HR: 1340h
AN: B13A-0172    [Abstracts]
TI: Hydrothermal Mineral Deposits From a Young (0.1Ma) Abyssal Hill on the Flank of the Fast-Spreading East Pacific Rise
AU: * Benjamin, S B
EM: benjamin_sara@yahoo.com
AF: UC Santa Barbara, Geological Sciences Building 526, Santa Barbara, CA 93106 United States
AU: Haymon, R M
EM: haymon@geol.ucsb.edu
AF: UC Santa Barbara, Geological Sciences Building 526, Santa Barbara, CA 93106 United States
AB: It has been estimated from heat flow measurements that at least 40% of the total hydrothermal heat lost from oceanic lithosphere is removed from 0.1-5 Ma abyssal hill terrain on mid-ocean ridge flanks. Despite the large magnitude of estimated hydrothermal heat loss from young abyssal hills, little is known about characteristics of hydrothermal vents and mineral deposits in this setting. This study describes the first abyssal hill hydrothermal samples to be collected on the flank of a fast-spreading ridge. The mineral deposits were discovered at "Tevnia Site" on the axis-facing fault scarp of an abyssal hill, located on $\sim$0.1 Ma lithosphere $\sim$5 km east of the East Pacific Rise (EPR) axis at 10\deg 20'N. Observations of Galatheid crabs, "dandelion" siphonophores, and colonies of dead, yet still intact, Tevnia worm tubes at this site during Alvin dives in 1994 suggests relatively recent hydrothermal activity. The deposits are friable hydrothermal precipitates incorporating volcanic clasts brecciated at both the micro and macro scales. The petrographic sequence of brecciation, alteration, and cementation exhibited by the samples suggests that they formed from many pulses of hydrothermal venting interspersed with, and perhaps triggered by, repeated tectonic events as the abyssal hill was uplifted and moved off-axis (see also Haymon et al., this session). Observed minerals include x-ray amorphous opaline silica and Fe-oxide phases, crystalline Mn-oxides (birnessite and todorokite), an irregularly stratified mixed layer nontronite-celadonite, and residual calcite in sediment-derived microfossils incorporated into the breccia matrix. This mineral assemblage suggests that the deposits precipitated from moderately low-temperature ($<$140\deg C) fluids, enriched in K, Fe, Si, and Mn, with a near-neutral pH. The presence of tubeworm casings at the site is evidence that the hydrothermal fluids carried H$_{2}$S, however no metal sulfide phases were identified in the samples. Although the fluids were actively venting from an abyssal hill distal to the ridge crest, the presence of Fe- and K-rich nontronite-celadonite suggests an axial fluid source. However, the observed textures, minerals, and microfossils, combined with the absence of copper, zinc, and sulfur minerals, clearly distinguishes these near-axis samples from hydrothermal deposits formed at higher temperatures ($>$350\deg C) on the mid-ocean ridge crest.
DE: 7220 Oceanic crust
DE: 8135 Hydrothermal systems (8424)
DE: 4804 Benthic processes/benthos
DE: 4832 Hydrothermal systems
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting