Figure 3

²⁰⁶Pb/²⁰⁴Pb versus A. ²⁰⁷Pb/²⁰⁴Pb and B. ²⁰⁸Pb/²⁰⁴Pb diagrams showing the evolution of the carbonate fraction of sample 185 from the 2.8 Ga Bulk silicate Earth⁷ to present (dash lines), and our model for the origin of HIMU OIB (solid lines). The model was constructed by metasomatizing (mixing) 99% of a slightly younger, 2.4 Ga evolving upper mantle (²⁰⁶Pb/²⁰⁴Pb = 13.728, ²⁰⁷Pb/²⁰⁴Pb = 14.614, ²⁰⁸Pb/²⁰⁴Pb = 33.210, 0.051 ppm Pb) ref.⁷ with 1% of a hypothetical Archaean limestone (²⁰⁶Pb/²⁰⁴Pb = 24.404, ²⁰⁷Pb/²⁰⁴Pb = 16.200, ²⁰⁸Pb/²⁰⁴Pb = 43.00; open square – M) containing the Pb of the most enriched, 8% partial melt from carbonated pelite (67.2 ppm; see ref.⁴¹ for details of experimental data, and modeling). The latter Pb concentration was used because of the dearth of experimental data for partial melting of limestone at high pressures. Each upper mantle and total silicate Earth symbol represents 0.4 Ga interval whereas each tick mark along the model mixing line represents 0.2 increment of melt contribution. See text for additional discussion.