Halogens in seawater, marine sediments and the altered oceanic lithosphere
Date
31/01/2018
Authors
Kendrick, Mark
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Springer Verlag
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.
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Keywords
Halogens, oceanic crust
Citation
Kendrick, M. A., 2018. Halogens in seawater, marine sediments and the altered oceanic lithosphere. In: Harlov, D. E. and Aranovich, L. Y. Eds.), The role of halogens in terrestrial and extraterrestrial processes Springer International Publishing.
Kendrick, M. A., 2018. Halogens in seawater, marine sediments and the altered oceanic lithosphere. In: Harlov, D. E. and Aranovich, L. Y. Eds.), The role of halogens in terrestrial and extraterrestrial processes Springer International Publishing.
Kendrick, M. A., 2018. Halogens in seawater, marine sediments and the altered oceanic lithosphere. In: Harlov, D. E. and Aranovich, L. Y. Eds.), The role of halogens in terrestrial and extraterrestrial processes Springer International Publishing.
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Book chapter
Book Title
The role of halogens in terrestrial and extraterrestrial processes
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Open Access
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