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

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 (2 sigma). The initial (187)Os/(188)Os 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 similar to 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 (187)Os/(188)Os values of ca. 25 at similar to 2687 Myr. In contrast, the surrounding gneisses and metasediments preserve present-day measured (187)Os/(188)Os 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. (C) 2000 Elsevier Science B.V. All rights reserved.