Oldoinyo Lengai rises impressively from the floor of the East African Rift in Northern Tanzania, and is famous as the only volcano on Earth to erupt carbonatite lavas. Carbonatites are extremely silica-poor, and contain ~30% mantle carbonate minerals, which makes these lavas as liquid as motor oil when they erupt at only 530°C. However, this fascinating magma system also erupts explosively every 20 to 30 years, producing ash primarily composed of nephelinite (a silica-undersaturated silicate magma rich in alkalis). In a recent paper in Earth and Planetary Science Letters, de Moor et al.  investigated the compositions of melt inclusions in nephelinite ash crystals, finding that these melts are extremely volatile-rich.
Analyses of volatiles by secondary ion mass spectrometry required synthesizing special matrix-matched standards for these enigmatic melt compositions. Silicate melts from Oldoinyo Lengai have very high concentrations of CO2 (between 2.7 wt% to 8.7 wt%). These high CO2 contents dwarf those found in typical silicate melts, which rarely contain over a few thousand ppm CO2. Whereas this carbon is undoubtedly from the mantle, ratios between carbon and trace elements in melt inclusions as well as gas compositions  indicate that such a mantle source is probably not particularly rich in carbon. Rather, trace elements in the ash samples indicate that extreme degrees of fractional crystallization took place under highly silica-undersaturated conditions in the presence of an immiscible carbon-rich fluid. This fluid buffered the CO2 content of the alkali-rich silicate melt, allowing CO2 and other volatiles to concentrate. Therefore, carbonatite lavas erupting between explosive eruptions derive from these carbon-rich fluids.
Surprisingly, melt inclusion analyses showed that the silicate melt is also water-rich, containing up to 10 wt% H2O. De Moor et al.  propose that water is the driving force behind the explosive eruptions at Oldoinyo Lengai, as water degassing drives crystallization. This result is also consistent with the water and CO2-rich nature of gas discharges from fumaroles at Oldoinyo Lengai. High degrees of crystallization can apparently plug the system, with volatiles unable to escape the magma, which in turn leads to explosive eruptions. Once the crystal-rich apex of mixed carbonatite-silicate magma systems is explosively expelled, volatiles and carbonate fluid can separate from the magma and resume passive degassing and effusive carbonatite eruptions.
Photo Credit: Maarten de Moor. Oldoinyo Lengai produced ash eruptions between September 2007 and July 2008, blanketing the surrounding landscape and affecting local Masai communities. A subsequent expedition in 2009 found carbonatite eruptions had resumed in the newly formed pit crater.