# Halogens in proto-Iceland plume basalts

E.L. Tomlinson, Estelle F. Rose-Koga, A.-S. Bouvier, D. O'farrell, J.T.

Caulfield, M.G. Jackson, Y. Moussallam, F.M. Stuart, J. Villeneuve

► To cite this version:

E.L. Tomlinson, Estelle F. Rose-Koga, A.-S. Bouvier, D. O'farrell, J.T. Caulfield, et al., Halogens in proto-Iceland plume basalts. Geochemical Perspectives Letters, 2025, 37, pp.12-17. 10.7185/geochem-let.2539. [https://doi.org/10.5308/729v2](https://doi.org/10.5308/729v2)

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1 **Halogens in proto-Iceland plume basalts**

2

3 Emma L. Tomlinson¹*, Estelle F. Rose-Koga², Anne-Sophie Bouvier³, Donal O'Farrell¹, John

4 T. Caulfield⁴, Matthew G. Jackson⁵, Yves Moussallam⁶, Finlay M. Stuart⁷ and Johan

5 Villeneuve⁸.

6

7 ¹Geology, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland.

8 ²Earth Sciences Institute of Orléans, University Orléans, CNRS, Orléans, France.

9 ³Institut des Sciences de la Terre, University of Lausanne, Lausanne, Switzerland.

10 ⁴School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane,

11 Queensland, Australia.

12 ⁵Department of Earth Science, University of California Santa Barbara, Santa Barbara,

13 California, USA.

14 ⁶Department of Earth and Environmental Sciences, Columbia University, New York, USA.

15 ⁷Scottish Universities Environmental Research Centre, East Kilbride, South Lanarkshire, UK.

16 ⁸Centre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine,

17 Vandœuvre-lès-Nancy, France.

18 *corresponding author

19

20 **Abstract**

21

22 Melt inclusions in olivine phenocrysts from high ³He/⁴He proto-Iceland plume (PIP) basalts

23 from Baffin Island and West Greenland have elevated Br/Cl ratios, extending to values

24 significantly above those found in mid ocean ridge (MORB) and ocean island basalts (OIB).

25 Chlorine isotope compositions are indistinguishable from MORB-source mantle and high

26 Br/Cl occurs in the absence of geochemical indicators for serpentinite, altered oceanic crust

27 or sediment. The highest Br/Cl occur in melt inclusions from basalts with the highest

28 ³He/⁴He. The high Br/Cl ratios are at the high end of the range of putative Earth-source

29 meteorites (CI- and CM-type carbonaceous chondrites). The PIP melt inclusion compositions

30 are consistent with the addition of a small volume of halogen-rich, high Br/Cl chondritic

31 material to halogen-depleted, low Br/Cl mantle.

32

33 **Introduction**

34

35 The halogens (F, Cl, Br, I) have proved to be valuable tracers of recycled marine sediment,

36 altered oceanic crust and serpentinite in the Earth's mantle (Kendrick et al., 2017). This

37 recycled crustal material tends to dominate the halogen inventory of the convecting mantle

38 (Kendrick et al., 2017) likely masking the primordial halogen inventory. The relative

39 abundances of Cl and Br in geochemically enriched intra-plate basalt glasses from Hawaii,

40 Samoa, Society and Pitcairn are essentially indistinguishable from mid-ocean ridge basalts,

41 implying that the convecting mantle is homogenous in terms of halogens (Kendrick et al.,

42 2014; Kendrick et al., 2015; Kendrick et al., 2025). However, the Earth's mantle is depleted

43 in halogens relative to chondritic meteorites. While the depletion of F aligns with its

44 condensation temperature, Cl and Br concentrations are an order of magnitude lower than

45 predicted by the volatility trend (Lodders and Fegley, 1998). One possible explanation for

46 this discrepancy is the presence of a halogen-rich reservoir in the deep Earth. The recent

47 discovery of elevated Br/Cl in melt-inclusion bearing olivine from high ³He/⁴He basalts from

48 Koko and Suiko in the Emperor Seamount (Broadley et al., 2019) chain provides the first

49 indication of halogen heterogeneity in the mantle that may be associated with a deep mantle

50 source.
The picritic flood basalts from Baffin Island (Clarke and Upton, 1971) and West Greenland (Larsen *et al.*, 1992) were erupted at around 61 Ma and are believed to be the earliest volcanic products from the Iceland plume. These proto-Iceland plume (PIP) basalts have the highest mantle-derived ³He/⁴He ratios recorded to date (Stuart *et al.*, 2003; Starkey *et al.*, 2009). The high ³He/⁴He PIP lavas have a large compositional range but tend to be geochemically depleted in terms of trace elements and radiogenic isotope compositions with only a few enriched basalts (Starkey *et al.*, 2009; Willhite *et al.*, 2019). In this contribution we report the F, Cl and Br compositions of melt inclusions in olivine phenocrysts from high ³He/⁴He basalts from Baffin island and West Greenland in an effort to characterise the heavy halogen composition of the ³He-rich deep mantle reservoir.

## Geological setting

The PIP basalts are sub-divided into a depleted N-type (La/Sm$_N$ <0.7) and more enriched E-type (Kent *et al.*, 2004). Both basalt types are depleted relative to the majority of OIB with respect to Sr-Nd-Pb isotopes (Starkey *et al.*, 2009; Willhite *et al.*, 2019). Based on incompatible trace elements and Sr-Nd-Pb-Os-O isotopes the N- and E-type magmas are not related by partial melting, fractional crystallisation or by crustal or lithospheric assimilation (Kent *et al.*, 2004). Instead, the compositional differences are attributed to melting of a heterogeneous mantle (Stuart *et al.*, 2003; Kent *et al.*, 2004). Herzberg and O'Hara (2002) calculate that the Baffin Island picrites formed by relatively high degrees of fractional melting of asthenospheric mantle, around 10-11% (depleted source) and 23-25% (enriched source) at 120-70 km depth.

The studied samples are olivine-phyric N- and E-type basalts from Padloping Island and Cape Searle, Baffin Island, and Disko Island and Nuussuaq, West Greenland and were previously studied by Stuart *et al.* (2003) and Starkey *et al.*, (2009, 2012). They are high-Mg (MgO >13 wt%) picritic lavas that were selected because of their high ³He/⁴He (26 to 49.5 R$_a$) and because their bulk rock compositions are minimally affected by crustal contamination, indicated by high Nb/Th (9-14) and low K/Nb (81-331). We opted to study melt inclusions, rather than whole rock basalts as they are more likely to record pristine volatile compositions. The host olivines are 300-500 µm diameter and have magnesian cores (100Mg/Mg+Fe mol. 86-87), indicating equilibrium with the least fractionated PIP basalt (Starkey *et al.*, 2012).

The elemental and isotopic (δ³⁷Cl) halogen compositions of PIP melt inclusions were analysed by secondary ion mass spectrometry (SIMS), see supplementary information. The dataset is provided in table S-1 and summarised in table 1. Halogen concentrations were calibrated using a suite of natural basalt glasses. The limit of detection (LoD) is calculated using the formula $LoD = 3.33σ/S$, where σ is the standard deviation of the intercept and S is the slope of the calibration curve. We report uncertainties as 2σ, where σ is calculated from the standard deviations of the slope and intercept of the calibration curve. Halogen ratios were calculated for individual melt inclusions and sample averages are calculated from these ratios. The Cl and Br concentrations of MPI-DING glass StHs6/80-G are within ±10% of the preferred values and precision for repeat analysis of F, Cl and Br in MPI-DING glass StHs6/80-G was typically ±10%.

## Halogen composition of PIP basalt melt inclusions
101 Average melt inclusion compositions in the PIP samples are F = 174-440 µg.g⁻¹, Cl = 78-100
102 µg.g⁻¹ and Br = 530-1100 ng.g⁻¹. Concentrations of Cl and Br are significantly lower than
103 those reported in enriched, EM-type and HIMU (high µ, µ = ²³⁸U/²⁰⁴Pb) glasses from Samoa,
104 Society and Pitcairn (Kendrick *et al.*, 2014; Kendrick *et al.*, 2015) and are at the lower end of
105 values reported for depleted OIB from Hawaii (Kendrick *et al.*, 2025) (Fig. 1a,b). Low
106 halogen concentrations have previously been recorded in basaltic glasses from E-type PIP
107 lavas from Baffin Island (Kendrick *et al.*, 2015). The PIP basalts analysed here have K/Cl of
108 27-62 and ⁸⁷Sr/⁸⁶Sr of 0.7030-0.7034 that plot at the enriched end of the MORB array (Fig.
109 1c).

110

111 Chlorine isotopes can be used as tracers of recycled chlorine in the mantle (John *et al.*, 2010).
112 The chlorine isotope composition (δ³⁷Cl) of three PIP basalt melt inclusions range from -0.2
to +0.6 ‰ (SMOC). This range overlaps the mantle value (−0.2 ± 0.5 ‰ (1σ); Sharp *et al.*,
113 2007) (Fig. 1d) and is lower than EM-type basalts which have more positive δ³⁷Cl (John *et*
114 al.*, 2010).
115

116

117 A key observation is that N- and E-type PIP Melt inclusions in samples have 1000Br/Cl ratios (6.7 to 12.4) that are significantly higher than recorded by MORB and OIB (2.8 ± 0.8; Kendrick *et al.*, 2017). These are also higher than Br/Cl ratios recorded in low ³He/⁴He E-type basalts from Baffin Island (Kendrick *et al.*, 2015) (Fig. 2a,b). The highest values are outside of uncertainty of MORB mantle value at 2σ. Importantly the samples with the highest ³He/⁴He have the highest Br/Cl ratios, although any correlation is obscured by the large uncertainty (Fig. 2c).

124

## Contamination

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126

127 The Br/Cl ratio of basaltic melt can increase during interaction with seawater-derived brines
128 in the crust, either before emplacement within the magma chamber or after emplacement
129 during alteration (Kendrick *et al.*, 2013a). However, only one of the studied basalts was
130 emplaced in a submarine setting, the others being subaerial or hypabyssal (table 1). In
131 addition, the PIP Melt inclusions do not show the low K/Cl and F/Cl ratios and the negative
132 correlations of these ratios with Br/Cl that characterize mixing with saline brine (Kendrick *et*
133 *al.*, 2013a) (Fig. 2a).

