lastname{Leroy}}
\addressSameAs{1}{Institut de min\'eralogie, physique des mat\'eriaux et
cosmochimie, Sorbonne Universit\'e, CNRS, 4 Place Jussieu, France}
\email[C. Leroy]{cl.clemenceleroy@gmail.com}

\author{\firstname{Benjamin}  \lastname{Cochain}}
\address{Institut de min\'eralogie, physique des mat\'eriaux et cosmochimie, Sorbonne Universit\'e, CNRS, 4 Place Jussieu, Paris, France}
\email[B. Cochain]{benjamin.cochain@gmail.com}

\author{\firstname{Tobias}  \lastname{Gr\"{u}tzner}\CDRorcid{0000-0003-1876-3367}}
\address{Institut f\"{u}r Geowissenschaften, Goethe-Universit\"{a}t Frankfurt, Germany}
\email[T. Gr\"{u}tzner]{tobias.gruetzner@outlook.com}

\author{\firstname{Qi}  \lastname{~Chen}}
\address{Department of Earth Science \& Environmental Change, University of Illinois at Urbana Champaign, Urbana, IL, USA}
\email[Q. Chen]{qichen22@illinois.edu}

\author{\firstname{Yoshio}  \lastname{Kono}\CDRorcid{0000-0001-5916-7524}}
\address{Department of Physics and Astronomy, Kwansei Gakuin University, Sanda, Japan}
\email[Y. Kono]{yoshiokono@kwansei.ac.jp}

\author{\firstname{Guoyin}  \lastname{Shen}\CDRorcid{0000-0001-5146-1147}}
\address{HPCAT, X-ray Science Division, Argonne National Laboratory, Argonne, USA}
\email[G. Shen]{gyshen@anl.gov}

\thanks{European Research council under the European Community's Seventh
Framework Programme (FP7/20072013 Grant Agreement No. 259649), ANR
Projet de Recherche Collaborative VOLC-HAL-CLIM (Volcanic Halogens:
from Deep Earth to Atmospheric Impacts, ANR-18-CE01-0018), EU Marie
Sk{\l}odowska-Curie Fellowship ``ExCliso'' (Project ID 101017762), CSC
scholarship (\#201806340094), DOE Office of Science by Argonne National
Laboratory under Contract No. DE-AC0206CH11357}

\begin{abstract} 
The speciation of iodine in basalts has been investigated by combining
{in situ} X-ray diffraction at high pressures and temperatures
up to 4.9 GPa and 1600 \textdegree C, and Raman spectroscopy on recovered
high pressure glasses at ambient conditions. Both methods point to
iodine being oxidized in basalts, whether molten or quenched as
glasses. Observed  interatomic distances and  Raman vibrational modes
are consistent with iodine being dissolved as complex iodate groups
alike polyiodates or periodates, not as $\mathrm{IO}_3^-$ groups.
Iodine speciation in basalts therefore seems to reflect a trend amongst
halogens, with lighter chlorine bonding to network modifying cations,
and bromine changing affinity from network modifying cations to oxygen
anions under pressure. In the absence of a fluid aqueous phase, iodine
could thus reach the Earth's surface in basaltic magmas as an oxide,
not as a reduced species.
\end{abstract}

\begin{altabstract} 
La sp\'{e}ciation de l'iode dans les basaltes a \'{e}t\'{e} \'{e}tudi\'{e}e en
combinant la diffraction des rayons X {in situ} \`{a} hautes pressions
et temp\'{e}ratures jusqu'\`{a} 4,9 GPa et 1600 \textdegree C, et la spectroscopie Raman
sur les \'{e}chantillons tremp\'{e}s et r\'{e}cup\'{e}r\'{e}s aux conditions
ambiantes. Les deux m\'{e}thodes indiquent que l'iode est oxyd\'{e} dans les
basaltes, qu'ils soient fondus ou tremp\'{e}s sous forme de verres. Les
distances interatomiques et les modes vibrationnels Raman observ\'{e}s
sont coh\'{e}rents avec la dissolution de l'iode sous forme de groupes
iodates complexes comme les polyiodates ou les p\'{e}riodates, et non sous
forme de groupes $\mathrm{IO}_3^-$. La sp\'{e}ciation de l'iode dans les basaltes
semble donc refl\'{e}ter une tendance parmi les halog\`{e}nes, le chlore plus
l\'{e}ger se liant \`{a} des cations modificateurs de r\'{e}seau, et le brome
changeant d'affinit\'{e}, se liant \`{a} des cations modificateurs de r\'{e}seau
puis \`{a} des anions d'oxyg\`{e}ne sous pression. En l'absence d'une phase
aqueuse fluide, l'iode pourrait donc atteindre la surface de la Terre
dans les magmas basaltiques sous forme d'oxyde et non d'esp\`{e}ce
r\'{e}duite.
\end{altabstract}


\keywords{\kwd{Iodine}
\kwd{Basalt}
\kwd{High pressure}}

\altkeywords{\kwd{Iode}
\kwd{Basalte}
\kwd{Haute pression}}

\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}
Halogens can provide key insights into magmatic processes ranging from
partial melting to volcanic eruptions \citep{Webster:2018} if their
behaviour is fully understood from fluorine to iodine. However, iodine
is the least investigated halogen in magmatic and volcanic processes,
in relation with its lowest abundance both in magmas
\citep{Kendrick:2012,Kendrick:2014} and in volcanic plumes
\citep{Aiuppa:2005}. It is nonetheless an important element as it
impacts atmospheric chemistry through ozone depletion
\citep{Solomon:1994}, having the largest ozone-depleting efficiency
\citep{Cuevas:2022}.


As for most elements, iodine speciation controls its solubility,
transport, and eventual elemental and isotopic fractionation between
coexisting phases. Progresses have been made towards the understanding
of its transfer between reservoirs, especially between magmas and
aqueous fluids to assess the extent of its degassing
\citep{Bureau:2000,Leroy:2019}, but not on its speciation in natural magmas. There
are besides no available {in situ} data on iodine speciation in
silicate melts,  i.e.\ under high temperature ($T$) and high
pressure ($P$) conditions. Available data on iodine speciation in
silicate glasses have been obtained using Raman, X-ray photoelectron
and/or X-ray absorption spectroscopies on borosilicate melts quenched
from high $P$--$T$ conditions \citep{Cicconi:2019,Morizet:2021}, in
order to understand and better predict iodine behaviour and eventual
mobility in nuclear waste glasses. Consequently, there is a lack of
data on iodine speciation in natural magmas in general, and in
particular at the high $P$--$T$ conditions at which they form and
ascend.

Here, we report {in situ} high $P$--$T$ synchrotron X-ray
diffraction data (XRD) on basaltic magmas, and Raman spectroscopy data
on recovered quenched glasses. High $P$ are necessary to dissolve
sufficient amount of iodine so that its effect on magmas properties can
be measured, but more importantly because arc magmas that are produced
at greater depths hence greater pressures than oceanic ridge basalts
are the most relevant to investigate iodine speciation, due to
recycling of marine sedimentary components that bring iodine to the arc
magma source \citep{Muramatsu:1998}.

The choice of Saint Vincent island (Lesser Antilles arc) basalt
\citep{Pichavant:2002} and of Mount Etna basalt (from 2002/2003 South
scoria \citep{Gennaro:2019}) was guided by the need to reflect a range
of volatile-rich  basalts,  with high MgO Saint Vincent basalt being
representative of one type of primary magma in subduction zones, and
Mount Etna alkali basalt representative of later stage basalt
differentiated through fractional crystallization and degassing.



\section{Materials and methods}

\subsection{Glass synthesis}

Starting natural basalt samples were ground, doped with NaI as iodine
source, and with deionised milli-Q water added in the case of Saint
Vincent basalt. When investigating the local environment of a trace
element in a magma, one must reach a compromise between lowest amount
possible and detection above noise level to avoid interaction between
iodine ions that would occur for elevated concentrations. Two iodine
levels were targeted, circa 3~wt\% for the Saint Vincent basalt, and
circa 1~wt\% for the Etna basalt. Since iodine solubility increases
with $P$, I-doping was done at 3.5~GPa and 1600~\textdegree C\ for the Saint
Vincent basalt using platinum capsules welded at both ends, and at 1
GPa and 1350~\textdegree C\ for the Etna composition using gold--palladium
capsules which have a lower $T$-stability but prevent  Fe loss to the
capsule unlike for Saint Vincent basalt that became almost FeO-free.
Having two different FeO content turned out to be essential in
assessing iodine local environment in the melt ({cf}.\ 
Section~\ref{section:XRD}). For both compositions, I-free glasses
were synthesized under the same conditions. High $P$--$T$ conditions
were generated by a Depth of the Earth piston cylinder press using a
half inch talc-pyrex cell assembly with a graphite heater; $T$ was
monitored using a W/Re thermocouple, run duration at high $T$ was one
hour.


