# Low δ30Si values in olivine-hosted melt inclusions trace sediment contributions to subduction zone melts

Anne-Sophie Bouvier, Martin Guitreau, Clémence Le Lay, Muriel Laubier, Estelle F. Rose-Koga

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Anne-Sophie Bouvier, Martin Guitreau, Clémence Le Lay, Muriel Laubier, Estelle F. Rose-Koga. Low δ30Si values in olivine-hosted melt inclusions trace sediment contributions to subduction zone melts. Earth and Planetary Science Letters, 2026, 679, 10.1016/j.epsl.2026.119884. insu-05537687

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Submitted on 5 Mar 2026

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# Low δ³⁰Si values in olivine-hosted melt inclusions trace sediment contributions to subduction zone melts

Anne-Sophie Bouvier<sup>a, *, b, c</sup>, Martin Guitreau<sup>b, c, d</sup>, Clémence Le Lay<sup>a</sup>, Muriel Laubier<sup>b</sup>, Estelle F. Rose-Koga<sup>d, e</sup>

## ARTICLE INFO

Editor: Dr Fang-Zhen Teng

**Keywords:**

Silicon isotopes
LA-MC-ICP-MS
Melt inclusions
Olivine
MORB
Subduction

## ABSTRACT

This study reports the first high-precision silicon isotope (δ³⁰Si) measurements of olivine-hosted melt inclusions (OHMIs) and their host olivine crystals using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). We analyzed samples from two contrasting tectonic settings: three mid-ocean ridge basalts (MORB) and four island arc volcanic rocks. Our results reveal systematic differences in silicon isotope compositions between these two environments. MORB samples show internally consistent δ³⁰Si values in both melt inclusions and host olivine crystals, with isotopic fractionation between phases suggesting equilibrium crystallization processes in MORB magmas. In contrast, arc samples display greater heterogeneity and systematically lower δ³⁰Si values that deviate from equilibrium fractionation factors between olivine and melt. The absence of a correlation with the degree of polymerization of the silicate melt (NBO/T) indicates that these isotopic signatures are not controlled by melt structure. The coupling of low δ³⁰Si and high δ¹⁸O values suggests significant contributions from subducted sedimentary materials, particularly siliceous components, to the arc magma source. Our results demonstrate that OHMIs preserve small-scale Si isotope heterogeneities which are invisible at the bulk rock scale. This work highlights the potential of *in situ* silicon isotope analysis of melt inclusions and their host crystals to trace magmatic processes and source contributions in different tectonic environments, hence providing new insights into the geochemical evolution of arc magmas and the role of subducted materials in mantle heterogeneity.

## 1. Introduction

Silicon is the second most abundant element on Earth and a major constituent of silicate minerals. It has three stable isotopes: ²⁸Si (92.23 %), ²⁹Si (4.68 %) and ³⁰Si (3.09 %). Silicon isotope geochemistry has mostly been used to characterize low temperature processes because they lead to large isotopic fractionation of up to 9 ‰ (Poitrasson, 2017). Fluids from surface environments often show enrichment in heavy Si isotopes due to the preferential uptake of light Si isotopes into clay minerals and/or micro-organisms in soils (e.g., Basile-Doelsch, 2006). Consequently, surface materials commonly exhibit Si isotopic compositions distinct from that of the mantle (δ³⁰Si = -0.29 ± 0.07 ‰, 2SD; Savage et al., 2010). High-temperature processes such as mantle melting and magma fractional crystallization, in contrast, induce small isotopic

fractionation (< 0.20 ‰; Armytage et al., 2012; Chakrabarti and Jacobsen, 2010; Fitoussi et al., 2009; Georg et al., 2007; Liu et al., 2025; Poitrasson and Zambardi, 2015; Qin et al., 2016; Savage et al., 2010; Savage et al., 2011; Savage et al., 2013a, b). Any variation larger than 0.20 ‰ in high temperature rocks is thus likely to reflect reworking or assimilation of material that was previously altered at low temperature (e.g., Guitreau et al., 2022; Savage et al., 2011; Trail et al., 2018; Ziegler et al., 2005).

To date, silicon isotope studies that focused on terrestrial basaltic material mostly rely on bulk analyses which display a narrow range of δ³⁰Si (between -0.39 ± 0.11 and -0.23 ± 0.05 ‰; e.g., Savage et al., 2010), therefore, preventing the observation of possible small-scale variability or primary heterogeneity before magma mixing or crustal assimilation. So far there has been no systematic *in-situ* measurements in

<footnote><sup>a</sup> Institut des Sciences de la Terre, Université de Lausanne, Switzerland</footnote>

<footnote><sup>b</sup> Laboratoire Magmas et Volcans, CNRS-UMR6524, IRD-UMR163, OPGC, Université Clermont Auvergne, F-63178 Aubière, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), 75005 Paris, France</footnote>

<footnote><sup>d</sup> Institut des Sciences de la Terre d'Orléans, ISTO, UMR7327, CNRS, UO, OSUC, F-45071, Orléans, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: anne-sophie.bouvier@unil.ch (A.-S. Bouvier).</footnote>
natural basaltic glasses from terrestrial samples. Only a few *in-situ* Si isotope studies have targeted olivine, mostly in extra-terrestrial material and in serpentinites (Geilert et al., 2021; Kadlag et al., 2021; Villeneuve et al., 2019; Villeneuve et al., 2020). This study aims to present the first *in-situ* silicon isotope data for terrestrial olivine crystals and their melt inclusions (OHMIs) in order to document the Si isotope compositions of melts before any potential influence of superficial processes.

