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Description: Interactions between dislocations in olivine and extrinsic cation vacancies created under hydrous or oxidizing conditions may influence the rheology of the Earth's upper mantle. In this study, we use atomic-scale simulations to calculate segregation energies for bare and protonated Mg vacancies to M1 and M2 sites in the core regions of [100](010) and [001](010) edge dislocations, and [100] and [001] screw dislocations. Calculated segregation energies are different for the two symmetry distinct M sites. The segregation energies calculated for the tightest binding M1 sites around [100] screw and [100] (010) edge dislocations are comparable to those calculated for the tightest binding M2 sites. Concentrations of M2 vacancy-related defects will thus be low in the core regions of these dislocations, given the comparatively high energy of these defects in the bulk lattice. In contrast, segregation energies for M2 defects to [001](010) edge dislocation cores are considerably lower than for equivalent M1 defects, and M2 vacancy concentrations around these dislocations will be similar to M1 vacancy concentrations. This means that the effect of magnesium vacancies on the mobility of the [001](010) edge dislocation may be significantly different to the effect on the mobility of the other dislocations considered.