3.1 Electron microprobe

Experimental run products were investigated for major element composition by the JEOL Superprobe 8200 electron microprobe (EMP) at the Dipartimento di Scienze della Terra, Università di Milano (Italy). The following natural standards were used: olivine for Mg, fayalite for Fe, omphacite for Na, ilmenite for Ti, K-feldspar for K, anorthite for Al and Ca, wollastonite for Si, pure Cr for Cr, scapolite for Cl, and niccolite for Ni. Beam conditions were 15 kV accelerating voltage and 5 nA current. The beam size was 1 µm for clinopyroxene and amphibole phases and up to 3 µm for the glass and mica. Counting time was 10 s for background and 30 s for the peak.

3.2 Micro-Raman spectroscopy

Water in amphiboles and glasses was determined by polarized micro-Raman spectroscopy. The analyses were carried out at the Laboratoire Magmas et Volcans (Clermont-Ferrand) using an inVia confocal micro-Raman spectrometer with a backscattered configuration designed by Renishaw and equipped with a 532.1 ± 0.3 nm diode laser (output power: 200 mW), a Peltier-cooled charge-coupled device (CCD) detector, a Rayleigh rejection edge filter, and a Leica DM 2500M optical microscope with a motorized XYZ stage. We used a high confocal setting (20 µm slit aperture), a 100× microscope objective and a grating of 2400 grooves/mm. This setting resulted in lateral and axial spatial resolutions of ∼1 and 2–3 µm at the sample surface and allowed a spectral resolution of ∼0.5 cm⁻¹. Laser power on the sample was reduced by filters to operate at 8 mW power. The spectra were acquired with polarized light using a half-wave plate. For water quantification in amphiboles and glasses, standards and samples were analyzed using the same analytical conditions. Daily calibration of the spectrometer was performed on a silicon standard (520.5 cm⁻¹ peak).

The spectra were recorded using WiRE™ 4.2 software in the wavenumber ranges from ∼50 to 1300 cm⁻¹ and from ∼2900 to 3900 cm⁻¹, which represent the vibration regions of the alumino-silicate network and of the O–H bond stretching, respectively. A total of 4 and 8 to 12 acquisitions of 15 s each were collected in the alumino-silicate and water spectral regions, respectively. After acquisition, cubic and linear baselines were fit to the spectra, respectively, in the water and alumino-silicate regions using PeakFit package software. Then the integrated intensities (i.e., the area) under the water peak (A_OH) and the silicate region (A_Si) were obtained and normalized to 1 s and 1 mW. The ratio $A_{\mathrm{OH}}/A_{\mathrm{Si}}$ was calculated for each acquired spectrum.

For quantification of water in glasses, an external calibration method was used following the procedure described in Schiavi et al. (2018). A total of 5 to 10 measurements were carried out on each sample. Basaltic, andesitic, and rhyolitic glasses with different water contents were used as reference standards (Médard and Grove, 2008; Schiavi et al., 2018) and analyzed several times during each analytical session in order to correct for the dependence of band intensities on delivered energy.

Due to the anisotropy of the amphibole and its complex structure, shape and intensity of Raman bands vary significantly from one crystal to another as a function of the orientation of the crystal lattice relative to the polarization plane of the laser source (Fig. 1). Consequently, the spectrum acquired on one polished face of a crystal is not fully representative of its water content. The method used to quantify water in amphibole adopts a statistical approach similar to that described by Bolfan-Casanova et al. (2014) for measuring water content in olivine using micro-Raman spectroscopy. In this approach, measurements are taken on a large number of amphibole grains differently oriented in two dimensions so that the average of these measurements simulates the analysis performed on individual standard amphiboles that are rotated and analyzed in three orthogonal directions. In each sample, several amphibole crystals, generally comprised of between 10 and 20 (but only 6 in the sample 18-15HM), were measured along different crystallographic orientations. Two measurements along orthogonal directions were taken systematically in each analyzed crystal to limit the effect of a potential preferential crystallographic orientation within the sample (Martinek and Bolfan-Casanova, 2021). Then average A_OH and $A_{\mathrm{OH}}/A_{\mathrm{Si}}$ values for amphiboles were calculated in each sample and converted to water contents using the calibration lines defined by the amphibole standards with known water contents (Fig. 2).

