Phase relationships of a silicate-bearing natrocarbonatite from Oldoinyo Lengai at 20 and 100 MPa

Silicate-bearing and silicate-free natrocarbonatites have been recently eru pted at Oldoinyo Lengai. Low-pressure (20 and 100 MPa) experiments were und ertaken to determine the phase relationships of a natural silicate-bearing natrocarbonatite (OL5) from Oldoinyo Lengai. Nepheline appears first on the liquidus. Over a protracted cooling interval (>200 degrees C), only silica te and oxide phases crystallize, with nyerereite and gregoryite precipitati ng at temperatures <635 degrees C. Results of this study, used in conjuctio n with the phase assemblage and mineral chemistry data available for natroc arbonatites from Oldoinyo Lengai, are used to test hypotheses relating sili cate-bearing natrocarbonatite to the silicate-free variety. Our preferred m odel involves low-P (similar to 20 MPa) in situ fractional crystallization of silicate minerals from a silicate-bearing natrocarbonatite parent magma to produce silicate-free natrocarbonatites. Subsequent evolution of low-den sity, silica-pool natrocarbonatite Is dominantly controlled by carbonate (n yerereite and gregoryite) crystallization and fractionation. The low densit y and viscosity of silicate-free; natrocarbonatite allow, these magmas to e rupt more easily than silicate-bearing natrocarbonatite.