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

Abstract

Silicate-bearing and silicate-free natrocarbonatites have been recently erupted at Oldoinyo Lengai. Low-pressure (20 and 100 MPa) experiments were undertaken to determine the phase relationships of a natural silicate-bearing natrocarbonatite (OL5) from Oldoinyo Lengai. Nepheline appears first on the liquids. Over a protracted cooling interval (>200°C), only silicate and oxide phases crystallize, with nyerereite and gregoryite precipitating at temperatures <635°C. Results of this study, used in conjunction with the phase assemblage and mineral chemistry data available for natrocarbonatites from Oldoinyo Lengai, are used to test hypotheses relating silicate-bearing natrocarbonatite to the silicate-free variety. Our preferred model involves low-P (~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-density, silica-poor natrocarbonatite is dominantly controlled by carbonate (nyerereite and gregoryite) crystallization and fractionation. The low density and viscosity of silicate-free natrocarbonatite allow these magmas to erupt more easily than silicate-bearing natrocarbonatite.