Early carbonatite magmatism at Oldoinyo Lengai volcano (Tanzania): carbonatite silicate melt immiscibility in Lengai I melt inclusions

Carbonatites are unusual C-rich alkaline magmas that have been reported throughout the geological record. Nevertheless, there is only one currently active carbonatite system on Earth: Oldoinyo Lengai stratovolcano in northern Tanzania (God's mountain in Maasai culture). Present-day Lengai carbonatites are natrocarbonatites. peculiar Na-rich carbonatites that. under atmospheric conditions, alter and leach to compositions similar to the more common Ca-carbonatites within weeks, preventing any long-term geological record of such Na-rich magmas. it follows that the oldest report of natrocarbonatites at Oldoinyo Lengai dates to the 19th century. Here, by using samples from the Lengai I cone (>11 ka), we show that immiscible silicate-carbonatite melts were already present at reservoir conditions at that time. Measurements of three-phase (carbonatite + silicate + gas) melt inclusions from Lengai I highlight that their chemical compositions were similar to those of immiscible melts recently present in the reservoir. Alkaline carbonatites in melt inclusions from both Lengai I and historical explosive eruptions are enriched in Ca relative to those historically effused at the surface and likely record higher equilibrium temperatures (>1100 degrees C). We also report chemical maps that qualitatively document elemental partitioning between immiscible silicate-carbonatite melts. We show that at the melt inclusions' entrapment conditions Si, Fe, K, Na, and CI are compatible with the silicate phase when C, Ca, P, Sr, Ba, and F are compatible with the carbonate phase.

Automated summary

Carbonatites are rare C-rich alkaline magmas, with only one active system on Earth currently. Research indicates that Oldoinyo Lengai volcano had reports of carbonatites dating back to the 19th century, and recent studies suggest the presence of immiscible silicate-carbonatite melts in the volcano's reservoir as early as >11 ka.