The boron isotope geochemistry of smectites from sodium, magnesium and calcium bentonite deposits

The mineralogical, chemical and isotopic analyses of smectites, with variable interlayer cation occupancies, from bentonite deposits in various depositional environments, reveal new insights into the boron sources and the fluids involved in bentonitization in marine and non-marine environments. Smectites from bentonites have non-exchangeable, structural boron concentrations of 0.2 to 196 mu g/g B. Smectites from sodium bentonites have higher boron concentrations ( > 30 mu g/g) than those from magnesium or calcium bentonites. Most smectites have a small, interstratified illitic component that has a major influence on boron concentrations, requiring the use of modified fluid-mineral boron partitioning coefficients, which indicate that bentonites formed from fluids of highly variable boron concentrations of < 0.1 mg/L B to > 100 mg/L B, and a chlorine content of 76.4 mg/L to 59,076 mg/L. The sodium bentonites formed from boron-rich saline fluids or brines whereas the fluids involved in calcium and magnesium bentonite formation have a more variable boron composition and salinity. The delta B-11 values of the structural boron in tetrahedral sites of smectites range from - 30.1 parts per thousand to + 12.2 parts per thousand. The smectites from terrestrial depositional settings have delta B-11 values of -30.1 parts per thousand to about 0 parts per thousand, whereas smectites from marine depositional settings have negative as well as positive delta B-11 values. The boron isotope values indicate that all examined bentonites from terrestrial depositional settings as well as many bentonites from marine depositional settings formed from basinal fluids or brines. The boron isotope geochemistry of smectites is demonstrated to be a tool for elucidating the fluids involved in the formation of clay mineral deposits. It also has great potential for tracing fluids in other settings involving authigenic clay minerals such as sedimentary basins and surficial crystalline rocks, as well as man-made applications such as in disposal sites for highly active nuclear waste.