Ferrimagnetic minerals above their respective Curie temperatures are paramagnetic. Deep crustal magnetic anomalies are constrained, therefore, to a depth detection limit which is defined by the Curie isotherm. Although magnetite, with a Curie temperature of 580¿C, is widely employed to map Curie isothermal surfaces it is a relatively rare mineral as stoichiometric Fe3O4. Solid solution of magnetite towards antiferromagnetic ulv¿spinel (Fe2TiO4) reduces the Curie temperature to a minimum of -153¿C and a high order of ferrimagnetic behavior can only be accomplished in plutonic rocks under the most exacting conditions of equilibrium exsolution and oxidation. Magnetic anomaly models need not be constrained by the 580¿C limit because the range in tmperatures may vary between 300¿C if maghemite (&agr;Fe2O3) dominates and 680¿C if hematite (&ggr;Fe2O3) is dominant. Because many magnetic anomaly (&ggr;Fe2O3) is dominant. Because many magnetic anomaly (&ggr;Fe2O3) is dominant. Because many magnetic anomaly (&ggr;Fe2O3) is dominant. Because many magnetic anomaly interpretations require Curie isothermal depths in excess of 580¿C, and because these may extend to the lower crust-upper mantle boundary, it is proposed that these anomalies may arise from partially serpentinized ultramafic bodies which have metal alloys as the prime magnetic source material with Curie temperatures in the range 620¿-1100¿C.