The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth's earliest stages of formation(1-8). Xe isotopic anomalies have been linked to degassing during accretion(2-4), but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts(9,10) and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated He-3/He-4 ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts(2-4,9,10). Portions of mantle involved in early high-pressure core formation would also be rich in FeO11,12, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs(13) with high He-3/He-4 ratios.