Juvenile lithospheric mantle enrichment and the formation of alkaline ultramafic magma sources: Re-Os, Lu-Hf and Sm-Nd isotopic systematics of the Norseman melnoites, Western Australia

Several 850-Ma melnoites occur in the Norseman region at the southeastern margin of the Archaean Yilgarn Craton, Western Australia. Major, trace element and Rb-Sr, Sm-Nd and Re-Os isotopic data from whole rocks and Re-Os and Lu-Hf isotopic data from mineral separates, including Mg-ilmenite macrocrysts and groundmass magnetite, are used to characterise their mantle source. Compositions of whole rocks and groundmass phlogopite and clinopyroxene show that although these rocks contain abundant Mg-ilmenite macrocrysts, they are not kimberlites. Macrocryst Mg-ilmenite geochemistry, coupled with the paucity of lithospheric mantle minerals, suggests that these macrocrysts are early-formed phenocrysts rather than xenocrysts. Mg-ilmenite geochemistry and estimated fO₂ are correlated with proximity to the cratonic margin, suggesting that the melnoite source was laterally heterogeneous on a scale of ~ 100 km. Whole rock initial ε_Nd values and ⁸⁷Sr/⁸⁶Sr ratios range from + 3.3 to + 5.4 and from 0.70181 to 0.70307, respectively, and Mg-ilmenite initial ε_Hf values range between - 4.8 and + 1.7. These data are consistent with the interpretation that the mantle source experienced the time-integrated enrichment of incompatible elements. Initial ¹⁸⁷Os/¹⁸⁸Os ratios for the uncontaminated on-craton melnoites, ilmenites and magnetites are radiogenic and range from 0.2133 to 0.2599 (γ_Os=+ 76 to + 115), whereas initial ¹⁸⁷Os/¹⁸⁸Os ratios for the Mg-ilmenite at the cratonic margin are significantly more radiogenic (¹⁸⁷Os/¹⁸⁸Os = 0.3918 and 0.4338; γ_Os=+ 223 and + 258). In contrast, the Lu-Hf isotopic data from Mg-ilmenite do not vary at the level of analytical uncertainty obtained between the on-craton and craton margin samples, suggesting the decoupling of Re and incompatible element enrichment through the 100-km section. The Lu-Hf, Sm-Nd and Re-Os isotopic systems also yield Mid-Proterozoic (1000-1400 Ma) model ages that are indistinguishable from the age of the Albany-Fraser Orogen (1100-1300 Ma) which truncates the Yilgarn craton at its southern margin. It is postulated that this episode of plate collision provided the necessary components (e.g. pyroxenites-websterites), via intra-mantle melt addition, to the lithospheric mantle to produce the observed geochemical and isotopic features in the melnoite source. This study highlights the ability of the Re-Os isotopic system, as opposed to the Lu-Hf and Sm-Nd isotopic systems, to fingerprint juvenile lithospheric mantle enrichment events and has important implications for the generation of ocean island-like Nd and Hf isotopic signatures in kimberlites and carbonatites.