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Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary


Ultralow-velocity zones (ULVZs) are regions of the Earth's core–mantle boundary about 1–10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10–20 per cent for compressional waves and 10–30 per cent for shear waves. It is also thought that such regions have an increased density of about 0–20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt2, core–mantle reaction zones3 or underside sedimentation from the core4. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core–mantle boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earth's interior by subduction5, sink to the bottom of the mantle and may explain all of the observed features of ULVZs.

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Figure 1: A 2.5-Gyr-old banded iron formation from Hamersley, Australia.
Figure 2: Schematic cross-section of the Earth's mantle showing formation of BIFs in the oceans and segregation at the core–mantle boundary.


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We thank P. Bown and M. Kendall for discussions. This work was performed under the auspices of the Deep Earth System Consortium. D.D. was supported by a Royal Society University Research Fellowship.

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Correspondence to David P. Dobson.

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Supplementary information

Supplementary Figure S1

Figure showing settling velocity with slab angle for the mixed diapir/conduit flow model. (PDF 12 kb)

Supplementary Figure Legend

This file contains the figure caption for Supplementary Figure S1, describing the diapir and conduit flow models. (DOC 22 kb)

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Dobson, D., Brodholt, J. Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary. Nature 434, 371–374 (2005).

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