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Mineralogical contraints on Curie isotherms in deep crustal magnetic anomalies

1978; American Geophysical Union; Volume: 5; Issue: 2 Linguagem: Inglês

10.1029/gl005i002p00105

1944-8007

Stephen E. Haggerty,

Geological and Geochemical Analysis

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 Fe 3 O 4 . Solid solution of magnetite towards antiferromagnetic ulvöspinel (Fe 2 TiO 4 ) 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 temperatures may vary between 300°C if maghemite (αFe 2 O 3 )dominates and 680°C if hematite (γFe 2 O 3 ) 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.