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135 The PIP basalt sequence was erupted through continental lithosphere and high grade crustal
136 basement (Robillard *et al.*, 1992) and the high Br/Cl ratios may have been generated during
137 interaction of the melts with the lithosphere and/or crust during ascent. Peridotite xenoliths
138 from the lithospheric mantle have high Br/Cl ratios, a feature that has been attributed to
139 metasomatism by subduction-related fluids (Broadley *et al.*, 2016). Similarly, while lower
140 continental crust has low halogen concentrations and low Br/Cl, organic matter within upper
141 continental crust has elevated Br/Cl (Han *et al.*, 2023). Assimilation of metasomatised
142 lithosphere or organic-rich upper continental crust could impart high Br/Cl ratios to the
143 basaltic melts. However, the PIP basalts in this study lack the elevated LILE/HREE ratios,
144 HFSE depletions and radiogenic Sr isotope compositions that are indicative of incorporation
145 of altered oceanic crust and lithosphere (Fig. 1c). Furthermore, it is difficult to explain the co-
146 occurrence of high Br/Cl and high ³He/⁴He by reactivation of subducted or crustal halogens.
147 Therefore, we suggest that the high Br/Cl ratios are likely to be characteristic of the high
148 ³He/⁴He PIP source.

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## Halogen composition of the proto-Iceland plume mantle source region

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The composition of the N-type and E-type mantle sources were estimated by accumulated fractional melt modelling assuming modal melting (see supplementary information). We use the composition of melt inclusions in samples PI-25 and CS-6 to represent melts from the depleted and enriched asthenospheric mantle, respectively, and assume melt fractions of 0.1 for the depleted melt and 0.25 for the enriched melt, as appropriate for melting of the PIP source (Herzberg and O'Hara, 2002). We use partition coefficients D<sub>F</sub>, and D<sub>Cl</sub> from Joachim et al. (2015) and consider Br and Cl to be similarly incompatible. The calculated halogen composition of the source mantle is shown in figure 3 and table S-6, and the 1000Br/Cl ratio is 9.3 (PI-25) and 6.9 (CS-6).

The halogen compositions of bulk silicate Earth (BSE) and depleted mantle (DM) are estimated from the currently available OIB and MORB data (table S-6). The calculated K, F and Cl contents of the mantle source of the N-type and E-type PIP basalts are broadly within range of most estimates of DM and BSE. However, the calculated Br content of the PIP sources is significantly higher than DM and BSE (Fig. 3), resulting in Br/Cl ratios that are 2-4 times higher than DM and BSE.

## Scenario 1: Addition of recycled halogens to the Iceland mantle plume source

Subducted sediments can have elevated Br/Cl ratios of 4.9±1.5 but with individual analyses extending to ~100 (Han et al., 2023). The δ³⁷Cl composition of subducted sediment ranges from -3.7 to +2.4 ‰ (Barnes and Sharp, 2017) and thus overlaps the mantle-like chlorine isotope composition of the PIP basalt melt inclusions and cannot exclude subducted sediment as the source of the high Br/Cl ratios. However, the PIP basalts are depleted and lack the elevated ⁸⁷Sr/⁸⁶Sr signatures typically associated with subducted sediment (Fig. 1c, Fig. 2b). Therefore, a recycled sediment origin for the higher Br/Cl ratios is only possible if the halogens are decoupled from lithophile trace elements during sediment devolatilization, however Cl is known to enhance the solubility of trace elements, including Sr, via chloride complexing such that Sr is compatible in saline fluid (e.g. Keppler (2017)). In addition, volcanic glasses that do have radiogenic ⁸⁷Sr/⁸⁶Sr compositions associated with the presence of recycled sediment do not have elevated Br/Cl contents (Kendrick et al., 2017).

Br/Cl ratios like those in the PIP melt inclusions are found in sedimentary marine pore fluids and in serpentinite (Kendrick et al., 2013b) (Fig. 2). Sedimentary pore fluids extend from mantle-like compositions to strongly negative δ³⁷Cl values of -8 ‰, with most of the available data sitting to the negative side of the terrestrial value (Barnes and Sharp, 2017), this means that sedimentary pore fluids are a possible, but less likely source of high Br/Cl in the PIP melt inclusions. Serpentinite is a more plausible candidate because the δ³⁷Cl averages -0.1‰. Broadley et al. (2019) report high Br/Cl in crushed olivine from high ³He/⁴He basalts from Suiko and Koko in the Emperor seamounts chain, where the halogens were inferred to reside in fluid and/or melt inclusions, they attribute high Br/Cl to incorporation of altered oceanic crust and dehydrated serpentinite into the high ³He/⁴He Hawaii plume source. However, serpentinites are characterised by strongly radiogenic ⁸⁷Sr/⁸⁶Sr compositions and low K/Cl that are not seen in the PIP basalts (Fig. 1c). Furthermore, the PIP basalts lack elevated Fe/Mn and depleted Ca associated with the presence of eclogite or pyroxenite and have primitive Nd isotope compositions, consistent with an absence of recycled oceanic crust (Jackson et al., 2010). Therefore, while serpentinite may be the source of the high Br/Cl ratios in the PIP melt inclusions, its contribution to the lithophile element inventory was minimal.
## Scenario 2: High Br/Cl mantle

An alternative interpretation is that elevated Br/Cl ratios are intrinsic to the deep mantle. In this scenario, the low halogen content of the convecting mantle is due to depletion during crust formation, while the lower Br/Cl is due to incorporation of low Br/Cl recycled material. One option is that the deep mantle source of the PIP lavas is a heavy halogen-poor, high Br/Cl lower mantle reservoir. Such a source must be heterogeneous to account for the presence of both N- and E-type PIP basalts both with high ³He/⁴He and high Br/Cl. Therefore, the PIP melts may be more easily explained by addition of a small volume of heavy halogen-rich, high Br/Cl deep mantle to halogen-poor mantle.

Current estimates for BSE suggest that the mantle is depleted in Cl and Br relative to chondritic meteorites. In addition, although Cl and Br have similar condensation temperatures, the limited available data suggest that chondritic meteorites have higher Br/Cl ratios than BSE, with most estimates being close to the average solar system value of 4.7 (Lodders and Fegley, 1998). The Br/Cl ratio of the PIP melt inclusions is within range but at the upper end of compositions reported for CI- and CM-type carbonaceous chondrites (Fig. 3b), the subtypes of carbonaceous chondrites that most closely resemble the source of volatile elements delivered to the Earth (Alexander et al., 2012). Together, this implies that the primordial mantle may have been more halogen-rich had higher Br/Cl ratios than the convecting mantle, and so incorporation of a small vestige of primitive mantle material into depleted/enriched mantle at the core mantle boundary could explain the unusual halogen composition of the PIP melts. Alternatively, the halogen-rich, high Br/Cl PIP source may reflect a deep mantle heterogeneity, perhaps related to the Earth's accretion and degassing history.

## Scenario 3: Addition of heavy halogens from the core

The low chlorine content of BSE may be reconciled with the composition of chondritic meteorites if the core is a significant reservoir for the heavy halogens (Yuan and Steinle-Neumann, 2024). Halogens may reside in the core if they partitioned into liquid metal during core formation. High-pressure experiments indicate that Cl shows lithophile to mildly compatible behaviour (Yuan and Steinle-Neumann, 2024). Bromine partitioning behaviour at high pressure is unknown, however if halogens become more compatible in metal with increasing atomic number, then the core could potentially have a high Br/Cl ratio. However, if Br is mildly compatible in the core, it would be difficult to explain why Br would be transferred out of the core into PIP source.

## Conclusions and implications

Melt inclusions in olivine from high ³He/⁴He PIP basalts are halogen-poor and have high Br/Cl ratios relative to MORB and the majority of OIB. The absence of elemental and radiogenic isotope evidence for recycled crustal material in the PIP basalt source, and the absence of elevated Br/Cl in OIB where recycled material is thought to be present, suggest that recycled halogens may not be the source of the elevated Br/Cl ratios in the PIP melt inclusions.

The high Br/Cl ratios are within range of those of chondritic meteorites that may have delivered volatiles to the early Earth. The PIP melt inclusion compositions are consistent with
251 the addition of a small vestige of halogen-rich, high Br/Cl chondritic material to halogen-poor, low Br/Cl mantle. The presence of a halogen-rich, high Br/Cl chondritic reservoir in deep Earth could explain the apparent deficit of heavy halogens in the Earth relative to chondritic meteorites. Alternatively, the halogen-rich, high Br/Cl mantle source may reflect heterogeneity related to the Earth's accretion history.

256
257 The occurrence of these high Br/Cl melts has implications for our understanding of the cycling and homogenisation of halogens in the mantle, as well as the halogen composition and degassing history of the Earth. The high Br/Cl component may be easier to detect in depleted OIB magmas, where the signature is less strongly overprinted by recycled halogens.

261
262 Acknowledgements

263
264 The authors thank the handling editor Romain Tartèse and Mark Kendrick and Ray Burgess for constructive and thoughtful reviews. This research was funded by SFI grant 15/ERC/B3131 'Halogen' to ELT. The iCRAG laboratory was supported by SFI/RI/3227.
266 ER-K acknowledges SYSTER-INSU-CNRS "Halogens history". Professor J. Godfrey Fitton is thanked for supplying samples collected in 1996 by Ian Snape, Coleen Cole and Sally Brown during an Edinburgh University expedition to Baffin Island supported by the Laidlaw-Hall Trust.

271
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365 Table and Figure captions

