Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. (15th February 2016)

• Record Type: Journal Article
• Authors: Rizo, H.; Walker, R.J.; Carlson, R.W.; Touboul, M.; Horan, M.F.; Puchtel, I.S.; Boyet, M.; Rosing, M.T.
• Is Part Of: Geochimica et cosmochimica acta. Volume 175(2016:Feb. 15)
• Page Start: 319
• Page End: 336
• Publication Date: 2016-02-15
• Subjects: Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2015.12.007
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: We report new data for W concentrations, stable W isotopic compositions, high-precision 182 W/ 184 W ratios, highly siderophile element (HSE) abundances and 187 Re– 187 Os systematics in a suite of 3.8–3.3 Ga mafic and ultramafic rocks from the Isua supracrustal belt, and the Paleoarchean terrane in the northwestern part of the belt. These data are compared with published data for 146 Sm– 142 Nd systematics in the same samples. The samples from the Isua supracrustal belt show well resolved excesses of 182 W/ 184 W of up to ∼21 ppm, consistent with previous W isotopic data reported byWillbold et al. (2011) . While there is abundant evidence that W was mobilized in the crust accessed by the Isua supracrustal suite, the isotopic anomalies are interpreted to primarily reflect processes that affected the mantle precursors to these rocks. The origin of the 182 W excesses in these rocks remains uncertain. The Isua mantle source could represent a portion of the post-core-formation mantle that was isolated from late accretionary additions (e.g., Willbold et al., 2011 ). However, the combined 182 W, Re–Os isotopic systematics and HSE abundances estimated for the source of the Isua basalts are difficult to reconcile with this interpretation. The W isotope variations were more likely produced as a result of fractionation of the Hf/W ratio in the mantle during the lifetime of 182 Hf, i.e., during the first 50 Ma of Solar System history. This could have occurred as a result of differentiation in an early magma ocean. The Isua suite examined is also characterized by variable 142 Nd/ 144 Nd, but the variations do not correlate with the variations in 182 W/ 184 W. Further, samples with ages between 3.8 and 3.3 Ga show gradual diminution of 142 Nd anomalies until these are no longer resolved from the modern mantle isotopic composition. By contrast, there is no diminishment of 182 W variability with time, suggesting different mechanisms of origin and retention of isotopic variations for these two extinct-radionuclide isotope systems. The presence of 182 W isotopic anomalies in rocks as young as 3.3 Ga, implies that early-formed, high Hf/W domains survived for more than 1 Ga in the convective mantle.