Abstract
We report here novel field spectroscopic measurements of the proportions of H 2O, CO 2, CO and SO 2 in gas emissions from Oldoinyo Lengai, the world's unique, active carbonatite volcano. We found that CO 2 constitutes <40 mol% of emissions from a lava lake, and 25 mol% from a cooler fumarole vent. These results suggest that H 2O is the predominant gas phase rather than CO 2, 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 CO 2/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 CO 2 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.
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