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# Experimental determination of lithium partitioning between plagioclase and hydrous rhyolitic melt

Maylis Dupont de Dinechin, Caroline Martel, Hélène Balcone-Boissard, Monika K Rusiecka, Rémi Champallier, Etienne Deloule

► To cite this version:

Maylis Dupont de Dinechin, Caroline Martel, Hélène Balcone-Boissard, Monika K Rusiecka, Rémi Champallier, et al.. Experimental determination of lithium partitioning between plagioclase and hydrous rhyolitic melt. Chemical Geology, 2025, 695, pp.123047. 10.1016/j.chemgeo.2025.123047. hal-05250479

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Research Paper

# Experimental determination of lithium partitioning between plagioclase and hydrous rhyolitic melt

Maylis Dupont de Dinechin<sup>a</sup>, Caroline Martel<sup>b,*</sup>, Hélène Balcone-Boissard<sup>a</sup>, Monika K. Rusiecka<sup>b,1</sup>, Rémi Champallier<sup>b</sup>, Etienne Deloule<sup>c</sup>

## ARTICLE INFO

Editor: Claudia Romano

Keywords:

- Lithium
- Plagioclase
- Partitioning
- Diffusion
- Decompression
- Cooling

## ABSTRACT

Lithium concentrations and partition coefficients were determined in plagioclase and Li-doped (300, 500, and 1500 ppm) H₂O-saturated rhyolitic melts experimentally produced at temperature of 800–875 °C and pressure of 50–150 MPa.

High bulk Li concentration of 1500 ppm have remarkable effects on the crystallization of rhyolitic melts, comparatively to lower concentrations: i) Ca-rich clinopyroxene crystallizes at the expense of plagioclase and Ca-poor clinopyroxene, ii) plagioclase anorthite content decreases, and iii) melt H₂O solubility increases significantly, resulting in lower crystallinities.

The Li content of plagioclase (70–900 ppm) and rhyolitic melt (300–1700 ppm) both increase with bulk Li content. The Li content in plagioclase increases with decreasing plagioclase anorthite content. The aqueous phase contains a negligible fraction of Li.

The plagioclase-rhyolitic melt partition coefficients ($K_{\text{plag/rhy}}^p$) range from 0.2 to 0.6 and are mostly constant with bulk Li concentration. At 875 °C, the $K_{\text{plag/rhy}}^p$ increase from ~0.3 to 0.6 with pressure decreasing from 150 to 50 MPa, while constant (0.2–0.4) with pressure at 800 °C. At 50–100 MPa, the $K_{\text{plag/rhy}}^p$ increase from 0.3 to 0.4 at 800 °C to 0.5–0.6 at 875 °C, while constant (0.3–0.4) with temperature at 150 MPa.

These results give information to interpret Li gradients in natural plagioclases with regard to processes such as magma degassing, decompression, and cooling. In particular, opposite Li gradients are expected in plagioclase during decompression-induced degassing (Li outward-gradients) and isobaric cooling (Li inward-gradients).

## 1. Introduction

Alongside commercial interest, Li is of relevance in a variety of geological processes involved in magmatism, metamorphism, hydrothermalism, and volcanism. In volcanology, Li has been used to trace syn-eruptive degassing and fluid-melt interaction in space and time, with promising perspectives on linking petrology to volcano monitoring. Pre-and syn-eruptive timescales can be estimated from compositional gradients in crystals and melts using models of diffusive relaxation. Following this objective, Li concentrations and isotope compositions have been measured in natural crystals, revealing a great variability of profiles (e.g., Smith and Brown, 1988), including contradictory core-rim gradients, which were attributed to processes such as magma ascent

(Genareau and Clarke, 2010), post-emplacement cooling (Gallagher and Elliott, 2009; Ellis et al., 2018), and volatile fluxing and degassing (Berlo et al., 2004; Kent et al., 2007; Vlastélic et al., 2011; Charlier et al., 2012; Giuffrida et al., 2018; Neukampf et al., 2022; Saalfeld et al., 2022). For example, drops in Li concentration at crystal edges were attributed to degassing since Li preferentially partitions into the fluid phase (Berlo et al., 2004; Kent et al., 2007; Genareau et al., 2009; Genareau and Clarke, 2010; Coogan, 2011). Conversely, rim Li enrichment was interpreted as variations in elemental partitioning between the crystal, the melt, and the gas phase (Charlier et al., 2012) or as Li reintroduction into the melt following a flushing process (Giuffrida et al., 2018). However, interpreting Li gradients remains challenging due to its high mobility, variable partitioning between coexisting melt, crystals and

<footnote><sup>a</sup> ISTE>P, UMR 7193, CNRS-Sorbonne Université, Paris, France</footnote>

<footnote><sup>b</sup> ISTO, UMR 7327, Univ Orléans, CNRS, BRGM, OSUC, F-45071 Orléans, France</footnote>

<footnote><sup>c</sup> CNRS, CRPG, UMR 7358, Université de Lorraine, BP 20, 54501 Vandoeuvre-lès-Nancy cedex, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: caroline.martel@cnrs-orleans.fr (C. Martel).</footnote>

<footnote n="1">¹ Present address: Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom</footnote>
fluid, and the lack of robust experimental constraints under realistic
magmatic condition. Recently, Dupont de Dinechin et al. (2023) have
pointed out that using Li gradients to interpret volcanological processes
and magmatic timescales via diffusion chronometry requires additional
information, such as accurate Li partitioning data between coexisting
crystal, melt, and fluid at known conditions of temperature and
pressure.

Crystal-melt Li partition coefficient, $K_{\text{Li}}^{\text{crystal/melt}}$ (defined as the ratio of the Li concentration in the fluid phase to that in the melt), has mostly been determined from natural samples, by measuring the Li content of coexisting crystals and glasses (melt inclusions and residual glasses). Among studied crystals, plagioclase is of high interest by its dominance in mafic to felsic magmas (e.g., Smith and Brown, 1988) and by its compositional sensitivity to various parameters, including temperature and melt H₂O content, which make it a useful tracer of magmatic processes such as cooling/heating and magma degassing. Plagioclase-melt partition coefficient, $K_{\text{Li}}^{\text{plag/melt}}$, determined from natural samples ranges from 0.1 to 0.7 (Smith et al., 2009; Brophy et al., 2011; Charlier et al., 2012; Cabato et al., 2013; Padilla and Gualda, 2016; Neukampf et al., 2019, 2023), reflecting Li preferential partitioning into the melt over the crystal. The variability in $K_{\text{Li}}^{\text{plag/melt}}$ mostly results from large variations in Li contents of plagioclase and quenched liquid, potentially reflecting chemical disequilibria, unknown conditions of pressure-temperature-composition, and post-eruptive modifications (Neukampf et al., 2022; Pichavant et al., 2024).

In contrast, $K_{\text{Li}}^{\text{plag/melt}}$ determined experimentally better capture equilibrium conditions and allow the effect of pressure, temperature, and phase composition to be constrained. Most of the experimental data originate from studies using mafic compositions at atmospheric pressure and temperatures above 1200 °C (e.g., Bindeman et al., 1998; Aigner-Torres et al., 2007), whereas experimental data involving felsic melts are scarce (e.g., Iveson et al., 2018, 2019; Pichavant, 2022). There are only two datasets on Li partitioning between plagioclase and a felsic melt in presence of an aqueous volatile phase, yielding $K_{\text{Li}}^{\text{plag/melt}}$ from 0.2 to 0.3 for pressures from 150 to 350 MPa at temperatures of 810–850 °C after Iveson et al. (2018) and $K_{\text{Li}}^{\text{plag/melt}}$ of 0.02–0.03 at 200 MPa and 800 °C after Gao et al. (2024).

As an alternative to the time-consuming (spanning all conditions)
and challenging (trace element analysis) experimental approach, trace-
element partition coefficients have been calculated thermodynamically
using lattice strain models (Blundy and Wood, 1994, 2003). For the Li
ionic radius (~0.9 Å) and charge (1+), the theory globally predicts $K_{\text{Li}}^{\text{plag/melt}}$ around 0.1 (Blundy and Wood, 2003; Dohmen and Blundy,
2014). Yet, the models mostly assume that Li in plagioclase substitutes in
the M-site at 8-fold coordination (i.e., as an alkali such as Na), although
possibilities as 5-fold coordination have been considered (e.g., Pohl
et al., 2024). Also, the lattice strain model cannot, in itself, be used to
predict for arbitrary conditions of pressure, temperature, and phase
composition, which sometimes requires specific design of experimental
runs to isolate the individual parameters (Dohmen and Blundy, 2014).

Fluid-melt Li partition coefficient, $K_{\text{Li}}^{\text{fluid/melt}}$ (defined as the ratio of the Li concentration in the fluid phase to that in the melt), is difficult to determine from natural samples since vapour is hardly preserved. Yet, the presence of fluid inclusions trapped in crystals may be a witness of the vapour phase composition in equilibrium with melt (e.g., Zajacz et al., 2008; Fiedrich et al., 2020). By measuring Li in natural fluid and melt inclusions, Zajacz et al. (2008) proposed $K_{\text{Li}}^{\text{fluid/melt}}$ from 1 to 50. Gas condensates may also be informative of the initial vapour compositions, which Vlastélic et al. (2011) analysed for natural trachytic pumices, suggesting $K_{\text{Li}}^{\text{fluid/trachyte}}$ from 60 to 135. Therefore, gas condensates and fluid inclusions both suggest Li affinity for the fluid phase.

