The rise and demise of the global elemental and isotopic heterogeneities in the early Earth's mantle: Lessons from the Archean komatiite Zoo

Igor S. Puchtel
Department of Geology, University of Maryland, College Park, MD 20742, USA

Komatiites, due to their unique properties and old ages, are among the best samples to constrain the evolution of the chemical composition of the early Earth's mantle. Isotopic and elemental signatures that existed in the early Earth’s mantle have been sampled by komatiitic magmas and preserved in the geological rock record. These mantle signatures have since been largely destroyed due to the dynamic nature of the planet. The new data provide robust evidence for the presence of remarkably ancient isotopic and chemical heterogeneities in the mantle within the first 2.0 Ga of Earth’s history. These heterogeneities reflect the combined effects of (i) the co-existence of diverse post-magma ocean silicate domains characterized by variably fractionated lithophile and siderophile element abundances; (ii) the presence of distinct reservoirs representing mantles and cores of differentiated planetesimals delivered to Earth during late accretion; and (iii) isotopic exchange across the core-mantle boundary. The near-complete disappearance of resolvable 142Nd anomalies and of decoupled 143Nd-176Hf isotopic signatures in the komatiite rock record by ~2.5 Ga indicates that the earliest silicate reservoirs, formed as a result of early magma ocean crystallization, had been largely destroyed by that time through vigorous convective mantle mixing, implying mixing rates of the mantle on the order of ~2.0 Ga (Figure). The shift from mostly positive 182W offsets in the pre-2.5 Ga mantle to no 182W offsets at ~2.5 Ga, as well as disappearance of HSE abundance anomalies by ~2.5 Ga, may indicate the combined effects of the onset of modern-style plate tectonics and core-mantle interaction that took over as the main driving force of creating 182W isotopic heterogeneities in the mantle. This study emphasizes the great compositional complexity of early Earth’s mantle in terms of both elemental abundances and isotope systematics; apparently, the early mantle was very dissimilar in composition to that of the modern BSE.