6.1. Protoliths and Metasomatism
The least metasomatised rock in the Svartberget body is garnet peridotite. Based on its chemical composition and mineralogy, it has been classified as a member of the so-called “Fe-Ti peridotites” that are considered to have been crustal magmatic rocks and were probably intruded into the Fennoscandian craton in the middle or late Proterozoic [
55,
61,
62]. This lithotype is distinctly different from the Mg-Cr-type orogenic peridotites in the WGR that were introduced into the gneisses tectonically, probably from the sub-Laurentian lithospheric mantle. The spectacular vein system with its zoned, websterite-dominated selvages was formed at P-T conditions similar to those pertaining in the upper mantle when the leading edge of the Baltica continental crust was subducted to depths of <100 km below Laurentia. The metasomatic agent was clearly a water-rich granitoid melt or melt-like fluid [
63,
64]. This locality is significant because the peridotite composition is broadly similar to mantle rocks, so it can act as an analogue for mantle metasomatism. Importantly, the size of the body and scale of the metasomatic features are on a mappable scale, offering an advantage over studies of smaller, fragmented xenoliths. The well-known Mg-Cr class of peridotites in the WGR [
44] largely lacks metasomatic products generated under mantle conditions by crust-derived fluids or melts (but see [
106] for micro-scale example), so the Fe-Ti class offers us the best case studies in the WGR.
The Årsheimneset mafic-ultramafic body lacks peridotite and is composed of a layered assemblage of metabasic eclogite and various opx-bearing pyroxenites. It may have been a layered mafic-ultramafic intrusion. The two websterite layers are texturally and compositionally very similar to the metasomatic selvages at Svartberget and to other, similar examples in the Molde area [
63] and display mica and garnet-rich veins indicative of an influx of silica and LIL-rich fluid or melt, so a similar origin by metasomatism of an olivine-rich precursor may be inferred. At this locality, the presence of quartz (coesite) eclogite allows comparisons to be made with previously published studies of halogen systematics of other eclogites in the WGR. The vein and depletion halo in this eclogite closely resembles those in eclogites in other subduction systems that have been interpreted as resulting from action of aqueous fluids with a dissolved terrigenous component [
80]. The field and petrographic characteristics of the Årsheimneset body are very similar to those in the much larger Eiksunddal massif approximately 40 km to the north [
46,
75], but Eiksunddal also has layers of Fe-Ti garnet peridotite, which encourages links between Årsheimneset and the Fe-Ti class of peridotites, including Svartberget.
In both bodies, the mineral phases investigated in the present study were clearly generated or modified by metasomatism under UHP conditions, so a major source of the halogens can be directly related to the previously identified, causative metasomatic agent.
6.5. Halogen Fractionation through Metamorphic and UHP Metasomatic Processes
Figure 8d shows two trends that depict the halogen evolution of a fluid phase during dehydration and hydration reactions [
60]. A residual fluid resulting from the formation of a metamorphic rock via dehydration reactions will have a significantly different halogen composition to a residual fluid resulting from the formation of a metamorphic rock via hydration reactions. During hydration reactions, fluids are subjected to a range of water activities, which allows for the fractionation of incompatible elements into the structure of mineral phases [
111]. Thus, as H
2O is consumed during the crystallisation of hydrous mineral phases, Br/Cl ratios will be unchanged until the activity of water in the system (
aH
2O) is reduced to a level which promotes the crystallisation of Cl-bearing mineral phases, thereby increasing the Br/Cl ratio of the coexisting fluid. The larger ionic radius of Br
-, relative to the OH
- ion, prevents significant incorporation of Br within mineral phases. In contrast to hydration reactions, dehydration reactions will result in the release of I from minerals and organic-rich sediments [
97,
109,
110], significantly increasing the I/Cl ratios of the coexisting fluid, e.g., the pore fluid field in
Figure 8. Br/Cl ratios are little affected due to the small concentrations of Br in mineral phases, relative to I, with which to enrich the resulting fluid.
