HR: 09:25h
AN: V41A-04    [Abstracts]
TI: Could Ultra-Low-Velocity Zones be Subducted Banded Iron Formations?
AU: * Dobson, D P
EM: d.dobson@ucl.ac.uk
AF: Department of Earth Sciences University College London, Gower Street, London, WC1E 6BT United Kingdom
AU: Brodholt, J P
EM: j.brodholt@ucl.ac.uk
AF: Department of Earth Sciences University College London, Gower Street, London, WC1E 6BT United Kingdom
AB: Ultra-low velocity zones (ULVZs) are regions of the Earth's core-mantle boundary (CMB) of about 1 to 10 km thick which exhibit seismic velocities which are lower than radial reference models by about 10 to 20% in V_p and 10-30% in V_s, together with a possible increase in density of 0-20%. A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt, core-mantle reaction zones, or underside sedimentation from the core. Here we suggest that ULVZs are relics of banded iron formations (BIFs) subducted to the CMB 2.8 to 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, BIFs were deposited in the world's oceans during the late Archaean and early Proterozoic. BIF has a mean density some 25% higher than the mantle, all the way from the Earth's surface to the CMB. As part of the ocean floor, BIFs would therefore be recycled into the Earth's interior by subduction. We estimate that as much as 500 m thickness of BIF could have precipitated on the ocean floor between 2800 and 1800 billion years ago. Such a volume of BIF would produce a 1.3 km thick layer surrounding the outer core, or a 10 km thick layre if it was concentrated into the regions where ULVZs are observed, consistent with estimated ULVZ volume. Recent models suggest that the core may be close to oxygen saturation, in which case BIF could stably pond on top of the outer core. Even if the core is undersaturated in O, capillary rise and diffusion arguments suggest that a km-thick BIF body could survive at the CMB for billions of years. If our model, which we argue is at least as likely as other candidate mechanisms, is correct the presence of subducted BIF in the form of FeO and post-stishovite would explain many of the physical (seismic wave speed, low V_s/V_p, high electrical conductivity) and chemical (high Pt content reservoir below deep plume sources) properties invoked for regions of the CMB. It is perhaps ironic that one of the most enigmatic features of the deepest interior of the Earth may owe its existence to some of the earliest and most primitive life.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly