Abyssal Serpentinites : Transporting Halogens from Earth ’ s Surface to the Deep Mantle

Serpentinized oceanic mantle lithosphere is considered an important carrier of water and fluid-mobile elements, including halogens, into subduction zones. Seafloor serpentinite compositions indicate Cl, Br and I are sourced from seawater and sedimentary pore fluids, while F may be derived from hydrothermal fluids. Overall, the heavy halogens are expelled from serpentinites during the lizardite–antigorite transition. Fluorine, on the other hand, appears to be retained or may be introduced from dehydrating sediments and/or igneous rocks during early subduction. Mass balance calculations indicate nearly all subducted F is kept in the subducting slab to ultrahigh-pressure conditions. Despite a loss of Cl, Br and I from serpentinites (and other lithologies) during early subduction, up to 15% of these elements are also retained in the deep slab. Based on a conservative estimate for serpentinite thickness of the metamorphosed slab (500 m), antigorite serpentinites comprise 37% of this residual Cl, 56% of Br and 50% of I, therefore making an important contribution to the transport of these elements to the deep mantle.


Introduction
The high water content and enrichment of fluid-mobile elements (FME) in arc magmas attest to their recycling from Earth's surface to the mantle.The subduction of hydrated rocks, including sediments, altered oceanic crust (AOC) and abyssal serpentinites, at convergent plate boundaries provides a mechanism for this recycling.Abyssal serpentinites, primarily composed of the hydrous phyllosilicate mineral serpentine, are formed by the hydration of mantle peridotites on or near the seafloor.Recent geophysical and geochemical data have emphasized their importance for a variety of subduction-zone-related phenomena, including seismic activity and the exhumation of ultrahigh-pressure (UHP) rocks [1].Furthermore, the high water content (up to 13 wt%) and wide stability (>100 km) [2] of serpentinites make them effective vehicles for the transport of FME, including halogens, into the mantle.This article reviews the incorporation of F, Cl, Br and I by the oceanic lithosphere during seafloor alteration and discusses the importance of abyssal serpentinites for halogen transfer from the Earth's surface to the deeper mantle.

Halogens as Tracers of Geological Processes
The halogens (F, Cl, Br, I) are highly reactive, volatile elements primarily concentrating in Earth's surface reservoirs, such as seawater (Cl, Br) and sediments (F, Br, I).They are also abundant in arc magmas compared to oceanic ridge magmas [3][4][5].In addition, high F contents have been reported in magmas originating from the deeper mantle, such as ocean island basalts (OIBs) [6] and kimberlites [7].
The halogens are relatively abundant in crustal rocks compared to mantle peridotites [8] because they are preferentially partitioned into the silicate melt during partial melting due to their incompatibility with mantle minerals.Chlorine, Br and I are also highly soluble in aqueous fluids, while the solubility of F is low [9].Owing to its smaller size and higher electronegativity, F is far more compatible in hydrous and nominally anhydrous minerals than the other halogens [10][11][12].As a consequence of their variable abundances and compatibilities, the halogens and their ratios (i.e., Br/Cl, I/Cl and F/Cl) are useful for tracing fluid sources and their migration in a variety of geological settings [13].The halogens, particularly Cl, are also important ligands to form soluble metal complexes that contribute to metal transport and formation of metal deposits [14].

Abyssal Serpentinite Formation
The composition and structure of the oceanic lithosphere is primarily controlled by the spreading rate along the Earth's >67,000 km [15] of oceanic ridges.At fast-spreading ridges (>5 cm/year) [16], the rate of magma supply exceeds that of plate divergence, and large quantities of mafic magmas are generated.This produces the classic "Penrose" style of oceanic lithosphere [17] where layers of pillow basalts, sheeted dykes and layered gabbros are overlain by variable sediment cover and underlain by mantle peridotite [18] (Figure 1A).This transition from gabbro to peridotite at ~6-7 km depth represents the seismic Mohorovicic discontinuity (Moho) [18].Hydrated mantle peridotites are observed along fractures and faults, which expose deep sections of oceanic lithosphere, such as Siqueiros Fracture Zone [19].
In contrast, the oceanic lithosphere produced in slow-(2-5 cm/year) and ultraslow-(<2 cm/year) [16] spreading ridges, such as the Mid-Atlantic Ridge (MAR) and South West Indian Ridge (SWIR), is much more heterogeneous [18].It is characterized by sea floor exposure of variably serpentinized mantle peridotite overlain by a thin layer of pillow basalt and sheeted dykes, as described for the MAR [20], or a tectonically thinned out layer of volcanic rocks underlain by up to 3 km of extensively serpentinized peridotite, as described for the SWIR [21] (Figure 1B).The rifting of plates at the ridges exposes mantle peridotites to hydration by seawater or submarine hydrothermal fluids yielding abyssal serpentinites, which may comprise up to 25% of the oceanic lithosphere formed at slow-spreading ridges [22].In these cases, the seismic Moho at shallower depths (1-4 km) may represent the serpentinization front [23].
Minerals 2019, 9 FOR PEER REVIEW 2 with mantle minerals.Chlorine, Br and I are also highly soluble in aqueous fluids, while the solubility of F is low [9].Owing to its smaller size and higher electronegativity, F is far more compatible in hydrous and nominally anhydrous minerals than the other halogens [10][11][12].As a consequence of their variable abundances and compatibilities, the halogens and their ratios (i.e., Br/Cl, I/Cl and F/Cl) are useful for tracing fluid sources and their migration in a variety of geological settings [13].The halogens, particularly Cl, are also important ligands to form soluble metal complexes that contribute to metal transport and formation of metal deposits [14].

