Boron isotope systematics of marine sediments

Abstract Boron contents and boron isotopic compositions were determined for modern and ancient (Permian to Miocene) marine sediments, including pelagic clay, calcareous ooze, siliceous ooze and neritic clay sediments. δ11B values of modern marine sediments range from −6.6 to +4.8‰. Isotopic variation is controlled by the simple mixing of four major constituents, detritus of continental origin, marine smectite, biogenic carbonates and biogenic silica. Detritus of continental origin, with an average δ11B value of −13 to −8‰, is the low-δ11B end-member constituent of marine sediments and its boron is largely controlled by the concentration of illite which originates from wind or fluvial transport. Marine smectite, biogenic carbonate and biogenic silica, on the other hand, represent the high-δ11B end members, with δ11B values of +2.3 to +9.2‰, +8.0 to +26.2‰ and +2.1 to +4.5‰, respectively. These high δ11B values are the result of the equilibrium uptake of boron from seawater. Spatial variations in boron isotopes in the Pacific sediments are essentially due to the distribution of the above four constituents. Although ancient argillaceous sediments (shale and slate) have boron contents that are identical with those of modern equivalents, boron contents of limestone and chert are distinctly lower than those of modern calcareous and siliceous oozes. Ancient marine sediments have systematically lower δ11B values (−17.0 to −5.6‰) than those of the modern sediments. The lower δ11B values can be caused by diagenesis, which induces (1) preferential removal of high-δ11B boron in calcium carbonate and silica during recrystallization and (2) boron isotopic exchange in the course of the smectite/illite transition. The observed boron isotopic compositions of ancient argillaceous sediments are distinctly different from those of fresh and altered MORB. Therefore, boron isotope systematics will be useful in identifying components from the descending oceanic slab involved in the formation of island arc magma and in investigating mantle-crust recycling through subduction processes.