Figure 1Polarized Raman spectra of the amphibole standard MGT4 in the regions of silicate network (a) and O–H stretching (b) vibrations, after baseline correction, as a function of the orientation of the crystal lattice relative to the polarization direction of the laser beam. The polarization of incident light was parallel to the polarization of scattered light. Polarized spectra were acquired on three orthogonal sections (100, 010, and 001). Representative spectra shown from bottom to top were collected, respectively, with the direction of laser polarization parallel to the c axis, b axis, and a axis^*. The a axis^* is distinguished from the true a axis because it is perpendicular to b axis and c axis. Note that the directions of the a axis and c axis do not coincide with those of the optical indicatrix axes in monoclinic amphiboles. The (top) spectrum with the most intense peaks at 228 and 665 cm⁻¹ displays the strongest OH band, whereas the (bottom) spectrum with the maximum peak at 750 cm⁻¹ is associated with the smallest OH band. Raman intensity was normalized to laser power and acquisition time. Spectra are vertically offset for clarity.

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Figure 2Calibration lines defined by kaersutitic amphiboles used as standards: correlation between water contents of amphibole determined by the OEA and (a) A_OH or (b) $A_{\mathrm{OH}}/A_{\mathrm{Si}}$. The three reference glasses also used as external standards during all Raman sessions are plotted. The factors used to correct the intensities of the reference glasses are reported in Table 3.

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Four kaersutitic amphibole crystals from Arizona (USA; Campbell and Schenk, 1950) and one kaersutitic amphibole collected from pyroclastites in the Ardèche volcanic province (France) were used as standards for the calibrations (Fig. 2). The five crystals were examined and oriented under a binocular microscope: they display prismatic habitus elongated parallel to the c axis, with the (110) face generally well developed and evident cleavage planes intersecting at 124 and 56^∘ (Deer et al., 1997). The method used to build the calibration is based on the theoretical considerations developed for quantitative absorbance measurements in anisotropic crystals by Libowitzky and Rossman (1996). These authors demonstrated that quantitative spectroscopic measurements in anisotropic media can be accomplished from polarized spectra on either three random, but orthogonal sections of a crystal or two orthogonal sections oriented parallel to each of the two axes of the indicatrix ellipsoid. As for the monoclinic amphiboles, orienting the crystals parallel to the axes of the optical indicatrix is no trivial task because only one of these three axes coincides with one of the crystallographic axes (Deer et al., 1997), i.e., the b axis. Therefore, in each kaersutite standard we made measurements on three orthogonal sections: the basal section perpendicular to the c axis and two planes parallel to the c axis, either the (100) and (010) faces or the (110) face and a plane orthogonal to it. If originally absent or not well developed, these planes were created by sectioning and polishing the crystal along defined directions. A large number of micro-Raman spectra were acquired (54 in total on 3 orthogonal faces; Fig. 1) by aligning the edge of each face with the instrument polarization axis and then rotating the crystal from 0 to 180^∘ in steps of 10^∘ each. Then the average values of A_OH and $A_{\mathrm{OH}}/A_{\mathrm{Si}}$ for the three faces taken together were calculated and used to build the calibration lines (Fig. 2). The uncertainty (standard deviation, 1σ) associated with the micro-Raman data is the result of errors related to (i) the analytical measurements; (ii) the dispersion of the relative intensities of the vibrational modes caused by the anisotropy of the minerals, which is intrinsic to the statistical method used for water quantification (Martinek and Bolfan-Casanova, 2021); and (iii) micro-Raman spectra treatment procedures (e.g., baseline fitting). The relative uncertainty is about 10 % and 12 % for amphiboles and glasses, respectively.

The spectral analysis of the kaersutitic standards revealed that the spectra displaying the most intense peak near 750 cm⁻¹ (see black spectrum in Fig. 1a) represent ∼67 % of all spectra acquired in each standard, the other ∼33 % being associated with the spectra showing the most intense peak at 228 cm⁻¹ and, to a lesser extent, at 665 cm⁻¹. Therefore, we took 67 % as a reference value to assess the level of randomness of the orientations used for water quantification on the samples, as detailed later in the text.

3.3 Organic elemental analyzer (OEA)

Water content of the five amphibole standards was determined by CHNS analyses (Organic Elemental Analyzer Flash 2000, Thermo Fisher) at the University of Milan and at the Laboratoire Magma et Volcans (Clermont-Ferrand). Samples were dried overnight to avoid any contamination with air humidity. Three measurements were performed for each standard.