Experimental data obtained at 650–950 °C and 50–400 MPa using Li-
rich rhyolitic melts (e.g., Macusani rhyolite or topaz-rhyolite) suggest
$K_{\text{Li}}^{\text{fluid/rhy}}$ from 0.1 to 13 (London et al., 1988; Webster et al., 1989). This
shows that Li preferentially partitions either in melt or fluid, depending
on the pressure-temperature conditions and the composition of the

aqueous fluid phase. For instance, Li partitioning is significantly
increased by adding halogens in the fluid phase (Webster et al., 1989),
while significantly decreased by adding CO₂ (Iveson et al., 2019).
Experimentally-determined $K_{\text{Li}}^{\text{fluid/melt}}$ was mostly calculated by mass
balance (from Li measurements in the bulk and the residual glasses) or,
recently, from direct Li measurements in the vapour and glass phases
(Gion et al., 2022).

Therefore, in order to better constrain crystal-melt-fluid partitioning
in a range of conditions relevant to volcanic processes and involving
felsic magmas, we conducted crystallization experiments at pressures
from 50 to 150 MPa and temperatures of 800 and 875 °C, starting from
Li-doped (300 to 1500 ppm) H₂O-saturated (2.5 to 5.1 wt%) rhyolitic
melts. We examined the influence of bulk Li content, pressure (or water
content), temperature, and phase composition, on Li concentration and
distribution between the plagioclase, the rhyolitic melt, and the aqueous
fluid. Such experiments pose analytical challenges for tracking and
measuring Li in different phases and under different conditions.

## 2. Methods

### 2.1. Starting material

The experiments were realized in two steps: i) a primary production of Li-doped glasses by fusion in an open Pt-crucible using a 1-atm furnace at 1200 °C (the starting material) and ii) crystallization experiments using these Li-doped melts sealed in Au-capsules with H₂O, by employing an internally-heated pressure vessel (IHPV).

The starting material is a rhyolitic glass used by Martel (2012), hereafter referred to as PEL-glass, which contains 75.7 wt% SiO₂, 2.4 wt % CaO, 3.6 wt% Na₂O, and 1.9 wt% K₂O (full composition reported in SM1) with an ASI ~1.65. In order of investigate the effect of the Li bulk content on Li crystal-melt-fluid partitioning, the PEL-glass was doped with different amounts (500, 1000, and 2000 ppm) of laboratory grade Li₂O powder (from Alfa Aesar company). The mixing of the PEL-glass and Li₂O powders was carried out in a vacuum tent to prevent the hygroscopic Li₂O powder from absorbing too much water from ambient humidity. The powder mixtures were placed in Pt-crucibles and melted for 1 h at 1200 °C in a 1-atm oven. The melt was rapidly and entirely quenched into a glass by cooling the Pt container with water (without contact between water and sample). Glass fragments were saved for chemical analyses whereas the rest was ground to a fine powder in an agate mortar to serve as starting glass material for the crystallization experiments.

For the crystallization experiments under pressure and temperature, about 2–3 mg deionized water (i.e., just enough to achieve H₂O saturation at run pressure and temperature) and 50 mg of the Li-bearing PEL-glass powder were introduced in Au capsules (2.5 mm inner diameter, 2.9 mm outer diameter, and 20 mm length), which were sealed both ends by arc welding (PUK technique). The capsules were weighed before and after welding, and after run, to check for H₂O-tightness (none of them showed weight variation over 1 µg, so that we considered that H₂O did not outgas during sample preparation and run).

### 2.2. Experimental methods

Each run consisted of three capsules containing different amounts of Li, which were placed in a sample holder and introduced in a vertically-working IHPV. The IHPV was pressurized with pure argon and pressure was measured using a pressure transducer calibrated against a Heise Bourdon gauge (accuracy of ±0.2 MPa). Using pure Ar gives oxidized conditions of at least 3 log units above the Ni-NiO (NNO) buffer under water saturated conditions, as reported in Martel, 2012). Temperature was measured by K-type or S-type thermocouples (precision of ±3 °C). Run duration varied from 6 to 16 days, mainly depending on temperature, and the samples were rapidly quenched by fusing the Pt wire holding the sample holder, in order to let it drop into the cold part of the
vessel (~50 °C, following a cooling rate of ~200 °C/s).

A set of six crystallization runs (18 samples) was performed at temperatures of 800 and 875 °C, and pressures of 50, 100, and 150 MPa (i.e., about 2.5, 3.8, and 4.9 wt% H₂O, respectively, calculated after the H₂O solubility model of rhyolitic melts; Liu et al., 2005) (Table 1). These parameters were chosen to be representative of silicic magma storage conditions (~2 to 6 km below the surface).

The main experimental concern was to nucleate plagioclase crystals large enough (i.e. > 10–20 µm) to allow analysis by secondary-ion mass spectrometry (SIMS). The isobaric experiments performed by Martel (2012) using Li-free PEL-glass samples showed that the plagioclases crystallizing at low pressures (< 100 MPa) and low temperatures (< 850 °C) were mostly below 5 µm in size, showing skeletal to dendritic shapes. The main reason for such crystal sizes is that crystallization under large effective undercooling (ΔT<sub>eff</sub> = liquidus temperature – run temperature; > 100–200 °C) promotes crystal nucleation at the expense of growth, i.e., extensive nucleation of small crystals. In contrast, crystallization under low undercooling (ΔT<sub>eff</sub> < 50–100 °C) promotes growth over nucleation by forming few large crystals (e.g., Hammer and Rutherford, 2002; Mollard et al., 2012). Only two among the six experiments were performed at a constant temperature of 875 °C (runs #1 and #2 in Table 1) that was close enough to the plagioclase saturation temperature (Martel, 2012) to keep ΔT<sub>eff</sub> below ~75 °C and thus allow crystallization of plagioclases larger than 10 µm. For the other experiments, we set overheating stages at temperatures slightly below the plagioclase liquidus given in Martel (2012), in order to promote nucleation of few plagioclases at ΔT<sub>eff</sub> < 50 °C. The overheating steps lasted from 2 to 4 days, calculated to be long enough to overcome plagioclase nucleation delays in rhyolitic melts (Rusiecka and Martel, 2022). Then temperature was isobarically decreased at a slow rate of ~1.5 °C/h (corresponding to typical cooling durations from 2 to 5 days) to the final temperature of 800 or 875 °C, in order to promote growth of the initial nuclei and limit syn-cooling new nucleation. The crystals continued growing during additional dwell time from 1 to 8 days at target temperature. These strategies were designed to optimize crystal size while minimizing disequilibrium zoning. We assumed that at least 1 day at final temperature was enough for Li to re-equilibrate within the crystals, given Li fast diffusivity in plagioclase (about 10<sup>-12</sup> to 10<sup>-13</sup> m²/s at 800 °C; Audétat et al., 2018; Pohl et al., 2024). Therefore, the experimental strategy was designed to produce large plagioclases re-equilibrated in Li at target temperature and pressure.

## 2.3. Analytical methods

Glass fragments of the Li-bearing starting glasses and fragments of the run products recovered after opening the capsules were mounted in epoxy resins and polished for textural and chemical analyses.

<table><caption>**Table 1**
Experimental run conditions.</caption><thead><tr><th rowspan="2">Run #</th><th rowspan="2">P<sup>a</sup><br>(MPa)</th><th rowspan="2">T1<sup>b</sup><br>(°C)</th><th rowspan="2">time1<sup>c</sup> (h)</th><th colspan="4">Cooling</th><th rowspan="2">time3<sup>d</sup> (h)</th></tr><tr><th>T<sup>b</sup><br>(°C)</th><th>ΔT<sup>b</sup><br>(°C)</th><th>time2<sup>d</sup> (h)</th><th>Rate<sup>d</sup><br>(°C/h)</th></tr></thead><tbody><tr><td>(1)875–150</td><td>150</td><td>875</td><td>0</td><td>875</td><td>0</td><td>0</td><td>0</td><td>138</td></tr><tr><td>(2)875–100</td><td>100</td><td>875</td><td>0</td><td>875</td><td>0</td><td>0</td><td>0</td><td>161</td></tr><tr><td>(4)875–50<sup>e</sup></td><td>50</td><td>975</td><td>92</td><td>875</td><td>100</td><td>94</td><td>1.1</td><td>162 + 17 + 18</td></tr><tr><td>(3)800–150</td><td>150</td><td>875</td><td>90</td><td>800</td><td>75</td><td>52</td><td>1.4</td><td>21</td></tr><tr><td>(5)800–100</td><td>100</td><td>925</td><td>68</td><td>800</td><td>125</td><td>78</td><td>1.6</td><td>105</td></tr><tr><td>(6)800–50</td><td>50</td><td>975</td><td>44</td><td>800</td><td>175</td><td>120</td><td>1.5</td><td>24</td></tr></tbody></table>

### 2.3.1. Scanning electron microscopy (SEM)

SEM analyses were performed using C-coated epoxy mounts by employing a ZEISS Supra 55VP electron microprobe (CCM platform) at Sorbonne Université, Paris, and a Zeiss Supra 55VP Carl Zeiss EVO MA10 (PARI platform) at IPGP, Paris. The SEM observations were performed at an accelerating voltage of 15 keV and magnification ranging from x50 to x1500, enabling objects as small as a few µm to be detected. The samples were checked for textural homogeneity and for the presence, distribution, and texture of the bubbles and crystals in the starting materials and the run products. Also, two types of SEM sessions were dedicated to the SIMS analyses: a pre-SIMS session to identify the largest crystal glass areas to be analysed by SIMS, and a post-SIMS session to locate the ion beam impacts (Fig. 1c). SEM observations were performed to assess the textural homogeneity of the run products and to locate large plagioclase crystals (>10 µm) embedded in homogeneous glass suitable for SIMS analysis. Special attention was given to select plagioclase-glass pairs located in close proximity, minimizing the spatial separation between phases. This protocol aimed to reflect local equilibrium conditions as accurately as possible. Pre-SIMS SEM sessions were used to map suitable areas, while post-SIMS SEM sessions allowed precise localization of the SIMS pits and identification of any measurements impinging on multiple phases. Data from ambiguous or heterogeneous areas were discarded from the dataset, and suspect measurements (e.g., anomalously high or low Li concentrations) were flagged for cautious interpretation, as detailed in Section 3.4 and Fig. 3.

### 2.3.2. Secondary-ion mass spectrometry (SIMS)

SIMS analyses of the crystal and glass Li concentration were performed using Au-coated epoxy by employing a Cameca IMS 1280 HR ion microprobe at CRPG, Nancy. A primary O- beam of 2 nA generated by a RF Hyperion source was focused on a 5–8 µm area. The positive secondary ions of ⁶Li, ⁷Li and ³⁰Si were measured with a single collector at a mass resolution (M/ΔM) of 2000 with an energy window of 30 eV, without (⁶Li, ⁷Li and ³⁰Si) and with (⁷Li and ³⁰Si) an energy offset of 60 eV. The measured masses were 5.5 for the Faraday cup (FC) background, ⁶Li without offset, ⁷Li without offset, ⁷Li with offset, 27.5 for the Electron Multiplier (EM) background, ³⁰Si without offset, ³⁰Si with offset. Each mass was measured for 4 s, with a waiting time of 1 s. The measurements are carried out with EM counting for counting rate lower than 100,000 counts/s, with FC for higher counting rates. Each measurement consisted of 8 successive acquisition cycles. Before each measurement, the primary beam centring, the sample charge, and the mass calibration, were automatically controlled after a 30-s pre-sputtering with a 10-µm sweep. The ⁷Li/⁶Li ratio was used as an internal control for Li measurement, and the ⁷Li/³⁰Si ratios were used to calculate the Li content using two silica-rich (> 70 wt% SiO₂) volcanic glasses (GB4 and UTR2; Jochum et al., 2006) and five silica-poor (49 wt% SiO₂) glasses (LSVEC10, LSVEC50, LSVEC100, LSVEC500, LSVEC1000; Decitre et al.,

<footnote><sup>a</sup> Run pressure.</footnote>

<footnote><sup>b</sup> T1 and T are the initial and final temperatures, respectively; undercooling ΔT is T1-T.</footnote>

<footnote><sup>c</sup> time1 and time3 are the dwell time at T1 and T, respectively; time2 is the duration of cooling from T1 to T.</footnote>

<footnote><sup>d</sup> Rate is the cooling rate as ΔT divided by time2.</footnote>

<footnote><sup>e</sup> Run (4)875–50, for which the rapid-quench failed twice (the sample bucket did not fall into the cold part of the vessel), so that the capsules were reloaded twice at P-T for extra 17 h and 18 h before ending with a successful rapid quench.</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Selected images of the samples. (a) PEL starting materials with 1500 ppm bulk Li, (b) run #1 (875 °C, 150 MPa, 1500 ppm bulk Li) showing the SIMS spots in plagioclase and glass, (c) run #6 (800 °C, 50 MPa, 1500 ppm bulk Li) showing skeletal crystals, and (d) run #5 (800 °C, 100 MPa, 600 ppm bulk Li). Plag for plagioclase, Hem for hematite, Cpx for clinopyroxene. Images from all samples are displayed in SM4.</figcaption></figure>

2001) as reference materials. The uncertainty of the Li abundance determinations was ±12 ppm. All SIMS analyses are reported in SM2.

### 2.3.3. Electron microprobe (EMP)

EMP analyses of the Li-bearing starting glasses were performed using C-coated epoxy mounts by employing a Cameca SX-Five and SX-100 at Service Camparis, Paris. The major elements (Si, Al, Ca, Na, K, Fe) were analysed using an accelerating voltage of 15 keV, a beam current of 10 nA, a beam diameter of 3 µm, and an acquisition time of 60 s. Minor elements (Mg, Ti) were analysed with a voltage of 15 keV, a beam current of 30 nA, and an extended counting time of 120 s to improve detection limits.

EMP analyses of the run products were performed using C-coated epoxy mounts by employing a JEOL JXA-IHP-200F from the MACLE-CVL platform at Orléans. All elements (Si, Al, Ca, Na, K, Fe, Mg, Ti) were analysed under the same conditions, with alkalis analysed first to minimize their migration under the electron beam. The EMP analytical conditions were set to an acceleration voltage of 15 kV, a beam current of 6 nA, and a focused beam (spot size of 2 µm) for the minerals and a defocused beam (10 and 20 µm diameter) for the glasses.

The analytical errors on the oxide analyses are 1 % relative for SiO₂ and Al₂O₃, 2 % for CaO, 3 % for FeO, MgO, and TiO₂, and 5 % for MnO, Na₂O and K₂O. For plagioclase, this results in a maximum uncertainty of ±2.5 mol% An, as reported in Martel (2012).

## 2.4. Calculation of the Li partition coefficients ($K_{Li}$)

### 2.4.1. Plagioclase-melt $K_{Li}$

The Nernst (or weight fraction) $K_{Li}^{plag/rhy}$ was determined by dividing the Li content of the plagioclase by the Li content of the glass, both measured by SIMS and given in ppm. Although this is the calculation mostly reported in the literature, we also considered the case of Li being incorporated into the plagioclase by substitution of other elements of similar charge and ionic radii, such as Na (McIntire, 1963; Banno and Matsui, 1973; Blundy and Wood, 1994). As a result, the equilibrium constant for the Li-Na substitution reaction can be expressed as:

$$K_{Li}^{plag/rhy} = \frac{a(Na_{rhy}^{+})}{a(Li_{rhy}^{+})} \times \frac{a(LiAlSi_{3}O_{8}(plag))}{a(NaAlSi_{3}O_{8}(plag))}$$

where a represents the activity of the respective component in the melt and solid phase. The activities are related to the mole fractions of the elements in the components, assuming ideal solution behaviour, as follows:

$$a_i = X_i \cdot \gamma_i$$

Here, $X_i$ represents the mole fraction of the component calculated from EPMA and SIMS analyses, and $\gamma_i$ is the activity coefficient, which can be assumed to be close to 1 for ideal solutions. $K_{Li}^{plag/rhy}$ thus
provides an insight into the relative stability of Li in the crystal compared to the melt phase.

### 2.4.2. Fluid-melt $K_{\text{Li}}$

The calculation of $K_{\text{Li}}^{\text{fluid/melt}}$ requires to determine the Li content of the vapour phase; note here that, strictly, the fluid is not composed of pure H₂O but contains significant amounts of components dissolved from the high-temperature melt. This was achieved by mass balance calculation from the Li contents of the starting material (294, 616, or 1495 ppm), the residual glass (measured by SIMS), and the plagioclase (measured by SIMS and neglecting the Li in the other phases, i.e., pyroxenes and hematite, on the ground that they represent mass fractions $\le 2$ wt%; Table 2). Then $K_{\text{Li}}^{\text{H2O/melt}}$ was determined by dividing the Li content calculated for the vapour phase by the Li content of the glass measured by SIMS.

### 2.4.3. Determination of the uncertainties

The uncertainties on the plagioclase-melt partition coefficients were obtained from the analytical and statistical uncertainties of the SIMS and EMP measurements, which were calculated using a Python code that propagates the errors using the Monte Carlo method. The maximum uncertainty of $\pm 0.06$ was applied to each $K_{\text{Li}}^{\text{lag/rhy}}$. The uncertainties on the fluid-melt partitioning are additionally depending on the errors propagated by mass balance calculation. Yet, it appears that the errors on the phase analysis (in wt%) are so large compared to the very small amounts of Li (in ppm) that Li in the fluid calculated by mass balance mostly gives negative values (SM3).

