HR: 14:10h
AN: U33B-03    [Abstracts]
TI: Lithospheric thickness, preservation of recycled oceanic lithosphere and the importance of water
AU: * Lee, C A
EM: ctlee@rice.edu
AF: Dept Earth Science, Rice University, Houston, TX 77005
AU: Lenardic, A
EM: adrian@rice.edu
AF: Dept Earth Science, Rice University, Houston, TX 77005
AB: It has been hypothesized that the viscosity structure of the mantle might be influenced by spatial variations of water content in the mantle. Thus, to what extent does the presence (or lack) of water play a role in controlling the dynamic and chemical evolution of Earth and other terrestrial planets? Fundamental to this question is how chemical differentiation processes control the distribution of water in the mantle. One place where dehydration of the mantle may be occurring is during the melting process associated with passive upwelling beneath mid-ocean ridges, which leaves a mantle residue that is depleted in meltable components and likely dehydrated. This depleted and dehydrated residuum manifests itself as a chemical and rheologic boundary layer, whose thickness should correlate with mantle potential temperature. Here, we suggest that the viscosity of this dehydrated chemical boundary layer would be high enough that vertical heat transfer through the boundary layer might occur only by conduction and not by convection. We further show that its thickness beneath oceans may limit the thickness of the oceanic thermal boundary layer, which includes the strong chemical boundary layer and the underlying convective sublayer. The presence of a depleted and dehydrated mantle residuum layer may also explain why the thickness of continents is lower than that predicted by simple conductive cooling arguments. If these conclusions are correct, scaling arguments and model results suggest that even after thermal re-equilibration with the ambient mantle, large segments of subducted oceanic lithosphere could be preserved for much longer times than predicted if oceanic lithospheric mantle was assumed to be hydrated and hence isoviscous with the ambient mantle. This implies further that there would be segments of preserved oceanic lithosphere throughout the mantle and that such lithosphere (crust + depleted residuum) might be occasionally sampled in plumes. We show using the first series transition metals (Fe, Mn, Ni, Co, Sc) that the oceanic lithospheric mantle, e.g., the complement to the oceanic crust, may indeed be a component in the source regions of many hotspot magmas. This is based on the fact that most trace-element and isotopic tracers are sensitive to crustal components, but the transition metals are sensitive to the mantle residuum due to their moderately incompatible to compatible behaviors. Compared to MORBs, most hotspot magmas have low Sc/Mn ratios and high (Fe, Co,Ni)/Mn ratios, which cannot be explained by high degree partial melting of a fertile mantle source. Instead, Sc/Mn and (Fe,Co,Ni)/Mn ratios respectively decrease and increase in peridotite residues as melt is extracted. Re-melting these depleted residues produce melts having metal systematics remarkably similar to those of hotspot magmas. One component in hotspot source regions may thus be represented by highly melt-depleted mantle, which we interpret to be recycled oceanic lithospheric mantle. These interpretations are consistent with the preservation of coherent sections of oceanic lithosphere in the mantle for long times and that dehydration of oceanic lithosphere may be a requisite for preservation. If the above interpretations and arguments are correct, it seems that water must play an important role in the dynamics and chemical differentiation of the Earth. A combination of seismology, geodynamics and geochemistry/petrology is needed to expand our understanding of how water influences mantle dynamics and chemical evolution.
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