The potential contribution to long wavelength magnetic anomalies from the lithospheric mantle

Abstract

Long-wavelength magnetic anomalies (LWMA) are broad scale variations of the magnetic field that are usually observed at high altitudes with amplitudes ranging up to about 20 nT. The source of these anomalies has typically been assumed to reside within the crust with negligible contributions from the lithospheric mantle. Recent work on the magnetization of mantle xenoliths suggests a possible lithospheric mantle contribution to LWMA, depending on tectonic setting and local geotherm. Here, we investigate the potential contribution of the lithospheric mantle to LWMA across two contrasting lithospheric geotherms: cratons and hotspots. To constrain the range of possible mantle contributions to satellite-observable LWMA in these settings, we develop a Monte Carlo suite of forward models of magnetic anomalies patterned after the mantle portions of the Siberian craton, the Hawai'i hotspot, and the French Massif Central plume. The models incorporate the current geotherms, the pressure dependence of the Curie temperature of magnetite, and the statistical distributions of rock magnetic data from mantle xenoliths in the specific regions. The Siberian craton shows the highest potential contribution of the mantle to LWMA with amplitudes of a few nT over wavelengths scaling to the long-axis of the craton. The Hawai'i hotspot region displays a potential contribution from the lithospheric mantle to satellite-measured magnetic anomalies on the order of 1 nT, while the Massif Central plume regions shows an insignificant contribution to satellite magnetic anomalies from lithospheric mantle. When compared with observed lithospheric total field anomalies derived from Swarm satellite data, the results show that the lithospheric mantle may contribute a few tens of percent to LWMA observed at satellite altitudes in specific regions depending on the average remanent magnetization in mantle minerals, the thickness of magnetized mantle (which depends on the geotherm), and the spatial extent of the region under consideration.