FINGERPRINTING SOURCES OF ARCHEAN ANORTHOSITES: AN IN-SITU ISOTOPIC STUDY COMBINING PB-PB ISOTOPES IN PLAGIOCLASE WITH LU-HF ISOTOPES IN ZIRCON

Constraining the nature of ancient crusts and tracing the isotopic evolution of the Archean mantle are of great interest for geologists today. We present in-situ analyses by LA-MC-ICPMS for Pb isotope compositions of preserved igneous plagioclase megacrysts and Hf isotope compositions of zircon grains from the Fiskenæsset and Nunataarsuk anorthosite complexes, southwestern Greenland, two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U-Pb zircon geochronology suggests the crystallization age of Fiskenæsset is 2936 ± 13 Ma (2σ, MSWD = 1.5) and Nunataarsuk is 2914 ± 6.9 Ma (2σ, MSWD = 2.0). For both Fiskenæsset and Nunataarsuk, the initial Pb isotope compositions of plagioclase megacrysts and the initial ε_Hf compositions of zircon grains extend beyond analytical uncertainty suggesting multiple sources contributed to the parent magma for both anorthosite complexes. Initial ε_Hf of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ε_Hf units. Plagioclase Pb isotope compositions from both anorthosite complexes share a depleted mantle end member yet diverge from this point: Fiskenæsset toward a high-µ, more radiogenic Pb crustal composition and Nunataarsuk toward a low-µ, less radiogenic Pb, crustal composition. Using the initial Pb isotope compositions of plagioclase megacrysts we are able to constrain the timing of mantle separation of the crustal end member. The Pb model age can be used in conjunction with the Hf isotope compositions to calculate the ¹⁷⁶Lu/¹⁷⁷Hf of the crustal source. The ¹⁷⁶Lu/¹⁷⁷Hf is characteristic of the nature of the source. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~ 3700 Ma) mafic crust with ¹⁷⁶Lu/¹⁷⁷Hf ~ 0.026. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~ 4200 Ma) mafic crust with ¹⁷⁶Lu/¹⁷⁷Hf ~ 0.032. Using our new in situ approach, contamination of mantle derived magma by ancient mafic crust has been discovered in both the Fiskenæsset and Nunataarsuk anorthosite complexes of Greenland. The isotope data presented here suggest the survival of Hadean and Eoarchean crust until ~ 2900 Ma. There is potential this old mafic crustal could be preserved at the surface within the Nunataarsuk and Fiskenæsset regions today.