Melt inclusions are small droplets of magma trapped during crystal growth. Because olivine is usually the first phase to crystallize during mafic magma crystallization, olivine-hosted melt inclusions are powerful tools to study the origin of mantle-derived magmas, as they represent near primary melts (e.g., Schiano, 2003). Here we analyzed Si isotopes using LA-MC-ICP-MS in olivine crystals and their melt inclusions from natural samples from mid-ocean ridge and modern arc settings. This study presents the first *in-situ* silicon isotope data in OHMI and isotope fractionation factor between natural olivine crystals and OHMIs.

## 2. Analytical techniques

### 2.1. Samples

MORB samples analyzed in this contribution are from three dredged basalts from the mid-Atlantic ridge: two picrites from the FAMOUS segment, close to each other (ARP73-10-03 and CH31-DR8; N-MORBs; Laubier et al., 2012), and one olivine-phyric lava from the 14° MAR triple junction, highly enriched in volatiles, with CO₂ present in vesicles, and water dissolved in the glass (CH77-DR6-203; E-MORB: Javoy and Pineau, 1991). All samples contain olivine crystals which host glassy melt inclusions. Thirty five of the MORB OHMIs presented here have already been analyzed for major elements and δ¹⁸O (Manzini et al., 2019). Amongst those, almost half of them are not in O isotopic equilibrium with their host crystal, with Δ¹⁸Ool-melt either < -1‰ or > -0.2‰. The remaining thirteen OHMIs are characterized for the first time.

Four arc samples have been selected for this study: SVN4b from St. Vincent (Lesser Antilles), IZ27-15 from Sukumoyama (North Izu Bonin), Sommata from La Sommata (Aeolian islands) and Aoba (Vanuatu). These OHMIs have already been characterized for major elements, δ¹⁸O, δ¹¹B and δ³⁷Cl, as well as their host olivine crystals for δ¹⁸O (Bouvier et al., 2019; Bouvier et al., 2022). Isotopic signatures show that the contribution of the subducted slab to mantle melting differs depending on the locality. Interestingly, the comparison of the δ¹⁸O values between each OHMI and its host olivine shows that none of the OHMIs from La Sommata is in isotopic equilibrium with its host olivine, whereas 17, 63, 79 and 46% of OHMI populations of Sukumoyama, Iwate, Aoba and St Vincent, respectively, are out of equilibrium with their host (Bouvier et al., 2022).

### 2.2. Sample preparation

Olivine crystals with naturally glassy melt inclusions with no visible crack were handpicked from their crushed matrix. Single olivine grains were polished to expose their melt inclusions, then pressed into 1-inch indium mounts with glass reference materials (BHVO-2G, BCR-2G and/or BIR-1G, depending on the mount) and San Carlos olivine (e.g., Schuessler and von Blanckenburg, 2014; Zambardi and Poitrasson, 2011). We checked for the absence of microcrystals within each melt inclusion using a Tescan Mira LMU field-emission scanning electron microscope (SEM) at the University of Lausanne, operated at 20 kV and a 0.2-0.4 nA probe-current. Finally, a Brucker GTA-K white light interferometer was used to check the topography of each mount.

### 2.3. Electron-microprobe analyzer (EMPA)

Major element compositions of olivine crystals and melt inclusions were acquired using a JEOL 8200 SuperProbe electron microprobe analyzer at the University of Lausanne. Analytical conditions for the

quantitative measurements of olivine were 15 kV, 20 nA, with a focused 1µm beam. Analytical conditions for glass analysis (melt inclusions) were 15 kV, 15 nA with a beam diameter of 10 µm. Counting times and standards used for each element, for olivine and glass, are compiled in Supplementary Material 1a. Major element concentrations are required to calculate SIMS instrumental mass fractionation (Manzini et al., 2017).

