Magnetic minerals: Understanding the processes of formation in soils and clays and identifying their presence in the rock record.

摘要

Magnetic minerals can form authigenically in soils and sediments, but the processes involved can be complex and are often a matter of debate. For example, anomalous concentrations in magnetic minerals have been described in soils and their origin has been ascribed to a range of processes including inorganic and biological precipitation. Also, the presence of secondary magnetic minerals in sedimentary rocks has been described and their origin has been related to a variety of geologic processes such as orogenic fluid migration, presence of hydrocarbons, and clay diagenesis. Developing a better understanding for the conditions by which magnetic minerals form in soils and sediments is important because of the information they can provide to decipher the geologic past on Earth and potentially on other planets.; This work presents results of biological formation of magnetite under a variety of laboratory conditions in a hydrocarbon-contaminated soil profile. Magnetite was formed only in biological samples but not in the abiotic: counterparts suggesting biological processes may be more important in the formation of magnetic minerals in soils than previously thought. A study on naturally occurring clays heated to temperatures equivalent to low burial conditions (62 and 98°C) resulted in increases in the bulk magnetic susceptibility (χ_b) in some smectites but not in any of the other tested clays. Furthermore, a closer look was taken at heated smectites and the authigenic magnetic grains that were formed in the process were rock magnetically characterized. The findings suggest that clay diagenetic processes occurring at low temperatures can lead to the formation of magnetite capable of carrying remanence and thus, provide a viable mechanism for the acquisition of secondary magnetizations in sedimentary rocks. Lastly, a rock magnetic comparison of two Paleozoic loessite-paleosol couplets is presented to investigate whether the enhanced χ _b trends in the paleosols as compared to the loessite can be confirmed in ancient deposits. The results for one couplet are similar to those from the Chinese Loess Plateau, which have an enrichment of ferrimagnetic material in the paleosol. The second couplet, however, reveals that the variations in the χ_b can be more complex and shows the importance of combining rock magnetic with sedimentologic evidence in order to identify the origin and preservation of χ_b patterns in geologic deposits.