The thermal diffusion of an Mg implant in Si has been measured with SIMS and compared to RIMS ( resonant ionisation mass spectrometry) measurements of Mg implantation and diffusion in Si wafers exposed to solar wind irradiation in the NASA Genesis mission. The Genesis samples show much more surface segregation that the samples annealed in the laboratory, due to diffusion and segregation of the implanted Mg to the heavily damaged near surface regions of the Genesis wafers. This Mg transport has been modeled by solving a set of stiff differential equations and found to agree with RIMS measurements for a Mg interstitial migration energy of 0.7 eV. (C) 2008 Elsevier B. V. All rights reserved.