# Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

Romain Sauvalle, Bruno Scaillet, Gaëlle Prouteau, Stéphane Scaillet, Raffaello Cioni, Zara Franceschini, Diletta Frascerra, Federico Sani, Giacomo Corti, Abate Melaku, et al.

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Romain Sauvalle, Bruno Scaillet, Gaëlle Prouteau, Stéphane Scaillet, Raffaello Cioni, et al. Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites. Journal of Petrology, 2024, 65 (11), 10.1093/petrology/egae112. insu-04827491

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Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

<table><tr><td>Journal:</td><td>Journal of Petrology</td></tr><tr><td>Manuscript ID</td><td>JPET-Jun-24-0109.R1</td></tr><tr><td>Manuscript Type:</td><td>Original Manuscript</td></tr><tr><td>Date Submitted by the Author:</td><td>n/a</td></tr><tr><td>Complete List of Authors:</td><td>Sauvalle, Romain; Institut des Sciences de la Terre d'Orleans, Scaillet, Bruno; CNRS/ISTO, PROUTEAU, Gaëlle; Institut des Sciences de la Terre d'Orléans, ISTO - UMR 6113 CNRS / UO, Scaillet, Stéphane; ISTO Cioni, Raffaello; Dipartimento di Scienze della Terra Franceschini, Zara; Dipartimento di Scienze della Terra; ISTO Frascerra, Diletta; ISTO Sani, Federico; Dipartimento di Scienze della Terra Corti, Giacomo; Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse Melaku, Abate; Addis Ababa University College of Natural Sciences, School of Earth Sciences; University of St Andrews, School of Earth and Environmental Sciences Andújar, Joan; ISTO- CNRS,</td></tr><tr><td>Keyword:</td><td>basalt, experimental petrology, Main Ethiopian Rift, Peralkaline, Phase equilibria</td></tr><tr><td>Journal of Petrology now offers Virtual Collections of published papers. You may choose up to three collections from the list below. Virtual collections will increase the visibility of your work.:</td><td>Intracontinental Rifts &lt; Province Themes, Experimental Petrology</td></tr></table>
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Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

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Romain Sauvalle¹*, Bruno Scaillet¹, Gaëlle Prouteau¹, Stéphane Scaillet¹, Raffaello Cioni², Zara Franceschini¹,², Diletta Frascerra¹, Federico Sani², Giacomo Corti³, Abate A. Melaku⁴,⁵, and Joan Andujar¹

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¹ Institut des Sciences de la Terre d'Orléans, UMR 7327 INSU-CNRS-BRGM – Université d'Orléans (France).

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² Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Florence (Italy).

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³Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Florence (Italy).

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⁴School of Earth Sciences, Addis Ababa University, Addis Ababa (Ethiopia).

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⁵School of Earth and Environmental Sciences, University of St Andrews, St Andrews, UK.

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*Corresponding author. Telephone (+33) 6 47 42 84 38.

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E-mail: [romain.sauvalle@etu.univ-orleans.fr](mailto:romain.sauvalle@etu.univ-orleans.fr)

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ABSTRACT

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Bimodal magmatism is characteristic of the geodynamic evolution of the Main Ethiopian Rift
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(MER), which is a reference area for the study of the processes leading to continental break-up
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before seafloor spreading. There are abundant emissions of basalts and rhyolites, which are in
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possible parent-daugther relationships. However, the P-T-H₂O conditions of production and
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storage of the basaltic end member remain unclear. Crystallization experiments have been
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conducted on an alkali basalt from the MER to define its pre-eruptive conditions and shed light
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on the compositional evolution of derivative liquids and source conditions. The experiments
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were performed at 100-200 MPa, 975-1080°C, varying H₂O/CO₂ ratios, corresponding to melt
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water contents of 1-5 wt%, at fO₂ slightly lower than the Fayalite-Magnetite-Quartz (FMQ)
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solid buffer. Comparison between the petrological attributes of the starting rock and the
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experiments shows that the basaltic magma was stored at 150-200 MPa, 1050 ± 10°C, with 1-
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2 wt% H₂O in melt, with fO₂ near FMQ prior to eruption. Geochemical modelling shows that
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the corresponding mantle source contained about 0.1 wt% H₂O, reflecting a metasomatized
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source. Extensive crystallization of such basalts produces SiO₂-rich liquids, which are not yet
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peralkaline, however. This underscores that extreme fractionation (>90 wt%) is required in
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order to produce peralkaline derivatives from mildly alkaline basalts. This extreme
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fractionation and the water-rich nature of the starting basalt readily explain the H₂O-rich
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condition of peralkaline rhyolites that have fueled caldera forming eruptions in the Rift.
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**Keywords**: Basalt; Experimental petrology; Main Ethiopian Rift; Peralkaline; Phase equilibria

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73 INTRODUCTION

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75 Peralkaline magmatic compositions, defined as rocks with a peralkalinity index >1
76 ((Na₂O+K₂O)/Al₂O₃, PI, molar %, Shand, 1927), are an important magmatic component of
77 extensional tectonic settings.

78 In Ethiopia, active rifting occurs along the East African Rift System (EARS), providing
79 great opportunities to study the link between magmatism and continental break-up. This study
80 focuses on the central segment of the Central Ethiopian Rift (CMER), where rifting maturity
81 is intermediate (Corti, 2009), the crust remaining thick (30-40 km) and is extensively filled by
82 mafic cumulates (Hutchison et al., 2018).

83 The recent volcanism in the area is mainly represented by voluminous peralkaline silicic
84 ignimbrites (comendites, pantellerites). These large volumes of ignimbrites attest to the huge
85 paroxysmic eruptions that occurred during the Pliocene and into recent times in Ethiopia
86 (Munesa Crystal Tuff, >800 km³, Woldegabriel et al., 1990; Franceschini, 2022). The halogen
87 and sulfur-rich nature of the peralkaline silicic compositions (up to 3 wt% F and 1 wt% Cl in
88 the Kenyan Rift, Scaillet & Macdonald, 2003) makes paroxysmal caldera-forming eruptions a
89 multi-faceted hazard, with direct risks associated with the huge volume of pyroclastic flow
90 and strong climatic effects (Ayalew et al., 2002; Macdonald & Scaillet, 2006). The volatile
91 and alkali-rich nature of peralkaline silicic melt also give rise to peculiar rheological
92 properties (peralkaline rhyolites are 100 to 1000 times less viscous than their metaluminous
93 counterparts, Di Genova et al., 2013). These properties promote extreme fractionation of
94 magmas by an effective crystal-melt segregation. This process leads to strong enrichments in
95 incompatible elements (HFSE, REE), making peralkaline magmas associated with ancient rift
96 zones some of the largest REE deposits in the world (Goodenough et al., 2016)

97 The mode of formation of these large volumes of highly differentiated rhyolites remains
98 unclear. Most authors invoke prolonged fractional crystallization (FC) of mildly alkaline
99 basalts as the main driving process for the genesis of peralkaline silicic melts (e.g., Civetta et
100 al, 1998). However, the occurrence of a compositional gap between basaltic and trachytic
101 compositions (Daly's Gap, Daly, 1925) contrasts with the compositional continuity expected
102 from FC. This gap has given rise to different petrogenetical models of peralkaline rhyolites, in
103 particular via partial melting of previous intrusives, or by assimilation of a crustal component
104 (e.g., Davies & Macdonald, 1987; Lowenstern & Mahood, 1991).

105 The recent mafic volcanism in the CMER is comparatively less studied than the felsic
106 endmember because it represents only a small fraction of the volcanic deposits, yet one that is
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a key component in the genesis of the observed peralkaline silicic compositions independently of the model used. Constraining this mafic volcanism is mandatory for understanding the magmatic activity and its evolution during rifting on a global scale.

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This study focuses on a young alkaline mafic sample from the Bofa stratoid series (Rooney, 2020), which is considered a plausible source for recent silicic peralkaline compositions. Equilibrium crystallization experiments were performed on this composition in an internally heated pressure vessel. The experiments address phase relationships and the compositional evolution of the main phases, including glass, at varying temperature and water activity, at a low pressure, and under reduced redox conditions. These allow to infer the storage conditions in terms of pressure, temperature, and water content of a typical mildly alkaline basalt of the MER, providing quantitative insights into the genesis of peralkaline rhyolites in a typical continental rifting setting.

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## GEOLOGICAL OUTLINE

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The Main Ethiopian Rift (MER, Fig. 1) is part of the East-African Rift System (EARS). It is bounded by the Afar Depression (north) and the Kenyan Rift (south). Magmatism associated with the EARS began about 45 Myr ago in the Eocene (e.g., Rooney, 2017). Rift opening was asynchronous (starting in the north, then propagating southward). This feature imparts remarkable compositional differences between magmas produced in rift segments of different maturity. During the Quaternary, in the north, around the Erta Ale segment, the continental crust has been strongly stretched and thinned (~16 km) and erupted basic magmas are close to tholeiitic compositions. The basalts of the less mature rift segments further south displaying a much thicker crust (30-40 km, e.g., Hutchison et al., 2018), such as the MER, tend to have transitional to mildly alkaline compositions.

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Two major magmatic stages are associated with the MER: a pre- and a syn-rift stage. The pre-rift stage, during the Oligocene, corresponds to the eruption of voluminous magmatic series, leading to thick basaltic plateaus, covering more than 600 000 km². The onset of rifting in the MER began only between 18 and 15 Ma ago (Wolfenden et al., 2004). The lavas are either basalts (mildly alkaline to transitional) or highly differentiated rhyolites (comendite, pantellerite), intermediate volcanic products being almost absent (leading to the so-called Daly compositional gap, Daly, 1925).

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From a tectonic perspective, two main types of faulting can be identified in the MER. The first, termed boundary faults (rift margins), has a N30-N40°E trend. The second one, termed
the Wonji fault belt (Corti, 2009), is characterized by a N12-N20°E trend with a left-lateral *en echelon* component, and is localised on the rift floor (Boccaletti et al., 1998). Young fissural mafic volcanism (1.6-0.1 Ma, Brotzu et al., 1986) and central volcanoes erupting peralkaline silicic products are associated with Wonji faults. Quaternary deformation in the MER is mainly localized on this *en-echelon* Wonji fault belt system, while the rifting style that operated during the Miocene-Pliocene is characterized by boundary faults developing asymmetric basins with diffuse volcanism along the rift (Corti et al., 2020).

Experiments were performed on an alkaline basalt (ETZ-13) collected in the central segment of the MER (eastern margin), from a phaneritic lava flow along the Golja Fault (Fig. 1) in Ethiopia in 2019 (Franceschini, 2022). Whole rock <sup>39</sup>Ar/<sup>40</sup>Ar dating performed on this sample (Franceschini, 2022) yields an age at 1.966 ± 0.674 Ma (see Supplementary Information, SI). This basaltic flow is part of the "Bofa Stratoid Series" defined by Rooney (2020) and is associated with an extensive mafic magmatic episode in the region at 1.6 ± 0.5 Ma, which occurred during the shift from boundary faults towards Wonji fault belt. These mafic volcanics, which cover the rift floor, may have been the parental magma of the more recent peralkaline felsic extrusives emitted during caldera forming events (Hutchison et al., 2016; Rooney, 2020).

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## 159 ANALYTICAL TECHNIQUES AND STARTING MATERIAL

### 160 Analytical techniques

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Experimental charges and the natural rock used as starting product were studied by SEM-EDS (Merlin Compact Zeiss) at ISTO. SEM-EDS study was necessary to conduct preliminary phase identification and textural analyses of the run products. The accelerating voltage was set at 15 kV and the working distance at 10 mm.

Analyses for major and volatile elements of the natural rocks and experimental charges were performed by electron microprobe analysis (EMPA, CAMECA SX-Five, ISTO) using an acceleration voltage of 15 kV and a sample current of 6 nA. Na and K were analyzed first and a ZAF correction procedure was applied. Na migration under the electron beam occurred mainly in hydrous compositions. For each analytical session, correction factors for Na migration were calculated from the analysis of hydrous glass standards of known water and alkali contents.

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Trace-element composition of bulk ETZ-13 is from Franceschini (2022) and were obtained by ICP-MS iCapQ at the CRPG (Nancy, France). The in-situ analysis of different mineral phases from ETZ-13 was performed by LA-ICP-MS, using an Agilent 8900 QQQ equipped with a RESOlution ArF 193 nm laser (ISTO, Orleans). The beam frequency was set at 10 Hz and fluency at 4 J/cm². Spot size was set at 30 µm for both the mineral targets and the glass standards. The NIST-610 (Jochum et al., 2010) was used as the reference material for this study. The NIST-612 glass standard (Jochum et al., 2010) and BCR-2G basaltic glass (Jochum et al., 2010) were used as secondary reference material to control the precision and accuracy of the analyses performed on ETZ-13. The time-resolved data was reduced using GLITTER!™ software (Achterbergh et al., 2000), with Si as an internal standard. Analyses performed on the secondary glass standards can be found in the supplementary data.

