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\DOI{10.5802/crgeos.226}
\datereceived{2023-02-24}
\daterevised{2023-06-22}
\dateaccepted{2023-06-22}
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\dateposted{2023-07-19}
\begin{document}

\begin{noXML}

%\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{Influence of volatiles (H$_{2}$O and CO$_{2}$) on shoshonite
phase equilibria}

\shortrunauthors

\author{\firstname{Francesco} \lastname{Vetere}\CDRorcid{0000-0002-0723-1990}\IsCorresp}
\address{Department of Physical Sciences, Earth and Environment,
University of Siena, 53100, Italy}
\address{Leibniz University of Hannover, Institute for Mineralogy,
Callinstrasse 3, Hannover, D-30167, Germany}
\email[F. Vetere]{francesco.vetere@unisi.it}

\author{\firstname{Olivier} \lastname{Namur}\CDRorcid{0000-0001-9129-3264}}
\addressSameAs{2}{Leibniz University of Hannover, Institute for Mineralogy,
Callinstrasse 3, Hannover, D-30167, Germany}
\address{Department of Earth and Environmental Sciences, KU Leuven,
3001 Leuven, Belgium}
\email[O. Namur]{olivier.namur@kuleuven.be}

\author{\firstname{Francois} \lastname{Holtz}\CDRorcid{0000-0002-2043-8603}}
\addressSameAs{2}{Leibniz University of Hannover, Institute for Mineralogy,
Callinstrasse 3, Hannover, D-30167, Germany}
\email[F. Holtz]{f.holtz@mineralogie.uni-hannover.de}

\author{\firstname{Renat} \lastname{Almeev}\CDRorcid{0000-0003-0652-9469}}
\addressSameAs{2}{Leibniz University of Hannover, Institute for Mineralogy,
Callinstrasse 3, Hannover, D-30167, Germany}
\email[R. Almeev]{r.almeev@mineralogie.uni-hannover.de}

\author{\firstname{Paola} \lastname{Donato}\CDRorcid{0000-0002-4287-7543}}
\address{Department of Biology, Ecology and Earth Sciences, University
of Calabria, 87036 Arcavacata di Rende CS, Italy}
\email[P. Donato]{paola.donato@unical.it}

\author{\firstname{Francesco} \lastname{Frondini}\CDRorcid{0000-0002-7539-9541}}
\address{Department of Physics and Geology University of Perugia,
piazza Universit\`{a}, 06100 Perugia, Italy}
\email[F. Frondini]{francesco.frondini@unipg.it}

\author{\firstname{Michele} \lastname{Cassetta}\CDRorcid{0000-0003-2078-655X}}
\address{Department of Computer Sciences, University of Verona, I-37134
Verona, Italy}
\address{Department of Industrial Engineering, University of Trento,
I-38122 Trento, Italy}
\email[M. Cassetta]{michele.cassetta@univr.it}

\author{\firstname{Alessandro} \lastname{Pisello}\CDRorcid{0000-0002-7772-4678}}
\addressSameAs{5}{Department of Physics and Geology University of Perugia,
piazza Universit\`{a}, 06100 Perugia, Italy}
\email[A. Pisello]{alessandropisello@gmail.com}

\author{\firstname{Diego} \lastname{Perugini}\CDRorcid{0000-0002-2888-6128}}
\addressSameAs{5}{Department of Physics and Geology University of Perugia,
piazza Universit\`{a}, 06100 Perugia, Italy}
\email[D. Perugini]{diego.perugini@unipg.it}

\begin{abstract}
Experiments
were performed at 500 MPa, 1080~\textdegree C and water
activities (\textit{a}H$_{2}$O) from 0.0 to 1.0, in
fluid-present and fluid-absent conditions, with the aim of constraining
the effect of volatiles on phase equilibrium assemblages of a
shoshonite from Vulcanello (Aeolian Islands, Italy). Experiments were
run both under reducing and oxidizing conditions and results show that
proportions, shapes and size of crystals vary as a function of the
volatile composition (\textit{X}H$_{2}$O and
\textit{X}CO$_{2}$) and volatile content. Clinopyroxene (Cpx) is the
main crystallising phase and is compositionally analogous to Cpx
crystals found in the natural rock. Plagioclase (Pl) is stable only for
water activity lower than 0.1, whereas Fe--Ti oxides are present in all
experimental runs, except for those where log \textit{f}O$_{2}$ was
lower than ${-}$9, ({${\Delta}$}NNO ${-}$0.11)
irrespective of the presence of CO$_{2}$. The addition of CO$_{2}$ (2.8
wt\%) in nominally dry experimental charges substantially reduces the
crystallinity by ca. 
1/3 compared to volatile free experiments. This
result has important consequences upon the physical properties of the
magma because it influences its viscosity and, as a consequence,
velocity during its travel to the Earth 
surface. 

Assuming that the widths of Vulcanello conduits vary from 0.5 to 1.5 m,
estimates of the ascent velocity vary in the range
1.5 ${\times}$ 10$^{-4}$--3.5 ${\times}$ 10$^{-2}$ m${\cdot}$s$^{-1}$ for
CO$_{2}$ free systems and from 
5.7 ${\times}$ 10$^{-4}$--1.3 ${\times}$ 10$^{-1}$ m${\cdot}$s$^{-1}$
for CO$_{2}$ bearing systems. 

Since shoshonitic magmas are common not only in the Italian volcanic
provinces (Aeolian Arc, Campi Flegrei, Ischia Island, Pontine Islands,
Monti Cimini, Monte Amiata, Capraia Island, Radicofani, Roccamonfina)
but also in different volcanoes worldwide (Yellowstone, Mariana Arc,
Kurile Arc, Tonga Arc, Andean Arc, Kamchatka Arc), we suggest that the
new data will be useful to better understand shoshonitic magma
behaviour under relevant geological scenarios. As such, we also suggest
that hazard evaluation should incorporate the probability of very rapid
ascent of {poorly-}evolved melts from depth. 
\end{abstract}

\keywords{\kwd{Vulcanello}
\kwd{Shoshonite}
\kwd{Phase equilibria}
\kwd{Mineral assemblage}
\kwd{H$_{2}$O--CO$_{2}$}}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

{V}olatile{s} dissolved in ma{gmas are k}ey parameters {to understand}
geophysical and geochemical signals {at depth}, as the{ir} exsolution
provides {the} {needed} driving force {for magma rise and, possibly,
trigger} explosive eruptions. {Being a multiphase system, most magmas
are composed of} phenocrysts {c}oexisting with a {volatile bearing}
silicate melt (e.g. H$_{2}$O, CO$_{2}$, S species, F, Cl, etc.\,\ldots){.
Phase equilibrium studies are of paramount importance to better
decipher} magma storage conditions {and physical properties during
ascent to the surface. Thus, the role of p}arameters govern{ing}
equilibrium between melt and phenocrysts {such as} pressure,
temperature, redox state, volatile conten{t, must be known}. {The best
way to constrain magma storage conditions of volcanic rocks and changes
occurring during ascent} is {to compare the natural solid assemblages}
{with} the phase assemblage and compositions obtained {through}
phase-equilibrium experiments in which all these parameters are
controlled  \citep[e.g.,][]{Marteletal2019}. Dissolved volatiles are
known to affect the rheological properties of magmas during ascent to
the Earth's surface and {H}$_{2}${O is the most important volatile in
igneous systems since it affects density, viscosity, as well as phase
equilibria, including crystallinity and liquidus temperature  
\citep[e.g.,][]{Burnham1981,Schulzeetal1997}. It is well known that the
amount of dissolved water in melts, as well as the mode of degassing in
magma conduits, both affect eruptive styles  [e.g., effusive vs.
explosive, \citealp{SissonLayne1992}; plinian vs. pelean,
\citealp{Marteletal1998}].} 