5.1 Calibration of water contents in amphibole standards

Micro-Raman spectra of kaersutitic amphibole standards display bands at ∼123, 165, 183, 228, 295, 320, 370, 430, 540, 665, 750, and 1012 cm⁻¹ (Fig. 1a). Relative intensities of the Raman bands of kaersutite strongly depend on the crystallographic orientation (Figs. 1, S1 in the Supplement), with the most intense peaks occurring at ∼228, 665, and 750 cm⁻¹. The strong Raman scattering near 750 cm⁻¹ is indicative of oxo-amphiboles and is related to the high number of Ti–O bonds (Leissner et al., 2015). The spectra in the OH region show a broad band located between 3600 and 3750 cm⁻¹, which consists of two or more components, with the most intense component centered near 3685 cm⁻¹ (Fig. 1b); the area of the OH bond (A_OH) varies significantly according to the orientation (Figs. 1b, S1).

The values of A_OH and $A_{\mathrm{OH}}/A_{\mathrm{Si}}$ (Fig. 2) show a linear correlation with the water contents (expressed as H₂O) of the kaersutite standards ranging from 0.82 wt % to 1.38 wt %, as determined by the OEA (Table 3). Therefore, these calibration lines can be used to determine the water content of unknown amphibole crystals. Following the method proposed by Bolfan-Casanova et al. (2014), we repeatedly analyzed three reference glasses with different water contents (Médard and Grove, 2008; Schiavi et al., 2018) throughout the Raman sessions in order to use them as external calibrants instead of kaersutitic standards, thus allowing us to reduce the time required for daily analysis of standards. The three glasses plot far from the linear calibrations defined by the kaersutites (Fig. 2), meaning that the Raman cross-section of these glasses is very different from that of the amphiboles and that correction factors for glass intensities are needed to align glasses and amphiboles on the same line. In other words, both A_OH and $A_{\mathrm{OH}}/A_{\mathrm{Si}}$ of glass standards must be multiplied by the correction factors reported in Table 3 in order to fall on the regression lines defined by the kaersutite standards. The correction factors include differences in composition, density, refractive index, and absorptivity (Bolfan-Casanova et al., 2014; Thomas et al., 2008) compared to amphiboles. After correction, these glass standards can be used as external calibrants for quantification of water in kaersutitic amphiboles.

Table 3Water content of the amphibole standards and reference glasses.

MGT: Hoover Dam (Arizona, USA). BV: Bas-Vivarais volcanic province (Ardèche, France). Reference glass compositions are reported in Schiavi et al. (2018)

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5.2 Water contents in synthesized glasses and amphiboles

The H₂O contents range from 2.20 ± 0.10 wt % to 5.03 ± 0.47 wt % in glasses and from 0.93 ± 0.08 wt %​​​​​​​ to 1.50 ± 0.12 wt % in amphiboles (Table 4, Fig. 7a). Representative micro-Raman spectra of glasses and amphiboles are shown in the Supplement (Figs. S2 and S3). In three samples, the percentage of amphibole spectra displaying the most intense peak near 750 cm⁻¹ does not differ much from that of the amphibole standards: 58 % of all acquired spectra in sample 17-7C, 69 % in sample 17-9C, and 73 % in 17-8HM. The spectra showing the strongest peak at 228 cm⁻¹ sum up to about 30 %, with the others associated with the highest peak at 665 cm⁻¹. This observation indicates that the sampling of various crystal orientations (i.e., the extent of anisotropy sampled) was good in these samples. In the sample 17-4C the percentage of spectra with maximum intensity at 750 cm⁻¹ increases to 86 %, likely because of a subtle preferential orientation of the amphiboles within the capsule. Instead, in the sample 18-15HM, where a small number (6) of amphibole crystals was available for analyses, the spectra with the most intense peak near 750 cm⁻¹ sum up to 50 % of the total spectra, while the other 50 % are represented by spectra having the most intense peak at 665 or 228 cm⁻¹. We used the reference value of 67 % provided by the analyses on the kaersutitic standards to correct the water contents estimated for the amphiboles of samples 17-4C and 18-15HM. For this correction, a weighted average for A_OH (and $A_{\mathrm{OH}}/A_{\mathrm{Si}}$) was calculated, imposing a contribution of 67 % to the spectra with maximum intensity at 750 cm⁻¹ and of 33 % to differently oriented spectra. The corrected values are provided in Table 4.

Table 4Mean H₂O contents in amphibole and glass determined by Raman spectroscopy in each experiment and calculated partition coefficients for H₂O and Cl (${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl, respectively).

