Feldspar-Ti-oxide metasomatism in off-cratonic continental and oceanic upper mantle

Metasomatism is responsible for enrichment of lithospheric mantle in incomp atible elements. The most common metasomatic products in mantle xenoliths a nd peridotite massifs worldwide are amphibole and mica providing mineral ho sts for alkalies, Ba, Nb. In contrast, xenoliths of spinel peridotite entra ined in basaltic rocks in southern Siberia (Russia) and the Kerguelen islan ds (Indian ocean) have metasomatic aggregates of alkali feldspar and Ti-ric h oxides (rutile, armalcolite, ilmenite) that commonly replace Al-spinel an d orthopyroxene or make up cross-cutting veins. Importantly, the feldspar-r ich aggregates also replace amphibole and mica formed by earlier metasomati c episodes. Armalcolite has not been previously reported in off-cratonic pe ridotite xenoliths. The unusual mineralogical composition of these metasoma tic products defines a specific type of mantle metasomatism, distinct from those commonly attributed to H2O-rich fluids, carbonate melts or Fe-Ti-rich silicate melts. This type of metasomatism operates both in continental and oceanic mantle and may be related to alkali-rich fluids/melts with low wat er activity (probably due to high CO2/H2O ratios). Electron microprobe anal yses have shown that the metasomatic rutile and armalcolite may have up to 4% of Nb2O5 and ZrO2. Feldspar analysed by laser ablation ICP-MS typically has high contents of light REE, Rb, Ba, Sr. Overall, the feldspar-rich meta somatic aggregates may be strongly enriched in incompatible trace elements, with HFSE largely hosted by the Ti-rich oxides. Precipitation of this mine ral assemblage can lead to unusual fractionations among incompatible elemen ts both in the metasomatic assemblage and associated fluids. (C) 1999 Publi shed by Elsevier Science B.V. All rights reserved.