However, water is not the only volatile component in magmas. Over tha
last decades researchers have focused their efforts on trying to
understand how different volatile ratios, such as H$_{2}$O/CO$_{2}$,
can influence the magma properties in terms of phase equilibria,
mobility of chemical elements, and the amount, shape and size of
crystals  \citep[e.g.][and references therein]{Kent2008,Marteletal2019}. 
The presence of CO$_{2}$ reduces water activity and it is
commonly assumed that the decreasing H$_{2}$O/CO$_{2}$ ratio in the melt
results in an increase of the liquidus temperature and crystal/melt
ratio in most silicate systems  
\citep[e.g.,][]{ClemensWall1981,ScailletEvans1999}. However, the results of
some experimental studies seem to indicate that the opposite may occur
in basaltic or depolymerized systems at very high CO$_{2}$
concentrations (low water activities) \citep{Giuffridaetal2017}. These
results are unexpected and contrast with our common assumption of the
role of CO$_{2}$--H$_{2}$O-bearing fluids on phase relationships. \looseness=-1

In order to expand the experimental dataset on the effect of CO$_{2}$
on phase equilibria, we present new H$_{2}$O- and CO$_{2}$-bearing high
temperature and high pressure crystallization experiments performed on
a shoshonitic composition from the Vulcanello peninsula (Island of
Vulcano, Aeolian Island, Italy). Experiments were targeted to
investigate the phase stabilities at pressure and temperature
conditions relevant for/to some of the southern Italy volcanoes such as
Vulcanello and, possibly, to other chemically similar volcanic systems.
We discuss experimental results in terms of phase equilibria but we
also address how both H$_{2}$O and CO$_{2}$ volatiles affect
rheological properties of the magma depending on their content. 

\subsection{Vulcanello magmatic source and products}

{Based on the analysis of f}luid inclusions in quartz xenoliths,
\citet{Zanonetal2003} provided evidence that the deepest level of magma
storage at Vulcanello is located between 17 and 21 km, close to the
Moho  [21--25 km; \citealp{Falsaperlaetal1985}] and exclude{d}
significant magma ponding at mid-crustal levels. {This is in accordance
with} the aeromagnetic data at Vulcano {pointing to a} magma reservoir
between 18 and 21 km depth  \citep{DeRitisetal2013}. {Moreover,}
\citet{Zanonetal2003} {provided and estimate for} Vulcanello magmas
{temperature of} 1083 ${\pm}$ 40~\textdegree C. 

{The formation of Vulcanello occurred during the last Eruptive Epoch of
the island of Vulcano  [Eruptive Epoch 8, \citealp{DeAstisetal2013}],
between AD 1100 to 1250  \citep{Arrighietal2006}, simultaneously to
effusive and explosive activity in the La Fossa crater 
\citep{Fusilloetal2015}.}

{The lava platform comprises several superimposed shoshonitic lava
flows, which, together with the products of explosive activity that
form a scoria cone, were the first subaerial products of the peninsula
(Vulcanello 1).} The rocks composing the lava platform are mainly
shoshonitic {and} contain large (up to 1.5 cm) xenocrysts of
plagioclase 
\citep{Davietal2009,DeAstisetal2013,Fusilloetal2015}.
{After a short time break, the activity resumed with explosive
eruptions (Vulcanello 2) which led to the formation of a second cone
during a single eruptive unit, showing 3-m-thick fallout deposit having
shoshonitic composition  \citep{Fusilloetal2015}. Following Vulcanello
2, no activity took place during ${\sim}$500~years. Subsequently, lava
emission resulted in the emplacement of the latitic Roveto lava flow,
that was followed by three explosive phases leading to the edification
of Vulcanello 3 cone. These} rock{s are} latitic {in} composition {with
the presence of} clinopyroxene{s} megacrysts  [ca. 1.5 cm;
\citealp{Fusilloetal2015}]. {The activity of Vulcanello 3 ended with
the emplacement of a final lava flow (Valle dei Mostri} latitic lava
{flow), which flowed in the same direction than the Roveto} latitic
lava flow  \citep{Fusilloetal2015,Nicotraetal2018,Davietal2009}.

\vspace*{-5pt}
\section{Experimental and analytical methods}\label{sec2}

\subsection{Starting material and experiments}\label{sec2.1}

The starting material of our experiments is a natural scoria lava from
Vulcanello 1. The bulk rock\break composition (Table~\ref{tab1}) is
characterized by SiO$_{2} = 53.40$ wt\% and Na$_{2}$O $+$ K$_{2}$O $=$
8.76 wt\%. In the total Alkali-Silica (TAS) diagram this composition
plots in the field of shoshonite ({Table~\ref{tab1}}). The modal
abundance of phenocrysts, mainly composed of clinopyroxene (Cpx) and
plagioclase (Pl), is 8--11~vol\%. The phenocrysts often contain
inclusions of Fe--Ti oxides. As reported in \citet{Davietal2009} the
composition of clinopyroxene {and feldspar} phenocrysts {ranges
between}
Wo$_{49\text{--}44}$En$_{36\text{--}42\text{--}36}$Fs$_{9\text{--}20}$
{and} An$_{5\text{--}46}$Ab$_{51\text{--}52}$Or$_{3\text{--}43}$,
respectively. Feldspar microphenocrysts show a {similar} composition to
that of phenocrysts
(An$_{3\text{--}35}$Ab$_{51\text{--}61}$Or$_{4\text{--}46}$). {O}livine
{has a compositional range} Fo$_{42\text{--}62}$ {and t}he Fe--Ti oxides
{have} Usp content {close to} 14 mol\%. The groundmass consists of
glass, leucite (Lc), Fe--Ti oxides, Cpx, Pl and small amount of olivine
(Ol). 

\begin{table}
\caption{\label{tab1}Compositions of starting glass}
\tabcolsep=7pt
\begin{tabular}{cccc}  
\thead
& \parbox[t]{1cm}{\centering VL0 (wt\%)} & std &
\parbox[t]{2.5cm}{\centering \citet{Vetereetal2007}
(wt\%)}\vspace*{2pt} \\ 
 \endthead
$n$ & 100 &  & 100 \\ 
SiO$_{2}$ & 53.40 & 0.41 & 53.47 \\ 
TiO$_{2}$ & 0.70 & 0.04 & 0.71 \\ 
Al$_{2}$O$_{3}$ & 15.78 & 0.24 & 15.48 \\ 
FeO & 8.04 & 0.36 & 8.39 \\ 
MnO & 0.10 & 0.12 & 0.10 \\ 
MgO & 4.88 & 0.15 & 4.88 \\ 
CaO & 8.81 & 0.22 & 8.51 \\ 
Na$_{2}$O & 3.66 & 0.23 & 3.66 \\ 
K$_{2}$O & 3.71 & 0.13 & 4.72 \\ 
P$_{2}$O$_{5}$ & 0.62 & 0.21 &  \\ 
LOI\vspace*{5pt} &  &  &  \\ 
Total & 99.76 &  & 99.92
\botline
\end{tabular}
\tabnote{Notes: Oxide components are given in wt\%. Errors represent
one standard deviation (std). Shoshonite studied by
\citet{Vetereetal2007} is also reported for comparison.}
\end{table}

Sample preparation prior to high-pressure experiments was performed at
the Petro-Volcanology Research Group laboratories of the University of
Perugia (PVRG; \url{http://pvrg.unipg.it}). About 100 g of crushed bulk rock
was melted in a Pt crucible in air at 1600~\textdegree C for 4 h in
a Pt$_{80}$Rh$_{20}$ crucible. Melting was performed in a Nabertherm HT
04/17 MoSi$_{2}$-heated box furnace. The melt was quenched to a glass
by pouring it on a brass plate. The glass was crushed, re-melted and
quenched again using the same procedure. This technique ensured
compositional homogeneity of the glass and avoided crystallization
\citep{Vetereetal2015a}. The composition of the glass and the absence of crystals
were controlled by electron microprobe imaging and measurements with a
CAMECA SX 100 at the Institute of Mineralogy University of Hannover
(see details below;\break {Table~\ref{tab1}}). 

