# Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: Insights from the Rum layered intrusion, Scotland

Amy P. Parker, Patricia L. Clay, Ray Burgess, Hélène Balcone-Boissard, Pierre Bürckel, Brian O'Driscoll

► To cite this version:

Amy P. Parker, Patricia L. Clay, Ray Burgess, Hélène Balcone-Boissard, Pierre Bürckel, et al., Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: Insights from the Rum layered intrusion, Scotland. Earth and Planetary Science Letters, 2019, 526, pp.1041-1057. 10.1016/j.epsl.2019.115769. insu-03586609

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# Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems

**DOI:**

10.1016/j.epsl.2019.115769

**Document Version**

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**Citation for published version (APA):**

Parker, A., Clay, P., Burgess, R., Balcone-Boissard, H., Bürckel, P., & O'Driscoll, B. (2019). Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: insights from the Rum layered intrusion, Scotland. *Earth and Planetary Science Letters, 526*. https://doi.org/10.1016/j.epsl.2019.115769

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Earth and Planetary Science Letters

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# Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: insights from the Rum layered intrusion, Scotland

Amy P. Parker<fnref n="1" />, Patricia L. Clay<fnref n="1" />, Ray Burgess<fnref n="1" />, Hélène Balcone-Boissard<fnref n="2" />, Pierre Bürckel<fnref n="3" /> and Brian O'Driscoll<fnref n="1" />

Amy.parker@manchester.ac.uk

## Highlights:

* NI-NGMS is a sensitive method for tracing halogen provenance and evolution in magmatic systems

* Rum halogens exhibit a broad scatter between mantle- and sedimentary-like halogens

* PGE-rich chromitites preserve evidence of a high-I (sedimentary) component

<footnote n="1">¹School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.</footnote>

<footnote n="2">²Sorbonne Universités, UPMC Université Paris 06, CNRS, Institut des Sciences de la Terre de Paris (ISTeP), 4 place Jussieu, 75005 Paris, France.</footnote>

<footnote n="3">³Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005 Paris, (France)</footnote>
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# Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: insights from the Rum layered intrusion, Scotland

Amy P. Parker¹, Patricia L. Clay¹, Ray Burgess¹, Hélène Balcone-Boissard², Pierre Bürckel³
and Brian O'Driscoll¹

¹School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK.

²Sorbonne Universités, UPMC Université Paris 06, CNRS, Institut des Sciences de la Terre de Paris (ISTeP), 4 place Jussieu, 75005 Paris, France.

³Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-75005 Paris, (France)

Amy.parker@manchester.ac.uk

**Keywords:** Halogens; noble gas isotopes; Rum Layered Suite; platinum-group elements; crustal contamination

## Abstract

The cycling of the halogens in sub-volcanic systems may play an important role in magmatic ore deposit formation and the emission of climate-altering gases via volcanic outgassing. However, the mobile and reactive nature of the halogens, as well as their typically low abundances in most intrusive igneous rocks, means that their behaviour in shallow crustal level magmatic systems is difficult to constrain. We present data from the ~60 Ma Rum Layered Suite, NW Scotland, utilising the neutron irradiation-noble gas mass spectrometry (NI-NGMS) technique, to provide insight into the provenance and behaviour of Cl, Br and I in magma chamber processes and precious metal enrichment. As a well-characterised and superbly exposed example of an open system sub-volcanic magmatic body, the Rum intrusion is an ideal locality to investigate halogen behaviour in this context. Our data reveal concentrations of <1 ppm to 124 ppm for Cl, <7 ppb to 484 ppb for Br and ~1 ppb to 363 ppb

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for I in the Rum rocks. Halogen ratios span a range of 2.3 x 10⁻⁴ to 1.5 x 10⁻² for Br/Cl and 4.0 x 10⁻⁵ to 7.4 x 10⁻² for I/Cl, defining an array from MORB-like to sedimentary-like halogen ratios. Chlorine and Br behave sympathetically in all lithologies, and in the rocks considered to be primary magmatic differentiates (i.e., peridotites, troctolites and gabbros), the variation in abundances is likely controlled by accessory quantities of primary hydrous magmatic phases crystallising from the mush interstitial liquid. Iodine behaviour is decoupled from that of Br and Cl, and combined with noble gas isotope data, suggests variable but pervasive contamination by a sedimentary source or sources during crystallisation of the Rum intrusion. In PGE-rich chromitites, high I/Cl ratios relative to their putative picritic parental melts point to the involvement of a crustal I source and hydrous fluids during chromitite formation. Our combined halogen and noble gas isotope datasets therefore reveal a record of high temperature processes associated with magmatic-crustal interactions and precious metal mineralisation, and provide new insights into volatile evolution during solidification of the Rum intrusion. A key implication of this work is that magma-crust interactions in mafic magmatic systems may lead to I concentration (i.e., from 12% to up to two orders of magnitude greater than MORB, in the Rum intrusion), before potential outgassing to the atmosphere.

# 1. Introduction

The heavy halogen group elements, chlorine, bromine, and iodine, are a suite of highly incompatible, reactive, and fluid mobile elements that are common constituents of high-temperature volcanic emissions (e.g., Aiuppa et al. 2009; Allard et al. 2016). Despite their relatively low abundances, halogen concentrations in silicate magmas have great significance in terms of releasing harmful and climate-altering volcanic gases to the atmosphere (von Glasow et al. 2009), as well as affecting magma viscosity and explosivity during volcanic eruptions (e.g., Pyle and Mather, 2009) and playing a role in rock alteration by hydrothermal fluids (Reed 1997). The halogens, especially Cl, also play an influential role in the transportation and redistribution of metals during the formation of magmatic ore
60 deposits (Halter and Webster 2004; Aiuppa et al. 2009). For example, the addition of 61 chloride salts to aqueous fluids produces negatively-charged ligands that complex with 62 positive metal ions, effectively increasing metal solubilities (cf. Lecumberri-Sanchez and 63 Bodnar 2018).

64 Layered mafic-ultramafic intrusions are considered to represent the solidified 65 remnants of basaltic magma chambers (O'Driscoll and VanTongeren 2017), and host the 66 majority of the world's exploited platinum-group element (PGE) deposits (Mungall and 67 Naldrett 2008). The crystallisation of layered intrusions is the result of chemical and physical 68 processes including magmatic differentiation and magma-crust interaction. These systems 69 therefore represent ideal natural laboratories for evaluating the geochemical evolution of 70 magma in sub-volcanic reservoirs. Layered intrusion-hosted PGE are principally associated 71 with base-metal sulfides that concentrate in stratiform 'reefs'; however, there is continued 72 debate over the mechanisms by which the PGE-enrichment occurs. Orthomagmatic models 73 focus on immiscible sulfide liquid as a scavenging agent for the PGE. Here, the driver for 74 sulfide liquid segregation is magma mixing or assimilation of crustal materials or the magma 75 chamber walls/floor-rock (Mungall and Naldrett 2008). Alternatively, metasomatic processes 76 involving the percolation of halogen-rich fluid through the footwall have been invoked (e.g., 77 for the J-M Reef of the Stillwater Complex, USA; Boudreau et al. 1986; Hanley et al. 2008). 78 However, the behaviour of Cl and especially Br and I during the formation of PGE-deposits 79 in layered intrusions remains poorly constrained. This is due in part to the highly mobile and 80 reactive nature of the halogens, as well as their low abundance in typical layered intrusion 81 lithologies, meaning that direct evidence of volatile activity is difficult to detect.

82 In order to elucidate the role of the halogens in PGE-reef formation, as well as 83 evaluate the processing of the halogens in sub-volcanic basaltic magmatic systems more 84 generally, we have carried out a detailed study of halogen abundances in the Rum Layered 85 Suite (RLS), NW Scotland. The heavy halogens range from moderately to highly 86 incompatible and therefore their distribution should be influenced by first order magmatic 87 processes, such as partial melting, fractional crystallisation and degassing (Aiuppa et al. 88
2009). As fluid mobile elements, they should also be strongly affected by the presence of aqueous fluids. The Rum intrusion is an ideal locality to carry out this work as it has undergone relatively little post-emplacement alteration and is otherwise well-characterised in terms of its petrology and geochemistry (e.g., Brown 1956; Bédard et al. 1988; Greenwood et al. 1992; Emeleus et al., 1996; O'Driscoll et al. 2009; 2010). We employ the neutron-irradiation noble gas mass spectrometry (NI-NGMS) technique to measure the heavy halogen (Cl, Br and I) abundances of PGE-enriched chromitite seams, picrite dykes (representative of the Rum parental melts), various magmatic differentiates (i.e., cumulates such as peridotites, gabbros), pegmatoids (representing late-stage volatile-rich magmas) and their mineral separates (e.g., olivine, amphibole, sulfides). The Rum intrusion was emplaced into metamorphic and sedimentary rocks of variable age and composition, and an ancillary aim of this study is to examine whether or not the halogens and noble gas isotope ratios can be used to trace magma-crust interactions.

