Boron isotopic composition of tourmalines from the 3.8-Ga-old Isua supracrustals, West Greenland: implications on the δ11B value of early Archean seawater

Abstract

The boron isotope composition of different occurrences of tourmalines in the 3.8-Ga-old Isua (West Greenland) supracrustals has been analysed by ion microprobe. Tourmalines in the mafic volcaniclastics have δ 11B values between −12.6 and +5.8% with a mean at −3.2±5.1%. These values are close to the values previously reported for tourmalines in the Isua acid volcanic tuffs (−6.7±1.4%) and indicate no difference in δ 11B value of the mantle or the continental crust between the present day and early Archean times. In contrast, tourmalines in a marine conglomerate have low and homogeneous δ 11B values of −20.2±2.2%. For these tourmalines, which are of authigenic origin having formed through metamorphism of pre-existing marine clays, a δ 11B of ∼ −8±7% can be calculated for the unmetamorphosed marine sediments deposited before 3.8 Ga at Isua. A value of +27±11% is calculated for the δ 11B value of seawater at 3.8 Ga. The large error arises because the δ 11B value of marine sediments are controlled by: (1) the ratio of detrital to chemically precipitated boron; and (2) the boron isotope fractionation between precipitated boron and dissolved boron in seawater. A high δ 11B similar to the present-day value of +40‰ seems rather unlikely, unless the fraction of detrital B in marine sediments was as high as today. A lower value of +27 ± 11‰ for early Archean seawater is consistent with mass balance of boron between seawater, mantle and continental crust.