The concentration profiles of 13 trace elements (Li, Na, Al, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Y) were determined in a natural mantle olivine xenocryst included in a hybridized hydrous dioritic magma during cooling from ∼1000 °C, which allow trace-element chemical diffusion coefficients to be evaluated relative to Mg-Fe interdiffusion under conditions that are difficult to access in the laboratory. The effective diffusion coefficients of many elements (Li, Ca, Sc, Mn, Co, Ni, and Y) fell within a factor of three of the corresponding Mg-Fe interdiffusion coefficient, in agreement with results from laboratory experiments at higher temperatures. By contrast, the concentration profiles for Na, Ti, and V implied much faster diffusion rates, while P showed no discernible diffusion. The Al and Cr profiles, which were well correlated with each other, were complex and variable on a small length scale due to local precipitation of spinel. These data show that the diffusion coefficients of cations in olivine are not simple functions of ionic charge or ionic radius, but they are likely controlled by the availability of suitable diffusion pathways.