HR: 15:10h
AN: V53E-07    [Abstracts]
TI: Trace Element Transfer In The Mantle Wedge: Evidence From Polyphase Inclusions In Garnet-Pyroxenites (Dabie-Shan, China)
AU: * Malaspina, N
EM: malaspina@dipteris.unige.it
AF: Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Dipartimento per lo Studio del Territorio e delle sue Risorse, Universit… di Genova, Genova, 16132 Italy
AU: Hermann, J
EM: joerg.hermann@anu.edu.au
AF: Research School of Earth Sciences, ANU, Mills Road, Canberra, ACT 0200 Australia
AU: Scambelluri, M
EM: marco.scambelluri@dipteris.unige.it
AF: Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Dipartimento per lo Studio del Territorio e delle sue Risorse, Universit… di Genova, Genova, 16132 Italy
AU: Compagnoni, R
EM: roberto.compagnoni@unito.it
AF: Dipartimento di Scienze Mineralogiche e Metrologiche, Università di Torino, via Valperga Caluso, 35, Torino, 10125 Italy
AB: Ultrahigh-pressure (UHP) associations of felsic and ultramafic rocks are natural laboratories to study element exchange between crust- and mantle-like systems at sub arc depths. We investigated garnet- orthopyroxenites from the Maowu ultramafic complex (Dabie Shan, China), hosted by UHP garnet-coesite gneisses. The pyroxenites have a peak assemblage of orthopyroxene (Opx2)+garnet (Grt2) ±clinopyroxene. Relic olivine and orthopyroxene (Opx1), associated with fine-grained garnet (Grt1), are included in Opx2. This indicates that Opx2 overgrows a previous garnet-harzburgite assemblage. The bulk rock Mg# and the high Ni content indicate that the orthopyroxenites derive from former mantle peridotites. The REE patterns are comparable with the ones of mantle rocks: an enrichment in LREE is their main distinctive feature. Such an enrichment is recorded by Opx2, which also contains appreciable Ba. The observed features indicate that the pyroxenites derive from harzburgite by the addition of a SiO2, Al2O3 and LREE-rich agent, most likely a hydrous granitic melt, sourced from the associated crustal rocks. This metasomatism occurred at UHP conditions (4.0-6.0GPa, 700-750°C) and produced porphyroblastic Ni- and LREE-enriched Opx2 and inclusion-rich Grt2. Polyphase inclusions found in Grt2 have negative crystal shapes and constant volume proportions of the infilling minerals (hydrous phases and 10-20vol% of opaque). We were able to re-homogenise these inclusions in a piston cylinder experiment at 3.5GPa and 900°C to a porous quench material. This indicates that garnet originally trapped a solute-rich aqueous fluid. The trace element patterns of the multiphase and of the experimentally re-homogenised inclusions are enriched in LILE, with spikes of Cs, Ba, and Pb, high U/Th ratios, and LREE. Such a signature suggests that the trapped fluid derived from crustal reservoirs and is very similar to the incompatible element enrichment observed in arc lavas. Absence of LILE in the whole rocks implies that the residual fluid largely escaped the system. The metasomatised ultramafic rocks represent a proxy for the trace element transfer from subducted crustal rocks to the mantle wedge at sub-arc depths. Hydrous granitic melts, likely produced by subducted sediments, will react with mantle wedge peridotites. The reaction produces a metasomatic LREE-enriched garnet-orthopyroxenite layer plus a residual aqueous fluid which concentrates the LILE and transports a crustal signature to the locus of partial melting within the mantle wedge.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005