The Br/Cl and I/Cl ratios obtained in this study (
Figure 8d) are generally consistent with the trend defined by hydration reactions in a previous study of the WGR [
60]. The evolution of halogen ratios during hydration reactions also mimics the trend of seawater evaporation [
60,
112], whereby
aH
2O is effectively lowered, without affecting the fluid Br/Cl ratio, until increasing salinity promotes the precipitation of halite, which then drives the Br/Cl and I/Cl ratios of the residual fluid higher. This trend indicates strong fractionation of Cl and Br between minerals and fluids [
60]. Thus, the crystallised hydrous phases would be expected to have a significantly lower Br/Cl ratios than the fluid. Using an example of halogen fractionation during halite precipitation, the (Br/Cl)
halite/(Br/Cl)
solution partition coefficient of 0.032, obtained at room temperature [
113], the Br/Cl ratio of the mineral responsible for fractionation is required to be ~8.6 × 10
−4 [
60]. This calculated Br/Cl ratio relies of course on the partition coefficient not changing at high pressure or temperature, which is uncertain. Applying the Br/Cl partition coefficient between halite and solution [
113] to samples in this study, the mineral responsible for fractionation would require a Br/Cl ratio ranging between 2.36 × 10
−3 and 2.46 × 10
−4, with an average value of 8.12 × 10
−4, a similar value to that obtained previously [
60].
Figure 8c shows that some mica separates in this study have Br/Cl ratios consistent with that required for efficient Cl and Br fractionation. Therefore, micas may be responsible for fractionation of Cl and Br during hydration reactions associated with metasomatism. The paucity of data for I in hydrous mineral phases makes assessing the fractionation of Cl from I difficult. Given the larger ionic radius of I, relative to Br, Cl may be expected to be fractionated from I more strongly than from Br. The low I/Cl ratios in micas and the fractionation trends shown in
Figure 8 do, however, appear to trend in the right direction, and suggest that mica may also be responsible for the efficient fractionation of Cl from I.
6.6. Halogen Systematics of Fluids in Subducted Crust and Mantle Metasomatism
The halogen composition of the fluid in this study is notably different to halogens retained in the slab during subduction as represented by Alpine eclogites and garnet peridotites [
17,
18] (
Figure 8c). The Alpine eclogites are from ophiolitic complexes and represent subducted oceanic lithosphere and are thus quite different in origin and evolution from the WGR eclogites, whose protoliths resided for a long period as intrusions within continental crust and were subducted along with it.
Taking the two examples described here as models for mantle rocks, and specifically sub-continental lithospheric mantle (SCLM), it is instructive to compare these results with SCLM-derived mantle xenoliths. The SCLM is expected to retain chemical heterogeneities introduced through subduction-related interactions between mantle and crustal sources [
114,
115] due to its isolated nature and because, unlike the mantle, it does not convect. The SCLM will, therefore, preserve halogens that are released from the slab during subduction and periodically be involved in mantle metasomatism during the ascent of fluids. Fluid inclusions in Siberian mantle xenoliths [
21] have high concentrations of halogens, suggesting that that the SCLM can be enriched in volatiles due to metasomatic processes. Their Br/Cl and I/Cl ratios are similar to samples of this study (
Figure 8d), while both show significant overlap with the halogen compositions of fluids trapped in minerals of the altered oceanic crust [
116]. This suggests that UHP metasomatism of mafic-ultramafic bodies in the WGR and metasomatism in the SCLM could have involved a component derived from seawater.
The sources of eclogitic fluids in the WGR are subject to debate, with suggestions that amphibolite-facies mafic protoliths and the gneissic country rock surrounding mafic-ultramafic bodies are potential origins [
58,
60,
63,
64]. Samples in this study show a trend from seawater-like Br/Cl ratios to more Br- and I-rich compositions, generally following a mixing trend defined by seawater and a hypothetical Br- and I-rich endmember (
Figure 8c). Most of the samples in this study lie on or close to the mixing line, which indicates that there may have been a seawater component in the metasomatic fluid, but the original seawater was processed and subsequently evolved either during subduction and coeval metasomatism. The process most likely driving the evolution of the seawater is the crystallisation of hydrous mineral phases (though not necessarily just Cl-rich mineral phases) causing the fractionation of halogens via the preferential removal of Cl from Br and I in the fluid.
