The Rich Isotopic Memory of Illite: An Example of Clay Dating and Fluid Fingerprinting from the Zimapán Basin in Central Mexico

Elisa Fitz-Díaz, Universidad Nacional Autónoma de México
John Cottle, University of California, Santa Barbara
Ben van der Pluijm, University of Michigan

Besides vein-forming minerals such as quartz and calcite, clay minerals are one of a few species that grow during deformation in the upper crust. Among the family of clay minerals, illite contains a particularly rich isotopic memory that can be applied to a variety of geologic studies. Illite contains potassium, a radioactive element that can be used to obtain the age of deformation through Ar-Ar dating. Illite also contains structural water (OH^- ions in the octahedral layer), which, through δ²H or δ¹⁸O analyses, can provide information on the source(s) of the fluids present during deformation. Finally, illite contains boron in its structure, which provides independent information on the nature of the fluid(s) that were active during deformation and/or ambient temperature conditions.

In order to understand the mechanisms and significance of illite crystallization in syn-tectonic veins, a structural, textural and isotopic study was conducted on rocks of the well-exposed and well-preserved Zimapan Basin of Central Mexico. This now-inverted basin contains a succession of Cretaceous deep-water marine carbonates, which were strongly folded during the Late Cretaceous. Structural observations identify two shortening events in these rocks (tight folds refolded in spaced open folds and two generations of axial plane cleavage). SEM analysis of shales that were sheared parallel to bedding during flexural folding, show that illite grains grew parallel to cleavage. Based on Ar-Ar illite age analysis (IAA) of neocrystallized clays these events are constrained to have occurred at 80-84 Ma and 75-77 Ma. New in-situ U-Th/Pb ages from monazite included in calcite and quartz from syntectonic stretching veins agree within uncertainty of the Ar-Ar data, supporting the robust nature of the IAA. δ²H analyses of both illite and fluid inclusions show that they grew in isotopic equilibrium with pore water that was a mixture of marine and meteoric sources, reflecting a combination of fluid re-cycling marine water meteoric water infiltration during basin inversion and regional deformation.