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.