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Ruthenium Isotopic Evidence for a Missing Late Accretion Component in the Mantle Source of Pilbara Craton

Early Archean komatiites from the Pilbara craton (Australia) exhibit lower highly siderophile element (HSE) concentrations than observed for younger komatiite formations [1]. The significant HSE depletion in 3.5-3.2 Ga-old komatiites from Pilbara suggests that these lavas derive from a mantle source that had not yet fully equilibrated with late accreted material [1]. This interpretation seems to be corroborated by the presence of 182 W excesses reported for some mafic rocks from Pilbara [2]. However, the 182 W excesses do not provide unequivocal evidence for incomplete mixing of late accreted material because they may also reflect early mantle differentiation processes [3]. In this regard, mass-independent Ru isotope variations represent a more powerful and indicative tool to identify primordial mantle heterogeneities arising from incomplete mixing of the ambient mantle with late accreted material [4]. Here we determined the Ru isotope compositions of komatiites from Pilbara and report new 187 Os/ 188 Os and HSE contration data for the same rocks. Komatiites from the Ruth Well, Louden/Bookingara and Coonterunah formations show a relative excess in 100 R u of 23 parts per million compared with the modern mantle composition. The magnitude of the 100 R u excess in these rocks is the same as observed for ultramafic