Os isotopes and the origin of the Tasmanian dolerites

Abstract

New Os isotope data obtained for oxides separated from samples of the Jurassic dolerites of Tasmania (Australia) are used to constrain the petrogenesis of the Ferrar continental flood basalt province. It is proposed here that the unradiogenic initial Os ratio (187Os/188Os = 0·145 ± 0·049 2σ) of these rocks is inconsistent with petrogenetic models involving the assimilation of continental crust by mantle-derived magmas. Instead, the results are more readily explained by models in which the introduction of upper-crustal materials into the mantle contaminates the source of these magmas before melting. The increasing body of Os isotope data for mantle-derived rocks also provides new opportunities to assess whether any distinction between the roles of asthenospheric vs lithospheric mantle in the genesis of continental flood basalts might be possible. It is concluded that, in the absence of an Os-rich phase with low Re/Os, the uncertainties in calculated initial ratios preclude the distinction between mantle sources in this particular case.