Rb/Sr data for seven basaltic provinces (K-Ar ages 50-0 Ma) in southeastern Australia imply isotopic heterogeneities in the mantle sources. The total range of 87Sr/ 86Sr is 0.7031-0.7054. Effects of crustal contamination are negligible, since the rocks analyzed represent primary or primitive magma compositions. The inferred scales of heterogeneity range from <1 km for small intraprovince variations, to in the order of 100 km for the larger differences between provinces. Correlation of regional high 87Sr/86Sr in basaltic rocks with the presence of amphibole-bearing upper mantle xenoliths suggests that the degree of metasomatic activity in the underlying mantle is a major control on the Rb/Sr and 87Sr/86Sr values of mantle source volumes and partial melts derived from these. Xenolith data also indicate that both pervasive metasomatism and the presence of crystallized melts or cumulates as veins and dykes in mantle wall rock are possible mechanisms for metasomatic additions. "Mantle isochrons" can be constructed both within some provinces and between provinces. However, episodic metasomatism in the mantle source regions, with correlated enrichment in Rb/Sr and 87Sr/86Sr, can produce artificial "isochrons" which may have no relevance to mantle differentiation events.