\subsection{High $P$--$T$ X-ray diffraction experiments}
The recovered glass from piston cylinder press experiments was
extracted from the platinum or gold--palladium capsule, crushed, and
loaded in either graphite capsule (Saint Vincent basalt) or in single
crystal diamond capsules with inner graphite caps and sealed under $P$
by Pt--5\%Rh caps (Etna basalt). High $P$--$T$ conditions
(Table~\ref{tab1}) were achieved using a Paris-Edinburgh press with
cell-assembly (Figure~\ref{fig:cellass}) as described in
\citet{Yamada:2011}. This cell-assembly is designed to optimize the
sample signal by using low absorbing materials along the X-ray path
(boron epoxy and hBN windows inside the MgO ring outside the graphite
heater), while $\mathrm{ZrO}_2$ parts away from the X-ray path insure
cell-assembly stability at high $P$--$T$ conditions. Talc powder was
added on top and bottom of graphite capsules, to act as $f\mathrm{O}_2$
buffer. Temperature was calculated from power-$T$ curve calibrated
against melting temperatures of salts \citep{Kono:2014}, and $P$ was
calculated from the cell volume of MgO cylinder surrounding sample
capsule \citep{Kono:2010}. Uncertainties on $P$ and $T$ are
respectively 0.3~GPa and 80~\textdegree C. While most runs were carried at
$T$ above the liquidus, two experiments were run between solidus and
liquidus $T$ (TGH25 and TGH26).


\begin{figure}
\includegraphics{fig01}
\caption{\label{fig:cellass}Sketch of the cell-assembly used for {in
situ} high $P$--$T$ X-ray diffraction experiments \citep{Yamada:2011}.
Sample (inner white rectangle) is packed either in a graphite capsule
or in a single crystal diamond cylinder (as drawn here, inner diameter:
1.0 mm), covered by inner graphite caps and sealed by Pt--Rh caps.}
\end{figure}

\begin{table*}[t!]
\caption{Chemical analyses (wt\%). Starting and recovered samples\label{tab1}}
\begin{tabular}{lcccccccccc}
\thead
Sample & SiO\tsub{2} & TiO\tsub{2}  & Al\tsub{2}O\tsub{3} & FeO & MgO &
CaO & Na\tsub{2}O & K\tsub{2}O & I &  Total$^*$\\ 
$P$--$T$ conditions & \multicolumn{10}{c}{(Standard deviations)}\\
\endthead
\multicolumn{11}{c}{Saint Vincent basalt (glass), starting and recovered samples from XRD experiments} \vspace*{4pt}\\
PC82$^{**}$ & 46.91  & 1.37  & 15.16  & 0.95  & 11.58  & 10.58 & 2.98& 0.50 & 2.97  &  96.59\\
3.5 GPa-1600~\textdegree C & (0.51) & (0.27) & (0.09) & (0.15) & (0.18) & (0.08) &(0.58) & (0.04) &  (0.22) &\\
PC83 & 47.95 & 1.36  & 15.57  & 1.24  & 12.33 & 10.94  & 2.84& 0.46 & 2.90& 95.59 \\
3.5 GPa-1600~\textdegree C & (0.92) & (0.10) &(0.47) & (0.13) & (0.19) & (0.34) & (0.18) & (0.07) & (0.14) & (1.28) \\
APS run 13& 47.16 &  1.30 & 15.42  & 0.70 & 12.42 & 10.78 & 2.55 & 0.38& 2.26   & 93.2  \\
4.7 GPa-1600~\textdegree C &  (0.86)& (0.07) & (0.40) &(0.10) &  (0.23) & (0.18) & (0.11) & (0.09) & (0.19) &   (1.13) \\
APS run 26  & 51.14  & 1.14 & 16.52 & 0.58 & 12.97 & 11.57  & 2.41 & 0.48 & -- & 97.06  \\
4.9 GPa-1600~\textdegree C  &  (0.93) & (0.11) & (0.32) &(0.15) & (0.16) & (0.27) & (0.11) & (0.05) & --  & (0.72) \vspace*{6pt}\\
\multicolumn{11}{c}{Etna basalt (glass), recovered samples from XRD experiments} \vspace*{4pt}\\
APS Etna2002 & 49.38 & 1.92 & 18.35 & 3.86 & 5.91 & 11.38 & 3.63 & 2.24 & --&  96.91\\
1.0 GPa-1250~\textdegree C & (0.46) & (0.15) & (0.40) & (0.37)& (0.14) & (0.25) & (0.16) & (0.24) & --&  (0.79)\\
APS 20BaM19 & 46.79 & 1.94 & 17.31 & 3.88 & 6.88 & 11.49 & 3.82 & 2.06 & 0.80 & 95.20\\
1.3 GPa-1120~\textdegree C  & (0.66) & (0.10) & (0.29) & (0.04) & (0.10) & (0.31) & (0.13) & (0.13) & (0.09)  & (0.81) \vspace*{6pt}\\
\multicolumn{11}{c}{Etna basalt (glass+clinopyroxenes), recovered samples from X-ray diffraction (XRD) experiments}\vspace*{4pt}\\
APS TGH25 glass& 49.73 & 2.00 & 19.20 & 7.04 & 3.55 & 8.40  & 4.73 & 2.91 & -- & 97.85\\
3.7 GPa-1450~\textdegree C  & (0.97) & (0.05) & (0.28) & (0.34) & (0.09) & (0.32) & (0.17) & (0.21) &  & (1.19)\\
APS TGH27 glass & 50.30 & 1.85 & 19.31 & 3.52 & 4.35 & 7.55 & 4.76 & 3.09 & 1.15 & 96.23\\
3 GPa-1550~\textdegree C  & (0.58) & (0.11) & (0.54) & (0.23) & (0.16) & (0.17) & (0.21) & (0.19) & (0.10)  & (0.17)
\botline
\end{tabular}
\tabnote{$^*$~Note that `Total' does not include water content, that varies from
1.6(0.3) to 3.8(0.5) wt\% as measured only for PC83 and PC82
respectively \citet{Leroy:2019}.}
\tabnote{$^{**}$~Data from \citet{Leroy:2019}.
Note that Etna basalt samples were recovered from XRD experiments still
embedded in their diamond capsule, hence high quality polishing could
not be achieved.}
\vspace*{4pt}
\end{table*}

