EVIDENCE FROM RE-OS ISOTOPES FOR PLUME LITHOSPHERE MIXING IN KAROO FLOOD-BASALT GENESIS

THERE is abundant evidence to link continental flood basalt provinces with mantle plume activity1,2; however, flood basalts often have radiogenic isotope signatures that are outside the range for plume sources as recorded in ocean-island basalts. These signatures are more readily attributed to source regions within the continental lithosphere, and there are strong indications that it is often the mantle lithosphere that contributes isotopically 'enriched' (low-Nd-143/Nd-144) material3,4. There are objections, however, to models that propose cold, refractory mantle lithosphere as an important source of basaltic magma5. The rhenium-osmium system may provide new constraints on the relative importance of plume and sub-continental lithospheric mantle (SCLM) in basalt genesis: samples of SCLM brought to the surface as xenoliths commonly have low Os-187/Os-188 ratios6,7, whereas ocean-island basalts tend to have higher than chondritic Os-187/Os-188, and old continental crust has even higher (more radiogenic) ratios. Here we report initial Os-187/Os-188 ratios for 190-Myr-old picrite basalts from the Nuanetsi region of the Karoo flood basalt province, which are most readily explained as mixtures between enriched SCLM and sub-lithospheric material, most probably derived from a mantle plume.