COMBINED PB AND S ISOTOPES IN ANCIENT SULPHIDES FROM SW GREENLAND

We have undertaken high-spatial resolution secondary ion mass spectrometry (SIMS) measurements of both Pb and S isotopes in sulphides from early Archaean samples at two localities in southwest Greenland. Pyrite from a 3.71 Ga sample of banded iron formation in the Isua Greenstone Belt, which has previously yielded unradiogenic Pb consistent with its ancient origin, contains sulphur with a mass independently fractionated (MIF) isotope signature (Δ³³S = +3.3 ‰). This reflects incorporation of atmospheric S modified by photochemical reactions in the early Archaean atmosphere and currently represents one of the most extreme positive excursion so far known from the early Archean rock record. Sulphides from a granulite facies quartz-pyroxene rock and an ultramafic boudin from the island of Akilia, in the Godthåbsfjord, have heterogeneous and generally radiogenic Pb isotopic compositions that we interpret to represent partial re-equilibration of Pb between the sulphides and whole-rocks during tectonothermal events at 3.6 Ga, 2.7 Ga and 1.6 Ga. Both these samples have Δ³³S = 0 (within analytical error) and therefore show no evidence for MIF sulphur. These data do not substantiate claims that a MIF sulphur signature in the Akilia quartz-pyroxene rock is consistent with a sedimentary protolith. Our study further illustrates the applicability of SIMS S isotope measurements in ancient rocks to elucidating early atmospheric parameters because of the ability to obtain combined S and Pb-isotope data. When information from geological context, petrography and chronology (i.e. by Pb isotopes) is combined and fully evaluated, Δ³³S signatures from sulphides can be interpreted with a higher degree of confidence.