RECONCILING 182W/184W VARIABILITY IN THE ARCHEAN MANTLE WITH W PARTITION COEFFICIENTS FOR METAL-SILICATE DIFFERENTIATION

Short-lived isotope systems, such as ¹⁸²Hf-¹⁸²W (T_½ = 8.9 Ma), provide important information on the earliest phases of planetary differentiation and evolution. Because of the short half-life of ¹⁸²Hf, variations in ¹⁸²W can be only produced during the first ~ 50 Ma of Earth’s history. Tungsten is siderophile while Hf is lithophile, so metal-silicate segregation is commonly invoked to explain the Hf/W changes needed to create ¹⁸²W/¹⁸⁴W variability. W is also substantially more incompatible during silicate crystal-liquid fractionation, so magma ocean crystallization also could lead to variations in Hf/W. Finally, the ¹⁸²W/¹⁸⁴W of the Earth, as well as the highly siderophile element (HSE) abundances in the mantle, were also modified to an unknown extent as a result of addition of late accreted materials.

In this study, we present high-precision ¹⁸²W data for mafic and ultramafic samples from the Isua supracrustal belt (southwest Greenland), with ages that range between 3.3 Ga and 3.8 Ga. The Isua samples show enrichments in ¹⁸²W of up to 15 ppm, relative to terrestrial standards and modern rocks, in general agreement with the previous results [1]. These excesses were previously interpreted to reflect the incomplete mixing of late-accreted materials into the mantle sources of these rocks during the period between 4.5 and 3.8 Ga. However, the HSE abundances in the studied samples suggest that their sources had HSE abundances similar to those in the estimates for the modern mantle. The “normal” HSE abundances of the Isua rocks suggest that their mantle source had already received the full complement of the late accretion component of HSE, and hence W. In this case, the ¹⁸²W/¹⁸⁴W ratio of the source of Isua’s samples before late accretion, assuming that the late accretion component represent ~0.5% of Earth’s mass, is calculated to have been between +23 to +32 ppm.

If the Hf/W fractionation was caused by silicate-metal differentiation, excesses of +23 to +32 ppm in ¹⁸²W require a partition coefficient for W between 36 and 37, assuming that the core formed in one stage at ~ 30 Ma after the Solar system formation. These partition coefficients are in the range of those experimentally determined (between 30 and 50) which include 2% of S at the end of the accretion [2].

1. Willbold et al., 2011, Nature. 2. Wade et al., 2012, Geochim. Cosmochim. Acta.