Resolving crystallisation ages of Archean mafic–ultramafic rocks using the Re–Os isotope system

Abstract

Rhenium–Osmium (Re–Os) isotope and elemental data are presented for mafic–ultramafic rocks from the central region of the Lewisian Archean terrain in northwest Scotland. These results give a best estimate for the time of emplacement of the mafic–ultramafic bodies of 2686.7±14.7 Myr (2σ). The initial 187Os/ 188Os isotope ratio of 0.10940±0.00076 indicates that such material possessed a chondritic Os isotope composition, which suggests that these rocks were formed by direct melting of mantle material, consistent with major and trace element constraints on their formation. Nevertheless, the Re–Os systematics of some of the mafic–ultramafic rocks in the Lewisian have been significantly disturbed, such that the original age information has been lost. These rocks lie on a regression line that defines an age of ∼3260 Myr, and a negative initial Os isotope composition, suggesting perturbation of the Re–Os system, either through assimilation or post-emplacement elemental exchange. Such a process also appears to have affected the Sm–Nd systematics in the same samples. Crustal assimilation can account for the observed Os and Nd isotope variations but only if the assimilated material possessed 187Os/ 188Os values of ca. 25 at ∼2687 Myr. In contrast, the surrounding gneisses and metasediments preserve present-day measured 187Os/ 188Os values of between 3 and 16. Rather, the spatial variation of initial Os and Nd isotope compositions suggests that isotope perturbation was caused by local sub-solidus element exchange between different lithologies, consistent with major element data and petrographic observations. Taken together, these results highlight the utility of the Re–Os isotope system for obtaining precise ages for Archean mafic–ultramafic rocks, and as a sensitive petrogenetic tracer capable of discriminating between assimilation or elemental exchange.