Na–bearing tremolites as reservoirs of fluid–mobile elements in the mantle wedge: inference from the Ochiai–Hokubo complex (Southwest Japan) in high–P schists

Abstract

We examined the importance of tremolites as reservoirs of LILE (large–ion lithophile elements) in the mantle wedge. The Ochiai–Hokubo complex in the Suo metamorphic rocks of greenschist to blueschist facies, Japan, is mainly composed of depleted lherzolites, which commonly contain tremolite. The complex is concentrically zoned in terms of degree of hydration and metamorphic mineral assemblage, showing two mineral zones, tremolite + olivine and clinopyroxene + antigorite from the center outward. The inner tremolite–bearing lherzolite is enriched with some LILE such as Na and Cs, which are stored mainly in tremolites (with up to 4 wt% Na₂O) and subordinately in chlorites. Relic primary clinopyroxenes are intact from the metasomatism. The fluids liberated from the high–P schists performed this modification on the lherzolites on progressive metamorphism, thus closely mimicking the mantle metasomatism by slab–derived fluids.