Potential Bias in Volcanic Paleomagnetic Records Due To Superimposed Chemical Remanent Magnetization

• Published in: Geophysical research letters 2024-06, Vol.51 (12), p.n/a
• Main Authors: Shcherbakov, V. P., Lhuillier, F., Gribov, S. K., Tselmovich, V. A., Aphinogenova, N. A.
• Format: Article
• Language: English
• Subjects: Air temperature; arai diagram; Bias; characteristic component; chemical remanent magnetization; Components; Cooling; Dynamo theory; Earth; Geomagnetic field; Magnetic field; Magnetic fields; Magnetization; Oxidation; Palaeomagnetism; Paleomagnetism; Remanence; Remanent magnetization; Rocks; Room temperature; Segments; Thermal stability; titanomagnetite; Volcanic rocks; zijderveld diagram
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL109630

Volcanic rocks, preserving paleorecords of Earth's magnetic field, are essential to constrain the working of the geodynamo, provided their primary signal was not biased. Using a thermomagnetometer, we simulate a situation where a sample's primary record, carried by a thermoremanence (TRM, acquired by cooling in air from 600°C to room temperature), is partly overprinted by a chemical remanence (CRM, acquired by 200 hr of isothermal exposure at 400°C). This situation leads to two directional and intensity components (in the form of linear segments) in the Zijderveld and Arai‐Nagata diagrams. In the case of unstable titanomagnetite grains prior to CRM acquisition, we show that both components can be strongly biased by up to ∼50° for paleodirections and ∼50% for paleointensities. In such a worst‐case scenario, the secondary CRM strongly overprints the primary TRM, rendering the common interpretation of Zijderveld and Arai‐Nagata diagrams in terms of characteristic components invalid. Plain Language Summary Volcanic rocks, the magnetic minerals of which can acquire a thermoremanent magnetization (TRM) from Earth's magnetic field during their initial cooling, are essential to constrain the working of the geodynamo through Earth's history. However, if the rock is subsequently reheated at moderate temperature in another ambient field, the initial record can be partly overprinted by a chemical remanence (CRM). Starting from a TRM applied on materials of various thermostability, we reproduced in laboratory conditions the acquisition of a CRM by annealing the rock in a controlled ambient field for 200 hr at 400°C. Rock‐magnetic and structural analyses at regular intervals, supplemented by continuous measurements of the remanent magnetization, indicate the creation of new phases as a result of oxidation processes. The paleomagnetic analysis of the final products reveals the existence of two distinct components that can be associated with the initial TRM and the secondary CRM. Whereas the paleomagnetic record of the initial TRM is trustworthy for the most stabilized initial products, directional (up to 50°) and intensity (up to 50%) biases are observed for the least stabilized initial products, illustrating a critical example where the interpretation of characteristic components (linear segments in the interpretation diagrams) is invalid. Key Points We overprint a thermoremanent magnetization (TRM) by cooling from 600°C with a chemical remanent magnetization by 200 hr