## 2.4. Laser ablation multi collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

The Si isotopic compositions of OHMIs and their host olivine crystals were measured by LA-MC-ICP-MS at Laboratoire Magmas et Volcans (LMV, Clermont-Ferrand, France) using a Resonetics Resolution M-50 coupled to a Thermo Scientific Neptune Plus, during three different sessions (February 2020, December 2020, and January 2025). Olivine crystals were analyzed by LA-MC-ICP-MS only because there were not enough available reference materials at the SwissSIMS lab to accurately correct for the large matrix effect during SIMS measurements (Villeneuve et al., 2019). LA-MC-ICP-MS measurements were done following the analytical protocol detailed by Guitreau et al. (2022; 2020) for the first session, but for the last two sessions we added ²⁷Al to the cup configuration to monitor that we are indeed measuring the inclusion and not the crystal (Supplementary Material 1b). Silicon isotopic fractionation induced by MC-ICP-MS instruments evolves non-linearly over time (Zambardi and Poitrasson, 2011) which requires a strict standard bracketing procedure to be applied to Si isotope measurements by LA-MC-ICP-MS. Because of matrix effect and issues with ablation of silica-rich material using nano-second lasers (e.g., Guitreau et al., 2020), we could not calibrate Si isotope analyses using the quartz reference material NBS28. Consequently, measured Si isotope compositions for OHMIs and olivine crystals were calibrated against BHVO-2G and San Carlos (SC) olivine (Schuessler and Von Blanckenburg, 2014), respectively, which were used as bracketing standards. All δ³⁰Si were subsequently re-calculated relative to NBS28 using BHVO-2G and SC δ³⁰Si reported values (Schuessler & Von Blanckenburg, 2014).

The depth of the pit formed during LA-MC-ICP-MS analyses (~20 µm) can be greater than the thickness of the targeted OHMI. The difference in Si counts between olivine and OHMIs being relatively small, it is important to monitor whether the analysis corresponds to glass only or whether some olivine was also sampled. During the first session, we only measured silicon isotopes and we attributed the decrease in Si count rate to olivine contribution (beneath the melt inclusion). To better constrain olivine contamination, we measured ²⁷Al concurrently to Si during the last two sessions (December 2020 and January 2025). Our typical internal error (on a single measurement, given as a 2 standard errors, 2 SE) is ± 0.15 ‰. Reference materials used for δ³⁰Si determination in melt inclusions were BHVO-2G, BIR-1G, BCR-2G and San Carlos olivine (Fig. 1; Supplementary Material 2a, b). The reference materials typically gave a reproducibility of ± 0.20 ‰ (two standard deviations, 2SD) both within individual sessions and across the three sessions. However, BIR-1G reference material measured value deviates from the published value (measured average δ³⁰Si of -0.61±0.32‰, average literature value: -0.27±0.07‰; Zambardi and Poitrasson, 2011) in the final session, leading to an overall reproducibility over the 3 sessions of ± 0.32 ‰ (Fig. 1; Supplementary Material 2a,b). It is important to note that the glass chip of BIR used in this last session differed from the one used in the first two sessions. This discrepancy suggests potential heterogeneity in Si isotopes within the BIR reference material.

## 3. Results

### 3.1. Major element compositions of melt inclusions and their host olivine crystals

A total of forty-eight OHMIs were analyzed, including thirty five OHMIs analyzed by Manzini et al. (2019). The thirteen new OHMIs from
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Average measured values for different reference materials (circles), compared to the published bulk values (squares). For BHVO-2G and SC olivines, 2 batches (one from University of Lausanne, UNIL, the other from Laboratoire Magmas et Volcans, LMV) were tested and treated separately to ensure both batches of the same reference material have the same Si isotopic composition. Reference values are from Schuessler and Von Blanckenburg, 2014 (BHVO-2G, SC) and Zambardi and Poitrasson, 2011 (BIR-1G and BCR-2G). The three different LA-MC-ICP-MS sessions are represented by different shades of greys. Overall, data from this study agree well with literature data, except for BIR-1G in the last LA-MC-ICP-MS session.</figcaption></figure>

MORB samples (ARP73-10-03 and CH31-DR8) are entrapped in olivine with forsterite content (Mg/(Mg+Fe) in moles) ranging from 89.3 to 92.0. The host olivine NiO (0.21 to 0.39 wt%) and CaO (0.31 and 0.39 wt %) contents are typical of magmatic olivines (e.g., Laubier et al., 2007). Before any petrogenetic interpretation, OHMIs must be corrected for post-entrapment crystallization (PEC), which refers to the fact that OHMIs underwent olivine crystallization after their entrapment. This process mainly affects Mg (major element in olivine) and Fe concentrations (e.g., Danyushevsky et al., 2000). The amounts of PEC were calculated using the Petrolog software (Danyushevsky and Plechov, 2011). The raw and PEC corrected major element compositions of all OHMIs are reported in Supplementary Material 3. The amount of PEC for the 13 new OHMIs ranges from 4.4 to 13.6 %, similar to previous studies (Laubier et al., 2012; Manzini et al., 2019). All the data presented hereafter are corrected for PEC. The OHMIs from ARP73-10-03 and CH31-DR8 MORB samples have basaltic compositions, similar to melt inclusions from the same samples published in Laubier et al. (2012) and Manzini et al. (2019). For comparison, CH77-DR6-203 melt inclusions display a similar range in SiO₂, but higher alkali contents (K₂O from 0.05 to 0.46 wt% and Na₂O from 2.07 to 2.60 wt%) than ARP73-10-03 and CH31-DR8 (Figure).

