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 εNd values and 87Sr/86Sr ratios range from +3.3 to +5.4 and from 0.70181 to 0.70307, respectively, and Mg-ilmenite initial εHf 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 (γOs=+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; γOs=+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.