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Mixing at mid-ocean ridges controlled by small-scale convection and plate motion


Oceanic lavas are thought to be derived from different sources within the Earth’s mantle, each with a distinct composition1,2,3,4. Large-scale plate motions provide the primary mechanism for mixing these sources, yet the geochemical signature of lavas erupted at different mid-ocean ridges can still vary significantly5,6. Geochemical variability is low where plate spreading rates are high, consistent with plate-scale mixing5,6. However, slow-spreading centres, such as the Southwest Indian Ridge in the Indian Ocean, are also geochemically homogeneous, which is inconsistent with plate-scale mixing6,7. Here we use numerical simulations of mantle flow to study mantle mixing at mid-ocean ridges, under conditions with variable plate length and spreading rate. Our simulations reveal that small-scale convection in the mantle contributes significantly to mantle mixing at slow spreading rates; faster plate velocities and smaller plates inhibit small-scale convection. We conclude that whereas fast-spreading ridge lavas are well mixed by plate-scale flow, slow-spreading ridge lavas are mixed by small-scale convection.

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Figure 1: Numerical experiment results: temperature fields and mixing efficiencies for cases with different half-spreading rates.
Figure 2: Numerical experiment results: averaged quantities and variability versus half-spreading rate.
Figure 3: Helium variability and predicted mixing times along mid-ocean ridges.


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This work benefited from comments by D. Graham. H.S. acknowledges the funds from the Deutsche Forschungsgemeinschaft (project SA 2042/2-1), from the Stifterverband für die Deutsche Wissenchaft, and from the Centre de Coopération Universitaire Franco-Bavarois. S.D.K. acknowledges the funds from the Humboldt Foundation. Figures were made with the Generic Mapping Tools (P. Wessel and W.H.F. Smith, EOS, Trans. AGU 76 (1995) 329).

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H.S. and S.D.K. conceived the study and designed the experiments. H.S. performed the experiments, analysed the data and developed the semi-analytic mixing model. H.S. and S.D.K. discussed the results and wrote the text.

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Correspondence to Henri Samuel.

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The authors declare no competing financial interests.

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Samuel, H., King, S. Mixing at mid-ocean ridges controlled by small-scale convection and plate motion. Nature Geosci 7, 602–605 (2014).

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