Abyssal Serpentinite Formation
The composition and structure of the oceanic lithosphere is primarily controlled by the spreading rate along the Earth's >67,000 km [15] of oceanic ridges.At fast-spreading ridges (>5 cm/year) [16], the rate of magma supply exceeds that of plate divergence, and large quantities of mafic magmas are generated.This produces the classic "Penrose" style of oceanic lithosphere [17] where layers of pillow basalts, sheeted dykes and layered gabbros are overlain by variable sediment cover and underlain by mantle peridotite [18] (Figure 1A).This transition from gabbro to peridotite at ~6-7 km depth represents the seismic Mohorovicic discontinuity (Moho) [18].Hydrated mantle peridotites are observed along fractures and faults, which expose deep sections of oceanic lithosphere, such as Siqueiros Fracture Zone [19].
In contrast, the oceanic lithosphere produced in slow-(2-5 cm/year) and ultraslow-(<2 cm/year) [16] spreading ridges, such as the Mid-Atlantic Ridge (MAR) and South West Indian Ridge (SWIR), is much more heterogeneous [18].It is characterized by sea floor exposure of variably serpentinized mantle peridotite overlain by a thin layer of pillow basalt and sheeted dykes, as described for the MAR [20], or a tectonically thinned out layer of volcanic rocks underlain by up to 3 km of extensively serpentinized peridotite, as described for the SWIR [21] (Figure 1B).The rifting of plates at the ridges exposes mantle peridotites to hydration by seawater or submarine hydrothermal fluids yielding abyssal serpentinites, which may comprise up to 25% of the oceanic lithosphere formed at slowspreading ridges [22].In these cases, the seismic Moho at shallower depths (1-4 km) may represent the serpentinization front [23].

