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.