BORON AND HALIDE SYSTEMATICS IN SUBMARINE HYDROTHERMAL SYSTEMS - EFFECTS OF PHASE-SEPARATION AND SEDIMENTARY CONTRIBUTIONS

Systematic studies of the distributions of B, deltaB-11, NH4, halides (Cl, Br, 1) and trace alkalis (Li, Rb, Cs) in vent fluids, combined wi th experimental data on super- and subcritical phase separation, provi de a method for separating the effects of interaction with basalts and / or sediments from those of phase separation. This allows a more gene ral understanding of geochemical processes in submarine hydrothermal s ystems, especially where a connection with sediment is not otherwise o bvious (e.g., Endeavour Segment, Juan de Fuca Ridge). Based on B and d eltaB-11 corrected for wallrock reactions, all published boron and chl oride data from mid-ocean ridge systems (MOR) (e.g., 11-degrees-N, 13- degrees-N and 21-degrees-N of the East Pacific Rise), except for the E ndeavour Segment, Juan de Fuca Ridge, are consistent with experimental phase separation data, suggesting a dominant control by the latter pr ocess. Fluids from sedimented ridge (SR) (e.g., Escanaba Trough and Gu aymas Basin), and from back-arc basins (BAB) (e.g., Mariana Trough, La u 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, a nd trace alkali metal contents (such as Li, Rb and Cs). High B concent rations and elevated deltaB-11 in Endeavour Segment can be explained b y a small, but distinguishable contribution from sediments, which is c onfirmed by slightly enhanced levels of Br, I and NH4.