Ten sites were drilled across the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, to investigate the nature of ridge-flank hydrothermal circulation and crustal evolution in a region of varying sediment-basement topography. The sites were divided into: i) the Hydrothermal Transition (HT) transect, a zone of gradually thickening sedimentary cover, extending from the ridge/flank boundary for a distance of 20 km to the east; ii) the Buried Basement (BB) transect, which consists of uniformly, flat lying basement, capped by a regionally continuous thick sedimentary sequence; and iii) the Rough Basement (RB) transect, represented by a region of basement highs and troughs, located c.100 km east of the ridge axis, and capped by a variable thickness of sediment.
Across the flank, a progressive sequence of alteration was recognised, produced as the hydrothermal system evolved from open oxidative conditions involving a slightly modified seawater derived fluid, to a more restricted/closed, non-oxidative environment in which more strongly modified seawater derived fluids interacted with the basement. Alteration commenced with the formation of chlorite at relatively high temperatures in non-oxidative to oxidative conditions, followed by the first wide spread stage of low temperature oxidative alteration with the formation of iron oxyhydroxides and celadonite. Progressive burial, along with continued alteration of the basement, restricted fluid circulation resulting in the formation of saponite ± pyrite over a range of temperatures under non-oxidative conditions. The final stage of alteration is represented by carbonate, which formed under non-oxidative conditions by interaction of strongly modified seawater and the crust.
Reprinted by permission of the Geological Society of London.