Re–Os systematics of the ca. 2.7-Ga komatiites from Alexo, Ontario, Canada

Abstract

The Re–Os isotope systematics of a suite of komatiitic flows from Alexo Township, in the Abitibi greenstone belt, Ontario were examined in order to characterize the partitioning behaviors of Re and Os during the generation and subsequent differentiation of komatiitic magma, and also to determine the initial Os isotopic composition of the late-Archean mantle source of these rocks. This suite of komatiites was suitable for these objectives because of their high Os concentrations, ≥∼1 ppb, and low 187Re/ 188Os ratios (ranging between 0.4 and 2.4). The Re–Os isotope results for whole-rock komatiites and chromite separates yield a model 3 isochron age of 2762±76 Ma. This age is in general agreement with previously determined U–Pb ages of zircons from associated felsic units and also with Sm–Nd and Pb–Pb isochron ages of these flows. This suite of Alexo komatiites, thus, demonstrates relatively limited mobility of Re and Os at the whole-rock scale, even after large-scale alteration and ambient lower greenschist facies metamorphism. The initial Os isotopic composition ( 187Os/ 188Os (I)=0.1080±0.0012) obtained from the regression is essentially chondritic ( γ Os=−0.1±1.0). Our results, therefore, suggest an absence of any large-scale deviation in initial Os isotopic composition of the mantle source of these komatiites relative to the putative chondritic composition of the contemporaneous convective upper mantle. For these komatiites, Os was moderately compatible with the mantle residue ( D Os mantle-melt∼2.1), whereas Re was incompatible ( D Re∼0.20). These D's are consistent with those previously estimated for other Archean komatiites. 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. The concentrations of Re and Os in Alexo komatiites are also comparable to those reported for komatiites from other greenstone belts. The apparent magmatic behaviors of both Re and Os in the whole-rock komatiites indicate that the Re–Os elemental systematics are primary features of these rocks. The Os isotopic results obtained for the Alexo komatiites, combined with those from the spatially associated Munro Township komatiites suggest a source for komatiites in the southern volcanic zone of the Abitibi greenstone belt that was dominated by Os with a chondritic isotopic composition. Also, the Os isotopic results, combined with previous Nd and Pb isotopic results, are consistent with derivation of the komatiites from either the contemporaneous convective upper mantle or a deep mantle source with isotopically similar characteristics. These results imply that if the chondritic Os isotopic composition of the mantle sources for the komatiites were set by a chondritic “late veneer”, most late accretion and subsequent homogenization within the mantle must have occurred prior to their eruption at ∼2.7 Ga.