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Highly heterogeneous depleted mantle recorded in the lower oceanic crust

Abstract

The Earth’s mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of mid-ocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle endmember. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here, we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high-spatial-resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection and melt transport in the mantle and the crust.

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Data availability

The data supporting the findings of this study are available within the Article and the Methods and in the PetDB data repository (http://www.earthchem.org/petdbWeb/search/readydata/MAR55S-52N_major_trace_isotope.csv).

Code availability

The code used to calculate adiabatic melting of a two-component mantle source, Melt-PX (ref. 52), can be accessed at https://doi.org/10.1002/2015JB012762.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Acknowledgements

This work was supported by the European Union’s Horizon 2020 research and innovation programme (Marie Skłodowska-Curie grant agreement No. 663830) and National Science Foundation (EAR-1834367) to S.L. and by the award NERC NE/R001332/1 to M.-A.M. We thank D. Muir, I. McDonald, T. Oldroyd and M. Jansen for their assistance on the scanning electron microscope, with LA-ICP-MS, with sample preparation and in using the micromill, respectively.

Author information

C.J.L. designed the study. S.L. and C.J.L. wrote the manuscript with input from M.-A.M., J.M.K. and G.R.D. S.L. and C.J.L. selected the samples. S.L. and M.C. performed the element maps. S.L. performed trace element analyses and geochemical modelling. S.L. and C.J.L. performed micromilling, and S.L., C.J.L., M.-A.M. and J.M.K. performed column chemistry and isotopic analyses.

Competing interests

The authors declare no competing interests.

Correspondence to Sarah Lambart.

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Fig. 1: Isotopic compositions of Atlantis Massif cumulate minerals.
Fig. 2: Isotopic compositions of cumulate minerals, abyssal peridotites and MORBs along the northern MAR and results of geochemical modelling.
Fig. 3: Intrasample heterogeneity.
Fig. 4: Frequency distribution of Nd isotopic compositions.
Fig. 5: Illustration of magma delivery from a two-component mantle to the crust.