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Weakening of calcium iridate during its transformation from perovskite to post-perovskite

Abstract

The lowermost part of the Earth’s mantle—the 200-km-thick D′′ layer—shows anomalous seismic properties1,2, and is rheologically distinct from the rest of the lower mantle3,4. The difference is thought to result from a phase transition from silicate perovskite to silicate post-perovskite3,4. However, the rheology of the latter phase remains to be documented owing to experimental difficulties in reproducing pressures equivalent to those in the lowermost mantle. Here we address this problem by conducting laboratory experiments that use calcium iridate, which has been shown to be an appropriate low-pressure analogue5,6. We find that the post-perovskite phase of this analogue is approximately five times weaker than its perovskite phase, and that it further weakens by a factor of two during the phase transformation; these are minimum estimates. If, as is likely, a similar weakening occurs in lower-mantle magnesium–silicate compositions, this could provide an explanation for the behaviour of the lowermost mantle as inferred from geophysical data.

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Figure 1: Variation of instantaneous strain rate (ε̇) with time.
Figure 2: Relative strength of the CaIrO3 sample and MgO reference versus time.
Figure 3: The possible effect of post-perovskite on flow in D′′.

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Acknowledgements

We acknowledge financial support from the following sources: EU, Marie Curie Fellowships contract HPMT-CT-2001-00231 (S.A.H.), NERC (S.A.H., D.P.D.), The Royal Society (D.P.D.), ESF EUROCORES program EuroMinScI, contract ERAS-CT-2003-980409 (N.P.W.), NSF grant EAR0809397 (L.L.). Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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Project planning: D.P.D. and J.P.B. On-line experiments: S.A.H., D.P.D., L.L., D.J.W. and L.W.; off-line experiments: N.P.W. Data analysis: S.A.H. and D.P.D.

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Correspondence to David P. Dobson.

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Hunt, S., Weidner, D., Li, L. et al. Weakening of calcium iridate during its transformation from perovskite to post-perovskite. Nature Geosci 2, 794–797 (2009). https://doi.org/10.1038/ngeo663

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