{The} NBO/$T$ parameter corresponds to the number of non-bridging oxygens
(NBO) per tetrahedrally coordinated cation ($T$) and {is a good proxy to
depict the structure of silicate melts in terms of polymerization} 
\citep{MysenRichet2005}. This ratio can be calculated as follows:
{\begin{equation}
\label{eq1}
\frac{\mathrm{NBO}}{T}=\frac{1}{T}\times {\sum} _{i=1}^{i}n\times 
{M}_{i}^{n+}
\end{equation}}\unskip
where $T$ is the total atomic abundance of tetrahedrally coordinated
cations, $M$ is the proportion of network modifying cations $i$,
with electrical charge $n+$ after subtraction of the portion
required for charge-balancing trivalent cations on tetravalent sites
\citep{MysenRichet2005}. Our starting composition has a NBO/$T$ of 0.39.
The fragility, $F$, of our starting composition, was also calculated
and has a value of 0.60. This parameter is useful to estimate the
deviation from Arrhenian behaviour upon cooling in a plot relating
viscosity and the scaled temperature ($T_{g}/T$) while approaching
the glass transition temperature $T_{g}$ 
\citep{Angell1995,GiordanoDingwell2003}. Both the NBO/$T$ and $F$ point to
a high degree of depolymerisation of our starting material.

In order to prepare volatile-bearing charges, we used the following procedure: 

The capsules (Au$_{80}$Pd$_{20}$; 5 mm diameter, 3 cm in length,
previously annealed at 1{100~\textdegree C} for 10 min and then welded
on one end)  w{ere} filled {with the} following procedure: ({a}) the
capsule {was} charg{ed} with the desired amounts of {${\pm}$}H$_{2}$O
{${\pm}$} Ag$_{2}$C$_{2}$O$_{4}$ {+} glass powder
(Ag$_{2}$C$_{2}$O$_{4}$ {is the} source for CO$_{2}$ {as the} silver
oxalate decomposes during heating and generates {the} carbon dioxide);
({b}) a steel piston {was used to gently} compact the charge; ({c}) {to
avoid any volatiles release,} the capsule was tightly squeezed on top,
rolled up in a wet tissue, and frozen by placing it into a bath of
liquid nitrogen; ({d}) the upper end of the capsule was welded shut
with a conventional graphite-arc welder. {In agreement with literature
data on the Vulcanello magmatic source (21--17 km depth and} 
1083 ${\pm}$ 40~\textdegree C, {see above) we performed experiments at 500
MPa and 1080~\textdegree C for 70 h, in order to shed light on
processes relevant to the magmatic source of Vulcanello magma.} 

\begin{figure}[t!]
%\vspace*{-2pt}
\includegraphics{fig01}
%\vspace*{-2pt}
\caption{\label{fig1} H$_{2}$O and CO$_{2}$ concentrations added to the
capsules before experiment (at 500 MPa and 1080~\textdegree C). The
shaded area represent the undersaturated conditions in accord with the
modeled H$_{2}$O and CO$_{2}$ concentrations at fluid saturation
obtained by \citet{Vetereetal2014,Vetereetal2011} for the same composition is also
shown (solid black curve). Orange and blue symbols indicate experiments
performed at reduced and oxidized conditions, respectively, (see
Table~\ref{tab3}). Green and black circles are nominally anhydrous 
VL46 and 47 experiments, respectively (Table~\ref{tab3}). Red dot refers
to the sample with the highest water content.}
%\vspace*{-2pt}
\end{figure}

\begin{table*}
\caption{\label{tab2}Results of microprobe analyses of residual glasses}
\tabcolsep=8.3pt\fontsize{9.6}{11.52}\selectfont
\begin{tabular}{lcccccccc}
\thead
 & VL32 & std & VL33 & std & VL34 & std & VL36 & std \\ 
\endthead
SiO$_{2}$ & 54.86 & 0.39 & 55.53 & 0.47 & 54.28 & 0.60 & 52.17 & 0.55 \\ 
TiO$_{2}$ & \00.71 & 0.05 & \00.73 & 0.04 & \00.73 & 0.06 & \00.71 & 0.04 \\ 
Al$_{2}$O$_{3}$ & 20.61 & 0.26 & 20.71 & 0.34 & 19.87 & 0.22 & 17.99 & 0.23 \\ 
FeOT & \01.81 & 0.57 & \01.21 & 0.27 & \01.99 & 0.31 & \02.95 & 0.18 \\ 
MnO & \00.10 & 0.06 & \00.09 & 0.04 & \00.15 & 0.09 & \00.13 & 0.11 \\ 
MgO & \01.72 & 0.15 & \01.51 & 0.57 & \02.13 & 0.37 & \03.07 & 0.46 \\ 
CaO & \04.70 & 0.31 & \04.13 & 0.27 & \05.16 & 0.27 & \06.23 & 0.10 \\ 
Na$_{2}$O & \06.32 & 0.26 & \06.66 & 0.16 & \06.08 & 0.21 & \05.42 & 0.17 \\ 
K$_{2}$O & \04.91 & 0.39 & \05.11 & 0.21 & \04.82 & 0.28 & \04.25 & 0.32 \\ 
P$_{2}$O$_{5}$ & \00.49 & 0.14 & \00.56 & 0.21 & \00.45 & 0.10 & \00.46 & 0.16\vspace*{5pt} \\ 

Total & 96.23 &  & 96.24 &  & 95.66 &  & 93.38 &  \\ 
H$_{2}$O & \03.77 &  & \03.76 &  & \04.34 &  & \06.62 &  \\ 
\hline
 & VL37 & std & VL38 & std & VL39 & std & VL41 & std \\ 
\hline
SiO$_{2}$ & 55.50 & 0.68 & 56.03 & 0.47 & 55.72 & 0.91 & 53.90 & 0.21 \\ 
TiO$_{2}$ & \00.45 & 0.13 & \00.49 & 0.10 & \00.49 & 0.09 & \00.62 & 0.05 \\ 
Al$_{2}$O$_{3}$ & 20.35 & 1.03 & 20.67 & 0.62 & 20.18 & 0.72 & 18.57 & 0.27 \\ 
FeOT & \04.32 & 1.07 & \04.12 & 0.70 & \03.96 & 0.58 & \04.64 & 0.23 \\ 
MnO & \00.12 & 0.05 & \00.09 & 0.10 & \00.09 & 0.10 & \00.10 & 0.11 \\ 
MgO & \01.29 & 0.38 & \01.14 & 0.23 & \01.10 & 0.32 & \02.00 & 0.37 \\ 
CaO & \03.99 & 0.40 & \03.69 & 0.42 & \03.60 & 0.32 & \04.66 & 0.17 \\ 
Na$_{2}$O & \06.65 & 0.35 & \06.74 & 0.38 & \06.95 & 0.17 & \05.87 & 0.10 \\ 
K$_{2}$O & \04.86 & 0.27 & \05.04 & 0.24 & \05.07 & 0.26 & \04.65 & 0.18 \\ 
P$_{2}$O$_{5}$ & \00.57 & 0.23 & \00.51 & 0.16 & \00.50 & 0.17 & \00.46 & 0.12\vspace*{5pt} \\ 

Total & 98.10 &  & 98.52 &  & 97.66 &  & 95.47 &  \\ 
H$_{2}$O & \01.90 &  & \01.48 &  & \02.34 &  & \04.53 &  \\ 
\hline
 & VL45 & std & VL46 & std & VL47 & std &  &  \\ 
\hline
SiO$_{2}$ & 51.36 & 0.99 & 56.02 & 0.83 & 55.69 & 0.44 &  &  \\ 
TiO$_{2}$ & \00.63 & 0.02 & \00.76 & 0.02 & \00.75 & 0.04 &  &  \\ 
Al$_{2}$O$_{3}$ & 15.01 & 0.24 & 18.10 & 0.34 & 18.55 & 0.64 &  &  \\ 
FeOT & \07.29 & 0.41 & \07.15 & 0.46 & \06.75 & 0.41 &  &  \\ 
MnO & \00.10 & 0.10 & \00.13 & 0.08 & \00.18 & 0.09 &  &  \\ 
MgO & \04.19 & 0.29 & \02.40 & 0.10 & \02.23 & 0.39 &  &  \\ 
CaO & \07.34 & 0.44 & \04.33 & 0.12 & \04.48 & 0.66 &  &  \\ 
Na$_{2}$O & \02.81 & 0.39 & \06.07 & 0.26 & \06.21 & 0.20 &  &  \\ 
K$_{2}$O & \03.61 & 0.13 & \04.01 & 0.08 & \04.01 & 0.27 &  &  \\ 
P$_{2}$O$_{5}$ & \00.51 & 0.04 & \00.74 & 0.03 & \00.75 & 0.07\vspace*{5pt} &  &  \\ 

