Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation: Preservation of primary signatures versus secondary disturbances

Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fracti ons of a BIF sample from the northeastern part of the Isua supracrustal bel t (West Greenland) yield an isochron of 3691 +/- 22 Ma (MSWD = 0.4). In com bination with previously published geochronological constraints for a minim um deposition age of similar to 3.71 Ga for volcanic sequences (Nutman et a l., 1997) comprising the studied BIFs, and supported by microtextural obser vations, this demonstrates an early Archean amphibolite facies metamorphic event in the supracrustal. PbSL data on magnetite from slightly discordant veins within the same iron stone, together with bull; Ph isotope values of paragenetically late pyrite, yield a 3.63 +/- 0.07 Ga (MSWD = 8.6) errorchr on, with significantly different Pb isotopic compositions from those obtain ed from the main magnetite layers, and this suggests that the discordant la yers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The flui ds controlling this redistribution of magnetite can be tentatively correlat ed with metasomatic alteration features produced during the widespread intr usion of 3.6 Ga granitic into 3.7 Ga tonalitic Amitsoq gneisses enclosing t he belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [epsilon Nd(3800) of +1.7 and +2.1], within error of p ublished results from adjacent basic units in the supracrustal suite. In co ntrast, the magnetite-dominated layers yield unrealistically high epsilon N d(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fract ionated, continental sources. These high epsilon Nd(3800), together with ra diogenic Sr leached from the magnetite-enriched separates, is ascribed to s econdary hydroxyapatite, which predominantly forms as crystal overgrowths i n the magnetite-rich bands. The timing of the hydrothermal event during whi ch apatite was deposited within the BIF remains uncertain, but a T-CHUR mod el age of 1.85 Ga from the apatite-dominated HCl leachate may point to a cl ose genetic relationship with local Proterozoic metamorphism and granite fo rmation by crustal remelting in the Isukasia area. Step leaching of magnetite from a similar silicate-oxide facies BIF from th e western part of the Isua supracrustal belt yield a Pb-Pb isochron age of 2.84 +/- 0.05 Ga (MSWD = 1.43). The agreement between the PbSL age and prev iously published field and isotopic evidence for a major late Archean metam orphic event affecting the western area suggests there was widespread equil ibration accompanying the intrusion of the 2.75-2.83 Ga granodioritic Ikkat toq gneisses west and southwest of the western limb of the Isua supracrusta l belt. We argue that the PbSL isochron ages date the peak of amphibolite facies me tamorphic events which, respectively, affected the eastern and western sect ions of the Isua supracrustal belt during the early and late Archean. Our r esults give additional support to the suggestion that the scatter on publis hed epsilon Nd(T) values from the Isua supracrustal belt and adjoining gnei sses can be assigned to post-formational hydrothermal processes and underli ne the need for care in the interpretation of Sm-Nd bulk data from polymeta morphic rocks to constrain isotopic models of early Earth's evolution. Copy right (C) 1999 Elsevier Science Ltd.