366

<table><thead><tr><th>Sample</th><th>CS-5</th><th>CS-6</th><th>PI-25</th><th>362077</th><th>332901</th></tr></thead><tbody><tr><td>Location</td><td>Cape Searle, Baffin Island</td><td>Cape Searle, Baffin Island</td><td>Padloping, Baffin Island</td><td>Nuussuaq, West Greenland</td><td>Disko, West Greenland</td></tr><tr><td>Member</td><td>Anaanaa</td><td>Anaanaa</td><td>Anaanaa</td><td>Naujánguit</td><td>Ordlingassoq</td></tr><tr><td>Deg. N</td><td>67.22</td><td>67.22</td><td>67.18</td><td>70.50</td><td>70.38</td></tr><tr><td>Deg. W</td><td>62.53</td><td>62.53</td><td>62.45</td><td>53.57</td><td>53.22</td></tr><tr><td>Emplacement<sup>s</sup></td><td>Dyke</td><td>Dyke</td><td>Subaerial lava flow</td><td>Submarine pillow lava</td><td>Subaerial lava flow</td></tr><tr><td>Olivine Fo</td><td>84.8 (1.2)</td><td>84.5 (0.2)</td><td>84.5 (01.0)</td><td>81.1 (6.8)</td><td>86.0 (1.6)</td></tr><tr><td>Type</td><td>E</td><td>E</td><td>N</td><td>N</td><td>E</td></tr><tr><td><sup>3</sup>He/<sup>4</sup>He (R/R<sub>a</sub>)*</td><td>26.30 (2.0)</td><td>37.90 (1.2)</td><td>49.50 (1.6)</td><td>44.00 (0.8)</td><td>45.80 (1.0)</td></tr><tr><td>n</td><td>8</td><td>3</td><td>32</td><td>4</td><td>2</td></tr><tr><td>SiO<sub>2</sub> (wt%)</td><td>51.38 (3.30)</td><td>49.06 (1.81)</td><td>49.84 (1.51)</td><td>49.64 (2.13)</td><td>52.96 (0.51)</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub> (wt%)</td><td>17.57 (2.46)</td><td>15.45 (0.34)</td><td>15.39 (0.90)</td><td>15.85 (0.93)</td><td>15.64 (0.01)</td></tr><tr><td>TiO<sub>2</sub> (wt%)</td><td>1.41 (0.27)</td><td>1.30 (0.05)</td><td>1.21 (0.36)</td><td>1.30 (0.10)</td><td>1.98 (0.17)</td></tr><tr><td>FeO<sub>t</sub> (wt%)</td><td>7.90 (2.48)</td><td>10.04 (0.08)</td><td>9.98 (0.91)</td><td>8.88 (4.40)</td><td>5.99 (0.30)</td></tr><tr><td>MnO (wt%)</td><td>0.16 (0.13)</td><td>0.18 (0.03)</td><td>0.19 (0.10)</td><td>0.17 (0.09)</td><td>0.12 (0.04)</td></tr><tr><td>MgO (wt%)</td><td>3.17 (2.88)</td><td>7.65 (0.77)</td><td>7.50 (1.17)</td><td>7.26 (2.52)</td><td>7.75 (0.03)</td></tr><tr><td>CaO (wt%)</td><td>14.99 (1.58)</td><td>12.77 (1.30)</td><td>13.19 (0.67)</td><td>13.08 (1.77)</td><td>12.38 (0.30)</td></tr><tr><td>Na<sub>2</sub>O (wt%)</td><td>2.03 (0.93)</td><td>2.00 (0.10)</td><td>2.06 (0.25)</td><td>2.04 (0.07)</td><td>2.64 (0.17)</td></tr><tr><td>K<sub>2</sub>O (wt%)</td><td>0.29 (0.14)</td><td>0.35 (0.18)</td><td>0.21 (0.10)</td><td>0.34 (0.11)</td><td>0.54 (0.01)</td></tr><tr><td>Total</td><td>98.88</td><td>98.80</td><td>99.56</td><td>98.56</td><td>99.98</td></tr><tr><td>n F/Cl/Br</td><td>4/4/0</td><td>2/3/3</td><td>11/11/4</td><td>1/3/4</td><td>2/3/2</td></tr><tr><td>F (µg.g<sup>-1</sup>)</td><td>211 (26)</td><td>174 (168)</td><td>174 (28)</td><td>216 NA</td><td>440 (14)</td></tr><tr><td>Cl (µg.g<sup>-1</sup>)</td><td>100 (22)</td><td>78 (17)</td><td>85 (32)</td><td>98 (31)</td><td>83 (42)</td></tr><tr><td>Br (ng.g<sup>-1</sup>)</td><td>&lt;LOD NA</td><td>530 (439)</td><td>961 (371)</td><td>1133 (782)</td><td>905 (366)</td></tr><tr><td>K/Cl</td><td>27 (14)</td><td>42 (13)</td><td>25 (18)</td><td>31 (10)</td><td>62 (27)</td></tr><tr><td>F/Cl</td><td>2.1 (0.3)</td><td>2.4 (2.4)</td><td>2.1 (0.6)</td><td>2.1 NA</td><td>5.7 (3.8)</td></tr><tr><td>1000Br/Cl</td><td>-</td><td>6.7 (5.0)</td><td>11.8 (4.9)</td><td>11.8 (5.9)</td><td>12.4 (0.7)</td></tr><tr><td>n</td><td>4</td><td>1</td><td>12</td><td>-</td><td>-</td></tr><tr><td>d<sup>37</sup>Cl (‰)</td><td>0.60 (0.21)</td><td>0.04 (0.43)</td><td>-0.21 0.64</td><td>-</td><td>-</td></tr></tbody></table>

367

368

369 Table 1: Average composition of PIP melt inclusions. <sup>s</sup>Emplacement information from LASS-

370 EVANS (2004), and PEDERSEN *et al.* (2017), *He data from STUART *et al.* (2003) and

371 STARKEY *et al.* (2009). Uncertainties are 2σ.
<figure><img src="image_2.png" /><figcaption>Figure 1: Compositions of the melt inclusions in PIP basalt olivine compared to glasses from depleted (grey; BROADLEY *et al.*, 2019, Kendrick *et al.*, 2025), EM-type (warm colours; STRONCIK AND HAASE 2004; JOHN *et al.*, 2010) and HIMU (blues; STRONCIK AND HAASE, 2004; JOHN *et al.*, 2010; KENDRICK *et al.*, 2014) basalts. MORB (green field) is from KENDRICK *et al.* (2017). Data sources for average SMS (subducted marine sediments), AOC (altered oceanic crust) and serpentinite fields are provided in table S-4.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2: Composition of melt inclusions in PIP olivines compared to depleted (grey), EM-type (warm colours) and HIMU (blues) OIB and to MORB (green field). Date sources as in figure 1.</figcaption></figure>

379

380

381

382
<figure><img src="image_4.png" /><figcaption>Figure 3: Calculated accumulated fractional melting path yielding the melt seen in N-type (PI-25), and E-type (CS-6) PIP basalts, shown in increments of F=0.1 from 0.1 (PI-25) and 0.25 (CS-6). Compared to estimates for DM, BSE and to peridotite xenoliths from intraplate settings and to CI- and CM-chondrites (table S-6).</figcaption></figure>
# Halogens in proto-Iceland plume basalts

E.L. Tomlinson, E.F. Rose-Koga, A-S Bouvier, D. O'Farrell, J.T. Caulfield, M.G. Jackson, Y. Moussallam, F.M. Stuart, J. Villeneuve

## Supplementary Information

* Methods

* Major element composition

* Halogen partitioning calculation

* Tables S-1 to S-6

* Figure S-1 to S-4

* Supplementary Information References

## Methods

### Sample preparation

Olivine crystals from the selected PIP lava flows are medium sized (300-500 µm) equant and tabular crystals. Olivine grains containing melt inclusions were hand-picked under transmitted light at 50-100× magnification, care was taken to avoid inclusions that were open or connected to cracks or spinel mineral inclusions. Melt inclusions are rounded, optically clear to pink glasses, ranging in size from 20 to 100 µm (Fig. S-1). Where possible, we selected clean, glassy inclusions. Melt inclusions in sample 332901 were re-homogenised using a Vernadsky-type microscope heating stage at 1-atm (Sobolev et al., 1980; Schiano, 2003) following the procedure described in Le Voyer et al. (2010). In short, a partially crystallized melt inclusion is put in a 1-atm oven in which the oxygen fugacity is kept low at $10^{-10}$ atm with purified He with Zr at 700°C to avoid oxidation of the host crystal and to facilitate a rapid quench. We increased the temperature at a rate of about 8°C/min and visually monitored the regular disappearance of crystals in the melt inclusion. Once all crystals had melted inside the melt inclusion, we kept the temperature constant for 5 more minutes and then quenched the experiment. The heating procedure was completed in less than 30 minutes. Selected olivine grains were mounted onto individual glass slides using Crystalbond 509 and then individually polished by hand using progressively fining grit SiC papers and then diamond paste to expose the melt inclusions. The melt inclusion-bearing olivines were liberated from the Crystalbond and mounted in a Struers Epofix resin mount before receiving a final ¼ µm diamond paste polish on an automated polisher. The mount was then analysed for major elements and chlorine isotopes. The melt inclusion-bearing olivine grains were then liberated from the epoxy mount and cleaned in acetone and distilled water before being pressed into indium (Rose-Koga et al., 2021) for elemental halogen analysis by SIMS.
## Major element analysis (EMPA-WDS and SEM-EDX)

Major element compositions of PIP melt inclusions were analysed using a JEOL 8600 electron microprobe equipped with 4 wavelength dispersive spectrometers (EMPA-WDS) at the Research Laboratory for Archaeology and the History of Art (RLAHA), University of Oxford. Data were acquired at an accelerating voltage of 15 KeV, a beam current of 5 nA and a beam diameter of 5 µm. Peak count times were 30-70 s on each peak except for Na (12 s). The instrument was calibrated for the beam conditions using a suite of appropriate mineral standards and count rates were corrected using the PAP absorption correction method. Volatile elements were also analysed by EMPA-WDS at the RLAHA. Analyses used peak counting times of 120 s for F, P and S and 30 s for Cl, however both F and Cl were below the detection limits of 0.11 and 0.03 %, respectively.

<table><thead><tr><th></th><th>Crystal</th><th>Peak count time (s)</th><th>Background count time either side of peak (s)</th></tr></thead><tbody><tr><td>Na</td><td>TAP</td><td>12</td><td>6</td></tr><tr><td>Mg</td><td>TAP</td><td>70</td><td>35</td></tr><tr><td>Al</td><td>TAP</td><td>50</td><td>25</td></tr><tr><td>Si</td><td>TAP</td><td>30</td><td>15</td></tr><tr><td>K</td><td>PET</td><td>30</td><td>15</td></tr><tr><td>Ca</td><td>PET</td><td>30</td><td>15</td></tr><tr><td>Ti</td><td>PET</td><td>30</td><td>15</td></tr><tr><td>Mn</td><td>LIF</td><td>40</td><td>20</td></tr><tr><td>Fe</td><td>LIF</td><td>40</td><td>20</td></tr><tr><td>F</td><td>LDE</td><td>120</td><td>60</td></tr><tr><td>P</td><td>TAP</td><td>120</td><td>60</td></tr><tr><td>Cl</td><td>PET</td><td>30</td><td>15</td></tr><tr><td>Cr</td><td>PET</td><td>60</td><td>30</td></tr><tr><td>S</td><td>PET</td><td>120</td><td>120</td></tr></tbody></table>

Additional major element analyses were undertaken at the iCRAG laboratory at Trinity College Dublin using a using a Tescan Mira XMU field-gun emission scanning electron microscope equipped with an XMAX 80 nm² energy dispersive (SEM-EDS) detector. Measurements were conducted using an accelerating voltage of 20 kV and a beam current of 300 pA. A suite of appropriate mineral standards from the Smithsonian Institute (JAROSEWICH *et al.*, 1980) was used to calibrate the instrument for quantitative analyses. Totals acquired were all within ±1 % of 100 wt%. Both EMPA-WDS and SEM-EDS methods were verified using the StHs6/80-G and GOR128-G MPI-DING reference glasses from the Max Planck Institute (Jochum *et al.*, 2006) and accuracy (%bias) and precision (%RSD) are typically better than ±5 % for both methods for all elements at concentrations above 0.5 wt%. Data is provided in table S-1.