## 3. Results

### 3.1. Li-bearing starting glasses

The experiments were realized in two steps: i) a primary production of Li-doped glasses by fusion in an open Pt-crucible using a 1-atm furnace at 1200 °C and ii) crystallization experiments using these Li-doped melts sealed in Au-capsules with H₂O, by employing a IHPV. During fusion in the 1-atm furnace, the Li initially introduced (2000, 1000, and 500 ppm) partially volatilized. Post-fusion SIMS measurements in the glasses along profiles of five points show homogeneous Li contents of 1528 ± 11, 616 ± 12, and 294 ± 3 ppm Li, hereafter referred as to PEL1500, PEL600, and PEL300 glasses, respectively (Table 2). This suggests Li loss during fusion from 24 to 42 % relative. All three post-fusion Li-doped starting glasses show slightly vesicular crystal-free glasses (Fig. 1a). Their major element compositions are reported in SM1.

### 3.2. Texture and phase assemblage of the run products

After the crystallization experiments using IHPV, selected SEM images of the run products are shown in Fig. 1 (SEM images of all samples are displayed in SM4). All run products contain vesicles, confirming that the samples were saturated with a fluid phase. Plagioclase morphology is rod-shaped in all samples, except at 800 °C and 50 MPa starting with PEL1500 where habits are skeletal (Fig. 1c). The size of the plagioclase crystals increases with decreasing bulk Li content, typically from below 15 × 50 µm (width by length) for initial Li content of 1500 ppm to above 20 × 100 µm for lower bulk Li contents (e.g., 1p to 1r in SM4).

The phase assemblages are reported in Table 2 and the pressure-temperature diagrams are shown in Fig. 2, in which the crystallization data obtained at 850 °C using the Li-free PEL-glass starting material of Martel (2012) (hereafter referred as to PEL0) have been added for comparison (runs performed under similar H₂O-saturation and highly oxidizing conditions, i.e. run T1500B, T1250, T1000, T750, T500, and T250 therein). All run products crystallized hematite, as expected for oxidized conditions of at least NNO +3 (as reported in Martel, 2012). Plagioclase crystallized in all products, but one at 800 °C and 150 MPa starting from PEL1500 (Fig. 2a). Clinopyroxene is present in all samples

crystallized from PEL1500 (Fig. 2a), but only appears at 800–850 °C and 75–100 MPa, and possibly at pressures $\le 50$ MPa (the crystals are too small for firm chemical identification), in samples crystallized from Li-poorer ($\le 600$ ppm) starting glasses (Fig. 2b).

### 3.3. Major element composition of the run products

The major element composition of the minerals and glasses is reported in SM1. Plagioclase anorthite molar contents (An) vary from An45 to An61 (Table 2). There is no systematic An decrease with temperature and pressure, contrary to expectation at equilibrium (e.g., Martel, 2012), reflecting that most of the plagioclases did not chemically re-equilibrate during the cooling ramp and dwell at target temperature. Consequently, plagioclases show heterogeneous compositions, ranging from the near-liquidus An50–60 composition at the time of their nucleation to more albitic compositions as re-equilibrating to target temperature (with variations up to 8 mol% An in runs with the highest cooling ramps; SM1). Hematite contains 1.6 to 6.2 wt% TiO₂. The clinopyroxene crystallized from PEL1500 is diopside (Wo45–49 En34–43, Wollastonite and Enstatite mol%, respectively, with 20–23 wt% CaO). The clinopyroxene crystallized from Li-poorer samples ($\le 600$ ppm bulk Li) is augite (Wo21–23 En59–64, with 9–10 wt% CaO). The residual liquid is rhyolitic with 77 to 81 wt% SiO₂. The SiO₂ content increases with decreasing pressure and temperature, as expected where silicate mineral crystallinity increases (Table 2). The ASI of experimental glasses is around 1.6 as the initial material.

### 3.4. Li concentration of plagioclase, melt, and fluid

The plagioclase crystals were just above the minimum size for SIMS analyses, which led to a high number of discarded analyses for which the SIMS spots were visibly impinging on glass (e.g., Fig. 1b). Plagioclase Li concentration increases from ~80–200 ppm for bulk Li content of 300 ppm, to ~100–400 ppm for bulk Li content of 600 ppm, and ~300–900 ppm for bulk Li content of 1500 ppm (Fig. 3). The Li concentration of the residual glass increases from ~300–400 ppm for bulk Li content of 300 ppm to ~400–700 ppm for bulk Li content of 600 ppm, and ~800–2000 ppm for bulk Li content of 1500 ppm (Fig. 3). Yet, among the 34 data of Li concentration, 4 are suspicious (although satisfactory visual SEM inspection of the SIMS spots), including: i) the high Li content of ~900 ppm recorded in a plagioclase that is nearly twice the amount measured in the other samples of the same series, and ii) the glass Li contents below the bulk concentrations (in the 800 °C experiments at 50 and 150 MPa from the series with 600 ppm bulk Li, and at 50 MPa from the series with 1500 ppm bulk Li). Although we suspect crystal-glass contamination for these analyses, we kept the Li concentrations in our dataset in absence of three-dimensional confirmation of contamination, but these values will be considered with caution hereafter.

According to the mass balance calculations, the Li content of the fluid phase is often negative (SM3), resulting in $K_{\text{Li}}^{\text{H2O/rhy}}$ being mostly <0.1 (Table 2), which reflects that the Li concentration in the fluid cannot be determined accurately by mass balance. Qualitatively, we speculate from these results that Li is only partitioning between crystal and melt, with no excess Li entering the fluid phase. These results, although not quantitatively robust, point to a limited Li partitioning into the fluid under our experimental conditions.

## 4. Discussion

This experimental study provides new insights into the behaviour of lithium in hydrous rhyolitic systems under shallow crustal conditions. Our results help to better understand how lithium partitions between plagioclase, melt, and fluid phases, and how this partitioning is influenced by key parameters such as bulk Li content, temperature, pressure, and melt composition. These processes are critical for interpreting Li zoning patterns in volcanic plagioclases and for using Li as a tracer of
<table><caption>**Table 2**

Li contents and partition coefficients.</caption><thead><tr><th rowspan="3">Run #<sup>a</sup></th><th rowspan="3">Starting material</th><th rowspan="3">Run products</th><th colspan="4">Plagioclase</th><th colspan="4">Rhyolitic glass</th><th colspan="2">Fluid</th><th colspan="2">Li partition coefficient<sup>f</sup></th></tr><tr><th colspan="3">wt% (mass balance)</th><th colspan="2">wt% (±sig)</th><th colspan="2">wt% (±sig)</th><th colspan="2">wt% (±0.03)</th><th colspan="2">wt% (±0.03)</th></tr><tr><th>An<sup>b</sup></th><th>mol%</th><th>sig</th><th>n</th><th>ppm</th><th>sig</th><th>n</th><th>ppm</th><th>sig</th><th>n</th><th>ppm</th><th>K</th><th>K'</th></tr></thead><tbody><tr><td></td><td>PEL1500</td><td>Gl</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td></td><td>PEL600</td><td>Gl</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td></td><td>PEL300</td><td>Gl</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>(1)875–150</td><td>PEL1500</td><td>Gl(97) + Plag(1) + Cpx(&lt;1) + Hem(2)</td><td>52.6</td><td>0.9</td><td>3</td><td>423</td><td>—</td><td>1</td><td>76.9</td><td>0.3</td><td>2</td><td>4.9</td><td>93.4(0.1)</td><td>1596</td><td>8</td><td>3</td><td>&lt;0</td></tr><tr><td></td><td>PEL600</td><td>Gl(96) + Plag(2) + Hem(2)</td><td>53.1</td><td>0.1</td><td>2</td><td>208</td><td>42</td><td>2</td><td>77.2</td><td>0.2</td><td>2</td><td>4.9</td><td>92.5(0.3)</td><td>631</td><td>11</td><td>7</td><td>0.33</td></tr><tr><td></td><td>PEL300</td><td>Gl(98) + Plag(&lt;1) + Hem(2)</td><td>53.4</td><td>2.1</td><td>2</td><td>77</td><td>—</td><td>1</td><td>77.1</td><td>0.1</td><td>2</td><td>4.9</td><td>92.3(0.3)</td><td>310</td><td>1</td><td>3</td><td>0.25</td></tr><tr><td>(2)875–100</td><td>PEL1500</td><td>Gl(87) + Plag(11) + Cpx(&lt;1) + Hem(2)</td><td>45.2</td><td>1.4</td><td>2</td><td>914</td><td>—</td><td>1</td><td>78.9</td><td>1</td><td>3.8</td><td>93.5(0.3)</td><td>1751</td><td>8</td><td>2</td><td>&lt;0</td><td>0.52</td></tr><tr><td></td><td>PEL600</td><td>Gl(84) + Plag(14) + Hem(2)</td><td>47.7</td><td>1.8</td><td>2</td><td>388</td><td>—</td><td>1</td><td>79.4</td><td>0.4</td><td>2</td><td>3.8</td><td>93.2(0.3)</td><td>677</td><td>10</td><td>2</td><td>0.57</td></tr><tr><td></td><td>PEL300</td><td>Gl(84) + Plag(14) + Hem(2)</td><td>50.2</td><td>1</td><td>181</td><td>—</td><td>1</td><td>79.0</td><td>0.1</td><td>2</td><td>3.8</td><td>94.3(0.1)</td><td>339</td><td>6</td><td>12</td><td>&lt;0</td><td>0.50</td></tr><tr><td>(4)875–50</td><td>PEL1500</td><td>Gl(96) + Plag(1) + Cpx(1) + Hem(2)</td><td>nd</td><td>—</td><td>—</td><td>550</td><td>—</td><td>1</td><td>77.4</td><td>0.1</td><td>2</td><td>2.5</td><td>95.8(0.3)</td><td>1653</td><td>14</td><td>3</td><td>0.33</td></tr><tr><td></td><td>PEL600</td><td>Gl(89) + Plag(9) + Hem(2)</td><td>60.7</td><td>1.4</td><td>6</td><td>457</td><td>2</td><td>2</td><td>79.1</td><td>1.3</td><td>3</td><td>2.5</td><td>95.9(0.9)</td><td>647</td><td>—</td><td>1</td><td>0.70</td></tr><tr><td></td><td>PEL300</td><td>Gl(89) + Plag(9) + Hem(2)</td><td>60.7</td><td>2.9</td><td>7</td><td>207</td><td>13</td><td>1</td><td>79.0</td><td>1.1</td><td>2</td><td>2.5</td><td>95.6(0.5)</td><td>372</td><td>—</td><td>1</td><td>0.53</td></tr><tr><td>(3)800–150</td><td>PEL1500</td><td>Gl(97) + Cpx(1) + Hem(2)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>77.5</td><td>—</td><td>1</td><td>5.1</td><td>90.3(0.3)</td><td>nd</td><td>—</td><td>nd</td><td>nd</td></tr><tr><td></td><td>PEL600</td><td>Gl(94) + Plag(4) + Hem(2)</td><td>47.1</td><td>—</td><td>1</td><td>168</td><td>—</td><td>1</td><td>78.0</td><td>0.4</td><td>3</td><td>5.1</td><td>91.8(0.8)</td><td>464</td><td>—</td><td>1</td><td>0.36</td></tr><tr><td></td><td>PEL300</td><td>Gl(94) + Plag(4) + Hem(2)</td><td>47.5</td><td>0.4</td><td>2</td><td>136</td><td>—</td><td>1</td><td>77.6</td><td>0.1</td><td>2</td><td>5.1</td><td>92.4(0.3)</td><td>325</td><td>—</td><td>1</td><td>0.42</td></tr><tr><td>(5)800–100</td><td>PEL1500</td><td>Gl(95) + Plag(3) + Cpx(&lt;1) + Hem(2)</td><td>nd</td><td>—</td><td>—</td><td>490</td><td>7</td><td>4</td><td>77.5</td><td>0.3</td><td>3</td><td>4.0</td><td>92.8(1.1)</td><td>1789</td><td>13</td><td>5</td><td>0.27</td></tr><tr><td></td><td>PEL600</td><td>Gl(83) + Plag(14) + Cpx(1) + Hem(2)</td><td>56.9</td><td>0.2</td><td>3</td><td>321</td><td>—</td><td>1</td><td>80.6</td><td>0.2</td><td>2</td><td>4.0</td><td>93.3(0.5)</td><td>760</td><td>5</td><td>6</td><td>0.42</td></tr><tr><td></td><td>PEL300</td><td>Gl(83) + Plag(14) + Cpx(1) + Hem(2)</td><td>58.6</td><td>1</td><td>105</td><td>—</td><td>1</td><td>80.5</td><td>0.2</td><td>2</td><td>4.0</td><td>94.1(0.6)</td><td>384</td><td>4</td><td>3</td><td>0.28</td><td>0.22</td></tr><tr><td>(6)800–50</td><td>PEL1500</td><td>Gl(80) + Plag(17) + Cpx(2) + Hem(2)</td><td>nd</td><td>—</td><td>—</td><td>320</td><td>—</td><td>1</td><td>77.6</td><td>1</td><td>2.7</td><td>93.5(0.3)</td><td>851</td><td>—</td><td>1</td><td>1000</td><td>0.38</td></tr><tr><td></td><td>PEL600</td><td>Gl(83) + Plag(15) + (Cpx?) + Hem(2)</td><td>53.8</td><td>1.7</td><td>3</td><td>113</td><td>—</td><td>1</td><td>80.2</td><td>0.6</td><td>2</td><td>2.7</td><td>95.2(0.7)</td><td>387</td><td>—</td><td>1</td><td>0.29</td><td>0.24</td></tr><tr><td></td><td>PEL300</td><td>Gl(80) + Plag(18) + Hem(2)</td><td>58.2</td><td>3.1</td><td>4</td><td>128</td><td>—</td><td>1</td><td>81.3</td><td>1</td><td>2.7</td><td>95.3(0.3)</td><td>364</td><td>—</td><td>1</td><td>&lt;0</td><td>0.34</td><td>0.27</td></tr></tbody></table>

Li contents and partition coefficients.

nd for not determined.

a Run number comprises the conditions of temperature and pressure, as given in Table 1.

b Plagioclase anorthite mole% (An) and glass SiO₂ content in wt% (SiO₂; recalculated on anhydrous basis), as reported from SM1.

c Li content in ppm measured by SIMS in plagioclase (Liplag) and glass (Lirhy), as reported in SM2; Li content calculated by mass balance in the H₂O vapour phase (LiH₂O).

d Melt H₂O content calculated at run P-T using the H₂O (Li-free) solubility model for rhyolites of Newman and Lowenstern (2002).

e EMPA raw totals of the glasses.

f Li partition coefficients, with K<sub>Li</sub><sup>plag/rhy</sup> = Li<sub>plag</sub>/Li<sub>rhy</sub> and K<sub>Li</sub><sup>H2O/rhy</sup> = Li<sub>H2O</sub>/Li<sub>rhy</sub> (see details in text).
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Phase diagrams as a function of pressure and temperature for the Li-bearing PEL rhyolite. (a) PEL1500 and (b) PEL600, PEL300, and PEL0 (at 850 °C, from Martel, 2012). Hem for hematite, Plag for plagioclase, Cpx for clinopyroxene with Dio and Aug being diopside and augite, respectively. The right Y-axis gives H₂O solubility calculated at 800 and 875 °C for the rhyolitic composition using the VolatilCalc formulation (Newman and Lowenstern, 2000). The experimental errors on pressure (±0.2 MPa) and temperature (±3 °C) are included in symbol size.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Li concentration in plagioclase and melt. Each symbol represents a run product obtained at constant pressure and temperature, for which plain and open symbols mark runs performed at 875 and 800 °C, respectively. Red, green, and blue symbols denote starting glasses containing 1528 ± 12, 616 ± 12, and 294 ± 12 ppm bulk Li (horizontal dashed lines), respectively. Point labels give the run pressure in MPa. The analytic error is of ±12 ppm Li on the SIMS measurements and is included in symbol size. Bracketed points are those for which contaminated analysis is suspected (see text). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

magma storage, ascent, and degassing.

In this section, we first discuss the influence of bulk Li content on crystallization processes and plagioclase composition (Section 4.1). We examine the controls on Li distribution between plagioclase and melt, focusing on the roles of temperature, pressure, and composition (Section 4.2). Based on these experimental constraints, we derive empirical partition coefficients for Li and evaluate their dependence on physical and chemical parameters (Section 4.3). Finally, we explore the implications of our results for understanding Li zoning in natural plagioclase and for reconstructing magmatic processes such as ascent dynamics, degassing, and post-eruptive evolution (Section 4.4).

Throughout the discussion, we compare our findings with previous experimental and natural studies (e.g., Coogan, 2011; Charlier et al.,

2012; Dohmen and Blundy, 2014; Iveson et al., 2018; Neukampf et al., 2022) to contextualize our results and highlight both consistencies and knowledge gaps that merit further investigation.

Iveson et al. (2018) mark the experiment with the most similar experimental conditions: T = 810–860 °C and P = 150–405 MPa and for a rhyodacitic system composition. The other experiments are based on more basic, hotter systems.

## 4.1. Li effect on rhyolitic melt crystallization

### 4.1.1. Phase assemblage and composition

Bulk Li concentrations have a significant effect on the crystallization sequence of PEL rhyolite in the investigated range of temperature and pressure, in particular regarding plagioclase and clinopyroxene appearance. In PEL experiment with 1500 ppm bulk Li at 150 MPa, plagioclase is absent at 800 °C and only in proportion of 1 wt% at 875 °C (Fig. 2a), suggesting that the plagioclase liquidus is likely very close to 800–875 °C and 150 MPa. In contrast, plagioclase is ubiquitous with contents >2 wt% at 800–875 °C and 150 MPa, when crystallized from PEL melts initially containing ≤600 ppm bulk Li (Fig. 2b), highlighting an extended stability field in Li-poorer melts (Fig. 2). Clinopyroxene is ubiquitous and the near-liquidus phase in the PEL1500 samples (Fig. 2a), whereas it is only stable at lower pressure and temperature in Li-poorer samples (Fig. 2b), highlighting that clinopyroxene stability is enhanced in Li-rich melts (≥ 1500 ppm Li).

