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 Austra lia have a large diversity in their chemical and Nd, Sr and Os isotopic com positions. Plains series olivine tholeiites and Cones series nepheline hawa iites have distinctive isotopic compositions and are clearly not related to each other by a simple genetic process. The Cones series nepheline hawaiit es have trace element abundances and Nd (epsilon (Nd) = +4.1 and +3.8), Pb (Pb-206/Pb-204 = 18.62 and 18.70) and Os (gamma (Os) = +6 and +7) isotopic compositions that fall within the range of ocean-island basalts (OIB). In c ontrast, the Plains series olivine tholeiites have lower epsilon (Nd) (+0.9 to +2.3), more radiogenic Sr (Sr-87/Sr-86 = 0.7045-0.7053), and considerab ly higher gamma (Os) (+42 to +250). The relatively low Os concentrations ob served in the Plains series olivine tholeiites [Os = 12-45 parts per trilli on (ppt)] compared with the more 'primitive' Cones series nepheline hawaiit es (Os = 160 and 250 ppt), render them more susceptible to contamination pr ocesses that can obscure their primary mantle signatures. Trace element tre nds and isotopic modelling suggest that the unusual geochemical signatures observed within the Plains series olivine tholeiites are the result of assi milation of continental crust possessing variable isotopic signatures. Howe ver, the Cones series ne-hawaiites have trace element and isotopic composit ions consistent with their derivation from melting of either a mantle plume (OIB-type source) or 'veined' sub-continental lithospheric mantle.