Revised source characteristics of Western Australian Archean igneous rocks from Pb isotopes

Characterising the source 238 U/ 204 Pb (μ) of igneous rocks based on a rock Pb isotope analysis and its age does not rely on the preservation of initial Pb isotopic compositions of an igneous rock (and its source region) and is applicable to any igneous rock with a known age. Published data from Archean igneous rocks from the Yilgarn and Pilbara Cratons, Western Australia, from komatiites to basalts to granites, yield model source μ values ranging from ∼8.2 to 9.4, compared to the inferred Archean mantle value of ∼8.4 for the Yilgarn Craton. The modelled source μ values of ultramafic to mafic magmas are sensitive to crustal contamination, whereas granites ( s.l. ) show the complete range of μ values. Granites with Sm–Nd isotope characteristics which suggest derivation from older crust, have the highest model source μ values. Analytical uncertainty in model source μ for Archean rocks, using currently available high-precision age and Pb isotope measurements, can be <0.005 μ units, compared with ∼0.05 for standard-bracketed TIMS data and >0.2 for laser-ablation ICP–MS analyses of K-feldspar from least-altered samples. This level of precision permits high-resolution characterisation of igneous source regions. Comparison of Pb- and Nd-isotope maps of the Yilgarn Craton demonstrates that model source μ can resolve additional source heterogeneity and terrane-scale boundaries beyond those evident from Nd isotopes alone. Despite the limited availability of optimal Pb isotope data, the results indicate that μ-based mapping provides a powerful complementary tool for characterising Archean igneous source regions and crustal architecture.