Influence of structural boron on ‘illite crystallinity’: results from mud volcanoes in Azerbaijan

Abstract Sedimentary volcanism is a widespread phenomenon on Earth that leads to the extrusion of fine-grained sediments, saline waters, and hydrocarbons in compressional environments. In the present study, mud volcanoes located in eastern Azerbaijan were investigated with a particular interest in boron (B) influence on illite crystallinity, compared with results reported in the literature for Northern Apennine mud volcanoes (Italy). Azerbaijan sediments have a predominant silt fraction and a mineralogy dominated by quartz, feldspar, calcite, and clay minerals (illite, mixed-layer illite smectite, smectite, and chlorite). Reichweite grade, measured by estimated illite percentage in Ilt-Sme, associated with a geothermal gradient of 18°C km –1 , indicates a sediment origin of 7–8 km, consistent with the depth of the Maikop Series, considered in the literature to be the main source rock of the erupted muds. Azerbaijan samples confirmed the inverse correlation between structural B in illite (53–182 μg g –1 ) and the Kübler index (KI) on illite (0.53–0.71°Δ2θ), previously observed for mud volcanoes in the Apennines. This suggests that a common process operates in these different environments, highlighting the role of B in illite crystallinity, and confirming the need to consider this interaction when using KI as a sediment depth marker in similar geological contexts.