Experimental homogenization of olivine-hosted melt inclusions
representative of near-primary basic and ultrabasic magmas is a powerful
approach to investigate the nature of their source regions and the melting
conditions in Earth's mantle. There is growing evidence that the total
CO2 contents of olivine-hosted melt inclusions may reach values of the
order of a single to several weight percent, especially in intraplate continental
basalts. To be able to homogenize melt inclusions with such high CO2
contents, we developed a technique allowing for heat treating of the melt
inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder,
using thick-walled Au80–Pd20 containers and molten NaCl as
the surrounding medium for the inclusion-bearing olivines. We applied this
technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais
volcanic province, French Massif Central. Thueyts melt inclusions were
chosen because of their high CO2 contents, as indicated by up to
1.19 wt % dissolved CO2 in the glasses and by the presence of
shrinkage bubbles containing abundant carbonate microcrystals in addition to
a CO2 fluid phase. The homogenization experiments were conducted at
pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘C,
and run durations of 30 min.
In all the melt inclusions treated at 2.5 GPa–1300 ∘C and half of
those treated at 2 GPa–1300 ∘C, we were able to completely
homogenize the inclusions, as indicated by the disappearance of the starting
bubbles, and we obtained total CO2 contents ranging from 3.2 wt % to
4.3 wt % (3.7 wt % on average). In all the other melt inclusions
(equilibrated at 1.5 or 2 GPa and 1300 ∘C or at
2.5 GPa–1275 ∘C), we obtained lower and more variable total
CO2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest
total CO2 contents, the size of the shrinkage bubble was in most cases
small (<5 vol %) to medium (<10 vol %): this is a
strong argument in favor of an origin of these melt inclusions by
homogeneous entrapment of very CO2-rich basanitic liquids
(∼ 4 wt %) at pressures of 2 to 2.5 GPa. The lower total
CO2 contents measured in some inclusions could reflect a natural
variability in the initial CO2 contents, due for instance to melt
entrapment at different pressures, or CO2 loss by decrepitation. An
alternative scenario is heterogeneous entrapment of basanitic liquid plus
dense CO2 fluid at lower pressures but still at least on the order of
1 GPa as indicated by dissolved CO2 contents up to 1.19 wt % in the
glasses of unheated melt inclusions. Whatever the scenario, the basanites
from the Bas-Vivarais volcanic province were generated in a mantle
environment extremely rich in carbon dioxide.
Introduction
Melt inclusions are tiny volumes of melt (± immiscible fluid) trapped
at depth within crystals growing in a magma. Once the inclusion is enclosed
in the host crystal, it may be preserved from most of the changes affecting
the magma until final cooling (e.g., Frezzotti, 2001). Thus, it provides
very valuable information on the composition and physical state of the magma
at the time of entrapment (e.g., Roedder, 1979; Frezzotti, 2001; Schiano,
2003; Kent, 2008; Métrich and Wallace, 2008; Audétat and Lowenstern,
2014; Esposito, 2021). In particular, melt inclusions are often the only way
to characterize the initial volatile contents of magmas at depth (H2O;
CO2; and S, F, or Cl species) because volcanic products at the surface are
almost fully degassed.
Many changes may take place in a melt inclusion after its entrapment. The
most common one is the formation of a shrinkage bubble during cooling (Fig. 1a–b), which is due to crystallization of the host mineral from the melt
onto the inclusion walls and to the greater thermal contraction of the melt
relative to its host mineral, among others. Upon cooling, the internal
pressure of the inclusion decreases, so at some point the melt becomes
saturated with a volatile phase, and a bubble nucleates (Roedder, 1979;
Lowenstern, 1995). The shrinkage bubble can store a significant fraction of
the volatiles initially dissolved in the melt; thus it is necessary to
measure or calculate this fraction to restore the initial volatile content
(e.g., Anderson and Brown, 1993; Hartley et al., 2014; Moore et al., 2015;
Maclennan, 2017). Recent studies found that an average of 20 % to 90 % of
the carbon dioxide initially dissolved in olivine-hosted melt inclusions is
present as a fluid phase in the shrinkage bubbles (e.g., Hartley et al.,
2014; Moore et al., 2015; Wallace et al., 2015; Tucker et al., 2019). In
addition to the volatile species present in the fluid phase filling the
shrinkage bubble, a significant amount of carbon and sulfur may also be
stored in micro- or nanocrystals on the bubble walls, mainly carbonates,
sulfides, and sulfates (Fig. 1c; Kamenetsky et al., 2002; Kamenetsky and
Kamenetsky, 2010; Moore et al., 2015; Esposito et al., 2016; Robidoux et
al., 2018; Venugopal et al., 2020; Schiavi et al., 2020).
(a) Euhedral olivine phenocryst from Thueyts showing a large melt
inclusion and a smaller one in its core and several small melt inclusions
and embayments along its rims. (b) Zoom on the large melt inclusion showing
the CO2-rich bubble and the characteristic shape of the Bas-Vivarais
melt inclusions: the melt inclusion is an irregular polyhedron with rounded
faces convex towards the interior of the inclusion. (c) Field-emission scanning electron microscope (SEM) microphotograph showing rhombohedral carbonate microcrystals on the bubble
wall of a melt inclusion from Lac d'Issarlès, Bas-Vivarais
(microphotograph taken by Dr. Letian Li with a Helios G4 PFIB CXe system at
the Thermo Fisher Scientific NanoPort in Eindhoven, the Netherlands). Scale
bars: 200 µm in (a), 50 µm in (b), and 2 µm in (c).
The mass of CO2 contained in the bubble and in the carbonate crystals
must be added to the CO2 dissolved in the glass to reconstruct the bulk
CO2 content in the inclusion at the time of entrapment. In the first
studies (e.g., Anderson and Brown, 1993; Shaw et al., 2010; see also
Wallace et al., 2015), computational methods were used to calculate the
amount of CO2 in the bubble based on the bubble volume and on the
assumption that bubble and melt were at equilibrium at the time of eruption.
More recently, the mass of CO2 in the shrinkage bubble has been
inferred from direct measurement of CO2 density by Raman spectroscopy
(e.g., Esposito et al., 2011; Steele-MacInnis et al., 2011; Hartley et al.,
2014; Moore et al., 2015; Aster et al., 2016). An alternative approach is to
homogenize the olivine-hosted melt inclusion at high temperature and to
analyze the CO2 dissolved in the glass after quenching. This has been
done both at room pressure using a high-temperature heating stage (Wallace
et al., 2015; Hanyu et al., 2020) and under pressure in an internally heated
pressure vessel (300–500 MPa, Mironov et al., 2015) or in a piston cylinder
(600 MPa, Hudgins et al., 2015; 500–800 MPa, Rasmussen et al., 2020). Hanyu
et al. (2020) combined homogenization in a heating stage, X-ray
microtomography to measure precisely the volumes of melt inclusions and bubbles
(after heating), and Raman spectroscopy to measure CO2 density in
bubbles. With these, they were able to estimate the total CO2 content
in the inclusions with an error of ∼ 10 %. Most of the
above studies were primarily concerned with the CO2 stored as a fluid
in the shrinkage bubble. A first attempt to estimate the contribution of
carbonate microcrystals to the bulk CO2 content of the melt inclusion
was made by visual inspection of optical photomicrographs to obtain a rough
estimate of the volume of carbonates (Moore et al., 2015). Recently, Schiavi
et al. (2020) developed a novel application of Raman microtomography
allowing for simultaneously identifying the microcrystals on the bubble walls
and estimating their volumes along with the bubble volume. They showed that
carbonates can occupy up to a third of the bubble cavity.
Although many olivine-hosted melt inclusions have CO2 contents of a
thousand to a few thousand parts per million (e.g., Maclennan, 2017), there is a growing
evidence that the total CO2 contents of melt inclusions may reach much
higher values, on the order of a single to several weight percent (Naumov et al., 2003;
Helo et al., 2011; Hudgins et al., 2015; Rasmussen et al., 2017; Hanyu et
al., 2020; Schiavi et al., 2020), especially in ocean island basalts and to
an even greater extent in intraplate continental basalts. These
volatile-rich melt inclusions cannot be homogenized at ambient pressure with
a heating stage (Schiavi et al., 2016; see also Rasmussen et al., 2020, and
references therein). So to be able to homogenize melt inclusions with such
high CO2 contents, we developed a technique allowing for heat treating
olivine-hosted melt inclusions under hydrostatic pressures up to 3–4 GPa in
a piston cylinder. Below we firstly present the details of the technique and
then we show the results of an application to CO2-rich melt inclusions
hosted in olivine phenocrysts in basanites from the Bas-Vivarais volcanic
province (Ardèche, France).
Experimental setupMelt inclusion homogenization at high pressure: choice of the
experimental configuration
The most conventional tool to homogenize melt inclusions is the microscope
heating stage, in which the inclusion can be monitored continuously under
the microscope during heating to determine precisely the homogenization
temperature (e.g., Danyushevsky et al., 2002; Massare et al., 2002;
Cannatelli et al., 2016). However, during heating at 1 atm, the
increase in the internal pressure of the melt inclusion can overcome the
strength of the host mineral and lead to its failure and to the decrepitation of
the inclusion (Ferrero and Angel, 2018); plastic deformation and volumetric
re-equilibration of the host mineral are also possible (e.g., Schiavi et
al., 2016; Drignon et al., 2021). Even if the host mineral does not fail and
the inclusion keeps its volatiles, heating at ambient pressure does not
allow for the total homogenization of CO2-rich melt inclusions and the
disappearance of the bubble. Different high-pressure techniques may be used
to avoid decrepitation during homogenization of volatile-rich melt
inclusions: gas pressure vessels, either externally heated (Student and
Bodnar, 2004; Thomas et al., 2006) or internally heated (Skirius et al.,
1990; Anderson et al., 2000; Mironov et al., 2015; Portnyagin et al., 2019);
piston-cylinder apparatuses (Sparks and Mavrogenes, 2005; Perchuk et al.,
2008; Bartoli et al., 2013, 2016; Cesare et al., 2015; Hudgins et al., 2015;
Ferrero et al., 2018; Rasmussen et al., 2020; Lewis et al., 2021; see also
Malaspina et al., 2006, for the case of solute-rich aqueous inclusions); and
hydrothermal diamond anvil cells (Li and Li, 2014). The principle is to
subject crystals containing unexposed melt inclusions to a confining
pressure using a gas, a liquid, or a solid medium in order to balance the
high internal pressure of the melt inclusion. Among these high-pressure
techniques, only the hydrothermal diamond anvil cell allows for continuous, in situ
observation of the changes that occur in the melt inclusion during heating
and homogenization (Li and Li, 2014). In the other high-pressure techniques,
the homogenized inclusions can only be observed after quenching. This
drawback is to some extent compensated for by much larger volumes of the
sample chamber (≥ 100 mm3 in gas pressure vessel and
∼ 10 mm3 in a piston cylinder vs. 0.1–1 mm3 in the
hydrothermal diamond anvil cell), which allow for several melt inclusion-bearing
crystals to be treated in a single experiment.
