Abstract
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.
Highlights
Seismic tomography and geochemical constraints provide us with a detailed picture of the physical and dynamic state of the present-day mantle (Coltice and Schmalzl, 2006; Kellogg and Turcotte, 1990; van der Hilst, 1999; van Keken and Zhong, 1999)
The Acasta Gneiss Complex (AGC) whole-rock W isotope data for the six felsic gneisses and two mafic gneisses are reported in Table 1 and Fig. 2
All AGC samples belonging to the 3960 Ma group have positive ε182W anomalies relative to present-day mantle and range between ε182W +0.06 to +0.15 (Fig. 2)
Summary
Seismic tomography and geochemical constraints provide us with a detailed picture of the physical and dynamic state of the present-day mantle (Coltice and Schmalzl, 2006; Kellogg and Turcotte, 1990; van der Hilst, 1999; van Keken and Zhong, 1999). Valuable information on the geodynamic regimes prevalent on the early Earth can be obtained by numerical simulations and analogue experiments Observational evidence to constrain the dynamics of the terrestrial Archean mantle has been scant, but high-precision determinations of differences in 142Nd/144Nd resulting from decay of short-lived 146Sm have recently opened a new window into the evolution of Hadean-early Archean rocks Investigating how early-formed reservoirs with discrete 146Sm/144Nd were dispersed and homogenised within the Earth over geological time-scales yields important information on the physical structure of the early terrestrial mantle (Bennett et al, 2007; Caro et al, 2003, 2005; Debaille et al, 2013; Harper and Jacobsen, 1992; Rizo et al, 2013).
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