Major element compositions of melt inclusions from La Sommata, St. Vincent Sukumoyama and Aoba were published in Bouvier et al. (2022) and reported in Supplementary Material 3. Their variability in SiO₂ is larger than that of MORB OHMI, with concentrations ranging from 47.4 to 51.4 wt%, and they display higher alkali contents (Na₂O ranging from 1.9 to 2.1 wt% and K₂O from 1.6 to 2.2 wt%; Fig. 2). Their host olivine crystals have variable Fo contents, ranging from 89 to 91% for La Sommata, from 80 to 87% for Aoba, from 83 to 84% for Sukumoyama and from 84.1 to 86.2% for St. Vincent.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** K₂O as a function of SiO₂ in OHMIs. Round symbols are for MORB samples, whereas square symbols are for arc samples. Melt inclusions from CH31-DR8 and part of the data from ARP73-10-03 are from this study, whereas the other data from this specific sample and for CH77-DR6-203 are from [1]: Manzini et al. (2019). [2]: Major elements for melt inclusions from arcs are from Bouvier et al. (2022).</figcaption></figure>
## 3.2. $\delta^{30}$Si in melt inclusions

All LA-MC-ICP-MS data for OHMIs are reported in Supplementary Material 4 and summarized in Supplementary Material 5. Note that Si isotope data were not corrected for PEC. With an expected maximum equilibrium fractionation factor between olivine and a basaltic melt at 1250°C of -0.07‰ (Qin et al., 2016) and considering a calculated PEC < 10%, correcting for PEC would result in a change of melt inclusion $\delta^{30}$Si <0.1‰, which is smaller than our LA-MC-ICP-MS analytical uncertainty. OHMIs from MORB samples show similar ranges of $\delta^{30}$Si values, with averages of -0.38 ± 0.16 ‰ (2SD) for ARP73-10-03, -0.43 ± 0.27 ‰ (2SD) for CH31-DR8 and -0.33 ± 0.25 ‰ (2SD) for CH77-DR6-203 (Fig. 3). Thirteen OHMIs were measured repeatedly, either during the same session or across the three different sessions. These measurements were consistent with each other within less than 0.20 ‰ (Supplementary Material 5). This consistency was observed both within a session and between sessions. However, 4 of the OHMIs exhibited less consistency, with values reproduced within 0.23 and 0.50 ‰ (2SD), higher than analytical uncertainty, which might reflect a small heterogeneity in those OHMIs.

The $\delta^{30}$Si of OHMIs from arc samples exhibit larger variations, with on average lower values compared to OHMIs from MORBs: average $\delta^{30}$Si values of -0.65 ± 0.71 ‰ (2SD) for La Sommata OHMIs, -0.75 ± 0.46 ‰ (2SD) for St. Vincent OHMIs, -0.54 ± 0.52 ‰ (2SD) for Sukumoyama OHMIs and -0.47 ± 0.38 ‰ (2SD) for Aoba OHMIs (Fig. 3). Due to their smaller size, only 4 OHMIs were measured repeatedly, and show reproducibility from 0.22 to 0.40 ‰, suggesting some degree of heterogeneity in these OHMIs (Supplementary Material 5).

## 3.3. $\delta^{30}$Si in the host olivines

All LA-MC-ICP-MS data for olivine crystals are reported in Supplementary Material 6 and summarized in Supplementary Material 5. Twenty-nine olivine crystals from MORBs (Fo$_{85}$-92) have been analyzed for $\delta^{30}$Si, which include 16 crystals from ARP73-10-03, 4 crystals from CH31-DR8 (both N-MORBs), and 9 crystals from CH77-DR6-203 (E-MORB). The overall range of $\delta^{30}$Si values in MORB olivine is narrow and similar for each MORB sample, leading to an average MORB value of -0.35 ± 0.14 ‰ (Fig. 3, Supplementary Material 5). Most olivine crystals

were analyzed more than once. The largest variability observed within an olivine was 0.30 ‰, 2SD (Supplementary Material 5, 6).

Olivine crystals from arc samples also show a narrow range of $\delta^{30}$Si values, independent of their Fo content. La Sommata olivines have average $\delta^{30}$Si values of -0.39 ± 0.18 ‰ (2SD), Aoba olivines average at -0.36 ± 0.32 ‰ (2SD), Sukumoyama at -0.25 ± 0.15 ‰ (2SD) and St. Vincent at -0.40 ± 0.16 ‰ (2SD, Supplementary Material 6). As for the MORB samples, most olivine crystals were measured at least twice, showing reproducible data, within 0.20 ‰ (2SD). Only 4 olivine crystals have variable $\delta^{30}$Si values with 2SD of 0.30-0.40 ‰ (Supplementary Data 6).

