Lack of late-accreted material as the origin of 182W excesses in the Archean mantle: Evidence from the Pilbara Craton, Western Australia

Tungsten Isotope Composition of Archean Crustal Reservoirs and Implications for Terrestrial μ182W Evolution

“…As can be seen in Figure 5, the AGC and CSBC rocks have similarly positive μ 182 W values over a time period slightly over 1 Ga. Furthermore, the μ 182 W values displayed by both regions of the Slave overlap values reported for TTG rocks from W. Greenland (Rizo et al, 2016; Tusch et al, 2019; Willbold et al, 2011); Saglek, Labrador (North Atlantic Craton; Liu et al, 2016); Nuvvuagittuq (Superior craton; Touboul et al, 2014); and the Pilbara craton (Archer et al, 2019; Rizo et al, 2019) as well as Neoarchean komatiites from Kostomuksha (Karelian craton; Touboul et al, 2012) and the Superior craton (Puchtel et al, 2018).…”

“…They interpreted this as evidence for the slow mixing into the mantle of a late‐accreted chondritic components. Later studies of komatiite HSE abundances (e.g., Archer et al, 2019; Puchtel et al, 2018; Puchtel, Blichert‐Toft, et al, 2016; Puchtel, Touboul, et al, 2016; Touboul et al, 2012) did not find an obvious secular trend with time in komatiite HSE abundances nor any obvious correlation between HSE abundance and W‐isotopic composition. Various models have attempted to reconcile these discrepancies and include models for mixing between an upper mantle that had obtained a late veneer with a lower mantle that had remained isolated (Willbold et al, 2015), as well as invoking uncertainties in mantle source HSE calculations to explain the apparent mismatch between source HSE abundances and W‐isotope compositions (Archer et al, 2019).…”

“…Thus, we did not endeavor to determine the HSE‐W‐isotope relationships in this suite of Slave craton samples. In addition, of the samples considered for HSE abundances by Maier et al (2009), later W‐isotope studies of these or related samples show the opposite W‐HSE correlation than expected for a gradual mixing in of late‐accreted components (Archer et al, 2019). Specifically, the southern African 3.5 Ga komatiites have zero to negative μ 182 W, while the 2.7 Ga komatiites, such as those in Kostomuksha or the Superior Province, have positive μ 182 W, within the range measured here for the Slave TTGs.…”

“…Thus, heterogeneous distributions in μ 182 W among Archean rock suites may be a more likely outcome of this scenario, and it can explain the regional variability in μ 182 W values within particular rock suites. Specifically, different komatiite suites are characterized by varying μ 182 W values, which are not correlated with HSE abundances (Archer et al, 2019). Also, felsic rocks from the Kaapvaal and North China cratons differ in μ 182 W from Slave craton rocks of the same ages (Mei et al, 2019; this work).…”

“…Analyses of W‐isotope compositions of ancient samples show many Archean (>2.5 Ga) rocks to have positive μ 182 W values of +10 to +15 (Archer et al, 2019; Dale et al, 2017; Liu et al, 2018; Mei et al, 2019; Rizo et al, 2019, 2016; Touboul et al, 2014; Tusch et al, 2019; Willbold et al, 2011, 2015) with some exceptions (Mei et al, 2019; Rizo et al, 2016). A prominent departure in the Archean is komatiites, displaying a range of μ 182 W values from negative (approximately −10 at 3,549 Ma Schapenburg; Puchtel, Blichert‐Toft, et al, 2016) to zero (at 3,500 Ma Komati, South Africa; Touboul et al, 2012) to positive (approximately +10 to +15 at 2,700 Ma Boston Creek and at 2,800 Ma Kostomuksha; Puchtel et al, 2018; Touboul et al, 2012) values.…”