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. δ 11B 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 δ 11B value of −13 to −8‰, is the low-δ 11B 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-δ 11B end members, with δ 11B values of +2.3 to +9.2‰, +8.0 to +26.2‰ and +2.1 to +4.5‰, respectively. These high δ 11B 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 δ 11B values (−17.0 to −5.6‰) than those of the modern sediments. The lower δ 11B values can be caused by diagenesis, which induces (1) preferential removal of high-δ 11B 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.
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