CONSTRAINTS ON MANTLE MELTING AT MIDOCEAN RIDGES FROM GLOBAL U-238 TH-230 DISEQUILIBRIUM DATA

The extent of radioactive disequilibrium between U-238 and its daughte r product Th-230 in midocean-ridge basalts from around the world is ne gatively correlated with axial ridge depth; local positive correlation s with inferred mantle source composition are also observed. The large r Th-230 excesses seen in samples from shallow ridges reflect a larger melt contribution from a garnet-bearing source which may be explained by a deepening of the solidus in hotter mantle regions. These observa tions provide important constraints for modelling melt generation at m id-ocean ridges.