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

Tobias Mattsson, Benoît Petri, Bjarne Almqvist, William McCarthy, Steffi Burchardt, J. Octavio Palma, Øyvind Hammer, Olivier Galland

Research output: Contribution to journalArticlepeer-review

6 Citations (Scopus)
5 Downloads (Pure)

Abstract

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.
Original languageEnglish
Article numbere2021JB021895
Number of pages23
JournalJournal of Geophysical Research: Solid Earth
Volume126
Issue number6
DOIs
Publication statusPublished - 9 Jun 2021

Keywords

  • AMS
  • MicroXCT
  • Distribution Anisotropy
  • AARM

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