Boron, bromine, and other trace elements as clues to the fate of chlorine in mid-ocean ridge vent fluids

Fluids from mid-ocean ridge hot springs typically have Cl concentrations which depart significantly from seawater values. These variations may be due in part to phase separation processes and/ or precipitation and dissolution of chloride-bearing minerals. Both of these processes likely produce systematic and recognizable variations in the distributions of trace elements which should be evident in vent fluid chemistries. To better understand how supercritical phase separation can affect trace element distributions, we conducted an experiment involving a Na-Ca-K-Cl fluid containing trace quantities of Sr, Ba, B, Li, and Br, which was allowed to separate into vapor and brine phases at 425, 440, and 450°C by systematically adjusting pressure. All of the measured trace elements were concentrated into the brine phase relative to the vapor phase. The relative order of partitioning into the brine was Ba > Sr > Ca > K > Na,Cl > Li > Br > B. Comparing the experimentally determined distribution of trace elements, especially B and Br, and Cl, with analogous data for vent fluids leads to the interpretation that both phase separation and mineral precipitation processes occur in ridge crest hot spring systems. Fluids with Cl greatly different from seawater may require a phase separation process at temperatures greater than 450°C to explain the B^SW (boron from a seawater source) and Br data. Furthermore, there is evidence that nearly all of the fluids venting from ridge crest systems, whether phase separation is indicated or not, appear to have experienced some degree of precipitation of a Cl mineral which may be similar to that formed in hydrothermal experiments. Relative concentrations of trace alkali and B^rock (boron derived from rock) in vent fluids suggest that fluids interact directly with recently cooled magma prior to phase separation but close to or within a cracking front region located slightly above the brittle/ductile transition zone surrounding the magma chamber. Vent fluids in such systems represent the combined chemistries of mixed brine, vapor, and hydrothermal fluid which has reequilibrated with the host rock at temperatures much less than those required for phase separation.