Investigating Influences on the Pb Pseudo‐Isochron Using Three‐Dimensional Mantle Convection Models With a Continental Reservoir

Abstract

Abstract For mid‐ocean ridge basalts and ocean island basalts, measurements of Pb isotope ratios show broad linear correlations with a certain degree of scatter. In 207Pb/204Pb—206Pb/204Pb space, the best fit line defines a pseudo‐isochron age (τPb) of ∼1.9 Gyr. Previous modeling suggests a relative change in the behaviors of U and Pb between 2.25 and 2.5 Ga, resulting in net recycling of HIMU (high U/Pb) material in the latter part of Earth's history, to explain the observed τPb. However, simulations in which fractionation is controlled by a single set of partition coefficients throughout the model runs fail to reproduce τPb and the observed scatter in Pb isotope ratios. We build on these models with 3D mantle convection simulations including parameterizations for melting, U recycling from the continents and preferential removal of Pb from subducted oceanic crust. We find that both U recycling after the great oxygenation event and Pb extraction after the onset of plate tectonics, are required in order to fit the observed gradient and scatter of both the 207Pb/204Pb—206Pb/204Pb and 208Pb/204Pb—206Pb/204Pb arrays. Unlike much previous work, our model does not require accumulations of subducted oceanic crust to persist at the core‐mantle boundary for long periods of time in order to match geochemical observations.

Keywords

• mantle convection
• mantle model
• Pb pseudo‐isochron
• recycled oceanic crust