Pre-Eruptive CO₂-Rich Fluid Interactions and Siderite Genesis at the Monoun Maar Volcano, Cameroon Volcanic Line: Insights from Stable Carbon and Oxygen Systematic

The Monoun Maar Volcano (MMV) is a polygenetic small-volume volcano within the Cameroon Volcanic Line (CVL), characterized by volcanic eruptions and CO₂-rich fluid interactions. Among its deposit sequence, a pyroclastic deposit unit is identified containing encrustations of ferrous carbonates, whose associated iron and genetic links remain poorly constrained. This study integrates petrographic, stable δ¹³C and δ¹⁸O isotopes, and palaeosalinity-paleotemperature modelling to elucidate the origin of siderite within this pyroclastic ejecta. Siderite occurs as pale yellow-brown massive aggregates within or at the border of some clasts, associated with pyroxene, plagioclase, and olivine. Isotopic data show δ¹³CVPDB values ranging from -6.0 to -5.2‰, consistent with a magmatic CO₂ source or the oxidative degradation of organic matter. δ¹⁸OVSMOW values are relatively high, around 31.4 to 32.3‰, suggesting a freshwater depositional environment influenced by meteoric water and magmatic CO₂. These isotopic values are also consistent with an authigenic carbonate formation likely affected by CO₂ emissions from submerged volcanic chimneys, although an allogenic origin might also be considered. Calculated palaeosalinity (Z = 115.2 to 117.34‰) is quite high, indicating saline hydrothermal fluids. Calculated paleotemperatures (T = 247.93 to 248.03°C) are consistent with hydrothermal systems or magmatic-hydrothermal interactions. The calculated palaeosalinity and paleotemperatures support the involvement of high-temperature, CO₂-rich fluids interacting with sediments and tephra from earlier eruptive phases at the bottom of the MMV crater. These findings contribute to a better understanding of the polygenetic eruptive evolution of the Monoun maar and highlight the role of carbonate mineralization in such volcanic settings.