Halide systematics of submarine hydrothermal vents

Abstract

CHLORIDE is the dominant anion in submarine hydrothermal fluids and plays a fundamental part in controlling their chemistry¹. The endmember Cl concentrations in virtually all submarine hot springs deviate from ambient sea water (∼541 mmol kg^–1) and range from 188 mmol kg^–1 at the ASHES vents on Axial Volcano² to 1,090 mmol kg^–1 at the Southern Juan de Fuca Ridge³. A variety of mechanisms have been proposed to explain these Cl enrichments and depletions including: rock hydration and dehydration⁴, phase separation^2–3,5–7, and the formation and dissolution of a retrograde soluble phase⁸. Support for the latter hypothesis came from hydrothermal fluid/basalt experiments which showed large changes in dissolved Cl at specific reaction conditions⁹ and also from the presence of Cl-rich minerals phases in ultramafic rocks¹⁰. We have carried out the first systematic study of Br and I in mid-ocean ridge hydrothermal vent fluids to constrain the models of halide diversity better by ascertaining their behaviour relative to Cl. The cohesiveness of the Br and Cl data and the similarity of Br/Cl to the seawater ratio suggests a single, non-fractionating process controls the salinity of sediment-starved mid-ocean ridge hydrothermal systems. The thermal decomposition of organic matter provides an additional source of Br and I in sediment-hosted systems.