Summary project C3: Preservation of ¹⁸²W heterogeneities in the Earth’s mantle – implications for the timing and nature of late accretion

Small ¹⁸²W excesses in Archean mantle-derived rocks have previously been interpreted to reflect crystal-liquid fractionation in an early magma ocean, high P-T metal-silicate equilibration at the base of a magma ocean, or missing late-accreted material with low ¹⁸²W. To distinguish between these disparate origins of ¹⁸²W excesses in Archean mantle sources, we investigated the ¹⁸²W composition, ¹⁴²Nd composition, Re-Os systematics, and highly siderophile element (HSE) abundances of Archean mantle-derived rocks from the Pilbara Craton, including samples from the Warrawoona Group and Ruth Well Formation. The measured µ¹⁸²W values (part-per-million deviations of ¹⁸²W/¹⁸⁴W from an Alfa Aesar W solution standards) for Pilbara volcanics are all ca. +10, similar to typical Archean values (+10-15) for mantle-derived rocks previously reported for other localities, including Nuvvuagittuq, Isua, Acasta, Saglek-Hebron, Kostomuksha, and Abitibi. The ¹⁴²Nd and Re-Os systematics of Pilbara volcanics rule out both crystal-liquid fractionation in a magma ocean and high P-T metal-silicate equilibration at the base of a magma ocean. Estimated HSE abundances, on the other hand, are ca. 50% lower than modern BSE, and the mantle source of these Archean rocks therefore was missing late-accreted material with low ¹⁸²W, resulting in the observed ¹⁸²W excesses. The typical Archean ¹⁸²W excesses observed for other mantle sources may also reflect a lack of late-accreted material, because it is unlikely that the nearly constant ¹⁸²W excess found in Archean mantle-derived rocks is produced by different and unrelated processes. Estimated HSE abundances for some of these sources are more variable, but this disparity can be explained by the uncertainties and assumptions involved with making HSE estimates, and/or decoupling of late-accreted W and HSE by mantle processes. Since 10-15 ppm ¹⁸²W excesses observed for many Archean samples most likely corresponds to mantle that received ca. 50% late accretion, the estimated pre-late accretion ¹⁸²W composition of the BSE must have been higher and similar to that of the Moon.