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Water-rich basalts at mid-ocean-ridge cold spots


Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts1,2 has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by ‘wet’ mantle plumes3,4,5. Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions of thermal minima derive from low degrees of ‘wet’ melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower ‘dry’ melting—a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at ‘cold spots’.

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We thank D. Brunelli for comments, and D. W. Forsyth for providing programs for temperature calculations. This work was supported by the Italian Consiglio Nazionale Ricerche, the US National Science Foundation and the EUROMARGINS programme.

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Correspondence to Enrico Bonatti.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Table

This table shows average chemical parameters obtained from basaltic glasses sampled along the eastern Romanche Ridge segment. (DOC 42 kb)

Supplementary Discussion

This provides a detailed description of the numerical modelling used. (DOC 2770 kb)

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Further reading

Figure 1: Distribution of Na8 and (H2O)8 in MORB glasses along the axis of the Mid-Atlantic Ridge from Iceland to the Equator.
Figure 2: Geometry of the passive-flow model.
Figure 3: Multibeam topography of the eastern Romanche ridge–transform intersection and predicted melt production beneath the ERRS.
Figure 4: Relationships between melt parameters predicted for MOR melting regimes and values obtained from the basalts sampled along the ERRS axis (Supplementary Table).


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