HR: 08:45h
AN: T41G-04 INVITED     [Abstracts]
TI: Oceanic Intraplate Volcanism: a consequence of mantle dynamics at scales small compared to deep mantle plumes and plates.
AU: * Parmentier, E
EM: EM_Parmentier@brown.edu
AF: Brown University, Department of Geological Sciences, Box 1846, Providence, RI 02912 United States
AB: Intraplate volcanism occurs at spatial and temporal scales not explained by relatively stationary, long lived hotspots that generate age progressive volcanic chains like Hawaii, which may be due to upwelling in plumes from the deep mantle. This small scale intraplate volcanism is most abundant on the Pacific plate and occurs over wide areas in the form of a) short volcanic chains aligned with plate motion along which volcanism persists for only 1-10 Myr and b) volcanic ridges, some of which are known to propagate at rates fast compared to plate motions. Recent intraplate volcanic activity is particularly high within the South Pacific Superswell, an area of relatively shallow seafloor for its age beneath which global seismic tomography indicates a large scale region of low seismic velocity. If low seismic velocity corresponds to low density, a large scale mantle upwelling is predicted. Radial seismic anisotropy is particular strong beneath the Pacific plate, and regions of faster horizontal than vertical shear wave velocity appear to correlate spatially with the abundance of seamounts. Several mechanisms to explain small scale volcanism have been suggested. One possibility may be thermal plumes that develop at the top of a hotter, but chemically more dense lower mantle upwelling (e.g. Davaille, et al., EPSL, 2002). Alternatively in regions where the mantle is initially at its melting temperature, decompression melting in upwellings that result from buoyancy associated with melting itself may occur (e.g. Raddick, et al., JGR, 2002). This spontaneous buoyant decompression melting, which might be termed "magmatic convective storms", should be limited by accumulation of chemically buoyant and creep resistant mantle depleted in Fe/Al and water, respectively, as melt is extracted. Modelling studies indicate that this mechanism can plausibly explain volcanism with the volumes and time scales observed. Water is much more soluable in the transition zone than in lower pressure upper mantle phases. Thus, an alternative mechanism currently being considered is the release of water due to the upwelling of water-rich transition zone mineral phases. Rapidly propagating volcanic ridges may be due to the triggering of buoyant melting by initial upwelling caused by small amounts of plate extension, an amount too small to explain melting due to the upwelling associated with extension alone. Alternatively, rapidly propagating volcanic ridges may be explained by fingering instabilities as horizontally moving low viscosity mantle displaces higher viscosity mantle (Weeraratne, et al., Fall AGU, 2003). Seismic observations must continue to play a major role in examining these possible mechanisms of intraplate volcanism.
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting