EarthRef.org Reference Database (ERR) -- White 1993

Concentrations of U, Th and Pb were determined by isotope dilution on 30 mid-ocean ridge basalt (MORB) glasses. The mean values of U-238/Pb-204 (mu) and Th-232/U-238 (kappa) in these glasses are 11.20 and 2.67 respectively. There are significant differences in these ratios between ocean basins. Mean kappa values increase in the order Atlantic < Pacific < Indian; mean mu is higher in Pacific MORB than in Indian and Atlantic MORB, but differences between the Atlantic and Indian are not significant. Mu correlates strongly with U concentration, but not with Pb concentration or with Pb-206/Pb-204. The correlation of mu with U but not with Pb-206/Pb-204 reflects the effect of partial melting and fractional crystallization on mu. Since the maximum value of U in the depleted mantle must be less than the bulk silicate earth value of 0.018 ppm, the mu-U correlation can be used to estimate the maximum mean value of mu in the depleted mantle (mu(DM)). This yields a maximum mu(DM) of 4.67. Similarly, the correlation between mu and Rb/Sr suggests a maxiMUM mu(DM) of 6.3. These results are consistent with values for mu(DM) derived using mass balance from the crustal and depleted mantle values of kappa, the depleted mantle Pb/Ce ratio, and the crustal abundance of Ce. The value of mu(DM) thus seems to be less than the bulk earth value, estimated to be 8 +/- 2. This is consistent with the incompatible-element-depleted character of this reservoir, but inconsistent with Pb isotope ratios, which record time-integrated values for mu of 8-9, and which indicate that mu has increased over time. This inconsistency requires that the depleted mantle be an open system with a residence time for Pb of 1 +/- 0.5 Ga. Neither the continental crust nor a primitive lower mantle have the appropriate isotopic composition to be the source of Pb in the depleted mantle. Mantle plumes do have the appropriate isotopic composition, and as little as 10% of the observed plume flux is needed to mix with the depleted mantle to supply the required flux of Pb.