Total & 91.85 &  & 99.70 &  & 99.61 &  &  &  \\ 
H$_{2}$O & \08.15 &  & \00.30 &  & \00.39 &  &  & 
\botline
\end{tabular}
\xxtabnote{Notes: H$_2$O represent the water content estimated by
difference methods. Microprobe data are reported for all the residual
glasses investigated in wt\% together with errors represented by one
standard deviation (std).}
\end{table*}

%\onecolumngrid

\begin{sidewaystable}
\vsptab
\caption{\label{tab3}Phase assemblage results derived from image analyses methods}
\tabcolsep=4.5pt
\begin{tabular}{cccccccccccccccc}  
\thead
 & \morerows{1}{\parbox[t]{.8cm}{\centering H$_{2}$O$_{\mathrm{in}}$  (wt\%)}} & 
 \morerows{1}{\parbox[t]{.8cm}{\centering CO$_{2}$ (wt\%)}} 
 & \xmorerows{1}{\textit{a}H$_{2}$O\tsup{\pounds}} & 
 \xmorerows{1}{log \textit{f}O$_{2}$} & 
 \xmorerows{1}{${\Delta}$QFM} & \multicolumn{4}{c}{CPx} 
 & \multicolumn{4}{c}{Plg} & Ox & 
 \morerows{1}{\parbox[t]{1cm}{\centering $\Phi_{\mathrm{tot}}$ (area\%)}} \\ 
\cline{7-10} \cline{11-14}
 & & &  &  &  & Fs & Wo & En & (area\%) & An & Or & Ab & $\Phi_{\mathrm{Plg}}$ (area\%) & $\Phi_{\mathrm{Ox}}$ (area\%) & \\ 
 \endthead
VL32\tsup{\#} & \03.5 & 0.651 & 0.36 & ${-}$10.80 & ${-}$1.37 & 17.31 & 46.40 & 36.28 & 16.1 &  &  &  &  &  & 16 \\ 
VL33\tsup{\#} & \03.5 & 0.291 & 0.35 & ${-}$10.82 & ${-}$1.39 & 13.25 & 47.03 & 39.71 & 16.3 &  &  &  &  &  & 16 \\ 
VL34\tsup{\#} & \04.2 & 0.092 & 0.44 & ${-}$10.64 & ${-}$1.21 & 16.11 & 45.87 & 38.00 & 14.3 &  &  &  &  &  & 14 \\ 
VL35\tsup{\#} & \05.5 & 0.919 & 0.61 & ${-}$10.35 & ${-}$0.92 &  &  &  & 10.7 &  &  &  &  &  & 11 \\ 
VL36\tsup{\#} & \06.5 & 0.558 & 0.74 & ${-}$10.18 & ${-}$0.75 &  &  &  & 10.2 &  &  &  &  &  & 10 \\ 
VL37 & \02.0 & 0.693 & 0.14 & \0${-}$7.64 & \mn1.79 &  &  &  & 15.8 &  &  &  &  & 1.7 & 17 \\ 
VL38 & \01.5 & 0.297 & 0.09 & \0${-}$8.01 & \mn1.42 &  &  &  & 18.1 &  &  &  &  & 1.7 & 22 \\ 
VL39 & \02.5 & 0.070 & 0.19 & \0${-}$7.38 & \mn2.05 &  &  &  & 18.7 &  &  &  &  & 1.9 & 21 \\ 
VL40 & \03.0 & 0.805 & 0.31 & \0${-}$6.94 & \mn2.49 & 15.67 & 41.34 & 42.96 & 17.0 &  &  &  &  & 1.5 & 18 \\ 
VL41 & \04.5 & 0.620 & 0.48 & \0${-}$6.57 & \mn2.86 & 16.45 & 47.85 & 35.68 & 14.0 &  &  &  &  & 1.4 & 15 \\ 
VL45\tsup{\$} & 13.0 & 0.000 & 1.00 & \0${-}$4.94 & \mn4.49 &  &  &  &  &  &  &  &  &  & \03 \\ 
VL46* & \00.0 & 2.830 & 0.01 & ${-}$10.26 & ${-}$0.83 & 17.33 & 42.91 & 39.75 & 15.3 & 33.58 & 9.60 & 56.82 & 5.9 &  & 21 \\ 
VL47* & \00.0 & 0 & 0.02 & \0${-}$8.92 & \mn0.51 & 17.90 & 42.42 & 39.68 & 21.6 & 32.91 & 9.40 & 57.69 & 8.8 &  & 30
\botline
\end{tabular}
\xxxtabnote{Crystals composition are also reported.}
\xxxtabnote{Note:
Crystallinity ($\Phi$) is reported in vol\% and derived from 500 MPa and
temperature of 1080~\textdegree C experiments. H$_2$O and CO$_2$ are reported in
wt\%. Water activity is also reported for all run while phase
compositions are reported for most of the experimental runs except for
those were crystal size was too small in order to be analyses.
\tsup{\#}Denote experiments performed at controlled \textit{f}O$_2$. 
*Denote experiments were no water was added to the charges.
\tsup{\$}Denote experiments with quench crystals. 
\tsup{\pounds}Water activity
is calculated from the amount of water estimated by the difference
method (see Table~\ref{tab2}).}
\end{sidewaystable}

%\twocolumngrid

A total of 13 experiments with variable volatile contents (see
{Figure~\ref{fig1}}) were performed at 500 MPa and
1080~\textdegree C ({Tables~\ref{tab2}} and
{\ref{tab3}}). Experiments were done in an internally
heated pressure vessel (IHPV) equipped with a rapid-quench sample
holder at the Institute fur Mineralogy, Leibniz University of Hannover
(Germany). Details for the IHPV are provided in \citet{Berndtetal2002}.
Temperature was measured using four unsheathed S-type thermocouples
(precision of temperature was ${\pm}$5~\textdegree C). The IHPV 
was pressurized with argon. Pressure \pagebreak was 
monitored\unskip\break \phantom{~}\break\clearpage\noindent 
using digital pressure
transducers having an uncertainty of about 1 MPa. The variation of
pressure during the experiments was ${\leq}$5 MPa. Some experiments
were performed at the intrinsic oxygen fugacity (\textit{f}O$_{2}$)
conditions of the pressure vessel which was found to be close to QFM+4
(the intrinsic \textit{f}O$_{2}$ corresponding to H$_{2}$O-saturated
conditions), with QFM being the quartz--fayalite--magnetite equilibrium 
\citep{Berndtetal2002}. At H$_{2}$O-undersaturated conditions,
\textit{f}O$_{2}$ is lower, depending on the prevailing water activity 
[QFM${-}$0.8 to QFM$+$3; \citealp{Botcharnikovetal2005}]. In our experiments
\textit{f}O$_{2}$ was calculated using the following relation: 
{\begin{equation}\label{eq2}
\log f\mathrm{O}_{2}=\log f\mathrm{O}_{2} (\text{at }
X\mathrm{H}_{2}\mathrm{O}_{\mathrm{in}}=1) - 2 \log
X\mathrm{H}_{2}\mathrm{O}_{\mathrm{in}}.
\end{equation}}\unskip
\citep{ScailletEvans1999}. We estimate that the maximum error on the
calculated \textit{f}O$_{2}$ is about 0.2--0.3 log units according to
\citet{Botcharnikovetal2005}. Other experiments were performed under
controlled \textit{f}O$_{2}$ conditions (QFM${-}$0.7 to QFM${-}$1.4) by adding
H$_{2}$ to the Ar pressure medium. The hydrogen fugacity
(\textit{f}H$_{2}$) in these experiments was monitored using a Shaw
membrane as described in \citet{Berndtetal2002}. The \textit{f}H$_{2}$
was adjusted to be \textit{f}H$_{2}=20$ bar at experimental P--T
conditions. Experiments were drop quenched with a cooling rate of
approximately 150--200~\textdegree C/s \citep{Berndtetal2002}. For one
experiment ({Table~\ref{tab3}}), it was not possible to avoid the
formation of quench crystals due to the relatively high amount of added
water (see results for details).


