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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Frei, Robert, Rosing, Minik, Stecher, Ole
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1999
• Subjects: Amîtsoq; Greenland
• Online Access: https://forskning.ruc.dk/da/publications/936c336e-36cb-4aff-b8ea-542b9aad59b3; https://doi.org/10.1016/S0016-7037(98)00290-7; https://hdl.handle.net/1800/936c336e-36cb-4aff-b8ea-542b9aad59b3

Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fractions of a BIF sample from the northeastern part of the Isua supracrustal belt (West Greenland) yield an isochron of 3691 ± 22 Ma (MSWD = 0.4). In combination with previously published geochronological constraints for a minimum deposition age of 3.71 Ga for volcanic sequences (Nutman et al., 1997) comprising the studied BIFs, and supported by microtextural observations, 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 bulk Pb isotope values of paragenetically late pyrite, yield a 3.63 ± 0.07 Ga (MSWD = 8.6) errorchron, with significantly different Pb isotopic compositions from those obtained from the main magnetite layers, and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3.6 Ga granitic into 3.7 Ga tonalitic Amîtsoq gneisses enclosing the belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [Nd(3800) of +1.7 and +2.1], within error of published results from adjacent basic units in the supracrustal suite. In contrast, the magnetite-dominated layers yield unrealistically high Nd(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fractionated, continental sources. These high Nd(3800), together with radiogenic Sr leached from the magnetite-enriched separates, is ascribed to secondary hydroxyapatite, which predominantly forms as crystal overgrowths in the magnetite-rich bands. The timing of the hydrothermal event during which apatite was deposited within the BIF remains uncertain, but a TCHUR model age of 1.85 Ga from the apatite-dominated HCl leachate may ...