To homogenize CO2-rich magmatic inclusions in olivine phenocrysts, we
chose a piston-cylinder apparatus rather than an internally heated pressure
vessel (e.g., Mironov et al., 2015), because the latter is limited to
pressures well below 1 GPa. The piston-cylinder technique offers excellent
performance in pressure and temperature (3–4 GPa and up to 1500 ∘C and more), high quench rate, large sample chamber, and relative ease of
use. Of the previous homogenization experiments in a piston cylinder, most
have been applied to garnet-hosted melt inclusions in high-grade metamorphic
rocks (e.g., Bartoli et al., 2013; Cesare et al., 2015; Ferrero et al.,
2018) and only a few to basaltic melt inclusions in olivine (Hudgins et
al., 2015; Rasmussen et al., 2020) or in plagioclase (Lewis et al., 2021).
For our application, we opted for a new configuration, in which the olivine
crystals are placed inside a thick-walled sample chamber and surrounded by
molten NaCl to ensure a hydrostatic environment during the experiment (Fig. 2). This configuration is designed to meet the following specifications:
protect crystals from non-hydrostatic stresses to prevent fracturing even at
confining pressures of several GPa, avoid chemical contamination of the melt
inclusions during the homogenization step, and allow for easy recovery of crystals
after the experiment. Compared to previous studies, the main differences are
(i) the use of a thick-walled, precious metal container (Fig. 2a) to
minimize deformation of the sample chamber during the experiment and (ii) the choice of molten salt as matrix. In most previous experiments,
thin-walled metal containers, or even just a graphite container
(Hudgins et al., 2015), were used, and the matrix was a silicate powder, with or
without added water: for example, silica powder in the case of garnet-hosted
melt inclusions (Bartoli et al., 2013; Cesare et al., 2015) or basalt
powder in the case of olivine-hosted melt inclusions (e.g., Mironov et al.,
2015). Nevertheless, Perchuk et al. (2008), Rasmussen et al. (2020), and
Lewis et al. (2021) have already used halide salts (NaCl, KBr, and CsI,
respectively) as a matrix in their high-pressure homogenization experiments.
(a) Two olivine phenocrysts are loaded in a thick-walled
Au80–Pd20 container along with a mixture of NaCl and powdered
olivine. The homogenization experiments are run at P–T conditions above the
melting curve of sodium chloride, ensuring hydrostatic conditions inside the
container. (b) Sketch of the 1/2 in. piston-cylinder assembly: 1 –
fired pyrophyllite, 2 – crushable MgO, 3 – graphite furnace, 4 – pyrex
cylinder, 5 – NaCl cell, 6 – brass ring, and 7 – Teflon foil.
A sketch of the sample chamber is shown in Fig. 2a. It is composed of a
thick-walled (∼ 0.75 mm) Au80–Pd20 container capped
by a 1 mm thick lid. The inner diameter of the container is 2.5 mm. The
sample chamber goes into a casing of fired pyrophyllite at the center of a
1/2 in. piston-cylinder assembly (Fig. 2b). The thick walls of
the capsule and the fired pyrophyllite are intended to minimize deformation
of the capsule during the experiment. To ensure hydrostatic conditions for
the olivine, the crystals are surrounded by a fluid medium during the
experiment. Among the low-melting-point compounds used as solvents to grow
large crystals of silicates in the flux-growth methods (PbO, V2O5,
tungstates, molybdates, borates, alkali halides, etc.; Carlson, 1986;
Skogby, 1999), we selected NaCl because the solubility of silicate
components in alkali halides is much lower than in the other solvents
(Carlson, 1986) and because sodium chloride is very easy to dissolve at the
end of the experiments. Accordingly, the olivine phenocrysts in the
homogenization experiments were surrounded by a mixture of 80 wt % reagent
grade NaCl and 20 wt % powdered olivine (same composition as the
phenocrysts). The purpose of adding small grains of olivine to NaCl was to
minimize the amount of dissolution undergone by the large olivine crystals.
Olivine phenocrysts hosting melt inclusions were selected, prepared, and
fully characterized before the experiment (see next section). Two
phenocrysts, about 1200 µm long, were loaded in the Au80–Pd20
container along with the dry NaCl–olivine mixture. On average, the ratio of
olivine (phenocrysts + powder) to olivine + NaCl is equal to 26 wt %.
After loading, the thick lid was pushed into the borehole of the container
using a bench vise. Finally, the lid and the container were welded together,
and the whole was placed in the center of the piston-cylinder assembly.
Choice and preparation of the starting olivine phenocrysts
All the olivine phenocrysts used in this study were collected in basanitic
tephras from the strombolian pyroclastic fallout of Thueyts volcano,
Ardèche, France. This volcano belongs to the quaternary intraplate
volcanic province of Bas-Vivarais, in the southeast of the French Massif
Central (e.g., Nomade et al., 2016). Thueyts olivine phenocrysts have high
forsterite contents (Fo > 85 %) and contain bubble-bearing,
CO2-rich melt inclusions (Fig. 1a–b). Indeed quantitative
characterization of volatiles by Raman microtomography yielded a total
CO2 content in excess of 2 wt % in a melt inclusion from Thueyts
(Schiavi et al., 2020). The high total CO2 content of Bas-Vivarais melt
inclusions is due to the combination of a high level of dissolved CO2
in the glass phase of the melt inclusions (in excess of 1 wt %; see
below), the presence of CO2 fluid in the bubbles, and the abundance of
carbonate microcrystals on the bubble walls (Fig. 1c). The dominant
carbonate phase is most often magnesian calcite, but dolomite or magnesite
are dominant in some bubbles (as determined from the wavenumbers of the
Raman modes; Perrin et al., 2016). Due to this high total CO2 content,
all our attempts to homogenize Thueyts inclusions at room pressure and
temperature up to 1230 ∘C failed: in no case was it possible to
make the CO2 bubbles disappear (see Supplement and Table S4
for details on heating experiments at 1 atm). Accordingly, Thueyts melt
inclusions are perfect candidates for homogenization at high pressures.
A population of olivine grains with melt inclusions was prepared for this
study. Some grains were ground and polished to expose the inclusions at the
surface in order to analyze the basanitic glasses before homogenization
using electron probe microanalysis (major elements, S, Cl, and F) and Raman
spectrometry (water and CO2). Olivine crystals (generally in the size
range 650–1750 µm) hosting several melt inclusions far from any cracks
and from the grain surface, not connected to the external melt, and randomly
distributed were selected for the experiments. These melt inclusions are
polyhedral; in general larger than 40 µm; and contain one or more
bubbles (Fig. 1b), which occupy on average 8.6 (±1.0) vol % of the
inclusion volume (from 3.3±0.7 vol % to 26.6±3.0 vol %). Before
the experiment, the olivines were doubly polished to remove the surrounding
glass and allow for clear observation of the melt inclusions inside. The melt
inclusions and the bubbles were measured and photographed using an optical
microscope.
Experimental techniques and run conditions
Our homogenization experiments cover pressure (P) and temperature (T) ranges
of 1.5 to 2.5 GPa and 1275 to 1300 ∘C, respectively (see Table 1
for a summary of the experimental parameters); the run duration was 30 min in
all the experiments. They can be separated into three groups. The first group
corresponds to our standard experimental conditions (P=2.5 GPa, T=1300∘C, t=30 min), for which we observed a complete
homogenization of the melt inclusions. The choice of 2.5 GPa was dictated by
the need to be above the volatile saturation pressure of a basanitic melt
that could contain up to 3 wt % CO2 or even more. To guide us in this
choice, a first-order estimate of the solubility of CO2 in our
basanitic composition was obtained using the models of Duan (2014) and
Eguchi and Dasgupta (2018), considering a temperature of 1300 ∘C
and pressures between 1.5 and 2.5 GPa. In the absence of water, Duan's model
yields CO2 solubilities ranging from 2.0 wt % at 1.5 GPa to 2.7 wt % at 2.0 GPa and to 3.2 wt % at 2.5 GPa. Assuming 2 wt % dissolved
water (close to the maximum water content measured in Thueyts melt
inclusions: 1.94 wt %; Table 2), it yields slightly lower CO2
solubilities: 1.9 wt %, 2.6 wt %, and 3.0 wt % CO2 at 1.5, 2.0,
and 2.5 GPa, respectively. The CO2 solubilities computed using the model of Eguchi
and Dasgupta (2018) are significantly larger: 3.1 wt %, 4.5 wt %, and 5.9 wt % at 1.5, 2.0, and 2.5 GPa, respectively (assuming a molar ratio of
Fe3+/ΣFe= 0.19; Gaborieau et al., 2020). These larger values
are in better agreement with the CO2 solubilities measured in
low-silica alkaline magmas (Thibault and Holloway, 1994; Brooker et al.,
2001). The CO2 solubility measured by Brooker et al. (2001) in a
Ca-rich nephelinite and an Mg-rich nephelinite is equal to 4.1 wt % and
4.6 wt %, respectively, at 2 GPa–1400 ∘C. In the case of a
tephrite close to our melt composition (SiO2: 44.1 wt %; CaO: 14.3 wt %; Na2O: 3.2 wt %; K2O: 3.5 wt %), Thibault and
Holloway (1994) report CO2 solubilities equal to 4.5 wt % at
1.5 GPa–1300 ∘C and 6.2 wt % at 2 GPa–1350 ∘C. From
this dataset, we estimate that it should be feasible to homogenize basanitic
melt inclusions with 3 wt % to 4 wt % CO2 at 2.5 GPa and possibly even at
2 GPa. Regarding the temperature, our choice of 1300 ∘C is based
on the temperature computed from the composition of unheated melt inclusions
corrected for post-entrapment crystallization and their hosts (Table 2)
using the olivine–liquid thermometer of Putirka et al. (2007) and assuming
equilibrium at a pressure of 2.5 GPa.
Summary of experimental parameters (run duration was 30 min in all
experiments). Cpx: clinopyroxene; Fo: forsterite; Ol: olivine.
a Mass ratio of olivine in the starting material; the mass of olivine
includes the mass of the large olivine grains and the mass of olivine powder
mixed with NaCl. b “Homogenization: total” means that all the melt inclusions were
fully homogenized (bubble-free) at the end of the experiment.
“Homogenization: partial (2/3)” means that only two out of three melt
inclusions were fully homogenized, with the third one containing a residual
bubble at the end of the experiment. In run HOP no. 18 at 1275 ∘C,
a daughter mineral (clinopyroxene) grew during the experiment.
c In HOP no. 1, we used two olivine grains without melt inclusions
(control experiment to evaluate the extent of olivine dissolution in molten
NaCl). d HOP no. 4 is the only experiment in which we added some water to the
mixture of NaCl and olivine powder in order to minimize water loss from the
melt inclusions during high-pressure homogenization. The amount of added
water was equal to 1.98 wt %, which corresponds to a ratio of
H2O/(H2O+NaCl) equal to 2.97 wt %.
e In HOP no. 16, the starting material was a cube of synthetic
forsterite embedded in pure NaCl. The cube was cut in a piece of forsterite
kindly provided by Kazuhiko Ito (Ito et al., 2003).
