Os isotopes and the origin of the Tasmanian dolerites

New Os isotope obtained for oxides separated from samples of the Furassic d olerites 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 (Os-187/Os-188 = 0.145 +/- 0.049 2 sigma) of these rocks is inconsistent with petrogenetic models involving the assimila tion of continental crust by mantle-derived magmas. Instead, the results ar e more readily explained by models in which the introduction of upper-crust al materials into the mantle contaminants 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 t he roles of asthenospheric vs lithospheric mantle in the genesis of contine ntal 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 initia l ratios precluded the distinction between mantle sources in this particula r case.