Fig. 4. Heavy iron subduction.

At left, a mid-ocean ridge produces basaltic crust, while the underlying oceanic peridotite residue has low Re/Os ratios and time-integrated unradiogenic Os (variably low ¹⁸⁷Os/¹⁸⁸Os) from prior mantle convection and melting. (1) Serpentinization at mid-ocean ridges, transforms, and bending-related faults leads to enrichment of isotopically heavy iron in magnetite and Fe-Ni alloys. (2) Subduction conveys these iron-rich phases to the transition zone/lower mantle. (3) Fe-Ni-C-S metallic melt with high δ⁵⁶Fe and unradiogenic Os is trapped as inclusions in large gem diamonds (CLIPPIR diamonds) such as those examined in this study. (4) Upward transport of diamonds by localized buoyancy associated with diamond formation or an external mechanism such as a plume, with ultimate exhumation to surface via kimberlite volcanism.