The least radiogenic terrestrial leads; implications for the early Archeancrustal evolution and hydrothermal-metasomatic processes in the Isua Supracrustal Belt (West Greenland)

Vein-type galenas from within metasomatized tonalitic gneiss sheets in a me tamorphosed oceanic crustal sequence of the arc-outbound Isua Supracrustal Belt (ISB), West Greenland reveal the hitherto least radiogenic terrestrial lead isotope compositions, These leads deviate from commonly applied avera ge continental crust evolution curves for lead and can be modeled by a sing le stage growth with starting parameters mu (U-238/Pb-204 = 7.70) and kappa (Tb-232/U-238 = 4.37), inferring model ages of 3.81 and 3.76 Ga for these galenas. The initial constraints are indicative of higher U/Pb and Th/U in the source of the hydrothermal fluids and arguably in the magmatic precurso rs of the gneisses. They support contentions regarding a time-integrated te rrestrial pre-history with an elevated Th/U in the early Earth's mantle, ne cessary to reconcile the difference between the observed Th/U and the time- integrated Th/U recorded by Pb isotope ratios of present depleted MORB mant le-derived rocks. In addition, they are in line with the requirement of pre ferential enrichment of U relative to Pb in a proto-continental crust, rela tive to depleted upper mantle in the early Earth's history. The age equivalency between the galena generations and abundant tonalitic g neiss sheets, and their mutual spatial relationship within the ISB. favors an interpretation of the least radiogenic terrestrial leads to closely appr oximate the initial Pb of 3.75 Ga and pre-3.8 Ga old tonalitic protoliths a nd therefore prompts a close genetic relationship between early Archean flu id flow and the magmatic events. The inferred 3.81 Ga model age of the leas t radiogenic galena Pb particularly agrees with previous U-Pb zircon geochr onological results on tonalite sheets within the supracrustals, which const rain a ca. 3.79-3.81 Ga minimum age for the formation to the Earth's oldest oceanic crustal sequence. In combination with 3.74 Ga Pb-Pb ages of post-d eformational hydrothermal-metasomatic minerals. our results imply that earl y Archean metasomatic activity is indistinguishable in time from the earlie st metamorphic overprint and from the ca. 3.75 Ga major pulse of tonalite, intrusions in the Isua area. (C) 2001 Elsevier Science B.V. All rights rese rved.