Glass compositions of unheated melt inclusions. The compositions
are corrected from post-entrapment crystallization with the software MIMiC
(melt inclusion modification corrections; Rasmussen et al., 2020), using a ratio of Fe3+/ΣFe equal to 0.19
(Gaborieau et al., 2020).
a Oxide concentrations are given in weight percent. Major and minor elements
are given after normalization to a total of 100 wt % (excluding volatile
elements). “Σ EPMA” is the original analytical total of the
electron probe microanalysis. b Volatile elements are given in parts per million (F, Cl, and S) or in wt %
(H2O and CO2); “b.d.l.” stands for below detection limit. All the
volatile elements are corrected using MIMiC, except F, which is not taken
into account by the software. Fluorine was corrected so that the ratio of
F / incompatible elements after correction is equal to the ratio of
F / incompatible elements of the electron probe microanalysis.
c “Total” equals the sum of major and minor oxides (Σ EPMA)
plus the volatile elements.
d The molar ratio of Mg2+/(Mg2++Fe2+) corresponds to the percentage of forsterite of the olivine (Ol-Fo) and the ratio of magnesium to iron in the glass of the melt inclusion (Mg#), where the total iron in the sample is considered Fe2+.
eXOl is the percentage of olivine added to melt composition by
the software MIMiC in the case of post-entrapment crystallization.
f Average composition of unheated melt inclusions.
In the second group of experiments, the run parameters were varied one by
one relative to their values in the standard experiments, with (i) three
experiments at lower pressure (one at 1.5 and two at 2 GPa, at T=1300∘C), (ii) one experiment at lower temperature
(1275 ∘C, at P=2.5 GPa), and (iii) one experiment with water
added to the NaCl–olivine mixture to evaluate the effect on water loss in
the melt inclusions (P=2.5 GPa, T=1300∘C).
The third group includes two experiments designed to evaluate the importance
of the interactions between olivine and molten NaCl: HOP no. 1 is identical
to the standard experiments (P=2.5 GPa, T=1300∘C, t=30 min), except that the two natural olivine crystals are devoid of melt
inclusions; HOP no. 16 was done under the same P–T–t conditions but with a
cube of synthetic forsterite in a matrix of pure NaCl. The purpose of these
two experiments was to verify that the amount of olivine dissolved in the
molten salt is small and to determine the reaction of olivine dissolution.
Target pressure and temperature, for instance 2.5 GPa and 1300 ∘C, were reached in three steps: (1) pressurization to 300 MPa at room
temperature, (2) heating to 900 ∘C and pressurization to 400 MPa
in ∼ 18 min, and (3) heating to target T at 25 ∘C min-1
and simultaneous pressurization to target P at ∼ 100 MPa min-1.
After 30 min at the target P and T (homogenization step), the experiments
were terminated by shutting off the power of the apparatus, resulting in an
average quench rate of 160 ∘C s-1 over the first 600 ∘C.
From the end of step 2 to the end of the homogenization step, the P–T
conditions were at all times above the melting curve of sodium chloride (from
801 ∘C at ambient pressure to 1227 ∘C at 2.5 GPa;
Akella et al., 1969), ensuring hydrostatic conditions inside the container.
At the end of the experiment, the container was slightly shortened and
deformed into a barrel. The lid was cut off, and the salt was dissolved in
water to recover the olivine crystals.
Analytical techniques
We describe below the main techniques used to characterize the melt
inclusions before and after high-pressure (HP) homogenization: electron
microprobe analysis and Raman spectroscopy. In addition to these techniques,
we used CHNS elemental analysis and Fourier-transform infrared spectroscopy (FTIR)
to measure the volatile contents of two new basanitic reference glasses that
were synthesized to complete our set of standards for quantification of
H2O and CO2 by Raman spectroscopy. These two techniques are
detailed in the Supplement.
Electron microprobe analysis (EMPA)
Melt inclusions, homogenized or not, and their host olivines were analyzed
for major and volatile (Cl, F, and S) elements with a CAMECA SXFive-TACTIS
electron microprobe using a 15 kV accelerating voltage. Major elements
analyses of olivine were performed with a 1 µm diameter focused beam
and a current of 15 nA, whereas melt inclusions were analyzed using a
defocused beam of 5–10 µm diameter and a current of 4–8 nA.
Volatiles were measured with a 5 µm beam size and a 40 nA current
(Rose-Koga et al., 2020) after all other analyses have been performed.
The raw compositions obtained by EMPA are given in the Supplement
(Tables S2 and S3). The compositions listed in Tables 2 and 3 have been
corrected for post-entrapment crystallization (PEC) or post-entrapment
dissolution (PED) of the host olivine: PEC (or more rarely PED) occurs after
melt entrapment in response to changing physicochemical conditions in the
magma during ascent and eruption (Sobolev, 1996; Danyushevsky et al., 2002)
or due to olivine–liquid exchanges that may occur at the end of the
experiments during quench. In the case of PEC, a layer of olivine
crystallizes at the expense of the liquid along the inclusion walls (or a
layer of olivine is dissolved in the liquid in the case of PED), resulting
in significant changes in the major element compositions of the liquid, in
particular its Fe and Mg contents. In this study, melt inclusion
compositions were corrected for PEC/PED using the software MIMiC (Rasmussen
et al., 2020). The correction involves adding increments of olivine back
into the liquid until the Fe–Mg partition coefficient between olivine and
liquid, KD, reaches the equilibrium value (calculated using the model of Toplis, 2005). The software MIMiC applies an iterative correction requiring
as input data the major element composition of the melt, its S, Cl, H2O,
and CO2 contents; the forsterite content of the host olivine; and the
oxidation state of the melt. For the latter, we used an oxygen fugacity of
+1 log unit relative to the quartz–fayalite–magnetite oxygen reference
buffer (ΔQFM + 1), as determined by Jannot et al. (2005) for the
Chaîne des Puys, another volcanic province of French Massif Central. In
addition to the corrected major element composition of the melt, the output
data include the degree of post-entrapment crystallization or melting. All
the natural melt inclusions from Thueyts experienced PEC, with a degree
comprised between 2.8 % and 11.8 % (7.3 % on average). In the case of
the compositions measured in the homogenization experiments, MIMiC output is
PEC for eight inclusions, and PED is output for the other five, with ranges from
3.0 % olivine crystallization to 3.7 % olivine dissolution (0.5 %
crystallization on average). It is possible that the natural melt inclusions
have suffered some Fe loss. We cannot evaluate to what extent, however,
because the FeO content of bulk rocks in the Bas-Vivarais volcanic province
is biased by systematic contamination by crustal and mantle xenocrysts.
Glass compositions of melt inclusions after high-pressure
homogenization experiments. The compositions are corrected from
post-entrapment crystallization or olivine dissolution with the software
MIMiC (Rasmussen et al., 2020), using a ratio of Fe3+/ΣFe equal to
0.19 (Gaborieau et al., 2020).
a Run conditions: “Stdd”, standard conditions
(2.5 GPa–1300 ∘C, no added water); “+ H2O”, 2.5 GPa–1300 ∘C with added water; “1.5 GPa”, 1.5 GPa–1300 ∘C;
“2.0 GPa”, 2.0 GPa–1300 ∘C; “1275 ∘C”,
2.5 GPa–1275 ∘C.
b Oxide concentrations are given in weight percent. Major and minor elements
are given after normalization to a total of 100 wt % (excluding volatile
elements). “Σ EPMA” is the original analytical total of the
electron probe microanalysis.
c Volatile elements are given in parts per million (F, Cl, and S) or in weight percent
(H2O and CO2); “b.d.l.” stands for below detection limit. All the
volatile elements are corrected using MIMiC, except F, which is not taken
into account by the software. Fluorine was corrected so that the ratio of
F / incompatible elements after correction is equal to the ratio of
F / incompatible elements of the electron probe microanalysis.
d “Total” equals the sum of major and minor oxides (ΣEPMA)
plus the volatile elements.
e Ol-Fo is the percentage of forsterite of the olivine after
experiment, defined as the molar ratio of Mg2+/(Mg2++Fe2+).
fXOl is the percentage of olivine added to or subtracted from melt
composition by the software MIMiC: olivine is added (positive figures) in
the case of post-entrapment crystallization; olivine is subtracted from the
melt composition (negative figures) to correct for the effect of olivine
dissolution.
f “Average Stdd” corresponds to the average values of the melt
inclusions homogenized under standard conditions (2.5 GPa–1300 ∘C).
Raman spectroscopy
Water and CO2 contents of the glass inclusions and their bubbles were
measured in back-scattered geometry using a Renishaw inVia confocal Raman
micro-spectrometer equipped with a 532.1±0.3 nm diode laser (180 mW
output power), a CCD (charge-coupled device) detector of 1040×256 pixels, a Rayleigh
rejection edge filter, and a Leica DM2500 M optical microscope with a
motorized XYZ stage. Daily calibration of the spectrometer was performed
using a silicon standard (520.5 cm-1 peak). During analysis, a grating
of 2400 grooves mm-1, a ×100 microscope objective (numerical
aperture = 0.9), a 20 µm slit aperture (high-confocality setting), and
a 3 mW laser power on the sample were used. Under these analytical
conditions, the spectral resolution was ∼ 0.5 cm-1, while
the lateral and vertical spatial resolutions were, respectively, 1 µm and
2–3 µm near the sample surface. The spectra of the glass were recorded
using the WiRE™ 4.4 (Windows-based Raman Environment) software in the wavenumber ranges from
∼ 100 to 1350 cm-1 (aluminosilicate network region) and
from ∼ 2900 to 3800 cm-1 (water region) for eight cycles
of 30 s. For the analysis of the CO2 bubbles present in unheated melt
inclusions and in two inclusions homogenized at medium pressure, the spectra
were acquired in the ∼ 200–1400 cm-1 range for four
cycles of 15 s. The Raman spectra measured in some inclusions are
contaminated by the signal of the host crystal. To correct for this effect,
we subtracted the purely olivine spectrum (measured in the same analytical
conditions as for the melt inclusions) from the glass spectra.
For quantification of H2O and CO2 in glasses, five basanitic
reference glasses with known CO2 and H2O contents (Schiavi et al.,
2018) were used as external standards and analyzed several times during each
session using the same analytical conditions. In order to extend the
calibration to the highest CO2 concentrations measured in studied
glass inclusions, two more basanitic reference glasses with higher CO2
contents were synthesized (BSN150121 and BSN210121, with 3.69 wt % and
4.46 wt % CO2, respectively; see Table S1 in the Supplement). Basanite
powder was loaded along with the required amount of oxalic acid dihydrate,
(COOH)2⋅ 2H2O, in thin-walled Au80–Pd20
capsules. After welding, the capsules were subjected to P= 2.5 GPa and T=1390∘C for 3 h in an end-loaded piston cylinder and
then quenched.
For determination of the CO2 content of the inclusion glasses, first Long's correction was applied to the raw data of the silicate band
(Long, 1977), then a cubic baseline was subtracted from spectra fixing two
peak-free anchor intervals at 100–250 and 1160–1350 cm-1. The spectra
of the glasses show three bands: a low-frequency band (200–625 cm-1), a middle-frequency band (625–830 cm-1), and a
high-frequency band (830–1200 cm-1). The dissolved CO32-
in the glass phase produces a peak near 1083 cm-1, whose intensity
increases with the CO2 content (Morizet et al., 2013). We performed
spectral peak fitting using WiRE™ 4.4 software, in order to identify
and quantify the spectral components of the silicate band. The entire
spectrum was fitted with 10 Gaussian components (9 for the silicate band
and 1 for the CO32- peak) using a multiple-peak fit tool and
following the method described by Morizet et al. (2013). This method
consists of several steps. (i) The number of components is determined. The
number of 10 components was chosen because it gave the best fit quality
(expressed as chi-square, χ2): tests with either more or fewer
components resulted in poorer quality fits with higher values of χ2. (ii) A first set of iterations is executed in which peak positions and
full widths at half maximum (FWHM) are fixed until χ2 remains
stable on a minimum value. (iii) A second set of iterations is performed with
peak positions, FWHM, and heights left free to evolve until the best fit
(i.e., the lowest χ2) is attained. The 10 components best
fitting the samples and reference glasses have the following positions
(Fig. S1 in the Supplement): (I) 422 cm-1 on average (400–458 cm-1,
minimum and maximum, respectively), (II) 512 cm-1 (489–528 cm-1),
(III) 580 cm-1 (574–587 cm-1), (IV) 725 cm-1 (686–768 cm-1), (V) 797 cm-1 (780–825 cm-1), (VI) 880 cm-1 (842–895 cm-1), (VII) 958 cm-1 (872–976 cm-1), (VIII) 1009 cm-1
(970–1033 cm-1), (IX) 1098 cm-1 (1028–1133 cm-1), and (X) 1085 cm-1 (1081–1088 cm-1). Component X corresponds to the
CO32- peak.
