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Fault-controlled hydration of the upper mantle during continental rifting

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Abstract

Water and carbon are transferred from the ocean to the mantle in a process that alters mantle peridotite to create serpentinite and supports diverse ecosystems1. Serpentinized mantle rocks are found beneath the sea floor at slow- to ultraslow-spreading mid-ocean ridges1 and are thought to be present at about half the world’s rifted margins2,3. Serpentinite is also inferred to exist in the downgoing plate at subduction zones4, where it may trigger arc magmatism or hydrate the deep Earth. Water is thought to reach the mantle via active faults3,4. Here we show that serpentinization at the rifted continental margin offshore from western Spain was probably initiated when the whole crust cooled to become brittle and deformation was focused along large normal faults. We use seismic tomography to image the three-dimensional distribution of serpentinization in the mantle and find that the local volume of serpentinite beneath thinned, brittle crust is related to the amount of displacement along each fault. This implies that sea water reaches the mantle only when the faults are active. We estimate the fluid flux along the faults and find it is comparable to that inferred for mid-ocean ridge hydrothermal systems. We conclude that brittle processes in the crust may ultimately control the global flux of sea water into the Earth.

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Figure 1: Comparison between the crustal thickness at which complete crustal embrittlement is predicted to occur3 and the maximum crustal thickness observed above or juxtaposed against serpentinized mantle at various North Atlantic magma-poor margins8.
Figure 2: Compressional (P-) wave velocities superimposed on coincident seismic reflection profiles illustrate the concentration of serpentinization beneath the hanging wall of normal faults (expansion of 6.5–7.5 km s−1 iso-velocity interval).
Figure 3: Water volume and amount of serpentinization associated with faults on the seismic reflection profile shown in Fig. 2b, assuming a two-dimensional structure.

Change history

  • 26 May 2016

    In the version of the Letter originally published, the following reference was mistakenly omitted in the Methods: '29. Minshull, T. A., Sinha, M. C. & Peirce, C. Multi-disciplinary, sub-seabed geophysical imaging. Sea Technol. 46, 27–31 (2005).' This should have been cited after '(OBSs)' in the sentence 'A grid of 72 ocean bottom instruments, comprising 44 four-component ocean bottom seismometers (OBSs) and 28 ocean bottom hydrophones (OBHs) was deployed on the seabed for three months to record these shots, with sample rates of 250 Hz and 200 Hz, respectively'. The original refs 29–33 have been renumbered accordingly. This has been corrected in the online versions of the Letter.

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Acknowledgements

We thank all who sailed with us on RV Marcus Langseth and FS Poseidon for their hard work at sea, M. Karplus for assistance with detailed survey design, and A. Krabbenhoft for assistance with data processing. This research was supported by the US National Science Foundation (OCE-1031769), the UK Natural Environment Research Council (NE/E016502/1 and NE/E015883/1) and the GEOMAR Helmholtz Centre for Ocean Research. Ocean bottom instruments were provided by the UK Ocean Bottom Instrumentation Facility and by GEOMAR. T.A.M. was supported by a Wolfson Research Merit award.

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D.S.S., T.J.R., T.A.M., D.K., D.J.S., C.R., J.M.B. and J.K.M. designed the seismic experiment. D.S.S. led the survey on RV Marcus Langseth and D.K. and C.P. led the deployment and recovery of seafloor instruments aboard FS Poseidon. G.B. conducted the seismic data analysis, with R.G.D providing assistance. T.J.R. compiled the North Atlantic seismic profiles and M.P.-G. carried out the numerical modelling. G.B. and T.A.M. wrote the first draft of the paper and all authors contributed to subsequent revisions.

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Correspondence to T. A. Minshull.

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Bayrakci, G., Minshull, T., Sawyer, D. et al. Fault-controlled hydration of the upper mantle during continental rifting. Nature Geosci 9, 384–388 (2016). https://doi.org/10.1038/ngeo2671

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