Near-liquidus crystallization of An45–60 plagioclase in Li-poor melts reduces the Ca content available for clinopyroxene crystallization, leading to a late crystallization of Ca-poor clinopyroxene (augite; 21–23 mol%Wo). In contrast, near-liquidus clinopyroxene in melts with ≥1500 ppm bulk Li leads to the crystallization of Ca-rich clinopyroxene (diopside; 45–50 mol%Wo) and the plagioclase stability field is slightly reduced (to below 150 MPa at 875 °C; Fig. 2a).

Although our experiments did not directly probe melt structure, the observed phase assemblages and variations in Ca partitioning are consistent with a Li-induced depolymerization of the silicate network, as suggested by studies of alkali-bearing melts (Mysen and Richet, 2019). In such systems, Li, like other alkalis, breaks Si–O–Si linkages and increases non-bridging oxygen content, potentially influencing element solubility, activity of melt-forming components, and crystallization sequences. This mechanism, if active, has important implications for interpreting mineral assemblages and compositions in natural volcanic systems where Li enrichment can occur during magma differentiation, fluid–melt interaction, or degassing. However, given that our experiments involve only ppm to tens of ppm Li, the extent to which Li may exert such a structural effect remains uncertain. At much higher Li concentrations (in the range of thousands of ppm), such a mechanism could become more significant, but this possibility still requires targeted experiments to be confirmed. This highlights the current ‘chicken-and-egg’ problem and underlines the need for further work.

Additionally, our results obtained from the isothermal experiments (for which plagioclase composition is homogeneous) suggest that plagioclase composition is not only controlled by pressure (water content) and temperature, as reported for instance in Martel (2012), but also depends on bulk Li content, with higher bulk Li contents resulting in less calcic plagioclases (i.e., a decrease by about 5–10 mol% An with bulk Li increasing from 0 to 1500 ppm; run #1 and #2 in Fig. 4a).

### 4.1.2. H₂O-Li₂O solubility and crystallinity

Although the H₂O contents of the residual glasses have not been measured, the differences to 100 wt% of the oxide totals of the glass EMP analyses may be attributed to dissolved volatiles, albeit not directly convertible into a concentration due to missing calibration with glass standards of known volatile contents; note that only data obtained during the same EMP session were compared, and that the following observation can only be considered as qualitative. The data show that for a given Li-bearing starting material, the difference to 100 wt%
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Relationships between Li concentration and plagioclase composition. (a) Effect of the bulk Li content, with Li-free samples reported from Martel (2012) (M12). Run #1 and #2 suggest that An is controlled by bulk Li content, with slightly less calcic plagioclases crystallizing from Li-richer melts. For the other runs (#3 to #6), the overheating stage did not allow full re-equilibration of the plagioclase to the composition expected at target temperature (see text). (b) Negative correlation between plagioclase An content and plagioclase Li content, as highlighted by the dashed lines for a given bulk Li content. The inset diagram compares our dataset of 300 ppm bulk Li samples (overheating stage ≤75 °C, i.e., runs #1, #2, #3) to the data of Coogan (2011) (black dots), thus extending the trend to less calcic plagioclases and higher bulk Li contents. (c) Li partitioning between plagioclase and melt is negatively correlated to An content. In (b-c), the labels give the bulk Li concentration in ppm and the blue-white squares represent data from run (#4) assuming a plagioclase equilibrium composition equal to that in Li-free samples (~An41). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

increases with increasing pressure, in agreement with the positive pressure dependence of H₂O solubility (Fig. 5a). More interestingly, the differences to 100 wt% are systematically shifted towards higher values (by about 1.5 wt%, i.e., twice the analytical error) for the 1500 ppm bulk Li samples with respect to those of lower bulk Li contents (Fig. 5a), suggesting higher H₂O solubilities in Li-richer samples. This highlights that melt Li content ≥1500 ppm enhances H₂O solubility, as expected in a melt becoming richer in alkalis (considering Li together with Na and K; Dingwell et al., 1997). Higher H₂O solubilities also likely explain the lower crystallinities observed (SM4) and calculated (by 3–7 wt% for run #2 and #4) in the 1500 ppm Li melts with respect to those with lower Li contents (Fig. 5b). Such an increase in H₂O content in evolved granitic/pegmatitic liquids has been suggested in some systems in presence of halogens or rare-earth elements (e.g., Pichavant, 1981; Schmitt et al., 2002; Thomas and Davidson, 2012), which is here confirmed

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Volatile content of the residual glasses and crystallinity at 800 °C as a function of pressure and bulk Li content of the rhyolite. (a) Difference to 100 wt % of the EMP analyses of the residual glasses, showing higher values when using the starting material with bulk 1500 ppm Li with respect to those with ≤600 ppm Li; note that the data were obtained during the same EMP calibration session for accurate comparison; error bars are the EMP analytical error. (b) Crystallinity calculated by mass balance as reported in Table 2, showing lower crystallinities in melts with 1500 ppm bulk Li.</figcaption></figure>

experimentally for Li-bearing rhyolitic melts.

## 4.2. Li content of plagioclase and melt: effect of bulk Li concentration, pressure, temperature, and composition

### 4.2.1. Bulk Li concentration

The Li contents of plagioclase and residual melt are both positively correlated with bulk Li content (Fig. 3). The residual glasses contain as much Li as the bulk starting glasses, which may be a useful information to retrieve initial Li contents from natural glasses such as melt inclusions.

### 4.2.2. Temperature

Temperature is thought to affect Li concentration by modifying the size of the host lattice sites through thermal expansion or contraction, or by inducing structural changes of the crystal or melt (e.g., Smith and Brown, 1988). At constant bulk Li content and constant pressure of 100 MPa, the Li content of plagioclase is systematically higher by at least ~100 ppm at 875 °C with respect to 800 °C, whereas the Li content of residual glass is systematically lower by ~100 ppm at 875 °C with respect to 800 °C (Fig. 3). This highlights that temperature favours Li
entering plagioclase rather than melt. This behaviour can be explained by the influence of temperature on the elastic properties of crystal lattices. Higher temperatures induced increased lattice vibrations and expansion of the plagioclase structure, which lowers the energetic penalty for incorporating small, incompatible cations like Li⁺. This interpretation follows the crystal lattice strain model proposed by Blundy and Wood (1994), which predicts that partition coefficients for trace elements increase with temperature when size mismatch is significant.

### 4.2.3. Pressure

It should be noted that pressure is closely linked to water content, making it difficult to isolate the effect of either parameter. In the literature, however, pressure has mostly been neglected in partitioning trace elements below 500 MPa (Dohmen and Blundy, 2014; Bédard, 2023), whereas water content has been shown to have an effect (Wood and Blundy, 2002), although these studies are not specifically dealing with Li in plagioclase. Our results suggest that decreasing pressure from 150 to 50 MPa at constant bulk Li concentration and constant temperature of 875 °C, systematically leads to an increase or maintaining Li content of the plagioclase and the residual glass (except for the suspicious 1500 ppm bulk Li sample at 100 MPa). This tendency is not systematically observed at 800 °C, but it should be noted that for this temperature, Li analysis is suspicious for three of the eight samples considered (Fig. 3).

### 4.2.4. Composition

The observed Li-dependence of plagioclase on temperature and pressure (water content) may, as suggested by Coogan (2011) and Dohmen and Blundy (2014), also reflect an inherent Li-dependence on plagioclase crystal chemistry in composition range from An45 to An61. These authors attributed this effect to charge balance, with the replacement of Na⁺ by Li⁺ being energetically favorable over the replacement of Ca²⁺ by Li⁺. We observe similar trends of Li in plagioclase, i.e., inversely correlated to An content (Fig. 4b, considering run #1 and #2 without overheating stages), although these results should be taken with caution because our experiments were not designed adequately to isolate the effect of plagioclase composition.

As demonstrated for the Li-dependence on plagioclase composition, melt Li content may be influenced by melt chemistry, as demonstrated experimentally (Watson, 1976; Ryerson and Hess, 1978), although further investigation is required to better characterize this effect (Blundy and Wood, 2003).

This trend reflects the crystal-chemical constraints on Li incorporation into feldspar structures. Lithium, as a small monovalent cation (Li⁺), is most likely to substitute for Na⁺ in the A-site of the feldspar structure, rather than for the larger and more highly charged Ca²⁺. In sodic-rich plagioclase, the structural environment is thus more favorable for Li incorporation, especially at elevated temperatures where the site is more flexible. This substitution mechanism is consistent with the models of Dohmen and Blundy (2014) and with the lattice strain theory of Blundy and Wood (1994), which both predict enhanced compatibility for small cations in structurally adapted sites.

Our experimental data agree well with these observations and further confirm that Li partitioning is not constant but depends on the evolving composition of the crystallizing plagioclase. This has important implications for interpreting natural Li zonation profiles, especially in feldspars exhibiting normal or oscillatory zoning in An content.

