Boron isotope geochemistry during diagenesis. Part I. Experimental determination of fractionation during illitization of smectite

Experiments were performed to measure the isotopic fractionation of boron between illite/smectite (I/S) clay minerals and water as a function of temperature (300 degrees and 350 degreesC) and degree of illitization. Corresponding changes in the oxygen isotopes were monitored as an indication of the approach to equilibrium. The kinetics of the B-isotope exchange follows the mineralogical restructuring of smectite as it recrystallizes to illite. An initial decline in delta B-11(I/S) occurs when the I/S is randomly ordered (RO). The delta B-11(I/S) values reach a plateau during R1 ordering of the I/S, representing a metastable condition. The greatest change in delta B-11(I/S) is observed during long-range (R3) ordering of the I/S when neoformation occurs. Values of delta B-11(I/S) measured on the equilibrium reaction products were used to construct a B-isotope fractionation curve. There is a linear correlation among data from these experiments and 1100 degreesC basaltic melt-fluid fractionation experiments (Hervig and Moore, 2000) that can be extrapolated to include adsorption experiments at 25 degreesC (Palmer et al,, 1987), Unlike other stable isotopic systems (e.g., oxygen) there is no mineral-specific fractionation of B-isotopes, bur rather a coordination dependence of the fractionation. Under diagenetic conditions B is predominantly in trigonal coordination in fluids but substitutes in tetrahedral sites of silicates. The preference of B-10 for tetrahedral bonds is the major fractionating factor of B in silicates. Copyright (C) 2001 Elsevier Science Ltd.