Osmium Isotopic Evidence for Crust–Mantle Interaction in the Genesis of Continental Intraplate Basalts from the Newer Volcanics Province, Southeastern Australia

Basalts from the Quaternary Newer Volcanics Province in southeastern Australia have a large diversity in their chemical and Nd, Sr and Os isotopic compositions. Plains series olivine tholeiites and Cones series nepheline hawaiites have distinctive isotopic compositions and are clearly not related to each other by a simple genetic process. The Cones series nepheline hawaiites have trace element abundances and Nd (εNd = +4·1 and +3·8), Pb (206Pb/204Pb = 18·62 and 18·70) and Os (γOs = +6 and +7) isotopic compositions that fall within the range of ocean-island basalts (OIB). In contrast, the Plains series olivine tholeiites have lower εNd (+0·9 to +2·3), more radiogenic Sr (87Sr/86Sr = 0·7045–0·7053), and considerably higher γOs (+42 to +250). The relatively low Os concentrations observed in the Plains series olivine tholeiites [Os = 12–45 parts per trillion (ppt)] compared with the more ‘primitive’ Cones series nepheline hawaiites (Os = 160 and 250 ppt), render them more susceptible to contamination processes that can obscure their primary mantle signatures. Trace element trends and isotopic modelling suggest that the unusual geochemical signatures observed within the Plains series olivine tholeiites are the result of assimilation of continental crust possessing variable isotopic signatures. However, the Cones series ne-hawaiites have trace element and isotopic compositions consistent with their derivation from melting of either a mantle plume (OIB-type source) or ‘veined’ sub-continental lithospheric mantle.