The CO2 concentration of unknown glass samples was obtained from an
independent linear relationship between the relative area of the
CO32- peak and the CO2 concentration of the known reference
glasses measured by CHNS elemental analysis and FTIR spectroscopy (see
Table S1). The calibration line (Fig. 3) providing the best
fit (R2=0.987) is
CCO2(wt%)=(AcompX/Atot)/2.5506,
where AcompX is the area of the CO32- peak and Atot is
the entire area of the silicate and carbonate bands.
Raman calibration line: plot of the relative area of the
CO32- peak of the Raman spectra (AcompX/Atot in Eq. 1) as a function of the CO2 concentration of the reference glasses. The
error bar represents the 95 % confidence interval.
In line with the results of Morizet et al. (2013), we found that precise
assignment of each Gaussian component to specific structural units of the
glass is not essential for our calibration, since this is built on the
AcompX/Atot ratio. Nonetheless, we highlight that the number and
positions of the components resulting from the fitting are in good agreement
with previous studies (Morizet et al., 2013, and references therein).
Moreover, Long's correction (Long, 1977) amplifies the intensity of the
bands at higher frequencies, including the CO32- peak, compared to
the low-frequency bands, which facilitates fitting the spectra around the
CO32- peak. Noticeably, the FWHM of the CO32- peak is
43–44 cm-1 for the reference glasses and ranges from 38 to 44 cm-1
in the studied samples; this is consistent with observations by Morizet et
al. (2013).
ResultsComposition of Thueyts melt inclusions
Analyses of 10 unheated melt inclusions (corrected for PEC) are reported in
Table 2 (see Table S2 for the raw data). All melt inclusions
have undergone post-entrapment crystallization to a variable degree: from
2.8 % to 11.8 %. Thueyts melt inclusions have a basanitic composition, with
a restricted range of oxide concentrations: 43.7 wt %–44.8 wt % SiO2,
6.1 wt %–8.1 wt % MgO, 8.5 wt %–10.6 wt % FeOT, 11.8 wt %–14.1 wt % CaO,
14.9 wt %–17.3 wt % Al2O3, 3.8 wt %–4.8 wt % Na2O, 1.5 wt %–2.2 wt % K2O, and 2.6 wt %–3.8 wt % TiO2. Basanites are the typical rock types
of the Bas-Vivarais volcanic province (Rochette et al., 1993) and are
interpreted as near-primary magmas ascending rapidly from mantle depths to
Earth's surface without any significant chemical differentiation in the
crust.
The melt inclusions are rich in volatiles, in particular in CO2. The
ranges of volatile concentrations measured in the glass phase of the
inclusions are as follows: from < 0.20 wt % to 1.19 wt % CO2, from
1.01 wt % to 1.94 wt % H2O, from 1291 to 2683 ppm S, from 1481 to 2098 ppm
Cl, and from 1038 to 1376 ppm F. Although such elevated CO2
concentrations have rarely been documented in the glass phase of melt
inclusions, they represent only a small part of the total amount of CO2
in Thueyts inclusions. Indeed, Schiavi et al. (2020) estimated that 66 %
of the total CO2 in a melt inclusion from Thueyts was stored in the
bubble as CO2 fluid and as carbonate microcrystals along the bubble
walls (Fig. 1c) and that the total CO2 was larger than 2 wt %. In
addition to carbon, 20 % of the sulfur is stored in the bubbles as
sulfide microcrystals.
For two unheated melt inclusions, we computed the mass of CO2 vapor
contained in the bubble by multiplying the bubble volume by the CO2
density. The later was measured by Raman spectroscopy using the relationship
between the Fermi diad split and CO2 density (Song et al., 2009). By
adding the CO2 vapor and the CO2 in the glass, we obtain 2.02 wt % and
2.69 wt % CO2 (Table S2). These are minimum values
because we did not take into account the CO2 stored in the carbonates,
but they confirm the abundance of CO2 in the Bas-Vivarais melt
inclusions.
Interactions between olivine and NaCl at high pressure and temperature
After NaCl dissolution at the end of the experiments, some grains come out
intact, while others are broken into a few pieces. The intact grains are
covered by a crust of prismatic crystals that is characteristic of the
chemical interactions between olivine and the NaCl matrix (Fig. 4a). In the
broken grains, the fractures show smooth olivine surfaces devoid of crust,
indicating that fracturing occurred late, during quench and unloading or
during capsule opening. The olivine grains were doubly polished to remove
the crust and to expose the homogenized melt inclusions. Some melt
inclusions observed before experiments were lost during polishing or because
of fracturing of the grains during decompression and unloading of the
piston-cylinder experiment.
(a) SEM photograph of one of the olivine grains recovered at the
end of experiment HOP no. 4 after NaCl dissolution: the grain is intact, and
its surface is fully covered by a crust of prismatic crystals (mostly
orthopyroxene). (b) SEM photograph of the forsterite parallelepiped at the
end of experiment HOP no. 16: the crystal is practically unchanged from its
original form and is partially covered by orthopyroxene needles. The crystal
broke at the end of the experiment, which explains the conchoidal fracture
surface on the lower side of the image. (c) Zoom showing the orthopyroxene
needles partially covering the surface of the forsterite crystal (run
HOP no. 16). Scale bars: 500 µm in (a), 400 µm in (b), and 90 µm in (c).
The scanning electron microscopy (SEM) photograph in Fig. 4a shows an
intact olivine grain, 2 mm long, recovered at the end of experiment
HOP no. 4. The olivine itself is not visible, as it is covered by a crust of
crystals welded together by films and patches of interstitial silicate
glass. The most abundant crystals are elongated prisms of orthopyroxene. In
addition to orthopyroxene, some clinopyroxene and oxide grains were also
observed. The growth of orthopyroxene, which is absent in the starting
material, indicates a process of incongruent dissolution of olivine during
the reaction with molten salt. In experiment HOP no. 4, some water was added
to the mixture of NaCl + olivine powder (Table 1), resulting in enhanced
interactions between olivine and molten salt. In other experiments under the
same conditions but without added water (such as HOP no. 1), the crust is
less developed but is also characterized by the predominance of
orthopyroxene rods.
In experiments with natural olivines, the interactions between the mineral
and the molten salt are made more chemically complex because of the presence
of some basaltic glass on the starting olivine crystals. This is why we
performed an experiment with a forsterite parallelepiped in pure NaCl
(HOP no. 16; Table 1). The SEM photographs in Fig. 4b–c show that the
parallelepiped remains practically unchanged from its original shape at the
end of the experiment and that it is partially covered by orthopyroxene
needles. Orthopyroxene presumably results from incongruent dissolution of
forsterite according to the following reaction:
2NaClsalt+Mg2SiO4,xl→Na2Osalt+MgCl2,salt+MgSiO3,xl,
where the subscripts salt and xl stand for molten salt and crystalline silicates,
respectively, and most of the sodium oxide is expected to be dissolved in
the molten salt (e.g., Kimura, 2011; Sheikh, 2016) and some in the
orthopyroxene. The fact that the dimensions and shape of the parallelepiped
hardly change during the experiment and the absence of rounding at the edges
and corners (Fig. 4b) demonstrate that the degree of dissolution of
forsterite is negligible. This observation supports the choice of molten
NaCl as a matrix for HP homogenization of melt inclusions hosted in Mg-rich
olivines and suggests that the addition of small grains of olivine to NaCl
to minimize the amount of dissolution undergone by the large olivine
crystals is not necessary.
Homogenization experiments: textures and phase assemblagesHomogenization experiments at 2.5 GPa–1300 ∘C–30 min
All experiments at 2.5 GPa–1300 ∘C gave consistent results and
ended in successful homogenization of the starting melt inclusions.
Photographs of a melt inclusion before and after experiment HOP no. 3 (2.5 GPa–1300 ∘C, no added water; Table 1) are shown in Fig. 5a–b.
Overall, the shape and size of the inclusions remain almost unchanged after
homogenization. In particular, the melt inclusion in Fig. 5a–b maintains
the same polyhedral shape during the experiment. In most cases, we lacked
well-defined reference points so that it was not possible to make very
precise measurements of length before and after the experiment
(Table S3). As illustrated in Fig. 5b, the main change in
the experiments at 2.5 GPa–1300 ∘C is the complete disappearance
of the bubble, with the whole volume of the inclusion being occupied by glass
(plus one or a few small grains of sulfide in most experiments; Fig. 5b and
Table S3). This suggests that all the CO2 initially
stored in the bubble has been dissolved in the silicate liquid. The
experiment at 2.5 GPa–1300 ∘C with added water (HOP no. 4) also
resulted in complete homogenization of the melt inclusions and total
resorption of bubbles.
Effect of temperature
The choice of 2.5 GPa and 1300 ∘C as standard P–T conditions
for our homogenization experiments is supported by the results of
experiments at lower pressure or temperature. The effect of a lower
temperature is illustrated by experiment HOP no. 18, which was conducted at
P= 2.5 GPa and T= 1275 ∘C instead of 1300 ∘C. In
this experiment, the melt inclusion partially crystallized, resulting in a
significant change in the composition of the liquid phase (see below). In
addition to olivine, the new mineral phase that grew at 1275 ∘C
is clinopyroxene (see Fig. S2). No bubble was
observed at the end of this experiment.
Effect of pressure
Three runs were conducted at a pressure lower than 2.5 GPa: HOP no. 13 and
HOP no. 20 at 2.0 GPa and HOP no. 10 at 1.5 GPa. Unlike the experiments at
2.5 GPa, melt inclusions in the experiments at 1.5 or 2 GPa show contrasting
behaviors. Indeed, in the same run, we can observe inclusions with a
residual bubble characteristic of an incomplete homogenization and
inclusions without a bubble. The first case is illustrated in Fig. 5c–d,
which shows a melt inclusion before and after the experiment at 1.5 GPa–1300 ∘C (HOP no. 10): a CO2 bubble smaller than the
initial bubble is still present at the end of the experiment. The two other
inclusions heat-treated at 1.5 GPa–1300 ∘C were completely
homogenized (that is, bubble-free; Table S3). At the end of
the experiments at 2 GPa, four out of five inclusions were completely
homogenized, but the last one still had a bubble, which was filled by a
dense CO2 phase (0.91 g cm-3). Unlike the bubbles before
homogenization, the bubbles remaining after the experiments are devoid of
carbonate microcrystals along their walls.