{In situ} high $P$--$T$ experiments were conducted using
energy-dispersive XRD on beamline 16-BM-B at the Advanced Photon Source
(Argonne, USA). The incident beam was collimated by tungsten slits 
(0.3~mm vertical ${\times}$ 0.1~mm horizontal) and the diffracted signal was
collected by an energy-dispersive germanium solid-state detector. In
the molten state, X-ray diffraction data were collected at different
$2\theta$ angles (2\textdegree, 2.7\textdegree, 3.5\textdegree,
5\textdegree, 7\textdegree, 10\textdegree, 15\textdegree,
20\textdegree, and 27\textdegree) thus covering up to 15~\AA$^{-1}$ in
$q$-space ($q=4\rmpi E \sin\theta/12.398$, where $E$ is the energy of
the X-rays in keV ranging up to 125~keV).

The multi-angle energy dispersive X-ray diffraction spectra were
converted into the structure factor $S(q)$ using analysis software
package (aEDXD) program developed by Changyong Park \citep{Kono:2014}.
The real-space radial distribution function, $g(r)$, that described the
short-order range structure (i.e.\ interatomic relations within
5--6~\AA) was obtained by Fourier Transform of the spline smoothened
$S(q)$:
{\begin{eqnarray}
g(r)=1+\frac{1}{4\rmpi r n }\int_0^{q_{\max}}q(S(q)-S_\infty)\sin(qr)\, 
\mathrm{d}q
\end{eqnarray}}\unskip
where $n$ is the atomic density in atoms per \AA$^3$ ($n=N_{\mathrm{A}}
M/\rho$, with $N_{\mathrm{A}}$ the Avogadro's constant, $M$ the mean
atomic molar mass of samples, and $\rho$ their mass density).


\subsection{Starting and recovered samples analyses}
Samples were polished for textural analyses  using a Zeiss Ultra 55
field emission scanning electron microscope (SEM) at OSU Ecce Terra,
Sorbonne Universit\'{e}, followed by chemical analyses
(Table~\ref{tab1}) carried at the Camparis center, Sorbonne
Universit\'{e}, using a Cameca SX-FIVE electron probe microanalyser
(EPMA) with accelerating voltage set at 15~keV, beam current at 4~nA,
and a defocussed beam ($7~\rmmu\mathrm{m}$ radius). 

Raman spectra were recorded on a Jobin Yvon Horiba HR460 spectrometer
using a single-grating monochromator with 1500 gratings/mm and an argon
laser (514.5~nm wavelength).


\section{Results}

\subsection{Quenched texture and composition of starting and recovered glasses}

Nanosize iodine droplets are observed on Saint Vincent basalt starting
glasses (PC82 and PC83, Figure~\ref{fig:SEM}a), and could either be
quench products or due to iodine oversaturation as those samples have
the highest iodine content (Table~\ref{tab1}). Such nanodroplets are not
observed in other samples, including Saint Vincent glass recovered from
XRD experiment which moreover was conducted at higher $P$ (APS run 13)
than during piston-cylinder press synthesis,  i.e.\ at conditions
of higher iodine solubility. Iodine was thus fully dissolved in molten
basalts probed by XRD. 


\begin{figure*}
{\vspace*{1pt}}
\includegraphics{fig02}
{\vspace*{1pt}}
\caption{\label{fig:SEM}SEM images of quenched glasses recovered from
piston-cylinder runs and {in situ} high $P$--$T$ X-ray diffraction
(XRD) experiments. (a) Starting sample PC82 (Saint Vincent basalt,
I-doped). (b) Saint Vincent basalt in graphite capsule recovered from
XRD experiments (left: run 13, I-doped, right: run 26, I-free). (c)
Etna basalt recovered from XRD experiments  (left: 20BaM19, I-doped, in
diamond capsule, right: Etna2002, I-free, extracted from diamond
capsule). Bright zones are either I\tsub{2} droplets (a), Pt bits
inherited from the previous piston-cylinder run using Pt capsules (b
right), or metallic Fe droplets (c).}
{\vspace*{1pt}}
\end{figure*}


Recovered Etna basalt samples from XRD experiments, either I-doped or
not, contain droplets of metallic iron (Figure~\ref{fig:SEM}c),
indicating reduction from FeO. This is not observed in recovered Saint
Vincent glasses, as those contained very little FeO in the starting
glass (less than 1~wt\%, Table~\ref{tab1}), and remained homogeneous
(Figure~\ref{fig:SEM}b).


\subsection{Melt structure: X-ray diffraction}\label{section:XRD}

To investigate the effect of iodine on melt structure, both I-doped and
I-free basaltic melts were probed.  In case of co-existence of melt and
crystals, the press was moved relative to X-ray beam position until
area free of crystals could be probed. Amongst the three pairs of
I-doped/I-free runs (Table~\ref{tab1}), two provided sufficiently high quality
data to extract \mbox{radial} distribution functions (APS runs 13/26 and APS
TGH25/27, Figure~\ref{fig:DRX}).  The structure factor, $S(q)$, of the I-doped melts
have a weaker first-sharp diffraction peak (highest intensity near 
2~\AA$^{-1}$ on Figure~\ref{fig:DRX} left panel) compared to the I-free
melts, indicative of a lesser degree of medium-range order
\citep{Salmon:1994}, in other words, a lesser degree of polymerisation.
APS runs 13 and 26 have different hydration levels in the starting
samples (1.6 vs 3.8~wt\% H\tsub{2}O), which also contributes to a
lesser degree of depolymerisation in I-doped APS run 13. However Etna
basalts were not hydrated prior to XRD experiments, but are similarly
impacted by the presence of iodine.\looseness=-1


\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig03}
{\vspace*{3pt}}
\caption{\label{fig:DRX}Right panel: structure factors, $S(q)$; left
panel: radial distribution functions, $g(r)$.}
{\vspace*{3pt}}
\end{figure*}


The effect of iodine on the radial distribution function $g(r)$
(Figure~\ref{fig:DRX} right panel) is less \mbox{pronounced.} The small
contribution near 3~\AA$^{-1}$ on $S(q)$ for run APS TGH27 stems from
dispersed Fe droplets in the magma, and translates into a contribution
peaking at 2.5~\AA$^{-1}$ on $g(r)$, consistently with reported XRD
data on molten Fe \citep{Sanloup:2000b} and with the observation of Fe
droplets on recovered samples (Figure~\ref{fig:SEM}c). Interestingly, we
do not observe this contribution of molten Fe for the I-free run APS
TGH25, attesting that reduction of Fe was less extensive in the I-free
basalt (Table~\ref{tab1}). Diffusion of hydrogen through the Pt--Rd
caps, and/or diffusion of C from the inner graphite caps, can not be
excluded as other causes of Fe reduction, but the difference between
I-free and I-doped samples is significant. \looseness=-1

To better evidence the contribution of iodine atoms to the radial
distribution function, we used the APS run13/26 datasets, that are not
impacted by the contribution of molten Fe to the XRD signal. The
difference between reduced radial distribution functions, $G(r)=4\rmpi r
\rho (g(r)-1)$, for I-doped and I-free basalts was calculated after
normalisation of $g(r)$ to the Si--O contribution  (Figure~\ref{fig:G_I}).
Note that while this procedure enables to evidence interatomic
distances, it is not sufficient to calculate accurate coordination
numbers. To do so, the full $g(r)$ should be simulated against the sum
of all partial pair distribution functions, but this is challenging for
such small~differences.  Two interatomic distances are \mbox{visible} at 
2.2~\AA\ and at 3.5~\AA\  (Figure~\ref{fig:G_I}), and due to the normalisation
procedure to the Si--O contribution, we cannot exclude that distances
shorter than 2~\AA\ also~exist. These contributions are either
I-related or enhanced contributions due to the presence of iodine. XRD
is sensitive to the electrons, the intensity of the signal evolves with
$Z^2$ ($Z$, atomic number). Iodine being a very heavy element, its
scattering is 4 times stronger than that of Fe, and 14 times stronger
than that of Si. Hence the likeliest possibility is that differences on
radial distribution function between I-doped and I-free basalts are due
to iodine, and not to other elements even if their abundances may vary
slightly. The potential contribution of Fe--O nevertheless needs to be
discussed, as its contribution to $g(r)$ in a basalt is at 2.07~\AA\ 
and 3.4~\AA\ (Fe--O and Fe--Fe interatomic distances respectively
\citep{Guillot:2007a}), all other main interatomic distances being
different (see position of main cation-oxygen interatomic distances on
Figure~\ref{fig:DRX}). However, it cannot be the case since FeO content is similar
between I-free and I-doped Saint Vincent basalt, and this content is
lower than 1 wt\% hence the expected contribution is very low. These
additional contributions in I-doped $G(r)$ can neither be attributed to
an eventual $P$ difference between I-doped and I-free basalts. The
maximum $P$ (4.9~GPa) reached in these experiments can induce changes
of coordination number for some cation-oxygen bonds, but it is way too
modest to induce a contraction of interatomic distances. Na--O for
instance contracts by 0.02\% between 0 GPa and 5 GPa
\citep{Karki:2018}, all other main cation-oxygen distances change\break 
even less. 

\begin{figure}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig:G_I}The iodine contribution to the reduced $G(r)$
radial distribution function, as obtained from the difference between
APS run 13 and APS run 26 XRD datasets.}
{\vspace*{2pt}}
\end{figure}


Amongst reported I--X bonds for iodine compounds, the 2.2~\AA\ distance
is shorter than iodine--iodine (2.7~\AA) or iodine--metal bonds (3.2
\AA--4.0~\AA), and closer to iodine--oxygen bonds reported for
crystalline iodates (1.8--2.2~\AA\ range), the longest I--O bonds
corresponding to medium intramolecular bonds with a single covalent
bond character \citep{Gauthier-Luneau:2010,Abudouwufu:2020}, indicative
that we are not looking at $\mathrm{IO}_{3}^{-}$ units here (I--O bond length nearer
1.8~\AA), but at more complex iodates. The longer 3.5 \AA\ distance is
too long for Na--I or Ca--I, but matches well with the I--I interatomic
distance in polyiodates such as
K\tsub{2}Na(IO\tsub{3})\tsub{2}(I\tsub{3}O\tsub{8})
\citep{Abudouwufu:2020}.

\begin{figure}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig:Raman}Raman spectra collected on an iodine
over-saturated sample (PC82), and on two recovered samples from {in
situ} X-ray diffraction experiments (I-doped 20BA/M19 and I-free
Etna2002), Raman spectra collected on I-doped and plain borosilicate
glasses (NBS20) \citep{Cicconi:2019} are shown for comparison. For
I-doped basaltic glasses, I\tsub{2} related modes are observed below
200~cm$^{-1}$ for PC82 (grey shaded area), and iodate-related modes are
marked by asterisks. Note that PC82-Saint Vincent basaltic composition
(3.8~wt\% H\tsub{2}O) is less polymerised, as seen by the increased
band near 1070~cm$^{-1}$. For borosilicate glasses, the strong bands in
the [740--840]~cm$^{-1}$ are related to borate rings and boroxol.}
{\vspace*{-2pt}}
\end{figure}

\subsection{Melt structure: Raman spectroscopy}
Some starting and recovered samples were analysed by Raman spectroscopy
(Figure~\ref{fig:Raman}). We note that it is difficult to obtain Raman
spectra on these natural compositions due to an inherent high level of
fluorescence, and in particular in the intramolecular water range 
(e.g.\ [2800--3600]~cm$^{-1}$). Iodine--iodine signature is visible on
starting sample PC82, with bands at 114~cm$^{-1}$ and 154~cm$^{-1}$,
consistently with the observation of nano-size iodine (I\tsub{2})
bubbles by SEM (Figure~\ref{fig:SEM}a), bands that are also
observed in the I-richest borosilicate glass in \citet{Cicconi:2019}
and were attributed to NaI bonds but seem more consistent with
I\tsub{2} signal. There is no visible iodine--iodine nor metal--iodine
contribution on any spectra measured on samples recovered from XRD
experiments. Instead there is the systematic presence of four small but
clear vibrational bands in I-doped samples at 615~cm$^{-1}$,
647~cm$^{-1}$ (this one on top of a broader band by comparison with
I-free samples), 1098 cm$^{-1}$, and 1177~cm$^{-1}$. Interestingly,
there is also a broad band circa 640--660~cm$^{-1}$ in I-doped
borosilicate glasses (Figure~\ref{fig:Raman}), albeit not discussed by
the authors. While we are most likely observing oxidized iodine in
these glasses, it is not in the form of 
$\mathrm{IO}_{3}^{-}$ units that
give rise to bands in the [700--800]~cm$^{-1}$ range
\citep{Cicconi:2019}, but of more complex iodate forms alike hydrated
periodate groups. For instance, mixed salt
Cs\tsub{2}[I(OH)\tsub{3}O\tsub{3}]${\cdot}$CsSO\tsub{4}(H)H\tsub{5}IO\tsub{6}
has its strongest band at 651~cm$^{-1}$, attributed to I--O
$\mathrm{IO}_{6}^{-5}$ symmetric stretching vibration
\citep{Romanchenko:2004}. The two highest Raman shift modes fall within
the range reported for $\delta$ I--O--H modes in octahedral periodates,
i.e.~[1050--1190]~cm$^{-1}$ \citep{Dengel:1993}.

