Summary: The results of the electron-probe microanalysis of the three minerals from orthopyroxene-amphibole-spinel coronas between olivine and plagioclase in a troctolite from Sulitjelma, Norway, are presented. They are shown to be incompatible with a late-magmatic origin for the coronas, but compatible with an origin by diffusion of material in solution in an aqueous medium across the olivine-plagioclase interface. This theory is discussed on a quantitive basis in the light of the microanalytical data.