Thermal effects of pyroxenites on mantle melting below mid-ocean ridges

Abstract

After travelling in Earth’s interior for up to billions of years, recycled material once injected at subduction zones can reach a subridge melting region as pyroxenite dispersed in the host peridotitic mantle. Here we study genetically related crustal basalts and mantle peridotites sampled along an uplifted lithospheric section created at a segment of the Mid-Atlantic Ridge through a time interval of 26 million years. The arrival of low-solidus material into the melting region forces the elemental and isotopic imprint of the residual peridotites and of the basalts to diverge with time. We show that a pyroxenite-bearing source entering the subridge melting region induces undercooling of the host peridotitic mantle, due to subtraction of latent heat by melting of the low-T-solidus pyroxenite. Mantle undercooling, in turn, lowers the thermal boundary layer, leading to a deeper cessation of melting. A consequence is to decrease the total amount of extracted melt, and hence the magmatic crustal thickness. The degree of melting undergone by a homogeneous peridotitic mantle is higher than the degree of melting of the same peridotite but veined by pyroxenites. This effect, thermodynamically predicted for a marble-cake-type peridotite–pyroxenite mixed source, implies incomplete homogenization of recycled material in the convective mantle.

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Fig. 1: Temporal variations of Nd isotopes and degree of melting along the VLS.
Fig. 2: Interpretative sketch of the upwelling mantle column below the VLS.
Fig. 3: Melt-PX24 numerical experiments for adiabatic melting of a two-component mantle source with lherzolite plus SD pyroxenite (M7-1629).
Fig. 4: Variation of the degree of melting estimated from mantle peridotite and associated basalts along the SWIR and the MAR from the Equator to the Azores hotspot region.
Fig. 5: The difference in the degree of melting estimated from genetically related basalts and peridotites (ΔF) versus the Nd isotopic composition of basalt.

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Acknowledgements

This work has been supported by Italian-PRIN prot. 2015C5LN35 and by the US National Science Foundation under grant no. OCE-05-51288. We are also grateful for the support of the Deep Energy community of the Carbon Observatory funded by the Alfred P. Sloan Foundation. We thank C. Langmuir, H. Dick, J. Warren and M. Seyler for stimulating insightful discussions and critical reading of an early version of the work. We are grateful to M. Ligi for his support on geophysics and S. Lambart for helping on Melt-PX. We also thank S. Lambart and A. Stracke for their constructive reviews that greatly improved the manuscript. This is Lamont-Doherty contribution number 8205.

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D.B. performed the modelling. A.C. analysed the samples. D.B. and A.C. processed the geochemical data and jointly wrote the paper. E.B. provided the opportunity and support for sea expeditions and work. All of the authors discussed the results and the interpretations.

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Correspondence to Daniele Brunelli or Anna Cipriani.

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Brunelli, D., Cipriani, A. & Bonatti, E. Thermal effects of pyroxenites on mantle melting below mid-ocean ridges. Nature Geosci 11, 520–525 (2018). https://doi.org/10.1038/s41561-018-0139-z

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