HR: 0800h
AN: V41D-0820    [Abstracts]
TI: Ancient mantle contribution to boninitic magma: evidence from Os isotopic compositions of Cr-spinel sands of boninites in Izu-Bonin arc
AU: Suzuki, K
EM: katz@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061, Japan
AU: Senda, R
EM: rsenda@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061, Japan
AU: * Shimizu, K
EM: shimmy@jamstec.go.jp
AF: IFREE, JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061, Japan
AB: Osmium (Os) isotope is a sensitive tracer of crust and sediment, because of significant contrast between a high Os isotope ratio of crust and sediments (¹⁸⁷Os/¹⁸⁸Os>0.5) and a low ratio of mantle (¹⁸⁷Os/¹⁸⁸Os<0.15). Therefore, Os isotope provides information on slab component contribution to the mantle source of island arc lavas. Radiogenic Os isotopic compositions are commonly found in volcanic lavas (Alves S., Schiano P., Capmas F. & Allegre C. J., (2002) Earth Planet. Sci. Lett. 198, 355-369)and peridotite xenoliths (Brandon A. D., Becker, H., Carlson, R. W. & Shirey S. B., (1999), Chem. Geol. 160, 387-407) in a subduction zone setting. Though most of the authors had attributed these high ¹⁸⁷Os/¹⁸⁸Os to the input of the slab component to the mantle source, some pointed out that assimilation of crustal materials during magma ascent possibly reproduce the elevated Os isotope ratios of the arc lavas (Lassiter J. C. & Luhr, J. F., (2001), Geochem. Geophys. Geosyst. 2, 2000GC000116). Since then, whether the high Os isotope ratios of arc lavas are caused by contribution of slab component or by assimilation of the overlying crust has long been highly debated (Woodhead J. & Brauns M., (2004), Earth Planet. Sci. Lett. 221, 309-323). Here we report the unradiogenic Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os < 0.125) of Cr-spinel sands from Chichi-Jima and Yome-Shima boninites, Izu-Bonin arc. As Cr-spinel is resistant to later alteration and weathering and, more importantly, is the early stage crystal in the fractional crystallization, it preserves the chamical and isotopic compositions of very primitive magma in its melt inclusion and spinel itself without any later stage crustal contamination. The obtained Os isotopic compositions are the lowest among the previously reported ¹⁸⁷Os/¹⁸⁸Os of arc magma. The unradiogenic Os isotopic compositions of boninite Cr-spinels obtained in this study are in the range of anciently depleted lithospheric mantle (Parkinson I.L., Hawkesworth C.J. and Cohen A.S. (1998) Science 281, 2011-2013) which strongly suggests that such ancient mantle contribute significantly to boninite magma generation.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 8185 Volcanic arcs
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting