A Comparative Os Isotopic Study of Al-undepleted Komatiites From the Abitibi Greenstone Belt and Ti-enriched Komatiites From the Finnish Lapland

The komatiites from ~ 2.7 Ga Alexo Township, in the Abitibi Greenstone Belt, Canada are dominantly of Al-undepleted type (AUK; Al₂O₃/TiO₂ ~ 19-21), whereas those from the ~ 2 Ga Finnish Lapland are Ti-enriched komatiites (TEK) that translate into significantly lower Al₂O₃/TiO₂ ratios (typically ~13). Also, the TEKs are more differentiated (MgO typically 14-18 wt.%) relative to the AUKs (MgO >~ 30 wt.%). We have compared the Os isotopic compositions, and Re and Os concentrations of the whole rocks and chromite separates from these two chemically distinct types of flows. Our objectives were: [1] to understand the differences, if any, in the partitioning behaviors of both Re and Os in these two types of komatiites; and [2] to determine if the observed heterogeneities in trace elements and REE characteristics for the TEKs (Hanski et al., 2001) are accompanied by any evidences for heterogeneities in their initial Os isotopic compositions. The whole-rock TEKs have significantly higher concentrations of both Re and Os (0.5-1.4 ppb of Re and 1-6.9 ppb of Os) relative to the AUKs (typically 0.2-0.3 ppb of Re and between ~1-2 ppb of Os). Also, the concentrations of Os (~63-161 ppb) and Re (up to ~ 13 ppb) in chromites in the TEKs are significantly higher, as compared to the AUKs (~11-27 ppb and <1 ppb, respectively). For the AUKs, Os was moderately compatible with the mantle residue (D_Os^mantle-melt ~ 2.1), whereas, Re was incompatible (D_Re^mantle-melt ~ 0.20). The apparent D_Os^{olivine/liquid} and D_Re^{olivine/liquid} in Alexo komatiites are 1.7 and 0.66, respectively. Our results, therefore, suggest that Os is slightly compatible whereas Re is moderately incompatible in olivine or a coprecipitating phase during magmatic differentiation of komatiitic liquids. On a plot of MgO versus Re and Os, the AUKs from the Alexo and Munro Townships, and the TEKs show a consistently incompatible behavior of Re in olivine, whereas the apparent compatibility of Os during the magmatic differentiation of the TEKs seems to be controlled by multiple factors, in addition to olivine fractionations. The initial ¹⁸⁷Os/¹⁸⁸Os for whole rocks and chromites in the AUKs are essentially chondritic (γ_Os = -0.1+/-1.0). Similarly, six chromite separates with the lowest ¹⁸⁷Re/¹⁸⁸Os ratios (<0.1) in the TEKs define a narrow range of γ_Os (between -0.3 to +0.1). The absence of significant crustal contamination is consistent with a LREE-depleted (ɛ_Nd ~ +4) mantle source for the TEKs. Combined, our results suggest that the mantle sources for either of these two komatiite types have not undergone any long-term fractionation in Re/Os ratio from the putative chondritic composition. These results also suggest an absence of any large-scale Os isotopic heterogeneity in the respective mantle sources for these two komatiite types.