Halogens in Seawater, Marine Sediments and the Altered Oceanic Lithosphere

Abstract

This chapter aims to provide a framework for understanding the distribution of halogens in the oceanic lithosphere. It reviews the concentration of F, Cl, Br and I in seawater, marine sediment pore waters, hydrothermal vent fluids, fluid inclusions from deeper in the crust, and the complementary solid-phase reservoirs of organic matter and minerals present in sediments and crustal rocks from varying depths. Seawater (3.4-3.5 wt. % salt) is depleted in F, weakly enriched in I and strongly enriched in Br and Cl compared to the primitive mantle. Sequestration of I and Br by phytoplankton lead to the storage of these elements in marine sediments which are the Earths dominant I reservoir. Regeneration of organic matter during diagenesis releases I and Br to marine sediment pore waters which can be advected into the underlying crust and lithosphere. In contrast, Cl is assumed to behave conservatively in pore waters and F is precipitated in sedimentary minerals meaning it is not significantly advected into the underlying basement. Vent fluids have salinities of 0.1-6 wt. % salts, which provide evidence for phase separation and segregation of vapours and brines in hydrothermal systems. The majority of vent fluids have Br/Cl ratios within 10% of the seawater value. However, elevated Br/Cl and I/Cl ratios indicate that some vent fluids interact with sediments, and depressed Br/Cl ratios suggest some vent fluids leach Cl from glassy volcanic rocks or halite. Vent fluids have F/Cl ratios scattered around the seawater value which reflects the generally low mobility of F during diagenesis and hydrothermal alteration. In comparison to vent fluids, fluid inclusions also provide evidence for phase separation but preserve a much greater range of salinity including brines with salinities as high as ~50 wt. % salt in many parts of the crust. The altered ocean crust has a F concentration of close to its initial value. In contrast, Cl is mobilised within layer 2 pillows and dykes and strongly enriched in layer 3 gabbros subjected to high temperature alteration. Amphibole is the dominant Cl host in the oceanic crust, with Cl concentrations of <500 ppm under greenschist conditions and up to wt. % levels under amphibolite conditions. The increasing Cl content of amphibole as a function of metamorphic grade most likely reflects a decreasing water/rock ratio and a general increase in fluid salinity as a function of depth in the crust. Amphibole preferentially incorporates Cl relative to Br and I; however, it is possible that I is enriched in absolute terms, and relative to Cl, in clay-rich alteration and biogenic alteration of glassy rocks in the upper crust. Serpentinites formed in the oceanic lithosphere can contain thousands of ppm Cl and some serpentinites preserve Br/Cl and I/Cl signatures very similar to sedimentary pore waters, indicating that all halogens have high compatibilities in serpentine. Fluorine is slightly enriched in serpentinites compared to peridotites which may indicate mobilisation of F from igneous lithologies in th...

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

“…[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].…”

“…[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.…”

“…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.…”

“…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.…”

“…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.…”