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

Abstract

Vein-type galenas from within metasomatized tonalitic gneiss sheets in a metamorphosed 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 average continental crust evolution curves for lead and can be modeled by a single stage growth with starting parameters μ ( 238U/ 204Pb=7.70) and κ ( 232Th/ 238U=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 precursors of the gneisses. They support contentions regarding a time-integrated terrestrial pre-history with an elevated Th/U in the early Earth's mantle, necessary to reconcile the difference between the observed Th/U and the time-integrated Th/U recorded by Pb isotope ratios of present depleted MORB mantle-derived rocks. In addition, they are in line with the requirement of preferential enrichment of U relative to Pb in a proto-continental crust, relative to depleted upper mantle in the early Earth's history. The age equivalency between the galena generations and abundant tonalitic gneiss sheets, and their mutual spatial relationship within the ISB, favors an interpretation of the least radiogenic terrestrial leads to closely approximate the initial Pb of 3.75 Ga and pre-3.8 Ga old tonalitic protoliths and therefore prompts a close genetic relationship between early Archean fluid flow and the magmatic events. The inferred 3.81 Ga model age of the least radiogenic galena Pb particularly agrees with previous U–Pb zircon geochronological results on tonalite sheets within the supracrustals, which constrain 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-deformational hydrothermal–metasomatic minerals, our results imply that early Archean metasomatic activity is indistinguishable in time from the earliest metamorphic overprint and from the ca. 3.75 Ga major pulse of tonalite intrusions in the Isua area.