# 4. Discussion

## 4.1. Influence of melt polymerization

The degree of melt polymerization has been suggested to play a major role in Si isotope fractionation (Savage et al., 2010), with higher $\delta^{30}$Si observed for lower NBO/T. Marianas and South Sandwich glasses have an NBO/T < 0.8, and the arc OHMIs from this study have an NBO/T ranging from 0.52 to 1.12 (as calculated from the raw EMPA data, before PEC correction, Supplementary Material 3). The absence of a correlation between NBO/T and $\delta^{30}$Si (Fig. 4a), either within samples or between samples, suggests that the variable $\delta^{30}$Si measured in arc OHMIs, with $\delta^{30}$Si values lower than bulk glass data, is not related to a change in the degree of polymerization due to post-entrapment processes (i.e., this is not an analytical artefact). If NBO/T is calculated using PEC-corrected major element compositions (Fig. 4b), which are supposed to represent the composition – and thus polymerization – of the melt before entrapment, a very weak positive correlation (R$^2$= 0.6) may be described between $\delta^{30}$Si and NBO/T for arc OHMIs, except for La Sommata OHMIs, whereas OHMIs from MORBs do not show any correlation (Fig. 4b). Bulk glass data from MORBs, the Marianas and South Sandwich (Savage et al., 2010) and Dazhonghe (Liu et al., 2022) are also reported in Fig. 4, and do not show any correlation between NBO/T and $\delta^{30}$Si. More importantly, any difference between in situ and bulk data within the arc samples, or between arcs, cannot be attributed to different NBO/T values.

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Box plots of Si isotope signatures measured in OHMIs (left) and olivine (right) for the different samples. The gray horizontal bar, going through the left and right panels, is the average values published for MORB (-0.27 ± 0.06 ‰; Savage et al., 2010; Liu et al., 2025), which also overlap with the average values published for modern island arc bulk basaltic glasses from the Marianas and South Sandwich samples (from -0.32 ± 0.05‰ to -0.25 ± 0.03‰; Savage et al., 2010).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. NBO/T calculated using the raw major element contents of OHMIs (as measured by EPMA) (a) and NBO/T calculated using OHMI compositions corrected for post-entrapment crystallization (b) as a function of δ³⁰Si (b). For comparison, data for MORB and arc bulk glasses from Savage et al. (2010) are reported as triangles. No PEC correction is required for those glasses, so the NBO/T of these samples are the same in both graphs.</figcaption></figure>

## 4.2. Si isotope signatures of MORB and arc OHMIs

The average values of δ³⁰Si obtained in the Atlantic MORB OHMIs (-0.37 ± 0.20 ‰; 2SD) are comparable, within uncertainty, to bulk published values from the Atlantic MORB glasses (-0.27 ± 0.06 ‰; Savage et al., 2010; Liu et al., 2025). OHMIs measured repeatedly during a session or in three different sessions show the same reproducibility, suggesting that each OHMI is homogeneous. Each OHMI population is relatively homogeneous, with 2 SD similar to that of the reference material (~0.20 ‰), except for CH31-DR8 which is the MORB sample with the lowest number of OHMIs analyzed. The similar δ³⁰Si between OHMIs from E-MORBs and N-MORBs show that the mantle enrichment in alkali and volatile elements beneath the 14° MAR triple junction did not significantly affect Si isotopes, suggesting a homogeneous and uniform MORB mantle, in agreement with Savage et al. (2010) and Liu et al. (2025). This consistency between OHMIs and bulk MORB data indicates that magmatic processes occurring after OHMIs entrapment (crystallization, mixing) do not significantly modify Si isotope signatures in MORB settings, allowing both analytical approaches to capture the same mantle source information.

All arc samples display a larger range of δ³⁰Si in OHMIs compared to those from MORBs (Fig. 3). A Welch's two tailed T-test shows that arc melt inclusions have statistically significant lower δ³⁰Si values compared to MORB samples (t-statistic = -3.41, degree of freedom = 21 and p-value = 0.003). Their δ³⁰Si values are also lower than those published for modern island arc bulk basaltic glasses from the Marianas and South Sandwich samples (from -0.32 ± 0.05 ‰ to -0.25 ± 0.03 ‰; Savage et al., 2010). The difference between δ³⁰Si in bulk basaltic glasses and in OHMIs could be explained by the fact that published glass data are not from the same arcs. Alternatively, this difference may also reflect the different sampling sizes, with OHMIs representing primitive, micro-scale melts, whereas bulk glasses reflect more evolved, homogenized magmas that, in this case, could have undergone mixing, crystallization, and possibly crustal assimilation.

The different δ³⁰Si values between OHMIs from MORB and those from arcs could be attributed to slab-derived components. Interestingly, Savage et al. (2010) observed that bulk samples from the Marianas and South Sandwich arcs do not show any correlation with the type of subducted sediments, despite these two arcs having very different subducting sediment endmembers (mainly clays vs. siliceous ooze, respectively). The authors concluded that the nature of subducted sediments did not influence the bulk δ³⁰Si of analyzed glasses. Nevertheless, arc rocks with variable SiO₂ contents (from 55.7 to 71.0 wt%) from a Paleozoic continental arc (Dazhonghe; Liu et al., 2022) display low δ³⁰Si

(-0.47 ± 0.05 to -0.71 ± 0.05 ‰), which were interpreted as reflecting the influence of sponge-rich sediments.

