Thermal Treatment of Clays Resulting in Magnetic Mineral Formation in a Smectite - An Analogy to low Burial Conditions

Widespread chemical remanent magnetizations (CRMs) are common in sedimentary rocks and several mechanisms such as the migration of orogenic fluids and the presence of hydrocarbons have been proposed to cause their formation. Another possible mechanism causing magnetite authigenesis and CRMs is the low temperature diagenetic conversion of smectite to illite during burial in sedimentary rocks. Although empirical evidence supports such a connection, genetic links between low temperature diagenesis (below 100oC) and changes in the magnetic signal have not been tested. In this study, bulk magnetic susceptibility of heated clays was monitored over time under a variety of experimental conditions. Gradual increases of bulk magnetic susceptibility in a heated nontronite were identified in all experimental settings. A heated sodium-montmorillonite showed an augmentation in the bulk magnetic susceptibility signal, but with a more complex response. No changes were observed in any of the other clays, including one smectite (iron-sodium nontronite). Hysteresis measurements combined with thermal demagnetization of low-temperature saturation isothermal remanent magnetization analyses, room temperature isothermal remanent magnetization, and anhysteretic remanent magnetization analyses of the nontronite indicate that the increase in bulk magnetic susceptibility is at least partially caused by the formation of ferrimagnetic material. The authigenic phase is interpreted as magnetite and is of superparamagnetic (SP) as well as remanence-carrying grain size. X-ray diffraction indicates that illitization of the smectite may not have occurred. Visible-infrared reflectance spectra have been measured and analyses of the spectra are underway. This laboratory study provides supporting genetic evidence for a potential connection between some clay diagenetic processes and the formation of some widespread CRMs.