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## Starting material

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ETZ-13 contains phenocrysts of plagioclase feldspar (Pl), magnetite (Mt), ilmenite (Ilm), clinopyroxene (Cpx), and iddingsite (altered olivine) with fresh olivine (Ol) cores. The groundmass consists of Pl microlites, Mt, Ilm, apatite (Ap) and devitrified glass (Gl). Mass balance calculations and image analyses (ImageJ software) lead to a calculated phenocryst content of ~16 wt% with around 6 wt% Ol, 4 wt% Pl; 3 wt% Cpx, 2 wt% Mt and 1 wt% Ilm.

Analyses performed on the mineral phases and the bulk rock composition for both major and trace elements are presented in Table 1 and 2.

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## Phase compositions

Glass: The groundmass is largely devitrified, with abundant microlites which likely crystallized during the cooling of the lava flows or within the conduit during magma uprise. Whenever possible, analyses of large glassy zones (>200 µm) provide average values for the glass composition with peculiar care taken to avoid phenocrysts or bubbles. The low standard deviation of the analyses (Table 1) is taken as evidence for representative glass compositions. These measured glass compositions are close to the whole rock composition, reflecting the low crystallinity of ETZ-13.

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**Olivine:** Most Ol are iddingsitized with however, fresh cores preserved in some crystals (Fig. 2). These are thought to have formed in the magma chamber and are used to constrain the pre-eruptive conditions. Fresh cores of Ol from ETZ-13 have a mean Fo content (calculated as Fo%=(Mg/(Mg+Fe+Mn))*100, molar) of 76.9 ± 1.0 (Table 1) and appear to be compositionally unzoned. The Ol are Ni-rich (mean 735 ppm) in ETZ-13 (Table 2) compared to the bulk rock (28.6 ppm).

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**Plagioclase:** Pl crystals are relatively large (sometimes > 1 mm long) compared to other phases. Major element mapping reveals that the crystals are zoned: the Pl cores of the ETZ-13 are strongly calcic (Table 1, Fig. 2), with An<sub>86.0±3.9</sub> (An%=(Ca/(Ca+Na+K))*100, molar), transitioning abruptly toward a thin less calcic rim at An<sub>77.4±2.3</sub> (Fig. 2). These rims are assumed to be in equilibrium with the melt prior to the eruption and were used to assess pre-eruptive conditions. In terms of trace element concentrations (Table 2), the thin rims have a much lower Sr and Ba content than the large calcic cores.

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**Oxides:** ETZ-13 contains Ilm and Mag with exsolution features and special care was taken to analyze oxides that appeared homogenous. Overall, oxides are the least abundant phenocrysts compared to Ol or Pl. Their compositions are shown in Table 1.

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**Clinopyroxene:** ETZ-13 contains Cpx in lesser amounts than either Ol or Pl. These Cpx are augites based on Morimoto (1989). They have CaO contents up to 20 wt% and a mean composition of En<sub>42.8</sub>Fs<sub>10.2</sub>Wo<sub>47.0</sub> (Table 1). In terms of trace elements, Cpx have high V abundances (>300 ppm, Table 2).

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**Apatite:** It is found only as microlites. A representative analysis of Ap is shown in Table 1. It contains fluorine (up to 6 wt%) and lesser amount of chlorine (<0.20 wt%).

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## Oxythermometry

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We used geothermometers as a first tool to assess the pre-eruptive conditions of the alkaline basalt used. Ilm-Mt oxythermometry (Ghiorso & Evans, 2008), Ol-liquid thermometry (Putirka, 2008) and Cpx thermometry (Wang et al., 2021) were used. These geothermometers are based on equilibrium constants. Therefore, for the calculated temperatures (and redox conditions) to be relevant, it is necessary to check for equilibrium between the considered phases. The Mt-Ilm pair passed the equilibrium test of Bacon & Hirschmann (1988). Regarding Ol, the Fe<sup>2+</sup>-Mg partition coefficients between olivine and glass are often used as a test for equilibrium (e.g., Sack, 1980; Ulmer, 1989). Because ETZ-13 is poorly crystallized, we use whole-rock ETZ-13 analyses (Table 1) rather than that of the devitrified groundmass. The Fe<sup>2+</sup> content of ETZ-13 was calculated following the method of
Kress and Carmichael (1991), with the temperature set at 1050°C and the fO₂ at FMQ. All iron in the olivine was assumed to be Fe²⁺. This method returns a mean K<sub>dFe-Mg</sub><sup>Ol/WR</sup> (partition coefficient of Fe-Mg between Ol and the whole-rock composition) of 0.28 ± 0.02, which is in good agreement with the 0.33 ± 0.03 expected for Ol-melt equilibrium in basaltic magmas (Ulmer, 1989). The K<sub>dFe-Mg</sub><sup>Cpx/WR</sup> value can also be used to test equilibrium between Cpx and the liquid (e.g. Klügel and Klein, 2005; Putirka et al., 2008), although it is less reliable than for olivine due to the more complex cationic exchanges controlling clinopyroxene composition. A value of 0.28 ± 0.08 is expected for equilibrium (retrieved from 1245 equilibrium crystallization experiments, Putirka et al. 2008) which is in good agreement with the value of 0.31 ± 0.03 determined from the Cpx of ETZ-13.

The calculated temperatures were used as a starting point for the experiments. The Cpx thermometer of Wang et al. (2021) yields the highest temperature (1106°C), while the Ol-liquid (Putirka, 2008) and Ilm-Mt (Ghiorso & Evans, 2008) models retrieve similar temperatures (1065 and 1051°C, respectively). An assumed pressure of 200 MPa was used for the calculations (see below). The results of the geothermometers are shown in Table 3.

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## EXPERIMENTAL STRATEGY

The evolution of peralkaline silicic melts is thought to occur at shallow crustal pressures, as inferred from field observations (Mahood, 1984), along with experimental (e.g., Scaillet & Macdonald, 2001 & 2003; Nekvasil et al., 2004; Caricchi et al., 2006, Di Carlo et al., 2010; Romano et al., 2018, 2020 & 2022), geochemical (e.g., Peccerillo, 2003; White et al., 2009; Hutchison et al., 2016c; Gleeson et al., 2017; Hutchison et al., 2018; Iddon et al., 2019) and geophysical (e.g., Mahatsente et al., 1999; Field et al., 2012; Hutchison et al., 2016c; Gleeson et al., 2017; Iddon et al., 2019) studies. However, the pressure conditions at which basaltic magmas fractionate to form peralkaline felsic derivatives are unclear.

Previous studies aiming at producing peralkaline compositions from basaltic magmas at high pressures (>500 MPa) remained far from reaching peralkalinity (Nekvasil et al., 2004; Caricchi et al., 2006), even after significant crystallization in the run products. Fractionation at low pressure could hinder amphibole stability in hydrous mafic magmas and promote massive plagioclase feldspar fractionation. This process could play a key role in the genesis of peralkaline magmas through Al depletion of the melt by massive feldspar fractionation, as suggested by Carrichi et al. (2006). Accordingly, all experiments were conducted at 200 MPa to constrain the compositional evolution of the basaltic sample at upper crustal pressure
conditions. Additionally, one experiment was performed at 100 MPa to assess the effect of pressure on the run products. The magmatic evolution of peralkaline magmas is generally interpreted as operating under reduced conditions (e.g., Macdonald et al., 2021), hence all experiments were conducted at an $f'O_2$ lying between FMQ-1 and FMQ-2 (i.e., one to two log units below the quartz-fayalite-magnetite solid buffer). Various water contents were explored, using different $H_2O/CO_2$ ratios to control water activity. The first experiment was performed at 1080°C as derived from different geothermometers (see above). Subsequent experiments were conducted at lower temperatures with an incremental decrease of 20°C with other parameters kept constant to evaluate the compositional evolution with decreasing temperature.

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## 282 Charge Preparation

The powdered rock was fused twice at 1400°C in air for 3-4 hours in a Pt crucible. The resulting glass was powdered in an agate mortar under acetone and the glass powder was used as starting material for the experiments. The glass composition was found to be close to the whole rock composition (Table 1), except for depletion in volatile elements. Au or $Au_{80}Pd_{20}$ capsules (1.5 cm long, 2.5mm inner, and 2.9 mm outer diameter) were used in an attempt to minimize iron loss due to the reduced nature of the experiments. Yet, up to ~2 wt% FeO was lost to the capsule (equivalent to ~14% weight fraction of the original total iron content) in high temperature charges. Such an Fe loss is less noticeable at low temperature. Water activity was controlled by the $H_2O/CO_2$ ratio, expressed as $X_{H_2O}$ (molar). Capsules were first loaded with deionized water, then silver oxalate as a source of $CO_2$, and finally with 30 mg of the glass powder. Each capsule had approximately 3 mg (10 wt.%) of fluid ($H_2O+CO_2$) to ensure fluid saturation and vary the $H_2O$ dissolved in the melt ($H_2O_{melt}$). For a given P and T, 5 capsules with $X_{H_2O}$ ranging from 1 to 0 (with $X_{H_2O}=1, 0.7, 0.5, 0.3, 0$ as reference for the capsules, respectively) were loaded to the vessel. The capsules were sealed by arc welding, weighed at each step, and after the experiments, to check for leaks. They were placed in a 120°C oven during at least 60 min prior to the experiments to homogenize the water distribution within the capsule.

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## 302 Equipment

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The experiments were conducted in an internally heated pressurized vessel at the Institut de la Terre d'Orléans (ISTO). The experiments are of equilibrium crystallization type and were
carried out at a fixed P-T for different durations (5-220 h) depending on temperature. A
mixture of H₂ and Ar was used as the pressure medium. Pure H₂ was loaded first to control
the fO₂ as a function of the Ar/H₂ ratio (Scaillet et al., 1992). The quantity of added H₂
necessary to reach the targeted redox conditions was based on previous experiments (see
below). The total pressure is measured with a factory-calibrated gauge and is known to within
± 20 bar. Temperature is measured continuously with two S-type thermocouples (± 10 °C).
Experiments were ended using a drop quench device (Di Carlo et al., 2006), which allows
cooling rates > 100 °C/sec. A transient short increase in pressure was used as evidence of
successful quenching, resulting from the sample holder falling into the lower cold part of the
vessel. Upon capsule opening, fragments of run product were mounted in epoxy resin,
polished and carbon coated for further analyses (SEM, EMPA). Fluid saturation was
highlighted by the presence of vesicles in the run products.

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## Attainment of equilibrium

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The use of finely powdered dry glass as a starting product is well known to favor bulk crystal-
liquid equilibrium on laboratory timescales (Pichavant et al., 2007). Bulk equilibrium is
highlighted in crystallization experiments by the euhedral shape of the crystals, the absence of
zoning, homogeneous compositions of crystals and glass, and low residuals of the mass balance
calculation. A failure in drop quench resulted in abundant quench crystals which shows that the
energy activation for nucleation in our experiments was low. In all successfully drop quenched
experiments no significant quench crystallization was observed in the run products.

Similarly to the natural rock (see Oxythermometry section), we used the Fe<sup>II</sup>-Mg partition
coefficients to test for equilibrium between olivine and glass (e.g., Sack, 1980, Ulmer et al.,
1989). The Fe<sup>II</sup> content of the experimental glasses was calculated according to the method of
Kress and Carmichael (1991), using the calculated fO<sub>2</sub> (see below) and the experimental
conditions for determining the Fe<sup>II</sup>/Fe<sup>III</sup> ratios as total iron is measured as FeO by the
microprobe. All iron in olivine was assumed to be Fe<sup>II</sup>. The calculated K<sub>dFe-Mg</sub><sup>Ol/Glass</sup> for each
experiment where olivine is present is plotted against temperature in Figure 3. The partition
coefficients agree within error with the values determined by Ulmer (1989) for basaltic liquids.

