Volatile emissions from the crater and flank of Oldoinyo Lengai volcano, Tanzania

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Abstract
As a comparison to airborne infrared (IR) flux measurements, ground‐based sampling of fumarole and soil gases was used to characterize the quiescent degassing of CO2from Oldoinyo Lengai volcano. Aerial and ground‐based measurements are in good agreement: ∼75% of the aerially measured CO2flux at Lengai (0.05–0.06 × 1012mol yr−1or 6000–7200 tonnes CO2d−1) can be attributed to seven large crater vents. In contrast to Etna and Vulcano Island, where 15–50% of the total CO2flux emanates diffusely through the volcanic flanks, diffuse emissions were measured only within 500 m of the crater rim at Lengai, contributing 2‐rich and H2O‐poor reported for any volcano, containing 64–74% CO2, 24–34% H2O, 0.88–1.0% H2, 0.1–0.4% CO and 2S, HCl, HF, and CH4. Volatile emissions of S, Cl, and F at Oldoiyno Lengai are estimated as 4.5, 1.5, and 1.0 × 107mol yr−1, respectively. Accuracy of the airborne technique was also assessed by measuring the C emission rate from a coal‐burning power plant. CO2fluxes were measured within ±10% near the plant; however, poor resolution at increased distances caused an underestimation of the flux by a factor of 2. The relatively large CO2fluxes measured for alkaline volcanoes such as Oldoinyo Lengai or Etna may indicate that midplate volcanoes represent a large, yet relatively unknown, natural source of CO2.