Reassessing the paleointensities of three Quaternary volcanic structures of the -Michoacán-Guanajuato Volcanic Field (Mexico) through a multimethodological analysis

Abstract

An approach that has acquired popularity in the last decade to ensure the quality of paleointensity determinations is the use of multimethodological analysis, especially when comparing intensity values obtained by methods with different physical bases such as the conventional Thellier-type experiments against the Shaw-family methods and the most recent multispecimen protocol. This research presents the reevaluation of the intensities of three monogenetic volcanic structures: the Rancho Seco and Jabalí cones and lavas, and the Malpaís Prieto fissure lava flow, all located in the Michoacán-Guanajuato volcanic field (central Mexico). This analysis is accompanied by an exhaustive study of rock-magnetic properties to obtain a complete characterization of the magnetization carriers and a better understanding of the intensity results. The three volcanic structures have previously published intensity data; one of them was obtained with the original Shaw method that nowadays is considered unreliable due to the absence of monitoring of thermal alteration during the experiment. The quality of the previously published data is discussed to be compared to our intensity values, which were obtained employing Thellier-type methods and the multispecimen protocol (MSP). A good concordance was found for the data obtained by more than one methodology for samples from the Jabalí volcano and the Malpaís Prieto lava flow, obtaining average intensity values of 43.7 ± 5.1 μT and 47.8 ± 3.1 μT, respectively. However, a difference of almost 30 μT was found between our MSP results (50.9 ± 2.8 μT) and previously published data using the IZZI method (20.0 ± 1.4 μT) for Rancho Seco volcano. An attempt was made to clarify this controversial result with the support of stepwise susceptibility against temperature curves analysis and the results obtained from a set of samples treated with the Tsunakawa-Shaw paleointensity method. The results obtained indicate that the low paleointensity values are associated with the thermal alteration of the magnetic mineralogy of the samples.