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Water content and fO₂, in the experimental charges

338
Each capsule contained fluids with different H₂O/CO₂ molar ratios (X<sub>H2O</sub>, see capsule preparation) allowing the effect of the melt water content (H₂O<sub>melt</sub>) on phase equilibria to be studied. Whenever possible, H₂O<sub>melt</sub> was determined using the by-difference method (Devine et al., 1995), with the same set of glass standards used for alkali-migration correction. As noted above, these standards (of composition close to ETZ-13) have known dissolved water contents and were used to infer the water content in the experimental glasses by correlating the sum difference to 100 with H₂O<sub>melt</sub>. However, at low temperatures and X<sub>H2O</sub>, the small size and homogeneous distribution of crystals, together with the low amount of glass, made the use of this technique difficult. Therefore, for all charges the H₂O<sub>melt</sub> was also calculated using the method of Scaillet & Macdonald (2001). First, the H₂O<sub>melt</sub> of each water-saturated charge was determined using the model of Lesne et al. (2011). The resulting H₂O<sub>melt</sub> coupled with the f°H₂O (fugacity of pure water) at the relevant P-T conditions (determined using the CORK equation of Holland & Powell (1991)), allows the calibration of an equation of the form f°H₂O (bars) = aH₂O<sub>melt</sub> (wt%)<sup>b</sup>. The retrieved coefficients for this study are a = 152.09 and b = 1.60 (R² = 0.989). At each P-T conditions explored, the fH₂O of water-undersaturated charges was determined using the following relationship: fH₂O = f°H₂O* X<sub>H2O</sub>. The H₂O<sub>melt</sub> is then given by the following equation: H₂O<sub>melt</sub> (wt%) = (fH₂O/152.09)<sup>1/1.60</sup>.

This method assumes ideal mixing of the H₂O-CO₂ fluid phase. In addition, the amount of H₂O lost from the fluid by dissolution in the melt is neglected. The calculated H₂O<sub>melt</sub> must therefore be considered as maximum values (e.g., Scaillet and Evans, 1999; Berndt et al., 2005; Scaillet & Macdonald, 2006; Andújar et al., 2017). The calculated H₂O<sub>melt</sub> are consistent with that determined by difference, whenever the use of this method was possible, considering an accuracy of the by-difference method on H₂O<sub>melt</sub> of ± 0.5 wt% (Devine et al., 1995).

Redox conditions lower than FMQ cannot be recorded by Co-Pd solid state sensors (Taylor et al., 1992). Therefore, given the reduced nature of the experiments, the redox conditions of each run were calculated using the water dissociation reaction:

$$K_w = fH_2O/(fH_2*fO_2^{1/2}) \quad (1)$$

The water dissociation constant (K<sub>w</sub>) is taken from Robie et al. (1978), and the fH₂O used is the same as that used for each run to determine H₂O<sub>melt</sub> using the water saturation models (either f°H₂O at water saturation or fH₂O = f°H₂O*X<sub>H2O</sub> for water undersaturated conditions, see above). From the numerous experiments performed at ISTO with H₂ membranes (Scaillet et al., 1992) it was empirically constrained that the fH₂ reached at the P-T of the experiment is 3 ± 1 times higher than the PH₂ load in the vessel at room temperature (e.g. Di Carlo et al.,
373 2010). Given the relative error of the fH₂ calculated using this relationship, the average error
374 of fO₂ is largely controlled by the fH₂ used for the calculation (2-4*PH₂). A factor of 3 was
375 used for internal consistency in the fO₂ calculated for each run. This results in an error of +0.4
376 (fH₂ = 2PH₂) or -0.3 (fH₂ = 4PH₂) relative to the FMQ buffer at the P-T conditions of interest.

377 Several capsules were nominally dry (XH2O=0). However, reduction of Fe2O3 of the
378 starting glass by the hydrogen of the pressure medium occurs during the experiments,
379 producing a small amount of water (~1 wt%). A nominal (baseline) H2Omelt = 1 wt% was used
380 for the residual glasses of nominally dry capsules, based on previous experiments and the
381 values determined via the by-difference method on some of the nominally dry glasses. The
382 corresponding fH2O (15.2 MPa, using the above relationship) was used to derive the redox
383 conditions of nominally dry charges. These values should be considered as maxima with
384 respect to the calculated fO2.

385 RESULTS

386 Phase proportions
387

387

388 Mineral phases identified in the run products of the experiments are Ol, Pl, Cpx, Ilm, Ap and
389 Amphibole (Amph). Phase proportions were calculated using constrained least squares mass
390 balance calculations (Albarède, 1995) with propagation of analytical error (Table 4). The low
391 residuals highlight the fact that no major phase has been overlooked. The calculated glass
392 proportion varies from 100 wt% down to 5.4 wt%. At 1080°C, the alkaline-basalt ETZ-13 is
393 at supra liquidus conditions regardless of water activity (using XH2O as a proxy). As expected,
394 charges with high water activities are much less crystallized than their water-poor
395 counterparts (Fig. 4). The increase in crystal content with decreasing temperature is not linear
396 (Fig. 4) and a sudden increase is associated when either Pl (low water activity, 13-H29-30,
397 13-H16 to H20, 13-H15, 13-H33-34, Table 4) or Amph (high water activity, 13-H31, 13-H32,
398 13-H33, 1000°C) crystallise. In fact, Pl represents up to 55.6 wt% of the charge (13-H34) and
399 Amph up to 53.1 wt% of the charge (13-H32), making them the dominant mineral phase by
400 more than a factor of 2. When Cpx and Ol coexist in the run-product, Cpx is always found in
401 similar (e.g. 13-H17 to 20) or slightly higher proportions (e.g. 13H-29 and 13-H30) than Ol.
402 Overall, Ol makes for 0.2 to 16.6 wt% in proportion of the charge (13-H23 and 13-H20,
403 respectively). Similarly, Cpx makes for 0.7 to 17.0 wt% of the charge (13-H28 and 13-H34,
404 respectively). Ilm and Ap remain minor phases in all the experiments in which they were
405 identified (up to 6.4 and 5.3 wt% in proportion, respectively). Apatite is often found as
inclusions in Ol and Amph and was sometimes too small to be correctly analyzed, yet its formal identification remained possible. An additional experiment was performed at 1060°C and 100 MPa. Halving the pressure almost halves the solubility of water in the melt. The water saturated glass at 200 MPa 1060°C has a H₂O_melt of 4.9 wt% while that at 100 MPa 1060°C has a H₂O_melt of 2.3 wt% (both measured by difference). This results in the crystallinity of the 100 MPa experimental run products being higher than their 200 MPa counterparts.

413

## 414 Phase relationships

415

The phase relationships of ETZ-13, at 200 MPa, at redox conditions around FMQ-1 are shown in Fig. 5. A hypothetical solidus was drawn based on the experimental results in terms of phase proportions. Ol is the liquidus phase and dominates the phase assemblage at high temperatures and water-rich conditions. It is followed by Ilm and Ap for high X<sub>H2O</sub> and Pl for low X<sub>H2O</sub>. In our experiments, Cpx is only stable for H₂O<sub>melt</sub> ≤ 4 wt%, while Amph is only stable for H₂O<sub>melt</sub> > 2.5 wt.% and at T < 1020°C. The coexistence of Amph and Cpx was only observed in two charges (13H-17 and 13-H33, Table 4). Ilm is stable starting from 1040°C at H₂O<sub>melt</sub> = 1 wt% and for all following sub-liquidus run products at lower temperature. Apatite shows a similar behavior but at 20°C degrees lower, being stable at 1020°C, H₂O<sub>melt</sub> = 1 wt% and all experiments at or below 1000°C. Addressing the phase relationships of ETZ-13 at 100 MPa would be quite speculative with a single experiment. However, it is possible to describe the similarities and differences at 1060°C between 100 MPa and 200 MPa. Here we use X<sub>H2O</sub> for a proper comparison as halving the pressures almost halves water solubility (Table 4). The charges at X<sub>H2O</sub> = 0.7 and 1 at 1060°C are at supra-liquidus conditions, regardless of pressure, while in charges with X<sub>H2O</sub> = 0.5, only Ol is observed. Pl is stable in charge 13-I4 (1060°C, 100 MPa, X<sub>H2O</sub> = 0.3) while it is absent in charge 13-H24 (1060°C, 200 MPa, X<sub>H2O</sub> = 0.3). Ilmenite was found at 100 MPa for X<sub>H2O</sub> = 0 while it is not present at 200 MPa (both at 1060°C), and in contrast, Cpx crystallized at 200 MPa and not at 100 MPa (also X<sub>H2O</sub> = 0, 1060°C).

435

## 436 Phase compositions

437

All the analyses performed on the experimental mineral phases can be found in the supplementary material.

440 *Plagioclase*
441

442

443

444

445

446

447

Pl composition ranges from labradorite (An₆₀) to bytownite (An₈₃). Temperature strongly affects the composition of the Pl, higher An content being associated to higher temperature (Fig. 6). An increase of H₂O_melt also strongly increases the An content of the Pl (Fig. 6). The most calcic feldspar crystallized in 13-H31 (975°C, 200 MPa, X_H₂O=1), showing a significant effect of H₂O_melt on the composition of Pl. For example, the An content of Pl decreases from 83 to 66 at 975°C from X_H₂O = 1 and X_H₂O = 0.3.

448

449

### Olivine

450

451

452

453

454

455

456

The experimental Ol ranges from Fo₄₂ to Fo₇₉, being homogeneous (± 1 % in Fo) in every charge. Overall Ol becomes less Mg-rich as temperature decreases (Fig. 7) for a given H₂O_melt. Similarly, at fixed temperature a decrease in H₂O_melt decreases MgO of Ol. The most magnesian Ol, Fo₇₉, is produced at 100 MPa 1060°C, X_H₂O = 0.5 (while at the same temperature and X_H₂O but at 200 MPa, it is ~Fo₇₂). The natural Ol Fo% content is also plotted in Fig. 7.

457

458

### Clinopyroxene

459

460

461

462

463

464

465

466

467

468

469

The experimental Cpx are either diopside or augite (based on the classification of Morimoto, 1989) with compositions in the range En₃₆-₄₅Fs₆-2₁Wo₄₂-₄₉. The calculated Mg# (as (Mg# = Mg/(Mg+Fe))), molar) ranges from 0.80 to 0.54. For a given temperature at 200 MPa, the Mg# systematically decreases with decreasing H₂O_melt, reflecting the increase in crystallinity but also the decrease in fO₂ (Table 4). Similarly, for a given X_H₂O, the Mg# tends to increase with increasing temperature (except at 1040°C and 1060°C X_H₂O = 0 where the Mg# of the Cpx is of 0.72 in both cases, Fig. 8a). The TiO₂ content of the Cpx increases with increasing temperature (Fig. 8b) while the iron content decreases with increasing temperature (Fig. 8c).

470

471

472

473

474

### Amphibole

The experimental Amph is calcic and Ti-rich. According to the IMA 2012 recommendations (Locock, 2013), the Amph that crystallized in the experiments are ferri-kaersutites. For a given temperature X<sub>Fe</sub> (as X<sub>Fe</sub> = Fe/(Fe+Mg+Mn), molar) of the Amph tends to decrease with increasing H₂O_melt. Similarly, X<sub>Fe</sub> decreases with increasing temperature.
475

476 *Ilmenite*

477

478 The only Fe-Ti oxide mineral that crystallized during the experiments is Ilm. The FeO~T~ content of the Ilm tends to decrease with increasing temperatures (Fig. 9a) and/or H~2~O~melt~ (i.e., fO~2~ as calculated for the experimental charges, Table 4) while the opposite is observed for TiO~2~ (Fig. 9b). Indeed, the redox conditions of the experiments strongly depend on the initial X~H~2O of the capsules, which controls the fH~2~O of the different runs (as fO~2~ = (fH~2~O / (K~w~*fH~2~))~2~, see “Water content and fO~2~ in the experimental charges” section for more details). Additionally, for a given temperature, MgO strongly decreases with decreasing H~2~O~melt~ (from 9.1 to 5.7 wt% at 1020°C for example). To a lesser extent the same effect is observed for a fixed X~H~2O and decreasing temperature. In the end, the MgO (wt%) in Ilm is strongly correlated to melt proportion (i.e. temperature and H~2~O~melt~), reaching 9.1 wt% in 13-H27 (1020°C, X~H~2O = 0.72, φ = 4.5 wt%).

489

490 *Glass*

491

492 The glass compositions of the run products range from basaltic to trachy-andesitic (based on the classification of Le Maître, 2002). At T ≥ 1040°C, the melt fraction remains very high (> 70%) and, in most run products, only Ol is stable. Only two nominally dry run products at 200 MPa at 1060 (13-H25) and 1040°C (13-H15) display mineral phases other than Ol. Therefore, the compositional evolution of the residual glass in the temperature interval 1080-1040°C is very limited and the melt remains basaltic, regardless of water activity. Experiments conducted at or below 1020°C yields significantly more evolved glasses, due to a larger degree of crystallization (Fig. 4). This comes along with a greater mineralogical diversity, with Pl, Cpx, oxides, Ap and Amph becoming stable (Fig. 5).

