Metal sources of black smoker chimneys, Endeavour Segment, Juan de Fuca Ridge: Pb isotope constraints

Hydrothermal chimney sulfides, vent cap chimney samples, Fe-oxide and basalts from sediment-starving Juan de Fuca Ridge, in the Endeavour segment, exhibit a range of Pb isotope ratios (206Pb/204Pb = 18.658–18.769; 207Pb/204Pb = 15.457–15.566; 208Pb/204Pb = 37.810–38.276). The data array is not parallel to the northern hemisphere mantle reservoirs indicating a possible sediment component within the sulfides. By assuming that the potential end-member sediment component has a 207Pb/204Pb (15.70) similar to Middle Valley sediment, it is suggested the potential end-member sediment component may have 206Pb/204Pb = 18.90; 208Pb/204Pb = 38.82. Basalt-derived Pb for the Endeavour segment hydrothermal system involves about 50/50 leaching of E-MORB and T2-MORB. Detailed observations show the Mothra field derives more Pb from T2-MORB than the Main Endeavour field does. According to the binary mixing model, the results show little Pb (<1.5%) or no Pb derivation from sedimentary sources. However, the high image, CH4 and Br/Cl ratios in hydrothermal fluids are consistent with a sediment component within the segment. Reconciling the Pb isotope data with the chemistry data of hydrothermal fluids, it is suggested that the sediment component may be located in a lower temperature recharge zone where Pb could not be mobilized from the sediment.