HR: 17:30h
AN: T34A-07    [Abstracts]
TI: Evidence for partial melting in the Earth's asthenosphere
AU: * Takahashi, E
EM: etakahas@geo.titech.ac.jp
AF: Research Center for the Evolving Earth and Planets, Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Hirano, N
EM: nhirano@geo.titech.ac.jp
AF: Research Center for the Evolving Earth and Planets, Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: In the beginning of plate tectonics, the boundary between the lithosphere and the asthenosphere was considered to correspond with the solidus of the Earth's mantle (McKenzie, 1967). The presence of partial melting in the asthenosphere has been supported by the occurrences of the seismic low velocity zone, the high electric conductive layer and the experimentally determined peridotite solidus in the presence of H₂O and CO₂ (e.g., Anderson and Sammis, 1970; Wyllie, 1988). More recently however, the significance of partial melting in the Earth's asthenosphere has been severely criticized based on laboratory measurements on Vs and Qs of mantle material as a function temperature and pressure (e.g. Gribb and Cooper, 2000; Faul and Jackson, 2005). Karato and Jung (1998) proposed that the presence or absence of small amounts of water in olivine is the dominant factor that controls the rheology of the mantle and the presence of partial melting in the asthenosphere is unnecessary. Here we report the evidence for partial melting in the asthenosphere based on detailed fieldwork. Young alkali basalt lava (6 Ma) was found on the 130Ma old subducting Pacific plate (Hirano et al 2001, GRL). We carried out four research cruises using JAMSTEC RVs in the Western Pacific Ocean (144-153°E, 37-40°N) in order to search for the eruption site of the young alkali basalt (Hirano et al, 2005 this meeting). A cluster of small volcanic cones composed of highly vesicular (30-60 vol.%) alkali basalt (0.05 to 1 Ma) was found on flat ocean floor (149.7°E 37.5°N, 6000m deep with 300 m thick sediments). Rock samples recovered from dredges and submersible dives show chemical compositions similar to those in the Hawaiian North Arch volcanic field (Clague et al, 1990; Frey et al., 2000). REE and other trace-elements indicate that these alkali basalt magmas were formed by very small degrees of partial melting at >100km depth. Although the distribution of the studied volcanic cones is presently limited (100×100 km), ocean bottom surveys with Seabeam and back-scattered images show evidence for similar young volcanic fields at several localities on the Pacific plate. We therefore conclude that these alkali basalts erupted from asthenosphere due to the flexure and fracturing of the Pacific plate near the trench (Hirano et al., 2001). According to McKenzie et al. (2005), the lateral temperature gradient in the asthenosphere underneath the Pacific plate is very small and the estimated temperature at the base of the plate (100-150 km depth) in our studied area ranges between 1300 to 1350°C. The high vesicularity of the recovered samples indicates the presence of at least 3-5 wt.% of CO₂ and H₂O in the magma. Because the solidus of mantle peridotite at 100-150 km depth is lower than 1300°C in the presence of CO₂ and H₂O, it is expected that a small amount of alkali basalt magma is ubiquitous in the asthenosphere underneath oceanic lithosphere.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3075 Submarine tectonics and volcanism
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005