501 Harker diagrams show the compositional evolution of the major elements versus SiO~2~, which is used here as a differentiation index (Fig. 10). Natural whole-rock compositions of the MER (WoldeGabriel et al., 1999; Trua et al., 1999; Peccerillo et al., 2003; Giordano et al., 2014; Hutchison et al., 2016; Franceschini, 2022) are also plotted to compare natural and experimental trends.

506 Overall, in the experiments at 200 MPa, MgO, FeO~T~, CaO, TiO~2~ and P~2~O~5~ decrease with increasing SiO~2~ (decreasing temperature, increasing crystal content) while Na~2~O, K~2~O, and Al~2~O~3~ increase with increasing SiO~2~ content. The compositional evolution of the
experimental glasses in terms of MgO, FeO<sub>T</sub>, TiO<sub>2</sub>, P<sub>2</sub>O<sub>5</sub>, CaO, Na<sub>2</sub>O and K<sub>2</sub>O matches relatively well the natural trend. However, the natural trend for Al<sub>2</sub>O<sub>3</sub> is not reproduced. The Al<sub>2</sub>O<sub>3</sub> content of the residual glasses tends to increase with increasing SiO<sub>2</sub> while it decreases with increasing SiO<sub>2</sub> for natural rocks. Decreasing the water content of the melt for a given temperature results in a similar compositional evolution versus SiO<sub>2</sub> as decreasing the temperature, both parameters resulting in an increase in crystal content. It is interesting to note that for CaO, the experimental glass compositions follow the natural trend down to 1000°C, below which the experimental CaO contents are slightly higher than the natural concentration for a given SiO<sub>2</sub>.

The compositional evolution of the liquid is mainly controlled by the crystallization of Pl+Cpx+Ol+Ilm+Ap-Amph, in that order of importance, at low X<sub>H2O</sub>, and by the crystallization of Amph+Pl+Ol+Ilm+Ap+Cpx at high X<sub>H2O</sub>. The crystallization of Ol+Cpx-Amph drives the compositional evolution of the liquid in terms of MgO and, to a lesser extent, FeO<sub>T</sub>. The crystallisation of Ilm is largely responsible for the depletion in TiO<sub>2</sub> and partly of FeO<sub>T</sub>. Furthermore, the strong increase in the SiO<sub>2</sub> content of the glass correlates with the stabilization of Ilm. This effect is further enhanced at lower temperatures with the massive crystallization of either Pl at low X<sub>H2O</sub> or Amph at high X<sub>H2O</sub>. These two phases decrease the CaO content of the glass with increasing SiO<sub>2</sub>. The crystallisation of Ap depletes the residual liquid in P<sub>2</sub>O<sub>5</sub>. In contrast, K<sub>2</sub>O is largely incompatible in all mineral phases, hence it strongly increases with decreasing melt content. The crystallization of Pl is the only phase that could decrease the Al<sub>2</sub>O<sub>3</sub> content of the melt to any significant extent, especially at high temperatures and/or high-water contents where Pl becomes more calcic. For example, decreasing the pressure to 100 MPa at 1060°C promotes the crystallization of calcic Pl at X<sub>H2O</sub> = 0.3 and X<sub>H2O</sub> = 0, in greater abundance than Ol. This results in a depletion of Al<sub>2</sub>O<sub>3</sub> from 15.8 wt% in the starting glass to about 14.3 wt% in the residual glass (1060°C, X<sub>H2O</sub> = 0). Similarly, at 200 MPa, 1040°C, and X<sub>H2O</sub> = 0, crystallization of calcic Pl causes the Al<sub>2</sub>O<sub>3</sub> content to drop to 15.5 wt%. These are the only two charges having lower Al<sub>2</sub>O<sub>3</sub> concentrations than the starting glass.

537

## 538 DISCUSSION

### 539 Pre-eruptive conditions of ETZ-13

540

Constraints on the pre-eruptive conditions of the alkali basalt ETZ-13 are obtained by comparing the phase assemblages in the run products with that of the natural rock. The
similarity of the crystal population in both the run products and the natural rock ETZ-13 provides a first guide to pre-eruptive conditions of ETZ-13. The phenocryst assemblage was well reproduced at 200 MPa, except for the lack of Mt. The natural rock contains both ilmenite and magnetite, which suggests that the experimental conditions were slightly too reduced. However, the mineral assemblage of Ol, Pl, Cpx in the phase diagram at 200 MPa is broadly replicated in the temperature range 1070-1030°C and for H₂O_melt 3.5-1.0 wt% H₂O_melt (Fig. 5).

This first estimate can be refined using the phase compositions and proportions. The natural rock ETZ-13 contains about 16 wt% of phenocrysts, which indicates near liquidus conditions (Fig. 2 and 5). Fo (mol%) in Ol, An (mol%) in Pl, and Mg# in Cpx, are influenced by both H₂O_melt and temperature. Ilm composition was not used because the redox compositions of the experiments appear to be slightly too reduced, and this strongly affects Ilm composition. The experimental Ol reach Fo contents close to that of the starting material (Fo₇₇±₁) at 1040°C (XH₂O = 0.3) and 1060°C (XH₂O = 0 but also XH₂O = 0.3 at 100 MPa, Fig. 7). The natural KₐdFe⁻Mg (using whole rock instead of glass) is in good agreement (within error) with the partition coefficient produced by the experimental olivine and glass (1040-1060°C). The natural Cpx is well reproduced at 1040 and 1060°C, both in terms of Mg# and KₐdFe⁻Mg (XH₂O = 0, Fig. 8). The case of Pl is more ambiguous for constraining the pre-eruptive temperature as its composition is clearly more dependent on H₂O_melt (Fig. 6). Indeed, a Pl composition close to that of the natural rims in ETZ-13 (An₇₇±₂, Fig. 6) is reproduced over a wide range of temperature (1000-1060°C). Together, these experimental trends constrain the pre-eruptive temperature of ETZ-13 to about 1050 ± 10°C, in good agreement with two-oxide (Ghiorso & Evans, 2008), Ol-liquid (Putirka, 2008), and Cpx (considering the standard error estimate of 36.6°C, Wang et al., 2021) geothermometers. With respect to water content, the experimentally constrained 1.5-2.0 wt% H₂O_melt for ETZ-13 (XH₂O = 0 or XH₂O = 0.3 at 1040 and 1060°C) is consistent with available melt inclusion data (e.g., Iddon et al., 2020; Colby et al., 2024).

For Pl, the step-like zonation (Fig. 2) suggests that An-rich cores were brought into contact and equilibrated with ETZ-13 magma soon prior to the eruption, developing a thin rim recording the pre-eruptive conditions of ETZ-13 (An₇₇±₃). Calcic Pl approaching natural cores has been reproduced at 975°C and H₂O saturation (Fig. 6), which therefore opens the possibility that such cores (An₈₆±₄) come from the colder, water-rich, part of the magma
575 reservoir, which were mixed back into hotter portions during recharge. Alternatively, such
576 calcic Pl cores may be inherited from a more mafic magma, perhaps parental to ETZ-13.

577 Cpx is expected to play an important role in the formation of peralkaline composition
578 (Gleeson et al., 2017; Macdonald et al., 2021 and references therein). More importantly, in
579 our experiments, it is also observed to co-crystallise with plagioclase as opposed to Amph.
580 Indeed, significant crystallization of plagioclase feldspar can be anticipated to produce the
581 characteristic Al depletion of the melt required to achieve peralkalinity.

582 At 1000°C and $X_{H_2O}$ = 0.7, Cpx is present in much higher proportion than Amph (8.9 vs 0.4
583 wt%, respectively, Table 4), and plagioclase crystallizes profusely (15.2 wt %). With
584 decreasing temperature and similar water content, one would expect an increase in the
585 proportion of Pl, as observed from 1060°C to 1000°C. However, from 1000°C to 975°C, at
586 high water content ($X_{H_2O}$=0.7), Cpx is unstable and the Pl content of the charge decreases
587 sharply relative to that of 1000°C (from 15.2 to 7.2 wt%, Table 4), in favour of Amph.

588 Amphibole seems thus to compete with plagioclase (and Cpx), and such an effect is likely
589 enhanced at higher pressures where Amph becomes more abundant (Caricchi et al., 2006). In
590 addition, significant oxide crystallization is expected for the production of the silica-saturated
591 peralkaline derivatives, especially when starting from an alkaline basalt like ETZ-13. In our
592 experiments, the presence of Amph limits the crystallization of Ilm compared to Cpx. This
593 can be explained by the fact that abundant crystallization of Amph imparts a larger decrease
594 in the FeO/MgO ratio of the melt, especially at low temperatures. Amph also contains much
595 more $TiO_2$ (up to 5.35 wt %) compared to Cpx, which presumably hinders the crystallization
596 of oxides.

597 Overall, this suggests that the T-H₂O evolution of the basaltic magmas during
598 differentiation in the MER does not intercept the field of amphibole stability at low pressure
599 (close to water saturation, starting from a temperature of 1000°C). Indeed, the experiments
600 suggests that significant amphibole crystallization would certainly results in the failure to
601 reach peralkaline composition. This is in line with the general absence of amphibole in mafic
602 magmas of peralkaline provinces (Macdonald et al., 2021).

603 The nominally dry charge of the single run performed at 100 MPa also reproduced
604 ETZ-13 well, except for Cpx, which is absent compared to the 200 MPa run at the same T-
605 $H_2O_{melt}$ conditions. The 200 MPa pressure was arbitrarily chosen to test shallow, upper crustal
606 conditions, but a slightly lower pressure could be compatible with the observed phase
607 assemblage as well by slightly increasing the $H_2O_{melt}$. For this reason, we conclude that a
608 storage pressure of 150-200 MPa for ETZ-13 is reasonable. This pressure range determined
for ETZ-13 is in very good agreement with recent thermodynamical modeling and thermogeobarometric studies performed on basaltic endmembers of the MER (Gleeson et al., 2017; Tadesse et al., 2023; Colby et al., 2024). Altogether, our experiments show that alkaline mafic compositions, potentially the source of the peralkaline rhyolites stored at low pressure (e.g., Scaillet & Macdonald, 2001), experience storage in the upper crust as well. Iddon et al. (2020) also highlighted this feature from H₂O-CO₂ systematics in Ol-hosted melt inclusions of basaltic rocks from the Kone and Aluto volcano in the MER. Similarly, Gioncada & Landi (2009) have shown that a storage pressure of 200 MPa is likely for the alkali-basalts of the peralkaline suite of Pantelleria. Melt inclusions from Pantelleria basalts are also water-rich (1.5-2 wt% H₂O_melt), compared to a typical MORB (0.1 wt%) for example, which is similar to the experimental pre-eruptive conditions we constrained for ETZ-13.

The water-rich nature of these mildly alkaline basalts is in line with the documented pre-eruptive H₂O-rich conditions of peralkaline rhyolites, either in the African rift (e.g., Wilding et al., 1993; Webster et al., 1993; Scaillet and Macdonald, 2001, 2006; Iddon et al., 2020) or in other settings (e.g., Pantelleria, Di Carlo et al., 2010, Mayor Island, Barclay et al., 1996; Baitoushan-Paektu, Horn and Schminke, 2000; Iacovino et al., 2016; Scaillet and Oppenheimer, 2023).

