238U/ 204Pb in MORB and open system evolution of the depleted mantle

Abstract

Concentrations of U, Th and Pb were determined by isotope dilution on 30 mid-ocean ridge basalt (MORB) glasses. The mean values of 238U/ 204Pb (μ) and 232Th/ 238U (κ) in these glasses are 11.20 and 2.67 respectively. There are significant differences in these ratios between ocean basins. Mean κ values increase in the order Atlantic < Pacific < Indian; mean μ is higher in Pacific MORB than in Indian and Atlantic MORB, but differences between the Atlantic and Indian are not significant. μ correlates strongly with U concentration, but not with Pb concentration or with 206Pb/ 204Pb . The correlation of μ with U but not with 206Pb/ 204Pb reflects the effect of partial melting and fractional crystallization on μ. Since the maximum value of U in the depleted mantle must be less than the bulk silicate earth value of 0.018 ppm, the μ-U correlation can be used to estimate the maximum mean value of μ in the depleted mantle ( μ DM ). This yields a maximum μ DM of 4.67. Similarly, the correlation between μ and Rb/Sr suggests a maximum μ DM of 6.3. These results are consistent with values for μ DM derived using mass balance from the crustal and depleted mantle values of κ, the depleted mantle Pb/Ce ratio, and the crustal abundance of Ce. The value of μ 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 μ of 8–9, and which indicate that μ 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.