{\vspace*{-2pt}}

\subsection{Electron microprobe and image analyses}\label{sec2.2}

All run-products were mounted in epoxy, ground flat and polished for
textural and chemical analyses. Glasses and minerals produced during
the experiments were analysed using a Cameca SX100 microprobe at the
Institute of Mineralogy. An accelerating voltage of 15~kV was used with
a beam current of 15~nA for silicate minerals and oxides and 6~nA for
glasses. The beam was defocused to at least 15~${\upmu}$m for glass
analyses and focused to ${<}$2 ${\upmu}$m for the analysis of
crystal phases. All elements were analysed with a counting time of 
10 s on peak. Standards used for calibration were Fe$_{2}$O$_{3}$,
MgO, MnTiO$_{3}$ (Mn and Ti), albite (Na), wollastonite (Si for mineral
and glass and Ca for glass), apatite (P and F for glass and F, Ca and P
for apatite), orthoclase (K), anhydrite (Ca and S). Standard deviations
reported in Table~\ref{tab2} were\break calculated based on 10 to 20 analysed
spots on the same sample. Raw data were corrected with the software
``Peak Sight'' and ``PAP'' matrix  \citep{PouchouPichoir1991}.
Precision and accuracy were determined by measuring reference glasses
VG-568 (rhyolite) and VG-2 (basalt) from Smithsonian standards
collection  \citep{Jarosewich2002}.

Due to the presence of crystals in experimental charges, the water
content of the glasses was estimated following the ``by-difference''
method described in \citet{Devineetal1995}. Following
\citet{Paratetal2008}, we estimate that the error on determination of
H$_{2}$O content is ${\sim}$0.5 wt\%. CO$_{2}$ in glasses were not
determined but estimated from the solubility curve in
Figure~\ref{fig1}.

Back-scattered electron (BSE) images were collected with a Philips FEG
(Field Emission Gun) Quanta 200F equipped with a Si/Li-SUTW detector
(EDAX, Philips Electronics) installed at the University of Calabria
(Italy). Representative BSE images were collected at different
magnifications (400$\times$ up to 1600$\times$) depending on the
crystal size.  Surface percentage of the different phases was determined
for all images using grey levels with Image-ProPlus 6.0 
\citep[see details in][]{Vetereetal2015a}. Seven to ten BSE images obtained in
different parts of the same section were evaluated for homogeneity. We
quantified surface fractions of glass, clinopyroxene, spinel,
plagioclase and gas bubbles. In total, 110 BSE images were analysed.

\section{Results}\label{sec3}

The crystalline phases observed in our experiments are clinopyroxene
(Cpx), plagioclase (Pl), and Fe--Ti oxides (Ox) in various proportions,
depending on the fraction of volatiles dissolved in the melt. The
results show that Cpx is the main phase with abundance increasing from
10.2\% to 21.6\% as \textit{a}H$_{2}$O decreases from 0.74
to nearly zero (Table~\ref{tab3}). Water activity also plays a major
role on the stability of Pl that is only present in experiments with a
melt H$_{2}$O content ${\leq}$1 wt\% (Table~\ref{tab3}). Plagioclase
has a maximum abundance of 8.8\% in the volatile-free sample (VL
47$^{*}$ in Table~\ref{tab3}). Minor Fe--Ti oxides
(${\leq}$2\%) are present in samples equilibrated under relatively
high oxygen fugacity conditions.

Based on the amounts of added volatiles and in agreement with
solubility model and data\break \citep[e.g.][]{Vetereetal2014} one can
distinguish between fluid-saturated (fluid present) and
fluid-undersaturated (fluid absent) runs with respect to H$_{2}$O $+$
CO$_{2}$ ({Figure~\ref{fig1}}).

Quenched melt (residual glass) is present in all run products with a
mode ranging from 70\% in VL47$^{*}$ to 97\% in 
VL45\tsup{\$} (Table~\ref{tab3}). This wide range of melt
fraction at constant pressure and temperature shows the importance of
water activity (\textit{a}H$_{2}$O) on the liquidus
temperature and crystallinity. All glass compositions, including the
starting composition, are listed in Tables~\ref{tab1}
and~\ref{tab2} and are illustrated
in {Figure~\ref{fig2}}. The compositions of residual glasses range from
51 to 56 wt\% SiO$_{2}$ (Table~\ref{tab2} and Figure~\ref{fig2}).
Glass compositions evolve as a function of the degree of crystallinity.
We however note that a larger crystallinity does not always correspond
to an enrichment of SiO$_{2}$ in the residual glass (compare
Table~\ref{tab2} and 3). We also note that in the experiment performed
under oxidizing and water-saturated (no CO$_{2}$) conditions, the
conditions are close to the liquidus and the melt has a composition
very similar to that of the starting glass (although comparing alkali
content in the VL45 and starting material we do note a difference of
ca. 0.5 wt\% of Na$_{2}$O possibly due to the EPMA measurements
conditions; red point in Figures~\ref{fig1} and~\ref{fig2}). Experiments performed at
reducing conditions and containing both water and CO$_{2}$ show a
SiO$_{2}$ trend that increases nearly linearly as the MgO content
decreases. The same observation can be done for, Na$_{2}$O and
K$_{2}$O. The total FeO decreases with decreasing MgO but FeO
concentrations are always higher in oxidized experiments than in
reduced experiments at a given MgO content, suggesting a possible
Fe-loss to the capsule wall at reducing conditions, as discussed below.
TiO$_{2}$ is relatively constant in reduced experiments but decreases
with decreasing MgO in oxidized runs. Due to the crystallization of the
main phase Cpx, CaO continuously decreases with MgO (with increasing
crystallinity). 


\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2} Change of the residual glass composition (in
wt\% of element concentrations) in the silicate melts. The analyses
are recalculated to a total of 100\%. The arrow on the CaO vs. MgO plot
indicates the vector pointing to the Cpx composition. The grey triangle
refers to the starting material composition while reddish and bluish
circles indicate experiments performed at oxidized and reduced
conditions, respectively, as reported in Tables~\ref{tab2}
and~\ref{tab3}. Red dot as in Figure~\ref{fig1}.} 
\end{figure*}