Our experimental data indicate that after more than 90 wt% of basalt crystallization the residual liquid is still not peralkaline (Fig. 11), stressing the need for a protracted fractional crystallization process to end up with alkali-rich derivatives (see below). Therefore, given the largely incompatible behaviour of H₂O during magma crystallization, second boiling of cooling peralkaline reservoirs seems inevitable. Note that this condition (of H₂O saturation) may be already reached during the cooling of the basaltic magma. For instance, 90 wt% crystallization of the alkali basalt having 1 wt% H₂O_melt, will produce a residual liquid which will be near H₂O saturation (7 wt% H₂O_melt at 200 MPa), coexisting with an exsolved fluid phase amounting to 0.3 wt% of the magma (at 200 MPa). The occurrence of an excess fluid phase in alkaline magmatic reservoirs is likely an important factor controlling eruption behaviour and readily explains the explosive character of many silicic eruptions in the Ethiopian rift. It also has implications for our understanding of the inner working of associated geothermal systems: continuous loss of magmatic volatiles prior to eruption, as documented for instance by Scaillet and Oppenheimer (2023) for the Baitoushan-Paektu volcano in North Korea, is facilitated by advective transport of bubbles toward the encasing rocks, which will supply H₂O (in addition to heat) to any overlying hydrothermal system.
643 Water content of the parental basalt of ETZ-13 and of its mantle source

644

645 As stated previously, the high pre-eruptive water content of ETZ-13 (1.5-2 wt%), a potential
646 source basalt of the peralkaline rhyolites, may account for the hydrous nature of the latter. The
647 mantle source plays a key role in the genesis of these peculiar magmatic compositions
648 associated with the Ethiopian Rift. A relatively primitive basalt such as ETZ-13 already
648 contains more than 10 times the water content of MORBs (e.g., Saal et al., 2002), indicating a
649 water-rich mantle source. It is possible to estimate the water content of a primitive parent
650 basalt of ETZ-13 (i.e., in equilibrium with the mantle) by simple geochemical modeling. In
651 the MER, basaltic magmas fractionate mainly olivine during their ascent (Furman et al.,
652 2006), which strongly influences the liquid line of descent in terms of MgO melt content.
653 Direct evidence for Ol fractionation lies in the typically low MgO and nickel (Ni) contents of
654 Ethiopian basalts (most basalts have < 8% MgO and < 100 ppm Ni, Trua et al., 1999; Ayalew
655 et al., 2002; Furman et al., 2006; Furman, 2007). Occasionally, however, primitive basalts are
656 found and interpreted as the putative parental basalt of a given volcanic system. The
657 quaternary basalt of the Galo-Salen unit (CMER) described by Trua et al. (1999), is such an
658 example, with an MgO content of 12 wt% and Ni of 263 ppm.
659

660 Assuming that the parental basalt of ETZ-13 has a composition similar in terms of
661 MgO and Ni to that of Galo-Salen unit, it is possible to estimate the amount of olivine that
662 crystallized to yield a melt composition similar to ETZ-13. Simple mass balance calculations
663 shows that the fractionation of 18 wt.% primitive Ol (MgO~40 wt%) is required to reach the
664 MgO content of ETZ-13 (5.87%).

665 Trace elements provide still more sensible proxies for geochemical modelling when
666 the partition coefficient of a given element between the melt and the mineral phase of interest
667 is well known. Ol strongly incorporates Ni compared to silicate melts and other mineral
668 phases, having partition coefficients ($\frac{C_{Ni}^{Ol}}{C_{Ni}^{melt}} = D_{Ni}^{Ol-melt}$) as high as 30 (Hart & Davis, 1978).

669 To a first approximation we use a partition coefficient of 25.7 for Ni between Ol and
670 melt, using $C_{Ol}^{Ni}/C_{bulk}^{Ni}$ (735.1/28.6, Table 2), since ETZ-13 is poorly crystallized. The
671 resulting partition coefficient of 25.7 corresponds to a minimum, as the bulk Ni content is
672 influenced by Ol. With an initial Ni content of 263 ppm in the parental magma, and
673 considering a constant $D_{Ni}^{Ol-melt}$ of 25.7, we can model the fraction of Ol that crystallized
674 during the ascent to reach the Ni content of ETZ-13 (28 ppm), using the standard Rayleigh
675 fractionation equation as:
676

$$ \frac{C_l}{C_0} = F^{D-1} \quad (2) $$

where $C_l$ and $C_0$ are the concentrations of Ni in both bulk ETZ-13 (28 ppm) and in the hypothetical parental basalt of ETZ-13 (263 ppm). D is the olivine/melt partition coefficient of Ni (25.7), F the fraction of melt remaining. Thus, 1-F corresponds to the fraction of olivine that would have crystallized during ascent from the parental basalt to produce ETZ-13 liquid. The resulting F is 91.5%, implying 8.5 wt.% Ol fractionated during ascent. This result is similar to the Ol proportion calculated using MgO (18%). The lower olivine fraction calculated using Ni could be explained by a lower $D_{Ni}^{Ol-melt}$ at higher MgO$_{melt}$ (Hart & Davis, 1978), a factor not considered here as $D_{Ni}^{Ol-melt}$ was assumed to be constant. It could also come from the fact that the used parental basalt is too poor in Ni.

No matter how approximate, these results unequivocably show that a hypothetical parental basalt (i.e., one in equilibrium with a mantle source) must be already quite rich in water itself. Specifically, a high water content of at least ~1 wt% for a putative parental basalt of ETZ-13, is inferred by accounting for the effect of olivine fractionation. Considering that water is perfectly incompatible during partial melting, the estimated water content of the upper mantle source undergoing 10% partial melting would be ~0.1 wt%. The metasomatism of the lithospheric mantle, possibly caused by a volatile-rich influx from the plume, could account for such a high water content (0.1% vs. 0.01 wt% in a "normal" mantle, e.g. Bell & Rossman, 1992). Melting of a metasomatized lithospheric mantle containing hydrous minerals, such as amphibole or phlogopite, may explain the high water content of the parental basalts. The Na-rich nature (high Na/K ratio) and the positive Ba anomaly often observed in basalts from the MER, such as ETZ-13 (Furman et al., 2006; Ayalew et al., 2016), favor an amphibole-rich source as opposed to a phlogopite-rich one (Rosenthal et al., 2009; Ayalew et al., 2016). Amph has been indeed found in fertile mantle xenoliths in the surrounding plateaus (Ferrando et al., 2008; Alemayehu et al., 2017), giving supporting evidence to a metasomatized-fertile lithospheric mantle source beneath the MER. Such a metasomatized mantle source would probably also contain significant amounts of CO$_2$. However, it is difficult to extrapolate the reasoning used for water to CO$_2$, as the CO$_2$ content in the experimental glasses is unconstrained. In addition, the low solubility of CO$_2$ in shallow magmatic reservoirs provokes its extensive degassing from melt (Iddon et al., 2020). This makes the melt inclusion data for CO$_2$ less straightforward to interpret in terms of CO$_2$ content of the mantle source.

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Implications for the genesis of peralkaline compositions in the MER

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The experiments performed on ETZ-13 provide insights into a possible pathway for the genesis of peralkaline rhyolites in the MER, starting from a typical mafic composition of the area. Indeed, phase relationships and composition show that a relatively primitive basaltic melt (8-18% Ol fractionation from a hypothetical parental basalt, see above) was temporarily stored in the upper crust (150-200 MPa). The low viscosity of peralkaline suites (due to high water content and excess of alkalis, Dingwell et al., 1998; Di Genova et al., 2013) coupled with the densely fractured and thinned crust that prevails in extensional settings are likely to favor magma ascent. At such low pressures and even for relatively high water content for a basaltic composition (1.5-2.0 wt% H₂O_melt), significant Pl fractionation is promoted that could induce the characteristic aluminum depletion of the melt (e.g., Caricchi et al., 2006). In our experiments at 200 MPa, the trend of most oxides fits well with the natural one. This suggests a very shallow origin for the genesis of the peralkaline derivatives in the MER. In fact, Romano et al. (2018) experimentally demonstrated that the metaluminous to peralkaline transition can occur in the trachytic compositional range. More importantly, they highlighted that such a transition occurs at low pressure (100 ± 50 MPa), under both hydrous and reduced conditions, similarly to our experiments. That the Al content of the melt increases with increasing differentiation in our experimental charges (Fig. 10) is in stark contrast to the decreasing trend observed in natural rocks, and shows that the investigated conditions are still not favourable to extensive Pl crystallisation, at least for equilibrium crystallization. As a result, peralkalinity was not achieved, even after extensive crystallization of the starting material (>90%, Fig. 11). One natural way to enhance the Pl effect is via continuous segregation of the mineral phases during crystallization (fractional crystallization, and especially for Pl, favored at low pressure). In addition, continuous mafic magma recharge from the source (caused by the extension-induced melting of the lithosphere) would maintain rather elevated temperatures, hence promoting the Al-rich nature of the Pl, as our experiments show, while prolonging the life of the crystallizing magma. Taken together, this could amplify Al depletion of the melt from an Al-depleted liquid such as that produced in our experiments at low pressures (200 MPa 1040°C, or at 100 MPa, 1060°C). This suggests that protracted fractional crystallization (FC) is indeed necessary (e.g., Civetta et al., 1998; Caricchi et al., 2006) to explain the genesis of peralkaline compositions. This does not preclude a role of the volatile elements, such as flushing by halogen-rich fluids (Bailey & Macdonald, 1970) during magma genesis or during remobilization of cumulates (Iddon et al., 2019).
The magma flux required to feed large magmatic chambers responsible for the paroxysmic eruptions in the MER, and the attendant crystal-melt segregation, may develop strongly zoned magmatic reservoirs, as suggested by many authors for peralkaline systems (e.g. for the MER, Iddon et al., 2019). This could indeed explain the famous "Daly Gap": intermediate compositions would be "trapped" in the magma chambers, while low-density rhyolitic compositions would be erupted during major eruptions (Franceschini, 2022). Meanwhile, occasional basaltic forerunners may by-pass this density filter and ascend to the surface through the densely fractured crust.

750

## 751 CONCLUSIONS

752

We performed equilibrium crystallization experiments on ETZ-13, a typical recent alkali-rich basaltic composition of the Main Ethiopian Rift. The experiments were conducted under reduced conditions ($fO_2$ < FMQ-1) and for the most part at 200 MPa. The temperature range investigated and the large range of water activity (from nominally dry to water-saturated) not only allowed to constrain likely pre-eruptive conditions for ETZ-13 but also to explore the compositional evolution of residual liquids arising from extensive crystallization. We estimate the pre-eruptive conditions of ETZ-13 at 1050 ± 10°C, 1.5-2 wt% $H_2O_{melt}$, 150-200 MPa and at redox conditions close to the FMQ buffer, in good agreement with the geothermobarometers used (two-oxides, Ghiorso and Evans, 2008; Ol-liquid, Putirka, 2008 and Cpx, Wang et al. 2021). This is the first experimental evidence that the alkaline mafic compositions of the MER, the inferred source of the voluminous silicic peralkaline magmas in the rift, are water-rich and can be stored at low pressures. Since trachytes and rhyolites in a similar context have also been shown to be stored in the upper crust (e.g., Scaillet & Macdonald, 2001; Romano et al., 2018), it can be inferred that the production of peralkaline compositions starting from a basalt likely occurs in the upper crust. First order evidence for this model is provided by the fact that the oxide trends produced by the experimental liquid evolution of ETZ-13, plotted in Harker diagrams, are broadly similar to the natural ones. Furthermore, $H_2O-CO_2$ systematics of melt inclusions from other volcano centers in the MER (Iddon et al., 2020) and in Pantelleria (Gioncada & Landi, 2010) also indicate upper-crustal, water-rich conditions for basaltic magma storage. Altogether, these features could be a prerequisite for the genesis of peralkaline composition. Yet, at 200 MPa, we did not observe the characteristic Al decrease of the liquid associated with the genesis of peralkaline compositions. This may indicate that equilibrium crystallization is not sufficient to account
for the genesis of peralkaline compositions. An initial stage of FC of massive amounts of calcic plagioclase (favored at low pressure and high H₂O_melt), followed by liquid extraction, associated to continuous mafic magma recharge in this natural open system could explain the decreasing Al trend of the residual melt. This is in keeping with the observation that silicic compositions are stored higher in the crust in the MER than the mafic compositions (Iddon et al., 2020).

782

## 783 DATA AVAILABILITY

784

The supplementary data underlying this article are available in the GFZ data service, at https://dataservices.gfz-potsdam.de/panmetaworks/review/f210766697a0bc096969d2e2f410e4c325c672e954a3bde893d6a8b0f9c20c1b/ (TEMPORARY REVIEW LINK BEFORE ACCEPTANCE).

## 789 ACKNOWLEDGEMENTS

790

The authors thank I. Di Carlo, P. Benoist-Julio, S. Erdmann and S. Janiec for their technical assistance with SEM, EPMA and LA-ICP-MS. The reviews of Pierangelo Romano and Carl Spandler helped to improve the manuscript and are gratefully acknowledged, along with the editorial handling of Georg Zellmer.

795

## 796 FUNDING

797

We acknowledge support from LabEx VOLTAIRE (ANR-10-LABX-100-01), EquipEx PLANEX (ANR-11-EQPX-0036) and the European Research Council (ERC) (Grant agreement No.101096688 [APATE][ERC-2022-ADG]).

801

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966 Rooney, T.O. (2017). The Cenozoic magmatism of East-Africa: Part I — Flood basalts and pulsed magmatism.

967 Lithos 286–287, 264–301. https://doi.org/10.1016/j.lithos.2017.05.014

968 Rooney, T.O. (2020). The Cenozoic magmatism of East Africa: Part II – Rifting of the mobile belt. Lithos 360–

969 361, 105291. https://doi.org/10.1016/j.lithos.2019.105291

970 Rosenthal, A., Foley, S.F., Pearson, D.G., Nowell, G.M., Tappe, S. (2009). Petrogenesis of strongly alkaline

971 primitive volcanic rocks at the propagating tip of the western branch of the East African Rift. Earth Planet. Sci.

