Ultramafic lamprophyre (UML) dykes from the Ferrar Province (Pensacola Mountains) of Antarctica preserve trace element and isotope signatures similar to Bouvet volcanic rocks, which are considered to reflect the palaeo composition of the Bouvet mantle plume. We report Sr, Nd, Pb, and Os isotope compositions for three ultramafic lamprophyre dykes emplaced at 183.2±2.2 Ma, coincident with the main Karoo–Ferrar magmatic event. The ultramafic lamprophyre dykes are characterized by high Ti, Cr, Ni, Nb/La, LaN/YbN, and Mg# values, and are the most primitive rocks of the Ferrar Province. The dykes have initial (183 Ma) 87Sr/86Sr ratios of 0.7044–0.7055, var epsilonNd of 4.6–4.8, 208Pb/204Pb of 39.6–40.3, and 187Os/188Os of 0.120–0.146 and contrast markedly with even the most primitive rocks of the Ferrar and Karoo provinces. The trace element and isotope characteristics have affinities to ocean island basalt (OIB) and the highly radiogenic character of 208Pb/204Pb and 206Pb/204Pb bear closest resemblance to Bouvet, which has previously been postulated as the plume responsible for the Ferrar Province. The ultramafic lamprophyres are believed to be the result of melting enriched Bouvet mantle plume material and represent one of the mantle end members in the Karoo–Ferrar province.