182Hf-182W decay system (t1/2 = 8.9 Ma) has proven very valuable in understanding the evolutionary history of the early Earth. The parent isotope, 182Hf, completely decays into 182W within the first 50 Ma of the solar system history, so variations in 182W/184W are only produced during this period or by later mixing of early-formed reservoirs. Many Archean terrestrial samples have 182W/184W ~10ppm higher than modern crustal values, implying that the mantle source of modern crust has experienced widespread addition of unradigenic material to decrease its 182W/184W since the Archean. Several theories have been put forth to explain this decrease in Δ182W, e.g. inmixing of late veneer material (Willbold et al., 2011) or Core-mantle interaction (Rizo et al., 2019). Here we report Δ182W data of mantle-derived rocks from the Pilbara Craton, Western Australia, focusing on the evolution of W isotopic compositions within this single Archean block through time. The Δ182W values were measured for a range of lithologies from the Pilbara region including Komatiite, Basalt and Gabbro, with ages ranging from 3.58-2.77 Ga. With an average Δ182W of +9.7 ± 2.2 ppm, the majority of the examined samples preserved relatively uniform 182W excesses. W has clearly experienced secondary mobility in many samples. Yet the W isotopic ratios of samples with magmatic W/Th (0.04-0.24) and disturbed W/Th ratios showed no discernible variations. The secondary tungsten enrichment event has been dated through tungsten-rich rutiles (as high as 13799ppm) found in the 3.5 Ga Mount Webber gabbros. The U-Pb dating of rutiles gave 207Pb/206Pb apparent ages at 3.0–2.9 Ga, showing the presence of a tungsten-rich fluid during the metamorphism at the time. Values of Δ182W = +12 ppm for three Mount Webber samples, which are dominated by W transported by this late metamorphic event, are inconsistent with the theory of a less radiogenic, homogenized Δ182W crustal fluid proposed by a previous study of the Pilbara Craton (Tusch et al, 2021). No systematic decrease in Δ182W values with time during Pilbara Craton formation (3.5-2.7Ga) is observed in our collection of samples. Further younger samples will be investigated, aiming to find the exact timing and style of Δ182W decrease in the upper mantle from typical Archean values of ~10 ppm to the modern composition.

Reference(s)
Willbold, M., Elliott, T. and Moorbath, S., 2011. The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment. Nature, 477(7363), pp.195-198. 

Rizo, H., Andrault, D., Bennett, N.R., Humayun, M., Brandon, A., Vlastélic, I., Moine, B., Poirier, A., Bouhifd, M.A. and Murphy, D.T., 2019. 182W evidence for core-mantle interaction in the source of mantle plumes. Geochem. Perspect. Lett., 11, pp.6-11. 

Tusch, J., Münker, C., Hasenstab, E., Jansen, M., Marien, C.S., Kurzweil, F., Van Kranendonk, M.J., Smithies, H., Maier, W. and Garbe-Schönberg, D., 2021. Convective isolation of Hadean mantle reservoirs through Archean time. Proceedings of the National Academy of Sciences, 118(2).