972 Lett. 284, 236-248. https://doi.org/10.1016/j.esp.2009.04.036

973 Saal, A. E., Hauri, E. H., Langmuir, C. H., & Perfit, M. R. (2002). Vapour undersaturation in primitive mid-

974 ocean-ridge basalt and the volatile content of Earth's upper mantle. Nature, 419(6906), 451-455.

975 https://doi.org/10.1038/nature01073

976

977 Sack, R.O. (1980). Some constraints on the thermodynamic mixing properties of Fe-Mg orthopyroxenes and

978 olivines. Contr. Mineral. and Petrol. 71, 257–269. https://doi.org/10.1007/BF00371667

979 Scaillet, B., Pichavant, M., Roux, J., Humbert, G., Lefèvre, A. (1992). Improvement of the Shaw membrane

980 technique for measurement and control of fH₂ at high temperatures and pressures. American Mineralogist, 77,

981 647-655.

982 Scaillet, B., Evans, B.W. (1999). The 15 June 1991 Eruption of Mount Pinatubo. I. Phase Equilibria and Pre-

983 eruption P–T–fO₂–fH₂O Conditions of the Dacite Magma. J. Petrol., 40, 381–411.

984 https://doi.org/10.1093/petroj/40.3.381

985 Scaillet, B., Macdonald, R. (2001). Phase Relations of Peralkaline Silicic Magmas and Petrogenetic

986 Implications. J. Petrol. 42, 825–845. https://doi.org/10.1093/petrology/42.4.825

987 Scaillet, B., Macdonald, R. (2003). Experimental Constraints on the Relationships between Peralkaline Rhyolites

988 of the Kenya Rift Valley. J. Petrol. 44, 1867–1894. https://doi.org/10.1093/petrology/egg062

989 Scaillet, B., Oppenheimer, C. (2023). Reassessment of the sulfur and halogen emissions from the Millennium

990 Eruption of Changbaishan (Paektu) volcano. Journal of Volcanology and Geothermal Research, 442, 107909.

991 https://doi.org/10.1016/j.jvolgeores.2023.107909

992 Shand, S.J. (1927). On the Relations between Silica, Alumina, and the Bases in Eruptive Rocks, considered as a

993 Means of Classification. Geol. Mag. 64, 446–449. https://doi.org/10.1017/S0016756800103760
994 Tadesse, A.Z., Fontijn, K., Caricchi, L., Bégué, F., Gudbrandsson, S., Smith, V.C., Gopon, P., Debaille, V., Laha, P., Terry, H., Yirgu, G., Ayalew, D., 2023. Pre-eruptive storage conditions and magmatic evolution of the Bora-Baricha-Tullu Moye volcanic system, Main Ethiopian Rift. Lithos, 442-443, 107088.
997 https://doi.org/10.1016/j.lithos.2023.107088

998 Taylor, J.R., Wall, V.J., Pownceby, M.I. (1992). The calibration and application of accurate redox sensors. Am. Mineral. 77, 284-295.

1000 Trua, T., Deniel, C., Mazzuoli, R. (1999). Crustal control in the genesis of Plio-Quaternary bimodal magmatism of the Main Ethiopian Rift (MER): geochemical and isotopic (Sr, Nd, Pb) evidence. Chem. Geol. 155, 201-231.
1002 https://doi.org/10.1016/S0009-2541(98)00174-0

1003 Ulmer, P. (1989). The dependence of the Fe2+-Mg cation-partitioning between olivine and basaltic liquid on pressure, temperature and composition: An experimental study to 30 kbars. Contr. Mineral. and Petrol. 101, 261-273. https://doi.org/10.1007/BF00375311

1006 Wang, X., Hou, T., Wang, M., Zhang, C., Zhang, Z., Pan, R., Marxer, F., Zhang, H. (2021). A new clinopyroxene thermobarometer for mafic to intermediate magmatic systems. Eur. J. Mineral., 33, 621-637.
1008 https://doi.org/10.519/ejm-33-621-2021

1009 Webster, J. D., Taylor, R. P., & Bean, C. (1993). Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma, Fantale volcano, Ethiopia. Contributions to Mineralogy and Petrology, 114, 53-62. https://doi.org/10.1007/BF00307865

1012 Wilding, M. C., Macdonald, R., Davies, J. E., & Fallick, A. E. (1993). Volatile characteristics of peralkaline rhyolites from Kenya: an ion microprobe, infrared spectroscopic and hydrogen isotope study. Contributions to Mineralogy and Petrology, 114, 264-275. https://doi.org/10.1007/BF00307761

1016 White, J.C., Parker, D.F., Ren, M. (2009). The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling. J. Volcanol. Geotherm. Res. 179, 33-55. https://doi.org/10.1016/j.jvolgeores.2008.10.007

1020 Woldegabriel, G., Aronson, J.L., Walter, R.C. (1990). Geology, geochronology, and rift basin development in the central sector of the Main Ethiopia Rift. Geol. Soc. Am. Bull. 102, 439-458. https://doi.org/10.1130/0016-7606(1990)102<0439:GGARBD>2.3.CO;2

1023 WoldeGabriel, G., Walter, R. C., Hart, W. K., Mertzman, S. A., & Aronson, J. L. (1999). Temporal relations and geochemical features of felsic volcanism in the central sector of the Main Ethiopian Rift. Acta Vulcanologica, 11, 53-68.

1026 Wolfenden, E., Ebinger, C., Yirgu, G., Deino, A., Ayalew, D. (2004). Evolution of the northern Main Ethiopian rift: birth of a triple junction. Earth Planet. Sci. Lett. 224, 213-228. https://doi.org/10.1016/j.epsl.2004.04.022

1028

1029

1030

1031

1032

1033

1034 **Figure 1:** Simplified map of the CMER (redrawn from Corti et al., 2020) and localization of the basalt, ETZ-13, used for the experiments.

1036

1037 **Figure 2:** (a) Iddingsitized Ol preserving fresh cores in ETZ-13. The black spots are LA-ICP-MS analyses. (b) Pl crystal showing an abrupt zonation as commonly observed in ETZ-13.
1039 Corresponding mean An contents for both cores and rims in ETZ-13 are reported on the figure.
1040

1041
1042 **Figure 3: Fe-Mg partition coefficients between the experimental olivine and the residual glass against temperature. The grey field represents the value of Ulmer (1989) for equilibrium in basaltic liquids.**
1043
1044

1045
1046 **Figure 4: Melt fraction of residual glass in the run products as a function of temperature for different XH₂O. The 100 MPa experiments are indicated with a red contour. The proportion of matrix glass in ETZ-13 is also shown.**
1047
1048

1049
1050 **Figure 5: Phase relationships as established from the experiments for ETZ-13 at 200 MPa ~FMQ-1. Phases lie inside their stability field. Grey dashed lines indicate the wt% of crystals.)**; Gl = Glass, Pl = Plagioclase, Ol = Olivine, Cpx = Clinopyroxene, Ilm = Ilmenite, Amph = Amphibole, Ap = Apatite. The approximate solidus position is shown as a dashed line. The pink rectangle delimits the inferred pre-eruptive T-H₂Oₘelt conditions. See text for more details.
1051
1052
1053
1054
1055

1056
1057 **Figure 6: An% content of the Pl in the run products as a function of temperature. Experiments at 100 MPa are indicated with a red contour. The An contents of the Pl core and rim in ETZ-13 are also represented. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature. Errors are smaller than symbol size (1σ).**
1058
1059
1060
1061

1062
1063 **Figure 7: Fo% content of the Ol in the run products as a function of temperature. The Ol that crystallized in the 100 MPa experiments are indicated with a red contour. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature. The Fo% content of the Ol in ETZ-13 are also represented. Errors are smaller than the symbols (1σ).**
1064
1065
1066
1067

1068
1069 **Figure 8: (a) Variation of Mg# (calculated as Mg#=Mg/(Mg+Fe)), molar) in function of temperature. (b) Variation of TiO₂ (wt%) content in Cpx in function of temperature. (c) Variation of FeOₜ (wt%) content in Cpx in function of temperature. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black dashed arrows illustrate the effect of temperature. Errors are smaller than the symbols (1σ).**
1070
1071
1072
1073

1074
1075 **Figure 9:** (a) Variation of FeO<sub>T</sub> (wt%) content in Ilm in function of temperature. (b) Variation of TiO<sub>2</sub> (wt%) content in Ilm in function of temperature. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature Errors are smaller than the symbols (1σ).

1079

1080 **Figure 10:** Harker diagrams of the experimental glasses. Natural whole-rock data from the MER are represented as the blue field (WoldeGabriel et al., 1999; Trua et al., 1999; Peccerillo et al., 2003; Giordano et al., 2014; Hutchison et al., 2016; Franceschini, 2022).
1081
1082
1083 The temperature of experiments is colour coded as indicated on the top panel and labelled
1084 next to some symbols. The experimental glasses at 100 MPa are shown with a red contour.

1085

1086 **Figure 11:** Evolution of the peralkalinity index (PI = Na+K/Al, molar) against residual melt fraction in the experiments. The experiment at 100 MPa and 1060°C is highlighted by a red contour.

1087

1088
<figure><img src="image_2.png" /><figcaption>Figure 1: Simplified map of the CMER (redrawn from Corti et al., 2020) and localization of the basalt, ETZ-13, used for the experiments.</figcaption></figure>

209x260mm (300 x 300 DPI)
<table><caption>**Table 1:** Major element compositions of whole-rock and mineral phases of sample ETZ-13.</caption><thead><tr><th>Element</th><th>ETZ-13</th><th>ETZ-13*</th><th>Matrix large zones "Glass" n=5</th><th>Ol</th><th>Pl (core)</th><th>Pl (rim)</th><th>Ilm</th></tr><tr><th></th><th>Bulk rock<sup>(a)</sup></th><th>Starting glass<sup>(b)</sup> n=9</th><th></th><th>n=14</th><th>n=14</th><th>n=12</th><th>n=7</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>44.25</td><td>44.20(21)</td><td>46.70(92)</td><td>37.59(41)</td><td>48.04(1.82)</td><td>51.58(1.06)</td><td>1.31(1.15)</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>15.26</td><td>15.82(12)</td><td>18.02(27)</td><td>0.05(3)</td><td>32.79(1.40)</td><td>30.05(90)</td><td>0.90(87)</td></tr><tr><td>FeO<sub>tot</sub><sup>1</sup></td><td>13.82</td><td>14.24(7)</td><td>11.20(79)</td><td>22.18(87)</td><td>0.51(8)</td><td>0.77(30)</td><td>46.65(1.76)</td></tr><tr><td>MgO</td><td>5.87</td><td>5.68(8)</td><td>4.22(31)</td><td>39.70(73)</td><td>0.15(10)</td><td>0.16(8)</td><td>4.35(0.29)</td></tr><tr><td>MnO</td><td>0.30</td><td>0.28(5)</td><td>0.18(4)</td><td>0.21(6)</td><td>0.02(4)</td><td>0.04(4)</td><td>0.32(11)</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>1.85</td><td>1.62(5)</td><td>2.02(11)</td><td>0.03(2)</td><td>0.04(4)</td><td>0.06(3)</td><td>0.07(9)</td></tr><tr><td>CaO</td><td>10.06</td><td>10.30(9)</td><td>9.48(42)</td><td>0.19(2)</td><td>15.52(1.23)</td><td>13.00(56)</td><td>0.26(21)</td></tr><tr><td>Na<sub>2</sub>O</td><td>3.35</td><td>3.56(4)</td><td>4.42(15)</td><td>0.02(2)</td><td>2.73(65)</td><td>4.06(35)</td><td>0.16(12)</td></tr><tr><td>K<sub>2</sub>O</td><td>0.79</td><td>0.74(8)</td><td>0.93(1)</td><td>0.01(2)</td><td>0.08(3)</td><td>0.20(0.06)</td><td>0.14(23)</td></tr><tr><td>TiO<sub>2</sub></td><td>3.71</td><td>3.65</td><td>2.74(34)</td><td>0.05(3)</td><td>0.08(5)</td><td>0.13(11)</td><td>45.56(1.56)</td></tr><tr><td>LOI<sup>2</sup></td><td>-0.53</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total</td><td>100.25</td><td>98.58</td><td>100.02</td><td>101.11</td><td>99.98</td><td>99.85</td><td>100.01</td></tr><tr><td>H<sub>2</sub>O tot. %</td><td>0.41</td><td>0.33</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cl</td><td>0.03</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>F</td><td>0.13</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Mg#<sup>3</sup></td><td>0.44</td><td>0.43</td><td>0.41</td><td></td><td></td><td></td><td></td></tr><tr><td>Fo<sup>4</sup></td><td></td><td></td><td></td><td>76.69(99)</td><td></td><td></td><td></td></tr><tr><td>An<sup>5</sup></td><td></td><td></td><td></td><td></td><td>86.03(3.88)</td><td>77.38(2.32)</td><td></td></tr><tr><td>En<sup>6</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Fs<sup>6</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Wo<sup>6</sup></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