## 4.3. Plagioclase-melt and fluid-melt Li partitioning: Influence of bulk Li concentration, pressure, temperature, and composition

### 4.3.1. Bulk Li concentration

$K_{Li}^{plag/rhy}$ is independent of Li concentration of the system at constant pressure and temperature, as shown by the relatively flat best-fit curves in Fig. 6a-b (except for the series at 875 °C and 50 MPa in Fig. 6a). This behaviour highlights that, up to bulk concentration of 1500 ppm, Li

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Influence of several parameters on plagioclase-rhyolite Li partition coefficient: (a-b) bulk Li concentration at 875 and 800 °C, respectively; Labels give the plagioclase anorthite content and the curves are best fits, (c) temperature, by averaging bulk Li concentrations, and (d) pressure, by averaging bulk Li concentrations (excepting the value at 1500 ppm, 875 °C, 50 MPa, that does not follow a constant trend with Li concentration). Red, green, and blue are data at pressure of 150, 100, and 50 MPa, respectively, and plain and open symbols are for data at 875 and 800 °C, respectively. The bracketed points in (a-b) are those whose Li analysis is suspected of contamination, as depicted in Fig. 3. Propagated errors on $K_{Li}^{plag/rhy}$ are ±0.06. In (c-d), the dashed fields contour $K_{Li}^{plag/melt}$ determined from natural plagioclase and dacitic to rhyolitic glasses; the plain field refers to $K_{Li}^{plag/rhy}$ determined experimentally; I18: Iveson et al. (2018), C13: Cabato et al. (2013), S09: Smith et al. (2009), B11: Brophy et al. (2011), C12: Charlier et al. (2012), N19: Neukampf et al. (2019), PG16: Padilla and Gualda (2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

obeys Henry's law (concentration-independent partition coefficient), as also reported by Bindeman and Davis (2000) and Iveson et al. (2018) in the system more felsic. For the series at 875 °C and 50 MPa, however, $K_{Li}^{plag/rhy}$ for 1500 ppm bulk Li samples is significantly lower (by about 0.2) than those containing 300 and 600 ppm bulk Li, which could suggest the onset of a non-Henrian behaviour at Li content between 600 and 1500 ppm. Yet, the lack of additional data and the overall small range of the $K_{Li}^{plag/rhy}$ (within less than 1 log unit) both call for further investigation to confirm this behaviour.

Moreover, substitution mechanism has been reported to play an important role for trace element partitioning and for the onset of Henry's law (e.g., Prowatke and Klemme, 2006). By considering Na-Li substitution (as defined above), $K_{Li}^{plag/rhy}$ ranges from 0.19 to 0.55, i.e., lower by less than 0.10 compared to $K_{Li}^{plag/rhy}$ (Table 2), so that any effect of such a substitution mechanism is likely to be negligible within the range of our experimental conditions.
### 4.3.2. Temperature

$K_{Li}^{\text{plag/rhy}}$, averaged across the three bulk Li concentrations that yield constant values (Fig. 6a-b), increase with temperature at pressures of 50 and 100 MPa, but remain constant at 150 MPa (Fig. 6c). This indicates that, at pressure below 150 MPa, Li would preferentially concentrate in melt as magma cools, rather than in plagioclase. In experiments carried out under warmer conditions (Bindeman et al., 1998; Dohmen and Blundy, 2014), Li does not appear any more compatible with a partition coefficient around 0.2.

### 4.3.3. Pressure

Evaluating the influence of pressure on $K_{Li}^{\text{plag/rhy}}$ (averaged across the three bulk Li concentrations) at constant temperature shows that $K_{Li}^{\text{plag/rhy}}$ decrease with increasing pressure (or water content) at 875 °C, while remaining constant at 800 °C (Fig. 6d). This suggests that during isothermal decompression at 875 °C, Li would preferentially concentrate into plagioclase rather than melt, whereas decompression at lower temperatures has no effect.

### 4.3.4. Composition

In line with Li concentrating in albitic plagioclase (Fig. 4b), $K_{Li}^{\text{plag/rhy}}$ shows an inverse correlation with plagioclase An content, decreasing by about 0.2 for every 5 mol% increase in An (Fig. 4c). Other experimental studies on Li partition coefficients in plagioclases did not systematically show a correlation between Kd and the major element composition of plagioclases (XAn) (Blundy, 1997; Bindeman et al., 1998; Bindeman and Davis, 2000; Aigner-Torres et al., 2007). Melt chemistry also plays a role: at 875 °C, $K_{Li}^{\text{plag/rhy}}$ is lower (0.2–0.3) for glasses with 77 wt% SiO₂ compared to those with 79 wt% SiO₂ (0.5–0.6; Fig. 6a), although no significant difference is observed at 800 °C (Fig. 6b). At 875 °C, this variation could provide an alternative explanation for the apparent onset of non-Henrian behaviour mentioned earlier, as the bulk Li-richest sample has a glass with 77 wt% SiO₂ whereas samples with lower bulk Li concentrations have 79 wt% SiO₂. However, this highlights the need for a more refined treatment of melt chemistry effect on Li partitioning. The impact of melt chemistry is also suggested by observing the variations in Kd (0.138 vs. 0.296) between experiments carried out on synthetic composition systems or on natural bulk composition for identical conditions of temperature (1300 °C) and palgioclase composition (An70) (Bindeman et al., 1998; Bindeman and Davis, 2000).

### 4.3.5. Fluid-melt partitioning

Although the calculation of Li in the fluid phase by mass balance may be too speculative to draw conclusions on fluid-melt Li partitioning, the low calculated $K_{Li}^{\text{H2O/Rhy}} < 0.1$ would agree with the ones reported from experiments using Li-rich rhyolites in aqueous fluids (i.e., 0.3–0.4, London et al., 1988; 0.2–0.9, Gion et al., 2022; 0.2, Iveson et al., 2019; see also the compilation by Dupont de Dinechin et al., 2023). Despite the lack of robust quantitative values, all mass balance results converge on the same qualitative outcome: under our experimental conditions (800–875 °C, 50–150 MPa, oxidizing, H₂O-saturated, Cl-free), Li does not partition significantly into the fluid phase. Yet, such a Li-incompatible behaviour is in contradiction with the $K_{Li}^{\text{fluid/melt}}$ of 1–50 determined from natural fluid inclusions (Zajacz et al., 2008) and $K_{Li}^{\text{fluid/trachyte}}$ of 60–135 measured from gas condensates (Vlastélic et al., 2011). Such $K_{Li}^{\text{fluid/melt}}$ differences between experiments and natural samples may be explained by (i) post-entrapment chemical disequilibria of the fluid and melt inclusions, (ii) complex compositions of natural fluids (containing halogens, carbonates, sulfurs, etc) that differs from simple aqueous fluids; for instance, Cl-bearing fluids have been demonstrated to significantly increase $K_{Li}^{\text{fluid/melt}}$ (Webster et al., 1989; Iveson et al., 2019), and (iii) low pressure-temperature conditions that increase $K_{Li}^{\text{fluid/melt}}$, as demonstrated by Webster et al. (1989) who reported $K_{Li}^{\text{H2O/Topaz/Rhy}}$ up to 13 at ~770 °C and ~50 MPa using a topaz-rhyolitic melt. In Charlier et al. (2012), the presence of brine, a Cl-rich fluid, mobilizes Li and concentrates it, reinforcing the idea that a ligant is needed to

mobilize Li. This finding supports the idea that Li partitioning into fluid strongly depends on fluid composition (e.g., presence of halogens) and pressure-temperature conditions. We thus propose that, in evolved, H₂O-rich but halogen-poor magmas under oxidizing shallow-crustal conditions, Li remains predominantly in the melt phase. This low affinity for the fluid should be considered when interpreting Li distributions in natural systems. However, we caution that our conclusion is qualitative and valid only under the specific experimental conditions used in this study.

### 4.4. Application to eruptive scenarios and recommendation for the interpretation of Li gradients in natural plagioclases

The $K_{Li}^{\text{plag/rhy}}$ determined experimentally at 800–875 °C and 50–150 MPa range from 0.2 to 0.6, in agreement with the 0.1–0.3 values determined experimentally from plagioclase in a H₂O-saturated rhyodacite at 810–850 °C and 150–350 MPa (Iveson et al., 2018), and with the range of 0.1–0.7 reported in the literature for coexisting plagioclase and dacitic-rhyolitic melt in natural samples (Dupont de Dinechin et al., 2023, and references therein). In contrast to the experimental data, $K_{Li}^{\text{plag/melt}}$ from natural samples does not correlate with temperature (Fig. 6c) or pressure (Fig. 6d). However, as noted earlier, Li contents in natural samples may reflect chemical disequilibrium, post-eruptive processes, and inaccuracies in determining the pressure-temperature conditions of magma crystallization. The experimental determination of fluid-melt-plagioclase Li partitioning could aid in tracing and predicting the fate of Li during eruptive processes such as magma ascent, degassing, and cooling. This would assist in interpreting the Li contents and gradients observed in crystals and glasses during magma evolution. Fig. 7 outlines two primary eruptive scenarios where the results of our study may be useful for predicting Li gradients in plagioclase, and alternative processes that could generate Li gradients in plagioclases are also discussed.

