Nd and Pb isotopic studies of an Archaean layered mafic-ultramafic complex, Western Australia, and implications for mantle heterogeneity

Nd and Pb isotopic ratios have been measured on the same samples from the Archaean layered Millindinna Complex in the western Pilbara Block of Western Australia. The Sm-Nd whole rock/mineral age is 2830 ± 20 Ma (2σ) and the initial ¹⁴³Nd ¹⁴⁴Nd ratio is 0.50892 ± 0.00002 (ε{lunate}Nd -0.7 ± 0.4). The dispersion in lead isotopic ratios is limited and the resulting Pb-Pb whole rock age is 2960 ± 20 Ma (2σ) with a calculated Mi of 8.3. Variations in calculated solTh U and initial ²⁰⁸Pb ²⁰⁶Pb ratios preclude a single source for this complex. Comparison with other Nd-Pb investigations of mafic/ultramafic complexes, particularly those in the time span from 2600-2900 Ma, reveals a trend from positive ε{lunate}Nd and low μ (7.7-7.9) values in Ontario (Munro Township, Alexo, Newton) to negative ε{lunate}Nd and higher μ values in the Suomussalmi, Millindinna and Stillwater Complexes. Although this trend could be the result of either mantle heterogeneity and/or crustal contamination, at the present time we favour crustal contamination.