Cl content (wt %) in amphiboles and glasses is reported in Table 2.

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Figure 7(a) Relation between H₂O contents (wt %) in amphiboles and quenched melt. Correlation between H₂O content (wt %) in amphibole and its TiO₂ (b) and Cl (c) contents (wt %). In (c) two separated correlations are visible based on the fO₂ of the system (dashed lines are a visual guide). H06: data from Hauri et al. (2006).

A significant negative correlation trend is shown between the content of H₂O and TiO₂ in amphiboles (Fig. 7b), potentially related to the Ti oxo-substitution mechanisms, and negative trends are shown in a H₂O_amph vs. Cl_amph diagram (Fig. 7c), likely due to their competition at the O(3) site of the amphibole (see “Discussion”).

6.1 Approach to equilibrium

In order to evaluate the approach to equilibrium, both textural relationships and chemical phase homogeneity of run products have been evaluated. As shown in Fig. 3, amphibole crystallizes directly from the melt forming single crystals embedded in the glass or develops on clinopyroxene as a rim. In both cases, the euhedral habitus of amphiboles suggests the equilibrium with the coexisting melt. Major element homogeneity within amphibole crystals was assessed by measuring electron microprobe (EMP) line profiles across some grains, whereas homogeneity of amphibole and glass within each experiment was checked by observing the variability in EMP analysis acquired at random spots throughout the capsule (see Cannaò et al., 2022). Additionally, Mg–Fe partitioning between glasses and amphiboles is consistent with bulk X_Mg of the starting glass and follows a T-dependent positive correlation (Fig. 5), in agreement with previous experiments of Tiepolo et al. (2000). Mass balance analysis further supports constraints against macroscopic fractionation within the charge, as expected in static fluid-saturated experiments (Schmidt and Ulmer, 2004). Further evidence of the achievement of equilibrium is based on trace element concentrations as reported in Cannaò et al. (2022; see their Fig. 5).

6.2 Ti oxo-substitution, Fe${}^{\mathrm{3}+}/$Fe_tot ratio, and OH incorporation in amphibole

The negative correlation between TiO₂ and H₂O content (Fig. 7b) strongly suggests that Ti oxo-substitution (1) is the dominant mechanism controlling the water content in the studied amphiboles. However, the significantly lower water content of amphibole in experiment 17-8HM (1050 ^∘C) compared to experiments with similar TiO₂ content, such as 17-4C and 17-9C (1015 and 1040 ^∘C, respectively) (Figs. 6b, 7b), suggests that the dehydrogenation mechanism (2) is also active, as already pointed out by Adam et al. (2007).

More oxidizing conditions, as in experiment 17-8HM, produce amphibole with higher Fe${}^{\mathrm{3}+}/$Fe_tot (up to 0.106, Table 5) as compared to amphibole synthesized at fO₂ close to the CCO buffer that presents lower Fe${}^{\mathrm{3}+}/$Fe_tot (from 0.056 to 0.082; Table 5). The incorporation of Fe³⁺ at the M(1) and M(3) sites triggers the deprotonation of two OH⁻ hydroxyls from the two associated O(3) sites in order to maintain the local charge balance (mechanism 2). Interestingly, in amphibole with lower TiO₂ content, as in the experiment 18-15HM (conducted at the same fO₂ conditions), this effect induced from local charge balance is not observed (Fig. 7b), suggesting that the dehydrogenation mechanism (2) is less pronounced, although the Fe${}^{\mathrm{3}+}/$Fe_tot ratio of 0.161 is the highest among our experiments (Table 5). This discrepancy can be explained by the different amount of the initial water content of the experiment or their experimental configuration (double- vs. triple-capsule techniques).

Table 5Calculated occupation of the amphibole T, B, and C sites and estimated Fe${}^{\mathrm{3}+}/$Fe_tot.

Recalculated according to the formula A_0–1B₂C₅T₈O₂₂W₂ (23 oxygen atoms per formula unit, a.p.f.u.​​​​​​​) using the method proposed in Li et al. (2020).

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At comparable water content (Table 4), the experiment 17-9C shows higher TiO₂ content compared to run 17-4C (Fig. 7b), which can be ascribed to a greater incorporation of Ti⁴⁺ at M(2,3) sites (which is not involved in the Ti oxo-substitution (1)) possibly via exchange vectors such as

This interpretation is consistent with the highest ^TAl average content shown by amphiboles from experiment 17-9C (2.150 a.p.f.u.​​​​​​​) among all experiments (Table 5). Moreover, the relative proportion of ^M(2,3)Al in our amphiboles relative to the ^M(2,3)Ti ranges from 1.2 to 1.6, falling within the range of relative proportions of the endmember exchanges proposed above.