Figure 1.
Simplified schematic diagram of representative cross sections of oceanic lithosphere produced at (A) fast- [17] and (B) slow- [21] spreading ridges.Serpentinite in the diagram represents either concentrated zones of serpentinization or diffuse serpentinization throughout the upper lithospheric mantle [24].Oceanic lithosphere produced at slow-and ultraslow-spreading ridges is particularly heterogeneous, with serpentinites commonly outcropping on the ocean floor [20,22].[17] and (B) slow- [21] spreading ridges.Serpentinite in the diagram represents either concentrated zones of serpentinization or diffuse serpentinization throughout the upper lithospheric mantle [24].Oceanic lithosphere produced at slow-and ultraslow-spreading ridges is particularly heterogeneous, with serpentinites commonly outcropping on the ocean floor [20,22].
In addition to ridges, abyssal peridotites are hydrated at the outer rise near trenches by fluid infiltration along faults in the bending slab [25,26] (Figure 2).Estimates for the volume of abyssal serpentinites entering subduction zones range from a conservative 1-2% of the upper 10 km of lithospheric mantle [6] to 15% of the upper 30 km [26].Serpentinization can also occur in mantle wedges by fluids released from subducting slabs [27] (Figure 2).
The formation of low-temperature (LT) serpentine phases, chrysotile and lizardite, occurs at temperatures below 400 °C [32].The varieties of serpentine phases are due to the different ways for the mineral to adjust geometric strain between the tetrahedral and octahedral sheets [33].Conditions that favour chrysotile formation over lizardite include a high water/rock ratio, isotopic shear stress and the hydration of microenvironments, such as veins and pores [32].
In close proximity of oceanic ridges, the fluid is dominated by seawater, as evidenced by serpentine 87 Sr/ 86 Sr ratios similar to that of present-day seawater [34].Oxygen isotope compositions are also used to evaluate the nature of the hydrating fluid.For example, most serpentinites have δ 18 O signatures between that of seawater (0‰SMOW) and anhydrous peridotites (~+5-6‰SMOW) [35].However, some serpentinites formed at fast-spreading ridges, such as Hess Deep, have higher δ 18 O values (~7‰), interpreted as evidence for hydration by evolved seawater, with possible contribution of magmatic fluids [36].The existence of magmatic fluids at Hess Deep is also suggested by low δ 13 C (−7.8‰ to −4.5‰) compared to 0‰ for dissolved carbonate in sea water and consistent with magmatic values [36].During its transport from ridges to subduction zones, abyssal sediments deposit on the oceanic lithosphere.These abyssal sediments, and those derived from the overlying plate through erosion near trenches, can also influence the composition of serpentinization fluids along bend-related faults of the outer rise or within the subduction channel at shallow depths [37].[25,26].The approximate depths of the lizardite (Liz)-antigorite (Atg) transition [28] and antigorite decomposition [2] are based on a geothermal gradient of ~5 • C/km [29].In cold subduction zones, fluids and fluid-mobile elements may persist in hydrous phase A after antigorite breakdown [30,31].Bls: blueschist; Ecl: eclogite.
The formation of low-temperature (LT) serpentine phases, chrysotile and lizardite, occurs at temperatures below 400 • C [32].The varieties of serpentine phases are due to the different ways for the mineral to adjust geometric strain between the tetrahedral and octahedral sheets [33].Conditions that favour chrysotile formation over lizardite include a high water/rock ratio, isotopic shear stress and the hydration of microenvironments, such as veins and pores [32].
In close proximity of oceanic ridges, the fluid is dominated by seawater, as evidenced by serpentine 87 Sr/ 86 Sr ratios similar to that of present-day seawater [34].Oxygen isotope compositions are also used to evaluate the nature of the hydrating fluid.For example, most serpentinites have δ 18 O signatures between that of seawater (0‰ SMOW ) and anhydrous peridotites (~+5-6‰ SMOW ) [35].However, some serpentinites formed at fast-spreading ridges, such as Hess Deep, have higher δ 18 O values (~7‰), interpreted as evidence for hydration by evolved seawater, with possible contribution of magmatic fluids [36].The existence of magmatic fluids at Hess Deep is also suggested by low δ 13 C (−7.8‰ to −4.5‰) compared to 0‰ for dissolved carbonate in sea water and consistent with magmatic values [36].During its transport from ridges to subduction zones, abyssal sediments deposit on the oceanic lithosphere.These abyssal sediments, and those derived from the overlying plate through erosion near trenches, can also influence the composition of serpentinization fluids along bend-related faults of the outer rise or within the subduction channel at shallow depths [37].
The variable uptake of Cl by serpentinites has been attributed to several factors, including water/rock ratio, fluid salinity, primary minerals (i.e., olivine versus pyroxene) and temperature.Preliminary studies suggested different Cl contents of bulk serpentinites were the result of different water/rock ratios [42]; however, this is debated because of comparable Cl contents among serpentine grains from partially (20%) and fully serpentinized abyssal peridotites [47].Furthermore, experimental serpentine products show similar Cl concentrations regardless of the water/rock ratio during the hydration of peridotites [50].Fluid salinity also appears to have a negligible impact on the uptake of structurally bound Cl [50]; however, when Cl-poor serpentinites are equilibrated with saline fluids at ambient temperature and pressure, the uptake of structurally bound Cl in the serpentinites is proportional to the fluid salinity [51].Chlorine contents have been shown to be higher in serpentine after orthopyroxene (bastite textures) compared to serpentine after olivine (mesh textures) in natural and experimental samples [39,49,51].Compared to olivine, orthopyroxene contains significant Al replacing Si in the tetrahedral site [52].Since Al has a larger ionic radius than Si, the substitution of Al for tetrahedral Si in lizardite reduces the distortion between the smaller tetrahedral and larger octahedral layers, which may allow the accommodation of the larger Cl ion (1.82 Å) in the OH − site.The influence of temperature on Cl uptake is illustrated by an increase in the Cl content in serpentinite with increasing temperature up to a maximum of 200 • C and then a decrease in Cl contents at higher temperatures (300-500 • C) [50].
In contrast to Cl, there are fewer data available for Br and I in serpentinites due to their much lower concentrations.Electron microprobe analyses can determine the Cl content of most serpentine grains but not Br and I; therefore, data of these elements is mostly limited to bulk rock compositions.Recent bulk rock analyses suggest concentrations of 0.16-6.8ppm Br (avg.1.2 ppm; Figure 3A) and 0.016-2.6ppm I (avg.0.78 ppm; Figure 3B) in seafloor and obducted abyssal serpentinites [44,48,53].Abyssal serpentinites have similar Br/Cl ratios to that of seawater, but their I/Cl ratios are much higher and overlap with those of sedimentary pore fluids (Figure 4A), suggesting a considerable contribution of sediments to serpentinizing fluids of abyssal serpentinites.[43,44,49,53].In situ data of serpentine grains are from [34,43].The F/Cl, Br/Cl and I/Cl ratios of the depleted mantle (grey fields) [6] and seawater (dashed lines) [54] are shown, along with the average concentrations of F, Cl, Br and I in the depleted mantle (black stars) [6].Figures modified after [55].