Boron isotopes in tourmaline from the ca. 3.7–3.8 Ga Isua supracrustal belt, Greenland: Sources for boron in Eoarchean continental crust and seawater

Abstract

Boron is highly concentrated in Earth’s crust relative to primitive mantle. However, when present-day crustal concentrations were achieved remains debatable. It has been proposed that seawater boron δ11B was lower than at present, consistent with a model relating increase in sea-water δ11B to the proportion of B extracted from Earth’s mantle into the oceans and crust. Our in situ ion microprobe analyses of tourmaline in 17 samples from the Eoarchean Isua supracrustal belt, Southwest Greenland, gave the following average δ11B with uncertainties ranging from ±0.4 to ±1.9‰: δ11B=−7.1 to −11.5‰ in amphibolite; δ11B=−10.5 to −25.3‰ in mica schist; δ11B=−19.2‰ in metachert (one sample), and δ11B=−21.9‰ in metaconglomerate (one sample). Tourmaline is largely schorl–dravite, rarely uvite–feruvite, and shows color and compositional zoning. δ11B varies from grain to grain in most samples; grains in a kyanite–staurolite schist are isotopically zoned, possibly because the rims incorporated B released by muscovite breakdown. The patterns in color-zoned tourmaline grains in our samples are not consistent with detrital origin of the cores, which rules out the possibility of there being tourmaline detritus from pre-existing continental crust in the studied samples. The tourmaline-bearing rocks are found in both the ca. 3700Ma northern and ca. 3800Ma southern terranes in the Isua supracrustal belt. Following an approach suggested by Chaussidon and Appel, we estimated Eoarchean seawater δ11B by calculating back from δ11B of tourmaline in metasedimentary rocks using fractionation of boron isotopes between clays and muscovite, tourmaline and aqueous fluid. This calculation gave an estimated δ11B≈+14±15‰ for Eoarchean seawater, 25‰ lower than present-day seawater (δ11B=+39.5‰). For comparison, an estimate obtained simply by direct comparison of δ11B for Eoarchean and Phanerozoic tourmaline presumed to have crystallized in similar environments gives δ11B≈+30‰ for Eoarchean seawater.At first, our results appear to be contradictory. On the one hand, tourmaline is as widespread and abundant in the Isua supracrustal rocks as it is in many younger complexes, which suggests that extraction of boron from the mantle had reached levels close to present day levels. On the other hand, there is no evidence of boron having been extracted prior to ca. 3800Ma and a seawater δ11B lower than present-day δ11B=39.5‰ implies that extraction of B from the mantle had not reached the present-day extent. The proto-arc scenario proposed by A. Nutman and co-authors offers a possible resolution to the contradiction by considering the abundance and isotopic composition of B in the Isua supracrustal rocks as a localized phenomenon that should not be interpreted to reflect B concentrations in an Earth-circling ocean. Accumulation of boron in seawater in a partially isolated basin associated with a proto-arc and the attendant volcanism might have been sufficiently rapid for the needed concentrations to be attained in a relatively short time.