HR: 08:55h
AN: V41G-04    [Abstracts]
TI: Non-hotspot volcano chains originating from small-scale sublithospheric convection
AU: * Ballmer, M D
EM: ballmer@tomo.ig.erdw.ethz.ch
AF: Institute of Geophysics, ETH Zurich, Zurich, 8093, Switzerland
AU: van Hunen, J
EM: jeroen.van-hunen@durham.ac.uk
AF: Univ.of Durham, Durham, DH1, 3LE, United Kingdom
AU: Ito, G
EM: gito@hawaii.edu
AF: SOEST, Univ.of Hawaii, Honolulu, HI 96822, United States
AU: Tackley, P J
EM: ptackley@ethz.ch
AF: Institute of Geophysics, ETH Zurich, Zurich, 8093, Switzerland
AU: Bianco, T A
EM: tbianco@hawaii.edu
AF: SOEST, Univ.of Hawaii, Honolulu, HI 96822, United States
AB: Although most of the intraplate volcanism in ocean basins is expressed in linear chains, not all of these can be attributed to a stationary hotspot. Many ridges do not show linear age progressions as predicted by this model (e. g., Cook-Austal, Magellan or Line Islands, and Pukapuka ridges). The well-studied Pukapuka ridges reside among other short-lived seamount-chains in the eastern part of the South Pacific Superswell. They are aligned by plate motion and by topography and gravity lineations with a wavelength of ~200 km. In order to account for these observations, three types of models have been put forward yet: lithospheric cracking, channelized return flow, and SSC. Gravity and tomography studies have rejected the lithospheric cracking model [Harmon et al., 2007], which furthermore presumes a reservoir of pre-existing partial melt in the asthenosphere. Channelized return flow might be a good explanation for the volcanism and the associated density anomalies. But it fails to explain, why many of the lineations are not associated with volcanism at all. The SSC-hypothesis is instead able to explain the gravity data and the volcanism. In the Earth's uppermost mantle SSC is likely to develop due to instabilities of the thickened thermal boundary layer below mature oceanic lithosphere (usually ~70 Ma). It is characterized by convective rolls aligning plate motion. Their onset is earlier (i.e. beneath younger and thinner lithosphere) for lower mantle viscosities (e.g. for hot or wet mantle) or adjacent to lateral thermal or compositional heterogeneity. In these cases, partial melt potentially emerges in the upwelling limbs of SSC. Partial melting changes the compositional buoyancy owing to melt retention and depletion of the residue. Therefore, it promotes upwelling and further decompression melting. In this study, we take the step towards fully thermo-chemical 3D-numerical models of SSC (using the FEM-Code CITCOM) with a realistic, temperature- and depth-dependent rheology in order to quantitatively test the SSC-hypothesis on intraplate volcanism. We explore the 3D-patterns of melting associated with SSC, the age of seafloor over which it occurs, and the rates of melt generation by varying the key parameters mantle viscosity and temperature, T_m. We also investigate the effect of lateral heterogeneity that locally reduces the onset age of SSC, and the effect of a rheology dependent on water and melt content. Melting due to SSC is predicted to emerge in elongated features (~750 km) parallel to plate motion and not just at a fixed spot. Therefore, irregular age progressions of the associated volcanism are predicted - contrary to the hotspot model. The seafloor age at which volcanism occurs is sensitive to T_m. For moderate T_m (1350 °C), volcanism develops beneath a relatively young lithosphere (~30 Myr), and higher T_m retards the onset of SSC and volcanism because of the stabilizing influence of a thicker residue from previous mid-ocean ridge melting (e. g., ~50 Myr for T_m=1410 °C). Mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism. Effective viscosity required to obtain km-high seamounts is ~2·10¹⁹ Pa·s, or significantly lower if stiffening due to exhaustion of water is considered. Our calculations predict many of the key observations of the Pukapuka ridges, and the volcano groups associated with the Cook-Austral, Line and Marshall Islands.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8148 Planetary volcanism (5480, 8450)
DE: 8178 Tectonics and magmatism
DE: 8415 Intra-plate processes (1033, 3615)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting