The oldest known diamond deposits occur in 2.7 Ga shoshonitic lamprophyre dikes and breccias in the southern Superior Province, Canada. Given the shallow mantle sources of shoshonite magmas, the diamonds are likely to be subductionrelated. During normal subduction, however, the diamond stability window lies below the source regions of typical shoshonites. Particle-in-cell finite-element modeling of normal and flat subduction indicates that shallow subduction angles optimise the conditions required for a diamond-lamprophyre association. Our results predict diamond formation in parts of the mantle wedge following flat subduction and two generations of diamonds in the subducted slab. Any of these diamond sources may be sampled by lamprophyric magmas during a short-lived period of thermal re-equilibration driven by orogeny. In contrast, normal subduction does not create a mantle wedge diamond population, and long-lived flat subduction without orogeny does not provide a means for lamprophyric magmas to access mantle wedge diamonds.