Cases of leaking melt inclusions
In most experiments, at least one melt inclusion was destroyed or did not
behave as a closed system during HP homogenization. This is due to
fracturing of the olivine phenocrysts during the preparation stage, the
loading stage, or the high-temperature stage of the experiment. The most
obvious case of failure corresponds to melt inclusions that show visual
signs of decrepitation (e.g., planes of inclusions in the adjacent olivine)
and/or chemical evidence of contamination by the molten salt, with Cl and
Na2O contents that may exceed 3 wt % and 9 wt %, respectively. More
rarely, especially in the experiments at 1.5 GPa (see below), we suspect
that some melt inclusions may have lost part of their volatile components
even though there is no visual evidence of decrepitation or chemical
evidence of NaCl contamination. To minimize the risk of leaking melt
inclusions during HP homogenization, only robust olivine phenocrysts should
be retained for experiments, i.e., those that are of sufficient size, that are free of
visible defects, and that contain a melt inclusion or a small number of melt
inclusions well in the center.
Compositions of homogenized melt inclusions: major elements, F, Cl, and SHomogenization experiments under standard conditions (2.5 GPa–1300 ∘C; no added water)
The compositions of homogenized melt inclusions are reported in Table 3
and illustrated in Figs. 6 and 7. Melt inclusions homogenized under our
standard conditions are characterized by low analytical totals (94.7 wt %
on average) and have basanitic compositions with 41.9 wt % SiO2,
8.4 wt % MgO, 11.3 wt % FeOT, 4.3 wt % Na2O, and 1.8 wt %
K2O (oxide concentrations are given after correction and normalization
to 100 wt % of the sum of the major elements). Compared to unheated melt
inclusions, the compositions after homogenization at 2.5 GPa–1300 ∘C (no added water) are significantly enriched in MgO
and FeOT and depleted in SiO2 (Fig. 6). The average F and Cl
contents (∼ 1230 and 1950 ppm, respectively) are comparable
to the F and Cl contents in the unheated inclusions (Fig. 7a). The
similarity of chlorine and Na2O contents before and after
homogenization demonstrates the absence of contamination of the melt
inclusions by molten salt.
(a–b) Optical microphotographs of a melt inclusion before (a) and
after (b) homogenization at 2.5 GPa–1300 ∘C–30 min
(HOP no. 3b-MIa). Note the complete disappearance of the bubble, which
indicates that all the CO2 initially stored in the bubble has been
dissolved in the liquid. The arrow points to a small sulfide grain. (c–d) Optical microphotographs of a melt inclusion before (c) and after (d)
homogenization at medium pressure (1.5 GPa–1300 ∘C–30 min;
HOP no. 10a-MIb): the pressure was not sufficient to completely homogenize
the inclusion, as shown by the persistence of a small CO2 bubble.
Homogenization experiments under non-standard conditions
The melt inclusions homogenized at lower pressure (1.5 or 2.0 GPa) yield
normalized compositions close to those measured in the experiments under
standard conditions but with higher EMPA totals. In contrast, the
compositions of the melt inclusions in the lower-temperature experiment and
in the experiment with added water differ significantly from the
compositions obtained under standard experimental conditions. In run
HOP no. 18 at 2.5 GPa–1275 ∘C, melt compositions are strongly
enriched in alkalis (∼ 6.0 wt % Na2O, 2.6 wt %
K2O; Fig. 6b), TiO2, Al2O3, P2O5, F, and Cl (Fig. 7a) and depleted in MgO and CaO relative to melt inclusions homogenized at
2.5 GPa–1300 ∘C. These features indicate significant liquid
crystallization at 1275 ∘C, in perfect agreement with the
observation of a large clinopyroxene crystal in the inclusion (Fig. S2 in the Supplement). This result confirms our choice of 1300 ∘C as standard
temperature. In run HOP no. 4 at 2.5 GPa–1300 ∘C with added water,
the average melt composition is enriched in SiO2 (44.0 wt %) and MgO
(10.7 wt %) and depleted in all incompatible elements in comparison with
the composition in the standard conditions (same P and T but with no added
water). If we compare now the melt in run HOP no. 4 with the average
composition of unheated melt inclusions, we find evidence of a significant
amount of olivine dissolution: strong enrichment in MgO and depletion in
CaO, Al2O3, and TiO2, at a nearly constant SiO2 level.
Major element compositions of unheated and homogenized melt
inclusions. Symbols are as follows: solid blue squares, unheated melt
inclusions; red circles, homogenization experiments at 2.5 GPa–1300 ∘C without water (solid red circles) or with added water (empty
red circles); solid grey diamond, experiment at 1.5 GPa–1300 ∘C;
solid grey triangle, experiment at 2 GPa–1300 ∘C; and solid grey
circles, experiment at 2.5 GPa–1275 ∘C. All the compositions are
corrected from PEC/PED and normalized to a total of 100 % (on a
volatile-free basis).
The case of sulfur
On average, the S content is ∼ 430 ppm lower in the
homogenized melt inclusions than in the unheated ones (1556 vs. 1985 ppm S,
respectively; Fig. 7b) and even 550 ppm lower if we consider only the
inclusions homogenized under standard conditions (1435 ppm S on average). In
many homogenized melt inclusions, S is still within the range of the values
measured in the unheated melt inclusions (from 1291 to 2683 ppm S), but in
some inclusions it is outside this range (down to 1109 ppm S). As most
homogenized melt inclusions contain one or a few grains of sulfide in
addition to the glass phase (Table S3), the lower average S
content presumably results from sulfide immiscibility during HP
homogenization. There are two notable exceptions (Fig. 7b). The first
corresponds to the melt inclusion in run HOP no. 10 at 1.5 GPa, which is rich
in sulfur (>2000 ppm) while containing sulfide grains. The
second exception corresponds to the experiment with added water (HOP no. 4),
in which we did not detect sulfide grains. The average S content in this
sample (1916 ppm) is close to that of natural samples and would be even
higher if we took into account the fact that the addition of water resulted
in excessive dissolution of the host olivine and thus in sulfur dilution.
(a) Fluorine (ppm) as a function of chlorine (ppm) in melt
inclusion glasses. (b) Sulfur (ppm) vs. H2O (wt %) in melt
inclusion glasses. Melt inclusions contaminated by NaCl during
homogenization experiments are not shown because of their high Cl contents
(more than 3 wt %). Symbols are as follows: solid blue squares, unheated
melt inclusions; red circles, homogenization experiments at
2.5 GPa–1300 ∘C without water (solid red circles) or with added
water (empty red circles); solid grey diamond, experiment at
1.5 GPa–1300 ∘C; solid grey triangles, experiment at
2 GPa–1300 ∘C; and solid grey circles, experiment at
2.5 GPa–1275 ∘C. The data are corrected for PEC/PED.
Compositions of homogenized melt inclusions: carbon dioxide
A first qualitative assessment of the CO2 content of the melt
inclusions after homogenization was done by analyzing the glass (not yet
exposed) through the olivine crystal by Raman spectroscopy. By comparison
with the Raman spectrum before homogenization, this allowed us to quickly
evaluate the success of the experiment and observe a significant increase in
the CO2 content of melt inclusions homogenized at high pressure. The
glass was then exposed, and its CO2 concentration was precisely
quantified by Raman spectroscopy (Fig. 8). The CO2 contents are
reported in Table 3, with additional data in Table S3 for melt
inclusions that were not analyzed with the electron microprobe (the
compositions in Tables 3 and S3 are slightly different because the former
have been corrected with the MIMiC software but the latter have not). The CO2
concentrations are plotted as a function of MgO contents in Fig. 9.
Comparison of the Raman spectrum of basanitic standard BSN150121
(3.69 wt % CO2) in grey with a spectrum representative of an unheated
melt inclusion (Thu3-6a) in blue and a spectrum after homogenization at 2.5 GPa–1300 ∘C (HOP no. 5a-MI) in red. The size of the
CO32- band at 1080 cm-1 in the glass homogenized at
2.5 GPa is as big as the CO32- band in the basanitic standard with
3.69 wt % CO2.
CO2 contents in glasses plotted as a function of MgO. Symbols
are as follows: solid blue squares, unheated melt inclusions; red circles,
homogenization experiments at 2.5 GPa–1300 ∘C without water
(solid red circles) or with added water (empty red circles); solid grey
diamonds, experiment at 1.5 GPa–1300 ∘C; solid grey triangles,
experiments at 2 GPa–1300 ∘C; and solid grey circles, experiment at
2.5 GPa–1275 ∘C. The solid grey diamond and empty diamond joined
by an arrow corresponding to melt inclusion HOP no. 10a-MIb: the empty diamond
shows the total CO2 content estimated by adding the CO2 contained
in the residual bubble to the CO2 dissolved into the glass. With the
exception of this one, all the melt inclusions presented are bubble-free and
completely homogenized. The grey area surrounded by a dashed line
corresponds to the melt inclusions that may have undergone CO2 loss
through cryptic decrepitation (see Sect. 4.5 for explanation). The MgO
contents correspond to the values after normalization to 100 wt % of the
sum of major and minor elements (on a volatile-free basis); the CO2
contents correspond to the values after PEC/PED correction (or to the raw
data when the glass composition was not analyzed with the electron
microprobe).
Experiments at 2.5 GPa–1300 ∘C
The CO2 contents of melt inclusions homogenized at 2.5 GPa–1300 ∘C (both without added water and with added water) are
extremely high: from 3.20 wt % to 4.27 wt % CO2 (Table 3; Fig. 9). This is
highlighted in Fig. 8, which compares the Raman spectrum of basanitic
standard BSN150121 (3.69 wt % CO2) with a spectrum representative of
an unheated melt inclusion and a spectrum after homogenization at 2.5 GPa–1300 ∘C. The striking feature in the later Raman spectrum is
the size of the CO32- band at 1080 cm-1, which is as big
as the CO32- band in the basanitic standard with 3.69 wt %
CO2. We emphasize that the bubble volume in the melt inclusions that
yielded these extreme values of CO2 was relatively small compared to
the inclusion volume: from 3.5 (±0.5) vol % to 13.3 (±0.6) vol % (8.9±0.8 vol % on average; Table S3).
The CO2 contents measured in the experiments at 2.5 GPa–1300 ∘C are slightly to significantly higher than the solubility value
of 3.2 wt % predicted by the model of Duan (2014), indicating that the model
underestimates the solubility of CO2 for compositions like ours. In
contrast, they are well below the solubility of 5.9 wt % CO2
calculated using the model of Eguchi and Dasgupta (2018). Considering the average
CO2 contents measured in the glass phase of unheated melt inclusions
(0.61 wt %) and in the melt inclusions homogenized at 2.5 GPa
(3.74 wt %), we estimate that in Thueyts melt inclusions, ∼ 84 % of the total CO2 is initially sequestered in bubbles, both as
CO2 fluid and as carbonate microcrystals.
Experiments at 2 GPa–1300 ∘C
In the two experiments at 2 GPa, four out of five melt inclusions were
completely homogenized. These four bubble-free inclusions yield a wide range
of CO2 contents: 1.39 wt %, 1.87 wt %, 3.31 wt %, and 3.88 wt %. This observation
suggests that the initial CO2 content is variable from one melt
inclusion to the other and that the CO2 solubility at 2 GPa–1300 ∘C is larger than 3.88 wt %. For comparison, Duan's model
predicts a much lower CO2 solubility (2.7 wt %; Duan, 2014), whereas
the model of Eguchi and Dasgupta (2018) yields a CO2 solubility equal to
4.5 wt %. The fifth inclusion contains a bubble with a dense CO2
phase (0.91 g cm-3; melt inclusion HOP no. 13a in Table S3),
and the glass shows a sharp CO32- band. Unfortunately, the
CO2 content of the glass could not be quantified by Raman spectroscopy
due to a fluorescence problem specific to this inclusion. We interpret
inclusion HOP no. 13a as the most CO2-rich melt inclusion of the set of
five inclusions tested at 2 GPa, with an initial CO2 content slightly
above the saturation value at this pressure.
Experiment at 1.5 GPa–1300 ∘C
For the experiment at 1.5 GPa, the three inclusions analyzed give
dissolved CO2 contents of 1.57 wt %, 1.90 wt %, and 2.14 wt %. The two poorest
inclusions were bubble-free, while the richest inclusion contains a bubble
of 2.9 (±0.7) vol %, with a CO2 density equal to 0.53 g cm-3 (Fig. 5d; inclusion HOP no. 10a-MIb in Table S3). Knowing the
total volume of the inclusion and the volume of the bubble, we calculated a
total content of 2.69 wt % CO2. This is only a semi-quantitative
estimate because of the complex shape of the inclusion and therefore the
large uncertainty in the volume. From these observations, we infer that the
initial CO2 content in melt inclusion HOP no. 10a-MIb was above the
CO2 solubility at 1.5 GPa–1300 ∘C and that the latter is
equal to ∼ 2.1 wt %. For comparison, the CO2
solubilities computed using the models of Duan (2014) and Eguchi and
Dasgupta (2018) are equal to 2.0 wt % and 3.1 wt %, respectively.
Experiment at 2.5 GPa–1275 ∘C
The two inclusions in experiment HOP no. 18 at 2.5 GPa–1275 ∘C
are bubble-free and have CO2 contents of 2.07 wt % and 2.93 wt % (Table 3). The higher value is close to the CO2 contents measured in
homogenized melt inclusions at 2.5 GPa–1300 ∘C, while the other value
is much lower (Fig. 9). The high degree of crystallization of the liquid in the
experiment at 1275 ∘C led to a significant increase in all
incompatible elements (Fig. 6b): +30 % for F, +37 % for Cl,
and +47 % for K2O (taking as reference the average composition of
melt inclusions homogenized under standard conditions). Considering a
closed-system behavior (but see below) and an average enrichment in
incompatible elements of 38 %, the corrected CO2 contents fall to
1.5 wt % and 2.1 wt %.
Cases of cryptic decrepitation?
Figure 9 shows a wide range of CO2 contents in the heated melt
inclusions: from 1.39 wt % to 4.27 wt %. It is common that inclusions
trap melts with different amounts of dissolved CO2, for instance when
olivine phenocrysts crystallize at various depths (Esposito, 2021).
Statistically, however, we note that all the inclusions homogenized at 2.5 GPa–1300 ∘C are systematically very rich in dissolved CO2
(≥3.20 wt %), whereas the inclusions homogenized at 1.5 GPa yield
much lower values: between 1.57 wt % and 2.69 wt % CO2. In the experiments
at 2 GPa, an intermediate situation is observed, with both CO2-rich
melt inclusions (3.31 wt % and 3.88 wt %) and CO2-poor ones (1.39 wt % and
1.87 wt %). It is therefore possible that some inclusions in the 1.5 and 2 GPa experiments have lost part of their CO2 when the pressure
difference between the inclusion and the molten salt exceeded a
decrepitation threshold on the order of a few hundred megapascals
(e.g., 250 MPa; Maclennan, 2017). Since there is no clearly visible evidence
of leakage, at least under the optical microscope, or signs of chemical
contamination, we call this hypothetical process cryptic decrepitation. In
addition to the experiments at 1.5 and 2 GPa, cryptic decrepitation could
also be involved in the experiment at 2.5 GPa–1275 ∘C. If we
assume that the melt inclusions in this experiment had initial CO2
contents in the range of the other experiments at 2.5 GPa (on average
3.74 wt % CO2), the crystallization of the liquid would cause a
significant increase in CO2 content (up to 4.9 wt % or 5.5 wt %
depending on whether we take F or K2O as a reference) and thus an
increase in the internal pressure in the inclusion that could ultimately
lead to decrepitation. In the future, it would be interesting to couple high-pressure homogenization experiments with high-resolution X-ray computed
tomography (e.g., Hanyu et al., 2020) to try to identify microtextures
characteristic of cryptic decrepitation and confirm the existence of this
process.
Compositions of homogenized melt inclusions: water
In all experiments except HOP no. 4 (the run with added water), a significant
water loss is observed in the melt: the water contents measured at the end
of the experiments are between <0.10 wt % and 0.95 wt % (on average
0.33 wt %) as compared to water contents between 1.01 wt % and 1.94 wt % (on
average 1.37 wt %) in unheated melt inclusions. Water loss is a well-known
problem in homogenization experiments in a heating stage and is due to rapid
diffusion of H+ through the host olivine at high temperature (Chen et
al., 2011; Gaetani et al., 2012; see also Portnyagin et al., 2008). In run
HOP no. 4, we added 2 wt % H2O to the system (corresponding to a ratio
of H2O to H2O + NaCl equal to 2.97 wt %; Table 1) to minimize
the gradient in water activity between the melt inclusion and its
environment. Water contents in HOP no. 4 melt inclusions (2.16 wt %–2.64 wt %
H2O) are significantly higher than in all experiments without added
water and even than in the unheated melt inclusions. As no water was found
in the bubbles of Thueyts melt inclusions (Schiavi et al., 2020), these
higher water contents may be attributed to a diffusive water gain during the
experiment. Therefore, by adding water to the surrounding environment, it is
possible to reduce or even reverse the water activity gradient between the
melt inclusion and the exterior (see also Mironov et al., 2015; Rasmussen et
al., 2020). A good knowledge of the activity–composition relation for
H2O in NaCl at high P and T would be required to calculate the exact
amount of added water required to cancel the water gradient between the melt
inclusion and the molten salt. The melt inclusions of experiment HOP no. 4
have the lowest EMPA totals (91.2 wt %–91.7 wt %) in perfect agreement with
their high contents in both CO2 and water.
DiscussionThe success of HP homogenization experiments of CO2-rich melt inclusions
Our experiments show that it is possible to homogenize melt inclusions very
rich in volatiles (here, up to 4.3 wt % total CO2) provided that a
sufficient pressure is applied. The minimum pressure required to homogenize
the most CO2-rich melt inclusions from Thueyts is between 2 and 2.5 GPa: at 2.5 GPa, we observe the complete disappearance of the initial
bubbles and the formation of homogeneous liquids whose Raman spectra have
CO32- peaks as big as those in our richest reference glasses (3.69 wt %
and 4.46 wt % CO2; Fig. 8). At P≤2 GPa, the most
CO2-rich melt inclusions retain a residual bubble at the end of the
experiments, and the liquid in equilibrium is not as rich in CO2 as at
2.5 GPa: such features indicate that the experimental pressure is
insufficient to dissolve the total CO2 content of
the inclusion in the liquid phase. These observations demonstrate that olivine-hosted melt
inclusions behave as closed systems for CO2 during HP homogenization
experiments and that olivine constitutes a tight container with respect to
CO2 exchanges (at the timescale of our experiments).
Because of the uncertainty in the physicochemical parameters at the time of
melt inclusion entrapment, it is unlikely to reproduce exactly the initial
composition of the inclusion at the end of a homogenization experiment. For
example, the melt inclusion can be overheated during HP homogenization,
which will result in excessive dissolution of the host olivine and thus in
CO2 dilution. Conversely, the melt inclusion may be underheated,
resulting in insufficient dissolution of the host crystal and
overconcentration of CO2. It is possible to correct for CO2
dilution by estimating the amount of excess olivine dissolved (e.g., Wallace
et al., 2015) or the deficit of dissolved olivine in the case of CO2
overconcentration.
Behavior of F, Cl, and S during HP homogenization experiments
If we take into account the effects of dissolution or crystallization of the
host crystal as a function of the experimental parameters, we find that
there is a good match between the concentrations of F and Cl in unheated
melt inclusions and those in homogenized melt inclusions (except in the case
of decrepitation and NaCl contamination by the molten salt). This indicates
that olivine-hosted melt inclusions behave as a closed system for these two
volatile elements for at least 30 min during HP homogenization.
A closed-system behavior is presumably the rule in the case of sulfur as
well, but this is more complicated to assess for two reasons. First, S
concentrations in the starting melt inclusions are not precisely known
because, in addition to the sulfur dissolved in the glass phase, sulfur is
also present as sulfide microcrystals along the bubble walls. It is well
known indeed that large amounts of sulfur may be stored in the bubbles of
melt inclusions (e.g., Esposito et al., 2016; Venugopal et al., 2020). In
the Bas-Vivarais province, FeS2 has been identified by Raman
microtomography in a melt inclusion from Thueyts and CuFeS2 in a melt
inclusion from the nearby Lac d'Issarlès volcano (Schiavi et al., 2020).
Schiavi et al. (2020) estimated that 20 % of the total sulfur was stored
in these sulfides in the melt inclusion from Thueyts. The second reason is
that a microscopic sulfide phase is present at the end of most
homogenization experiments (see Table S3), suggesting the
formation of an immiscible sulfide liquid in the melt inclusion at high
temperature. This process is well known in the literature and has been
explained in different ways based on the interrelationship between water
loss, iron loss, oxygen fugacity, and the solubility of sulfur (Danyushevsky
et al., 2002; Baker and Moretti, 2011; Wallace and Edmonds, 2011). As a
result of sulfide immiscibility, the S content in the silicate glass of the
experiments homogenized under standard conditions is 550 ppm lower than in
the glass phase of unheated melt inclusions (on average 1435 ppm vs. 1985 ppm, respectively; Fig. 7b). Based on these average values and considering
that 20 % of the total sulfur in the starting melt inclusions is stored
as sulfide microcrystals lining the bubble walls, it is estimated that the
total sulfur content is about 2500 ppm and that, in homogenization
experiments under our standard conditions, the sulfide phase accounts for
slightly more than 40 % of total sulfur.
Another lesson from the experiments is that the addition of water to
minimize the gradient in water activity between the melt inclusion and the
external environment is also beneficial for the homogenization of sulfur.
Indeed, in run HOP no. 4 with ∼ 2 wt % added water, there is
no evidence of the formation of an immiscible sulfide liquid in the melt
inclusion at high temperature, and all the sulfur is dissolved in the
silicate glass at the end of the experiment (Fig. 7b). Further experiments
with added water would be necessary to confirm this observation and to
establish the link between water loss or gain, oxygen fugacity, and sulfur
solubility.
Behavior of H2O during HP homogenization experiments
With respect to water, the two main findings of our study are the following: (1) a rapid
and systematic loss of water from melt inclusions in experiments without
added water, in agreement with previous work (Portnyagin et al., 2008; Chen
et al., 2011; Gaetani et al., 2012), and (2) the possibility of minimizing
this loss, or even increasing the water content in the melt inclusions, by
adding water to the surrounding medium (see also Rasmussen et al., 2020). In
our case, the initial water contents of the melt inclusions are low (1.37 wt % on average), and our priority was not to keep constant the water
concentrations during the experiments but to estimate the total CO2
content of the melt inclusions. Nevertheless, keeping water during
homogenization experiments is important in order to fully recover the melt
composition and the temperature at the time of melt inclusion entrapment
(Danyushevsky et al., 2002; Chen et al., 2011). This is all the more
important as water loss can be accompanied by a significant decrease in the
silica content of the melt inclusion and thus lead to an underestimation of
the SiO2 content of the original magma (Portnaygin et al., 2019).
