Credit: © Paul Morin (University of Minnesota)

The isotopic decay system of uranium and thorium to lead, with its wide range of half-lives between about 4 and 14 billion years, is widely used for tracing the extraction and recycling of continental crust, but it has been riddled with a plethora of interlinked paradoxes. A recent model study of the mantle helps disentangle the complexities of lead cycling.

Richard O'Connell from Harvard University and colleagues1 numerically simulate the formation, evolution and destruction of geochemical heterogeneities within four reservoirs of the solid Earth — upper and lower crust, depleted mantle and lower mantle. They find that the model explains the available data best if about 60% of the lead from the subducting oceanic crust is assumed to be transferred to the mantle wedge. The researchers determine a critical length scale below which existing heterogeneities would eventually be mixed — for example, in a volcanic eruption — and would therefore not appear in rock samples. They find that mid-ocean-ridge basalts are composed of a mixture of a handful of large-scale reservoirs such as the oceanic crust and a background material consisting of all heterogeneities that are smaller than the critical length.

In addition, the simulations suggest that previously postulated distinct primordial mantle reservoirs may in fact correspond to this background material, which resembles a well-stirred marble cake.