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## 2. Geological setting and sample details

The Rum Layered Suite (RLS; Fig. S1) formed at 60.53 ±0.08 Myr (Hamilton et al. 1998) during opening of the North Atlantic and magmatism associated with the present-day Iceland mantle anomaly, as part of the British Palaeogene Igneous Province (BPIP). The RLS intruded into a suite of rocks comprising Archean basement (Lewisian Complex) gneisses and Neoproterozoic (Torridonian Supergroup) sandstones, as well as a succession of Mesozoic sediments (Emeleus et al. 1996). The Torridonian rocks are largely unmetamorphosed and comprise arkosic sandstones and shales. The Mesozoic sedimentary rocks include a condensed sequence of Triassic breccias and sandstones (~80 m thick; Emeleus et al. 1996), and fault-bound slivers of Lower Jurassic limestone, sandstone and mudstone which are thermally metamorphosed by the RLS (Fig. S1c).

The RLS exhibits evidence for open system magma chamber behaviour and is subdivided into three portions; the Eastern, Western and Central Layered Intrusions (Fig. S1b). The Eastern Layered Intrusion (ELI; Fig. S1d) comprises 16 individual cyclic units, each
thought to represent a new influx of basaltic and/or picritic composition magma, with a
peridotite base and troctolite (±olivine gabbro and anorthosite) top (Brown 1956; Emeleus et
al. 1996). Platinum-group element-enriched Cr-spinel seams (chromitite), typically 2-4 mm
thick, occur at the boundaries between some units (e.g. 7-8 and 11-12; e.g. O'Driscoll et al.
2009). Anhedral accessory base-metal sulfide grains (<0.1 mm) occur within these
chromitite seams, and are the primary hosts for the PGE mineralisation (O'Driscoll et al.
2009). The chromitite seams were originally interpreted as the product of crystal settling onto
the plagioclase-rich magma chamber floor (e.g., Brown 1956); however, recent
investigations have advocated an in situ crystallisation model for the chromitites (O'Driscoll
et al. 2010).

Thirty samples were selected for halogen abundance measurements, including bulk
rock and mineral separates, from various lithologies throughout the intrusion (see Table S1
for further details). An important goal of this study is to investigate the halogen geochemistry
of the PGE-reefs, so detailed transects across the Unit 7-8 and 11-12 chromitite seams were
carried out including footwall and hanging wall lithologies. The mineralogy and field
relationships of these rocks have been previously described (e.g., O'Driscoll et al. 2009,
2010). Magmatic differentiates from parts of the Rum intrusion devoid of chromitite seams
were also sampled (e.g., Unit 9). A selection of picrite dykes, considered proxies for the
parental magmas that formed the Rum magma chamber, was included. Olivine separates
were analysed from two picrite samples (RM_16_010 and RM_16_012). Sulfide-bearing
troctolites (RM17) of the contact zone formed due to contamination of magma by S-bearing
sedimentary rocks (Hulbert et al. 1992), so were analysed to investigate a potential country
rock halogen signature. Similarly, samples from sulfide-bearing peridotite in a satellite plug
(outside the RLS; RM_16_017; Fig. S1) were analysed. Finally, a suite of late-stage
pegmatoids were analysed. Some of the pegmatoids occur at the tops of the macro-rhythmic
units (Units 10, 11) and are considered to represent compaction-driven expulsion of
interstitial melt from the underlying crystal mush (Emeleus et al. 1996). Another pegmatoid
(RM_16_021) comes from the Central Intrusion, part of a suite that was dated by Hamilton et
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al. (1998) as the youngest magmatism associated with the RLS. A variety of country rocks, including Jurassic mudrocks, Triassic sediments and Torridonian Supergroup sandstones, were also analysed.

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## 3. Analytical techniques

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### 3.1 Electron microscopy

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In order to gain a qualitative overview of halogen distribution in key samples (e.g., the PGE-rich chromitite seams), X-ray mapping was carried out using the Cameca SX-100 electron microprobe in Manchester. Chlorine maps were conducted at an acceleration voltage of 15 kV and 200 nA beam current.

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### 3.2 Halogen abundances and noble gas isotopes by neutron irradiation

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A detailed description of the NI-NGMS technique and our sample preparation methodology is provided in the supplement (S1.1) and in Ruzié-Hamilton *et al.* (2016). NI-NGMS utilises the neutron conversion of Cl, Br and I into their respective noble gas proxy isotopes [(n, γ, β) reaction; producing <sup>38</sup>Ar from Cl, <sup>80,82</sup>Kr from Br, and <sup>128</sup>Xe from I]. The method enables measurement of the heavy halogens and the natural noble gases simultaneously.

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Measurements of the halogens were conducted on the Thermo Scientific Argus VI™ noble gas mass spectrometer at the University of Manchester. Data were corrected for blanks (conducted after every 3-4 measurements) and mass discrimination (monitored daily using air calibrations). Blank contributions were typically <<1 % (but up to 3%) for <sup>38</sup>Ar, and <<0.1 % for <sup>80,82</sup>Kr and <sup>128</sup>Xe. Data were also corrected for decay of <sup>37</sup>Ar and <sup>39</sup>Ar, air contributions, and neutron-induced reactions producing <sup>40</sup>Ar, <sup>39</sup>Ar, <sup>38</sup>Ar and <sup>36</sup>Ar, monitored by the K and Ca salts placed in the irradiation (Table S2). Individual replicate data is provided together with specific sample details in Table S2 of the supplement. Between two and five replicate analyses have been averaged for each sample. Halogen elemental ratios are reported as weight ratios and uncertainties quoted to 1 standard error of the mean of the replicate averages provide a measure of sample heterogeneity.

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### 3.3 Halogen abundances after pyrohydrolysis extraction

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The halogen abundances of a selection of Rum sedimentary rocks were measured using the pyrohydrolysis technique, a detailed description of which is given in the supplement (S1.2) and in Balcone-Boissard et al. (2009) and Schnetger and Muramatsu (1996). Hydrolysis solutions were analysed by ion chromatography for F and Cl and inductively coupled plasma-mass spectrometry (ICP-MS) for Br and I. This method was employed for the sedimentary samples rather than NI-NGMS due to the technical difficulties of producing high-precision measurements in materials containing abundant organic materials and carbonates (e.g., Mesozoic mudstones).

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## 4. Results

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### 4.1 Mineral-scale observations

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The volume proportion of interstitial minerals (representing crystallised interstitial melt) is variable for the peridotites, troctolites and gabbros. However, hydrous minerals (e.g., biotite, serpentine and amphibole) are a ubiquitous accessory phase in all of these rocks, occurring at the boundaries or triple grain junctions between primocrysts and interstitial phases (Fig. S2a). No apatite is observed. No textural evidence for degassing or fluid escape is observed in any of these rocks. The picrite dykes are mainly olivine-phyric, with a fine-grained groundmass comprising intergrown plagioclase and clinopyroxene, but one sample (RM_16_011) is aphyric and exhibits a spinifex-type 'quench' texture suggesting rapid solidification (Fig. 1a). The pegmatoids contain minor amounts of hydrous phases (biotite and amphibole) and apatite, except for pegmatoid (RM_16_021), which has abundant amphibole and biotite, as well as apatite and zircon (Fig. 1b).

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Chromitite seams are always associated with a greater proportion (1-2 vol.%) of hydrous minerals than their host rocks. Biotite and amphibole occur as anhedral crystals in interstitial areas to Cr-spinel crystals, as spherical inclusions in Cr-spinel and adjacent olivine crystals (Fig. S2b,c), and in thin (<1 mm) veins that cross-cut the primary minerals
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(Fig. S2d). The CI X-ray maps of the Unit 7-8 boundary chromitite also reveal the presence of rare apatite grains (<50 µm in size), commonly associated with biotite (Fig. 1c,d). The apatite has moderate to low CI contents of <0.4 wt.%. Biotite grains (<1 mm) also appear to have relatively low CI contents (<0.1 wt.%). Element mapping of CI highlights extensive and pervasive alteration veining along olivine grain boundaries and intra-crystal fractures.

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## 4.2 Heavy halogen abundances and ratios for the Rum intrusion and country rocks

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The halogen abundance and noble gas isotope data for the Rum samples are presented in Table 1 and in Figures 2-4. Chlorine, Br and I concentrations range from 1 ppm to 124 ppm, 7 to 484 ppb and <1 to 363 ppb, respectively, in bulk samples and mineral separates. In general, Br and Cl abundances are positively correlated (Fig. 2a); however, I and Cl are not correlated (Fig. 2b). Intra-sample variation was not significant in most cases (e.g., within ~15%) and likely reflects natural sample variability. This is particularly true for some bulk samples, for instance the B2 Peridotite, where concentrations are low (i.e., <10 ppm Cl) and therefore the variation over repeat analyses (2-10 ppm Cl over 5 analyses ~62% variation) is higher.

