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 the crust. Overall, the altered oceanic lithosphere reaching subduction zones is estimated to have a maximum median Cl content of ~400 ppm, and it is estimated to have a F/Cl ratio of ~0.25 compared to ~2 in pristine crust. It is therefore estimated that approximately 90% of the Cl present in altered oceanic lithosphere is introduced during seawater alteration.