Extreme peralkalinity in delhayelite- and andremeyerite-bearing nephelinite from Nyiragongo volcano, East African Rift

• Extremely peralkaline glass at Nyiragongo has formed by differentiation of nephelinitic magma.
• Glass coexists with rare Ba and Ti bearing minerals, some of which are unique to Nyiragongo.
• Long crystallization history to solidus temperatures around 600 °C
• Cooling at highly reducing conditions
• Unique mineralogy is due to extreme peralkalinity, low oxygen fugacity and high halogen activity.

Highly peralkaline leucite nephelinite from the active volcano Nyiragongo in the Virunga province of the East African Rift contains globules of iron- and volatile-rich, highly peralkaline silicate glass with (Na + K)/Al up to 18 which has formed as a late differentiate of less peralkaline precursors, probably by fractional crystallization at a shallow level in the volcanic system. A number of uncommon minerals coexist with this glass (kalsilite, kirschsteinite, chlorbartonite, götzenite, delhayelite, umbrianite, zirconian cuspidine, andremeyerite (BaFe2Si2O7), other Ba–Fe–Ti silicate minerals, and unnamed alkali-barium phosphate and Zr–Nb–Ti silicate minerals). These minerals are members of late magmatic assemblages that have survived sub-solidus recrystallization. Combeite occurs as a near-solidus mineral.Low-variance mineral assemblages in Nyiragongo nephelinite define a cooling trend from eruptive temperatures ≥ 980 °C to the solidus of extremely peralkaline residual liquids at ca. 600 °C, followed by sub-solidus recrystallization and metasomatism down to ca. 500 °C. Oxygen fugacity well below the QFM buffer (QFM-2 to -3) persisted throughout the magmatic crystallization stage, but increased to above QFM during the final stage of postmagmatic recrystallization.Highly alkaline, volatile-rich minerals such as delhayelite, götzenite and cuspidine were stabilized by a combination of high peralkalinity and elevated activity of chlorine and fluorine; these conditions persisted to sub-solidus temperatures.