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*Picrites.* Picrite samples (n=3) have Cl concentrations of 23-75 ppm, 89-229 ppb Br and 4-19 ppb I. Picrite olivine separates (n=2) display systematically lower halogen contents (Cl = 6-13 ppm; Br = 43-48 ppb; I = 0.5-2 ppb). The picrites define a trend to high Cl and low $^{40}$Ar (Fig. 3a), distinct from most other RLS rocks.

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*Rum intrusion magmatic differentiates.* Peridotites (n=3) overlying the chromitite seams have halogen contents of 3-16 ppm Cl, 2-78 ppb Br and 1-4 ppb I. Troctolites and olivine gabbros (n=3) range from ~1-83 ppm Cl, 14-84 ppb Br and 3-60 ppb I. In the Unit 1 troctolite (RM17), for which a sulfide separate and bulk rock were analysed, the sulfide shows higher I concentrations compared to bulk rock (24 ppb to 6 ppb, respectively) whilst Cl is similar (~4 ppm). Chromitites (n=4) have halogen contents of 2-3 ppm Cl, 9-35 ppb Br and 10-102 ppb I. The Unit 7-8 chromitite has high I (75 ppb) compared to Br (29 ppb) and Cl (2 ppm; Fig. 2a,b). In contrast, the Unit 11-12 chromitite seam has lower Br (10 ppb) and I
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(17 ppb). Chromitites have low Cl/$^{36}$Ar ratios and define a trend towards high $^{40}$Ar and low Cl (Fig. 3a, b). The anorthosites (n=3), some of which are closely associated with the chromitite seams (n = 4), display higher concentrations of Cl (e.g., 11 ppm in the Unit 7 anorthosite) compared to the chromitite seams, but variable I (29 ppb Unit 7 anorthosite, 50 ppb Unit 11 anorthosite).

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Satellite intrusion rocks. Peridotite plugs (n=2) have halogen abundances of 16 and 99 ppm Cl, 67 and 394 ppb Br and 21 and 50 ppb I.

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Pegmatoids. Pegmatoids (n=4) yield highly variable halogen abundances. Chlorine contents range from 2-53 ppm, Br ranges from 7-144 ppb and I from 2-363 ppb. Associated mineral separates, including biotite (n=1) and amphibole (n=3) also show variation. Amphiboles have halogen contents of 1-39 pm Cl, 7-20 ppb Br and 2-104 ppb I. Biotite (124 ppm Cl, 29 ppb Br and 200 ppb I) is also Cl- and I-rich.

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## 4.3 Halogens in the Rum intrusion country rocks

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Halogens, including F, were analysed for a selection (n=7) of RLS country rocks including Mesozoic sandstones and limestones, as well as Torridonian sandstones (Table S3). The sedimentary rocks all exhibit concentrations of the heavy halogens that are at the upper end of the array defined by the RLS rocks (Fig. 2). One calcareous sandstone of Mesozoic (Triassic) age has a high I/Cl ratio of 0.51 resulting from an I concentration of 3227 ppb and a relatively low Cl concentration (6 ppm).

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## 4.4 Noble gas isotope systematics

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Noble gas ratios, such as $^{130}$Xe/$^{36}$Ar and $^{84}$Kr/$^{36}$Ar, have well-defined compositions for air, MORB, seawater and sedimentary rocks (Fig. 4). The RLS data exhibit a degree of scatter, with no consistent variation, and the sample set overall yields ranges between ~ 17-125 x 10⁻³ for $^{84}$Kr/$^{36}$Ar and 0.2-4.8 x 10⁻³ for $^{130}$Xe/$^{36}$Ar. Hence, the data range from MORB-like ($^{84}$Kr/$^{36}$Ar = 0.05, $^{130}$Xe/$^{36}$Ar ~1 x 10⁻³, Moreira et al., 1998; Holland and Ballentine, 2006) to the field for submarine sediments ($^{84}$Kr/$^{36}$Ar = 0.1 - 0.2 and $^{130}$Xe/$^{36}$Ar = 2-3 x 10⁻³; Matsuda

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and Nagano, 1986; Staudacher and Allègre, 1988), with some clustering of the data close to
the seawater value $^{84}$Kr/$^{36}$Ar = 0.04, $^{130}$Xe/$^{36}$Ar = 5 x 10$^{-4}$ (Holland and Ballentine, 2006;
Kendrick et al., 2013).

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## 5. Discussion

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Halogen ratios (Br/Cl and I/Cl) are useful for discriminating between specific reservoirs (e.g., MORB, sediments, etc.; Tables 1, S2; Fig. 2c). The RLS displays significant variations in halogen concentrations and ratios throughout the intrusion, with a broad general trend from mantle-like values (2.86 ±0.6 and 60 ±30 x 10⁻⁶ wt. Br/Cl and I/Cl; Kendrick et al. 2012; 2013; 2014) in the picrites towards fields for sedimentary rock compositions (Fig. 2c). The ELI exhibits elevated I/Cl compared to the picrite samples and MORB and OIB values in general. This is especially true of certain pegmatoids and chromitite horizons, where I concentrations are significantly higher and decoupled from Cl and Br (Fig. 2b). In the following sections, we discuss these trends in the context of primary magmatic (i.e., magma differentiation-related) processes, mineralogical controls on the halogens and secondary processes (including crustal contamination and late-stage alteration).

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### 5.1 *The Rum parental magmas and primary magmatic processes*

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**Parental melt characteristics.** The RLS and associated intrusions, including the picrite dykes and satellite plugs (Emeleus et al. 1996; Upton et al. 2002; Meyer et al. 2009), are the result of Palaeogene volcanism associated with opening of the North Atlantic Ocean and proto-Iceland plume activity. Picrites reveal Br/Cl and I/Cl ratios that are similar to established mantle values (e.g., Kendrick et al. 2012, 2013, 2014), consistent with previous suggestions for the composition of the Rum mantle source region (O'Driscoll et al. 2009). Br/Cl and most of the I/Cl sample ratios lie close (within an order of magnitude) to the MORB trend (Fig. 2a,b), further supporting a primary origin for the picrite halogens. The picrite samples exhibit amongst the highest Cl concentrations in our Rum dataset. Given that these rocks represent the primitive magmas that formed the Rum intrusion, we suggest that Cl has been

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subsequently lost during the magma chamber processes responsible for crystallisation of the
cumulate lithologies (peridotites, troctolites and gabbros). Conversely, the picrites have <11
ppb iodine, a factor of 2-10 lower than I in many of the Rum intrusion rocks.

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*Magmatic differentiation.* Bromine and Cl abundances correlate well throughout the Rum intrusion rocks (Fig. 2a), from typically low Br and Cl in the chromitites and anorthosites, to higher Br and Cl in the picrites and peridotites. Troctolite and gabbro samples vary along this spectrum. Iodine and Cl, however are not well-correlated and show no trend with compositional group (except for the picrites, which lie close to the MORB line; Fig. 2b). This suggests that the process(es) or phase(s) (minerals, inclusion, etc.) controlling Br and Cl distribution in these rocks is the same, whilst I behaviour is different. Here, we evaluate the potential for fractional crystallisation to explain the halogen variations observed.

The halogens are highly incompatible elements ($D_{halogen}^{mineral/melt} << 1$) and thus should behave similarly during progressive crystallisation of magma. Major element fractional crystallisation indices (e.g., Ni and $Al_2O_3$ versus MgO content; Fig. S3a,b) correlate well with mineral abundances in the cumulates. However, halogen abundances and ratios show no correlation with MgO content (Fig. S4a-c). Calculations based on estimated bulk partition coefficients for Cl and Br show no significant fractionation of these elements from one another during crystallisation of the parental melt (Table S4, Fig. S5), in agreement with the Rum data (Fig. 2a). Therefore, halogen distributions in the cumulates are not explained by simple fractional crystallisation processes, in line with previous studies on basaltic glass (e.g., Schilling et al., 1980, Kendrick et al., 2013), which have shown that halogens are not fractionated by melting and crystallisation processes.

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*Shallow-level magmatic degassing and fluid-melt partitioning.* Rum cumulates are Cl-deficient compared to the picrites considered to be their parental magma(s). Here we examine if shallow magmatic degassing processes can explain these differences. Halogen degassing into the vapour phase is expected only at pressures <1 MPa (Edmonds et al., 2009), but partitioning of halogens into aqueous fluid is considered. Chlorine partitions

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preferably into the fluid phase over the melt ($D_{Cl}^{fluid/melt}$ >>1) across most of the range of P-T conditions relevant to shallow-crustal magmatic systems. Limited data for Br and I partitioning between fluid and melt exist for felsic magmas (e.g., albitic melt) and indicate that $D_{Cl}^{fluid/melt}$ ~8, $D_{Br}^{fluid/melt}$ ~18 and $D_{I}^{fluid/melt}$ ~ 104 (Bureau *et al.*, 2000). Therefore, in the presence of aqueous fluid, the halogens should degas in the order I>>Br>Cl, meaning even minor degassing could influence the halogen budget of the RLS.

