Abstract

AbstractAnisotropy 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 distribution anisotropy. 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 anisotropy 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.

Highlights

  • The analysis of magnetic fabrics by means of anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic and isothermal remanence magnetization (AARM and anisotropy of isothermal remanent magnetization (AIRM)) are routinely employed for rock fabric determination

  • We evaluated the spatial distribution of grains, that is, distribution anisotropy (DA), by calculating directional cosines l, m, n of the vector defined by two grain centers, for each couple of grains in the sample segmented using Blob3D

  • In this study we investigated the source of the Anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and AIRM fabrics in the Cerro Bayo trachyandesite by using μXCT and novel and established statistical methods to analyze the petrofabric

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Summary

Introduction

The analysis of magnetic fabrics by means of anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic and isothermal remanence magnetization (AARM and AIRM) are routinely employed for rock fabric (or petrofabric) determination. Journal of Geophysical Research: Solid Earth et al, 2004; Schöpa et al, 2015) For paramagnetic minerals such as pyroxene, amphibole, and biotite, the AMS principal axes usually correlates to crystallographic axes and are produced by the Fe spatial ordering in the crystal lattice (Biedermann, 2018; Biedermann et al, 2015; Borradaile & Jackson, 2010). This relation does not hold for ferrimagnetic minerals (in particular magnetite) because the (low-field) AMS may have several sources, in this case domain state and distribution anisotropy (DA) are two key factors (Ferré, 2002; Hrouda, 1982; Potter & Stephenson, 1988; Stephenson et al, 1986)

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