According to these authors, the coupled loss of silica and water is caused
by the growth along the inclusion walls of a defect olivine, in which
vacancies in metal sites are occupied by hydrogen. In the case of water-rich
melt inclusions such as in island-arc systems, this effect could lead to
concomitant depletion of melt in H2O and SiO2 of up to 4 wt %–5 wt %
(Portnyagin et al., 2019).
In our standard experiments at 2.5 GPa–1300 ∘C, the SiO2
content of the glasses is 41.9 wt % (on a volatile-free basis), and the
H2O content is 0.16 wt % on average (Table 3). In comparison, the
SiO2 and H2O contents of unheated melt inclusions are equal to
44.4 wt % and 1.37 wt % (Table 2). On its own, water loss during the
experiments cannot explain the decrease in SiO2 from 44.4 wt % to
41.9 wt %, but, following Portnyagin et al. (2019), it could contribute to
a decrease in SiO2 in melt on the order of 1 wt %. The other factor
that contributes to the decrease in SiO2 in our experiments is the
incorporation into the silicate liquid of the carbonate microcrystals
contained in the bubbles, as discussed in the next section. The importance
of water and the coupling between H2O and SiO2 are also well
demonstrated by our experiment with added water (HOP no. 4), in which both
H2O and SiO2 are significantly higher than in our standard
experiments: 2.40 wt % vs. 0.16 wt % for H2O and 44.1 wt % vs.
41.9 wt % for SiO2. Given the importance of water and its impact on
other compositional parameters, more experiments with added water are needed
to properly control water concentrations after homogenization and thus
better constrain the original melt composition of trapped liquids and the
trapping temperature.
Importance of HP homogenization for major and trace elements
In the case of melt inclusions such as those of Bas-Vivarais, where
large quantities of carbonate microcrystals and to a lesser extent of
sulfides are observed inside the bubbles (Fig. 1; Schiavi et al., 2020), a
significant effect of homogenization on melt composition can be anticipated
not only for volatile elements but also for major and trace elements too and
probably also in terms of isotope ratios, particularly those of carbon and
sulfur. For trace elements, all the elements present in carbonates and
sulfides are concerned, the first of which is Cu because chalcopyrite was identified
in most of the bubbles in the melt inclusions studied by Schiavi et al. (2020). Regarding the major elements, the main elements concerned are Ca, Mg
(contained in carbonates), and Fe (contained in carbonates and sulfides). To
illustrate the effect of homogenization on major elements, the bulk
composition of a bubble was calculated and then mixed in different
proportions with the average composition of Thueyts melt inclusions (Table 4). To compute the bubble composition, we used the masses of carbonate,
FeS2, and fluid CO2 given by Schiavi et al. (2020) for a melt
inclusion from Thueyts, and we considered a composition of 70 %
CaCO3, 27 % MgCO3, and 3 % FeCO3 (mol %) for the
carbonate phase. With these parameters, we obtain the bulk composition Bub 33 (33 stands for the percentage of carbonates in the bubble: 33 vol %): 51.3 wt % CO2, 34 wt % CaO, 9.4 wt % MgO, 3.7 wt %
FeO, and 1.6 wt % S. We “homogenized” this composition with the average
composition of Thueyts melt inclusions to produce mixtures with 2, 3, and
4 wt % CO2 (Table 4). Besides the increase in total CO2 content,
the main effects of homogenization are a decrease in SiO2 from 44.3 wt % to
42.8 wt % (on a volatile-free basis) and an increase in CaO from 13.1 wt % to
15.1 wt %. The MgO content increases slightly, and the FeOT content is
nearly constant. These chemical trends are not very sensitive to the choice
of carbonate composition (dolomite vs. Mg-calcite).
Theoretical composition of bubbles in melt inclusions from Thueyts
(Bub 33 and Bub 42) and bulk compositions (BMI and BMI*; bulk melt inclusion) obtained by
homogenizing these bubbles with the glass phase of Thueyts melt inclusions.
For BMI and BMI*, the number in the suffix corresponds to the weight percentage
of CO2 in the mixture: 2 wt %, 3 wt %, or 4 wt % CO2.
a Bubble composition Bub 33 is based on the masses of carbonate,
FeS2, and CO2 fluid reported by Schiavi et al. (2020) in the
bubble of a melt inclusion from Thueyts, which contains 33 vol %
carbonate. For the carbonate phase, we consider a mixture of 70 mol %
CaCO3, 27 mol % MgCO3, and 3 mol % FeCO3.
b MI is the average glass composition measured in unheated melt
inclusions from Thueyts (corrected for PEC). For all the silicate liquids in
the table (but not for the bubble), the major and minor elements (SiO2
to P2O5) are normalized to a total of 100 %; the total before
normalization is given in the line “Σ maj.” (major).
c BMI-2 to BMI-4 are obtained by mixing Bub 33 and MI in different
proportions to obtain bulk compositions with 2 wt %, 3 wt %, or 4 wt % CO2,
respectively.
d Bubble composition based on the same data as for Bub 33 but
considering 42 vol % of carbonates instead of 33 vol % to take into
account the uncertainty in Raman tomography (±9 vol %; Schiavi et
al., 2020).
e BMI*-4 is a composition with 4 wt % CO2 obtained by mixing Bub 42 with the average melt inclusion composition MI recalculated with
1.2 wt % dissolved CO2 (see text for explanation).
f HOP Stdd is the average composition of melt inclusions homogenized under standard conditions, with all iron recalculated as FeO.
g Weight percentages of bubble and silicate glass required to produce the mixtures with 2 wt %, 3 wt %, and 4 wt % CO2. The volume percentages of the
whole bubble and of CO2 fluid (fl.) are given for comparison and were
computed using a bulk density of 1.15 g cm-3 for Bub 33 or
1.41 g cm-3 for Bub 42, a density of 2.8 g cm-3 for the silicate
liquid, and a percentage of fluid CO2 of 67 vol % in Bub 33 and
58 vol % in Bub 42.
In terms of weight, the proportions of bubbles needed to reach 2 wt % to 4 wt %
CO2 after homogenization range from 2.7 % to 6.7 % (Table 4). In terms
of volume, this corresponds to proportions of 6.4 % to 14.8 % if we
consider the whole bubble and 4.3 % to 9.9 % if we consider only the
CO2 fluid. The main parameters that control these proportions are the
CO2 content of the silicate glass and especially the percentage of
carbonates in the bubble, which strongly affects the bulk density of the
bubble. The percentage of carbonates is variable and can exceed 50 vol %
in some bubbles of Bas-Vivarais (unpublished data). Thus, we made a
mixing calculation considering a bubble with 42 vol % carbonates and a
silicate glass with 1.2 wt % CO2 (the maximum measured in unheated
melt inclusions), instead of 33 vol % carbonates and 0.61 wt % dissolved
CO2 in the calculations above. With these new parameters, the bubble
volume required to reach 4 wt % CO2 after homogenization falls to
10.7 vol % (6.2 vol % of CO2 fluid), instead of 14.8 vol %.
These calculations are exploratory because there are currently very few
constraints on the compositions and quantities of carbonates and sulfides
contained in melt-inclusion-hosted bubbles and thus on the bulk composition
and density of bubbles. However, if we compare the compositions of unheated
melt inclusions and of melt inclusions homogenized under standard
conditions, we see that the dissolution of the bubble components into the
silicate liquid contributes to the evolution of major elements, in
particular the decrease in SiO2 and the slight increase in CaO. On the
other hand, the high concentrations of FeOT and MgO in the homogenized
melt inclusions require in addition a dissolution of the host olivine. In
the mixing calculations of Table 4, it is interesting to note that it is not
possible to reach SiO2 contents as low as in our standard
homogenization experiments (41.9 wt % SiO2), even in the most
CO2-rich compositions (BMI-4, 42.8 wt % SiO2; BMI*-4,
43.0 wt % SiO2). Therefore, some of the decrease in SiO2 content
(∼ 1 wt %) observed in these experiments may be coupled to
water loss, as proposed by Portnyagin et al. (2019).
The need for a multi-analytical-technique approach
High-pressure homogenization is a powerful tool to access the total CO2
content of volatile-rich melt inclusions. There is, however, an intrinsic
limitation to this technique which comes from the impossibility to accurately measure
the parameters of the melt inclusions before homogenization:
glass composition, water content, and nature and abundance of microcrystals in
the bubbles. This limitation stems from the need to keep the inclusion at a
significant depth in the host crystal to avoid decrepitation. The inability
to individually characterize each melt inclusion prior to HP homogenization
is a source of uncertainty due to the significant heterogeneity in major and
trace elements in melt inclusion suites, even within the same sample, the
same crystal, or the same melt inclusion assemblage (e.g., Helo et al., 2011;
Esposito et al., 2014; Wallace et al., 2015). Therefore, prior to any series
of HP homogenization experiments, a thorough petrographic study of the suite
of melt inclusions (location, shape, and size of melt inclusions; bubble
sizes) is required, and a number of representative inclusions must be
characterized by conventional glass analysis techniques (EPMA, vibrational
spectroscopies, ion microprobe) and other less conventional techniques such
as Raman microtomography, X-ray computed tomography, and dual-beam approaches (focused
ion beam and field-emission scanning electron microscopy) to quantify the
content of the bubbles (Schiavi et al., 2020).
In the future, it would be highly beneficial to systematically couple HP
homogenization experiments with high-resolution X-ray computed tomography
(e.g., Hanyu et al., 2020; Drignon et al., 2021). Prior to homogenization,
this nondestructive technique would allow for accurate (≤ 0.5 µm)
measurement of the shape and size of melt inclusions and bubbles,
characterization of additional phases (crystals, immiscible phases), and
possibly discarding melt inclusions with hidden defects. After
homogenization, X-ray computed tomography should be used to determine the
final shape and size of the melt inclusions, to identify any residual or
newly grown phases (bubbles, sulfides, silicate crystals, etc.) and quantify
their volumes, to detect decrepitation microtextures, etc. The benefits will
include the following: (1) accurate quantification of size and shape variations in melt
inclusions during homogenization that will help confirm excess dissolution
or crystallization of the host crystal, (2) the ability to precisely
calculate the total CO2 content of melt inclusions with a residual
bubble through reliable measurement of volumes, and (3) the possibility of
detecting experimental failure even though there is no visual evidence under
the optical microscope.
