Abstract
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.