6.3 Water contents in amphibole, H₂O and Cl partition coefficients between amphibole and liquid (${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$, ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl)

The achievement of equilibrium conditions allows us to use the micro-Raman-derived water contents in amphibole and glasses to calculate the H₂O partition coefficients between amphibole and liquid (${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$). This, in turn, provides an evaluation of the affinity of water in amphibole as a function of Cl content of the system. The calculated ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ (Table 4) ranges from 0.29 ± 0.06 in experiment 18-15HM to 0.52 ± 0.04 in experiment 17-9C (Fig. 8). No significant variation is observed as a function of variable fO₂ conditions. The calculated ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ indicates that water has a moderate affinity for amphibole. Data obtained from EMPA analyses were used to calculate the ${}^{\mathrm{Amph}/\mathrm{L}}$D of Cl (Fig. 6a), which ranges from 0.248 ± 0.017 in experiment 17-7C to 0.439 ± 0.107 in experiment 17-4C (Table 4), with an average value of 0.38 ± 0.10 (Fig. 6a). Similar to ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$, no significant variation is observed as a function of variable fO₂ conditions. The amphibole–liquid partition coefficient for Cl is highly debated in the literature, and a wide range of ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl is provided (from 0.03 to 0.62; Dalou et al., 2014; Van den Bleeken and Koga, 2015; Bénard et al., 2017). Our data fall at the upper limit of the literature data (median at 0.26), supporting the moderate incompatibility of Cl for amphibole.

Figure 8Correlation between ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and Cl (a) and TiO₂ (b) contents (wt %) in amphibole. H06: data from Hauri et al. (2006); T99: data from Tiepolo (1999). The diagram in (c) shows the correlation between ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl, giving their exchange coefficient (${}^{\mathrm{Amph}/\mathrm{L}}$K${}_{D{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}$). ${}^{\mathrm{Amph}/\mathrm{L}}$K${}_{D{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}$ obtained from this work is 1.27, although it becomes lower than unity considering all data (this study and data from Hauri et al., 2006).

Compared to previous studies at water-undersaturated conditions that obtained ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ between 0.12 and 0.24 (Hauri et al., 2006), the present study suggests higher H₂O partition coefficients (Fig. 8a and b). It should be noted that the work of Hauri et al. (2006) is the only experimental work reporting ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$. These experiments (runs 1442, 1446, and 1452) were performed at variable T ranging between 1000 and 1050 ^∘C and P from 0.5 to 2.0 GPa, using a basanitic glass as starting material and producing amphibole with mainly Ti-pargasitic composition (Adam and Green, 1994). Unpublished experiment-derived SIMS (secondary ion mass spectrometry) analyses on ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ (Tiepolo, 1999) report an average value of 0.26 ± 0.04 (n=22) for amphibole, characterized by significantly lower Cl (below 0.05 wt %) and variable TiO₂ contents (gray fields in Fig. 8a and b). These experiments were performed at equilibrium T from 950 to 1075 ^∘C and fixed P of 1.4 GPa. The wide T range used for these experiments may explain the high variability in TiO₂ content of these amphiboles reported in Fig. 8b.

Remarkably, positive correlations are observed between ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and Cl and TiO₂ contents in amphiboles (Fig. 8a and b) resembling the negative trends shown between the H₂O content in amphibole and its Cl and TiO₂ contents (Figs. 6b, 7b and c). Although our ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ has relatively higher uncertainty compared to SIMS data (Hauri et al., 2006), the correlation with Cl and TiO₂ can be linearly fitted, giving a good fit quality parameter, i.e., R²=0.872 and 0.870 (Fig. 8), respectively. This evidence points to a significant effect of Cl (and TiO₂) in the hydrogenation of the amphibole, thus controlling its ability to incorporate water. Despite this evidence, a positive correlation between ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl is shown in Fig. 8c, where the exchange coefficient between H₂O and Cl (${}^{\mathrm{Amph}/\mathrm{L}}$Kd${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}=$ ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ / ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl) for our data results in a preferential incorporation of water in amphibole compared to Cl (${}^{\mathrm{Amph}/\mathrm{L}}$Kd${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}\approx \mathrm{1.27}$). Remarkably, considering the data from Hauri et al. (2006), which extend the range of the ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ and ${}^{\mathrm{Amph}/\mathrm{L}}$D_Cl towards lower values (Fig. 8c), the resulting ${}^{\mathrm{Amph}/\mathrm{L}}$Kd${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}$ significantly changes (${}^{\mathrm{Amph}/\mathrm{L}}$Kd${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}/\mathrm{Cl}}\approx \mathrm{0.76}$), suggesting a preferential incorporation of Cl with respect to water. The explanation for such contrasting behavior is difficult to explain with the available data, and more detailed investigations are required to better unravel this conundrum.

