Rock Magnetism Of Ankerite Cemented Concretions In Shale

Abundant carbonate concretions are distributed in the MesoproterozoicXiamaling formation in Xishan, Beijing and the Cretaceous Nenjiang Formation inSongliao basin. These concretions reveal typical inverse magnetic fabric. Suchunusual fabric may be related to some mineral which could be the indicator of thesedimentary environment during concretion formation. We carefully investigate themineral composition and rock magnetic properties of the concretions, in an attempt toidentify the mineral responsible for the inverse magnetic fabric. Besides, the rockmagnetic behavior of concretions during thermal treatment is examined.XRD and electron microbe results reveal that the concretions are cemented byankerite, which are further confirmed by thin section staining technique. Rockmagnetic results reveal that the magnetic carrier in the concretions is magnetite. Theanisotropy of anhysteretic remanence (AAR) and the anisotropy of magneticsusceptibility(AMS)show different patterns, indicating the ferrimagnetic mineralsmay not be responsible for the inverse magnetic fabric. The characteristics ofmagnetic susceptibility and AMS change with temperature are similar to that of otherferroan carbonate minerals. Electron back-scattered diffraction (EBSD) results show acorrelation between the degree of anisotropy of AMS and the degree of preferredcrystalline orientation (CPO). So we conclude that the inverse magnetic fabric isdominated by the ankerite cement in the concretions.Like samples containing siderite, the magnetic susceptibility of concretionsincrease rapidly after heated in high temperature, with the interchange between K1and K3axis of the AMS ellipsoid. The curie points of the resultants of heatingrevealed by κ-T curves are different. Magnetite or maghemite is produced by theheating of Nenjiang concretions, while a ferrimagnetic phase with the curie point ofabout490℃is formed by the heating of Xiamaling concretions. We conjecture themineral might be a mangnese-ferrite produced by heating of ankerite containingmanganese. The magnetite doped with Mn2+has lower curie point, just like Ti intitanomagnetite. All results indicate that the ankerite is more stable than siderite whenheated, and the composition of ankerite may affect the oxidation product and magnetic properties during thermal treatment.The ankerite cement in Xiamaling concretions and Nenjiang concretions retaintheir original properties, which indicates the concretion may not have suffered seriousweathering or heating processes. The temperature at which the magnetic susceptibilityand AMS of ankerite start to change may provide with us the upper limit ofpaleotemperature of the region.