HR: 1340h
AN: T53A-1398    [Abstracts]
TI: Across-arc geochemical variation of basalt in the Izi-Oshima to Niijima cross chan, norhtern Izu arc
AU: * Kimura, J
EM: jkimura@riko.shimane-u.ac.jp
AF: Department of Geoscience, Shimane University, Nishikawatsu 1060, Matsue, 690-8504 Japan
AU: Kent, A J
EM: adam.kent@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-55 United States
AU: Chijiiwa, H
EM: s029207@matsu.shimane-u.ac.jp
AF: Department of Geoscience, Shimane University, Nishikawatsu 1060, Matsue, 690-8504 Japan
AU: Lowe, M
EM:
AF: Department of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-55 United States
AB: The foundations of the Izu-Bonin (Ogasawara)-Mariana (IBM) arc date back ~45 Ma and have been subjected to subduction of the Pacific Plate along the eastern rim of the Philippine Sea Plate. The present IBM arc has been active since ~17Ma and volcanic islands are located both along and across the arc. In the northern Izu Arc five cross-chain islands are aligned obliquely across the arc. In order of increasing distance from the arc these are Izu-Oshima (IO), Toshima (TS), Udonejima (UD), Niijima (NJ), and Kozushima (KZ). Basalt lavas occur on IO, TS, UD and NJ and are all dated (or estimated) as late Pleistocene to recent, and are thus well suited for examination of the present across-arc geochemical variation. We have analyzed twenty basalt lavas (SiO2 = 49-51 wt.%, Mg# = 39-57) from these islands for major and trace elements, and demonstrate a number of interesting across arc geochemical variations. Slab fluid signals, as shown by [Pb/Ce]n, [Sb/Gd]n and [Ba/Th]n are highest in the volcanic front at IO (6.6, 10 and 9.8) and decrease towards the rear arc (1.5-1.9, 1.6-18 and 1.8-2.1), consistent with greatest input from the slab fluid signal in the volcanic front basalts. [La/Yb]n is low in the volcanic front IO (0.6 on average). It increases in TS (3.1), whereas decrease through UD (2.6) to NJ (1.6). [Nb/Yb]n and [Nb/Zr]n behave almost same perhaps reflecting different degree of partial melting. However, these ratios do not exactly correlate each other indicating different slab sediment contribution. Source heterogeneity is unlikely because [Nb/Ta]n and [Zr/Hf]n remain broadly constant over the cross-chain. Overall, slab fluid contribution is maximum in the volcanic front and decreases abruptly in the rear-arc. Apparent degree of partial melting is lower in the rear-arc and contribution of slab sediments may also play a role in subtle chemical variation found in rear-arc basalts. Interpretations based on trace elements will be tested by analyzing Sr-Nd-(Hf)-Pb isotopes.
DE: 8140 Ophiolites (3042)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005