(a) Bromine and Cl concentrations in all serpentinites are elevated with respect to the average depleted mantle but are strongly correlated along a linear trend defined by the Br/Cl ratio of the mantle and seawater, suggesting their coherent behaviour during serpentinization (hydration) and subduction (dehydration).On average, the obducted abyssal serpentinites having lower Br and Cl concentrations than the seafloor samples, and the subducted serpentinites have lower concentrations than the obducted samples.(b) All samples have elevated I concentrations with respect to the depleted mantle, and most samples also have I/Cl ratios greater than the mantle.Iodine and Cl broadly correlate among the seafloor and obducted abyssal serpentinites.The seafloor and obducted abyssal serpentinites have similar I concentrations, while their subducted counterparts extend to lower values.(c) Individual lizardite grains have higher F content than antigorite, but bulk rock values for lizardite-and antigorite serpentinites are similar.The higher F content of some subducted serpentinites may indicate an influx of F during antigorization from other dehydrating lithologies.[43,44,49,53].In situ data of serpentine grains are from [34,43].The F/Cl, Br/Cl and I/Cl ratios of the depleted mantle (grey fields) [6] and seawater (dashed lines) [54] are shown, along with the average concentrations of F, Cl, Br and I in the depleted mantle (black stars) [6].Figures modified after [55].(a) Bromine and Cl concentrations in all serpentinites are elevated with respect to the average depleted mantle but are strongly correlated along a linear trend defined by the Br/Cl ratio of the mantle and seawater, suggesting their coherent behaviour during serpentinization (hydration) and subduction (dehydration).On average, the obducted abyssal serpentinites having lower Br and Cl concentrations than the seafloor samples, and the subducted serpentinites have lower concentrations than the obducted samples.(b) All samples have elevated I concentrations with respect to the depleted mantle, and most samples also have I/Cl ratios greater than the mantle.Iodine and Cl broadly correlate among the seafloor and obducted abyssal serpentinites.The seafloor and obducted abyssal serpentinites have similar I concentrations, while their subducted counterparts extend to lower values.(c) Individual lizardite grains have higher F content than antigorite, but bulk rock values for lizardite-and antigorite serpentinites are similar.The higher F content of some subducted serpentinites may indicate an influx of F during antigorization from other dehydrating lithologies.[43,44,48,53].(a) Br/Cl and I/Cl ratios of the seafloor and obducted abyssal serpentinites overlap with those of sedimentary pore fluids (see [24] for review), while the Br/Cl ratios of the subducted serpentinites and metamorphic peridotites extend to lower values, similar to the depleted mantle [6].(b) F/Cl ratios of all serpentinites are much higher than seawater [54], suggesting input from F-rich hydrothermal fluids (e.g., Escanaba Trough high 3 He/ 4 He pore fluids) [56] on the seafloor, or F mobilized from incoming plate sediments [53] and igneous lithologies during shallow subduction.On average, the subducted antigorite serpentinites and metamorphic peridotites have higher F/Cl ratios relative to their lizardite-bearing counterparts.Bulk serpentinite data are from [43,44,48,53].(a) Br/Cl and I/Cl ratios of the seafloor and obducted abyssal serpentinites overlap with those of sedimentary pore fluids (see [24] for review), while the Br/Cl ratios of the subducted serpentinites and metamorphic peridotites extend to lower values, similar to the depleted mantle [6].(b) F/Cl ratios of all serpentinites are much higher than seawater [54], suggesting input from F-rich hydrothermal fluids (e.g., Escanaba Trough high 3 He/ 4 He pore fluids) [56] on the seafloor, or F mobilized from incoming plate sediments [53] and igneous lithologies during shallow subduction.On average, the subducted antigorite serpentinites and metamorphic peridotites have higher F/Cl ratios relative to their lizardite-bearing counterparts.
Few reports document the bulk F content in modern seafloor and obducted abyssal serpentinites.Obducted abyssal serpentinites in Tertiary subduction complexes in Dominican Republic and the Alps have concentrations (7-31 ppm F) [48,53] similar to those of the depleted mantle (12 ± 2 ppm) [6], whereas seafloor serpentinites from the Logatchev Hydrothermal Field along the Mid-Atlantic Ridge are enriched in F (50-130 ppm; Figure 3C).A similar enrichment is also observed for individual serpentine minerals from Logatchev and the Mid-Atlantic Ridge Kane (MARK) fracture zone (50 to 470 ppm; Figure 3C) [38,47].The variable enrichment of F in abyssal serpentinites likely reflects the composition of the serpentinizing fluid.Seawater is generally low in F (<2 ppm) [54]; therefore, abyssal peridotites primarily hydrated by seawater are also expected to be low in F relative to the other halogens.On the contrary, abyssal serpentinites formed near volcanic centres, such as Logatchev and the MARK area, may have elevated F contents and F/Cl ratios due to a contribution of hydrothermal fluids from volcanic rocks, which have been shown to have higher F/Cl ratios than seawater (Figure 4B) [56].
Additional F enrichment may occur during serpentinization at the outer rise or within the shallow subduction channel by F mobilized from overlying sediments or igneous lithologies.In general, F content and F/Cl ratios of sediment pore fluids are low (similar to seawater) due to the low solubility of F in aqueous fluids [9].However, enrichment of F in water is expected during the dissolution of marine carbonates in water below the carbonate compensation depth, which contain up to 1700 ppm F [57] and high F/Cl ratios (~4-8) [58].
Similar to the heavy halogens, F is considered to reside in the serpentine OH − site, but it may also be incorporated into the O 2− site due to their similar ionic radii.Charge imbalance is compensated for by the coupled substitution of Al 3+ and F − (1.33 Å) with Si 4+ and O 2− (1.21 Å), as suggested for F in pyroxenes [10].
Recently published halogen budget calculations for a representative section of the oceanic lithosphere are based on a Penrose-type oceanic lithosphere typically produced at fast-spreading ridges [24].It comprises 400 m of mature or lithified marine sediment, 1500 m of pillow basalts and sheeted dykes, 4500 m of gabbros and 500 m of serpentinites.In this model, serpentinites contribute 31% of the total I in the lithosphere, 23% Br, 26% Cl and 2% F (Figure 5).However, the degree of serpentinization is still poorly constrained, and this is considered a conservative lower limit for the contribution of serpentinites to the overall halogen budget of the oceanic lithosphere.Oceanic lithosphere produced at slow-spreading ridges likely contains a much larger volume of abyssal serpentinites.If we apply the Muller model [21] for oceanic lithosphere produced at slow-spreading ridges, which comprises up to 3000 m of serpentinized mantle peridotites, the contribution of serpentinites increases to 73% for I, 69% for Br, 78% for Cl and 14% for F (Figure 5).Mass balance calculations for a convergence rate of 5 cm/year, bulk plate density of 2.8 g/cm 3 and global trench length of 44,000 km [3] yield a global F influx of 6.41 × 10 12 g/year regardless of the model used (Table 1).On the contrary, the total global influx of Cl, Br and I entering subduction zones with the Muller model (3.55 × 10 13 g/year) doubles in comparison to that with the Penrose model (1.79 × 10 13 g/year; Table 1), illustrating the importance for constraining the volume of serpentinites entering subduction zones.1. Numbers in bars denote the percent contribution of serpentinite to the total budget for each element.

