Osmium isotopic evidence for crust-mantle interaction in the genesis of continental intraplate basalts from the Newer Volcanics Province, southeastern Australia
Js. Mcbride et al., Osmium isotopic evidence for crust-mantle interaction in the genesis of continental intraplate basalts from the Newer Volcanics Province, southeastern Australia, J PETROLOGY, 42(6), 2001, pp. 1197-1218
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.