## Elemental halogen analysis

Halogen concentrations of PIP melt inclusions were analysed by secondary ion mass spectrometry (SIMS) using a CAMECA ims1280-HR at Centre de Recherches Pétrographiques et Géochimiques. The samples were sputtered with a Cs⁺ primary beam at an accelerating voltage of 10 kV and focused as a 15 µm beam size on the sample. Each spot was sputtered either for 300 sec with a 6µm rastered beam of 0.5 nA in 2022 or
for 60 sec with a 5µm rastered beam of a stronger primary intensity of 3nA in 2019. The electron gun was used for charge compensation at the surface of the sample. Negative secondary ions were extracted through a potential of 10 kV and measured in peak jumping mode on several EM detectors. Analytical conditions are shown below.

For analysis of Br, energy filtering was set between +20 to +30 eV to reduce matrix effects. The mass resolving power was set to about 21,000 to separate mass interferences from hydrides (SeH), oxides (CuO) and metals (FeMg, VMg, CrAl and FeAl) on Br. In the 2019 session, Br concentrations were measured in a dedicated Br analysis in which we measured masses 76.85 (background), 77 ($^{29}$Si$^{16}$O$_3$), 78 ($^{30}$Si$^{16}$O$_3$), 79 (Br) and 81 (Br); these masses were selected to minimize the hysteresis of the magnet. In 2022, we measured Br in the same analysis as F and Cl, so we measured mass $^{79}$Br, and $^{81}$Br in addition to 17.8 (background), $^{18}$O, $^{19}$F, $^{27}$Al, $^{30}$Si, $^{35}$Cl and $^{79}$Br and $^{81}$Br, we used a long wait time of 4s before analysis of $^{79}$Br to allow the magnet to settle.

Halogen analyses were calibrated using a suite of natural basalt glasses: VG-2, also known as USNM 111240/52 (Straub and Layne, 2003; Rose-Koga et al., 2020) and Macquarie basalt glasses 25603, 40428, 47963 and 60701 (Kendrick et al., 2012; Kendrick et al., 2017). The reference values for these glasses and the measured raw counts are summarised in table S-2 and the resulting calibration curves are shown in figure S-2. We measured both $^{79}$Br and $^{81}$Br (Fig. S-3), however we use $^{79}$Br to quantify Br in the studied samples as this reproduced the known Br contents of the quality control standards more closely. All errors are quoted as 2σ and include both the analytical uncertainty and the propagated error on the calibration curve.

<table><thead><tr><th></th><th>10-11 April 2019</th><th>12 April 2019</th><th>11-12 July 2022</th></tr><tr><th>Date</th><th>Br</th><th>Volatile elements</th><th>Halogens</th></tr></thead><tbody><tr><th>Masses</th><td>76.85 (background)<br/>$^{29}$Si$^{16}$O$_3$, $^{30}$Si$^{16}$O$_3$, $^{79}$Br, $^{81}$Br</td><td>11.8 (background FC),<br/>11.9 (background EM)<br/>$^{12}$C, $^{17}$O, $^{16}$O$^{1}$H, $^{18}$O, $^{19}$F, $^{27}$Al,<br/>$^{30}$Si, $^{32}$S, $^{35}$Cl,</td><td>17.8O (background)<br/>$^{18}$O, $^{19}$F, $^{27}$Al, $^{30}$Si, $^{35}$Cl, $^{79}$Br,<br/>$^{81}$Br,</td></tr><tr><th>Energy filtering</th><td>20 eV</td><td>20 eV</td><td>30 eV</td></tr><tr><th>Primary beam current</th><td>3.4 nA</td><td>1 nA</td><td>0.5 nA</td></tr><tr><th>Pre sputter</th><td>60 s</td><td>60 s</td><td>300 s</td></tr><tr><th>Raster area</th><td>5 x 5 µm</td><td>10 x 10 µm</td><td>6 x 6 µm</td></tr><tr><th>MRP</th><td>21000</td><td>7500</td><td>22000</td></tr><tr><th>Analysis time</th><td>4 s (background)<br/>4 s, 4 s, 12 s, 12 s</td><td>2 s, 1 s (background)<br/>4 s, 2 s, 4 s, 4 s, 2 s, 2 s, 4 s, 4 s, 3 s</td><td>4 s (background)<br/>3 s, 5 s, 3 s, 3 s, 4 s, 10 s, 10 s</td></tr><tr><th>Wait time before</th><td>2 s for all</td><td>2 s, 1 s (background)<br/>1 s for all</td><td>4 s (background)<br/>1 s, 1 s, 1 s, 1 s, 1 s, 4 s, 2 s</td></tr><tr><th>No. cycles</th><td>12</td><td>10</td><td>10</td></tr><tr><th>Approx. analysis time</th><td>12 minutes</td><td>18 minutes</td><td>18 minutes</td></tr></tbody></table>

The limit of determination was calculated from the calibration curve using the formula $LoD = 3.33 \times \sigma/S$, where $\sigma$ is the standard deviation of the intercept and S is the slope of the calibration curve. Measured reference material compositions are provided in table S-1. Accuracy is typically better than ±10 %bias for Cl in MPI-DING glasses StHs6/80-G and KL2-G, while the measured concentrations of F in both glasses are slightly low but are within error of the reference values reported by Jochum et al. (2006). For Br, accuracy is typically better than ±10 %bias for StHs6/80-G (0.8 ± 0.2 µg.g$^{-1}$; Jochum et al., 2006), but is worse in KL2-G (0.2 µg.g$^{-1}$; Jochum et al., 2006) at +30 %bias, suggesting that Br concentration may be overestimated at
low abundance. This is particularly true for the 2019 Br data which should be treated with caution as no other Br quality control standard was analysed in this session. Precision for repeat analysis of F, Cl and Br in MPI-DING glass StHs6/80-G is typically better than ±15 %RSD.

Errors on individual halogen analyses are derived from uncertainty on the linear fit of the appropriate calibration curve and errors on halogen ratios for individual melt inclusions are determined by propagating this calibration uncertainty. For the studied melt inclusions, absolute halogen concentrations may be affected by post entrapment crystallisation (see below), therefore we calculate halogen ratios for individual inclusions where both elements were measured, the average sample values are then the averages of those ratios in melt inclusions from olivines from a given sample lava flow.

## Chlorine isotopic analysis

Chlorine isotope compositions were analysed in 2017 using a CAMECA ims1280-HR at the SwissSIMS laboratory, University of Lausanne following a method similar to that of Manzini et al. (2017). Samples were sputtered with a 1.5 nA Cs⁺ primary beam with an accelerating voltage of 10kV, initially the beam was rastered over an area of 25 µm for 250 s to remove surface Cl contamination. Each analysis consists of 70 cycles of 10 sec, using a rastered beam over an area of 10 µm. It results of a total time per analysis of 17 minutes, included automated centring of the secondary beam in field and contrast apertures. Secondary ions were extracted into the mass spectrometer through a potential of 10 kV, through an energy slit set to a width of 50 eV and were counted on two electron multiplier detectors simultaneously. The mass resolving power was set to ~4800. Glass standards B-21, B-20D (basalts; John et al., 2010) were used for calibration, B-20D was closest to the sample glass composition and so was used to correct for mass fractionation. Additionally, StHs6/80-G and T1-G (andesite and granodiorite; Jochum et al., 2006), for which no bulk δ³⁷Cl value exists but was proven to give reproducible data, was used to monitor the instrument stability. A drift correction was applied for mount 2 where 4 permil shift was observed for StHs6/80-G. Reproducibility of the StHs6/80-G after drift correction was ~0.4 permil (2 σ). Errors of individual analysis are calculated as two standard errors of the 70 cycles and depends on the Cl content (correlated to count rate) of each melt inclusion compositions.

## Major element compositions

Host olivines have magnesian cores with forsterite (Fo, Mg/Mg+Fe) of 86-87, most of the studied samples span a fairly narrow compositional range with only weak normal zoning towards their rims (Fo₈₃-Fo₈₅), however olivines from sample 332901 extend to lower forsterite contents (Fo₇₆). The observed forsterite contents are comparable to the composition of olivine expected to be in equilibrium with sample DUR1, a phenocryst-poor lava sample considered to be the most unfractionated of the PIP lavas (Starkey et al., 2012). The olivines have CaO contents of 0.26-0.37 wt%. High-Mg PIP olivines are considered phenocrysts, rather than xenoliths from the peridotite mantle, because they show smooth continuous trends in Cr₂O₃ and CaO over the range of forsterite, consistent with derivation from the same magmatic system (Larsen and Pedersen, 2000; Starkey et al., 2012). In addition, PIP olivines are characterised by higher CaO for a given forsterite content than olivine from mantle xenoliths from West Greenland (Bernstein et al., 2006), consistent with a high-temperature, low pressure magmatic origin (Jackson and Shirey, 2011).

The melt inclusions have MgO 2.7-8.7 wt%. This is lower than the whole-rock compositions of their host lavas reported by (STARKEY et al., 2009), suggesting that they have undergone post entrapment crystallisation (PEC) during which the encapsulated melt crystallises olivine at the inclusion/phenocryst interface during cooling e.g. (Sobolev, 1996). Indeed, thin recrystallisation rims are observed in back-scattered electron images of the majority of inclusions. To calculate the liquid line of decent (LLD) of the
melt inclusions, equilibrium olivine was subtracted in 0.1 % increments from the whole rock composition of DUR1, a phenocryst poor PIP lava considered to represent the liquid composition (Starkey *et al.*, 2012). The composition of equilibrium olivine was calculated using an equilibrium constant $K_D$ = 0.3 (Roedder and Emslie, 1970). Four of the samples, those with glassy melt inclusions, lie on the calculated LLD, suggesting their compositions are consistent with perfect fractional crystallisation of 2-10 % (PI-25, CS-6 and 362077) and 5-25% (CS-5) olivine (Fig. S-4). For samples in which melt inclusions show small degrees of PEC (5-10 %), melt inclusions show a limit range of major element and halogen compositions. The highest K and Cl contents are found in melt inclusions with the lowest MgO, consistent with evolution on the LLD during PEC of olivine.

Sample 332901 lies off the LLD and has melt compositions suggesting diffusive equilibration between the trapped melt and the host olivine, melt inclusions in this sample were originally crystalised and were homogenised for this study. We have not corrected for PEC because although the halogens are incompatible element during olivine crystallisation and their concentrations will increase in the melt, their ratios in the melt will be unchanged at $F>0.1$, which is the minimum degree of melting proposed for the PIP source by Herzberg and O'Hara (2002).

