Boron and halide systematics in submarine hydrothermal systems: Effects of phase separation and sedimentary contributions

Systematic studies of the distributions of B, δ¹¹B, NH₄, halides (Cl, Br, I) and trace alkalis (Li, Rb, Cs) in vent fluids, combined with experimental data on super- and subcritical phase separation, provide a method for separating the effects of interaction with basalts and/or sediments from those of phase separation. This allows a more general understanding of geochemical processes in submarine hydrothermal systems, especially where a connection with sediment is not otherwise obvious (e.g., Endeavour Segment, Juan de Fuca Ridge). Based on B and δ¹¹B corrected for wallrock reactions, all published boron and chloride data from mid-ocean ridge systems (MOR) (e.g., 11°N, 13°N and 21°N of the East Pacific Rise), except for the Endeavour Segment, Juan de Fuca Ridge, are consistent with experimental phase separation data, suggesting a dominant control by the latter process. Fluids from sedimented ridge (SR) (e.g., Escanaba Trough and Guaymas Basin), and from back-arc basins (BAB) (e.g., Mariana Trough, Lau Basin and Okinawa Trough), when compared with mid-ocean ridge data, show expected effects of organic matter and/or sediment contributions. This is particularly noticeable from enhanced levels of Br, I, NH₄, and trace alkali metal contents (such as Li, Rb and Cs). High B concentrations and elevated δ¹¹B in Endeavour Segment can be explained by a small, but distinguishable contribution from sediments, which is confirmed by slightly enhanced levels of Br, I and NH₄.