Abstract

The incorporation of boron (B) and nitrogen (N) into illite is the key demand-side process responsible for the diagenetic budget of these elements in sedimentary basins, with important implications for pore-water chemistry, natural-gas composition, and borehole geophysics. The purpose of the present study was to take advantage of recent advances in quantitative mineral analysis of sedimentary rocks which have opened new possibilities for investigating this particular process. In order to avoid complications with recycled (detrital) N and B, clays from pyroclastic horizons of sedimentary rocks (bentonites) were used. The B and N contents in illite-smectite were measured in samples from different sedimentary basins, representing a complete range of diagenetic alteration. The bulk-rock chemical measurements, performed on raw rock samples in order to avoid any loss of exchangeable B and N, were referred to the contents of illite-smectite clays and to the content of illite alone, both measured by a combination of XRD and chemistry-based techniques.

Both B and N (as NH4+) are present in illite, so their contents in illite-smectite clay increase in a more or less linear manner with progressing illitization. Thus, during diagenesis, the illite-smectite clay is a net consumer of B and N from the pore water. The amount of N in individual illite layers decreases during diagenesis and the amount of B either decreases or remains stable. Bentonitic illite must acquire both B and N from outside of the bentonite bed. In one diagenetic cycle, bentonitic illite fixes up to 800–1000 ppm B and up to >1% N expressed as (NH4)2O, corresponding to >20% of the fixed cation sites.

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