Abstract
Fault zones in the ocean crust are commonly hypothesized to act as high-permeability conduits that focus fl uid fl ow in oceanic hydrothermal systems. However, there has been little direct study of faults in crust formed at fast-spreading ridges. Here we describe the geology and geochemistry of an ~40-m-wide fault zone within the uppermost sheeted dike complex exposed at Pito Deep (northeastern Easter microplate). Titanium in quartz thermometry gives temperatures of 392 ± 33 °C for quartz precipitation, indicating that this fault zone focused upwelling fl uids at temperatures similar to those of black-smoker vent fl uids. Correlated enrichment in 87 Sr/ 86 Sr and MgO in fault breccias, along with 87 Sr/ 86 Sr ratios higher than in average vent fl uids, provide evidence for mixing between high-temperature upwelling fl uids and a seawater-like fl uid within the fault zone. Large high-temperature fl uid fl uxes are required to maintain high temperatures during mixing. If this fault zone is representative of upfl ow zones beneath hydrothermal vents on the East Pacifi c Rise, then it is possible that vent fl uids evolve thermally and chemically during their ascent and may not record the precise conditions at the base of the hydrothermal system.
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