## Halogen partition calculation

Experimental work demonstrates that F is more compatible than Cl in mantle minerals and that both become less incompatible with increasing temperature (Joachim *et al.*, 2015). We use partition coefficients determined at 1500°C and 2.3 GPa, conditions appropriate for partial melting in the OIB source mantle (DASGUPTA *et al.*, 2007). We use $D_F^{ol}$, $D_F^{opx}$, $D_{Cl}^{ol}$, and $D_{Cl}^{opx}$ at 1500 °C from figure 4 of Joachim *et al.* (2015). Following Joachim *et al.* (2015) and based on the lower temperature datasets of Hauri *et al.* (2006); Dalou *et al.* (2012); Dalou *et al.* (2014), we assume that $D_F^{cpx}/D_F^{opx}$ is 1.5 and $D_{Cl}^{cpx}/D_{Cl}^{opx}$ is 5. Following KENDRICK *et al.* (2012) we assume that Br and Cl are similarly incompatible ($D_{Br} = D_{Cl}$) in olivine, orthopyroxene and clinopyroxene. In practice, Br may be less compatible than Cl on account of its larger ionic radius, however even if the partition coefficient of Br is two orders lower than that of Cl, this will only affect the calculated composition of the source below ~$F=0.1$, lower than the degree of melting for both E and N-type PIP magmas proposed by Herzberg and O'Hara (2002). Table S-5 shows the average modal abundances of olivine, orthopyroxene and clinopyroxene in spinel peridotites (McDonough, 1990), excluding minor garnet/spinel, along with the mineral and resulting bulk F, Br and Cl partition coefficients used for accumulated fractional melting modelling. Table S-6 shows the calculated compositions of the PI-25 and CS-6 source regions compared to estimates for the depleted mantle (DM) and bulk silicate Earth (BSE) and data for intraplate lithospheric xenoliths.
Supplementary Tables

**Table S-1** Major element, elemental halogen, chlorine isotope compositions of PIP melting inclusions, along with major element compositions of host olivine and published whole rock trace element and isotope compositions.

**Table S-2** Raw data and reference values used for calibration of F, Cl and Br. Reference values are from: VG-2, also known as USNM 111240/52 (Straub and Layne, 2003; Rose-Koga et al., 2020) and Macquarie basalt glasses 25603, 40428, 47963 and 60701 (Kendrick et al., 2012; Kendrick et al., 2017). Errors on reference values are 2σ. NA – data not available. The limit of determination calculated for each session is also shown.

<table><thead><tr><th rowspan="2"></th><th colspan="3">F</th><th colspan="3">Cl</th><th colspan="4">Br</th></tr><tr><th><sup>19</sup>F /<sup>30</sup>Si</th><th>Error (σ)</th><th>F (µg.g<sup>-1</sup>)</th><th><sup>35</sup>Cl /<sup>30</sup>Si</th><th>Error (σ)</th><th>Cl (µg.g<sup>-1</sup>)</th><th><sup>81</sup>Br /<sup>30</sup>Si</th><th>Error</th><th><sup>79</sup>Br /<sup>30</sup>Si</th><th>Error</th><th>Br (µg.g<sup>-1</sup>)</th></tr></thead><tbody><tr><td colspan="13"><strong>12/07/2022</strong></td></tr><tr><td>VG2</td><td>1.76E-01</td><td>1.09E-02</td><td>334 ± 14</td><td>2.68E-01</td><td>1.65E-02</td><td>306 ± 13</td><td>4.14E-04</td><td>1.98E-05</td><td>2.95E-04</td><td>1.55E-05</td><td>NA</td></tr><tr><td>VG2</td><td>1.85E-01</td><td>2.03E-02</td><td>334 ± 14</td><td>2.57E-01</td><td>1.52E-02</td><td>306 ± 13</td><td>3.92E-04</td><td>2.44E-05</td><td>2.86E-04</td><td>1.99E-05</td><td>NA</td></tr><tr><td>VG2</td><td>1.86E-01</td><td>1.28E-02</td><td>334 ± 14</td><td>2.54E-01</td><td>1.42E-02</td><td>306 ± 13</td><td>3.59E-04</td><td>2.26E-05</td><td>2.49E-04</td><td>1.72E-05</td><td>NA</td></tr><tr><td>25603</td><td>6.09E-01</td><td>2.96E-02</td><td>769</td><td>7.00E-01</td><td>3.94E-02</td><td>799 ± 17</td><td>7.80E-04</td><td>4.89E-05</td><td>6.10E-04</td><td>3.96E-05</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td>7.16E-01</td><td>4.25E-02</td><td>769</td><td>7.19E-01</td><td>4.84E-02</td><td>799 ± 17</td><td>7.95E-04</td><td>3.89E-05</td><td>5.57E-04</td><td>2.93E-05</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td>5.62E-01</td><td>3.12E-02</td><td>769</td><td>6.91E-01</td><td>4.63E-02</td><td>799 ± 17</td><td>7.85E-04</td><td>4.94E-05</td><td>5.67E-04</td><td>3.67E-05</td><td>2.64 ± 0.06</td></tr><tr><td>60701</td><td>4.69E-01</td><td>4.13E-02</td><td>536</td><td>3.55E-01</td><td>2.02E-02</td><td>435 ± 10</td><td>3.98E-04</td><td>1.99E-05</td><td>2.42E-04</td><td>1.34E-05</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td>3.93E-01</td><td>3.18E-02</td><td>536</td><td>3.48E-01</td><td>1.97E-02</td><td>435 ± 10</td><td>4.16E-04</td><td>3.03E-05</td><td>2.27E-04</td><td>1.83E-05</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td>4.01E-01</td><td>1.60E-02</td><td>536</td><td>3.51E-01</td><td>1.96E-02</td><td>435 ± 10</td><td>3.97E-04</td><td>2.56E-05</td><td>2.31E-04</td><td>1.52E-05</td><td>1.38 ± 0.04</td></tr><tr><td>40428</td><td>6.59E-01</td><td>5.02E-02</td><td>729</td><td>4.45E-01</td><td>2.44E-02</td><td>494 ± 11</td><td>4.27E-04</td><td>2.17E-05</td><td>2.72E-04</td><td>1.90E-05</td><td>1.58 ± 0.04</td></tr><tr><td>40428</td><td>6.33E-01</td><td>3.37E-02</td><td>729</td><td>4.49E-01</td><td>2.60E-02</td><td>494 ± 11</td><td>4.56E-04</td><td>3.04E-05</td><td>2.61E-04</td><td>1.79E-05</td><td>1.58 ± 0.04</td></tr><tr><td>40428</td><td>4.35E-01</td><td>3.16E-02</td><td>729</td><td>4.44E-01</td><td>2.49E-02</td><td>494 ± 11</td><td>4.65E-04</td><td>3.07E-05</td><td>5.04E-04</td><td>3.35E-05</td><td>1.58 ± 0.04</td></tr><tr><td>47963</td><td>1.16E+00</td><td>8.40E-02</td><td>1262</td><td>1.22E+00</td><td>8.14E-02</td><td>1356 ± 30</td><td>1.23E-03</td><td>7.29E-05</td><td>1.02E-03</td><td>6.11E-05</td><td>4.57 ± 0.11</td></tr><tr><td>47963</td><td>1.08E+00</td><td>6.33E-02</td><td>1262</td><td>1.06E+00</td><td>6.89E-02</td><td>1356 ± 30</td><td>1.27E-03</td><td>7.93E-05</td><td>7.58E-04</td><td>4.93E-05</td><td>4.57 ± 0.11</td></tr><tr><td>LoD (µg.g<sup>-1</sup>)</td><td colspan="3">64</td><td colspan="3">51</td><td colspan="3">0.13</td><td colspan="2">0.49</td></tr><tr><td colspan="13"><strong>13/07/2022</strong></td></tr><tr><td>47963</td><td>1.19E+00</td><td>9.10E-02</td><td>1262</td><td>1.05E+00</td><td>6.33E-02</td><td>1356 ± 30</td><td>1.23E-03</td><td>6.97E-05</td><td>1.29E-03</td><td>7.33E-05</td><td>4.57 ± 0.11</td></tr><tr><td>47963</td><td>1.08E+00</td><td>6.74E-02</td><td>1262</td><td>9.28E-01</td><td>4.60E-02</td><td>1356 ± 30</td><td>1.28E-03</td><td>6.83E-05</td><td>1.31E-03</td><td>6.86E-05</td><td>4.57 ± 0.11</td></tr><tr><td>60701</td><td>4.62E-01</td><td>3.60E-02</td><td>536</td><td>2.76E-01</td><td>1.51E-02</td><td>435 ± 10</td><td>4.38E-04</td><td>1.93E-05</td><td>3.86E-04</td><td>1.89E-05</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td>6.51E-01</td><td>3.86E-02</td><td>536</td><td>3.15E-01</td><td>1.39E-02</td><td>435 ± 10</td><td>4.66E-04</td><td>2.57E-05</td><td>4.19E-04</td><td>2.46E-05</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td>6.81E-01</td><td>3.70E-02</td><td>536</td><td>3.25E-01</td><td>1.76E-02</td><td>435 ± 10</td><td>4.44E-04</td><td>2.63E-05</td><td>3.99E-04</td><td>2.43E-05</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td>5.07E-01</td><td>3.00E-02</td><td>536</td><td>3.35E-01</td><td>1.82E-02</td><td>435 ± 10</td><td>4.36E-04</td><td>2.56E-05</td><td>3.92E-04</td><td>2.36E-05</td><td>1.38 ± 0.04</td></tr><tr><td>25603</td><td>6.53E-01</td><td>3.07E-02</td><td>769</td><td>5.92E-01</td><td>2.47E-02</td><td>799 ± 17</td><td>7.92E-04</td><td>5.00E-05</td><td>7.74E-04</td><td>4.92E-05</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td>5.45E-01</td><td>6.55E-02</td><td>769</td><td>5.87E-01</td><td>3.60E-02</td><td>799 ± 17</td><td>8.01E-04</td><td>4.35E-05</td><td>7.85E-04</td><td>4.36E-05</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td>7.06E-01</td><td>4.88E-02</td><td>769</td><td>6.32E-01</td><td>3.78E-02</td><td>799 ± 17</td><td>7.45E-04</td><td>4.34E-05</td><td>7.65E-04</td><td>4.56E-05</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td>6.37E-01</td><td>4.66E-02</td><td>769</td><td>6.16E-01</td><td>3.51E-02</td><td>799 ± 17</td><td>7.27E-04</td><td>3.04E-05</td><td>7.50E-04</td><td>3.21E-05</td><td>2.64 ± 0.06</td></tr><tr><td>VG2</td><td>2.29E-01</td><td>1.45E-02</td><td>334 ± 14</td><td>2.24E-01</td><td>1.20E-02</td><td>306 ± 13</td><td>4.20E-04</td><td>2.29E-05</td><td>4.71E-04</td><td>2.62E-05</td><td>NA</td></tr><tr><td>VG2</td><td>2.44E-01</td><td>9.64E-03</td><td>334 ± 14</td><td>2.39E-01</td><td>1.33E-02</td><td>306 ± 13</td><td>3.91E-04</td><td>2.08E-05</td><td>4.45E-04</td><td>2.45E-05</td><td>NA</td></tr><tr><td>VG2</td><td>2.07E-01</td><td>1.33E-02</td><td>334 ± 14</td><td>1.95E-01</td><td>9.46E-03</td><td>306 ± 13</td><td>3.74E-04</td><td>1.71E-05</td><td>3.82E-04</td><td>2.00E-05</td><td>NA</td></tr><tr><td>VG2</td><td>1.93E-01</td><td>9.13E-03</td><td>334 ± 14</td><td>2.23E-01</td><td>1.19E-02</td><td>306 ± 13</td><td>3.95E-04</td><td>2.03E-05</td><td>4.36E-04</td><td>2.42E-05</td><td>NA</td></tr><tr><td>LoD (µg.g<sup>-1</sup>)</td><td colspan="3">113</td><td colspan="3">49</td><td colspan="3">0.19</td><td colspan="2">0.07</td></tr></tbody></table>

