Abstract

To unravel the rare earth element (REE) redistribution during high-pressure–low-temperature metamorphism, we have analyzed by ion microprobe all the minerals from representative Fe-gabbros from Ligurian metaophiolites (northwestern Italy). Contrary to what is observed for fresh Fe-gabbros, the clinopyroxene contribution to the whole-rock REE inventory of blueschists and eclogitized Fe-gabbros is minor or negligible. In both blueschists and eclogites, REE are redistributed among newly formed minerals. In blueschists, lawsonite is the major REE carrier and concentrates almost all the light REE (LREE) in the rock, whereas titanite plays an important role for middle and heavy REE (MREE, HREE). In eclogites, LREE and HREE mainly reside in accessory allanite and garnet, respectively. The development of blueschist and eclogite facies mineral assemblages in mafic oceanic crust, due to subduction to 35–65 km depths, was not accompanied by release of significant amounts of LREE to the overlying mantle wedge. Owing to the stability of lawsonite and epidote at ultra-high-pressure conditions, subduction of mafic rocks is considered an effective mechanism to bring LREE to great depth within the mantle.

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