Eight experiments were performed at volatile saturated conditions and
six at volatile undersaturated conditions (Figure~\ref{fig1}).
{Figure~\ref{fig3}} shows the variation of crystal content as a
function of water content. The general trend indicates that the higher
the amount of water in the melt, the lower the amount of crystals, as
expected from the numerous previous phase relationships conducted so
far. No significant difference between fluid-absent and fluid-present
runs is detected in terms of crystal content in samples with an amount
of water added to the system that is larger than 1.0 wt\%. Thus, we did
not detect a substantial difference in crystal content between
experiments performed at fluid-saturated and fluid-undersaturated
conditions if the \textit{a}H$_{2}$O is similar. For example,
we investigated the effect of increased CO$_{2}$ concentration in
water-rich samples (i.e. up to 3.7~wt\% H$_{2}$O; sample VL32 and 
VL33, see Table~\ref{tab3}) by performing two experiments with a total
amount of CO$_{2}$ from 6500 ppm (fluid-saturated) and 2900 ppm
(fluid-undersaturated). No difference between CO$_{2}$-rich and
CO$_{2}$-poor samples was observed in terms of crystallinity. In
particular, both melts showed Cpx contents of ${\sim}$16\%
(Table~\ref{tab3}). In contrast, our experiments show that, {at} {very
low H}$_{2}${O content}, the presence of CO$_{2}$ causes a substantial
decrease of crystal content. A difference up to 10 {area}\% in crystal
content is observed between samples VL46 and VL47 (see Table~\ref{tab3}), where
the first charge contains a large amount of CO$_{2}$ (2.8 wt\%,
nominally dry, no water added) while VL47 is nearly volatile free. It
is emphasized that th{is} difference of {10 area}\% in crystallinity
determined by image analysis  is also consistent with mass balance
calculations. Using {the GeoBalance program [}Excel VBA program for
mass balance calculation;
\citealp{Lietal2020}] the residual melts in
VL47 can be best reproduced if 6 {wt}\% {Pl} and 30 {wt}\% {Cpx}
crystallize from the starting material (the mineral compositions from
experimental products were considered for the calculation, see
Table~\ref{tab3}). For VL46, mass balance calculation indicates the
crystallization of 3 {wt}\% {Pl} and 25 {wt}\% {Cpx}, point{ing} to the
possible role of CO$_{2}$ on phase proportions as discussed\break below. 

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3} Evolution of crystallinity vs. water content of
experiments presented in Table~\ref{tab3}. Note the variation on
crystallinity between CO$_{2}$ bearing and CO$_{2}$ free sample in
nominally dry experimental charges. Symbols as in Figure~\ref{fig1}.}
\end{figure}

{The a}lphaMELTS code was used to {further} {elucidate} the phase
evolution {observed} in {this study.} The alphaMELTS (V. 1.9) software
provides a simple text-based interface to subroutine versions of the
algorithms MELTS 
\citep{AsimowGhiorso1998,SmithAsimow2005,Ghiorsoetal2002,GhiorsoSack1995}. 
{It} allows to calculate equilibrium assemblages along a thermodynamic
path set by the user (details {are provided by}: 
\url{http://melts.ofm-research.org/} and 
\url{https://magmasource. caltech.edu/forum/}).
{Results of the simulations are shown in} {{Figure~\ref{fig4}}}
{comparing the crystals content measured via Image Analyses and
calculated by using aplhaMELTS. There is a good correlation between
calculated and \mbox{measured} data, although VL46 sample shows a calculated
crystallinity higher than 40 vol\% (expected from equilibrium
thermodynamic calculation) whereas the experimentally observed
crystallinity is much lower (Figure~\ref{fig4} and Table~\ref{tab3}).} 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4} Comparison between Image Analyses measurements
and calculat amount of crystals by alphaMELTS approach. Note the good
correlation for most of the sample. VL46 sample, instead shows  a large
departure relatively to the 1:1 line (green circle).}
\end{figure}

\section{Discussion}\label{sec4}

\subsection{Effect of volatiles on phase equilibria}\label{sec4.1}

In order to test for equilibrium condition in our experiments we can
refer to models proposed by {\citet{Putirka2008}}. {Tests for
equilibrium between Cpx and a coexisting liquid can be made by
comparing observed and predicted values for Fe--Mg exchange, or
K}$_{D}${(Fe--Mg)}$^{\text{cpx--liq}}${, which should be 0.28 $\pm$ 0.08
for Cpx (Putirka,2008; considering FeO$=$FeO}$_{\mathrm{tot}}${). Our
results show} values ranging from 0.236 and 0.248 when using
compositions in Table~\ref{tab2}, indicating compositions close to
equilibrium. 

The glasses of our isothermal experiments show geochemical trends that
are entirely controlled by the degree of crystallinity that itself
depends on the composition and amount of volatiles in the system. The
evolution trends for most of the elements (Ca, Mg, Si, Al) are
consistent with the crystallization of Cpx (Figure~\ref{fig2}), which
predominates the solid phase. Ti is almost constant in experiments
performed at reduced conditions whereas its abundance is lower in the
experiments performed under oxidized conditions, due to the
crystallization of Fe--Ti oxides in the latter. The composition of
melts from two experiments (VL46 and VL47 at very low H$_{2}$O content;
Tables~\ref{tab2} and~\ref{tab3}) deviates slightly from the general
trend for some elements and can be explained by the presence of Pl in
the run products ({Figure~\ref{fig5}}).

\begin{figure*}
\vspace*{2pt}
\includegraphics{fig05}
\caption{\label{fig5} BSE images of sample VL46 and VL47 (see
Table~\ref{tab3}) after annealing at 1080~\textdegree C and pressure of
500 MPa. In particular, as 2.8 wt\% CO$_{2}$ is added to the system,
the crystal content decreases by about 10 area\%, and also the relative
amount of single crystal phases (Cpx and Pl) decreases (VL46*,
Table~\ref{tab3}). Clinopyroxene amount varies from 21.6 to 15.1 area\%
while plagioclase slightly decreases from 8.8 to 5.9 area\% (VL46* and
VL47*, Table~\ref{tab3}).}
\vspace*{5pt}
\end{figure*}

The amount of CO$_{2}$ that can be dissolved in melts is lower than
that of H$_{2}$O and the solubility of both volatiles has been
investigated as a function of pressure in a variety of melt
compositions  
\citep[e.g.,][]{Brookeretal2001,KingHolloway2002,HollowayBlank1994, 
Dixonetal1995a,Dixonetal1995b,Jakobsson1997,Morizetetal2010,
BakerAlletti2012,Schanofskietal2019}. 
Carbon species (CO$_{2}$, CH$_{4}$, and CO) have low solubility in
basalt and shoshonite melts [at 500 MPa ${<}$ 4500 ppm CO$_{2}$,
\citealp{Shishkinaetal2014,Vetereetal2014,Behrensetal2009,Botcharnikovetal2006}] 
and carbonate minerals are not stable in mafic melts. It could
therefore be anticipated that (for low CO$_{2}$ melt concentrations)
CO$_{2}$ has no effect on phase compositions and proportions when
compared to CO$_{2}$-free dry systems. Indeed, samples with high water
contents do not show any important differences in terms of
crystallinity between CO$_{2}$-bearing and CO$_{2}$-free samples
(Figure~\ref{fig3}). This indicates that water plays the most important
role on controlling the crystallization temperatures and that the role
of minor to moderate CO$_{2}$ concentrations is subordinate. However,
for systems with low water and relatively high CO$_{2}$ levels, this
observation seems not to be valid. The difference of 10\% in
crystallinity found between nominally dry samples (fluid absent and
pure CO$_{2}$ fluid) implies that CO$_{2}$ does not behave as a fully
inert component, only reducing water activity. Following the
determinations of \citet{Husenetal2016}, it is emphasized that the
melts synthesized at nominally dry fluid-absent conditions in the
internally-heated pressure vessel used at Hannover probably contain at
least 0.5 wt\% H$_{2}$O. Absolutely dry conditions can never be
realized because entrapment of adsorbed water during capsule
preparation cannot be avoided and because hydrogen can diffuse through
the capsule material at high temperature. This process is partially
related to reduction of ferric iron to ferrous iron in the melt during
the high $T$--$P$ experiments following the reaction
{\begin{equation}\label{eq3}
\mathrm{H}_{2}({g}) + \mathrm{Fe}_{2}\mathrm{O}_{3}({m}) = 2 \mathrm{FeO}({m}) +\mathrm{H}_{2}\mathrm{O}({m})
\end{equation}}\unskip
where \textit{m} refers to the melt phase and \textit{g} the gas phase.
Thus, the difference between fluid-absent and fluid-present experiments
can be considered as a minimum value and may be even larger than
determined in this study, if absolutely H$_{2}$O-free systems are\break
considered.  

Although experimental conditions are very different, our observations
are in agreement with\break observations of \citet{Fiegeetal2015} showing
that the addition of ${\sim}$2000 ppm of CO$_{2}$ to a hydrous
basaltic melt (5 wt\% H$_{2}$O) has a small effect on crystal content
(Cpx, Spl) and glass fractions (during decompression). Nevertheless,
when evaluating in detail those data, we can see that samples
containing only H$_{2}$O show relatively higher abundance of Cpx
compared to those containing both H$_{2}$O and CO$_{2}$. Experiments
simulating the effects of CO$_{2}$ fluxing conducted by
\citet{Giuffridaetal2017} also indicate that the addition of CO$_{2}$
to a partially crystallized basalt can influence the phase proportion
and result in a partial dissolution of Cpx, in contrast to the common
assumption.  \citet{Vetereetal2015b} noted that the crystallinity of
Etna basalt decreases as CO$_{2}$ is added to the system, which is in
agreement with the data from this study on a shoshonitic system. In
particular, in those experiments, Pl was the most abundant mineral
phase, but Pl was absent when only CO$_{2}$ was added to the
experimental charge. In addition, CO$_{2}$ appeared to lower Cpx
abundance  \citep{Vetereetal2015b}. Thus, CO$_{2}$ may not act as a
complete inert phase in mafic systems in which it is mainly
incorporated as carbonate\break species. 

A closer look on data presented in Figure~\ref{fig3} allows us to
further discuss the effect of the CO$_{2}$ in magmatic systems. In
fact, Figure~\ref{fig3} shows, at least for added water content higher
than 2 wt\%, a nearly linear correlation between crystallinity and
added water content, independently on \textit{f}O$_{2}$. In
{Figure~\ref{fig6}} we show that, if the amount of added CO$_{2}$ is
larger than 0.65~wt\% and added water content is close or lower than 2
wt\%, the crystallinity of the run products becomes lower than what
would be predicted from the nearly linear trend between crystallinity
and added water, as highlighted by the dashed black and red lines
(Figure~\ref{fig6}). 


\begin{figure}
%\vspace*{-3pt}
\includegraphics{fig06}
\caption{\label{fig6} {Insight on the evolution of crystallinity vs.
water content of experiments presented in Table~\ref{tab3}. Note the
variation on crystallinity (red and black dashed curves) between
samples having water content lower than 3.5~wt\% with CO$_{2}$
${>}$ 0.65 wt\% and CO}$_{2} < 0.65$ wt\%. Equations curves
are also reported.}
%\vspace*{-3pt}
\end{figure}

More in details, the flushing experiment performed by
\citet{Giuffridaetal2017}, showed that a CO$_{2}$-rich fluid phase
leads to an increase of the amount of Cpx and a decrease of the
abundance of Pl at 300 MPa. This decrease of Pl proportion is
associated with a change in An content. Although the conditions are
very different, as described above, this proves an effect of CO$_{2}$
on phase stability. {Even if small, we also
observed a change in An content from CO}$_{2}$
{free to CO}$_{2}$
{bearing (VL46 to VL47) as observed in}
\citet{Giuffridaetal2017} from high to low pressure experiments on Etna
basalt. The glass compositions presented in Table~\ref{tab2} also
highlight some differences between VL46 and VL47 samples. The SiO$_{2}$
and Al$_{2}$O$_{3}$ concentrations are lower in VL46 whereas FeO and
CaO concentrations are higher in VL46. This leads to a small variation
of NBO/$T$ (0.33 to 0.28) but could mirror the small mineral chemistry
variations that we observed.  Finally, a liquidus temperature
depression, caused by dissolved CO$_{2}$ in melt, is expected and this
depression may range from negligible to several hundred degrees
depending on silicate melts composition and pressure 
\citep{Eggler1975,Eggler1976,MysenBoettcher1975, EgglerKadik1979,
EgglerRosenhauer1978, Boettcheretal1987, Boettcher1984}. 
The extent of this depression is linked to the solubility and
solubility mechanism(s) of CO$_{2}$ in the melt. For instance, the
liquidus temperature depression of the CaMgSi$_{2}$O$_{6}$--CO$_{2}$
system compared to the volatile free system illustrates this mechanism 
\citep{EgglerRosenhauer1978, MysenRichet2005}.

Finally, it is emphasized that the difference in liquidus temperature
for the CO$_{2}$ bearing and CO$_{2}$ free compositions (samples VL46
and VL47) observed experimentally at 500 MPa could not be reproduced
with alphaMELTS, with $T_{L}$ of 1223~\textdegree C and
1231~\textdegree C,  respectively. Thus, the application of
thermodynamic models to predict the role of  high CO$_{2}$
concentrations on phase stability in water-poor systems needs to be
reconsidered. This appears to be particularly relevant in Italians
volcanic systems, which are often reacting with carbonates. 

\subsection{Implications for magma dynamics}\label{sec4.2}

The data presented above (literature data and our new experimental
results) can have profound implications upon the rheological behavior
of magmas. In particular, viscosity is highly influenced by the amount
of solid phases. This, in turn, might influence the way magma ascends
towards the Earth surface and eventually erupts.

The Vulcanello peninsula formed by effusions from a 600 m long, 0.5 to
1.5 m wide eruptive fissure (dike)  \citep[][and reference 
therein]{Vetereetal2007}. Based on the analysis of fluid inclusions in quartz
xenoliths we assume for the storage of the shoshonitic magma a trapping
depth between 21.5 and 17 km (corresponding to the Moho depth) 
\citep{Zanonetal2003,Peccerilloetal2006}. As such, a possible deep
storage reservoir could be hypothesised from which, during the eruptive
phase of Vulcanello 1, the magma moved directly to the surface, without
intermediate storage into the crust 
\citep{Davietal2009,Nicotraetal2018}. %\pagebreak 
Taking into account the water
free samples VL46 and VL47 (Figure~\ref{fig5}), having a crystal
content of about 20 and 30 area\%, respectively, we can calculate the
effective viscosity\break of the crystal bearing system using the models
proposed by \citet{Sato2005} and \citet{Vonaetal2011} [${\eta}_{r}
= \eta_{\mathrm{eff}}/\eta_{m}$ where $\eta_{r}$ is the
relative viscosity, $\eta_{\mathrm{eff}}$ is the effective viscosity of
the liquid containing a fraction of crystals, and $\eta_{m}$ is
the viscosity of the melt using the model of 
\citealp{Vetereetal2007}]. Moreover, as required by the
\citet{Vonaetal2011} model, using image analysis technique on BSE
images collected after experiments, one can extract and characterize
the crystal shapes from the input grey-scale slides, without
considering the possible presence of nanometric crystals 
\citep{Cassettaetal2023}. Procedures are described in 
\citet{DellinoLaVolpe1996} and \citet{Loncaric1998}. The aspect ratio
(AS $=$ major axis/minor axis) parameter providing information on the
particles elongation was determined following 
\citep{CoxBudhu2008}. Results indicate that for our experiments the
average AS for Pl is 2.5 and for Cpx is 1.7 while Fe--Ti oxides (when
present) have AS close to 1. By taking into account these results,
viscosity values of 10$^{4.0}$--10$^{4.4}$ Pa${\cdot}$s are obtained for system
containing 20 and 30 area\% of crystals, respectively by using the
\citet{Vonaetal2011} model. These values are comparable to those
obtained using the model proposed by  \citet[][i.e. 10$^{3.9}$ and
10$^{4.2}$ Pa${\cdot}$s]{Sato2005}.

{Figure~\ref{fig7}} shows the variation of viscosity of the shoshonitic
magma as a function of the crystal and volatile contents. The increase
in viscosity is clearly related to the decrease of the volatile content
and the relative increase of solid phases. 

