Diffuse degassing at Longonot volcano, Kenya: Implications for CO₂ flux in continental rifts

Magma movement, fault structures and hydrothermal systems influence volatile emissions at rift volcanoes. Longonot is a Quaternary caldera volcano located in the southern Kenyan Rift, where regional extension controls recent shallow magma ascent. Here we report the results of a soil carbon dioxide (CO₂) survey in the vicinity of Longonot volcano, as well as fumarolic gas compositions and carbon isotope data. The total non-biogenic CO₂ degassing is estimated at < 300 kg d^− 1, and is largely controlled by crater faults and fractures close to the summit. Thus, recent volcanic structures, rather than regional tectonics, control fluid pathways and degassing. Fumarolic gases are characterised by a narrow range in carbon isotope ratios (δ¹³C), from − 4.7‰ to − 6.4‰ (vs. PDB) suggesting a magmatic origin with minor contributions from biogenic CO₂. Comparison with other degassing measurements in the East African Rift shows that records of historical eruptions or unrest do not correspond directly to the magnitude of CO₂ flux from volcanic centres, which may instead reflect the current size and characteristics of the subsurface magma reservoir. Interestingly, the integrated CO₂ flux from faulted rift basins is reported to be an order of magnitude higher than that from any of the volcanic centres for which CO₂ surveys have so far been reported.