\section{Discussion}
Iodine induces changes in the silicate melt network structure, observed
here by a lesser degree of polymerisation as probed by X-ray
diffraction, and reported on the basis of Raman spectroscopy on Fe-free
alkali-rich felsic magmas \citep{Faranda:2023}, and Fe-free
borosilicate glasses \citep{Cicconi:2019,Morizet:2021}. While this
behaviour is noticeable for experimental levels of I-doping in the
order of 1 or more wt\%, and is a relevant property in the context of
nuclear waste glasses, it is not expected for natural levels of I
content in magmas. The network modifying role of I however does
indicate that I is not retained passively in the voids or in the ring
structure of magmas, nor that it removes Na or K ions from the melt
oxides otherwise the opposite effect would be observed,  i.e.\ 
enhanced polymerisation.


The coexistence of reduced iodide (I$^-$) and oxidized iodate
($\mathrm{IO}_{3}^{-1}$) species in SiO\tsub{2} poor (14--33~wt\%
SiO\tsub{2}) borosilicate glasses \citep{Cicconi:2019} synthesized at
1.5~kbar in the B\tsub{2}O\tsub{3}--SiO\tsub{2}--Na\tsub{2}O system,
was reported based on combined Raman and X-ray Absorption Near Edge
Structure (XANES) spectroscopies. However, as mentioned above, the
Raman data also showed vibrational modes corresponding to
$\mathrm{IO}_{6}^{-5}$ groups. More complex high $P$ aluminoborosilicate
glasses in the system
SiO\tsub{2}--Al\tsub{2}O\tsub{3}--B\tsub{2}O\tsub{3}--CaO--Na\tsub{2}O
investigated by X-ray  photo-electron  and extended X-ray absorption
spectroscopies have a large predominance of iodide species
\citep{Morizet:2021}, in addition to iodates for their most SiO\tsub{2}
poor composition (NH glass, 43~wt\% SiO\tsub{2}), although reconciling
information from both types of spectroscopies proved difficult. It is
important to note that borosilicate glasses are not ideal analogues for
basaltic melts, but they share the same characteristic to have a low
degree of polymerisation and high I solubility. Last but not least,
\citet{Leroy:2019} observed only I\tsub{2} signals on {in situ}
Raman spectra of I-doped haplogranite melt coexisting with hydrous
fluid. This points out that I speciation depends on magma
polymerisation,  e.g.\ on the SiO\tsub{2}${+}$Al\tsub{2}O\tsub{3}
content, and also on its FeO content. This, in turn, could underpin the
lower solubility of I in SiO\tsub{2}-rich well polymerised magmas
\citep{Cicconi:2019,Leroy:2019}.


The potential role of FeO is highlighted here by the amount of reduced
iron that is higher for I-doped samples than I-free samples ran for
similar duration at high $T$ (TGH25 and TGH26, Table~\ref{tab1}), indicating
that FeO was the likely source of oxygen to form periodates, leading to
the formation of metallic Fe. This effect is unfortunately not clear
for I-free Etna2002 compared to I-doped 20BA/M19, with similar FeO
content in the quenched glass which could result from Etna2002 shorter
run duration. In the case of Saint Vincent basalt experiments, the
starting glass was already almost FeO free (Table~\ref{tab1}) after
piston-cylinder press iodine and water doping using platinum capsules.
Hence the most obvious source of oxygen was water for this composition.


