The spectra of isotopic heterogeneities along the mid-Atlantic Ridge Arnaud Agranier a , Janne Blichert-Toft a, * , David Graham b , Vinciane Debaille c , Pierre Schiano c , Francis Albare `de a a Ecole Normale Supe ´rieure, 69007 Lyon, France b Oregon State University, Corvallis OR 97331, USA c Laboratoire Magmas et Volcans, 63038 Clermont-Ferrand, France Received 28 March 2005; received in revised form 2 July 2005; accepted 4 July 2005 Available online 18 August 2005 Editor: R.D. van der Hilst Abstract Due to the advent of large-throughput plasma source mass spectrometers, extensive sets of high-precision Pb, Nd, and Hf isotopic data can now be produced on mid-ocean ridge basalts. A first such set of isotopic results on the mid-Atlantic Ridge is examined here in combination with literature data. The spectra of the data vs. latitude are strongly colored. When combined with conventional scatter plots, they reveal two contrasting types of behavior. The signal of the Icelandic hot spot is clearly identifiable on the spectra of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb/ 204 Pb, the first principal component of Pb isotopes, 87 Sr/ 86 Sr, and 3 He/ 4 He, over length scales of 6–108 (type A spectrum). In contrast, the power decreases in a near-continuous manner for the isotopic signals of Nd and Hf, and for the second Pb isotope principal component (type B spectrum). We interpret type B spectra as a dynamic cascade, in which the size of mantle heterogeneities is continually reduced upon stretching and refolding of the mantle by convection. The power law coefficient of this spectrum has an exponent of  1 indicative of a Batchelor regime and reflects a smooth, but chaotic, probably steady-state flow. Since the isotopic properties of the type B spectra also characterize the EM I+EM II mix of the Dupal anomaly, we infer that they depict the convective northward dispersal (dreeling offT) of the Southern Hemisphere anomalous mantle. D 2005 Elsevier B.V. All rights reserved. Keywords: mid-Atlantic Ridge; MORB; Hf isotopes; Pb isotopes; mantle heterogeneities; mixing; DUPAL; periodograms 1. Introduction Because basalts erupted along mid-ocean ridges (MORB) tap the asthenospheric mantle almost con- tinuously over 50,000 km, their geochemical proper- ties offer a unique representation of the history of a 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.07.011 * Corresponding author. E-mail address: jblicher@ens-lyon.fr (J. Blichert-Toft). Earth and Planetary Science Letters 238 (2005) 96 – 109 www.elsevier.com/locate/epsl