Complex Sm-Nd and Lu-Hf isotope systematics in metamorphic garnets from the Isua supracrustal belt, West Greenland

摘要

Combined Sm-Nd and Lu-Hf isotope analyses of metamorphic garnets from the Earth's oldest oceanic crystal sequence (the Isua supracrustal belt, West Greenland) yield errorchrons with apparent late Archean (2.8 Ga) crystallization dates. This contrasts with field and other geochronological evidence that link garnet growth to an early Archean (3.7 Ga) metasomatic-metamorphic event, but agreers more closely with the timing of a regional thermal overprint known to have affected both the Isua supracrustal belt and adjacent gneisses. Sm-Nd geochronological evidence presented here shows that the garnets were never reported after garnet growth. The apparent partial reopening of the Sm-Nd isotope system within bulk garnets if instead explained by variable resetting of metamict allanite inclusions and their differing relative abundance within the garnet hosts. Allanite precipitated from metasomatic alteration fluids that percolated through he rocks during the emplacement of tonalite sheets. While light-rare-earth-element-rich allanite inclusions can affect the Sm-Nd system of bulk garnet, its low heavy-rare-earth element and Hf contents produce no measurable effect on the Lu-Hf system. Rather, the abundant common Hf in zircon, also present as inclusions in the garnets, affected the Hf budget to the extent of significantly lowering apparent Lu-Hf garnet ages. Whether the zircon inclusions are older (i.e., inherited) or contemporaneous (i.e., metasomatic) with the host garnets cannot be resolved with the present data. Despite the complications caused by inclusions in the garnets analyzed here, this study shows evidence for a metasomatic event at 3.7 Ga and a widespread resetting event at 2.3 Ga, but no evidence for a meaningful geologic event at 2.8 Ga. The 2.8 Ga Sm-Nd and Lu-Hf correlation lines obtained for the Isua garnets are interpreted as mixing liens between, respectively, metamict allanite and metamict zircon, both reset at 2.3 Ga, and garnet crystallized at 3.7 Ga. This study emphasizes the need for caution in the interpretation of Sm-Nd and Lu-Hf geochronological data involving garnets. Highly resistant inclusions, such as zircon, are unlikely to re-equilibrate with their host mineral during later tectometamporphic overprints and can severely bias parent-daughter element budgets. Likewise, metamictization of Th-U-rich inclusions (such as allanite and some zircons) within host minerals (in this case garnet) renders the inclusions increasingly prone to open-system behavior during later thermal events, leading to apparent initial isotopic disequilibrium between the inclusion phase and its less affected host. Finally, an initial ε_(Nd) value at 3.7 Ga of -14 for garnets not containing allanite inclusions suggests the presence in the Isua area of very early differentiated crust.