Iodine speciation in basalts as an oxide is quite unique amongst
halogens elements. X-ray absorption spectroscopy measurements on
glasses recovered from high $P$--$T$ conditions have mostly targeted
chlorine and bromine speciation so far. Chlorine bonds to network
modifying cations \citep{Evans:2004,Thomas:2022}, in particular to Ca,
Fe and Mg, to a lesser extent to Na, and in the case of the most
SiO\tsub{2} rich melts also to Si \citep{Thomas:2022}. The higher
affinity of chlorine for alkaline-earth cations than for alkaline
cations was confirmed for borosilicate glasses \citep{Jolivet:2023}.
Bromine speciation in silica-rich magmas changes between low $P$, with
Br--Na bonds and a hydration shell, to a closer oxygen environment above
2 GPa \citep{Cochain:2015,Louvel:2020} although whether oxygen belongs
to water molecules or to the silicate network could not be deciphered.
Bromine speciation besides remains to be investigated in basaltic
compositions. The bulk silicate Earth (BSE) contains approximately ten
times more Br than I \citep{Kendrick:2017,Guo:2021}, the Br/I ratio
raises in MORB and arc basalts (circa 50, \citep{Kendrick:2014}), and
even slightly more in atmospheric volcanic plumes (58--87,
\citep{Aiuppa:2005}). Differences in Br and I speciation in magmas
could, at least partially, underpin this behaviour.


\section{Conclusion}
While iodine is generally thought as present in nature either as iodide
(I$^{-}$), iodate ($\mathrm{IO}_{3}^{-}$), or elemental iodine
(I\tsub{2}), we show here that it is stable in basaltic melts and
glasses as a more complex iodate such as polyiodates or
orthoperiodates. It is thus possible that this peculiar speciation of
iodine might have been missed in past investigations, in particular
when using spectroscopic methods that require the collection of data on
reference materials and for which only I$^{-}$, I\tsub{2} and
$\mathrm{IO}_{3}^{-}$ were looked for.   An alternative to reference
materials is to use an input model, but this is very difficult to have
for non-crystalline materials. In this respect, the X-ray diffraction
method used here is not standard- nor model-dependent. But it can only
be applied to the study of the heaviest---hence most scattering---I
element. Indeed, with solubilities reaching only a few wt\% in
compressed basalts, its detection is clear but remains small,
demonstrating that any lighter halogen element will not be detectable
by this method. Iodine as orthoperiodate is observed in basaltic melts
generated between 1.5~GPa and 4.9 GPa in the present study, but
similarity in Raman spectroscopy data shows that this oxidized state
extends to SiO\tsub{2}-poor  borosilicate melts at 1.5~kbar
\citep{Cicconi:2019}. It does not however extend to SiO\tsub{2}-rich
compositions, as only I\tsub{2} signal was observed on Raman spectra
measured on compressed I-doped haplogranite melt \citep{Leroy:2019},
and mostly as iodide in SiO\tsub{2}-rich high $P$ aluminoborosilicate
glasses \citep{Morizet:2021}. 

Interstingly, formation of iodates has also been observed during
circulation of water--iodide solutions through volcanic rock cores at
ambient conditions \citep{Neil:2020}, on the basis of UV spectroscopy
measurements. This was interpreted as resulting from the retention of
oxidized I by minerals in volcanic rocks, concomitantly with reduction
of ferric ions. In conjunction with the present results, this points
out that future studies should be dedicated to elucidating
simultaneously the speciation of I and Fe in magmas to fully understand
I behaviour in petrologic and volcanic processes.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Acknowledgments}

Natural basaltic samples from Saint Vincent and Mount Etna were
provided by G. Prouteau from Institut des Sciences de la Terre
d'Orl\'eans. We acknowledge K. Curtis-Benson for providing parts for
cell assemblies and arranging shipments before and after experiments at
the Advanced Photon Source, K. B\'{e}neut for use of the Raman
spectroscopy platform at IMPMC, O. Boudouma for SEM measurement and N.
Rivodini for EPMA analyses at OSU Ecce Terra, Sorbonne Universit\'{e}.
This work was supported by the European Research council under the
European Community's Seventh Framework Programme (FP7/20072013 Grant
Agreement No. 259649 to CS), TG was supported by the ANR Projet de
Recherche Collaborative VOLC-HAL-CLIM (Volcanic Halogens: from Deep
Earth to Atmospheric Impacts, ANR-18-CE01-0018) and is grateful for an
EU Marie Sk{\l}odowska-Curie Fellowship ``ExCliso'' (Project ID
101017762), QC was supported by CSC scholarship (\#201806340094). HPCAT
operations are supported by DOE NNSA's Office of Experimental Sciences.
The Advanced Photon Source is a U.S. Department of Energy (DOE) Office
of Science User Facility operated for the DOE Office of Science by
Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

\CDRGrant[ERC]{259649}
\CDRGrant[ANR]{ANR-18-CE01-0018}
\CDRGrant[EU]{101017762}
\CDRGrant[CSC]{\#201806340094}
\CDRGrant[DOE]{DE-AC02-06CH11357}

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