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

摘要

Metasomatism is responsible for enrichment of lithospheric mantle in incompatible elements. The most common metasomatic products in mantle xenoliths and peridotite massifs worldwide are amphibole and mica providing mineral hosts for alkalies, Ba, Nb. In contrast, xenoliths of spinel peridotite entrained in basaltic rocks in southern Siberia (Russia) and the Kerguelen islands (Indian ocean) have metasomatic aggregates of alkali feldspar and Ti-rich oxides (rutile, armalcolite, ilmenite) that commonly replace Al-spinel and orthopyroxene or make up cross-cutting veins. Importantly, the feldspar-rich aggregates also replace amphibole and mica formed by earlier metasomatic episodes. Armalcolite has not been previously reported in off-cratonic peridotite xenoliths. The unusual mineralogical composition of these metasomatic products defines a specific type of mantle metasomatism, distinct from those commonly attributed to H_2O-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 water activity (probably due to high CO_2/H_2O ratios). Electron microprobe analyses have shown that the metasomatic rutile and armalcolite may have up to 4% of Nb_2O_5 and ZrO_2. Feldspar analysed by laser ablation ICP-MS typically has high contents of light REE, Rb, Ba, Sr. Overall, the feldspar-rich metasomatic aggregates may be strongly enriched in incompatible trace elements, with HFSE largely hosted by the Ti-rich oxides. Precipitation of this mineral assemblage can lead to unusual fractionations among incompatible elements both in the metasomatic assemblage and associated fluids.