Spatially variable carbon transport pathways in a volcanic and geothermal environment, Menengai Caldera, Kenya

AbstractMagmatic volatiles in geothermal settings ascend through permeable pathways developed from various tectonic and volcanic processes, resulting in structurally complex fracture networks controlling emissions of CO₂ and other magmatic volatiles. To better understand these controls, we examine spatial patterns and isotopic signatures of fumaroles and diffusely degassing CO₂ in the Menengai Geothermal Field, Kenya. Sequential Gaussian simulation analyses support a total CO₂ emission likely exceeding 389.7 t d⁻¹, with heavier δ13C-CO2 values suggesting primarily mantle‑derived carbon contributions. Degassing pattens mimic major rift structures, including NNW‑ to NNE‑trending faults and secondary E‑ to ENE‑trending structures. Fumarole lineaments, however, exhibit substantial scattering, with some observed lineament patterns falling outside the typical rift‑related structural fabric. We suggest that the broad distribution of CO₂ degassing (10s of km2 scale) is influenced by a large, ≥6.5 km‑deep igneous body, while deep volcanic and tectonic structures allow the upward migration of volatiles when hydraulically linked to this source. Lineaments observed in CO₂ flux patterns (1000s of m2 scale) support shallow CO₂ transport along blind rift‑parallel faults that terminate within post‑caldera lavas. From this depth, volatiles likely ascend through a permeability network derived from volcanic structures developed in post‑caldera lava flows, governing the local‑scale (100s of m² scale) patterns of CO₂ degassing. We conclude that a range of tectonic‑ and volcanic‑related factors, operating and interacting across multiple scales, control the spatial patterns of CO₂ degassing at Menengai, with possible implications for other geothermal systems within the East African Rift System (EARS), particularly the Kenyan Rift.