Rock magnetic investigation of possible sources of the Bangui magnetic anomaly

Abstract

International audienceThe Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3 A/m with maximum thicknesses of 38 and 54 km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures

The effect of hydrostatic pressure up to 1.61 GPa on the Morin transition of hematite‐bearing rocks: Implications for planetary crustal magnetization

“…How does these results can help to understand the nature of crustal magnetism and planetary magnetic anomalies? Knowing the behavior of hematite in different pressure‐temperature conditions allows geological mapping of magnetic anomalies of hematite‐rich surfaces such as Bangui magnetic anomaly in Central African Republic [ Ouabego et al, ]. As shown here, T M reaches room temperature at ~1.5 GPa.…”

“…We present here new magnetic data on the hydrostatic pressure dependence of the Morin transition up to 1.61 GPa obtained on a well‐characterized multidomain (MD) hematite‐bearing rock sample from banded iron formation, which is possibly responsible for the largest large‐scale magnetic anomaly on Earth [ Demory et al, ; Ouabego et al, ]. We used a nonmagnetic high‐pressure cell of piston‐cylinder type for hydrostatic pressure application and a Superconducting Quantum Interference Device (SQUID) magnetometer for remanent magnetization measurements under pressure in the course of zero‐field warming of the cell with sample from 243 K to T 0 .…”

“…Hematite (αFe 2 O 3 ) is a common mineral in paleomagnetic and rock magnetic studies. It is abundant in both igneous and sedimentary terrestrial rocks [Dunlop and Özdemir, 1997] and may carry large crustal magnetization on Earth [McEnroe et al, 2004;Ouabego et al, 2013]. This mineral is also stable in the Martian surface conditions [Bandfield, 2002;Chevrier et al, 2004;Wiens et al, 2015], and it has been proposed as a candidate magnetic mineral for the Martian magnetic anomalies [Dunlop and Kletetschka, 2001].…”

Thermoremanence acquisition and demagnetization for titanomagnetite under lithospheric pressures

“…This change could have an important impact on magnetic crustal models using deep sources, especially since most of the biggest magnetic anomalies on Earth are located inside cratons [ Ravat et al , ; Langel and Hinze , ; Ouabego et al , ], where the corrected Curie depth has a chance to be the most different from the current one, and where the potentially overmagnetized part of lithosphere is the thickest. Stronger magnetization at large depths could also mean smaller sources in some models.…”

“…Our work shows a significant increase starting between 337 and 675 MPa (depending on the main magnetic mineral); this implies that rocks located below a depth limit oscillating between 13 and 20 km could have a magnetization 2 times higher than what is currently admitted. This change could have an important impact on magnetic crustal models using deep sources, especially since most of the biggest magnetic anomalies on Earth are located inside cratons [Ravat et al, 1991;Langel and Hinze, 1998;Ouabego et al, 2013], where the corrected Curie depth has a chance to be the most different from the current one, and where the potentially overmagnetized part of lithosphere is the thickest. Stronger magnetization at large depths could also mean smaller sources in some models.…”