Self‐Reversed Magnetization in Sediments Caused by Greigite Alteration

• Published in: Geophysical research letters 2023-06, Vol.50 (12), p.n/a
• Main Authors: Chang, Liao, Pei, Zhaowen, Xue, Pengfei, Wang, Shishun, Wang, Zhaoping, Krijgsman, Wout, Dekkers, Mark J.
• Format: Article
• Language: English
• Subjects: chemical remanent magnetization; Fluids; Geochronology; Geological time; Grains; Greigite; Igneous rocks; Iron; Iron sulfides; Magnetic field; Magnetic fields; Magnetite; Magnetization; magnetostratigraphy; Miocene; Nanoparticles; Oxygenation; Percolation; Planetary magnetic fields; Polarity; Pyrrhotite; Records; remagnetization; Remanence; Remanent magnetization; Sediment; Sediments; self‐reversal; Sulphides; Tectonics
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL103885

Multipolarity remanence in greigite‐bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite‐bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single‐polarity” to “multi‐polarity” samples. The inward alteration of sulfide grains is topotactic and is size‐dependent with higher alteration in smaller grains. We propose a multi‐phase self‐reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self‐reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications. Plain Language Summary Some magnetic minerals in nature can be magnetized opposite to the external geomagnetic and planetary magnetic fields—a peculiar phenomenon called “self‐reversal.” A self‐reversal magnetization process is typically observed to occur in igneous rocks during cooling in an external field. Here, using magnetic and microscopic analyses we demonstrate that sediments containing authigenic ferrimagnetic iron sulfide mineral—greigite—can acquire a self‐reversed magnetization during progressive surface alteration of greigite nanoparticles. Surface alteration produces new “magnetic shells” that are magnetized opposite to the magnetization of the parent greigite core through magnetic interactions due to the close contact between the core and shell. Post‐depositional sedimentary processes, for example, percolation of fluids or oxygenation could trigger surface alteration that leads to “self‐reversal” and complicate the primary magnetization records. This self‐reversal mechanism can explain the commonly reported anomalous magnetization records of authigenic greigite; it is very useful for correct interpretations of tectonic and paleoenvironmental processes, and geological age frames of iron sulfide bearing sediment sequences. Key Points Surface alteration of diagenetic greigite to magnetite and pyrrhotite causes magnetic softe