HR: 13:40h
AN: V23D-01    [Abstracts]
TI: Large-throughput, Precise Mass Spectrometers and new Constraints on Mantle Dynamics
AU: * Albarede, F
EM: albarede@ens-lyon.fr
AF: Ecole Normale Superieure, 46 Allee d'Italie, Lyon, 69007 France
AB: The advent of large-throughput MC-ICP-MS is fast paving the way for the production of much larger sets of high-precision Sr, Pb, Nd, and Hf isotopic data than possible just a few years ago. In particular, mid-ocean ridges can now be covered with a dense array of data of consistently high quality, which, in turn, allows spectral methods to be employed, thereby bridging the gap with modern physics of mixing and mantle geodynamics. A first such set of isotopic results on the Mid-Atlantic Ridge from 78 degrees north to 55 degrees south (Agranier et al., EPSL, in press) is examined here in combination with literature data. The spectra of the data vs. latitude are strongly colored. Two contrasting types of behavior are observed: 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 ⁴He/³He, over length scales of 6-10 degrees (type A spectrum). In contrast, the power decreases in a near-continuous manner for the isotopic signals of Nd and Hf, and 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 `convective' regime, in which the variance of tracer distributions flows down the wavelength scale in proportion to the power present at each wavelength. This regime reflects a smooth, yet chaotic flow at steady-state in which heterogeneities are continuously produced at long wavelengths by subduction. Regrettably, sampling along mid-ocean ridges is still not dense enough to identify at the upper end of the spectra the typical wavelength at which melting processes starts reducing local variance. We argue that increasing the isotopic coverage of mid-ocean ridges and extending it to the whole ridge system should reveal which parts of the observed isotopic heterogeneities can be assigned to lagrangian stirring on the one hand, and to the presence of sinking plates and lithospheric rafts on the other hand. Such an endeavor should also shed light on the transient vs. steady-state nature of these anomalies, and help us understand better the scale distribution of melting processes. [This work is dedicated to Stan Hart, who already 35 years ago understood the potential of radiogenic isotopes in oceanic basalts as a geodynamic tool.]
DE: 1038 Mantle processes (3621)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005