6.4 The effect of Cl on the hydrogenation of amphibole

The measured water contents in both amphiboles and glasses in our experimental study are similar or lower compared to data provided by Hauri et al. (2006) (Fig. 7a); the calculated ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$, however, is significantly higher (Fig. 8) and, interestingly, dependent on Cl (and TiO₂) contents in amphibole (Fig. 7b, c). In particular, our data highlight that for high Cl contents, dehydrogenation mechanisms are discouraged, and incorporation of water in amphibole is enhanced (Fig. 8). It should be noted that for comparable Cl content in amphibole, experiment 17-8HM performed at oxidized conditions (ΔFMQ = +1.7) is characterized by higher ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ compared to experiment 17-7C, which runs at ΔFMQ = −2.6 (Fig. 8a), suggesting a relevant role of the fO₂ condition of the system in influencing the affinity of water in amphibole.

The positive correlation between the Cl content in amphibole and ${}^{\mathrm{Amph}/\mathrm{L}}$D${}_{{\mathrm{H}}_{\mathrm{2}}\mathrm{O}}$ is unexpected as (i) Cl and water compete for the O(3) occupancy, as clearly shown by the negative correlation between H₂O and Cl content in amphibole (Fig. 7c), and (ii) the presence of abundant Cl in the system lowers the fH₂, and, thus, the incorporation of H in amphibole should decrease or at least be limited (Popp et al., 1995b). The deviation of our data from these effects can therefore be explained by crystallographic reasons. Indeed, the presence of abundant Cl in amphibole greatly influences its crystal structure (Oberti et al., 1993; Hawthorne and Oberti, 2007; Oberti et al., 2007) and, in turn, might influence the Ti oxo-substitution (1) that regulates water incorporation in amphibole. Because M(1) and M(3) sites are coordinated by the O(3) site, the occupancy of these three sites is interdependent. In particular, two M(1) and one M(3) sites are locally coordinated by two O(3) sites, located above and below the strip of octahedra, respectively. The radius of Cl⁻ (1.81 Å; Shannon, 1976) is much larger compared to the other anions that can be hosted at the anionic sites (F${}^{-}=\mathrm{1.33}$ Å, OH${}^{-}=\mathrm{1.37}$Å, O${}^{\mathrm{2}-}=\mathrm{1.40}$ Å); therefore, the presence of Cl at the O(3) site determines its expansion by stretching of the <M(1)–O(3)> and <M(3)–O(3)> bonds (Oberti et al., 1993; Hawthorne and Oberti, 2007). This expansion is facilitated by the presence of larger cations in M(1) and M(3), leading to the preferential incorporation of Mg²⁺ and, especially, Fe²⁺ at these sites, rather than smaller cations like Al³⁺, Fe³⁺, and Ti⁴⁺ that are instead relocated to the M(2) sites (Giesting and Filiberto, 2016; Henry and Daigle, 2018). In particular, the presence of Cl⁻ at one of the two O(3) sites does not allow the entrance of Ti⁴⁺ in M(1), ultimately impeding the release of the OH⁻ hosted at the associated O(3) site via reaction (1). Although the incorporation of Ti⁴⁺ at the M(1) site is hampered at high Cl concentration, the observed increase in the TiO₂ content in amphibole associated with Cl (Fig. 6b) is explained by considering that a significant fraction of Ti⁴⁺ may also be hosted at the M(2) site of the amphibole (Hawthorne et al., 1998; Tiepolo et al., 1999). Remarkably, this crystallographic feature appears to be linked to the fO₂ of the system and is more pronounced for experiments performed at high fO₂.

The coupled structural changes induced by the incorporation of Cl in amphibole also affect its trace element incorporation, as recently documented in a companying paper by Cannaò et al. (2022).