Episodic magmatism and serpentinized mantle exhumation at an ultraslow-spreading centre

Abstract

Mid-ocean ridges spreading at ultraslow rates of less than 20 mm yr−1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow-spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2 to 5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s−1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P- to S-wave velocity (Vp/Vs) ratio. Here we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow-spreading Mid-Cayman Spreading Centre. We suggest that high Vp/Vs ratios greater than 1.9 and continuously increasing P-wave velocity, changing from 4 km s−1 at the seafloor to greater than 7.4 km s−1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough therefore provides a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle covering about 25% of the seafloor in this region.

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Fig. 1: Cayman Trough bathymetry and layout of seismic experiment.
Fig. 2: Seismic results.
Fig. 3: P-wave properties of magmatic and amagmatic domains.
Fig. 4: Vp/Vs ratio as a proxy for rock types and mantle serpentinization.

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Acknowledgements

Funding for this project was obtained from the German Science Foundation (DFG), supporting RV METEOR cruise M115, from the US National Science Foundation (NSF) under grant OCE-1356895, and from the British Natural Environment Research Council (NERC) under grant NE/K011162/1. The authors thank the captain, officers, crew and scientific party of RV METEOR for their assistance during the CAYSEIS cruise.

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I.G., N.W.H., H.J.A.v.A., C.Pe. and A.D. planned the survey and obtained the funding. All co-authors contributed to collecting data at sea and discussed results. C.Pa., I.G. and M.S. processed the data. I.G. and M.S. conducted seismic inversions and error analysis. C.Pe. conducted the analysis of the gravity data. I.G., N.W.H. and C.Pe. wrote the paper with input from H.J.A.v.A. and all authors commented on the manuscript.

Correspondence to Ingo Grevemeyer.

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Grevemeyer, I., Hayman, N.W., Peirce, C. et al. Episodic magmatism and serpentinized mantle exhumation at an ultraslow-spreading centre. Nature Geosci 11, 444–448 (2018). https://doi.org/10.1038/s41561-018-0124-6

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