HR: 16:45h
AN: U34A-04    [Abstracts]
TI: The survival of geochemical mantle heterogeneities
AU: * Albarede, F
EM: albarede@_ns-lyon.fr
AF: Ecole Normale Superieure, 46 allee d'Italie, Lyon, 69007 France
AB: The last decade witnessed major changes in our perception of the geochemical dynamics of the mantle. Data bases such as PETDB and GEOROC now provide highly constrained estimates of the geochemical properties of dominant rock types and of their statistics, while the new generation of ICP mass spectrometers triggered a quantum leap in the production of high-precision isotopic and elemental data. Such new advances offer a fresh view of mantle heterogeneities and their survival through convective mixing. A vivid example is provided by the new high-density coverage of the Mid-Atlantic ridge by nearly 500 Pb, Nd, and Hf isotopic data. This new data set demonstrates a rich harmonic structure which illustrates the continuing stretching and refolding of subducted plates by mantle convection. Just as for oceanic chemical variability, the survival of mantle geochemical heterogeneities though mantle circulation can be seen as a competition between stirring and renewal. The modern residence (renewal) times of the incompatible lithophile elements in the mantle calculated using data bases vary within a rather narrow range (4-9 Gy). The mantle is therefore not currently at geochemical steady-state and the effect of its primordial layering on modern mantle geochemistry is still strong. Up to 50 percent of incompatible lithophile elements may never have been extracted into the oceanic crust, which generalizes a conclusion reached previously for $^{40}$Ar. A balance between the buoyancy flux and viscous dissipation provides frame-independent estimates of the rates of mixing by mantle convection: primordial geochemical anomalies with initial length scales comparable to mantle depths of plate lengths are only marginally visible at the scale of mantle melting underneath mid-ocean ridges ($\approx$~50~km). They may show up, however, in hot spot basalts and even more in melt inclusions. Up to 50 percent primordial material may be present in the mantle, but scattered throughout as small ($<$~10~km) domains, strongly sheared and refolded, and interlayered with younger recycled material. The exploration of the fine-scale geochemical structure of the mantle and the quest for preserved remnants of very old mantle arise as the strongest priorities of deep Earth geochemistry.
DE: 8130 Heat generation and transport
DE: 7207 Core and mantle
DE: 3919 Equations of state
DE: 1212 Earth's interior--composition and state (8105)
DE: 1507 Core processes (8115)
SC: Union [U]
MN: 2004 AGU Fall Meeting