Identification of ultra-fine magnetic particles in weakly magnetic carbonates using time-decay of viscous remanence

Abstract:

In some geological and environmental processes, such as diagenesis, very low grade metamorphism, pedogenesis, anthropogenic pollution, new ultra-fine magnetic minerals may be formed. The variation in content of these minerals has been routinely investigated by frequency-dependent magnetic susceptibility. Although being quite reliable for most rocks, frequency-dependent susceptibility reaches its limit when applied to very weakly magnetic rock types, e.g. carbonates. Assuming a broad size distribution of the ultra-fine magnetic particles spanning across the SP/SSD boundary we suggest assessing their content by quantification of time-decay of viscous remanent magnetization. Using artificially-imparted magnetization we usually obtain much stronger signal compared to that of magnetic susceptibility. For that purpose we employed a LDA5/PAM1 Pulse Magnetizer coupled with a JR6 Spinner Magnetometer (both manufactured by Agico, Inc.). Both instruments are simultaneously controlled thus they work in the same time frame. Magnetic remanence is measured repeatedly as a function of time and exponential decay curves are fitted on the acquired data and the relative ratio of viscous and non-viscous particles is estimated. The proposed method is tested on two sets of samples representing biostratigraphically well-established sections across Silurian shallow-water limestone facies in the Prague Synform (Czech Republic). Sampling interval comprises so-called Lau Event which belongs to one of the major environmental and biological perturbances in the Phanerozoic Ocean. This level is also associated with very strong geochemical changes, so-called global Middle Ludfordian Carbon Isotope Excursion, recognized in numerous areas worldwide. Other geophysical methods applied include high-resolution magnetic susceptibility measurements and gamma-ray spectrometry, supplemented by rock magnetic measurements (ARM/IRM) and frequency-dependent magnetic susceptibility.