<table><thead><tr><th></th><th>Mt</th><th>Cpx</th><th>Ap Microlites n=7</th></tr><tr><th></th><th>n=9</th><th>n=9</th><th></th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>0.06(4)</td><td>46.00(30)</td><td>0.28(5)</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>4.27(1.88)</td><td>8.29(34)</td><td>0.01(1)</td></tr><tr><td>FeO<sub>tot</sub><sup>1</sup></td><td>71.67(2.88)</td><td>8.76(44)</td><td>0.47(11)</td></tr><tr><td>MgO</td><td>3.72(83)</td><td>13.02(10)</td><td>0.26(9)</td></tr><tr><td>MnO</td><td>0.47(10)</td><td>0.16(0.04)</td><td>0.12(8)</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.07(9)</td><td>0.00(0)</td><td>42.54(1.22)</td></tr><tr><td>CaO</td><td>0.04(8)</td><td>19.92(0.23)</td><td>53.00(1.90)</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.02(3)</td><td>0.63(3)</td><td>0.06(3)</td></tr><tr><td>K<sub>2</sub>O</td><td>0.01(1)</td><td>0.00(0)</td><td>0.01(2)</td></tr><tr><td>TiO<sub>2</sub></td><td>19.73(2.92)</td><td>2.60(14)</td><td>0.02(3)</td></tr><tr><td>LOI<sup>2</sup></td><td></td><td></td><td></td></tr><tr><td>Total</td><td>96.57</td><td>99.39</td><td>102.30</td></tr><tr><td>H<sub>2</sub>O tot. %</td><td></td><td></td><td></td></tr><tr><td>Cl</td><td></td><td></td><td>0.18(4)</td></tr><tr><td>F</td><td></td><td></td><td>5.47(67)</td></tr><tr><td>Mg#<sup>3</sup></td><td></td><td>0.73(1)</td><td></td></tr><tr><td>Fo<sup>4</sup></td><td></td><td></td><td></td></tr><tr><td>An<sup>5</sup></td><td></td><td>42.77</td><td></td></tr><tr><td>En<sup>6</sup></td><td></td><td>10.19</td><td></td></tr><tr><td>Fs<sup>6</sup></td><td></td><td>47.04</td><td></td></tr><tr><td>Wo<sup>6</sup></td><td></td><td></td><td></td></tr></tbody></table>

Notes: (a) Bulk rock analysis (conducted by Franceschini (2022), ICP-MS, CRPG, Nancy, France. (b) Starting glass EPMA analyses. Oxides concentrations are normalized to 100 wt%. Standard deviations (1σ) are indicated in brackets in terms of smallest unit cited. Pl = Plagioclase, Ol = Olivine, Cpx = Clinopyroxene, Ilm = Ilmenite, Mt = Magnetite, Ap = Apatite. ¹Total iron content as FeO. ²Loss on ignition. ³Mg#=(MgO/(MgO+FeO<sub>tot</sub>)). ⁴In Ol, Fo=100(Mg/(Mg+Fe+Mn)), molar. ⁵In Pl, An=100(Ca/(Ca+Na+K)), molar. ⁶En, Fs and Wo calculated for Cpx following Morimoto (1989).
<table><caption>**Table 2 :** Trace element compositions of whole-rock and mineral phases of sample ETZ-13.</caption><thead><tr><th>Element<br>(ppm)</th><th>Bulk<br>rock<sup>(a)</sup></th><th>OI<sup>(b)</sup></th><th>PI (core)<sup>(b)</sup></th><th>PI (rim)<sup>(b)</sup></th><th>Cpx<sup>(b)</sup></th></tr><tr><td></td><td></td><td>n=3</td><td>n=8</td><td>n=7</td><td>n=4</td></tr></thead><tbody><tr><td>V</td><td>234.3</td><td>12.65(2.43)</td><td>0.27(10)</td><td>2.08(1.60)</td><td>329.43(3.75)</td></tr><tr><td>Ni</td><td>28.6</td><td>735.14(5.68)</td><td>0.46(34)</td><td>0.51(29)</td><td>17.48(17.01)</td></tr><tr><td>Cr</td><td>44.8</td><td>30.02(84)</td><td>0.47(47)</td><td>-</td><td>2.39(22)</td></tr><tr><td>Rb</td><td>9.46</td><td>0.06(3)</td><td>0.31(5)</td><td>0.16(4)</td><td>0.31(40)</td></tr><tr><td>Ba</td><td>285.90</td><td>6.97(4.73)</td><td>66.11(3.04)</td><td>26.23(4.56)</td><td>0.78(57)</td></tr><tr><td>Th</td><td>1.91</td><td>0.01(0)</td><td>0.01(1)</td><td>0.00(0)</td><td>0.03(3)</td></tr><tr><td>U</td><td>0.52</td><td>0.07(4)</td><td>0.01(0)</td><td>0.00(0)</td><td>0.02(2)</td></tr><tr><td>Nb</td><td>23.52</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Ta</td><td>1.98</td><td>0.01(1)</td><td>0.01(1)</td><td>0.00(0)</td><td>0.04(4)</td></tr><tr><td>La</td><td>30.47</td><td>0.48(30)</td><td>1.46(6)</td><td>0.76(7)</td><td>0.90(24)</td></tr><tr><td>Ce</td><td>68.81</td><td>0.17(10)</td><td>2.64(11)</td><td>1.40(19)</td><td>4.11(1.29)</td></tr><tr><td>Pb</td><td>2.09</td><td>0.03(1)</td><td>0.24(3)</td><td>0.12(4)</td><td>0.22(34)</td></tr><tr><td>Pr</td><td>10.53</td><td>0.10(6)</td><td>0.31(2)</td><td>0.15(3)</td><td>0.75(6)</td></tr><tr><td>Sr</td><td>885.18</td><td>1.62(1.04)</td><td>1514.69(43.20)</td><td>1018.21(77.69)</td><td>53.62(3.06)</td></tr><tr><td>Nd</td><td>50.36</td><td>0.43(26)</td><td>1.19(12)</td><td>0.62(9)</td><td>4.95(26)</td></tr><tr><td>Sm</td><td>11.95</td><td>0.08(4)</td><td>0.21(2)</td><td>0.11(2)</td><td>2.17(12)</td></tr><tr><td>Zr</td><td>200.41</td><td>3.71(2.12)</td><td>1.41(2.53)</td><td>0.47(66)</td><td>33.36(14.72)</td></tr><tr><td>Hf</td><td>4.94</td><td>0.07(5)</td><td>0.20(4)</td><td>0.03(3)</td><td>1.62(44)</td></tr><tr><td>Eu</td><td>3.85</td><td>0.03(3)</td><td>0.59(6)</td><td>0.22(2)</td><td>0.81(3)</td></tr><tr><td>Gd</td><td>10.88</td><td>0.15(10)</td><td>0.10(4)</td><td>0.08(2)</td><td>2.84(5)</td></tr><tr><td>Tb</td><td>1.49</td><td>0.02(1)</td><td>0.01(0)</td><td>0.01(0)</td><td>0.45(2)</td></tr><tr><td>Dy</td><td>8.07</td><td>0.18(9)</td><td>0.04(1)</td><td>0.03(1)</td><td>2.70(13)</td></tr><tr><td>Y</td><td>38.22</td><td>0.94(51)</td><td>0.15(2)</td><td>0.15(4)</td><td>12.10(36)</td></tr><tr><td>Er</td><td>3.54</td><td>0.11(6)</td><td>0.01(0)</td><td>0.01(1)</td><td>1.23(7)</td></tr><tr><td>Yb</td><td>2.60</td><td>0.12(7)</td><td>2.60(0)</td><td>0.01(1)</td><td>0.88(9)</td></tr><tr><td>Lu</td><td>0.37</td><td>0.02(1)</td><td>0.00(0)</td><td>0.00(0)</td><td>0.12(1)</td></tr></tbody></table>

Notes: (a) ICP-MS, CRPG, Nancy, France (Franceshini, 2022). Uncertainties are <20% (b) LA-ICP-MS. Standard deviations (1σ) are indicated in brackets in terms of smallest unit cited. n.d. = not determined.
<figure><img src="image_3.png" /><figcaption>Figure 2: (a) Iddingsitized Ol preserving fresh cores in ETZ-13. The black spots are LA-ICP-MS analyses. (b) Pl crystal showing an abrupt zonation as commonly observed in ETZ-13. Corresponding mean An contents for both cores and rims in ETZ-13 are reported on the figure.</figcaption></figure>

157x87mm (300 x 300 DPI)
<table><caption>**Table 3:** Results of different thermometers (and oxybarometer) used to infer the pre-eruptive conditions of the starting compositions used for the experiments.</caption><thead><tr><td>Method</td><td>ETZ-13</td></tr></thead><tbody><tr><td>Two-oxides calculated temperature (°C)</td><td>1051</td></tr><tr><td>Two-oxides calculated redox condition (ΔFMQ) (Ghiorso and Evans, 2008)</td><td>-0.80</td></tr><tr><td>Olivine-liquid thermometer (°C) (Putirka et al., 2008)</td><td>1065</td></tr><tr><td>Clinopyroxene thermometer (°C) (Wang et al., 2021)</td><td>1106</td></tr></tbody></table>

Notes: FMQ is the Quartz-Magnetite-Fayalite solid buffer and the Δ notation gives the log unit difference.
<figure><img src="image_4.png" /><figcaption>Figure 3: Fe-Mg partition coefficients between the experimental olivine and the residual glass against temperature. The grey field represents the value of Ulmer (1989) for equilibrium in basaltic liquids.</figcaption></figure>

144x90mm (300 x 300 DPI)
<table><caption>Table 4: Proportions of phases (wt%) and experimental parameters in the run products.</caption><thead><tr><th>Run</th><th>X<sub>H2O</sub><sup>1</sup></th><th>H<sub>2</sub>O<sub>melt</sub> by difference<sup>3</sup></th><th>H<sub>2</sub>O<sub>melt</sub> calc<sup>4</sup>(wt%)</th><th>fH<sub>2</sub>O (MPa)</th><th>fH<sub>2</sub><sup>5</sup> (MPa)</th><th>Log fO<sub>2</sub><sup>6</sup></th><th>ΔFMQ<sup>7</sup></th><th>Phase assemblage and abundances (wt %)<sup>8</sup></th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td colspan="10"><b>M6, 1080°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 6 h</b></td></tr><tr><td>13-H6</td><td>1.00</td><td>3.9</td><td>4.9</td><td>201.2</td><td>3.0</td><td>-10.9</td><td>-1.06</td><td>Gl</td><td></td></tr><tr><td>13-H7</td><td>0.63</td><td>3.1</td><td>3.7</td><td>126.7</td><td>3.0</td><td>-11.2</td><td>-1.37</td><td>Gl</td><td></td></tr><tr><td>13-H8</td><td>0.54</td><td>2.7</td><td>3.4</td><td>108.6</td><td>3.0</td><td>-11.5</td><td>-1.66</td><td>Gl</td><td></td></tr><tr><td>13-H9</td><td>0.35</td><td>1.9</td><td>2.6</td><td>70.4</td><td>3.0</td><td>-11.9</td><td>-2.10</td><td>Gl</td><td></td></tr><tr><td>13-H10</td><td>0.00</td><td>1.5</td><td>1*</td><td>15.2*</td><td>3.0</td><td>-13.4</td><td>-3.30</td><td>Gl</td><td></td></tr><tr><td colspan="10"><b>M11, 1060°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 6 h</b></td></tr><tr><td>13-H21</td><td>1.00</td><td>4.9</td><td>4.9</td><td>191.1</td><td>3.0</td><td>-11.5</td><td>-1.08</td><td>Gl</td><td></td></tr><tr><td>13-H22</td><td>0.67</td><td>3.0</td><td>3.9</td><td>133.4</td><td>3.0</td><td>-11.8</td><td>-1.39</td><td>Gl</td><td></td></tr><tr><td>13-H23</td><td>0.55</td><td>2.6</td><td>3.4</td><td>109.5</td><td>3.0</td><td>-12.1</td><td>-1.69</td><td>Gl (99.8), Ol (0.2)</td><td></td></tr><tr><td>13-H24</td><td>0.36</td><td>2.0</td><td>2.6</td><td>71.6</td><td>3.0</td><td>-12.5</td><td>-2.13</td><td>Gl (99.0), Ol (1.0)</td><td></td></tr><tr><td>13-H25</td><td>0.00</td><td>1.6</td><td>1*</td><td>15.2*</td><td>3.0</td><td>-13.7</td><td>-3.32</td><td>Gl (89.9), Pl (4.5), Ol (3.5), Cpx (2.0)</td><td>0.81</td></tr><tr><td colspan="10"><b>M7, 1040°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 10 h</b></td></tr><tr><td>13-H11</td><td>1.00</td><td>4.7</td><td>4.9</td><td>196.9</td><td>3.0</td><td>-11.8</td><td>-1.11</td><td>Gl</td><td></td></tr><tr><td>13-H12</td><td>0.69</td><td>3.0</td><td>3.9</td><td>135.8</td><td>3.0</td><td>-12.1</td><td>-1.42</td><td>Gl (99.8), Ol (0.2)</td><td>0.11</td></tr><tr><td>13-H13</td><td>0.54</td><td>3.5</td><td>3.3</td><td>106.8</td><td>3.0</td><td>-12.4</td><td>-1.71</td><td>Gl (99.4), Ol (0.6)</td><td>0.16</td></tr><tr><td>13-H14</td><td>0.32</td><td>2.8</td><td>2.4</td><td>63.0</td><td>3.0</td><td>-12.9</td><td>-2.16</td><td>Gl (99.0), Ol (1.0)</td><td>0.03</td></tr><tr><td>13-H15</td><td>0.00</td><td>2.0</td><td>1*</td><td>15.2*</td><td>3.0</td><td>-14.0</td><td>-3.34</td><td>Gl (80.7), Pl (10.1), Ol (5.1), Cpx (2.9), Ilm (1.3)</td><td>0.39</td></tr><tr><td colspan="10"><b>M15, 1020°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 12 h</b></td></tr><tr><td>13-H26</td><td>1.00</td><td>4.8</td><td>4.9</td><td>194.5</td><td>3.0</td><td>-12.2</td><td>-1.14</td><td>Gl</td><td></td></tr><tr><td>13-H27</td><td>0.72</td><td>3.1</td><td>3.8</td><td>140.0</td><td>3.0</td><td>-12.5</td><td>-1.44</td><td>Gl (95.5), Ol (3.4), Ilm (1.1)</td><td>0.11</td></tr><tr><td>13-H28</td><td>0.56</td><td>2.8</td><td>3.3</td><td>108.9</td><td>3.0</td><td>-12.8</td><td>-1.74</td><td>Gl (95.0), Ol (2.7), Ilm (1.2), Cpx (0.7),</td><td>0.16</td></tr><tr><td>13-H29</td><td>0.31</td><td>1.8</td><td>2.1</td><td>60.2</td><td>3.0</td><td>-13.2</td><td>-2.18</td><td>Gl (60.2), Pl (21.0), Cpx (10.5), Ol (4.8), Ilm (3.4),</td><td>0.03</td></tr><tr><td>13-H30</td><td>0.00</td><td>1.1</td><td>1*</td><td>15.2*</td><td>3.0</td><td>-14.3</td><td>-3.35</td><td>Gl (33.28), Pl (36.5), Cpx (15.7), Ol (7.7), Ilm (5.5), Ap (0.8)</td><td>0.39</td></tr><tr><td colspan="10"><b>M10, 1000°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 24 h</b></td></tr><tr><td>13-H16</td><td>1.00</td><td>5.1</td><td>4.9</td><td>192.1</td><td>3.0</td><td>-12.4</td><td>-1.16</td><td>Gl (87.4), Ol (5.5), Amph (2.7), Ap (2.3), Ilm (2.2)</td><td>0.18</td></tr><tr><td>13-H17</td><td>0.69</td><td>3.9</td><td>3.8</td><td>132.5</td><td>3.0</td><td>-12.7</td><td>-1.47</td><td>Gl (56.0), Pl (15.2), Ol (10.3), Cpx (8.9), Ilm (4.8), Ap (4.2), Amph (0.4)</td><td>0.07</td></tr><tr><td>13-H18</td><td>0.55</td><td>3.2</td><td>3.3</td><td>105.6</td><td>3.0</td><td>-13.0</td><td>-1.76</td><td>Gl (46.6), Pl (23.2), Ol (11.6), Cpx (9.0), Ilm (5.1), Ap (4.5)</td><td>0.09</td></tr><tr><td>13-H19</td><td>0.27</td><td>-</td><td>2.1</td><td>51.8</td><td>3.0</td><td>-13.4</td><td>-2.20</td><td>Gl (35.4), Pl (31.2), Ol (14.1), Cpx (8.5), Ilm (5.5), Ap (5.3)</td><td>0.39</td></tr><tr><td>13-H20</td><td>0.00</td><td>-</td><td>1*</td><td>15.2*</td><td>3.0</td><td>-14.6</td><td>-3.36</td><td>Gl (16.8), Pl (48.9), Ol (16.6), Cpx (8.6), Ilm (4.8), Ap (4.2)</td><td>0.20</td></tr></tbody></table>
<table><caption>Table 4 : continued</caption><thead><tr><th>Run</th><th>X<sub>H2O</sub><sup>1</sup></th><th>H<sub>2</sub>O<sub>melt</sub> by<br>difference<sup>3</sup></th><th>H<sub>2</sub>O<sub>melt</sub><br>calc<sup>4</sup> (wt%)</th><th>fH<sub>2</sub>O<br>(MPa)</th><th>fH<sub>2</sub><sup>5</sup><br>(MPa)</th><th>Log fO<sub>2</sub><sup>6</sup></th><th>ΔFMQ<sup>7</sup></th><th>Phase assemblage and abundances (wt %)<sup>8</sup></th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td colspan="10"><b>M16, 975°C, 200 MPa, PH<sub>2</sub> = 1 MPa (target = FMQ-1), 48 h</b></td></tr><tr><td>13-H31</td><td>1.00</td><td>5.6</td><td>4.9</td><td>190.2</td><td>3.0</td><td>-12.6</td><td>-1.18</td><td>Gl (51.8), Amph (38.9), Ap (3.6), Pl (3.2), Ilm (2.4), Ol (0.1)</td><td>0.02</td></tr><tr><td>13-H32</td><td>0.66</td><td>-</td><td>3.7</td><td>63.8</td><td>3.0</td><td>-12.9</td><td>-1.49</td><td>Gl (38.2), Amph (53.1), Pl (7.2), Ap (3.0), Ilm (0.9), Ol (0.6)</td><td>0.13</td></tr><tr><td>13-H33</td><td>0.52</td><td>-</td><td>3.2</td><td>52.4</td><td>3.0</td><td>-13.2</td><td>-1.78</td><td>Gl (17.3), Amph (32.0), Pl (28.4), Cpx (16.3), Ilm (4.5), Ap (1.0), Ol (0.5)</td><td>0.09</td></tr><tr><td>13-H34</td><td>0.27</td><td>-</td><td>2.1</td><td>34.3</td><td>3.0</td><td>-13.6</td><td>-2.22</td><td>Gl (5.4), Pl (55.6), Cpx (17.0), Ol (12.9), Ilm (6.4), Ap (2.8)</td><td>0.51</td></tr><tr><td colspan="10"><b>M117, 1060°C, 100 MPa, PH<sub>2</sub> = 0.8 MPa (target = FMQ-1), 6 h</b></td></tr><tr><td>13-I1</td><td>1.00</td><td>2.3</td><td>3.1</td><td>95.2</td><td>2.4</td><td>-12.1</td><td>-1.46</td><td>Gl</td><td></td></tr><tr><td>13-I2</td><td>0.75</td><td>2.2</td><td>2.4</td><td>63.8</td><td>2.4</td><td>-12.4</td><td>-1.81</td><td>Gl</td><td></td></tr><tr><td>13-I3</td><td>0.64</td><td>2.1</td><td>2.1</td><td>52.4</td><td>2.4</td><td>-12.6</td><td>-1.98</td><td>Gl, Ol</td><td></td></tr><tr><td>13-I4</td><td>0.28</td><td>2.0</td><td>1.7</td><td>34.3</td><td>2.4</td><td>-13.0</td><td>-2.35</td><td>Gl (94.6), Ol (3.0), Pl (2.4)</td><td>0.48</td></tr><tr><td>13-I5</td><td>0.00</td><td>1.3</td><td>1*</td><td>15.2*</td><td>2.4</td><td>-13.7</td><td>-3.05</td><td>Gl (73.3), Pl (20.2), Ol (4.8), Ilm (1.7)</td><td>0.28</td></tr></tbody></table>

Notes: Gl = Glass, Pl = Plagioclase, Ol = Olivine, Cpx = Clinopyroxene, Ilm = Ilmenite, Amph = Amphibole, Ap = Apatite. ¹Initial mole fraction of H₂O of the C–O–H fluid loaded to the capsule. ²Residual of the mass-balance calculations. ³H₂O<sub>melt</sub> determined by difference (wt%). ⁴H₂O<sub>melt</sub> calc at H₂O saturation using the model of solubility of Lesne et al. (2011). The H₂O calc in water undersaturated capsules is determined using the methodology of Scaillet & Macdonald (2006). ⁵fH₂=3*PH₂ (Di Carlo et al., 2010). ⁶Log of the oxygen fugacity (bar) determined using the water-dissociation equation. ⁷log fO₂ (experiment)-logfO₂ of FMQ buffer calculated at P and T from O'Neill (1993). ⁸Calculated by mass-balance (Albarède, 1995).
<figure><img src="image_5.png" /><figcaption>Figure 4: Melt fraction of residual glass in the run products as a function of temperature for different XH2O. The 100 MPa experiments are indicated with a red contour. The proportion of matrix glass in ETZ-13 is also shown.</figcaption></figure>

215x142mm (300 x 300 DPI)
<figure><img src="image_6.png" /><figcaption>Figure 5: Phase relationships as established from the experiments for ETZ-13 at 200 MPa ~FMQ-1. Phases lie inside their stability field. Grey dashed lines indicate the wt% of crystals.); Gl = Glass, Pl = Plagioclase, Ol = Olivine, Cpx = Clinopyroxene, Ilm = Ilmenite, Amph = Amphibole, Ap = Apatite. The approximate solidus position is shown as a dashed line. The pink rectangle delimits the inferred pre-eruptive T-H2Omelt conditions. See text for more details.</figcaption></figure>

153x138mm (300 x 300 DPI)
<figure><img src="image_7.png" /><figcaption>Figure 6: An% content of the PI in the run products as a function of temperature. Experiments at 100 MPa are indicated with a red contour. The An contents of the PI core and rim in ETZ-13 are also represented. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature. Errors are smaller than symbol size (1σ).</figcaption></figure>

204x145mm (300 x 300 DPI)
<figure><img src="image_8.png" /><figcaption>Figure 7: Fo% content of the OI in the run products as a function of temperature. The OI that crystallized in the 100 MPa experiments are indicated with a red contour. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature. The Fo% content of the OI in ETZ-13 are also represented. Errors are smaller than the symbols (1σ).</figcaption></figure>

199x133mm (300 x 300 DPI)
<figure><img src="image_9.png" /><figcaption>Figure 8: (a) Variation of Mg# (calculated as Mg# = Mg/(Mg+Fe)), molar) in function of temperature. (b) Variation of TiO2 (wt%) content in Cpx in function of temperature. (c) Variation of FeOT (wt%) content in Cpx in function of temperature. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black dashed arrows illustrate the effect of temperature. Errors are smaller than the symbols (1σ).</figcaption></figure>

172x162mm (300 x 300 DPI)
<figure><img src="image_10.png" /><figcaption>Figure 9: (a) Variation of FeOT (wt%) content in Ilm in function of temperature. (b) Variation of TiO2 (wt%) content in Ilm in function of temperature. The vertical blue dashed arrow shows the effect of increasing melt water content, while the black one illustrates the effect of temperature Errors are smaller than the symbols (1σ).</figcaption></figure>

166x63mm (300 x 300 DPI)
<figure><img src="image_11.png" /></figure>

Figure 10: Harker diagrams of the experimental glasses. Natural whole-rock data from the MER are represented as the blue field (WoldeGabriel et al., 1999; Trua et al., 1999; Peccerillo et al., 2003; Giordano et al., 2014; Hutchison et al., 2016; Franceschini, 2022). The temperature of experiments is colour coded as indicated on the top panel and labelled next to some symbols. The experimental glasses at 100 MPa are shown with a red contour.

253x279mm (300 x 300 DPI)
<figure><img src="image_12.png" /><figcaption>Figure 11: Evolution of the peralkalinity index (PI = Na+K/Al, molar) against residual melt fraction in the experiments. The experiment at 100 MPa and 1060°C is highlighted by a red contour.</figcaption></figure>

170x113mm (300 x 300 DPI)