Understanding Re–Os systematics and model ages in metamorphosed Archean ultramafic rocks: A single mineral to whole-rock investigation

Abstract

187Rhenium–187Os model ages are used to infer the timing of crust-mantle differentiation events and evolution of lithospheric mantle. However, ancient samples often have long and complex histories of metamorphism, metasomatism and deformation that may be problematic if these processes caused disturbance the Re–Os system. Such processes have been shown to disturb the Re–Os systematics of ophiolitic chromitites, but the effect on stratiform chromitites has not previously been investigated. Here we present a detailed petrographic and Re–Os isotopic study of chromitites, chromite-bearing meta-peridotites and single chromite grains from the Ujaragssuit nunât layered ultramafic intrusion, southwest Greenland, which has experienced intense deformation and at least two episodes of metamorphism up to amphibolite facies. We report the first ever Re–Os isotope and platinum-group element concentration data for single chromite grains achieved by single grain dissolution and isotope dilution. Micrometric Os-, Ir-, Ru – sulphide and sulpharsenide inclusions in chromite occur in the massive chromitites; these accessory phases host a significant portion of the Os, Ir and Ru in these samples. The remaining Os-Ir-Ru budget, along with Pt and Pd, appears to be homogeneously distributed within the chromite, occurring either in the lattice itself, as evenly distributed sub-micrometric alloy inclusions in chromite (unresolvable using the techniques applied in this study) or a combination of both. Rhenium is hosted in silicates, as predicted by previous studies. Both single-grain and whole-rock Re–Os isotope systematics yield unfeasibly young Re–Os model ages due to their radiogenic 187Os/188Os compositions. This could result from country rock contamination of the original melt from which the Ujaragssuit nunât intrusion crystallised, and/or from disturbance of the Re–Os isotope systematics of these rocks during regional metamorphic events at ∼2.8 and/or ∼3.75Ga. We conclude that it is vital to establish a complete sample history before interpreting Re–Os isotope data and the validity of model ages, as the Re–Os system is sensitive to disturbance. It is feasible that in some cases single chromite grains or chromite mineral separates may offer a more robust and meaningful isotopic record than silicates, because Re is preferentially hosted in silicates relative to chromite. Alternatively, the 190Pt–186Os decay system, applied to whole-rock samples, may provide a more reliable chronometer than 187Re–187Os model ages in samples that have experienced crustal contamination and/or metamorphic disturbance.