Os–Pb–Nd isotope and highly siderophile and lithophile trace element systematics of komatiitic rocks from the Volotsk suite, SE Baltic Shield

Highly siderophile element (HSE; Ru, Pd, Re, Os, Ir, and Pt) abundances and Re–Os isotope systematics in combination with lithophile trace element and Sm–Nd and Pb–Pb isotopic data are reported for komatiites and basalts from the Volotsk suite in the SE Baltic (Fennoscandian) Shield. The new Sm–Nd and Pb–Pb whole-rock isochron ages of 2850 ± 84 and 2861 ± 150 Ma, respectively, define the emplacement age of the volcanic sequence. This age is statistically indistinguishable from the emplacement age of the lower mafic-ultramafic sequences of the adjacent Sumozero-Kenozero greenstone belt. The Re–Os internal isochron derived from the analysis of a whole rock B2–4 cumulate and three chromite separates yields an age of 2878 ± 81 Ma, which is consistent with the ages obtained from the lithophile element isotopic systems. The komatiites of the Volotsk suite were derived from a mantle source characterized by depletions in highly incompatible lithophile trace elements, with the magnitude of the time-integrated LREE depletion (initial ɛNd = +2.7 ± 0.2) typical of this period in the Earth's mantle history. Yet this source had an initial γ187Os = −0.4 ± 0.4 and, thus, evolved with a time-integrated Re/Os that was identical, within uncertainty, to that of the carbonaceous chondritic reference. In addition, the calculated relative abundances of Os, Ir, Ru, and Pt in the komatiite source are similar to chondrites and may be explained by late (post-core segregation) accretion to the Earth of materials with high absolute and chondritic relative abundances of HSE or by re-entrainment of the core. If the former interpretation is correct, the late accreted materials were well homogenized within the upper mantle by 2.9 Ga. Like many other Archean komatiites, the studied rocks have radiogenic Nd isotopic composition, indicative of their long-term relative depletions in highly incompatible lithophile trace elements, yet the long-term Re/Os in their source remained near-chondritic. Modeling indicates that the komatiite source must have experienced only a small degree of partial melting (2–3%) and extraction of basaltic melts ca. 150 Ma prior to emplacement of the komatiitic sequences. Larger degrees of melting would otherwise have led to loss of a substantial fractionation of Re in the source, which would then have evolved to a subchondritic 187Os/188Os by the time of komatiite formation.