The fractionation trend in
Figure 8d shows that the starting composition of the fluid responsible for metasomatism must have been slightly more I rich than seawater, falling between modern seawater and MORB I/Cl values. Therefore, prior to halogen fractionation, it is likely that the fluid acquired I from I-rich minerals and organic material during the early stages of subduction. Following the addition of I, hydrous mineral crystallisation may have reduced the
aH
2O in the fluid to such an extent that it could, given the large quantities of Cl in seawater, lead to Cl saturation in the residual fluid. A Cl-saturated fluid will significantly increase mineral-fluid Cl partition coefficients and promote the incorporation of Cl into solid phases [
60,
111]. The incorporation of Br and I into solid phases is little affected due to the much lower concentration of these halogens in seawater—70 ppm Br and 0.06 ppm I [
117]. This notion of Br and I inactivity is consistent with the relatively narrow ranges in Br/I ratios across samples in this study (
Figure 7 and
Figure S1). The effects of preferential incorporation on the halogen composition can be seen in
Figure 8d, where crystallised solid phases will have low Br/Cl and I/Cl ratios, while the residual fluid will evolve to higher Br/Cl and I/Cl ratios. As previously mentioned, micas analysed here have Br/Cl and I/Cl ratios low enough to efficiently fractionate Cl, suggesting that micas may be important in the fractionation of Cl from Br and I.
Mica crystallisation alone is, though, unlikely to account for all of the observed halogen fractionation. It is likely that other Cl-bearing phases such as amphibole or apatite, could fractionate Cl from Br and I during metasomatism. The Br and Cl content of apatites in mantle peridotites [
118] give Br/Cl ratios of 0.1–10 × 10
−3, which is low enough to generate residual fluids with high Br/Cl ratios consistent with those observed in this study and could be a plausible explanation for similar values obtained from brine inclusions in diamonds [
94], and in mantle xenoliths from Siberia [
21]. It is, therefore, reasonable to suggest that other hydrous phases may also be able to fractionate Cl from Br and I. In the Lindås Nappe (LN) near Bergen, which is a slice of Baltica basement granulite that underwent early Scandian transformation to eclogite, fluid inclusions from quartz veins have up to 30 wt% dissolved salts and no visible water vapour in any inclusions, nor showed any clathrate formation during microthermometry [
58]. The authors suggested that water has been selectively removed from the fluid in this region, which is consistent with fractionation and hydration trends suggested here and elsewhere [
21,
60]. Fluids in the WGR may better represent a more primitive eclogitic fluid than the evolved composition at the Lindås Nappe [
58].
Fluid mobility within the WGR is on a much larger scale than in other tectonic settings. For example, in Alpine eclogites, fluid mobility operated on the scale of centimetres, with eclogites acting as closed systems with respect to fluids in the region [
119,
120,
121]. In contrast, field relations in the Norwegian Caledonides indicate fluid mobility on the scale of metres to kilometres [
37,
60]. It is, therefore, prudent to consider whether eclogites of the WGR may have acted as open systems with respect to external fluids. An open system allows for the possibility of repeated fluid infiltration and loss from the system, resulting in complex controls on the composition of metasomatic fluids over time. The elevated I/Cl signatures in garnet from sample QC36A (
Figure 8) are unlike any other sample in this study and may indicate that more than one fluid composition has interacted this sample, and by implication other UHP eclogites and peridotites of the WGR. The halogen ratios of this garnet suggest that its halogen composition was inherited from pore fluids, and may reflect an early I-rich fluid, which was not modified by further halogen fractionation. The sample is from near the limit of metasomatism in a selvage and may have retained some pre-metasomatic garnet that has retained some earlier fluid. Therefore, multiple fluids of various compositions may have existed and interacted with the Svartberget peridotite. Given that the majority of samples in this study closely follow a trend of apparent fractionation and hydration, it can be assumed that the system appears largely closed, with perhaps a minor component involving external fluid sources.
Numerous saline groundwaters and brines from Precambrian shields have high Br/Cl ratios, similar to samples in this study, and low I/Cl ratios [
100,
101]. Groundwater and fluid inclusion leachates from the Stripa granite in Sweden also show high Br/Cl ratios [
99,
102] but have I/Cl ratios similar to samples in this study. These shield and granitic groundwaters have distinctly different halogen signatures to halogen-rich oil field brines and mine waters that have previously been shown to have elevated I/Cl ratios [
98,
102,
103,
104,
105], similar to marine pore fluids. The high I/Cl and low Br/Cl ratios of oil field brines, mine and sedimentary formation waters precludes them from being the origin of halogen signatures in this study. The high Br/Cl ratios in Precambrian shields and granitic groundwaters suggest that the fluid observed here could have originated in these settings. Shield brines have been suggested to form through intense evaporation of seawater, with consequent Cl precipitation and residual Br enrichment, with potential sequestration of I in organic matter before the fluids enter basement material [
102,
122]. It is suggested further that the Stripa granite halogen composition formed in a similar fashion and represents a modified analog to the Precambrian shield brines [
102]. The higher I/Cl ratios in the fluids of the Stripa granite may result from some interaction with organic matter, or limited sequestration of I in organic matter, before entering basement rocks. The process forming these Br-rich groundwaters and brines is, therefore, similar to seawater evaporation and fractionation/hydration trends [
60] suggested earlier to explain similar features of the WGR fluids (
Figure 8d). The pre-Caledonian evolution of western Baltica is interesting in this regard, because of the extensive marine transgression across the region and deposition of the organic-rich late Cambrian Alum Shale Formation [
31]. This shale can be traced well into the WGR [
31] and, while it has not been recognised around the two localities under study, it did very likely cover the WGR basement prior to the initiation of Caledonian orogenesis. This offers the possibility of a source of seawater and organic matter, with evaporation during the ensuing regression.