Experimental constraints (Webster *et al.* 1999; Alletti *et al.*, 2009) suggest that the maximum Cl capacity of a melt analogous to the Rum parental magma is ~2.15 wt.% (Fig. S6a), whilst melt inclusion data indicate that mafic silicate melts contain a maximum of 0.5-1.0 wt.% Cl (Webster *et al.*, 1999). These values exceed the ~75 ppm measured in picrite RM_16_011, suggesting either an initially Cl-poor melt or degassing of Cl into an aqueous fluid (e.g., a first boiling process driven by H₂O exsolution). At ~2 km depth (4.9 x 10⁷ Pa), a reasonable depth of emplacement for the Rum magma chamber (Holness 1999), ($D_{Cl}^{fluid/melt}$ = 9.1 (Fig. S6b; Alletti *et al.*, 2009), suggesting that early aqueous fluids exsolved from the RLS magmas could have removed significant Cl (Fig. S6c). Despite experimental and melt inclusion constraints suggesting the potential for high Cl concentrations, petrographic and geochemical evidence for the aqueous fluid required to extract the halogens is absent from the Rum cumulates. For instance, only small amounts (<1 vol. %; see above) of hydrous magmatic minerals (e.g., biotite and apatite) are present. Furthermore, there is no evidence of fluid inclusions and the loss on ignition (LOI) values for the cumulates are low (typically <0.5 wt.% and up to 1.70 wt. %; O'Driscoll *et al.* 2009). We therefore suggest that the Rum parental melt was either Cl-poor or very efficiently degassed during pre-crystallisation first boiling, due to the high ($D_{Cl}^{fluid/melt}$, along with the H₂O that drove degassing, a process for which no field or petrographic evidence now remains.

*Mineralogical controls on halogen distributions.* Previous work on the trace element geochemistry, as well as the radiogenic and stable isotope systematics of the RLS have shown that low temperature alteration and/or hydrothermal processes did not play a major
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role in during the evolution of the Rum intrusion (e.g., Palacz 1985; Greenwood et al. 1992; Meyer et al. 2009; O'Driscoll et al. 2009). The mineralogy and textures of RLS rocks (particularly the cumulates) support this suggestion; alteration is generally minor and spatially confined (e.g., localised alteration veins in chromitites; Fig. 1b), with no evidence for widespread low-temperature modification or recrystallisation.

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The cumulate rocks are mixtures between primocrysts and interstitial melt. In line with other work on these rocks (Emeleus et al. 1996), we interpret the accessory hydrous phases present as reflecting volatile enrichment in the residual liquid, during progressive crystallisation of the crystal mush. By contrast, the chromitites contain microstructural evidence for volatile mobility, including greater amounts of interstitial biotite and apatite, and secondary biotite-amphibole-bearing veins. In order to determine if the hydrous phases might control the bulk rock halogen abundances, contributions from these phases can be compared to bulk rock halogen abundances. The modal proportions of apatite, biotite and amphibole required to account for bulk rock Cl, are calculated using halogen concentrations (Fig. S7 and Table S5a) in these phases, estimated from the amphibole and biotite separates analysed from the Rum pegmatoids (these rocks are not directly petrogenetically related, but aid a first order approximation). Apatite was approximated using the median halogen composition of OH apatite (Kusebach et al., 2015). Apatite can account for the lowest bulk Cl abundances (<0.5 vol.%), but not Cl in the highest abundance samples, which would require ~10 vol.% apatite. Apatite can account for the highest Br and I abundances (<3.9 vol.%); however, a lack of correlation between bulk rock P₂O₅ and any of the heavy halogens suggests that apatite alone does not control halogen contents (Fig. S7d). Biotite and amphibole can account for the lowest I abundances only (<0.6-1.1 vol.%). Therefore, whilst the full range of Br and I abundances in the Rum rocks can be best accounted for by apatite, biotite, which is ubiquitously present, is a more realistic carrier for halogens in all but the highest concentration samples (the latter do contain apatite). A caveat is that Br and I whole rock abundances show no correlation in these rocks (which would be expected if they were hosted by the same phase) yet Cl and Br are strongly correlated. Thus, we conclude
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that the heavy halogen budgets of the cumulates are controlled in general by hydrous
accessory phases related to the final stages of crystallisation of the interstitial liquid, and the
precise nature of the control on the variations between lithologies is likely a function of
contributions from mixtures of these phases in each whole rock sample.

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## 5.2 Magma-crust interactions and the high I component in the RLS

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The Rum I source. Chlorine and Br are present in MORB-relative abundances in the RLS rocks. Iodine, however, is decoupled from Cl and Br in its abundance and ratio patterns and requires an alternative explanation. We therefore consider the role of crustal contamination to explain the high I seen in some lithologies (e.g., Déruelle et al. 1992; Muramatsu and Wedepohl 1998). Studies based on different radiogenic isotopic techniques (e.g., Sr, Nd, Os) have demonstrated that the RLS rocks preserve evidence for crustal contamination (Palacz 1985; Meyer et al. 2009; O'Driscoll et al. 2009). Palacz (1985) estimated contamination of the Rum parental magma by up to 7% Lewisian Complex gneiss to account for Sr isotope signatures in the RLS cumulates. Similarly, O'Driscoll et al. (2009) suggested 5-8% contamination of a suite of Rum rocks by Lewisian gneiss on the basis of Os isotope data. Upton et al. (2002) calculated ≤4% crustal contamination of a suite of picrite dykes, including the M9 picrite from this study, on the basis of Sr, Nd and Pb isotopes. Here, we consider the role that contamination by Torridonian Supergroup and Mesozoic sedimentary country rocks may have had. Whilst we have no halogen data for the basement Lewisian gneisses, it is unlikely that they contributed significant amounts of heavy halogens, particularly I, to RLS magmas (because of pre-intrusion metamorphism and their initially low abundances – they mainly have igneous protoliths).

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Due to its biophilic behaviour, I is enriched in organic sediments (e.g., marine shales) and is a sensitive indicator of contamination by sediments (Li and Schoomaker 2003; Muramatsu and Wedepohl 1998; Kendrick et al. 2017). The I concentrations of the sediments that may have yielded the fluid contaminant range from 11-183 ppb (n=3, Jurassic mudstones and calcareous sediments), 33-3227 ppb (n=2, Triassic sandstone and
393 calcareous sediment) and 47-122 ppb I (n=2, Torridonian sandstones). For comparison, the
394 bulk continental crust has an estimated I concentration of 700 ppb (Rudnick and Gao, 2003).
395 Assuming solid assimilation of country rock, different degrees of contamination would be
396 required to account for the variation in I enrichment relative to the picrites (maximum I = 11
397 ppb). Iodine enrichment is not lithology dependant; high-I (>45 ppb) samples include
398 peridotite, troctolite, anorthosite and chromitite (Table 1). Using average I concentrations for
399 each of the country rock reservoirs, assimilation of 0.1-6.2% of the Triassic rocks best
400 accounts for the elevated I abundances seen in the RLS rocks in terms of consistency with
401 previous studies (Tables S3; S5b; Palacz 1985; Meyer et al. 2009; O'Driscoll et al. 2009).
402 Torridonian sandstones and Jurassic sediments require greater degrees of assimilation
403 (>10% and 15%, respectively) to account for even the lowest levels of I enrichment in the
404 RLS rocks. The Rum intrusion was emplaced close to the unconformity between the
405 Lewisian gneisses and the Torridonian rocks, and clearly interacted with Mesozoic rocks too,
406 so it is likely that mixtures of these different reservoirs interacted with the Rum parental
407 melts (see below).

408 Direct country rock assimilation can explain the ubiquitous enrichment of I in the Rum
409 samples, relative to MORB. The picrites show the lowest I concentrations, lying closest to
410 the MORB reference line (Fig. 2b), and supporting their interpretation as proxies for the Rum
411 parental melts. However, assimilation of solid country rock would not obviously fractionate I
412 from Br and Cl (i.e., addition of halogens would be congruent) in the manner shown by the
413 data (Fig. 2b), so it is useful to further consider the nature of the contaminant that introduced
414 I into the RLS magmas. As noted above, the cumulate lithologies do not preserve any field
415 or petrographic evidence for significant fluid flow. However, it is possible that the pervasive I
416 enrichment in the RLS rocks reflects addition to the parental melts of fluid generated by
417 dehydration of country rocks such as the Triassic sediments, prior to crystallisation of the
418 cumulates. Based on what little is known about I partitioning (e.g., in felsic systems a high
419 fluid-melt partition coefficient of D ~ 104; Bureau et al., 2000), even small amounts of fluid
420 extracted during devolatilisation of sediments would likely contain enough I to account for the
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relative enrichment over the picrites, whilst accounting for the low LOI and low abundances
of hydrous phases in the RLS rocks. It is worth noting that the Jurassic country rocks occur
in fault-bound blocks in close proximity to the intrusion and may have undergone
devolatilisation-related I loss, relative to Cl (11 ppb versus 34 ppm, respectively; Fig. 2c),
based on comparisons with a global marine shale average (Li and Schoonmaker 2003).