Homogeneous vs. heterogeneous melt inclusion entrapment
As our study shows, homogenization at high pressure (up to 3–4 GPa) in
a piston cylinder is a powerful approach to characterize melt inclusions very
rich in volatile elements. One must be careful, however, in the
interpretation of the results because there is a risk of mistakenly
homogenizing melt inclusions resulting from heterogeneous entrapment. In the
case of heterogeneous entrapment, the bubble is not a shrinkage bubble but
comes from a primary bubble trapped with the silicate liquid at the time of
the inclusion formation. Heterogeneous entrapment can involve a wide variety
of immiscible fluids, such as silicate melt–CO2 fluid, silicate
melt–carbonate melt, silicate melt–sulfide melt, and silicate melt–aqueous
solutions (e.g., Roedder, 1992; Kamenetsky and Kamenetsky, 2010). In some
cases, it is straightforward to identify melt inclusions issued from
heterogeneous entrapment, such as when the CO2 bubble occupies a large
part of the inclusion volume or the bubble / melt inclusion volume ratio
varies greatly in nearby, coeval melt inclusions (e.g., Bodnar and Student,
2006; Steele-MacInnis et al., 2017; Esposito, 2021). In other cases, it is
more difficult. In general, the bubble / melt inclusion volume ratio below
which a melt inclusion is considered likely to result from homogeneous
entrapment is ≤10 vol %: for example, 5 vol % for Hartley et al. (2014) and Moore et al. (2015) or 6 vol % to 8 vol % for Aster et al. (2016).
Recently, Hanyu et al. (2020) made a series of homogenization of
CO2-rich melt inclusions in olivine phenocrysts from ocean island
basalts and used a threshold of 12 vol % to distinguish homogeneous from
heterogeneous entrapment. At this stage, it is difficult to set the exact
threshold value to be considered for our melt inclusions because of the lack
of a model describing the formation and evolution of shrinkage bubbles in
the specific case of a CO2-rich magma rising rapidly from mantle
depths.
In the population of melt inclusions studied, there is a positive
correlation between bubble size and inclusion size, with bubble / melt
inclusion ratios mostly between 3 vol % and 10 vol % (Fig. 10a). These
characteristics are consistent with an origin by homogeneous entrapment for
the majority of melt inclusions but do not exclude cases of heterogeneous
entrapment. Some bubbles exceed 10 vol %, and there are two melt
inclusions with large bubbles (>20 vol %) that must
correspond to heterogeneous entrapment. Most of the melt inclusions used for
the homogenization experiments had bubbles <10 vol %, with only
two inclusions exceeding the 12 vol % threshold (Fig. 10b). Two major
results that emerge from Fig. 10b are that there is no positive
correlation between bubble volume percentage and total CO2 content and
that most of the highest CO2 contents are related to small- (<5 vol %) to medium-sized bubbles (<10 vol %).
(a) Plot of bubble volumes (µm3) as a function of melt
inclusion volumes (µm3) for both the unheated melt inclusions and
the melt inclusions used in the homogenization experiments (same symbols as
in Fig. 9). Isovolumetric lines corresponding to bubble / melt inclusion
volume ratios of 1 %, 3 %, 10 %, and 30 % are also shown. Bubble volumes were
calculated from the bubble diameters assuming a spherical shape. Melt
inclusion volumes and associated errors were computed following the method
described by Moore et al. (2015). Most of the melt inclusions have complex
shapes that were approximated by an ellipsoidal shape. (b) Plot of the total
CO2 contents measured in the homogenization experiments and the
CO2 contents measured in the glasses of unheated melt inclusions as a
function of bubble sizes (in vol %). The dashed line at 12 vol % is the
threshold used by Hanyu et al. (2020) to separate homogeneous from
heterogeneous entrapment.
The volumetric analysis above and the relationship between bubbles' percentage by volume
and total CO2 contents provide strong arguments in favor of homogeneous
entrapment of very CO2-rich basanitic liquids (∼ 4 wt %) at pressures of 2 to 2.5 GPa. As low-silica alkaline liquids are
expected to be generated at pressures mostly in the range 2–4 GPa depending
on the model (Dasgupta et al., 2007; Foley et al., 2012; Mallik and
Dasgupta, 2014; Pilet, 2015), such entrapment pressures correspond to
near-source mantle depths. In this scenario of homogeneous entrapment, the
lower total CO2 contents measured in some inclusions could reflect a
natural variability in the initial CO2 contents, due for instance to
melt entrapment at different pressures. Alternatively, melt inclusion
entrapment could be heterogeneous and occur at lower pressures but still at
least on the order of 1 GPa as indicated by dissolved CO2 contents up
to 1.19 wt % in the glasses of unheated melt inclusions. In this case, the
wide range of total CO2 contents (from 1.39 wt % to 4.27 wt %) would
result from the entrapment of CO2 bubbles in addition to the silicate
liquid. The presence of these bubbles could be due either to deep gas
exsolution during ascent of CO2-rich primary magmas or to CO2
fluxing from depth. Whatever the scenario, one cannot escape the conclusion
that the basanites from the Bas-Vivarais volcanic province were generated in
a mantle environment rich in carbon dioxide.
It is beyond the scope of this paper to make a definitive choice between
these different petrogenetic scenarios. This will be the focus of a
subsequent paper based on systematic studies of melt inclusions from several
volcanoes of the Bas-Vivarais province (homogenization experiments at medium
to high pressure; trace and major element analyses of unheated and
homogenized inclusions).
Insights into the mantle source of Bas-Vivarais volcanism and the origin of carbon dioxide
Thueyts melt inclusions are characterized by very high CO2 contents.
High CO2 contents have already been reported in other alkaline magmas
from different continental intraplate settings but not to the same extent
as in the Bas-Vivarais volcanic province: for instance, up to 1.8 wt % in
basanites from Ross Island, within the West Antarctic Rift System (Rasmussen
et al., 2017), or 2.5 wt % in Montcineyre basanite, in the French Massif Central
(Schiavi et al., 2020). In basanites from the East African Rift, Hudgins et
al. (2015) measured total CO2 contents up to 1 wt % and calculated
initial CO2 contents in the primary magmas of 3 wt % to 5 wt % or 5 wt % to
10 wt % on the basis of CO2–Nb and CO2–Cl systematics,
respectively.
The abundance of CO2 in Thueyts melt inclusions is in good agreement
with current petrogenetic models of low-silica alkaline lavas. It is now
well accepted that low-silica magmas such as nephelinites and basanites
cannot be produced directly from dry lherzolites and that the presence of
volatiles and/or recycled components in the source region is required to
explain their major element compositions, their high content of incompatible
trace elements, and their isotopic signature (e.g., Pilet, 2015). The nature
of the recycled components (oceanic crust, delaminated lower crust) is still
a matter of debate, but the participation of a carbonate component is
increasingly recognized as being critical (Dasgupta et al., 2007; Zeng et
al., 2010; Mallik and Dasgupta, 2014; Hammouda and Keshav, 2015; Xu et al.,
2020; Brenna et al., 2021). For example, Xu et al. (2020) concluded that the
Cenozoic low-silica alkaline lavas of eastern China were produced in two
stages: first partial melting of recycled oceanic crust (eclogite) in the
presence of pervasive carbonate melt that yielded relatively silica-rich
melts and then reaction between these silica-rich melts and peridotite to
generate silica-undersaturated alkaline magmas. Alternatively, low-silica
alkaline lavas could be produced directly by low-degree melting of a
metasomatized peridotite enriched in CO2 (0.1 wt %–0.25 wt % CO2) by
the addition of a carbonatitic melt (Dasgupta et al., 2007).
Direct evidence of carbonatitic magmatism in the French Massif Central is
very rare: a few occurrences of carbonatites mixed with Cenozoic volcanic
products, with negligible erupted volumes (Chazot and Mergoil-Daniel, 2012),
or subtle geochemical signatures of metasomatism by carbonatitic melts or by
carbonated alkali basalt melts in some xenoliths from the lithospheric
mantle (e.g., Uenver-Thiele et al., 2017; Puziewicz et al., 2020). The
generation of low-silica alkaline magmas, such as the basanites of the
Bas-Vivarais volcanic province, and the abundance of CO2 in these
magmas are arguments in favor of a more pervasive presence of interstitial
carbonatitic melt in the deeper mantle of the French Massif Central, beneath
the lithosphere–asthenosphere boundary.
Conclusions
In this paper, we present a new technique to homogenize volatile-rich
olivine-hosted melt inclusions in a piston-cylinder apparatus at pressures
up to 3–4 GPa. We applied this technique to olivine phenocrysts from Thueyts
basanite, of the Bas-Vivarais volcanic province, in the French Massif Central. Thueyts
melt inclusions were chosen because of their high CO2 contents, as
indicated by up to 1.19 wt % dissolved CO2 in the glasses and by the
presence of shrinkage bubbles containing a multitude of carbonate
microcrystals in addition to CO2 fluid. In all the melt inclusions
treated at 2.5 GPa–1300 ∘C and half of those treated at
2 GPa–1300 ∘C, we were able to completely homogenize the melt
inclusions, and we obtained total CO2 contents ranging from 3.20 wt % to 4.27 wt % (3.7 wt % on average). In all the other melt inclusions
(equilibrated at 1.5 or 2 GPa and 1300 ∘C or at
2.5 GPa–1275 ∘C), we obtained lower and more variable total
CO2 contents (1.39 wt % to 2.93 wt %), either because of a natural
variability in the initial CO2 contents or because of CO2 loss by
decrepitation. The abundance of CO2 in the primary magmas of the
Bas-Vivarais volcanic province is in good agreement with the presence of a
carbonatite component in the mantle source regions of low-silica alkaline
magmas, as advocated by most current petrogenetic models. High-pressure
homogenization of melt inclusions representative of near-primary mantle
magmas is a powerful approach to investigate the nature of their source
regions and the melting conditions.
Data availability
Most of the relevant data are included in the article (in Tables 2 and 3) or the Supplement (Tables S2, S3, and S4). Raw data not included can be provided by the corresponding author upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/ejm-34-325-2022-supplement.
Author contributions
DL designed the experiments at high pressure, and RB carried them out. RB
prepared the samples and performed the measurements with FS. RB and NC
carried out the experiments with the heating stage. CF performed the measurements
with the CHNS. RB, DL, FS, and NC analyzed the data. RB and DL prepared the
manuscript with contributions from all other co-authors.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Probing the Earth: experiments and mineral physics at mantle depths”. It is not associated with a conference.
Acknowledgements
The authors thank the following people for their help and technical
assistance: Jean-Luc Devidal (electron probe microanalysis), Emmy Voyer
(scanning electron microscopy), Franck Pointud and Antoine Mathieu
(management and maintenance of experimental facilities), and Jean-Louis Fruquière and Cyrille Guillot (machine shop). We also thank Hans Keppler
(Bayerisches Geoinstitut), Ali Bouhifd, Mickaël Laumonier, Manon Hardiagon, and Emmanuelle Defive for scientific discussions and advice. The
field-emission SEM image in Fig. 1c was made with a Helios G4 PFIB CXe
DualBeam at the Thermo Fisher Scientific NanoPort in Eindhoven, the
Netherlands: Letian Li and Laurent Vassé are warmly thanked for
their hospitality and expertise. We are grateful to the two reviewers,
Rosario Esposito and Maxim Portnyagin, for their constructive comments, and
to Max Wilke for editorial handling and patience as well as Elisabetta Rampone.
Roxane Buso thanks the
region of Auvergne-Rhône-Alpes for funding her doctoral thesis. This is a contribution of the
Laboratory of Excellence ClerVolc (Clermont-Ferrand Centre for Volcano Research; no. 535).
Financial support
This research has been supported by the Région Auvergne-Rhône-Alpes (Pack Ambition Recherche; grant no. VolcAURA-LS172914).
Review statement
This paper was edited by Max Wilke and reviewed by Rosario Esposito and Maxim Portnyagin.
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