Major and Trace Element Geochemistry and Os Isotopic Compositions of Komatiites From Dundonald Beach, Abitibi Greenstone Belt, Canada

We have examined the major and trace elements, and Os isotopic compositions of a suite of cumulate and spinifex textured komatiitic rocks from the Dundonald Beach area, part of the ∼2.7 Ga Abitibi greenstone Belt, Ontario, Canada. This suite of rocks forms a series from peridotitic komatiites (MgO ∼ 42 wt.% on a volatile-free basis) to komatiitic basalts (MgO ∼8 wt.%). Based on major element oxide ratios (e.g. Al₂O₃/TiO₂ ∼21-26 and CaO/Al₂O₃ typically <= 1) and unfractionated HREE characteristics (e.g. (Gd/Yb)_N ∼0.9-1.1), these rocks are similar to the spatially associated Al-undepleted komatiites from Alexo and Munro Townships. Also, these rocks are strongly LREE-depleted ((La/Sm)_N = 0.41-0.67; (Ce/Yb)_N = 0.41-0.70)) and have variable total REE (4-22 ppm). A strong negative correlation between Mg# and total REEs suggests that the REE patterns of these rocks are primary features of their mantle source. The Re-Os isotope results for whole-rock komatiites and chromite separates from a single flow yield a model 3 isochron age of 2606 +/- 55 Ma. This age is slightly younger ( ∼50 Ma) compared to the U-Pb zircon ages of the associated volcanics reported from the presumed extension of the same Kidd-Munro assemblage in Alexo and Munro Townships. The initial ¹⁸⁷Os/¹⁸⁸Os ratio (0.1090 +/- 0.0019) obtained from the regression is essentially chondritic (γ _Os(T) = -0.2 +/- 1.7). The peridotitic komatiites have the highest Os concentrations and low ¹⁸⁷Re/¹⁸⁸Os ratios (up to ∼4.2 ppb and < 0.5, respectively) among the whole rocks, whereas the komatiitic basalts have relatively low Os concentrations ( ∼0.3 ppb) and high ¹⁸⁷Re/¹⁸⁸Os ratios ( ∼3.1-11.9). For these komatiites, Os was compatible with the mantle residue (D_Os^mantle-melt ∼7.6), whereas Re was moderately incompatible (D_Re ∼0.6), typical of most komatiitic magmas. The absence of a strong correlation between Os and Ni concentrations in the whole-rocks suggests that the distribution of Os in these rocks is not primarily controlled by fractionation of olivine. The apparent D_Re^ol+chmt/liq. ( ∼0.7), on the other hand, suggests that Re was moderately incompatible in olivine and/or chromite during the differentiation of komatiitic magmas. A chondritic initial Os isotopic composition for the mantle source for these komatiites is consistent with that previously reported for the komatiites from Alexo and Munro Townships. Our Os isotopic results for Dundonald komatiites, combined with those reported for Alexo and Pyke Hill komatiites, therefore, suggest that a major portion of the ∼2.7 Ga mantle source for the komatiites in the Abitibi greenstone belt was dominated by Os with chondritic isotopic compositions. Also, the LREE-depleted, yet chondritic Os isotopic composition for the mantle source of these komatiites is indistinguishable from the projected chondritic composition of the contemporaneous depleted convective upper mantle.