Lithophile and siderophile element systematics of Earth’s mantle at the Archean–Proterozoic boundary: Evidence from 2.4 Ga komatiites

Abstract

New Os isotope and highly siderophile element (HSE) abundance data, in combination with lithophile trace element and Sm–Nd, Lu–Hf, and Hf–W isotope data, are reported for komatiitic basalts from the Vetreny Belt and tonalites from the adjacent Vodla Block in the Fennoscandian Shield. Komatiitic basalts define a Re–Os isochron with an age of 2407±6Ma and an initial γ187Os=+1.7±0.2 (2SE). The Pt–Os data for chromite separates yield an average initial ε186Os=+0.03±0.02 (2SE). The 147Sm–143Nd and 176Lu–176Hf data for the komatiitic basalts give isochron ages and initial ratios of, respectively, 2403±32Ma and ε143Nd=−0.90±0.09, and 2451±79Ma and ε176Hf=+0.4±0.2 (2SE). Bulk tonalites are characterized by average initial γ187Os, ε143Nd, and ε176Hf values of +304±64, +1.8±0.6, and +2.5±1.6 (2SE), respectively, when calculated for the ∼3.21Ga age of the rocks. The komatiitic basalts and tonalites have μ142Nd values of, respectively, +0.5±2.8 and −0.4±5.2 (2SD). By contrast, both the komatiitic basalts and tonalites exhibit positive 182W anomalies of +7.1±4.5 and +12.6±4.5ppm (2SD), respectively.The komatiitic basalts were derived from a komatiitic parental magma with ∼27wt.% MgO; it was modified by both assimilation of the tonalites and fractional crystallization en route to the surface. Lithophile trace element data constrain the degree of crustal contamination to be 4.0±0.4%. Highly siderophile element abundance data indicate that crustal contamination must have had a negligible effect on the Os isotopic composition of the komatiitic parental magma. By contrast, the Nd, Hf, and W isotope systematics of the komatiitic parental magma were strongly modified as a result of assimilation of the tonalites. The positive initial ε143Nd and ε176Hf values of the tonalites indicate that they formed via melting of a precursor with time-integrated suprachondritic Sm/Nd and Lu/Hf. This precursor was most likely ancient mafic crust. The large positive 182W anomaly present in the tonalites requires that the precursor crust incorporated a primordial component with Hf/W that became fractionated, relative to the bulk mantle, within the first 50Ma of Solar System history.The absolute HSE abundances in the mantle source of the Vetreny komatiite system are estimated to be 66±7% of those in the present-day Bulk Silicate Earth. This observation, coupled with the normal 182W/184W composition of the komatiitic basalts, when corrected for crustal contamination (μ182W=−0.5±4.5ppm), indicates that the W-HSE systematics of the Vetreny komatiite system most likely were established as a result of late accretion of chondritic material to Earth. Our present results, combined with isotopic and chemical data available for other early and late Archean komatiite systems, are inconsistent with the model of increasing HSE abundances in komatiitic sources as a result of slow downward mixing into the mantle of chondritic material accreted to Earth throughout the Archean. The observed HSE concentration variations rather reflect sluggish mixing of diverse post-magma ocean domains characterized by variably-fractionated lithophile and siderophile element abundances.