Continues over...
<table><thead><tr><th rowspan="2"></th><th colspan="3">F</th><th colspan="3">Cl</th><th colspan="4">Br</th></tr><tr><th><sup>19</sup>F /<sup>30</sup>Si</th><th>Error (σ)</th><th>F (µg.g<sup>-1</sup>)</th><th><sup>35</sup>Cl /<sup>30</sup>Si</th><th>Error (σ)</th><th>Cl (µg.g<sup>-1</sup>)</th><th><sup>81</sup>Br /<sup>30</sup>SiO<sub>3</sub></th><th>Error</th><th><sup>79</sup>Br /<sup>30</sup>SiO<sub>3</sub></th><th>Error</th><th>Br (µg.g<sup>-1</sup>)</th></tr></thead><tbody><tr><td colspan="13"><strong>11/04/2019</strong></td></tr><tr><td>47963</td><td></td><td></td><td></td><td></td><td></td><td></td><td>3.70E-02</td><td>7.32E-04</td><td>3.52E-02</td><td>8.02E-04</td><td>4.57 ± 0.11</td></tr><tr><td>47963</td><td></td><td></td><td></td><td></td><td></td><td></td><td>3.65E-02</td><td>7.49E-04</td><td>3.52E-02</td><td>7.99E-04</td><td>4.57 ± 0.11</td></tr><tr><td>25603</td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.31E-02</td><td>4.62E-04</td><td>2.03E-02</td><td>6.92E-04</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.30E-02</td><td>5.42E-04</td><td>2.05E-02</td><td>4.04E-04</td><td>2.64 ± 0.06</td></tr><tr><td>25603</td><td></td><td></td><td></td><td></td><td></td><td></td><td>2.32E-02</td><td>4.24E-04</td><td>1.92E-02</td><td>4.58E-04</td><td>2.64 ± 0.06</td></tr><tr><td>60701</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.60E-02</td><td>3.69E-04</td><td>1.24E-02</td><td>2.90E-04</td><td>1.38 ± 0.04</td></tr><tr><td>60701</td><td></td><td></td><td></td><td></td><td></td><td></td><td>1.59E-02</td><td>1.83E-04</td><td>1.25E-02</td><td>3.76E-04</td><td>1.38 ± 0.04</td></tr><tr><td>LoD (µg.g<sup>-1</sup>)</td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.38</td><td></td><td>0.18</td><td></td><td></td></tr><tr><td colspan="13"><strong>12/09/2019</strong></td></tr><tr><td>47963</td><td>3.27E+00</td><td>1.77E-02</td><td>1262</td><td>1.99E+00</td><td>1.44E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>47963</td><td>3.26E+00</td><td>1.88E-02</td><td>1262</td><td>1.98E+00</td><td>1.60E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>47963</td><td>3.26E+00</td><td>2.11E-02</td><td>1262</td><td>1.98E+00</td><td>1.44E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40428</td><td>1.89E+00</td><td>1.27E-02</td><td>729</td><td>7.01E-01</td><td>5.67E-03</td><td>494 ± 11</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40428</td><td>1.89E+00</td><td>1.14E-02</td><td>729</td><td>7.00E-01</td><td>4.21E-03</td><td>494 ± 11</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40428</td><td>1.89E+00</td><td>1.20E-02</td><td>729</td><td>6.97E-01</td><td>4.85E-03</td><td>494 ± 11</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.34E-01</td><td>2.98E-03</td><td>334 ± 14</td><td>3.93E-01</td><td>2.30E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.32E-01</td><td>3.72E-03</td><td>334 ± 14</td><td>4.00E-01</td><td>2.59E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.46E-01</td><td>2.64E-03</td><td>334 ± 14</td><td>4.11E-01</td><td>2.97E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40428</td><td>1.89E+00</td><td>1.31E-02</td><td>729</td><td>6.99E-01</td><td>4.89E-03</td><td>494 ± 11</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>40428</td><td>1.88E+00</td><td>1.27E-02</td><td>729</td><td>6.95E-01</td><td>4.61E-03</td><td>494 ± 11</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.36E-01</td><td>4.66E-03</td><td>334 ± 14</td><td>4.12E-01</td><td>2.87E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.27E-01</td><td>4.33E-03</td><td>334 ± 14</td><td>3.89E-01</td><td>2.16E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VG2</td><td>6.26E-01</td><td>4.47E-03</td><td>334 ± 14</td><td>4.01E-01</td><td>3.10E-03</td><td>306 ± 13</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>47963</td><td>3.27E+00</td><td>1.96E-02</td><td>1262</td><td>2.01E+00</td><td>1.45E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>47963</td><td>3.26E+00</td><td>1.97E-02</td><td>1262</td><td>1.99E+00</td><td>1.72E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>47963</td><td>3.26E+00</td><td>1.74E-02</td><td>1262</td><td>2.00E+00</td><td>1.23E-02</td><td>1356 ± 30</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>LoD (µg.g<sup>-1</sup>)</td><td>44</td><td></td><td></td><td>13</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>**Table S-3** Raw data and drift correction for calibration of δ³⁷Cl. Reference values for B2-1 and B20D are +0.59 ‰ (± 0.33 ‰) and +0.95 ‰ (± 0.33 ‰), respectively (JOHN *et al.*, 2010). MI – melt inclusion.</caption><thead><tr><th></th><th>(nA)</th><th><sup>37</sup>Cl/<sup>35</sup>Cl<br>H2/L1</th><th><sup>35</sup>Cl/Coeff<br>L1</th><th><sup>37</sup>Cl/Coeff<br>H2</th><th>yield<br>lse</th><th>δ<sup>37</sup>Cl raw<br>%<sub>00</sub></th><th>raw value<br>bias to StH</th><th>IMF</th><th>δ<sup>37</sup>Cl corrected<br>%<sub>00</sub></th><th>δ<sup>37</sup>Cl corrected<br>2se</th></tr></thead><tbody><tr><td colspan="11"><strong>Reference material mount</strong></td></tr><tr><td>TiG</td><td>1.76</td><td>3.281E-01</td><td>2.15E-02</td><td>1.271E+05</td><td>9.489E-01</td><td>4.171E+04</td><td>9.44E-01</td><td>7.227E+04</td><td>26.64</td><td>0.43</td></tr><tr><td>TiG</td><td>1.76</td><td>3.281E-01</td><td>2.16E-02</td><td>1.268E+05</td><td>8.773E-01</td><td>4.160E+04</td><td>8.81E-01</td><td>7.194E+04</td><td>26.59</td><td>0.43</td></tr><tr><td>B2-1</td><td>1.76</td><td>3.265E-01</td><td>1.12E-02</td><td>1.014E+06</td><td>7.138E-01</td><td>3.311E+05</td><td>7.22E-01</td><td>5.768E+05</td><td>21.46</td><td>0.22</td></tr><tr><td>B2-1</td><td>1.75</td><td>3.267E-01</td><td>1.20E-02</td><td>9.984E+05</td><td>5.781E-01</td><td>3.262E+05</td><td>5.88E-01</td><td>5.697E+05</td><td>22.15</td><td>0.24</td></tr><tr><td>B20D</td><td>1.75</td><td>3.268E-01</td><td>1.04E-02</td><td>7.552E+05</td><td>5.053E-01</td><td>2.468E+05</td><td>5.13E-01</td><td>4.326E+05</td><td>22.52</td><td>0.21</td></tr><tr><td>B20D</td><td>1.74</td><td>3.269E-01</td><td>9.21E-03</td><td>7.081E+05</td><td>6.404E-01</td><td>2.314E+05</td><td>6.44E-01</td><td>4.066E+05</td><td>22.71</td><td>0.18</td></tr><tr><td>StH</td><td>1.73</td><td>3.283E-01</td><td>1.42E-02</td><td>2.943E+05</td><td>5.678E-01</td><td>9.664E+04</td><td>5.68E-01</td><td>1.699E+05</td><td>27.29</td><td>0.28</td></tr><tr><td>StH</td><td>1.73</td><td>3.286E-01</td><td>1.45E-02</td><td>2.818E+05</td><td>5.442E-01</td><td>9.260E+04</td><td>5.45E-01</td><td>1.631E+05</td><td>28.13</td><td>0.29</td></tr><tr><td>SC</td><td>1.72</td><td>2.803E-01</td><td>8.84E-01</td><td>4.960E+03</td><td>2.193E-01</td><td>1.391E+03</td><td>8.60E-01</td><td>2.880E+03</td><td>-122.86</td><td>17.67</td></tr><tr><td colspan="11"><strong>Sample mount 1</strong></td></tr><tr><td>StH mount1</td><td>1.69</td><td>3.285E-01</td><td>1.45E-02</td><td>2.806E+05</td><td>6.061E-01</td><td>9.217E+04</td><td>6.06E-01</td><td>1.656E+05</td><td>27.88</td><td>0.29</td></tr><tr><td>CS5-3</td><td>1.70</td><td>3.269E-01</td><td>1.94E-02</td><td>1.575E+05</td><td>5.628E-01</td><td>5.148E+04</td><td>5.65E-01</td><td>9.269E+04</td><td>22.93</td><td>0.39</td></tr><tr><td>CS5-4</td><td>1.70</td><td>3.269E-01</td><td>2.05E-02</td><td>1.598E+05</td><td>5.016E-01</td><td>5.223E+04</td><td>5.13E-01</td><td>9.420E+04</td><td>22.79</td><td>0.41</td></tr><tr><td>bg mount1</td><td>1.69</td><td>3.145E-01</td><td>2.94E-01</td><td>5.525E+03</td><td>1.610E+00</td><td>1.729E+03</td><td>1.31E+00</td><td>3.263E+03</td><td>-16.08</td><td>5.88</td></tr><tr><td>CS5-5S</td><td>1.69</td><td>3.266E-01</td><td>2.27E-02</td><td>1.381E+05</td><td>4.428E-01</td><td>4.510E+04</td><td>4.50E-01</td><td>8.189E+04</td><td>22.04</td><td>0.45</td></tr><tr><td>CS5-5L</td><td>1.68</td><td>3.270E-01</td><td>2.06E-02</td><td>1.388E+05</td><td>4.596E-01</td><td>4.538E+04</td><td>4.59E-01</td><td>8.258E+04</td><td>23.13</td><td>0.41</td></tr><tr><td>StH mount1</td><td>1.67</td><td>3.285E-01</td><td>1.49E-02</td><td>2.847E+05</td><td>5.654E-01</td><td>9.353E+04</td><td>5.67E-01</td><td>1.704E+05</td><td>27.97</td><td>0.30</td></tr><tr><td>CS6-1</td><td>1.67</td><td>3.267E-01</td><td>2.13E-02</td><td>1.303E+05</td><td>4.266E-01</td><td>4.256E+04</td><td>4.31E-01</td><td>7.787E+04</td><td>22.16</td><td>0.43</td></tr><tr><td colspan="11"><strong>Sample mount 2</strong></td></tr><tr><td>StH mount2</td><td>1.65</td><td>3.285E-01</td><td>1.44E-02</td><td>2.835E+05</td><td>7.090E-01</td><td>9.315E+04</td><td>7.08E-01</td><td>1.723E+05</td><td>27.95</td><td>0.29</td></tr><tr><td>PI-25_M15</td><td>1.65</td><td>3.266E-01</td><td>2.12E-02</td><td>1.315E+05</td><td>5.735E-01</td><td>4.294E+04</td><td>5.78E-01</td><td>7.988E+04</td><td>21.99</td><td>0.42</td></tr><tr><td>PI-25_M17</td><td>1.64</td><td>3.273E-01</td><td>2.31E-02</td><td>1.453E+05</td><td>5.797E-01</td><td>4.758E+04</td><td>5.85E-01</td><td>8.853E+04</td><td>24.13</td><td>0.46</td></tr><tr><td>StH