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7} Variation of viscosity vs. water content for
samples reported in Table~\ref{tab3}. Symbols are identical to those
reported in Figures~\ref{fig2} and~\ref{fig3} (see text for details).}
\end{figure}
%\pagebreak

Considering that magma ascent in dykes  is strongly conditioned by dyke
width, {magma} ascent in narrow dykes {could} {be obstructed} by magma
crystallization. Critical dyke widths were calculated by
\citet{Petfordetal1994} for magma {with felsic composition showing}
similar {v}iscosities as those investigated here  
\citep[see also][]{Scailletetal1996}{. Results} {show that} dykes thinner than 1
m {can hardly} propagate from very large depths. {We are aware that}
narrow dykes~ (${<}$1 m) cannot be approximated to a smooth tabular
shape {with a constant} dyke width, due to the natural roughness of
rock fracture planes. {However, by doing the identical exercise as in
\citet{Petfordetal1994}, and taking into account (a) basaltic magma
temperature, (b) freezing point, (c) rock country temperature
(1200~\textdegree C, 700~\textdegree C and 300~\textdegree C respectively),
(d)~the latent heat for basalt (400 J/g) and (e) the s}pecific heat of
basaltic magma (1.0 J${\cdot}$g$^{-1}{\cdot}$\textdegree C$^{-1}$), the critical
size for dikes reduces to 0.3 m. Thus, we have some constrains for
the following discussion.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig08}
\vspace*{-2pt}
\caption{\label{fig8} (a,b) Velocities and ascent times evolution vs.
conduits size for Vulcanello (a) and hypothetic  volcanic systems with
dike widths ${\leq}$3 m and  a density contrast between magma and
country rock $\Delta\rho=200$ kg/m$^{3}$ (b). Please refer to text for
details.}
\vspace*{-2pt}
\end{figure*}

In order to calculate a possible ascent velocity (\textit{u};
m${\cdot}$s$^{-1}$) of these magmas we can use the relation of
\citet{ListerKerr1991} for laminar flow:
{\begin{equation}\label{eq4}
u=(w^{2}/3\eta)\Delta\rho g.
\end{equation}}\unskip

As stated above, the eruptive fissure at Vulcanello has width 
${\leq}$1.5 m. If now we consider a density contrast between magma and country
rock of $\Delta\rho=275$~kg/m$^{3}$ as reported in
\citet{Vetereetal2007}, and $\eta= 10^{3.5}$ Pa${\cdot}$s for a system
containing melt with 0.3~wt\% H$_{2}$O (see also Table~\ref{tab3}
samples VL46, VL47), we can estimate a possible rise velocity of such
magmas, assuming that the width of the conduits varies from 0.1 to 1.5
m. Results are presented in {Figure~\ref{fig8}a} where the ascent
velocity is found to vary in the range \textit{u}(CO$_{2}$
{free}) $=$ 1.5 ${\times}$ 10$^{-4}$--3.5 ${\times}$ 10$^{-2}$ m/s, and
\textit{u}(CO$_{2}$ {bearing}) $=$ 
5.7 ${\times}$ 10$^{-4}$--1.3 ${\times}$ 10$^{-1}$m/s. The estimated velocity is subject to an
abrupt increase if we consider H$_{2}$O--CO$_{2}$ bearing systems.
Considering that at Vulcanello the water content is estimated between
0.3 and 1.9 wt\% [analysis of melt inclusions from shoshonitic samples
of Vulcanello in \citealp{Gioncadaetal1998} and
\citealp{Clocchiattietal1994a,Clocchiattietal1994b}] and considering a
temperature of 1080~\textdegree C  [the estimated temperature for
Vulcanello magmas is 1083 ${\pm}$ 40~\textdegree C,
\citealp{Zanonetal2003}], a crystallinity of 17 area\% can be expected
for H$_{2}$O- and CO$_{2}$-bearing samples (e.g. VL37;
Table~\ref{tab3}). At these conditions the magma viscosity is about
10$^{2.8}$ Pa${\cdot}$s and its ascent velocity (1.5 ${\times}$ 10$^{-2}$--3.3
m/s as $w$ varies from 0.1 to 1.5 m, respectively, Figure~\ref{fig7})
would be faster than in a volatile-free magma {(}see
\textit{u}(CO$_{2}$ {free}) given above{)}. Thus, in case of a
nominally dry magmatic system rising in conduits having width 1.5 m,
the time to reach the surface starting from a depth of 20 km is
slightly lower than 4 years. However, if CO$_{2}$ is present, this
duration decreases to ${\sim}$300 days, and ${<}$2 h are estimated
assuming that 2.0 wt\% H$_{2}$O $+$ 0.6~wt\% CO$_{2}$ are dissolved in
the melt. Although these short timescales cannot be applied to the
Vulcanello magma, which has lower volatile  
\citep{Clocchiattietal1994a,Clocchiattietal1994b}, they could be
relevant to other shoshonitic volcanic systems. 

As a further example, by making identical considerations as above, we
can consider a hypothetic scenario with a viscosity $\eta =
10^{3.5}$ Pa${\cdot}$s (for a system containing melt $+$ 0.3 wt\% H$_{2}$O see
Table~\ref{tab3}, samples VL46, VL47). Thus, if dyke widths are
${\leq}$3 m and the density contrast between magma and country rock is
$\Delta\rho=200$ kg/m$^{3}$ and adopting Equation~(\ref{eq4}), we can have
an estimate of the possible rise velocity as presented in
{Figure~\ref{fig8}b}. In this case, results are:
\textit{u}(CO$_{2}$ {free}) $=$ 1.1 ${\times}$ 10$^{-4}$--1.0 ${\times}$ 10$^{-1}$ 
m/s, and \textit{u}(CO$_{2}$
{bearing}) $=$ 4.2 ${\times}$ 10$^{-4}$--3.7 ${\times}$ 10$^{-1}$
m/s. Finally, if we consider H$_{2}$O--CO$_{2}$ bearing systems with
concentrations similar to the experiments VL32 or VL33
(Table~\ref{tab3}) and a temperature of 1080~\textdegree C, the lower
crystallinity allows magma to have viscosity in the order of 10$^{2}$
Pa${\cdot}$s and, as consequence, to rise much faster in the range
4.7 ${\times}$ 10$^{-2}$--42.9 m/s as \textit{w} varies
from 0.1 to 3 m, respectively (Figure~\ref{fig8}). It is important to
keep in mind that in dynamic magmatic systems (during ascent) the
CO$_{2}$ release is relatively rapid and possibly can locally
accelerate magma ascent.

\section{Conclusions}\label{sec5}

Crystallization experiments of a natural shoshonite were performed at
500 MPa, and 1080~\textdegree C and water activities
\textit{a}H$_{2}$O from 0.01 to 1.00. Results show that
clinopyroxene is the main crystallizing phase and plagioclase is found
to be a stable phase only for\break water activity lower than 0.1, while
spinel is present in all the runs except in those where log
\textit{f}O$_{2}$ was lower than ${-}$9 (${\Delta}$NNO ${-}$0.11).

Interestingly, at low water activity (below 0.1) the addition of
CO$_{2}$ at constant water content reduces the crystallinity of
shoshonitic systems. This behaviour is not predicted by classical
thermodynamic models. Thus, in the case of CO$_{2}$-rich and
H$_{2}$O-poor systems, magmas become less viscous due to the relative
decrease in crystallinity. The application of viscosity models to our
experimental dataset allows us to infer the possible ascent velocity
and ascent time of this type of shoshonitic H$_{2}$O-poor magmas from
Vulcanello. 

Since the magmatic composition used for our study is chemically similar
to many Italian volcanic systems (Aeolian Arc, Campi Flegrei, Ischia
Island, Pontine Islands, Monti Cimini, Monte Amiata, Capraia Island,
Radicofani, Roccamonfina) and also to other systems worldwide
(Yellowstone, Mariana Arc, Kurile Arc, Tonga Arc, Andean Arc, Kamchatka
Arc), we suggest that our observations will be useful to better
understand shoshonitic magma behaviour under relevant geological
scenarios. The probability of very rapid ascent of less-evolved melts
from depth has to be taken into account for future volcanic crisis and
hazard evaluation.

\section*{Conflicts of interest}

Authors have no conflict of interest to declare.

\section*{Acknowledgements}

This study was funded by the ``\textit{Piano di Sostegno alla Ricerca
2022 per finanziamenti a progetti di ricerca Curiosity-driven
(F-CUR\_CREAMI)''} to FV.

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