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*Evidence of crustal contamination from noble gas isotope signatures.* The RLS noble gas isotope data lend support to crustal contamination sourcing some of the halogens. Many samples (including chromitites, cumulates and pegmatoids) trend towards and into a field in $^{130}\text{Xe}/^{36}\text{Ar}$ versus $^{84}\text{Kr}/^{36}\text{Ar}$ space defined by oceanic sediments (Fig. 4). Elevated $^{130}\text{Xe}/^{36}\text{Ar}$ ratios are often measured in marine shales due to their high organic content (e.g., Ozima and Podosek 2002); therefore, the enriched $^{130}\text{Xe}/^{36}\text{Ar}$ and $^{84}\text{Kr}/^{36}\text{Ar}$ ratios may suggest assimilation of Mesozoic sedimentary rocks into the Rum parental magmas. The Ar isotope data are also useful here. We calculated age-corrected $^{40}\text{Ar}/^{36}\text{Ar}$ ratios, based on the anticipated ingrowth of radiogenic Ar (over 60.53 ±0.08 Ma; Hamilton et al. 1998) for the RLS and measured K concentrations (Table S2). The K-contents of the RLS samples are generally low (~40-3000 ppm; with the exception of some pegmatoid samples), and therefore ingrowth cannot have led to the $^{40}\text{Ar}/^{36}\text{Ar}$ ratios observed, over ~60 Ma. A wide range (330-9300) in $^{40}\text{Ar}/^{36}\text{Ar}$ ratios is observed (Table S2; Fig. 3a), and the age-correction for $^{40}\text{Ar}$ results in only a small change to the $^{40}\text{Ar}/^{36}\text{Ar}$ ratio, on the order of 0.1-1%. The lowest, air-like ratios ($^{40}\text{Ar}/^{36}\text{Ar}$ air = 298.6; Lee et al., 2006), occur in the ELI pegmatoids and satellite plug and the highest ratios are exhibited by some cumulates (Fig. 3a). Mineral separates, such as amphibole, clinopyroxenes and sulfides, have ratios that are generally elevated slightly above air, whilst bulk samples preserve higher and more variable $^{40}\text{Ar}/^{36}\text{Ar}$ ratios of ~2000 to 9300 (Fig. 3a). This may indicate siting of $^{40}\text{Ar}$ in grain boundary or fine-matrix material. $^{40}\text{Ar}$ is generally not correlated with K (Fig. 4b), and the $^{40}\text{Ar}/\text{K}$ ratios measured in most of the RLS samples far exceed those expected for a ~60 Ma intrusion ($^{40}\text{Ar}^*/\text{K} \approx 6.34 \times 10^{-9}$; Fig. 4b).

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The excess <sup>40</sup>Ar component probably either comes from the mantle source of the Rum intrusion (maximum <sup>40</sup>Ar/<sup>36</sup>Ar ~ 44,000; Moreira *et al.*, 1998) or from the surrounding crustal rocks. We suggest that the latter is a more likely scenario, given previously documented evidence for crustal contamination in RLS (Palacz 1985; Meyer *et al.* 2009; O'Driscoll *et al.*, 2009), and also a positive correlation between excess <sup>40</sup>Ar and I abundance in a subset of the Rum samples (Fig. 3d). Of the three possible K-bearing crustal sources of the excess <sup>40</sup>Ar (Archean gneisses, Torridonian sandstones and Mesozoic sediments), it is worth noting that the Torridonian sediments locally contain tourmaline, which is known to host significant <sup>40</sup>Ar (several orders of magnitude greater than co-genetic crustal minerals; Saito *et al.*, 1984). Together with the fact that many of the Torridonian sandstones are arkosic (K-rich), they represent at least one potential source for the high <sup>40</sup>Ar concentrations observed in the RLS rocks. Alternatively, as noted above, the <sup>40</sup>Ar-I relationship seen in the Unit 11-12 rocks (i.e., Fig. 3d) may point to multiple crustal source signatures (<sup>40</sup>Ar from Lewisian gneisses, I from Mesozoic sediments) being brought together.

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### 5.3 Implications for PGE-enrichment and sub-volcanic processing of the halogens

The variations in Rum halogen geochemistry, particularly in the chromitites and associated lithologies (Fig. 5), offer new insights into the mechanisms of precious metal concentration in sub-volcanic systems. The development of PGE-rich chromitite seams in layered intrusions has been attributed to *in situ* reaction of incoming melt with the footwall rocks, i.e., the magma chamber floor, in an essentially metasomatic process (Mathez and Kinzler, 2017). It is important to note in this respect the greater abundances of hydrous phases within Rum chromitite seams, compared to the footwall and hanging wall rocks (Fig. 1c,d). The two key observations from a halogen standpoint for the Rum PGE-rich lithologies are their relatively low Cl and relatively high I abundances. Previous work proposed that the Rum chromitites and anorthosites formed following downward percolation of picritic magma and dissolution of the felspathic footwall (O'Driscoll *et al.* 2010). If the Unit 7 footwall rocks had an I/Cl ratio similar to that of the Unit 10 pegmatoid (RM_16_022) prior to emplacement of the Unit 8
magma(s), a reasonable proposition given that there is no chromitite at the base of Unit 11, assimilation of pegmatoidal material could have triggered localised volatile exsolution to produce the hydrous phases in the Unit 7-8 chromitite seam. The relatively low Cl abundances in the chromitites together with localised petrographic evidence for fluid flow probably reflect subsequent volatile loss (Figs. 5, S2d). Furthermore, given the arguments made above for high I concentrations being a consequence of crustal contamination, it would seem that the footwall assimilation process recycled these crustal I signatures to the resultant chromitites. The noble gas isotope data support this conclusion, with well-developed coupled enrichments observed between excess <sup>40</sup>Ar and I in the Unit 11 anorthosite (Fig. 3d). The Unit 11 anorthosite and Unit 11-12 chromitite plot closely together in I/Cl versus Br/Cl space; suggesting a petrogenetic association between both that is not evident at the Unit 7-8 boundary. This discrepancy could be a consequence of assimilation of compositionally variable footwall.

Models for PGE-enrichment in layered intrusions (notably the Stillwater intrusion) appeal to Cl-rich fluids exsolved from the magma to transport and concentrate the PGE into reefs (e.g., Boudreau *et al.* 1986). Such (saline) fluids are known to have high solubilities for the PGE (up to ppm levels; Hanley *et al.* 2005), which could account for the high levels of enrichment in Pt and Pd in the Rum chromitites (e.g., several ppm in the Unit 7-8 chromitite; O'Driscoll *et al.* 2009). However, comparatively little is known about the relationship between the halogens and the base metal sulfides that sequester the PGE in high-temperature magmatic systems. In particular, there are open questions around the ability of crystallising sulfide melt to fractionate the halogens from one another, and whether or not base metal sulfides can host significant halogen concentrations (Mungall and Brenan 2003). Limited experimental evidence suggests halogens preferentially partition into silicate melts during sulfide-silicate melt immiscibility, although sulfide melts are capable of dissolving up to 2000 ppm halogens (Mungall and Brenan 2003; Lecumberri-Sanchez and Bodnar 2018). It has been demonstrated that I may be moderately chalcophile (Fuge and Johnson 1984). Whilst Cl may also display chalcophile behaviour, at lower temperatures exsolved halides mix with
aqueous fluids (resulting in high Cl/Br ratio fluids; Mungall and Brenan 2003; Lecumberri-Sanchez and Bodnar 2018); hence, Cl is more easily lost to a fluid phase. Sulfide separates from the Unit 1 troctolite (RM17) and the satellite peridotite plug (RM_16_17) yield higher Br/Cl and I/Cl ratios compared to their respective whole rock values (Fig. 2c, Table 1). Sulfides from the troctolite in particular appear to be enriched in I, by a factor of ~4, compared to their bulk rock counterpart (~24 ppm versus ~6 ppm; Fig. 2b). These troctolites and the satellite peridotites have δ³⁴S values of -14.8‰ and -18.3‰, respectively, attributed by Hulbert et al. (1992) to incorporation of biogenic S from the adjacent Jurassic sediments. It would seem likely that the I enrichment in these rocks also reflects a crustal contamination signature (Fig. S1c). Sulfides are a relatively minor phase in the chromitites (<5 vol.%), so it is unlikely that sulfides alone can account for the entire chromitite I budget. However, the possibility that I may exhibit chalcophile tendencies in high-temperature magmatic systems highlights the need for further experimental work on silicate melt-sulfide melt and sulfide melt-sulfide I partitioning behaviour, to shed light on this relatively poorly constrained part of the I geochemical cycle.

