Insights into early Earth from Barberton komatiites: Evidence from lithophile isotope and trace element systematics

Abstract

Major, minor, and lithophile trace element abundances and Nd and Hf isotope systematics are reported for two sets of remarkably fresh, by Archean standards, samples of komatiitic lavas from the 3.48Ga Komati and the 3.27Ga Weltevreden Formations of the Barberton Greenstone Belt (BGB) in South Africa. These data are used to place new constraints on the thermal history of the early Archean mantle, on the timing of its differentiation, and on the origin and chemical nature of early mantle reservoirs and their evolution through time.Projected moderate to strong depletions of highly incompatible lithophile trace elements and water in the mantle sources of both komatiite systems, combined with the partitioning behavior of V during lava differentiation, are consistent with anhydrous conditions during generation of the komatiite magmas. Komati and Weltevreden lavas are inferred to have erupted with temperatures of ∼1600°C, and, thus, represent the hottest known lavas on Earth. The calculated mantle potential temperatures of ∼1800°C for both komatiite systems are 150–200°C higher than those of contemporary ambient mantle. Combined, these observations are consistent with the origin of these BGB komatiite magmas in mantle plumes in the lower mantle.New Sm–Nd and Lu–Hf isotopic data allow precise determination of initial ε143Nd=+0.46±0.10 and +0.50±0.11 and initial ε176Hf=+1.9±0.3 and +4.7±0.8 for the Komati and the Weltevreden system komatiites, respectively. These positive initial values reflect prior fractionation of Sm/Nd and Lu/Hf in the mantle early in Earth history. Conversely, μ142Nd values are 0.0±2.4 and +2.2±4.1 for the Komati and the Weltevreden systems, respectively. These values overlap, within uncertainties, those of modern terrestrial rocks, thus, limiting the magnitudes of possible Sm/Nd fractionations generated by early Earth processes in the sources of these rocks. Combined 142,143Nd and Hf isotope and lithophile trace element systematics are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated Sm/Nd and Lu/Hf at ca. 4400Ma. These domains were likely generated as a result of crystallization of a primordial magma ocean, with Mg-perovskite and minor Ca-perovskite acting as fractionating phases. The inferred mantle domains were evidently mixed away by 2.7Ga on the scale of mantle reservoirs sampled by late Archean komatiite lavas emplaced worldwide.