Metal sources in the Middle Valley massive sulphide deposit, northern Juande Fuca Ridge: Pb isotope constraints

Massive sulphides drilled from the Bent Kill deposit during Ocean Drilling Program Leg 139 at Middle Valley, northern Juan de Fuca Ridge, have Pb-206/ Pb-204 = 18.43-18.86; Pb-207/Pb-204 = 15.43-15.63; Pb-208/Pb-204 = 38.08-38 .60. These span 35% of the total Pb isotope range measured in hydrothermal sulphides from the East Pacific Ocean. Pb-205/Pb-204 ratios overlap the ran ge of local basalts but Pb-208/Pb-204,d Pb-207/Pb-204 require a contributio n from a more radiogenic source that is similar to the overlying sedimentar y sequence. Three samples have lower Pb-207/Pb-204 ratios than the mean bas alt value which may reflect the preservation of heterogeneities in the loca l basalts. The Pb isotopic compositions of primary pyrrhotites are indistin guishable from the secondary pyrite-dominated assemblage which recrystallis ed from low temperature hydrothermal fluids. This may be explained by the i mportance of the sulphosalts as a host for Pb in the deposit. Clastic sulph ides from sediments at Bent Hill are at the radiogenic end of the massive s ulphide range suggesting that surficial sulphides may not provide an unbias ed sample of the past hydrothermal circulation responsible for massive sulp hide deposition. If the ph content of sediment is 100 times greater than th e basalts the sulphide Pb isotopes imply that the hydrothermal fluids have extracted Pb from 60 to 900 times more basalt than sediment. Lead in the ma ssive sulphide deposit has been extracted from 2 x 10(8)-3 x 10(8) m(3) of oceanic crust demonstrating the local nature of hydrothermal circulation at mid-ocean ridges. The He-3/He-4 of hydrothermal fluids in the sulphides (6 .2 x 10(-6)-7.8 x 10(-6)) show only half the variation between the sediment and basalt end-members displayed by Pb isotopes. This is consistent with v olatile acquisition from magma degassing, while the Pb is leached during hy drothermal fluid interaction with crystallised oceanic crust. (C) 1999 Else vier Science B.V. All rights reserved.