In order to further investigate the origin of the low Si isotope values measured in OHMIs from arcs, the δ³⁰Si values were compared to δ¹⁸O values obtained in the same OHMIs (Fig. 5; Bouvier et al., 2022). Indeed, Liu et al. (2022) showed that the influence of the nature of subducted sediments can be identified when the δ³⁰Si data are coupled to δ¹⁸O. Silica-rich sediments, like marine sponges, typically display low δ³⁰Si (down to -6.74 ‰, median value of -4.2 ‰; Cassarino et al., 2018) associated with high δ¹⁸O (up to +42 ‰; e.g., Yanchilina et al., 2021). Silica-rich sediments can thus be distinguished from modern authigenic clays, which exhibit lower δ¹⁸O (+30 ‰; Isson and Rauzi, 2024) and higher δ³⁰Si than marine sponges (average value of -2 ‰; Geilert et al., 2024). On the contrary, radiolarites tend to have positive δ³⁰Si (-0.2 to +2 ‰; e.g., Bole et al., 2020a) and elevated δ¹⁸O (+20 - +35 ‰; Bole et al., 2020b). Mixing trends between DMM and these sediment endmembers are represented in Fig. 5. For comparison, we also report the composition of oceanic crust altered at high temperature, taken as the most extreme δ¹⁸O and δ³⁰Si values from Yu et al. (2018). It should be noted that Yu et al. (2023) reported slightly more extreme δ³⁰Si values (-0.58‰ instead of -0.37‰ used here) for altered oceanic crust, however the authors did not provide δ¹⁸O values. In Fig. 5, most OHMIs plot close to the mixing lines between DMM and the different sediment endmembers (radiolarites, modern authigenic clays, median composition of sponge-rich sediments or the extreme composition of sponge-rich sediments), suggesting that the lower δ³⁰Si values most probably reflect the greatest influence of subducted sediments. Four OHMIs are shifted from the DMM-sediment mixing lines, but would plot along a mixing trend between the high temperature altered oceanic crust (AOC; Yu et al., 2018) and sponge-rich sediments. The mixing trend between AOC and sponge-rich sediments could reflect the bulk slab composition (e.g., mélange melting model; Nielsen and Marschall, 2017). However, the isotopic values required to fit the 4 OHMIs are rather extreme (i.e., the δ¹⁸O values < +3.5 ‰ only represent a small portion of the AOC; Gao et al., 2012). Moreover, mixing between DMM and a slab mélange would imply almost 100% of the slab mélange to explain the 4 OHMIs, which is not realistic. In fact, one OHMI, surrounded by a circle in Fig. 5, displays geochemical evidence for local assimilation of crust altered at high temperature, based on the high P content, low Fo (~83), and low δ¹⁸O of its host olivine (+2.6 to +3.1 ‰) (Bouvier et al., 2022). The 4 OHMIs that fall outside any possible mixing line between DMM and sediments may instead represent the additional influence of a high temperature solute-poor fluid from within the arc crust, affecting the δ¹⁸O of the melt but not its δ³⁰Si, thereby decoupling O and Si isotope systematics.
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Si isotope signatures as a function of O isotope signatures measured in the same OHMIs, modified from Liu et al. (2022). The two bulk data from Liu et al. (2022) are reported for comparison, as well as the DMM and one of the most extreme compositions of the altered AOC (δ¹⁸O of +3.0 ‰ and δ³⁰Si of -0.37 ‰, Yu et al., 2018). Mixing lines between DMM (δ¹⁸O: Eiler (2001), δ³⁰Si: Savage et al. (2010)) and different sediment end-members are represented by 4 dashed arrows: modern authigenic clay (δ¹⁸O: Isson and Rauzi (2024), δ³⁰Si: Geilert et al., (2024)), sponge-rich sediments (for the median and for the lowest δ³⁰Si values: Cassarino et al. (2018); δ¹⁸O: Eiler et al. (2005)), and radiolarites (δ¹⁸O and δ³⁰Si values from Bole et al. (2020a,b).</figcaption></figure>

The influence of sediment types on Si and O isotopes in OHMIs as suggested in Fig. 5 is consistent with previous studies. In fact, the sediments from DSDP site 374, representing the sediment subducting beneath the Aeolian arc, are mainly composed of clays, evaporites and carbonates, with rare occurrences of radiolarian and sponges (Hsu et al., 1978). Carbonates and evaporites will not contribute to the δ³⁰Si signature, so the sediment signature is likely to be derived from clays only. Indeed, five out of 6 Sommata OHMIs plot around the DMM-clays mixing line, suggesting that 10-15 % of the Si isotope signature are derived from clays. The last OHMI falling outside the mixing line might represent a melt with a flavor of sponge sediments overprinted by a signature from high temperature (HT) fluids in the crust (Fig. 5). Melts from Sukumoyama (Honshu, N. Izu Bonin) should also record the influence of siliceous ooze and clays, based on the nature of the sediments recovered from ODP Site 579 (e.g., Plank and Langmuir, 1998). This seems consistent with our data, although only 2 OHMIs have been analyzed for both δ³⁰Si and δ¹⁸O. Subducting sediments beneath the Southern Lesser Antilles also contain mainly carbonates and clays, as well as sponges in the first 100m (ODP Site 144 report, Hayes et al., 1972). One of the three Lesser Antilles OHMI has a δ³⁰Si value consistent with a clay influence, whereas the 2 others seem to have recorded a sponge-rich influence as well as the superimposed imprint of HT fluids. The sediments that are being subducted beneath the Vanuatu are mainly composed of volcanicastics and clays, but also contain radiolarians and sponges (DSDP site 286 report; Andrews et al., 1975). Indeed, one data plot along a mixing line between DMM and radiolarite, whereas 3 others plot along a mixing line between DMM and the sponge-rich sediments with the lowest δ³⁰Si value. The last data point is from the OHMI which records the influence of HT fluids in the arc crust (Fig. 5).