#### 4.4.1. Cooling

During magma cooling from 875 to 800 °C, $K_{Li}^{\text{plag/rhy}}$ decreases by about 0.2–0.3, which is better marked at pressure $\le$ 100 MPa (Fig. 6c). This will result in Li depletion from plagioclase core to rim (recording concentrations from 875 to 800 °C, respectively) by about few tens to hundreds of ppm, whereas melt enriches by about a hundred of ppm (Fig. 3). Yet, if cooling is sufficiently long for plagioclase crystallization or re-equilibration in major elements (> 10 days according to our experimental data), the newly-formed plagioclase will be more albitic (e.g., Martel, 2012) and thus richer in Li (by several hundreds of ppm from An55 to An45; Fig. 4b and Coogan, 2011). According to this scenario, the plagioclase core-to-rim Li-depleting effect of cooling (Fig. 7b) could be compensated by a Li-enrichment in crystallizing An-poorer plagioclases (Fig. 7c).

#### 4.4.2. Magma ascent

Decompression of a volatile-saturated melt induces degassing, which can occur under either closed-system conditions (where the gas phase remains in contact with the melt) or open-system conditions (where exsolved volatiles escape from the system). Our phase-equilibrium experiments were performed in the presence of a slightly oversaturated aqueous fluid phase, and are best interpreted as simulating stepwise closed-system degassing, where the excess fluid is removed at each pressure step.

At 875 °C, we observe that $K_{Li}^{\text{plag/melt}}$ increases with decreasing pressure (or decreasing H₂O content; Fig. 6d). This trend predicts an enrichment of Li in plagioclase rims during open-system degassing induced by magma ascent (Fig. 7a). In contrast, at 800 °C, Li concentrations in both plagioclase and melt appear unaffected by pressure changes (Fig. 6d), suggesting little to no Li redistribution under cooler ascent scenarios. Concurrently, isothermal decompression of H₂O-saturated melts leads to increasingly sodic (albitic) plagioclase, as
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Li gradients in plagioclase predicted from plagioclase-melt Li partition coefficients ($K_{Li}^{plag/rhy}$) for different magmatic scenarios involving a H₂O-saturated Li-doped rhyolitic melt: (a) isothermal decompression or melt H₂O decrease, creating Li-enriched rims, (b) isobaric cooling, creating Li-depleted rims, (c) inverse correlation between plagioclase Li concentration and An content, suggesting that the crystallization of An-poorer plagioclase, by cooling or decompression, would result in higher Li concentration in plagioclase, and (d) the decrease of $K_{Li}^{fluid/melt}$ anticipated in presence of a brine at low pressure, which forces Li to re-enter melt and plagioclase (Charlier et al., 2012).</figcaption></figure>

documented in previous studies (e.g., Martel, 2012). Since sodic compositions favor Li incorporation (Coogan, 2011), decompression could further enhance Li uptake into the plagioclase structure. Therefore, decompression-induced open-system degassing would likely result in Li enrichment of plagioclase, whether at rims or in newly formed crystals, by about 100–200 ppm for decompression from 150 to 50 MPa (i.e., ~30 % to ~100 % relative enrichment), whereas the residual melt would record a lower Li enrichment by less than about 100 ppm (up to ~30 % relative; Fig. 3). However, these interpretations are based on experiments conducted with a pure H₂O-rich fluid, lacking volatile li-gands such as Cl, which are commonly present in natural magmatic systems. Chlorine has been shown to complex with Li in the fluid phase, thereby promoting its extraction from the melt (Charlier et al., 2012). The absence of Cl in our experiments likely prevented Li loss to the fluid, resulting in an overestimate of Li retention during degassing.

This discrepancy could explain the contrast with natural observations, which frequently show Li depletion in both plagioclase and melt during magma ascent and open-system degassing (Giuffrida et al., 2018; Neukampf et al., 2022). Thus, while our results are internally consistent, they may not fully capture the fluid–melt–crystal dynamics of volatile-rich natural systems, especially in Cl-bearing conditions.

#### 4.4.3. Alternative scenarios

Two main processes likely to create Li gradients in plagioclase are the

presence of a brine and post-eruptive processes, as discussed below.

**Brines.** The fluid composition, in particular halogen and Cl concentrations, may explain the Li concentration in the phases and in the fluid in particular (section 4.3.5). The simulated scenario concerns a magma in equilibrium with a NaCl-bearing aqueous phase. At high pressure (> ~150 MPa), the aqueous phase is in the state of a supercritical fluid. At lower pressure, this supercritical phase crosses a two-fluid solvus, leading to phase separation into a supercritical vapour and a subcritical hydrosaline liquid commonly referred to as a brine (Veksler, 2004). The brine becomes more saline with decompression and eventually precipitates halite in equilibrium with a Cl-enriched Li-bearing gas phase at pressures <22 MPa (Chou, 1987). Charlier et al. (2012) proposed that the breakdown of the liquid–vapour equilibrium at the critical pressure of 22 MPa causes changes in the melt-chloride and chloride-associated components (such as LiCl), overall leading to a decrease in $K_{Li}^{fluid/melt}$ that forces Li to partition into the melt. Based on this scenario, Charlier et al. (2012) interpreted rim-to-core Li-enrichment of plagioclases as reflecting syn-eruptive processes by which the Li from the brine is re-entering melt and its crystal cargo at low pressure (Fig. 7d). In retrospect, the observation of such Li-inward gradients in plagioclases could attest for the presence of a brine.

**Post-eruptive processes.** In volcanic rocks, Li has been identified to be highly mobile long after eruption and readily diffuses into phenocrysts as groundmass crystallization increases the Li concentration of the residual melt (Ellis et al., 2018). This interpretation is supported by our findings that the Li content of plagioclase increases with increasing Li content of the melt (Fig. 3).

### 5. Concluding remarks

* There is a marked effect of Li on rhyolitic systems at high bulk concentrations of about 1500 ppm, on crystalline phase stability (extended clinopyroxene stability), on phase composition (Ca-rich versus Ca-poor clinopyroxene), and on H₂O solubility (higher solubilities than expected in Li-free melts).

* The Li contents of plagioclase and melt both increase with bulk Li content; the Li in melt approximating bulk Li content.

* $K_{Li}^{plag/rhy}$ are mostly constant with bulk Li concentration, obeying Henry's law

* $K_{Li}^{plag/rhy}$ decrease with temperature at low pressures (50–100 MPa), suggesting a preferential Li partitioning into the melt upon magma cooling, whereas temperature-independent at higher pressure of 150 MPa.

* $K_{Li}^{plag/rhy}$ decrease with pressure at 875 °C, whereas remaining constant at 800 °C.

* Li does not partition significantly into the fluid in presence of a H₂O fluid phase.

* The experimental $K_{Li}^{plag/rhy}$ cover the range of $K_{Li}^{plag/rhy}$ reported in the literature for natural samples; Li outward gradients (enriched rims) in natural plagioclases may result from degassing during isothermal decompression, whereas Li inward gradients (depleted rims) may reflect magma cooling, the presence of brines, or post-eruptive groundmass crystallization.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2025.123047.

### CRediT authorship contribution statement

**Maylis Dupont De Dinechin:** Writing – original draft, Formal analysis, Data curation, Conceptualization. **Caroline Martel:** Writing – review & editing, Supervision, Conceptualization. **Hélène Balcone-Boissard:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization. **Monika K. Rusiecka:** Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. **Rémi Champallier:** Writing – review & editing, Methodology,
Conceptualization. Etienne Deloule: Writing – review & editing, Methodology, Data curation.

## Funding

This work was supported by the French National Research Agency (ANR) as part of the LabEx VOLTAIRE project (LABX-100-01) and the ANR 'V-CARE' project (ANR-18-03-0010CE; G Boudon), the INSU LiVolc, and the French Ministry of Higher Education and Research and Innovation for the doctoral grant of M. Dupont de Dinechin.

## Declaration of competing interest

Maylis Dupont de Dinechin reports financial support was provided by French National Research Agency. Maylis Dupont de Dinechin reports financial support was provided by French Government Ministry of National Education. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

We would like to thank M. Pichavant, B. Dubacq, F. Costa, and A. Brunelle for helpful discussions. We extend our gratitude to I. di Carlo and S. Erdmann for technical assistance with the EMP instruments from the facilities of the MACLE-CVL platform co-funded by the European Union and Centre-Val de Loire Region (FEDER). We would also like to thank O. Boudouma for his invaluable help with the SEM on CCM platform, N. Rividi for the EMP analyses at the CAMPARIS platform, and all the staff at the LG-SIMS-Nancy platform for the Li analyses. We are also grateful to two anonymous reviewers for their constructive comments and advice that significantly improved the manuscript, and to the associate editor, C. Romano, for her valuable handling of the manuscript.

## Data availability

No data was used for the research described in the article.

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