Ubiquitous ultra-depleted domains in Earth’s mantle

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Abstract

Partial melting of Earth’s mantle generates oceanic crust and leaves behind a chemically depleted residual mantle. The time-integrated composition of this chemically depleted mantle is generally inferred from basalts produced at mid-ocean ridges. However, isotopic differences between oceanic mantle rocks and mid-ocean ridge basalts suggest that mantle and basalt composition could differ. Here we measure neodymium isotope ratios in olivine-hosted melt inclusions from lavas of the Azores mantle plume. We find neodymium isotope ratios that include the highest values measured in basalts, and suggest that melts from ultra-depleted mantle contribute to the isotopic diversity of the erupted lavas. Ultra-depleted melts have exceedingly low preservation potential during magma extraction and evolution due to progressive mixing with melts that are enriched in incompatible elements. A notable contribution of ultra-depleted melts to the Azores mantle plume therefore implies that variably depleted mantle is the volumetrically dominant component of the Azores plume. We argue that variably depleted mantle, sometimes ranging to ultra-depleted compositions, may be a ubiquitous part of most ocean island and mid-ocean ridge basalt sources. If so, Earth’s mantle may be more depleted than previously thought, which has important implications for the rate of mass exchange between crust and mantle, plume dynamics and compositional stratification of Earth’s mantle.

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Fig. 1: Schematic diagram showing the melting area underneath mid-ocean ridges.
Fig. 2: 143Nd/144Nd of Azores melt inclusions compared to those of Azores and Mid-Atlantic ridge basalts and abyssal peridotites.
Fig. 3: Rare-earth element concentrations in melt inclusions and lavas.

Data availability

The authors declare that the data supporting the findings of this study are available within the article and the Supplementary information.

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Acknowledgements

This work was supported by the German Research Foundation (grant no. GE2817/6-1). F.G. acknowledges support from Europlanet 2020 RI, which has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 65420. J.M.K. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 759563. C. Beier is thanked for contributing samples from Pico Island.

Author information

A.S. and F.G. conceived and designed the study. A.S., F.G. and J.M.K. wrote the manuscript and contributed to data interpretation. F.G. and J.B. conducted the electron microprobe and LA-ICPMS analyses. F.G. and A.S. conducted the MC-ICPMS analyses. F.G. and J.M.K. conducted the TIMS analyses.

Correspondence to Andreas Stracke.

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Supplementary information

Supplementary dataset Tables 1 and 2.

REE and Nd isotope data.

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Stracke, A., Genske, F., Berndt, J. et al. Ubiquitous ultra-depleted domains in Earth’s mantle. Nat. Geosci. 12, 851–855 (2019) doi:10.1038/s41561-019-0446-z

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