Quantification of induced and remanent magnetizations in the lithospheric mantle and consequences for long wavelength magnetic anomalies

The interpretation of long wavelength magnetic anomalies [LWMA] (Magsat, CHAMP, SWARM) requires an understanding of magnetic mineralogy in the lithospheric mantle and reliable models of induced and remanent sources. The Curie depth (200°C<Tc<700°C) is estimated by surface heat flow, by magnetization contrasts, or by spectral magnetic depth determination of near-surface data. Discrepancies between these approaches are attributed to variations in the magnetic composition in the lower crust and neglect the contribution of the mantle. This contribution in low geotherms such as cratons and forearcs may be significant. Forty representative, non-serpentinized xenoliths have been studied. These specimens are from oceanic hot-spot, continental mantle plume, island arc and craton sources. A magnetic inventory shows that diamagnetic phases (Pl) contribute little to low-field susceptibility [K] (Kdia < -1.6 x 10-6 [SI]) while the main paramagnetic phases (Ol, Cpx, Opx) contribute Kpara ≈ 500 x 10-6 [SI]. In Pl-free peridotites, the total contribution of paramagnetic phases to K is determined by 2 independent approaches: from the high-field slope of an hysteresis experiment and by calculating Kpara = ∑ Ki Vi. These diamagnetic and ferromagnetic components display limited variability with peridotite facies, rock type or tectonic setting. Silicates account for 56-97% (average ≈ 85%) of the magnetic susceptibility depending on rock composition. When present, the paramagnetic contribution of chromite ≈ 1%. The remaining contribution to K arises from variable amounts of primary magnetite (Mt) and pyrrhotite (Po). These phases, although present in 10s to 100s of ppm, contribute significantly to the rock magnetic properties because they have large intrinsic susceptibilities (≈ 1 to 4 [SI] for magnetite). Stoichiometric Mt has been identified as microscopic exsolutions in the lattice of olivine and accounts for 2 to 43% (average ≈ 8%) of K. These specimens display contrasting NRM, Mr, Ms depending on their tectonic setting. The remanent magnetization carried by Mt is characterized by a Tc of 580°C at 1 atm. and 596°C at a depth of 30 km. In low geotherm settings (<20°C/km), the Curie depth can be below the Moho. In such cases the lithospheric mantle layer comprised between the Moho and the Curie depth contributes to both induced and remanent magnetization. At the Curie depth (corresponding to 853 K) the induced magnetization has decreased according to the Curie-Weiss law to about Kpara-853K ≈ 175 x 10-6 [SI] while the remanent component remains constant. Below the Curie depth, the remanent component goes to 0 and the induced component continues to decrease with increasing temperature. The magnetic susceptibilities of lithospheric mantle minerals suggest they need to be considered to adequately interpret LWMA in low geotherm settings.