Tungsten isotope composition of the Acasta Gneiss Complex

High-precision tungsten (¹⁸²W/¹⁸⁴W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε¹⁸²W values between +0.06 to +0.15. Two ca. 3600 Ma felsic samples from this terrane have ε¹⁸²W ∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε¹⁸²W data are correlated with ε¹⁴²Nd (Roth et al., 2014) and we attribute this variability to incomplete metamorphic homogenisation of the 3960 Ma protolith with more recent material in a 3370 Ma tectono-thermal event. Notably, the value of the positive ε¹⁸²W anomalies seen in the 3960 Ma AGC samples that are least affected by metamorphic homogenisation is comparable to that observed in other early Archean rocks (Isua Supracrustal Belt, Greenland; Nuvvuagittuq Supracrustal Belt, Canada) and the late Archean Kostomuksha komatiites (Karelia). This demonstrates a globally constant signature. We infer that the presence of a pre-late veneer mantle represents the most straightforward interpretation of a uniform distribution of εW182∼+0.15 value in Archean rocks of different ages. We show that such a notion is compatible with independent constraints from highly siderophile element abundances in mafic and ultra-mafic Archean mantle-derived rocks. The absence of anomalous ε¹⁸²W and ε¹⁴²Nd so far measured in samples younger than ca. 2800 Ma suggests progressive convective homogenisation of silicate reservoirs. The downward mixing of an upper mantle rich in late-delivered meteoritic material might account for these combined observations.