HR: 11:20h
AN: T32A-05    [Abstracts]
TI: Growth and Construction of Oceanic Crust at Atlantis Bank, Southwest Indian Ridge
AU: * Schwartz, J J
EM: joshua.j.schwartz@gmail.com
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, Wy 82071 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, Wy 82071 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, Wy 82071 United States
AU: Miranda, E A
EM: emiranda@uwyo.edu
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, Wy 82071 United States
AU: Grimes, C B
EM: cgrimes@uwyo.edu
AF: University of Wyoming, Dept. 3006, 1000 University Ave, Laramie, Wy 82071 United States
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 93 Water Street, Woods Hole, MA 02543
AB: Magmatic zircon is a common accessory mineral in oceanic crustal rocks including gabbro, oxide gabbro, diabase and felsic veins. Its presence in these rocks provides an exceptional opportunity to document crustal growth processes at slow-spreading mid-ocean ridges. We present nineteen Pb/U zircon SHRIMP-RG ion probe ages of lower crustal rocks collected by manned submersible, ROV, dredging and ODP drilling from a 20 x 30 km² area of Atlantis Bank, Southwest Indian Ridge, which allow us to constrain the growth and construction of oceanic crust. Weighted average ²⁰⁶Pb/²³⁸U ages of these samples range from 10.7 to 13.9 Ma, with errors of 0.1-0.6 m.y. (<1 - 4%). At least 75% of these gabbros accreted within error of the predicted sea-surface magnetic age, whereas up to 25% are between 700,000 and 2.5 m.y. older. In one sample, we identified zircon with inherited cores as much as 1.5 m.y. older than their corresponding rims. There is no observable correlation between age and lithology, and the anomalously old samples are not from any specific part of Atlantis Bank; they appear to be randomly distributed amongst the non-anomalous age samples and come from various structural depths. We consider two models to explain the presence of these anomalously old rocks: i) a stochastic intrusion model whereby magma was intruded at different spatial locations within the rift valley as the plates spread apart, resulting in the entrapment of older lower crust by subsequent intrusions; and/or ii) a model in which some gabbroic bodies originally crystallized at depths of ~5-18 km below the base of the crust in a thick, cold, axial lithosphere and were subsequently uplifted along flow-lines and intruded by shallow-level magmas during the creation of Atlantis Bank. In this model, the difference in time between the Pb/U zircon crystallization age and the magnetic age is a proxy for the depth at which zircon crystallized (assuming a constant mantle upwelling rate during the construction of Atlantis Bank over ~1.3 m.y.). We prefer the latter model, although aspects of both models may apply.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1115 Radioisotope geochronology
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: Fall Meeting 2005