Os Isotopic Composition of Late-Archean Komatiites From Alexo in the Abitibi Greenstone Belt, Canada

We have examined the Re-Os isotope systematics and major and trace element characteristics of a suite of komatiitic flows from Alexo area in the Abitibi greenstone belt. The objective of this study was to determine the initial Os isotopic composition of the late-Archean mantle source(s) of these rocks. Most of the samples have high Os concentrations and low Re/Os ratios. Moreover, some of these rocks have high Cr concentrations, and, therefore, direct determination of initial Os isotopic composition from primary chromites is possible. The Re-Os isochron results for whole rocks and chromites yield an age of 2760 +/- 79 Ma. This age is in general agreement with Pb-Pb and Sm-Nd isochron ages of 2690 +/- 15 Ma and 2752 +/- 87 Ma, respectively (Dupre et. al., 1984). The initial Os isotopic composition (¹⁸⁷Os/¹⁸⁸Os_(I) = 0.1081 +/- 0.0016) obtained from this isochron is essentially chondritic (γ _Os = -0.5 +/- 1.5). Our results, therefore, suggest an absence of any large-scale heterogeneity in initial Os isotopic composition of the mantle source(s) of these komatiites. Duplicate analyses of a differentiated (Mg# 0.66) and yet presumably cogenetic basaltic komatiite (¹⁸⁷Re/¹⁸⁸Os > 8.14 and Os < 0.46 ppb) yield relatively radiogenic initial γ _Os values of +15.9 and +12.6. Such radiogenic initial isotopic compositions in an otherwise chondritic association may indicate: [1] open-system behavior due to hydrothermal alteration and low-grade metamorphism and/or [2] crustal contamination. The latter interpretation is consistent with recently studied komatiites and associated magmatic ores from this area (Lahaye et. al., 2001).