The Rum intrusion was emplaced at a depth of ~2 km and likely underlay a significant volcanic edifice (Emeleus et al., 1996), so the implications of our halogen dataset extend past magma chamber processes and ore genesis, to elucidating the distribution of halogens in sub-volcanic systems prior to and/or during eruption. In particular, I abundances in Rum samples are 12% to two orders of magnitude greater than typical MORB compositions (based on a global range of 1-90 ppb reported by Kendrick et al., 2013), suggesting that crustal contamination might have led to pre-concentration of the halogens in the Rum system before possible injection into the atmosphere on eruption (Aiuppa et al., 2009; Bureau et al., 2000; Pyle and Mather, 2009). Iodine has significantly deleterious effects on atmospheric composition, including tropospheric ozone depletion and the formation of aerosols (e.g., Vogt, 1999). It is worth noting that Rum represents a relatively small (~700 km³; Emeleus et al., 1996) end-member compared to layered intrusions such as the Bushveld Complex (South Africa), with an estimated magma volume of ~1 × 10⁶ km³
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(Cawthorn, 2015). Establishing the abundances and distributions of the heavy halogens in these larger layered intrusions may represent an important avenue for future work, by revealing whether or not the associated magmatism had the potential to introduce significant quantities of I (and Cl, Br) into the atmosphere at punctuated periods in Earth history.

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# 6. Conclusions

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In a suite of Rum picrites, cumulates, pegmatoids and chromitites, Cl and Br abundances are present in MORB-relative proportions and correlate well with one another. Their distribution can be accounted for by mineral phases observed in the rocks (e.g., biotite, amphibole and apatite). In the cumulates, these are primary magmatic hydrous phases representing final crystallisation of interstitial liquid in the crystal mush. Iodine behaviour is decoupled from that of Cl and Br; elevated I abundances (and I/Cl ratios relative to MORB) are observed in all samples except for the picrites. The I enrichment points to crustal contamination of the Rum magmas, for which Triassic sedimentary rocks are a likely candidate (maximum 3227 ppm I). Noble gas isotope compositions, particularly $^{40}$Ar/$^{36}$Ar, also suggest crustal assimilation and in some samples correlate well with I abundance (Fig. 3d). While mass balance calculations suggest that direct solid assimilation fits with previous estimates for the degree of crustal contamination of Rum magmas (based on radiogenic isotopes), this does not explain the fractionation of I from Cl and Br. Instead, addition of a fluid derived by country rock dehydration might better explain the pervasive I enrichment.
High I abundances indicate the presence of a crustal component in the PGE-rich chromitites, and together with petrographic evidence for the presence of a fluid phase, suggests reprocessing of pegmatoidal footwall rocks during chromitite petrogenesis. In summary, our study reveals insights into the relative importance of magmatic, crustal contamination and degassing processes in controlling halogen abundances and fractionation in layered intrusions. The importance of aqueous fluids in transporting and redistributing the halogens is emphasised, with implications for their metal-carrying capacity and role in forming magmatic PGE deposits.
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561 Acknowledgements

562 This manuscript forms a component of A. Parker's PhD thesis undertaken at the University
563 of Manchester, supported by the Natural Environment Research Council grant number
564 NE/L002469/1. Bastian Joachim is thanked for helpful discussions on halogen partitioning.

565 Halogen analyses after pyrohydrolysis were supported by IPGP multidisciplinary program
566 PARI and by Region Île-de-France SESAME Grant no. 12015908. Alan Boudreau and an
567 anonymous reviewer are thanked for their very constructive reviews, as is Tamsin Mather for
568 editorial comments, all of which greatly improved earlier versions of the manuscript.

569

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Kendrick, M. A., Hémond, C., Kamenetsky, V.S., Danyushevsky, L., Devey, C.W., Rodemann, T., Jackson, M.G., and Perfit, M.R., (2017). Seawater cycled throughout Earth's

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656 mantle in partially serpentinized lithosphere. *Nature Geoscience*, 10(3), 222-228. DOI:
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658

659 Kusebach, C., John, T., Whitehouse, M.J., and Engvik, A.K., (2015) Apatite as a probe for
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661 to Mineralogy and Petrology*, 170, 34, 20 pp.

662

663 Lecumberri-Sanchez P., and Bodnar, R.J., (2018). Halogen Geochemistry of Ore Deposits: 664 Contributions Towards Understanding Sources and Processes. In: Harlov, D.E. and 665 Aranovich L., (Eds.) The Role of Halogens in Terrestrial and Extraterrestrial Geochemical 666 Processes. Springer International Publishing, Switzerland.

667 Lee, J.Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.S., Lee, J.B., and Kim, J.S. 668 (2006) A redetermination of the isotopic abundance of atmospheric Ar. *Geochimica et 669 Cosmochimica Acta*, 70(17), 4507-4512.

670 Li, Y-H and Schoonmaker, J.E., (2003) Chemical Composition and Mineralogy of Marine 671 Sediments. Treatise on Geochemistry, Executive Editors: Heinrich D. Holland and Karl K. 672 Turekian. pp. 407. Elsevier, p.1-35

673 Martin, J.M., and Meybeck, M., (1979) Elemental mass balance of material carried by major 674 World rivers. *Marine Chemistry*, 7(3): 173-206.

675

676 Mathez, E.A. and Kinzler, R.J., (2017) Metasomatic chromitite seams in the Bushveld and 677 Rum layered intrusions, *Elements*, 13(6), 397-402.

678 Matsuda, J., Nagao, K., (1986). Noble-gas abundances in a deep-sea sediment core from 679 eastern equatorial Pacific. *Geochemical Journal*, 20, 71–80. DOI: 10.2343/geochemj.20.71
680 Meyer R., Nicoll G.R., Hertogen J., Troll V.R., Ellam R.M., Emeleus C.H., (2009). Trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the North Atlantic Igneous Province: an example from the Isle of Rum, NW Scotland. *Geological Magazine*, 146(3), 382–399. DOI: [10.1017/S0016756809006244](https://doi.org/10.1017/S0016756809006244)

684 Moreira, M., Kunz, J., and Allègre, C. (1998) Rare gas systematics in popping rock: isotopic and elemental compositions in the upper mantle. *Science*, 279, 1178-1181.

686 Mungall, J.E. and Brenan, J.M., (2003). Experimental evidence for the Chalcophile behavior of the halogens. *Canadian Mineralogist*, 41(1), 207–220. DOI: [10.2113/gscanmin.41.1.207](https://doi.org/10.2113/gscanmin.41.1.207)

688 Mungall, J.E. and Naldrett, A. J., (2008). Ore Deposits of the Platinum-Group Elements. *Elements*, 4, 253–258. DOI: [10.2113/GSELEMENTS.4.4.253](https://doi.org/10.2113/GSELEMENTS.4.4.253)

690 Muramatsu, Y. and Wedepohl, K.H., (1998). The distribution of iodine in the Earth's crust. *Chemical Geology*, 147, 201-216. DOI: [10.1016/S0009-2541(98)00013-8](https://doi.org/10.1016/S0009-2541(98)00013-8)

692 Muramatsu, Y., Doi, T., Tomaru, H., Fehn, U., Takeuchi, R., and Matsumoto, R., (2007) Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates. *Applied Geochemistry*, 22, 534-556.

695 O'Driscoll, B., Day, James M.D., Daly, S.J., Walker, R.J., and McDonough, W.F., (2009). Rhenium-osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly. *Earth and Planetary Science Letters*, 286(1-2), 41–51. DOI: [10.1016/j.epsl.2009.06.013](https://doi.org/10.1016/j.epsl.2009.06.013)

700 O'Driscoll, B., Emeleus, C.H., Donaldson, C.H., and Daly, S.J. (2010). Cr-spinel seam petrogenesis in the Rum layered suite, NW Scotland: Cumulate assimilation and in situ crystallization in a deforming crystal mush. *Journal of Petrology*, 51(6), 1171–1201. DOI: [10.1093/petrology/egg013](https://doi.org/10.1093/petrology/egg013)
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746 **Figure captions**

747 Figure 1. (a) Photomicrograph (in cross-polarised light) of a Rum picrite with a groundmass
748 comprising ‘sprays’ of olivine and plagioclase, suggesting rapid solidification. (b)
749 Photomicrograph (in plane-polarised light) of apatite and biotite in a Rum pegmatoid. Ap =
750 apatite, Bt = biotite, OI = olivine, Plg = plagioclase, Mt = magnetite. (c) Backscatter electron
751 micrograph image from approximately 2 mm above the main Unit 7-8 boundary seam (arrow
752 points to location of seam). (d) Cl element map (quantified) of the same area as (c). Note the
753 presence of elevated Cl concentrations in the groundmass around olivine and Cr-spinel
754 crystals, and in fractures in olivine primocrysts.

