Os isotope constraints on the origin of the 2.7 Ga Boston Creek Flow, Ontario, Canada

Abstract

The Re/Os isotopic systematics of the ca. 2.7 Ga Boston Creek flow of the Abitibi Greenstone Belt of Ontario are examined. The approximately 100-m thick Fe-rich komatiite flow became differentiated as it crystallized, and ranges from an olivine cumulate at the base to a gabbrodiorite near the top. The melt from which this flow crystallized was enriched in FeO, and was likely derived from an Fe-rich reservoir within the Archean mantle. The melt was also enriched in light and heavy rare earth elements, Zr, Hf, and many other incompatible trace elements. Whole rock samples believed to be most representative of parental melt compositions indicate that the parental melt contained 15–17 wt.% MgO and that concentrations of Re and Os were approximately 0.6 and 0.2 ppb, respectively. The Re and Os concentrations are somewhat lower than concentrations estimated for most komatiitic and picritic liquids. As is also common for differentiated komatiitic, picritic and basaltic flows, there is a positive correlation between Os concentration and MgO. There is a weak inverse correlation between Re and MgO. The Re/Os systematics of most samples appear to have remained closed since crystallization. Seven samples define an isochron with an age of 2708±13 Ma (2 σ), which is within uncertainty of the zircon age previously established for the rocks. Three of the samples plot well above this isochron and most likely indicate late-stage addition of radiogenic 187Os from mineralized rocks. The initial ratio of the isochron is 0.1044±6 (2 σ) equivalent to an initial γ Os value of −3.8±0.5. The very depleted initial ratio, compared to current models for upper mantle evolution of Os, requires derivation of the Os from a long-term Re depleted source. The Re depletion in the source probably occurred 500 to 1000 Ma prior to the generation of the flow. The depleted initial ratio is consistent with derivation of the parental melt from early Archean, melt-depleted subcontinental lithospheric mantle. The Fe and incompatible trace element enrichment may have occurred during a metasomatizing event that caused renewed melting at 2.7 Ga. Alternatively, the source of the flow could represent a portion of the mantle that retained some characteristics of early Earth formation, such as majorite fractionation from a magma ocean, or an initially higher FeO content. In the latter instance, an Archean melt depletion event is still probably necessary to explain the depleted Os isotopic composition.