Geochemistry of Boron in Pelitic Sediments

Abstract

One hundred and seventy four shale and argillaceous carbonate samples have been analyzed spectrographically for boron. The samples represent argillaceous sediments varying in age from the Proterozoic to the Cretaceous. A negative correlation between per cent CO2 and boron suggests that boron is mainly distributed in the argillaceous fraction and in the silica fraction. The B 2497.73A° line, because of an Fe coincidence at 2497.82A°, was found to be unsuitable for boron determinations in either the first or second orders of a Jarrel-Ash Wadsworth-mount spectrograph (Dispersion 5.2 and 2.6 A/Mm. respectively). Consequently, an analytical method utilizing B 2496.78a° in the second order spectrum was used. The boron results of this method compared to a chemical method appear to be systematically lower at concentrations above 24 ppm. boron. For example, two samples analyzed chemically contained 24 and 117 ppm. boron whereas the same samples analyzed spectrographically contained 24 and 79 ppm. boron. Absolute boron values probably could be recalculated by adjusting the working curve. Shales containing an apparent boron content above 75 ppm. appear to indicate a marine or hypersaline depositional environment. One section, Whitehorse F. (Triassic) has been misclassified into brackish or fresh water depositional environment based on low (below 75 ppm.) boron content. However, the mineralogy of this section is atypical (very high carbonate content) and the results may be misleading. Samples from the Fernie, Banff, Mount Hawk, Thorold and Grimsby, and the sub-Devonian all contain more thin 75 ppm. thus indicating marine depositional environments. A brackish water environment is suggested for shales from the Littleton and Kaakapau (Blackstone) Formations. One shale section, the Proterozoic (Miette Group), may have been deposited under a fresh water environment.