The halogen signatures in this and previous studies of the WGR may, therefore, originate from fluids released during subduction and HP to UHP metamorphism of the continental crust of the Fennoscandian shield and its sedimentary cover. This study adds the contribution of anatexis under UHP conditions, as indicated by the evidence from Svartberget. It has been argued [
63] that the UHP conditions make it likely that supercritical fluids were generated by anatexis rather than silicate melts or aqueous fluids, which is supported by the high concentrations of high field strength elements in the metasomatic veins (as seen in the abundant apatite and monazite), a possible result of the strong solvent action of supercritical fluids. The large, combined size of the UHP domain outcrop within the WGR and their correspondence with ultra-coarse, decussate, metasomatic garnet websterites (
Figure 1) suggest that subduction of giant UHP terrains might have a significant geochemical influence on the SCLM in collisional orogens, even though subduction was transient. This is reinforced by emerging evidence for earlier, extensive diamond-facies UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides [
35] thought to record one or more arc-continent collisions with Baltica during early to middle Ordovician, pre-Scandian closure of Iapetus.
The continental crust is a potential major source of both H2O and volatiles in the form of sediment and basement pore waters. It is an important alternative fluid and halogen source to the oceanic lithosphere, especially when transient continental subduction episodes occur during arc-continent and continent collisions. Such crust could induce widespread metasomatism of the SCLM during subduction. Overall, despite the equivocal origin of the fluid responsible for metasomatism of the SCLM, the process exerting a controlling factor on the evolution of mantle halogen compositions appears similar for fluid found in different crustal settings: evaporation or hydrous mineral crystallisation induces the precipitation of Cl following a decrease in aH2O, which is then followed by a subsequent enrichment in Br and I of the residual fluid. I can also be either inherited from fluid interaction with organic material, or sequestered into organic material early in the subduction process.
The wide range of halogen compositions observed in mantle sources suggests that regions of the mantle are more heterogeneous than previously suggested [
21,
123]. The range of halogen compositions of the subducting crust [
17,
18,
116] does not match the narrow range in halogen compositions of MORB and OIB source regions [
12,
14,
123]. It has been argued that subduction does not introduce major halogen heterogeneity into the mantle [
18]. However, this would require halogens to be decoupled from other volatile species, i.e., nitrogen, oxygen and sulphur that show heterogeneity in the mantle [
124]. The high Br/Cl and I/Cl ratios of fluids in this study suggest, by analogy, that the SCLM may retain a significant quantity of devolatilised Br and I. The Siberian SCLM is enriched in Cl, Br and I by factors of 125, 675 and 100 times, respectively, relative to the depleted MORB mantle (DMM) [
21]. The Br- and I-rich nature of such samples of the sub-continental material indicates that the SCLM represents a major halogen reservoir and, therefore, must be considered in future estimates of global volatile budgets and fluxes. The UHP mafic and ultramafic bodies described in this contribution offer a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred into the SCLM.
An obvious target for future halogen studies is the Mg-Cr class of orogenic peridotites in the WGR, generally considered to have been entrained from the SCLM [
44]. To date, the only published study of metasomatism relating to the Scandian subduction episode in these rocks is from the Bardane body on Fjortoft island, which shows micro-scale veining with similar large-ion-lithophile enriched composition and micro-diamond [
106], indicative of a supercritical fluid with a continental or terrigenous sediment source. The veinlets are associated with MPIs bearing solid-phase assemblages similar to Svartberget, including phlogopite, Cl-apatite and diamond [
125]. To date, there have been no detailed studies of halogens in these rocks.