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Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume


The compositional differences between mid-ocean-ridge and ocean-island basalts place important constraints on the form of mantle convection1,2. Also, it is thought that the scale and nature of heterogeneities within plumes and the degree to which heterogeneous material endures within the mantle might be reflected in spatial variations of basalt composition observed at the Earth's surface. Here we report osmium isotope data on lavas from a transect across the Azores archipelago which vary in a symmetrical pattern across what is thought to be a mantle plume. Many of the lavas from the centre of the plume have lower 187Os/188Os ratios than most ocean-island basalts and some extend to subchondritic 187Os/188Os ratios—lower than any yet reported from ocean-island basalts. These low ratios require derivation from a depleted, harzburgitic mantle, consistent with the low-iron signature of the Azores plume. Rhenium-depletion model ages extend to 2.5 Gyr, and we infer that the osmium isotope signature is unlikely to be derived from Iberian subcontinental lithospheric mantle. Instead, we interpret the osmium isotope signature as having a deep origin and infer that it may be recycled, Archaean oceanic mantle lithosphere that has delaminated from its overlying oceanic crust. If correct, our data provide evidence for deep mantle subduction and storage of oceanic mantle lithosphere during the Archaean era.

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Figure 1: Map of the Azores archipelago showing bathymetry and islands sampled (a), along with the average 187Os/188Os and 87Sr/86Sr ratio for each island plotted along a northwest–southeast transect across the plume (b).
Figure 2: Fe2O3 content versus 187Os/188Os ratio, showing the broad positive correlation (r2 = 0.3) in the Azores lavas.
Figure 3: Plot of 187Os/188Os versus 206Pb/204Pb for the Azores lavas (including data from ref. 12).


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We thank W. White for supplying samples from Flores and Corvo, M. Gilmour for assistance with the analytical work, and A. Hofmann for comments on the manuscript. This work was supported by the NERC (N.W.R. C.J.H. and I.J.P.) and by a Royal Society university research fellowship (S.P.T.).

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Correspondence to Simon Turner.

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Schaefer, B., Turner, S., Parkinson, I. et al. Evidence for recycled Archaean oceanic mantle lithosphere in the Azores plume. Nature 420, 304–307 (2002).

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