Tungsten isotope composition of the Acasta Gneiss Complex

Abstract

Editor: B. Marty Keywords: Archean mantle crust late veneer tungsten isotopes Acasta Gneiss Complex High-precision tungsten ( 182 W/ 184 W) isotope measurements on well-characterised mafic and felsic samples of the ca. 3960 Ma Acasta Gneiss Complex (AGC; Northwest Territories, Canada) show radiogenic ε 182 W values between +0.06 to +0.15. Two ca. 3600 Ma felsic samples from this terrane have ε 182 W ∼ 0 and are the oldest samples so far documented to have a W isotopic composition indistinguishable from that of the modern mantle. The ε 182 W data are correlated with ε 142 Nd (Roth et al., 2014) and weattribute 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 ε 182 W 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 ε 182 W ∼ +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 ε 182 W and ε 142 Nd 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.

Highly siderophile element and 182 W evidence for a partial late veneer in the source of 3.8 Ga rocks from Isua, Greenland

“…It should also be noted that the inclusion of iron meteorite material in this model is similar to the model of retention of some giant impactor core material in Earth's mantle, proposed by Willbold et al (2015). That model also produces ε 182 W of ~0.13 at the HSE contents that we propose for the Isua mantle, if using an initial mantle value similar to that of the Moon (~0.25).…”

“…This finding perhaps suggests that local remobilisation of W is the most likely explanation for elevated W/Th ratios in at least some samples. One alternative possibility is that the presence of elevated W/Th in Isua, Acasta, Komati and Nuvvuagittuq samples could reflect an inherent Archaean magmatic feature (Willbold et al, 2015), though no mechanism has yet been proposed.…”

“…Recently, the extinct 182 Hf-182 W decay system (t 1/2 = 8.9 Ma), which so far has mainly been used as a chronometer of core formation (Kleine et al, 2009), has emerged as a new tool with which to investigate late accretion (Willbold et al, 2011;Kruijer et al, 2015;Touboul et al, 2015;Willbold et al, 2015). During core formation, lithophile Hf separated from moderately siderophile W, causing Hf/W fractionation which, following decay of 182 Hf, led to distinct 182 W signatures in the mantle and core of planetary bodies.…”

“…This is because the addition of a late veneer with a composition and mass as inferred from HSE systematics lowered the 182 W/ 184 W of Earth's mantle by ~0.1 to ~0.4 ε-units (Willbold et al, 2011;Touboul et al, 2012;Kruijer et al, 2015;Touboul et al, 2015). Several recent studies have identified 182 W excesses of this magnitude in some Archaean samples and, in some cases, these signatures have been attributed to derivation of these samples from sources that lack some portion of the late veneer (Willbold et al, 2011;Willbold et al, 2015). However, the origin of these 182 W excesses is not fully understood and in other cases have been ascribed to very early differentiation events within Earth's mantle, rather than late accretion (Touboul et al, 2012;Touboul et al, 2014;Rizo et al, 2016).…”

“…The origin of the 182 W enrichment in these fluids remains unclear, however, and could be related to either a lower proportion of late veneer or early differentiation. In contrast, the constancy of 182 W excesses (~+0.15; Isua, Kostomuksha, Nuvvuagittuq and also Acasta), has been interpreted to reflect a global signature of a pre-late veneer mantle, enriched in HSE by giant impactor core material (Willbold et al, 2015). Recently, negative ε 182 W has been reported -the first such finding -in Schapenburg komatiites (Barberton greenstone belt) and this appears coupled with depletions in 142 Nd, suggesting a signature of early mantle differentiation (2016).…”