A Mount Fuji basaltic tree-mold presents the mineralogical record associated with a large oxygen fugacity gradient during the cooling of the basalt. Magnetic exchange anisotropy is postulated to be characteristic of cation excess in some spinel titanomagnetites. The exchange anisotropy is characterized by loop shifting along the field and magnetization axes when cooled in a magnetic field, large coercivity increase, and magnetic viscosity which has discrete activation fields and logarithmic to linear decay characteristics. The cryogenic loop characteristics attributable to exchange anisotropy are not found in the tree-mold where iron and ilmenite are the main phases. The coupling has been created in the laboratory with ordinary basalt artificially reduced in Hydrogen gas. The coupling is not found in spinel oxidation non-stoichiometry. These results imply that we are able to identify the presence of spinel that have equilibrated in the low oxygen fugacity range of the spinel stability field where cation excess is characteristic. ¿ 2000 American Geophysical Union