Anomalous magnetic properties in rocks containing the mineral siderite: Paleomagnetic implications

Abstract

Sampling and magnetic measurement of a 1‐m bed in a new road cut of the Upper Cretaceous Austin Chalk (northeastern Texas) has yielded anomalous magnetic results. Initial measurement of the anisotropy of magnetic susceptibility (AMS) indicated unusually high anisotropies and low bulk susceptibilities characteristic of a magnetocrystalline anisotropy which might be expected for siderite (FeCO3). Natural remanent moments (NRM) for these samples were low (<1 × 10−4 A m2), and directions were typical for samples which had acquired a normal geomagnetic field overprint at the site. Periodic remeasurement of the NRM yielded an increase in moment for some samples and periodic reversals of RM in a direction parallel or antiparallel to the ambient field in the laboratory. Isothermal remanent moments of these samples saturates at low induction values (∼200 mT). AMS remeasurement over a period of weeks to months revealed a general decrease in the anisotropy magnitudes, an increase in susceptibility, and a change in principal axis orientations. These data were compared with heating (oxidation) experiments on Austin Chalk, siderite ore, and clastic sediments with siderite cements. All of these results are consistent with the X‐ray diffraction and Mössbauer data, which indicate the presence of siderite in the Austin Chalk samples. The Mössbauer spectra of the samples obtained at room temperature and 78°K indicate the presence of approximately 80% pyrite, 10% siderite, and 10% of a clay component (possibly chlorite) when only the iron‐bearing mineral components are considered. After sampling, exposure to the air, and subsequent oxidation in the laboratory, the siderite in Austin Chalk samples appears to have altered to γFe2O3 (maghemite) or Fe3O4 (magnetite), increasing the magnetic moment and changing the NRM and AMS directions in the samples. The continuing changes appear to reflect a conversion from the less stable γFe2O3 to αFe2O3 (hematite) or oxidation of Fe3O4 to αFe2O3. We infer from our study that oxidation of siderite‐bearing limestone (e.g., during the formation of unconformities or due to low‐magnitude tectonic events) may produce a very stable secondary magnetic moment in the rock. Such changes may be very important when interpreting paleomagnetic data from limestones.