755

756 **Figure 2.** Bromine (a) and I (b) normalised to Cl and the Br/Cl and I/Cl (c) ratios for all Rum
757 samples. Mineral separates from picrites, troctolites and pegmatoids are shown as open
758 symbols of the same shape and outline colour as the bulk rock equivalent. MORB data
759 points are small black dots and the reference ratio line is solid black. Data are from Kendrick
760 et al. (2012; 2013; 2014). The seawater ratio reference line is dashed and taken from Li and
761 Schoonmaker (2003). Marine sediments and pore fluids (pore fluids describe the typically Br-
762 and I-rich fluid phase recovered from sedimentary cores; n =134, grey crosses) data are
763 from Muramatsu et al. (2007), Martin et al. (1993) and Kastner et al. (1990). Average
764 continental crust is represented as a brown-outlined star and is from Rudnick and Gao
765 (2003). Subcontinental lithospheric mantle (SCLM) from Burgess et al. (2002) is represented
766 by red star symbols. In (c), a global marine shale average composition is depicted as a
767 black outline triangle (Li and Schoonmaker, 2003).

768

769 **Figure 3.** (a) Age-corrected $^{40}$Ar/$^{36}$Ar ratio as a function of the molar Cl/$^{36}$Ar (x 10$^{-6}$) in the
770 RLS rocks by sample type. Mineral separates from picrites, troctolites and pegmatoids are
771 shown as open symbol of the same shape and outline colour as the bulk rock equivalent.
772 Two trends are observed for the bulk rock and mineral separates (inset) with different $^{40}$Ar/Cl
773 ratios. The $^{40}$Ar/$^{36}$Ar ratio in air ($^{40}$Ar/$^{36}$Ar$_{air}$ = 298.56; Lee et al., 2006) is shown by the dashed
black line. The potential influence of crustal input to the elevated ⁴⁰Ar/³⁶Ar ratio is shown schematically. (b) Molar ⁴⁰Ar/Cl as a function of the molar K/Cl ratio by sample type. The lack of correlation indicates the ⁴⁰Ar is decoupled from the K-content and therefore not the result of in situ K-decay (excess Ar). The ⁴⁰Ar/K ratio for the 60 Ma is ~ 6.34 x 10⁻⁹, which is essentially at the origin in this plot. The mineral separates (inset) show a distinct trend compared to the bulk rock samples. (c) The molar ⁴⁰Ar/K (x 10⁻⁵) ratio as a function of the molar I/K ratio (x 10⁵) in the RLS. Mineral separates from the pegmatoids show a high I/K ratio weakly correlated with their low ⁴⁰Ar/K ratio - see also inset and (d). (d) Excess ⁴⁰Ar (mol/g) as a function of I (mol/g) concentration in the Unit 11-12 peridotite, anorthosite and chromitites. The long dash line (R²=0.99) is a linear fit to the B2 chromitite data (black squares) and one B2 anorthosite point (blue squares). The short dash line (R²=0.99) is a linear fit to the B2 peridotite data (green squares) and data for three B2 anorthosite samples.

786

787

788

**Figure 4.** ¹³⁰Xe/³⁶Ar versus ⁸⁴Kr/³⁶Ar isotope plot of all RLS rocks. Mineral separates from picrites, troctolites and pegmatoids are shown as open symbols of the same shape and outline colour as the bulk rock equivalent. Reference values, including, MORB (orange outline stars) include Bravo Dome and 'popping rock', seawater (grey outline star) and air (blue outline star); data from Moreira *et al.* (1998), Holland and Ballentine (2006) and Ozima and Podosek (2002). The marine sediments range (grey field) is a reference range from Ozima and Podosek (2002).

789

790

791

792

793

794

795

796

**Figure 5.** Halogen data (Cl data in ppm and Br and I data in ppb) as a function of the stratigraphic column of units 7 to 14 of the Eastern Layered Intrusion of Rum, including schematic diagrams of unit boundary textural relationships where the PGE-rich chromitites occur (redrawn after O'Driscoll *et al.* 2009). Halogens show the full range (e.g., all replicate analyses; Table S2) of data. PGE data of O'Driscoll *et al.* (2009) are included for comparison.

797

798

799

800

801
<figure><img src="image_4.png" /><figcaption>Figure 1
Click here to download high resolution image</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 2
Click here to download high resolution image</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 3
Click here to download high resolution image</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 4
Click here to download high resolution image</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 5**
Click here to download high resolution image</figcaption></figure>
<caption>**Table 1**
Click here to download Table: Table 1-revised.xlsx</caption>

