EVIDENCE FROM HAFNIUM ISOTOPES FOR ANCIENT SUB-OCEANIC MANTLE BENEATH THE RIO-GRANDE RIFT

UNDERSTANDING the nature of the mantle underlying continents is important for balancing the inventory of the Earth's main chemical reservoirs1,2 and for constraining aspects of mantle convection, such as the extent to which continental lithosphere is coupled to the asthenosphere beneath3-5. Strontium, neodymium and lead isotope studies of basaltic lavas from regions of continental extension, such as the southwestern United States, have identified at least two components of the sub-continental mantle6: one composed of ancient lithosphere, enriched by the addition of metasomatic fluids or crustal material, and an asthenospheric component with Sr, Nd and Pb isotope compositions matching those of ocean-island basalts. The lutetium-hafnium isotope system7-9 provides additional information not obtainable from the other systems; in particular, by betraying the presence of garnet during melting10,11 (hence constraining the depth of previous melting events of the source mantle). Here we report that basalts derived from upwelling asthenosphere in the region of the Rio Grande rift, southwestern United States, have Nd and Hf isotope ratios that lie significantly off the ocean-island Nd-Hf array10. We interpret the low Hf-176/Hf-177 ratios in these basalts as reflecting derivation from ancient asthenospheric mantle that melted at shallow levels beneath the oceans. Hafnium isotopes in continental basalts may thus provide evidence for large-scale overriding by continents of sub-oceanic mantle.