Modeling lead isotopic heterogeneity in mid-ocean ridge basalts

We have investigated the evolution of the U–Th–Pb system in a 4-reservoir model with an isolated mantle of bulk silicate Earth composition, a depleted mantle, and a two-layered continental crust. These models not only quantify the mean composition of the reservoirs, but also explicitly track the creation, evolution and destruction of heterogeneities (sub-reservoirs) within the reservoirs. The heterogeneities are sampled statistically during melting that produces basalts, and the model results are directly compared with observed Pb-isotope arrays.The resolution of the Pb paradox and a satisfactory fit to the MORB 207Pb–206Pb pseudo-isochron require the removal of a large fraction (∼ 50%) of lead from subducted oceanic crust. The pseudo-isochron slope represents a mixing line between the “background matrix” (average of small length scale sub-reservoirs which appear in roughly the same relative proportion in all samples) in the MORB source and the larger-scale sub-reservoirs (which appear in varying proportion in different samples). The background matrix is similar in isotopic composition to the apparent components FOZO and C, and hence these components need not be in the lower mantle. The kappa conundrum only presents a problem if the mantle is treated as a closed system.