mount2</td><td>1.63</td><td>3.292E-01</td><td>1.57E-02</td><td>2.799E+05</td><td>6.887E-01</td><td>9.213E+04</td><td>6.92E-01</td><td>1.712E+05</td><td>30.07</td><td>0.31</td></tr><tr><td>bg mount2</td><td>1.63</td><td>3.200E-01</td><td>2.88E-01</td><td>3.982E+03</td><td>9.707E-02</td><td>1.272E+03</td><td>3.36E-01</td><td>2.438E+03</td><td>1.43</td><td>5.77</td></tr><tr><td>PI-25_M19</td><td>1.63</td><td>3.275E-01</td><td>2.06E-02</td><td>1.387E+05</td><td>6.249E-01</td><td>4.542E+04</td><td>6.26E-01</td><td>8.517E+04</td><td>24.60</td><td>0.41</td></tr><tr><td>PI-25_M110</td><td>1.63</td><td>3.276E-01</td><td>2.02E-02</td><td>1.452E+05</td><td>7.090E-01</td><td>4.757E+04</td><td>7.15E-01</td><td>8.937E+04</td><td>24.91</td><td>0.40</td></tr><tr><td>PI-25_M111</td><td>1.62</td><td>3.275E-01</td><td>2.10E-02</td><td>1.344E+05</td><td>6.070E-01</td><td>4.402E+04</td><td>6.08E-01</td><td>8.310E+04</td><td>24.70</td><td>0.42</td></tr><tr><td>StH mount2</td><td>1.62</td><td>3.294E-01</td><td>1.46E-02</td><td>2.767E+05</td><td>6.976E-01</td><td>9.111E+04</td><td>6.96E-01</td><td>1.713E+05</td><td>30.59</td><td>0.29</td></tr><tr><td>PI-25_M112</td><td>1.61</td><td>3.276E-01</td><td>2.66E-02</td><td>8.322E+04</td><td>3.447E-01</td><td>2.726E+04</td><td>3.53E-01</td><td>5.170E+04</td><td>25.01</td><td>0.53</td></tr><tr><td>PI-25_M115</td><td>1.61</td><td>3.277E-01</td><td>2.45E-02</td><td>9.846E+04</td><td>1.296E+00</td><td>3.227E+04</td><td>1.30E+00</td><td>6.133E+04</td><td>25.22</td><td>0.49</td></tr><tr><td>PI-25_M116</td><td>1.60</td><td>3.276E-01</td><td>1.66E-02</td><td>2.208E+05</td><td>7.701E-01</td><td>7.234E+04</td><td>7.75E-01</td><td>1.381E+05</td><td>25.18</td><td>0.33</td></tr><tr><td>StH mount2</td><td>1.60</td><td>3.296E-01</td><td>1.47E-02</td><td>2.744E+05</td><td>6.889E-01</td><td>9.043E+04</td><td>6.88E-01</td><td>1.718E+05</td><td>31.32</td><td>0.29</td></tr><tr><td>PI-25_M117</td><td>1.59</td><td>3.277E-01</td><td>2.10E-02</td><td>1.342E+05</td><td>6.107E-01</td><td>4.397E+04</td><td>6.17E-01</td><td>8.443E+04</td><td>25.39</td><td>0.42</td></tr><tr><td>PI-25_M120</td><td>1.59</td><td>3.279E-01</td><td>1.47E-02</td><td>2.721E+05</td><td>6.163E-01</td><td>8.921E+04</td><td>6.20E-01</td><td>1.716E+05</td><td>26.11</td><td>0.29</td></tr><tr><td>PI-25_M119L</td><td>1.59</td><td>3.278E-01</td><td>2.09E-02</td><td>1.347E+05</td><td>6.475E-01</td><td>4.416E+04</td><td>6.52E-01</td><td>8.476E+04</td><td>25.64</td><td>0.42</td></tr><tr><td>PI-25_M119S</td><td>1.58</td><td>3.279E-01</td><td>2.11E-02</td><td>1.326E+05</td><td>6.141E-01</td><td>4.348E+04</td><td>6.20E-01</td><td>8.372E+04</td><td>25.92</td><td>0.42</td></tr><tr><td>StH mount2</td><td>1.58</td><td>3.299E-01</td><td>1.48E-02</td><td>2.701E+05</td><td>7.012E-01</td><td>8.910E+04</td><td>6.98E-01</td><td>1.713E+05</td><td>32.16</td><td>0.30</td></tr><tr><td>StH mount2</td><td>1.57</td><td>3.298E-01</td><td>1.48E-02</td><td>2.690E+05</td><td>6.788E-01</td><td>8.873E+04</td><td>6.78E-01</td><td>1.708E+05</td><td>32.03</td><td>0.30</td></tr><tr><td>bg mount2</td><td>1.57</td><td>3.396E-01</td><td>8.83E-02</td><td>7.039E+03</td><td>3.029E+00</td><td>2.394E+03</td><td>3.06E+00</td><td>4.486E+03</td><td>62.54</td><td>1.77</td></tr><tr><td>StH mount2</td><td>1.57</td><td>3.299E-01</td><td>1.49E-02</td><td>2.667E+05</td><td>6.903E-01</td><td>8.798E+04</td><td>6.89E-01</td><td>1.700E+05</td><td>32.16</td><td>0.30</td></tr><tr><td>StH mount2</td><td>1.56</td><td>3.299E-01</td><td>1.49E-02</td><td>2.655E+05</td><td>6.748E-01</td><td>8.760E+04</td><td>6.77E-01</td><td>1.702E+05</td><td>32.30</td><td>0.30</td></tr></tbody></table>
<table><caption>**Table S-4** Published values for key mixing reservoirs used to general fields in figures 1 and 2.</caption><thead><tr><th>K/Cl</th><th>average</th><th>1σ</th><th>Min.</th><th>Max.</th><th>reference</th></tr></thead><tbody><tr><td><strong>Subducted marine sediments (SMS)</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>UCC</td><td>63</td><td>65</td><td></td><td></td><td>Rudnick and Gao (2014)</td></tr><tr><td>UCC</td><td>41</td><td></td><td></td><td></td><td>Hans Wedepohl (1995)</td></tr><tr><td><strong>Altered oceanic crust (AOC)</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Altered oceanic crust</td><td></td><td></td><td>0.3</td><td>2.0</td><td>Kendrick et al. (2013a)</td></tr><tr><td><strong>Serpentinite</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Serpentinite</td><td>0.12</td><td>0.1</td><td>0.0</td><td>0.3</td><td>Kendrick et al. (2013b)</td></tr><tr><td><strong>1000Br/Cl</strong></td><td>average</td><td>1σ</td><td>Min.</td><td>Max.</td><td>reference</td></tr><tr><td><strong>Subducted marine sediments (SMS)</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>UCC</td><td>4.9</td><td>1.5</td><td>2</td><td>265</td><td>Han et al. (2023)</td></tr><tr><td>UCC</td><td>4.3</td><td></td><td></td><td></td><td>Rudnick and Gao (2014)</td></tr><tr><td>UCC</td><td>2.5</td><td></td><td></td><td></td><td>Hans Wedepohl (1995)</td></tr><tr><td>sedimentary rock</td><td>5.7</td><td>5.4</td><td></td><td></td><td>Kendrick et al. (2017)</td></tr><tr><td><strong>Altered oceanic crust (AOC)</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Layer 3 AOC</td><td>1.9</td><td>1.2</td><td></td><td></td><td>Kendrick (2018)</td></tr><tr><td>Altered oceanic crust</td><td></td><td></td><td></td><td>&lt;2.5</td><td>Kendrick et al. (2014)</td></tr><tr><td>Altered oceanic crust</td><td></td><td></td><td>0</td><td>3.5</td><td>Kendrick (2019)</td></tr><tr><td>Altered oceanic crust</td><td>1.7</td><td>1.6</td><td></td><td></td><td>Chavrit et al. (2016)</td></tr><tr><td><strong>Serpentinite</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Serpentinite</td><td>3.3</td><td>0.9</td><td></td><td></td><td>Kendrick (2018)</td></tr><tr><td>Serpentinite</td><td>5.5</td><td>1.9</td><td>2.7</td><td>10.4</td><td>Kendrick et al. (2013b)</td></tr><tr><td>Serpentinite</td><td>4.7</td><td>7.2</td><td></td><td></td><td>Chavrit et al. (2016)</td></tr><tr><td><strong>δ<sup>37</sup>Cl</strong></td><td>average</td><td>1σ</td><td>Min.</td><td>Max.</td><td>reference</td></tr><tr><td>Sediment and metasediment</td><td></td><td></td><td>-3.7</td><td>2.4</td><td>Bouvier et al. (2019)</td></tr><tr><td>Altered oceanic crust</td><td></td><td></td><td>-1.6</td><td>1.8</td><td>Bouvier et al. (2019)</td></tr><tr><td>Serpentinite</td><td></td><td></td><td>-2.7</td><td>2.4</td><td>Bouvier et al. (2019)</td></tr><tr><td>MORB</td><td></td><td></td><td>-1.04</td><td>0.37</td><td>Sharp et al. (2007)</td></tr><tr><td><strong><sup>87</sup>Sr/<sup>86</sup>Sr</strong></td><td>average</td><td>1σ</td><td>Min.</td><td>Max.</td><td>reference</td></tr><tr><td>subducting sediments</td><td></td><td></td><td>0.7053</td><td>0.7080</td><td>Plank and Langmuir (1998)</td></tr><tr><td>Altered oceanic crust</td><td></td><td></td><td>0.7035</td><td>0.7075</td><td>Staudigel et al. (1995)</td></tr><tr><td>Serpentinite</td><td></td><td></td><td>0.7037</td><td>0.7095</td><td>Hattori and Guillot (2007)</td></tr><tr><td>DM</td><td></td><td></td><td>0.7020</td><td>0.7024</td><td>Zindler and Hart (1986)</td></tr><tr><td><strong><sup>3</sup>He/<sup>4</sup>He</strong></td><td>average</td><td>1σ</td><td>Min.</td><td>Max.</td><td>reference</td></tr><tr><td>DM</td><td></td><td></td><td>11</td><td>7</td><td>Marty and Zimmermann (1999)</td></tr></tbody></table>
<table><caption>**Table S-5** Partition coefficients used in source mantle calculation. Note, our bulk D<sub>F</sub> and D<sub>CI</sub> differ from those in (JOACHIM *et al.*, 2015) (D<sub>F</sub> = 0.08 and D<sub>CI</sub> = 0.03), the reason for this are unclear, however their bulk partition coefficients can be reproduced using D<sub>F</sub><sup>cpx</sup>/D<sub>F</sub><sup>opx</sup> = 5 and D<sub>CI</sub><sup>cpx</sup>/D<sub>CI</sub><sup>opx</sup> = 1.5, rather than 1.5 and 5, respectively.</caption><thead><tr><th></th><th>X</th><th>D<sub>F</sub></th><th>D<sub>CI</sub></th><th>D<sub>Br</sub></th><th>D<sub>K</sub></th></tr></thead><tbody><tr><td>Olivine</td><td>0.62</td><td>0.028</td><td>0.029</td><td>0.029</td><td>0.0003</td></tr><tr><td>Orthopyroxene</td><td>0.24</td><td>0.066</td><td>0.027</td><td>0.027</td><td>0.0024</td></tr><tr><td>Clinopyroxene</td><td>0.12</td><td>0.098</td><td>0.136</td><td>0.136</td><td>0.0081</td></tr><tr><td>Bulk</td><td>0.98</td><td>0.045</td><td>0.041</td><td>0.041</td><td>0.0017</td></tr></tbody></table>

