HR: 08:00h
AN: MR31D-01 INVITED    [Abstracts]
TI: Localization Mechanisms of Melting and Melt Migration in the Mantle Wedge at Convergent Plate Boundaries
AU: * Parmentier, E M
EM: em_parmentier@brown.edu
AF: Department of Geological Sciences, Brown University, Providence, RI 02912, United States
AU: Cagnioncle, A
EM: amandine@brown.edu
AF: Department of Geological Sciences, Brown University, Providence, RI 02912, United States
AB: Along-strike magmatic segmentation with a 50-100 km scale has been recognized for several convergent plate boundaries, including the Aleutians, the Cascades (Marsh, 1979), and Japan (Tamura et al., 2002). Mantle wedge seismic velocity structure beneath Japan is also segmented with low seismic velocity columns beneath volcanic segments. Identifying plausible buoyant flow mechanisms that may lead to magmatic segmentation is thus an important aspect of understanding melting and melt migration in these settings. Reaction infiltration instability during melting has been identified as a possible mechanism of melt channelization (Aharonov et al. 1995; Spiegelman et al., 2001), but the predicted scale of several compaction lengths (1-200 m) appears to be too small for this mechanism alone to explain the observed segmentation. Thermal convection in the mantle wedge (Honda and Yoshida, 2005) may be driven by cool downwellings ~100 km in scale that develop at the base of the overriding plate. Decompression melting is thus localized in upwelling regions between the cool downwelling sheets aligned with plate motion. Convective flow in the mantle wedge of an appropriate scale might also be driven by instability of thickened crust beneath the magmatic arc (Behn et al., 2007), but a direct connection of this mechanism with melt production has not yet been explored. Instability may also result from buoyancy that develops along the top of the downgoing plate. Marsh and Carmichael (1974) proposed that melt generated along the top of the downgoing plate ascended diapirically; however recent evidence does not generally favor melting of the downgoing plate. Dehydration of the plate however releases hydrous fluid that should rise buoyant into the overlying mantle. Buoyant solid flow may hence develop due to the presence of hydrous fluid or low density serpentine (and other hydrous phases) formed in cool mantle just above the top of the downgoing plate. Serpentine formation involves a significant volume increase and serpentenized mantle is therefore less dense than dry mantle containing an equivalent amount of free water. Exothermic serpentine formation is limited by the heat of reaction. However, decreasing pressure in buoyantly ascending mantle allows the continuing formation of serpentine, thus leading to buoyant instability. Ongoing theoretical studies are exploring the conditions required for instability, the wavelength at which it would occur, and the physical properties of mantle materials that would make it possible.
DE: 8145 Physics of magma and magma bodies
DE: 8185 Volcanic arcs
DE: 8434 Magma migration and fragmentation
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting