The spectra of isotopic heterogeneities along the mid-Atlantic Ridge

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 206Pb/ 204Pb, 207Pb/ 204Pb, 208Pb/ 204Pb, the first principal component of Pb isotopes, 87Sr/ 86Sr, and 3He/ 4He, over length scales of 6–10° (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 (‘reeling off’) of the Southern Hemisphere anomalous mantle.