<table><caption>Table 1. Sample lithology/provenance details and K and halogen abundances/ratios. Data are averages and the uncertainties are 1 sigma SEM</caption><thead><tr><th>Sample ID</th><th>Sample lithology/provenance*</th><th>Sample type</th><th>K (ppm)</th><th>CI (ppm)</th><th>Br (ppb)</th><th>I (ppb)</th><th>K/CI (wt.)</th><th>Br/CI (wt. x10<sup>-3</sup>)</th><th>I/CI (wt. x10<sup>-3</sup>)</th></tr></thead><tbody><tr><td colspan="10"><strong>Picrites</strong></td></tr><tr><td>M9 Picrite</td><td>M9 Picrite, WLI</td><td>Bulk</td><td>451 ± 69</td><td>23.1 ± 2.0</td><td>89.9 ± 5.4</td><td>19.4 ± 0.5</td><td>19.4 ± 1.3</td><td>3.89 ± 0.06</td><td>0.84 ± 0.02</td></tr><tr><td>RM_16_010 Olivine</td><td>Picrite (olivine), ELI</td><td>Separate</td><td>125 ± 19</td><td>6.1 ± 0.5</td><td>43 ± 0.2</td><td>2.1 ± 0.1</td><td>20.5 ± 1.4</td><td>7.1 ± 0.08</td><td>0.34 ± 0.01</td></tr><tr><td>RM_16_011</td><td>Picrite (fine grained), ELI</td><td>Bulk</td><td>2908 ± 94</td><td>74.5 ± 4.7</td><td>229 ± 14</td><td>11.2 ± 1.2</td><td>39.1 ± 1.2</td><td>3.07 ± 0.04</td><td>0.15 ± 0.01</td></tr><tr><td>RM4 (RM_16_012)</td><td>Picrite, WLI</td><td>Bulk</td><td>484 ± 19</td><td>36.1 ± 3.8</td><td>181.6 ± 35.4</td><td>3.9 ± 0.6</td><td>13.6 ± 2</td><td>5.03 ± 0.16</td><td>0.11 ± 0.01</td></tr><tr><td>RM_16_012 Olivine</td><td>Picrite (olivine), WLI</td><td>Separate</td><td>17 ± 8</td><td>13.3 ± 2.2</td><td>47.8 ± 5</td><td>0.5 ± 0.5</td><td>1.2 ± 0.4</td><td>3.59 ± 0.12</td><td>0.04 ± 0.01</td></tr><tr><td colspan="10"><strong>Rum intrusion rocks</strong></td></tr><tr><td>B2 Peridotite</td><td>Unit 12 peridotite, ELI</td><td>Bulk</td><td>71 ± 6</td><td>4.8 ± 1.3</td><td>18.6 ± 1.2</td><td>3.0 ± 0.8</td><td>19.5 ± 5</td><td>3.9 ± 0.16</td><td>0.63 ± 0.07</td></tr><tr><td>B2 Anorthosite</td><td>Unit 11 anorthosite, ELI</td><td>Bulk</td><td>283 ± 39</td><td>6 ± 1.1</td><td>31.1 ± 4.7</td><td>50.1 ± 17</td><td>50.8 ± 9.1</td><td>5.2 ± 0.18</td><td>8.38 ± 0.83</td></tr><tr><td>B2 Chromite</td><td>Unit 11-12 Chromitite, ELI</td><td>Bulk</td><td>77 ± 3</td><td>2.1 ± 0.2</td><td>8.6 ± 0.5</td><td>10.1 ± 3.4</td><td>37.5 ± 2.5</td><td>4.14 ± 0.06</td><td>4.84 ± 0.49</td></tr><tr><td>RM11 Chromitite</td><td>Unit 11-12 Chromitite, ELI</td><td>Bulk</td><td>120 ± 54</td><td>3.4 ± 0.3</td><td>15.1 ± 1.7</td><td>18.1 ± 14.9</td><td>34.2 ± 12.9</td><td>4.45 ± 0.09</td><td>5.31 ± 0.10</td></tr><tr><td>U9 Troctolite</td><td>Unit 9 troctolite, ELI</td><td>Bulk</td><td>179 ± 6</td><td>83.1 ± 14.3</td><td>484.5 ± 77.8</td><td>60 ± 14.2</td><td>2.3 ± 0.5</td><td>5.83 ± 0.19</td><td>0.72 ± 0.07</td></tr><tr><td>U9 Olivine Gabbro</td><td>Unit 9 gabbro ELI</td><td>Bulk</td><td>181 ± 21</td><td>20.7 ± 4.5</td><td>92.1 ± 21.6</td><td>8.4 ± 1.2</td><td>10 ± 2.9</td><td>4.44 ± 0.18</td><td>0.41 ± 0.03</td></tr><tr><td>U9 Peridotite</td><td>Unit 9 Peridotite, ELI</td><td>Bulk</td><td>53 ± 13</td><td>16.4 ± 3</td><td>84.2 ± 14.2</td><td>3.9 ± 0.4</td><td>3.5 ± 1.4</td><td>5.14 ± 0.17</td><td>0.24 ± 0.01</td></tr><tr><td>U9 Anorthosite</td><td>Unit 9 anorthosite, ELI</td><td>Bulk</td><td>111 ± 4</td><td>6.9 ± 0.6</td><td>30.6 ± 5.1</td><td>2.5 ± 0.4</td><td>16.2 ± 0.9</td><td>4.46 ± 0.09</td><td>0.37 ± 0.02</td></tr><tr><td>U7-8 Peridotite</td><td>Unit 8 peridotite, ELI</td><td>Bulk</td><td>52 ± 1</td><td>3.4 ± 0.1</td><td>14 ± 0.6</td><td>1.1 ± 0.1</td><td>15.4 ± 0.2</td><td>4.15 ± 0.03</td><td>0.32 ± 0.01</td></tr><tr><td>C8 Chromite-Peridotite</td><td>Unit 8 chromitite-peridotite boundary, ELI</td><td>Bulk</td><td>319 ± 126</td><td>1.9 ± 0.2</td><td>23.5 ± 0.5</td><td>102.1 ± 19.1</td><td>160 ± 49</td><td>12.16 ± 0.15</td><td>52.85 ± 3.23</td></tr><tr><td>RM_16_008</td><td>Unit 7-8 chromitite, ELI</td><td>Bulk</td><td>135 ± 36</td><td>2.3 ± 0.7</td><td>35 ± 4</td><td>48.2 ± 2.2</td><td>60.1 ± 1.7</td><td>15.39 ± 0.58</td><td>21.19 ± 1.86</td></tr><tr><td>C8 Anorthosite</td><td>Unit 7 anorthosite, ELI</td><td>Bulk</td><td>624 ± 48</td><td>10.6 ± 5.2</td><td>19.8 ± 7.3</td><td>28.7 ± 4.4</td><td>74.7 ± 32</td><td>1.87 ± 0.14</td><td>2.71 ± 0.47</td></tr><tr><td>RM17 Troctolite</td><td>Unit 1 troctolite, ELI</td><td>Bulk</td><td>665 ± 245</td><td>4.1 ± 1.2</td><td>36.5 ± 25</td><td>6.3 ± 4.4</td><td>156.1 ± 15.7</td><td>8.8 ± 1.14</td><td>1.53 ± 0.28</td></tr><tr><td>RM17 Sulfide</td><td>Unit 1 troctolite (sulfide), ELI</td><td>Separate</td><td>164 ± 8</td><td>3.6 ± 0.6</td><td>42 ± 10.8</td><td>23.5 ± 9.7</td><td>46.7 ± 10</td><td>11.51 ± 0.42</td><td>6.44 ± 0.77</td></tr><tr><td>RM7 Olivine gabbro</td><td>Gabbro (cpx-rich), CI</td><td>Bulk</td><td>119 ± 8</td><td>0.7 ± 0.02</td><td>8.6 ± 1.6</td><td>2.6 ± 0.6</td><td>166.8 ± 5.9</td><td>12.1 ± 0.24</td><td>3.58 ± 0.24</td></tr><tr><td>RM7 Cpx</td><td>Gabbro (cpx-rich), CI</td><td>Separate</td><td>44 ± 6</td><td>1.2 ± 0.1</td><td>15.7 ± 0.8</td><td>4.0 ± 0.4</td><td>37.6 ± 5.1</td><td>13.52 ± 0.09</td><td>3.41 ± 0.10</td></tr><tr><td colspan="10"><strong>Satellite intrusion rocks</strong></td></tr><tr><td>RM_16_013</td><td>Satellite peridotite plug, West Sgaorishal</td><td>Bulk</td><td>186 ± 20</td><td>98.7 ± 11.9</td><td>393.8 ± 61.7</td><td>49.7 ± 0.3</td><td>1.9 ± 0.4</td><td>3.99 ± 0.1</td><td>0.50 ± 0.02</td></tr><tr><td>RM_16_017</td><td>Satellite peridotite plug, West Sgaorishal</td><td>Bulk</td><td>568 ± 112</td><td>16.4 ± 1.2</td><td>66.7 ± 1.5</td><td>20.5 ± 2.0</td><td>34.4 ± 4.3</td><td>4.08 ± 0.04</td><td>1.25 ± 0.04</td></tr><tr><td>RM_16_017 Sulfide</td><td>Satellite peridotite plug (sulfide), West Sgaorishal</td><td>Separate</td><td>314 ± 11</td><td>7.6 ± 0.2</td><td>42 ± 2.6</td><td>13.9 ± 2.8</td><td>41.2 ± 2.6</td><td>5.5 ± 0.05</td><td>1.83 ± 0.10</td></tr><tr><td colspan="10"><strong>Pegmatoids</strong></td></tr><tr><td>RM_16_022 A</td><td>Unit 10 pegmatoid, ELI</td><td>Bulk</td><td>295 ± 12</td><td>2.1 ± 0.05</td><td>7.1 ± 0.8</td><td>158 ± 62.2</td><td>139.4 ± 8.7</td><td>3.32 ± 0.05</td><td>74.31 ± 8.27</td></tr><tr><td>RM_16_022 Amphibole</td><td>Unit 10 pegmatoid (amphibole), ELI</td><td>Separate</td><td>56 ± 8</td><td>1.0 ± 0.1</td><td>nd</td><td>53.8 ± 6.1</td><td>57.5 ± 0.6</td><td>nd</td><td>55.15 ± 2.78</td></tr><tr><td>RM_16_024</td><td>Unit 11 Pegmatoid, ELI</td><td>Bulk</td><td>586 ± 8</td><td>4.1 ± 0.5</td><td>8.1 ± 1.0</td><td>1.9 ± 0.4</td><td>146.6 ± 16.3</td><td>1.98 ± 0.06</td><td>0.47 ± 0.03</td></tr><tr><td>RM_16_024 Amphibole</td><td>Unit 11 Pegmatoid (amphibole), ELI</td><td>Separate</td><td>226 ± 32</td><td>3.8 ± 0.2</td><td>6.9 ± 0.2</td><td>2.1 ± 1.1</td><td>59 ± 6</td><td>1.82 ± 0.01</td><td>0.55 ± 0.08</td></tr><tr><td>RM3 Pegmatoid</td><td>Pegmatoid, WLI</td><td>Bulk</td><td>2569 ± 907</td><td>38.3 ± 7.8</td><td>143.7 ± 30</td><td>58.7 ± 20.6</td><td>62.5 ± 18.7</td><td>3.75 ± 0.07</td><td>1.53 ± 0.18</td></tr><tr><td>RM_16_021 A</td><td>Pegmatoid, CI</td><td>Bulk</td><td>861 ± 94</td><td>53.2 ± 4.5</td><td>85.8 ± 37.4</td><td>362.7 ± 81.9</td><td>16.6 ± 2.3</td><td>1.61 ± 0.08</td><td>6.82 ± 0.47</td></tr><tr><td>RM_16_021 Amphibole</td><td>Pegmatoid (amphibole), CI</td><td>Separate</td><td>187 ± 70</td><td>18.6 ± 4.3</td><td>20.4 ± 1.5</td><td>103.7 ± 13</td><td>9.7 ± 1.5</td><td>1.1 ± 0.04</td><td>5.57 ± 0.44</td></tr><tr><td>RM_16_021 Biotite</td><td>Pegmatoid (biotite), CI</td><td>Separate</td><td>34739 ± 6184</td><td>124 ± 26.3</td><td>28.9 ± 6.3</td><td>200.7 ± 40.1</td><td>280 ± 49.9</td><td>0.23 ± 0.04</td><td>1.62 ± 0.13</td></tr></tbody></table>

<footnote>*ELI, WLI and CI are Eastern Layered Intrusion, Western Layered Intrusion and Central Intrusion, respectively</footnote>