Halogen Behaviour During Serpentinite Subduction
Available data of FME for subducted abyssal serpentinites are mostly from oceanic lithosphere produced at slow-spreading ridges, such as serpentinites of the Greater Antilles extinct arc in Dominican Republic [48], ophiolitic massifs of the Western Alpine-Northern Apennine chain in Italy [41,43,44,47,53] and the Cerro del Almirez massif in Spain [55].Buoyant oceanic lithosphere formed at such slow-spreading ridges subducts at shallow angles and forms accretionary prisms, where the upper portion of the incoming oceanic lithosphere with sediments is scraped off and obducted [62].The remaining serpentinites (and oceanic crust) are subducted to form subducted abyssal serpentinites.  1. Numbers in bars denote the percent contribution of serpentinite to the total budget for each element. 1 Thicknesses of lithological units from [24]; 2 Compilation of halogen concentrations of sediments, AOC and lizardite serpentinites of incoming slab from [24] and references therein; 3 Himalayan Tso Morari UHP sediments (unpublished data of Page, Hattori and Guillot, 2019); sampling and bulk chemistry described in [59]; halogen contents determined using methodology described in [48,60]; 4 Halogen data for eclogitic basalt and gabbro from Western Alps [61]; 5 Halogen data for antigorite serpentinites from Dominican Republic and Western Alps [48,53]; 6 Thicknesses of lithological units from [21]; AOC: altered oceanic crust; Liz: lizardite; Atg: antigorite.

Halogen Behaviour During Serpentinite Subduction
Available data of FME for subducted abyssal serpentinites are mostly from oceanic lithosphere produced at slow-spreading ridges, such as serpentinites of the Greater Antilles extinct arc in Dominican Republic [48], ophiolitic massifs of the Western Alpine-Northern Apennine chain in Italy [41,43,44,47,53] and the Cerro del Almirez massif in Spain [55].Buoyant oceanic lithosphere formed at such slow-spreading ridges subducts at shallow angles and forms accretionary prisms, where the upper portion of the incoming oceanic lithosphere with sediments is scraped off and obducted [62].The remaining serpentinites (and oceanic crust) are subducted to form subducted abyssal serpentinites.The FME data of abyssal serpentinites from fast-spreading ridges are virtually nonexistent, with the exception of those from Hess Deep of the Pacific plate [36].To date, there is no data for their subducted equivalents; however, serpentinization of oceanic lithosphere from fast-spreading ridges is suggested along deep fracture zones and during infiltration of fluids along extensional faults at the outer rise [26].These serpentinites likely reside much deeper in the oceanic lithosphere, stay cooler for much longer than those near the top of the slab and as a consequence may transport FME even deeper in the mantle than summarized in this section.

Lizardite-Antigorite Transition
During shallow (<15 km) subduction, abundant water is expelled from sediments during their compaction and refluxed back to the Earth's surface (Figure 2) [63,64], but water structurally bound in hydrous minerals remains in subducting metasediments, altered oceanic crust and serpentinites.As the slab descends, the lizardite +/− chrysotile transition to antigorite occurs at ~300-400 • C [28] and is considered to be accompanied by the release of additional water [32] and certain FME [65].This is supported by much higher concentrations of Cl, Br and I in seafloor and obducted abyssal lizardite serpentinites than subducted abyssal antigorite serpentinites [41,48,53,55].This is also in agreement with lower Cl contents in other subducted lithologies (i.e., metasediments and metabasites) compared to their pre-subduction equivalents [60,[66][67][68].The transition is temperature-dependent.Therefore, it may occur at relatively shallow depths, 20-30 km, in warm subduction zones (~15 • C/km), or much deeper (60-80 km) in cold subduction zones (~5 • C/km; Figure 2).Some antigorite serpentinites display lower Br/Cl and I/Cl ratios compared to their lizardite-bearing counterparts (Figure 4A), suggesting a preferential loss of Br and I relative to Cl during the serpentine phase transition [44,53,55], whereas others have slightly higher ratios (Figure 4A) [48].Given the limited data available for HP serpentinites and the uncertainty regarding the exact composition of their protoliths, it is difficult to fully assess these subtle differences in halogen behaviour.The wide range of serpentinite compositions likely reflects variations in the timing and sources of serpentinization.Differences in halogen behaviour may also be attributed to different geothermal gradients and, consequently, varying depth of stability of host minerals among subduction zones.Regardless of these differences, overall, all studies indicate a loss of the heavy halogens during the transformation to antigorite.The halogens released from the slab during shallow subduction may be refluxed back to the ocean or transferred to the overlying mantle wedge where they are incorporated into forearc mantle serpentinites.Shallow (<25 km) forearc serpentinites have elevated halogen concentrations relative to the deleted mantle [44,48,69,70].These serpentinites are considered an important transient reservoir for the halogens and other FME in the mantle wedge [71], and their dehydration in the deeper mantle may influence partial melting and volcanic front formation, as indicated by their similar FME enrichment patterns to those of arc magmas [27].
In contrast to the heavy halogens, bulk F contents of subducted abyssal antigorite serpentinites are similar to, if not higher than, those of obducted abyssal lizardite serpentinites (Figure 3C) [48,53].Antigorite serpentinites also have consistently higher F/Cl ratios (Figure 4B), indicating a preferential retention of F and/or an additional influx during antigorite formation.During subduction, additional F may be mobilized from metasedimentary or crustal rocks.The improved accommodation of F relative to the heavier halogens is attributed to its smaller size and the crystallographic differences of lizardite and antigorite.In lizardite, the entire Si-O tetrahedral sheet is distorted to match the Mg-O octahedral sheet, resulting in a flat crystal structure, but in antigorite, individual Si-O tetrahedra are periodically reversed, yielding a more wavy structure with stronger bonds between layers and a tighter crystal structure overall [33].As a result, the larger halogens may be more readily expelled from the hydroxyl sites during the phase transition [69], while the smaller F ion (1.33 Å), which has a similar ionic radius to OH − (1.35 Å), is more likely to remain in the antigorite crystal structure.The retention of F in serpentine minerals during this transition is supported by in situ analyses of coexisting lizardite and antigorite grains from Western Alps that show similar F content in both serpentine phases [47].

Antigorite Dehydration
As the slab subducts farther into the mantle, antigorite eventually dehydrates to metamorphic peridotite at ~600-700 • C [2].In warm subduction zones, this corresponds to a slab depth of ~80 km [30], but in cold subduction zones, the geothermal gradient is much smaller (~5 • C/km) [29], and abyssal serpentinites are thermodynamically stable to depths >150 km (Figure 2) [30,31].Cold subduction zones are also characterized by much thicker oceanic lithosphere, resulting in a larger temperature difference (~200 • C) between the top and bottom of the slab [29].Consequently, serpentinites of the lower section of the slab may remain stable to depths >200 km.
The dehydration of antigorite is accompanied by the release of fluids that become progressively depleted in Br and I relative to Cl [43,53], suggesting Br and I are preferentially lost during the early stages of antigorite breakdown.These fluids may be incorporated into the overlying mantle wedge, as evidenced by similar Br/Cl and I/Cl ratios to fluids in deep mantle wedge peridotites [72].Yet, some Cl (<160 ppm), Br (<0.2 ppm) and I (<0.07 ppm) are retained in the dehydrated peridotite residues of subducting slabs and likely contained in desiccated fluid inclusions of olivine and pyroxene [55].
Fluorine residing in the antigorite OH − site is also expected to be released during serpentinite dehydration, but F in the O 2− sites may be retained in nominally anhydrous minerals after antigorite decomposition.Indeed, high F (up to 130 ppm) has been reported in secondary olivine after antigorite decomposition [73].The higher F in bulk HP serpentinites may also be attributed to the occurrence of Ti-clinohumite [53], a F-bearing accessory mineral of HP ultramafic rocks [74], or contained in chlorite as observed for the Cerro del Almirez (Spain) metamorphic peridotites [55].

