Decrypting magnetic fabrics (AMS, AARM, AIRM) through the analysis of mineral shape fabrics and distribution anisotropy

The fieldwork was supported by the DIPS project (grant no. 240467) and the MIMES project (grant no. 244155) funded by the Norwegian Research Council awarded to O.G. O.P.'s position was funded from Y-TEC.Anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AARM and AIRM) are efficient and versatile techniques to indirectly determine rock fabrics. Yet, deciphering the source of a magnetic fabric remains a crucial and challenging step, notably in the presence of ferrimagnetic phases. Here we use X-ray micro-computed tomography to directly compare mineral shape-preferred orientation and spatial distribution fabrics to AMS, AARM and AIRM fabrics from five hypabyssal trachyandesite samples. Magnetite grains in the trachyandesite are euhedral with a mean aspect ratio of 1.44 (0.24 s.d., long/short axis), and > 50% of the magnetite grains occur in clusters, and they are therefore prone to interact magnetically. Amphibole grains are prolate with magnetite in breakdown rims. We identified three components of the petrofabric that influence the AMS of the analyzed samples: the magnetite and the amphibole shape fabrics and the magnetite spatial distribution. Depending on their relative strength, orientation and shape, these three components interfere either constructively or destructively to produce the AMS fabric. If the three components are coaxial, the result is a relatively strongly anisotropic AMS fabric (P’ = 1.079). If shape fabrics and/or magnetite distribution are non-coaxial, the resulting AMS is weakly anisotropic (P’ = 1.012). This study thus reports quantitative petrofabric data that show the effect of magnetite distribution anisotropy on magnetic fabrics in igneous rocks, which has so far only been predicted by experimental and theoretical models. Our results have first-order implications for the interpretation of petrofabrics using magnetic methods.Publisher PDFPeer reviewe

El Pinacate volcanic field, Northwest Mexico: An example of a shield cluster; [Campo volcánico El Pinacate, noroeste de México: Un ejemplo de un cúmulo de escudos]

El Pinacate volcanic field, Northwest Mexico, includes a large shield volcano (Sta. Clara) and more than 400 vents, most of which are cinder cones and a few maars of Pleistocene and recent age. Previous studies of the distribution of those vents focused on the identification of preferred orientations of vent alignments, paying little attention to the identification of clusters of vents within the field. In this work several methods of spatial distribution analyses are combined together to describe the spatial structure of the distribution. As a result, four main structures are identified, each interpreted as a long-lived volcanic system that has remained active throughout the history of the volcanic activity on the region. Interaction of the axes of activity within those structures with an older listric fault that crosses the field from NW to SE is responsible for the apparent difference in distribution observed in the north and south halves of the field. Differences to the depth of the Curie isotherm also contribute to the observed differences on the vent distribution at the surface. Although the influence of tectonic stresses is very important in controlling the location of activity on this field, in this work we show that it is not necessary to invoke a change in the orientation of those stresses to explain the evolution of the region. © This is an open access article under the CC BY-NC-SA license (https://creativecommons.org/licenses/by-nc-sa/4.0/)Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]