Open-path Fourier transform spectroscopy of gas emissions from Oldoinyo Lengai volcano, Tanzania

We report here novel field spectroscopic measurements of the proportions of H2O, CO2, CO and SO2 in gas emissions from Oldoinyo Lengai, the world's unique, active carbonatite volcano. We found that CO2 constitutes <40mol% of emissions from a lava lake, and 25 mol% from a cooler fumarole vent. These results suggest that H2O is the predominant gas phase rather than CO2, as reported in previous studies based on conventional sampling (Trans. Am. Geophys. Union 69 (1998) 1466; J. Geophys. Res. 101 (1996) 13819), though it is possible that water is introduced by remelting of older hydrated lava flows. We also observed rapid variations in CO2/CO molar ratios (between 450 and 750 in 1 h) in the lava lake emissions, which could reflect mixing of gases exsolved from deep and shallow magma. Lengai's measured CO2 flux (J. Geophys. Res. 101 (1996) 13819; Geology 23 (1995) 933) exceeds the time-averaged magma discharge rate, suggesting efficient separation of carbon and water-rich fluids from unerupted silicate magma. This may play an important role in parental magma differentiation. (C) 2002 Elsevier Science Ltd. All rights reserved.