Halogen Transfer to the Deeper Mantle
Preliminary mass balance calculations suggest nearly all subducted Cl is returned to surface reservoirs through arc volcanism [3,63]; however, these studies do not account for the significant Cl in abyssal serpentinites.More recent calculations that include abyssal serpentinites suggest inputs of Cl (and Br and I) in subduction zones may exceed volcanic outputs [53,75], and they also support the retention of nearly all subducted F (up to 95%) beyond subarc depths.
Here we show that subducted serpentinites are an important reservoir for halogens in the metamorphosed slab and a vehicle for their transport to the deeper mantle.Considering a model for oceanic lithosphere produced at fast-spreading ridges [24], serpentinites of 500 m thickness constitute 50% of residual I, 56% of Br, 37% of Cl and 2% of F in the eclogitized slab (Table 1; Figure 6).Yet, as previously mentioned, the volume of serpentinized peridotites entering subduction zones is not well defined.If serpentinite thickness is considered to be 3000 m, which is more representative of oceanic lithosphere produced at slow-spreading ridges [21], the serpentinite contribution increases to 86% for I, 94% for Br, 87% for Cl and 13% for F (Table 1; Figure 6).
Minerals 2019, 9 FOR PEER REVIEW 11 oceanic lithosphere produced at fast-spreading ridges [24], serpentinites of 500 m thickness constitute 50% of residual I, 56% of Br, 37% of Cl and 2% of F in the eclogitized slab (Table 1; Figure 6).Yet, as previously mentioned, the volume of serpentinized peridotites entering subduction zones is not well defined.If serpentinite thickness is considered to be 3000 m, which is more representative of oceanic lithosphere produced at slow-spreading ridges [21], the serpentinite contribution increases to 86% for I, 94% for Br, 87% for Cl and 13% for F (Table 1; Figure 6).1. Numbers in bars denote the percent contribution of serpentinite to the total residual halogens in the slab.
Mass balance calculations confirm nearly all F is retained to HP-UHP conditions and suggest up to 14% Cl, 12% Br and 5% I may also be retained to such depths in the mantle, presumably beyond subarc depths (Table 1).These findings are in agreement with previous evidence for halogen retention beyond the depth of arc fronts based on their concentrations and ratios in magmas from the deeper mantle, including back-arc basin basalts (BABB) and ocean island basalts (OIB) [76][77][78].Mass balance calculations confirm nearly all F is retained to HP-UHP conditions and suggest up to 14% Cl, 12% Br and 5% I may also be retained to such depths in the mantle, presumably beyond subarc depths (Table 1).These findings are in agreement with previous evidence for halogen retention beyond the depth of arc fronts based on their concentrations and ratios in magmas from the deeper mantle, including back-arc basin basalts (BABB) and ocean island basalts (OIB) [76][77][78].Despite a loss of Cl, Br and I during early subduction, HP antigorite serpentinites can still retain appreciable amounts of these elements, along with significant F, compared to their peridotite protoliths [43,48,49,55,71].Given its stability to ~200 km in cold subduction zones, antigorite may provide a reservoir for these elements to depths beyond those associated with most volcanic arcs (~100 km) [79], and some water and FME may be transported even deeper (up to 300 km) by hydrous phase A after antigorite decomposition (Figure 2) [31].

Figure 1 .
Figure1.Simplified schematic diagram of representative cross sections of oceanic lithosphere produced at (A) fast-[17] and (B) slow-[21] spreading ridges.Serpentinite in the diagram represents either concentrated zones of serpentinization or diffuse serpentinization throughout the upper lithospheric mantle[24].Oceanic lithosphere produced at slow-and ultraslow-spreading ridges is particularly heterogeneous, with serpentinites commonly outcropping on the ocean floor[20,22].

Figure 3 .
Figure 3. Halogen concentrations of seafloor and obducted abyssal lizardite serpentinites (Liz srp) and subducted antigorite serpentinites (Atg srp).Bulk serpentinite data are from[43,44,49,53].In situ data of serpentine grains are from[34,43].The F/Cl, Br/Cl and I/Cl ratios of the depleted mantle (grey fields)[6] and seawater (dashed lines)[54] are shown, along with the average concentrations of F, Cl, Br and I in the depleted mantle (black stars)[6].Figures modified after[55].(a) Bromine and Cl concentrations in all serpentinites are elevated with respect to the average depleted mantle but are strongly correlated along a linear trend defined by the Br/Cl ratio of the mantle and seawater, suggesting their coherent behaviour during serpentinization (hydration) and subduction (dehydration).On average, the obducted abyssal serpentinites having lower Br and Cl concentrations than the seafloor samples, and the subducted serpentinites have lower concentrations than the obducted samples.(b) All samples have elevated I concentrations with respect to the depleted mantle, and most samples also have I/Cl ratios greater than the mantle.Iodine and Cl broadly correlate among the seafloor and obducted abyssal serpentinites.The seafloor and obducted abyssal serpentinites have similar I concentrations, while their subducted counterparts extend to lower values.(c) Individual lizardite grains have higher F content than antigorite, but bulk rock values for lizardite-and antigorite serpentinites are similar.The higher F content of some subducted serpentinites may indicate an influx of F during antigorization from other dehydrating lithologies.

