Barium-rich phlogopite in a mantle derived xenolith of the Upper Canada Mine kimberlite, Ontario, Canada

Implications for Ba-reservoir in the upper mantle

Abstract

Barium-rich phlogopite occurs in a mantle derived xenolith incorporated into the Upper Canada Mine kimberlite, Ontario, Canada. The phlogopite is characterized by high BaO (up to 4.3 wt.%) and Na₂O (up to 2.2 wt.%) and extremely low TiO2 (0.02 wt.%) contents with a relatively high ratio (0.95) of Mg/(Mg+Fe). The chemical characteristics distinguish the phlogopite from Ba-rich micas reported from various alkalic mafic-ultramafic igneous rocks and
suggest that the phlogopite has been formed by mantle metasomatism involving Ba- and alkali-rich, Ti-poor hydrous fluids. The Ba content of phlogopite in this study is about five times higher than that of phlogopite previously reported from mantle derived xenoliths. A compilation of data suggests a distinct correlation between Ba and K in mantle derived xenoliths and various continental volcanic rocks (alkali basalts, ultrapotassic basic rocks, lamprophyres, lamproites, and kimberlites). A range of Ba/K in the xenoliths and-the volcanic rocks examined is comparable with that of mantle phlogopite, suggesting that phlogopite may be a principal reservoir for Ba as well as K in the upper mantle source regions.