Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere

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Abstract

Strong evidence exists that water is carried from the surface into the upper mantle by hydrous minerals in the uppermost 10–12 km of subducting lithosphere, and more water may be added as the lithosphere bends and goes downwards. Significant amounts of that water are released as the lithosphere heats up, triggering earthquakes and fluxing arc volcanism1. In addition, there is experimental evidence for high solubility of water in olivine, the most abundant mineral in the upper mantle, for even higher solubility in olivine’s high-pressure polymorphs, wadsleyite and ringwoodite2, and for the existence of dense hydrous magnesium silicates that potentially could carry water well into the lower mantle3 (deeper than 1,000 km). Here we compare experimental and seismic evidence to test whether patterns of seismicity and the stabilities of these potentially relevant hydrous phases are consistent with a wet lithosphere. We show that there is nearly a one-to-one correlation between dehydration of minerals and seismicity at depths less than about 250 km, and conclude that the dehydration of minerals is the trigger of instability that leads to seismicity. At greater depths, however, we find no correlation between occurrences of earthquakes and depths where breakdown of hydrous phases is expected. Lastly, we note that there is compelling evidence for the existence of metastable olivine (which, if present, can explain the distribution of deep-focus earthquakes4,5,6,7) west of and within the subducting Tonga slab8 and also in three other subduction zones, despite metastable olivine being incompatible with even extremely small amounts of water (of the order of 100 p.p.m. by weight7). We conclude that subducting slabs are essentially dry at depths below 400 km and thus do not provide a pathway for significant amounts of water to enter the mantle transition zone or the lower mantle.

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Figure 1: Schematic summarizing key points presented in this study.

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Acknowledgements

We thank L. Dobrzhinetskaya, P. Silver, J. Smyth, T.-L. Tseng, H. Zhang and J. Zhang for discussions. H.W.G. thanks D. LeFay for technical support. This work was supported by NSF grants EAR0552011, EAR-0652626, EAR-0125938 (H.W.G.), EAR9909362, EAR0551995 (W.-P.C.) and EAR552002 (M.R.B.). W.-P.C. acknowledges a distinguished visiting professorship, funded by the National Science Council of Taiwan, at the Academia Sinica.

Author information

The concepts underlying this paper have developed through discussions among the authors over the last few years. Interpretation of experimental results is primarily due to H.W.G.; global seismic results and artwork are primarily due to W.-P.C. and M.R.B. H.W.G. wrote the first draft of the manuscript; all authors participated in manuscript revision.

Correspondence to Harry W. Green II.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text on Abundance of DHMS in deep Slabs and Potential Rehydration of the Upper Slab by Subsequent Dehydration of the Slab's Interior. The file also contains additional references and Supplementary Figure 1 with a legend. (PDF 134 kb)

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Green II, H., Chen, W. & Brudzinski, M. Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere. Nature 467, 828–831 (2010) doi:10.1038/nature09401

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