Allegre's Lead Paradox Revisited

Allegre (1969), using a generalized Concordia plot, was first to note that in a 4.55 Ga old Earth, Pb has been removed from the mantle in preference to U, even though magmatism tends to do the opposite. He noted that this "contradiction" might require a separate reservoir where the missing lead is stored. This "contradiction", more commonly expressed by the conventional Holmes-Houtermans diagram, has become known as the "lead paradox." Several models have been proposed to resolve it, ranging from late "core pumping" of Pb (as suggested by Allègre) to Pb storage in the mantle transition zone (Murphy et al. 2003) or in ancient parts of the lower continental crust. The idea of late core pumping has recently been revived by Wood & Halliday (2005) who suggested that Pb was sequestered through late sulfide segregation into the core. Here we propose that crystallization of Ca-perovskite, accompanied by segregation of a dense silicate melt toward the core-mantle boundary, can account for the apparently elevated U/Pb ratio of the accessible silicate Earth, particularly if the partition coefficient for U and Th in Ca-perovskite is as high as 400 as suggested by Corgne and Wood (2005). Such a dense liquid is the inferred consequence of the measured crossover of melting temperatures of silicate perovskite and ferro-periclase at about 1200 km depth, and the predicted Fe-rich eutectic and low melting temperatures in the lowermost mantle (Boehler, 2000). In addition, lower-mantle melt segregation with residual Ca-perovskite will cause a decrease in Nb/Ta from the primitive (chondritic) value in the accessible mantle, another, more recently discovered puzzle of mantle geochemistry. Downward segregation of a dense melt fraction and final solidification of the lowermost mantle may have been a slow process requiring more than 100 Ma, and involving a substantial fraction of the mantle. We suggest that this process served to stabilize the D'' reservoir storing solar noble gases (Tolstikhin & Hofmann, 2005) and subchondritic 142Nd/144Nd ratios (Boyet & Carlson, 2005), which these authors have ascribed to subduction of a primordial crust at an even earlier stage. Allegre, C.A. (1969) Earth Planet. Sci. Lett. 5, 261-269. Boehler, R. (2000) Reviews Geophys. 38, 221-235. Murphy, D.T. et al. (2003) J. Petrol. 44, 39-53. Tolstikhin, I.N. & Hofmann, A.W. (2005) Phys. Earth Planet. Int. 148, 109-130. Boyet, M. & Carlson, R.W. (2005) Science 309, 576-581. Corgne, A & Wood, B.J. (2005) Contrib. Min. Pet. 149, 85-97.