Evidence of Hydrocarbon-Rich Fluid Interaction with Clays: Clay Mineralogy and Boron Isotope Data from Gulf of Cadiz Mud Volcano Sediments

Clay dehydration at great depth generates fluids and overpressures in organic-rich sediments that can release isotopically light boron from mature organic matter, producing(10)B-rich fluids. The B can be incorporated into the tetrahedral sites of authigenic illite during the illitization of smectite. Therefore, the crystal-chemical and geochemical characterization of illite, smectite or interlayered illite-smectite clay minerals can be an indicator of depth (temperature) and reactions with the basin fluids. The aim of this study was to determine the detailed clay mineralogy, B-content and isotopic composition in illite-smectite rich samples of mud volcanoes from the Gulf of Cadiz, in order to evaluate interactions of hydrocarbon-rich fluids with clays. Molecular modeling of the illite structure was performed, using electron density functional theory (DFT) methods to examine the phenomenon of B incorporation into illite at the atomic level. We found that it is energetically preferable for B to reside in the tetrahedral sites replacing Si atoms than in the interlayer of expandable clays. The B abundances in this study are high and consistent with previous results of B data on interstitial fluids, suggesting that hydrocarbon-related fluids approaching temperatures of methane generation (150 degrees C) are the likely source of B-rich illite in the studied samples.