HR: 0800h
AN: V31A-1427    [Abstracts]
TI: Does Fluid-Induced Eclogitization of Subducted Lower Oceanic Crust Produce the Slab Component of Arcs?
AU: * Schenk, V
EM: vs@min.uni-kiel.de
AF: Institut fuer Geowissenschaften and SFB 574, Universitaet Kiel, Olshausenstr. 40, Kiel, 24098 Germany
AU: John, T
EM: tj@min.uni-kiel.de
AF: Institut fuer Geowissenschaften and SFB 574, Universitaet Kiel, Olshausenstr. 40, Kiel, 24098 Germany
AB: To investigate fluid-induced transformation processes and associated trace element mobilization, co-genetic gabbros and eclogites of Zambia have been used. The rocks represent relics of subducted lower oceanic crust and gradual stages of the prograde gabbro-to-eclogite transformation are preserved by disequilibrium textures of incomplete reactions. No evidence for prograde blueschist- or amphibolite-facies mineral assemblages was found in the eclogites. Instead, fine-grained intergrowths of eclogite-facies minerals replacing plagioclase indicate the direct eclogitization of gabbroic precursors. Eclogitization occurred at 630-690øC and 2.6-2.8GPa and was accompanied by a channelized fluid flow that produced veins of the peak metamorphic assemblage. Evidences for aqueous fluids with variable salinities, in cases up to brine compositions, were found. Although all of the mafic rocks were subducted, only those gabbros that were infiltrated by fluids were eclogitized. Hence, the eclogites and their veins represent relict fluid pathways through subducted oceanic crust, providing direct evidence of channelized fluid flow within a slab. The gabbros and eclogites have MORB-like trace element patterns and initial Nd and Hf isotope compositions. In some eclogites, however, the LREE have been strongly fractionated from the HFSE and HREE, an effect that cannot be of magmatic origin but must have occurred during metamorphism. Eclogitization was limited by fluid availability, and the fluid flow through the rock is the most likely mechanism for LREE fractionation. Model fluid-rock ratios reveal that the fractionated rocks reacted with an amount of fluid up to 80% of their mass to create the most depleted REE patterns. The lower gabbroic part of the oceanic crust is an unlikely source for such a large volume of fluid and thus we hypothesise that the fluid originated in the underlying serpentinised lithospheric mantle. If, after triggering eclogitization, the resulting LREE-rich, HFSE+HREE-poor slab fluid reaches the zone of partial melting in the mantle wedge, it may contribute significantly to the arc signature. We will evaluate whether the trace element mobilization during fluid-induced eclogitization could be generally responsible for producing the slab component in arc magmas.
DE: 8045 Role of fluids
DE: 3660 Metamorphic petrology
DE: 3924 High-pressure behavior
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting