Mineral resorption triggers explosive mixed silicate–carbonatite eruptions

Abstract

Historic eruptions of Earth's only active carbonatite volcano, Oldoinyo Lengai (Tanzania), have repeatedly switched from low energy carbonatite lava extrusion to highly energetic explosive silicate volcanism, most recently in 1966–67 and 2007–08. The explosive eruptions produce strongly Si-undersaturated peralkaline silicate ashes with unusually high (Na + K)/Al of 3.4–6.3 when compared to the average peralkalinity of ∼0.8 in the East African Rift System. A series of experiments in the carbonatite–clinopyroxene system at 750–1150°C, 0.1 GPa, reveal that augitic clinopyroxene breaks down peritectically at >900°C yielding strongly peralkaline conjugated silicate- and carbonatite melts. The clinopyroxene-derived silicate melt dissolves (Na,K)2O from the (Na,K)2CO3-component of the carbonatite leading to high peralkalinities and to liberation of excess CO2, since the solubility of carbon dioxide in silicate liquids is ≪1 wt.% at subvolcanic pressures. Carbonatite injection into subvolcanic clinopyroxene-rich crystal mushes hence explains the occurrence of strongly peralkaline silicate melts and provides a mechanism for CO2-driven explosive eruptions. The silicate melt compositions mostly depend on the (Na + K)/Ca ratio of the intruding carbonatite, the silicate ashes erupted in 1966–67 and 2007–08 require an interaction of a clinopyroxene-rich crystal mush with a slightly less evolved alkali-carbonatite than presently erupted at Oldoinyo Lengai. The mechanism identified here, where mineral breakdown induced melt hybridization triggers volatile saturation and highly explosive volcanism is generally applicable to igneous systems that involve carbonatites or other low-viscosity CO2-bearing alkaline silicate melts.