Modern ocean island basalt–like 182W signature in Paleoarchean mafic rocks: Implications for the generation, preservation, and destruction of early mantle heterogeneities

Abstract

Komatiites and picrites generated by high degrees of mantle partial melting serve as potential probes of Earth’s deep mantle. Tungsten (W) isotopes in these rocks offer a rare chance to better understand early differentiation, late accretion, core-mantle interaction, and subsequent evolution of Earth’s mantle. We present new W isotope data for Archean komatiites and basalts from the Barberton (South Africa) and Suomussalmi (Finland) Greenstone Belts and Permian picrites from the Emeishan large igneous province (China). The Paleoarchean samples from the Barberton Greenstone Belt have modern ocean island basalt (OIB)−like μ182W values ranging from −20.4 to +5.6, whereas the Mesoarchean komatiites from the Suomussalmi Greenstone Belt show μ182W values of −2.2 to +11.3. The Permian Emeishan picrites give μ182W values of −7.1 to +3.1. Our data, combined with the published global data set, show that W isotope heterogeneity in the mantle has existed throughout Earth’s history, with positive μ182W values transitioning to near-zero in the upper mantle by the end of the Archean. The negative μ182W values of Paleoarchean samples in the Barberton Greenstone Belt and modern OIBs likely result from either early differentiation or core-mantle interaction. The incorporation of a plume-delivered negative μ182W component and enhanced mantle mixing is a viable mechanism to explain the transition of μ182W values in the upper mantle from positive to near-zero, while recycling of crustal materials into the mantle would result in a shift of negative μ182W values of the lower mantle closer to zero since the onset of plate tectonics. The latter process could possibly explain the slightly negative to near-zero μ182W values of the Emeishan picrites and some kimberlites. The well-resolved negative μ182W anomalies observed in this study provide important insights into the generation, preservation, and obliteration of W isotope heterogeneities in the lower mantle.