Platinum is fractionated from osmium primarily as a consequence of processes involving sulfide and metal crystallization. Consequently, the ¹⁹⁰Pt¹⁸⁶Os isotope system (¹⁹⁰Pt → ¹⁸⁶Os + α) shows promise for dating some types of magmatic sulfide ores and evolved iron meteorites. The first ¹⁹⁰Pt ¹⁸⁶Os isochrons are presented here for ores from the ca. 251 Ma Noril'sk, Siberia plume, and for group IIAB magmatic iron meteorites. Given the known age of the Noril'sk system, a decay constant for ¹⁹⁰Pt is determined to be 1.542 × 10⁻¹²a⁻¹, with ±1% uncertainty. The isochron generated for the IIAB irons is consistent with this decay constant and the known age of the group. The ¹⁸⁶Os/¹⁸⁸Os ratios of presumably young, mantle-derived osmiridiums and also the carbonaceous chondrite Allende were measured to high-precision to constrain the composition of the modern upper mantle. These compositions overlap, indicating that the upper mantle is chondritic within the level of resolution now available. Our best estimate for this ¹⁸⁶Os/¹⁸⁸Os ratio is 0.119834 ± 2 (2σ_M). The ¹⁹⁰Pt/¹⁸⁶Os ratios determined for six enstatite chondrites average 0.001659 ± 75, which is very similar to published values for carbonaceous chondrites. Using this ratio and the presumed composition of the modern upper mantle and chondrites, a solar system initial ¹⁸⁶Os/¹⁸⁸Os ratio of 0.119820 is calculated. In comparison to the modern upper mantle composition, the ¹⁸⁶Os/¹⁸⁸Os ratio of the Noril'sk plume was approximately 0.012% enriched in ¹⁸⁶Os. Possible reasons for this heterogeneity include the recycling of Pt-rich crust into the mantle source of the plume and derivation of the osmium from the outer core. Derivation of the osmium from the outer core is our favored model.