Hafnium isotopic variations in East Atlantic intraplate volcanism
Résumé
The broad belt of intraplate volcanism in the East Atlantic between 25A degrees and 37A degrees N is proposed to have formed by two adjacent hotspot tracks (the Madeira and Canary tracks) that possess systematically different isotopic signatures reflecting different mantle source compositions. To test this model, Hf isotope ratios from volcanic rocks from all individual islands and all major seamounts are presented in this study. In comparison with published Nd isotope variations (6 epsilon Nd units), (176)Hf/(177)Hf ratios span a much larger range (14 epsilon Hf units). Samples from the proposed Madeira hotspot track have the most radiogenic Hf isotopic compositions ((176)Hf/(177)Hf(m) up to 0.283335), extending across the entire field for central Atlantic MORB. They form a relatively narrow, elongated trend on the Nd vs. Hf isotope diagram (stretching over > 10 epsilon Hf units) between a depleted N-MORB-like endmember and a moderately enriched composition located on, or slightly below, the Nd-Hf mantle array, which overlaps the proposed "C" mantle component of Hanan and Graham (1996). In contrast, all samples from the Canary hotspot track plot below the mantle array ((176)Hf/(177)Hf(m) = 0.282943-0.283067) and form a much denser cluster with less compositional variation (similar to 4 epsilon Hf units). The cluster falls between (1) a low Hf isotope HIMU-like endmember, (2) a more depleted composition, and (3) the moderately enriched end of the Madeira trend. The new Hf isotope data confirm the general geochemical distinction of the Canary and Madeira domains in the East Atlantic. Both domains, however, seem to share a common, moderately enriched endmember that has "C"-like isotope compositions and is believed to represent subducted, < 1-Ga-old oceanic lithosphere (oceanic crust and possibly minor sediment addition). The lower (176)Hf/(177)Hf ratio of the enriched, HIMU-like Canary domain endmember indicates the contribution of oceanic lithosphere with somewhat older recycling ages of a parts per thousand yen1 Ga