<table><caption>**Table S-6** Composition of the N-PIP and E-PIP source calculated using the accumulated fractionation model based on samples PI-25 and CS-6, respectively. Compared to estimates for the composition of BSE, DM, lithospheric xenoliths and chondritic meteorites. Errors are 1 standard deviation.</caption><thead><tr><th></th><th>Method/Location</th><th>F (µg.g<sup>-1</sup>)</th><th>Cl (µg.g<sup>-1</sup>)</th><th>Br (µg.g<sup>-1</sup>)</th><th>1000Br/Cl</th></tr></thead><tbody><tr><td colspan="6"><strong>PIP source</strong></td></tr><tr><td>N-PIP source (PI-25, F=0.1)</td><td>Min-melt partitioning</td><td>19</td><td>9.2</td><td>0.104</td><td>11.3</td></tr><tr><td>E-PIP source (CS-6, F=0.25)</td><td>Min-melt partitioning</td><td>44</td><td>20</td><td>0.133</td><td>6.8</td></tr><tr><td colspan="6"><strong>Depleted mantle</strong></td></tr><tr><td>Schilling et al. (1980)</td><td>Min-melt partitioning</td><td>65</td><td>7.3</td><td>0.02</td><td>2.74</td></tr><tr><td>Jambon et al. (1995)</td><td>Canonical ratio</td><td>-</td><td>~6</td><td>0.0075</td><td>1.25</td></tr><tr><td>Saal et al. (2002)</td><td>Canonical ratio</td><td>16 ± 3</td><td>1 ± 0.5</td><td>-</td><td>-</td></tr><tr><td>Salters and Stracke (2004)</td><td>Canonical ratio</td><td>11 ± 5</td><td>0.51 ± 0.09</td><td>-</td><td>-</td></tr><tr><td>Shimizu et al. (2016)</td><td>Canonical ratio</td><td>8.00</td><td>0.40</td><td>-</td><td>-</td></tr><tr><td>Joachim et al. (2015)</td><td>Min-melt partitioning</td><td>3-14</td><td>0.6-16</td><td>-</td><td>-</td></tr><tr><td>Kendrick et al. (2017)</td><td>Canonical ratio</td><td>12 ± 2</td><td>5 ± 2</td><td>0.013 ± 0.006</td><td>2.60</td></tr><tr><td>Guo and Korenaga (2021)</td><td>Log-log linear</td><td>54 ± 4.8</td><td>67 ± 26</td><td>0.049 ± 0.010</td><td>0.73</td></tr><tr><td colspan="6"><strong>Bulk Silicate Earth</strong></td></tr><tr><td>McDonough and Sun (1995)</td><td>Canonical ratio</td><td>25</td><td>17</td><td>0.05</td><td>2.94</td></tr><tr><td>Palme and O'Neill (2014)</td><td>Mass balance + ratio</td><td>25 ± 10</td><td>30 ± 12</td><td>0.075 ± 0.038</td><td>2.50</td></tr><tr><td>Lyubetskaya and Korenaga (2007)</td><td>Canonical ratio</td><td>18 ± 8</td><td>1.4 ± 0.5</td><td>0.0036 ± 0.0004</td><td>2.57</td></tr><tr><td>Kendrick et al. (2017)</td><td>Mass balance</td><td>17 ± 6</td><td>26 ± 8</td><td>0.076 ± 0.025</td><td>2.92</td></tr><tr><td>Guo and Korenaga (2021)</td><td>Log-log linear</td><td>57 ± 4.8</td><td>81 ± 24</td><td>0.091 ± 0.019</td><td>1.12</td></tr><tr><td colspan="6"><strong>Intraplate lithospheric xenoliths</strong></td></tr><tr><td>Broadley et al. (2016)</td><td>West Antarctic</td><td></td><td></td><td></td><td>0.7-10</td></tr><tr><td>Kobayashi et al. (2019)</td><td>Eifel</td><td></td><td></td><td></td><td>0.5-4.5</td></tr><tr><td>Kobayashi et al. (2019)</td><td>Potrillo</td><td></td><td></td><td></td><td>0.1-4</td></tr><tr><td>Kobayashi et al. (2019)</td><td>San Carlos</td><td></td><td></td><td></td><td>3-7</td></tr><tr><td colspan="6"><strong>CI Chondritic meteorites</strong></td></tr><tr><td>Wasson et al. (1997)</td><td>Compilation</td><td>64</td><td>680</td><td>3.6</td><td>5.29</td></tr><tr><td>Lodders and Fegley, 1998)</td><td>Compilation</td><td>60</td><td>717</td><td>3.34 ± 0.34</td><td>4.66</td></tr><tr><td>Palme and Zipfel (2022)</td><td>Orgueil</td><td>-</td><td>600 ± 68</td><td>2.68 ± 0.51</td><td>4.47</td></tr><tr><td>Palme and Zipfel (2022)</td><td>Ivuna</td><td>-</td><td>585</td><td>5.68</td><td>9.71</td></tr><tr><td>Palme and Zipfel (2022)</td><td>Alais</td><td>-</td><td>710</td><td>4.39</td><td>6.18</td></tr><tr><td>Palme and Zipfel (2022)</td><td>Tonk</td><td>-</td><td>510</td><td>9.01</td><td>17.7</td></tr><tr><td colspan="6"><strong>CM Chondritic meteorites</strong></td></tr><tr><td>Wasson et al. (1997)</td><td>Compilation</td><td>38</td><td>160</td><td>2.6</td><td>16.3</td></tr><tr><td>Lodders and Fegley (1998)</td><td>Compilation</td><td>38</td><td>430</td><td>3</td><td>7.0</td></tr><tr><td>Reed and Allen (1966)</td><td>Mighei</td><td>-</td><td>350 ± 10</td><td>3.5 ± 1</td><td>10.0</td></tr><tr><td>Goles et al. (1967)</td><td>Mighei</td><td>-</td><td>430</td><td>3.9</td><td>9.1</td></tr><tr><td>Goles et al. (1967)</td><td>Mighei</td><td>-</td><td>510 ± 40</td><td>2.64 ± 0.03</td><td>5.2</td></tr></tbody></table>
Supplementary Figures

<figure><img src="image_5.png" /><figcaption>PI25-12 (mount 1)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>CS-5 3 (mount 3)</figcaption></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>CS-6 1 (mount 3)</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

**Figure S-1:** Examples of the studied melt inclusions in transmitted light and Backscatter electron image.
<figure><img src="image_11.png" /><figcaption>**Figure S-2:** Raw data for the calibration glass standards (in 2019: glasses VG-2, Macquarie 40428 and 47963; in 2022: glasses VG-2, Macquarie 25603, 40428, and 60701), the resulting calibration curves and their 95 % confidence intervals calculated by linear regression.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure S-3:** Comparison between Br concentrations determined for unknowns (individual samples and quality control standards) using <sup>81</sup>Br and <sup>79</sup>Br, errors are propagated from the calibration curve.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure S-4:** Composition of PIP melt inclusions (this study) compared to the calculated liquid line of descent of melt DUR1 due to removal of olivine (+ markers show 5 % increments of olivine fractionation). Whole rock compositions are from Starkey *et al.* (2009). Also shown is the composition of melts in equilibrium with an olivine of a fixed composition of Fo<sub>86</sub>.</figcaption></figure>
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