REE behavior in warm and cold subducting oceanic crust

Abstract

In situ laser probing of minerals in a lawsonite-bearing eclogitic metabasite and two epidote-bearing eclogites reveals metamorphic reaction-controlled REE mobility and redistribution. In the lawsonite-bearing eclogitic metabasite, the garnet shows typical core-to-rim HREE depletion and a Tb-Er + Y enriched outer rim. Inclusions across the garnet reveal that formation of the rim coincided with the disappearance of epidote and titanite and the appearance of sodic pyroxene and rutile, possibly representing the blueschist-eclogite facies transition. The lawsonite is characterized by a flat REE pattern in the core but its rim shows remarkable HREE depletion due to garnet nucleation. In contrast, in the epidote-bearing eclogite, lawsonite is found only as inclusions in the garnet and was otherwise consumed along the prograde path. The garnet outer rim is characterized by MREE (Sm-Tb) enrichment caused by the lawsonite breakdown, while omphacite and rutile were stable, suggesting eclogite-facies dehydration. Thus, in warm subduction zones, the LREE may be largely released at shallow depths due to lawsonite breakdown and fluxed into the hydrated mantle wedge, where they can contribute to arc volcanism. In cold subduction zones, however, some LREE and MREE are retained in the slab and released at depths well beyond the arc; element flux to the sub-arc mantle in such subduction zones may be dominated by other sources, such as dehydration of the serpentinized part of the slab.