The Si isotope compositions of OHMIs from arc volcanoes thus bring new constraints on the slab contribution, this insight was not apparent based on bulk rock data from modern subduction samples. The fact that South Sandwich glasses did not record the signature from sponge-rich sediments although subducting sediments are only composed of siliceous ooze (Plank and Langmuir, 1993) may not necessarily indicate that these sediments did not contribute to magma generation. Instead, this could reflect efficient homogenization of Si isotope signatures during magmatic differentiation and mixing processes that occurred after

OHMIs entrapment and before the eruption. Under certain circumstances, such as higher sediment melt fluxes, different melting conditions, or limited magma homogenization, sedimentary signatures can be preserved at the bulk rock scale, as demonstrated by the compositions of Dazhonghe bulk glasses.

### 4.3. Si isotope fractionation between olivine and melt

Si isotope fractionation between olivine hosts and their melt inclusions (Δ³⁰Siolivine-melt) is predicted to be -0.07 ‰ at 1250°C for a basaltic series (Qin et al., 2016) and was further measured at 0.001 ± 0.029 ‰ for MORBs by Liu et al. (2025). The average Δ³⁰Siolivine-melt recorded in ARP73-10-03 is +0.03 ± 0.19 ‰, +0.05 ± 0.24 ‰ for CH31-DR8 and -0.03±0.09 ‰ for CH77-DR6-203 (Supplementary Material 5). These values are consistent with the values measured by Liu et al. (2025) and calculated by Qin et al. (2016). Thus, most MORB OHMIs and their hosts are in isotopic equilibrium, although 4 sample-pairs deviate from the expected equilibrium value by more than ±0.2 ‰ (Fig. 6), consistent with previously documented O-isotope disequilibrium (Manzini et al., 2019).

Because the major element compositions of OHMIs from arc samples differ from those of MORB (Fig. 2), we used OHMIs PEC-corrected major element compositions (Supplementary Material 3) to calculate equilibrium fractionation factors from first-principles models (Qin et al., 2016). The theoretical equilibrium value at 1100°C would lead to Δ³⁰Siolivine-melt = -0.03‰ for La Sommata (highest alkali content, Fig. 3), indistinguishable from the MORB equilibrium within LA-MC-ICP-MS uncertainty. Fig. 6 shows that olivine-OHMIs δ³⁰Si relationships in arc samples span a wider range of Δ³⁰Siolivine-melt than MORB. As discussed in section 4.2, neither NBT/O nor PEC-corrected data correlate with δ³⁰Si. This is valid also for Δ³⁰Siolivine-melt, which indicate that PEC did not modify the Si isotope signatures of the OHMIs, nor the fractionation between OHMIs and their host olivines.

The larger Δ³⁰Siolivine-melt values than predicted by first-principles model for arc samples, with measured values ranging from -0.2 ± 0.25 ‰ to +1.2 ± 0.25 ‰ (Fig. 7), can thus be interpreted in terms of magmatic processes. Similar O-isotopic disequilibrium was reported in the same OHMIs, with no relationship with NBT/O and PEC (Bouvier
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** δ³⁰Si in olivine plotted against δ³⁰Si in melt inclusion. The black line represents the theoretical equilibrium fractionation value Δ³⁰Si_olivine-melt = 0.001 ± 0.029 ‰ for MORB (Liu et al., 2025). For comparison, Δ³⁰Si_olivine-melt = +0.2 ‰ (fitting most arc data) and Δ³⁰Si_olivine-melt = +0.6 ‰ (fitting the most extreme data, except one) are plotted. Each point corresponds to a single OHMI (average measurement when a melt inclusion was analyzed multiple times) associated with the δ³⁰Si of its host olivine.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Comparison of Si isotope fractionation (Δ³⁰Si Ol-melt) and O isotope fractionation (Δ¹⁸O Ol-melt) between olivine and melt. The dashed lines represent equilibrium values for MORB (for Si isotopes: Liu et al., 2025; for O isotopes: value from Bouvier et al., 2022). It should be noted that equilibrium values for alkali basalts only slightly differ from the values reported for MORBs (see text for details) and they would almost overlap if plotted. O isotope data are from Manzini et al. (2019) and Bouvier et al. (2022), and were measured in the same OHMIs and olivines by SIMS.</figcaption></figure>

et al., 2022). The apparent disequilibrium was suggested to result from OHMI entrapment in remobilized, yet partly dissolved, olivine crystals which initially crystallized from a melt with a different, possibly homogenized, oxygen and silicon isotopic composition. The volume of olivine in equilibrium with the melt inclusions for O isotopes is very small, resulting in a thin band of less than 10 microns around the OHMIs, making it difficult to measure (Bouvier et al., 2022). When compared to the published Δ¹⁸O_olivine-melt in the same OHMIs (Fig. 7), both MORB and arc samples exhibit variable degrees of apparent disequilibrium in both isotopic systems. While no systematic relationship is observed between Δ³⁰Si_olivine-melt and Δ¹⁸O_olivine-melt across all samples, the

simultaneous Si- and O-isotopes disequilibrium in many arc OHMIs supports their entrapment in remobilized olivine crystals. These features reflect the complexity of arc magma plumbing systems, where olivine crystals may grow in homogenized melts with MORB-like δ³⁰Si signatures, whereas OHMIs capture snapshots of small, compositionally distinct, melt batches injected into melt lenses or mush zones. Olivine-melt inclusions disequilibrium, either isotopic or elemental (e. g., trace elements; Wieser et al., 2019), highlights the ability of OHMIs to record micro-scale heterogeneities that are not preserved in bulk melts, underscoring the role of crystal recycling and melt injections in arc magma evolution.
## 5. Conclusion

This study presents the first LA-MC-ICP-MS δ³⁰Si data for olivine-hosted melt inclusions (OHMIs) and their host olivine crystals from three MORB and four island arc samples. OHMIs from MORB samples exhibit homogeneous δ³⁰Si compositions both within individual samples and between samples, consistent with published bulk glass data (Savage et al., 2010). Olivine crystals from MORBs also display homogeneous compositions, but show a more restricted range of δ³⁰Si values compared to their hosted melt inclusions. The resulting Si isotope fractionation between melt and olivine (MORB Δ³⁰Siolivine-melt = +0.03 ± 0.25 ‰) agrees well with theoretical predictions (Qin et al., 2016) and empirical determination (Δ³⁰Siolivine-melt = +0.01 ± 0.029 ‰, Liu et al., 2025). In contrast, OHMIs from arc samples are more heterogeneous and exhibit systematically lower δ³⁰Si values compared to MORB OHMIs. These values are lower than bulk rock data from modern arcs (Savage et al., 2010) but similar to those reported for ancient continental arc glasses. Notably, no correlation was observed between δ³⁰Si and the NBO/T ratio of the OHMIs, suggesting that the low δ³⁰Si values in arc samples are not related to melt polymerization. While the Si isotope fractionation factor between OHMIs and olivine in MORB samples (MORB Δ³⁰Siolivine-melt) matches theoretical values, the fractionation factors for arc samples are predominantly higher than expected (arc Δ³⁰Siolivine-melt = -0.2 ± 0.25 ‰ to +1.2 ± 0.24 ‰). This discrepancy may be explained by the entrapment of OHMIs within remobilized olivines that crystallized from different, homogenized, melt batches, as previously suggested for these samples based on phosphorus and oxygen isotope data (Manzini et al., 2019, 2019; Bouvier et al., 2022). The systematically lower δ³⁰Si values in arc OHMIs, when considered alongside δ¹⁸O measurements from the same inclusions, is interpreted as reflecting the influence of subducted sediments, particularly siliceous (sponge-rich) sedimentary components in the arc source region. This study demonstrates that OHMIs and bulk rocks provide complementary insights: OHMIs capture early-stage heterogeneities and sediment signatures that are homogenized in erupted magmas. Future work considering solution MC-ICP-MS analyses of bulk arc basalts coupled with in-situ Si isotopes analysis of melt inclusions and their hosts from the same samples is required to verify the potential offset between primary melt inclusions, their host, and associated bulk rocks, allowing to refine the conditions under which crystals, and possibly bulk rocks may preserve the sediment melt signature.

### CRediT authorship contribution statement

**Anne-Sophie Bouvier:** Writing – original draft, Validation, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Martin Guitreau:** Writing – review & editing, Validation, Methodology, Funding acquisition, Formal analysis. **Clémence Le Lay:** Investigation, Formal analysis. **Muriel Laubier:** Writing – review & editing, Resources. **Estelle F. Rose-Koga:** Writing – review & editing, Resources.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work has been founded by the Agassiz foundation (University of Lausanne). ASB and CL are thankful to Martin Robyr for his assistance during the EMPA sessions. ASB thanks N. Métrich and A.R.L. Nichols for St. Vincent and Sukumoyama samples, respectively. We thank P. Savage and an anonymous reviewer for their constructive comments and suggestions, which greatly improved the quality of this manuscript. We are

grateful to the editor, Dr Fang-Zhen Teng, for the efficient handling of our submission. MG acknowledges funding from French Agence National de la Recherche through the funded project "Zircontinents" (ANR-17-CE31-0021) and ClerVolc (ANR-10-LABX-0006). This is contribution no. 742 of the ClerVolc program of the international Research and Center for Disaster Sciences and Sustainable Development of the University of Clermont Auvergne.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2026.119884.

### Data availability

Data will be made available on request.

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