Compared to other incompatible lithophile isotope systems the Re-Os isotopic system is very well suited for tracing extraction and subduction of Mid-Ocean Ridge Basalt (MORB). During partial melting Re is mildly incompatible whereas Os is strongly compatible resulting in high Re/Os elemental ratios in MORB and correspondingly low Re/Os ratios in the depleted solid residue left behind. As ¹⁸⁷Re decays to ¹⁸⁷Os the ¹⁸⁷Os/¹⁸⁸Os ratios of MORB and depleted mantle residue will diverge. MORB develops high, radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios while the depleted mantle residues develop relatively low, un-radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios. When MORB is subducted back into the upper mantle re-equilibration with the depleted mantle residue is expected to take place, but the time scales and length scales on which this re-equilibration occurs are poorly constrained. We report osmium isotope compositions of more than 700 mantle-derived Os-rich platinum-group element alloys thought to represent the upper mantle. Our data form a wide, essentially Gaussian distribution demonstrating that, with respect to Re-Os isotope systematics, the upper mantle is extremely heterogeneous. Depleted and enriched domains can remain un-equilibrated on a time scale of billions of years. Effective equilibration between these domains probably requires high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both Os and Re. The Gaussian shape of the Os isotope distribution is a signature of a random mixing process between depleted and enriched domains in a plum-pudding configuration in the upper mantle, rather than the result of individual melt depletion events. Our data lend strong support to the view that secondary metasomatic melt-rock processes define not only the major and trace element chemistry of mantle derived rocks but also their Re-Os isotope systematics. These processes can mask primary melt depletion features related to previous times that the material passed through an upper mantle region with high degrees of partial melting.