HR: 0800h
AN: V51B-0522    [Abstracts]
TI: Melting Behaviors of Recycled Subducted Crust in the Mantle: Constraints from Melting Phase Relations of Pyroxenites
AU: * Kogiso, T
EM: kogisot@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: Univ. Minnesota, 310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
AB: Ocean island basalts (OIB) have isotopic and trace element signatures that are considered to originate in subducted mafic oceanic crust. Recent experimental studies on mafic lithologies (pyroxenites) revealed that diversity in major element composition of OIB can also be explained by partial melting of a variety of pyroxenites derived from subducted mafic crust. Most part of subducted oceanic crust has MORB-like (silica-excess) composition, but the uppermost part of the subducted oceanic crust suffers extraction of small degree partial melts or siliceous hydrous fluids during subduction, resulting in formation of bimineralic pyroxenite which consists solely of garnet and clinopyroxene. Partial melts from MORB-like pyroxenite at high pressures have silica-saturated compositions, which have similarities to silica-rich tholeiitic OIB [1], whereas partial melts from bimineralic pyroxenite have silica-undersaturated compositions similar to alkalic OIB lavas [2]. On the other hand, subducted oceanic crust is hybridized with surrounding mantle peridotite through mechanical and diffusive interactions with peridotite, yielding olivine-bearing pyroxenite. To investigate melting behaviors of such olivine-bearing pyroxenite, we have conducted partial melting experiments on a mixture of bimineralic eclogite (B-ECL1) with 5 % Fo60 olivine (bulk Mg\# = 61) at 5 GPa. The experimental results show that addition of olivine to B-ECL1 expands the stability of garnet in the residue, resulting in producing liquids richer in SiO$_{2}$ and alkalis and poorer in Al$_{2}$O$_{3}$ than those of B-ECL1. At $1600\deg$C, the degree of partial melting of the B-ECL1 + olivine mixture is higher than that of B-ECL1, suggesting that the solidus temperature is lowered by addition of olivine. However, experiments on a Mg-rich pyroxenite (MIX1G: bulk Mg\# = 79), which has more olivine component, have shown that the solidus of MIX1G is higher than $1600\deg$C [3]. These results suggest that bimineralic pyroxenite can produce larger amounts of partial melts when it is mixed with smaller amounts of olivine. This further indicates that upwelling mantle containing recycled crust with bimineralic composition would begin to melt at the boundary between pyroxenite and peridotite, producing strongly alkalic liquids first. [1] Pertermann and Hirschmann 2003, J.Petrol. 44, 2173. [2] Kogiso and Hirschmann 2002, EOS 83, V71C-12. [3] Kogiso et al. 2003, EPSL 216, 603.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting