Possible influence of clay contamination on B isotope geochemistry of carbonaceous samples

Abstract

The authors report results from an experimental study on mixtures of pure endmembers of natural clay and carbonate. The scientific rationale is an evaluation as to what extent B contents and B isotopes of carbonate samples may be obscured as a result of contamination with clay, particularly since both authigenic carbonates and biogenic carbonates (e.g. microfossil tests) often contain some clay. Three aliquots of a series of samples (each containing 0, 20, 40, 60, 80, 100% clay) were analyzed. Set 1 was washed with distilled, de-ionized water; for set 2 the HCl soluble parts were dissolved in 2 M HCl after washing; set 3 was completely digested with 30M HF prior to a series of ion exchanges. Isotope data of the endmembers are 6.6‰ (100% marble) and −4.6‰ (100% clay), with the clay being the dominant B source (ca. 50 ppm compared with <2 ppm in the carbonate). For set 1, only the B adsorbed to the clay was washed out with the de-ionized water (δ 11B adsorbed=12.9–14.1‰±0.5‰), while no B was mobilized from the carbonate. The HCl-dissolvable B in washed samples of set 2 show slightly increasing B contents with higher clay contents, suggesting that dissolution of the marble as well as B mobilization from the clay account for this trend. δ 11B isotopes tend towards more negative values when clay content increases, indicating that some structurally-bound B is lost from the sheets of linked (Si, Al)O 4 tetrahedra of the clay mineral. This result shows that not only B adsorption, but possibly diffusion or weathering of broken edges of clay minerals releases some structurally bound B of clay minerals. Set 3, where bulk samples were completely HF-digested, shows as expected a linear increase in B concentrations and decreasing δ 11B ratios with increasing clay content. The overall results suggest that relatively small amounts of clay (e.g. as contamination in a microfossil test) have no significant impact on the B content and δ 11B measured for the carbonate, but that care has to be taken if clay exceeds 10wt.% (e.g. carbonate concretions, chimneys, etc.).