Figure 3 .
Figure 3. Halogen concentrations of seafloor and obducted abyssal lizardite serpentinites (Liz srp) and subducted antigorite serpentinites (Atg srp).Bulk serpentinite data are from[43,44,49,53].In situ data of serpentine grains are from[34,43].The F/Cl, Br/Cl and I/Cl ratios of the depleted mantle (grey fields)[6] and seawater (dashed lines)[54] are shown, along with the average concentrations of F, Cl, Br and I in the depleted mantle (black stars)[6].Figures modified after[55].(a) Bromine and Cl concentrations in all serpentinites are elevated with respect to the average depleted mantle but are strongly correlated along a linear trend defined by the Br/Cl ratio of the mantle and seawater, suggesting their coherent behaviour during serpentinization (hydration) and subduction (dehydration).On average, the obducted abyssal serpentinites having lower Br and Cl concentrations than the seafloor samples, and the subducted serpentinites have lower concentrations than the obducted samples.(b) All samples have elevated I concentrations with respect to the depleted mantle, and most samples also have I/Cl ratios greater than the mantle.Iodine and Cl broadly correlate among the seafloor and obducted abyssal serpentinites.The seafloor and obducted abyssal serpentinites have similar I concentrations, while their subducted counterparts extend to lower values.(c) Individual lizardite grains have higher F content than antigorite, but bulk rock values for lizardite-and antigorite serpentinites are similar.The higher F content of some subducted serpentinites may indicate an influx of F during antigorization from other dehydrating lithologies.

Figure 4 .
Figure 4. Halogen ratios of seafloor and obducted abyssal lizardite serpentinites (Liz srp), subducted antigorite serpentinites (Atg srp) and metamorphic peridotites after serpentinite dehydration.Bulk serpentinite data are from[43,44,48,53].(a) Br/Cl and I/Cl ratios of the seafloor and obducted abyssal serpentinites overlap with those of sedimentary pore fluids (see[24] for review), while the Br/Cl ratios of the subducted serpentinites and metamorphic peridotites extend to lower values, similar to the depleted mantle[6].(b) F/Cl ratios of all serpentinites are much higher than seawater[54], suggesting input from F-rich hydrothermal fluids (e.g., Escanaba Trough high3 He/ 4 He pore fluids)[56] on the seafloor, or F mobilized from incoming plate sediments[53] and igneous lithologies during shallow subduction.On average, the subducted antigorite serpentinites and metamorphic peridotites have higher F/Cl ratios relative to their lizardite-bearing counterparts.

Figure 4 .
Figure 4. Halogen ratios of seafloor and obducted abyssal lizardite serpentinites (Liz srp), subducted antigorite serpentinites (Atg srp) and metamorphic peridotites after serpentinite dehydration.Bulk serpentinite data are from[43,44,48,53].(a) Br/Cl and I/Cl ratios of the seafloor and obducted abyssal serpentinites overlap with those of sedimentary pore fluids (see[24] for review), while the Br/Cl ratios of the subducted serpentinites and metamorphic peridotites extend to lower values, similar to the depleted mantle[6].(b) F/Cl ratios of all serpentinites are much higher than seawater[54], suggesting input from F-rich hydrothermal fluids (e.g., Escanaba Trough high3 He/ 4 He pore fluids)[56] on the seafloor, or F mobilized from incoming plate sediments[53] and igneous lithologies during shallow subduction.On average, the subducted antigorite serpentinites and metamorphic peridotites have higher F/Cl ratios relative to their lizardite-bearing counterparts.

Minerals 2019, 9 FOR PEER REVIEW 9 Figure 5 .
Figure 5. Summary of F, Cl, Br and I budget calculations for an incoming slab according to Penrosetype and Muller-type models of oceanic lithosphere produced at fast-and slow-spreading centers, respectively.The Penrose model consists of 400 m sediment, 6000 m igneous crust and 500 m lizardite serpentinites.The Muller model consists of 400 m sediment, 3500 m igneous crust and 300 m lizardite serpentinites.Average halogen concentrations for each unit are reported in Table1.Numbers in bars denote the percent contribution of serpentinite to the total budget for each element.

Figure 5 .
Figure 5. Summary of F, Cl, Br and I budget calculations for an incoming slab according to Penrose-type and Muller-type models of oceanic lithosphere produced at fast-and slow-spreading centers, respectively.The Penrose model consists of 400 m sediment, 6000 m igneous crust and 500 m lizardite serpentinites.The Muller model consists of 400 m sediment, 3500 m igneous crust and 300 m lizardite serpentinites.Average halogen concentrations for each unit are reported in Table1.Numbers in bars denote the percent contribution of serpentinite to the total budget for each element.

Figure 6 .
Figure6.Summary of F, Cl, Br and I budget calculations for a metamorphosed slab according to Penrose-type and Muller-type models of oceanic lithosphere.See Figure4for descriptions of unit thicknesses.Average halogen concentrations for each unit are reported in Table1.Numbers in bars denote the percent contribution of serpentinite to the total residual halogens in the slab.

Figure 6 .
Figure6.Summary of F, Cl, Br and I budget calculations for a metamorphosed slab according to Penrose-type and Muller-type models of oceanic lithosphere.See Figure4for descriptions of unit thicknesses.Average halogen concentrations for each unit are reported in Table1.Numbers in bars denote the percent contribution of serpentinite to the total residual halogens in the slab.

Table 1 .
Summary of global mass balance calculations.