2026 Articles

Accurate geomagnetic paleointensities retrieved by the RESET method from a 1915 Mount Lassen dacitic lava dominated by non-single-domain titanomagnetites

Chen, Xiaowei; Wang, Huapei; Wen, Chen; Coe, Rob; Kent, Dennis V.

Acquiring accurate absolute paleointensities of the geomagnetic field is essential for studying the evolution of the geodynamo in Earth's core and its thermal history. Thermoremanence is a preferred magnetic recording mechanism, but most igneous rocks are dominated by non single domain (non-SD) ferromagnetic grains (e.g., magnetite larger than ~100 nanometers), for which there is no strict theory for obtaining unbiased paleointensities. The Repeated Thellier-series Experiment (RESET) method has been recently developed to specifically deal with non-SD effects and thermal alterations so as to provide an accurate paleointensity. To test its capability, we studied a 1915 CE dacitic flow from Mount Lassen (located in northern California, USA, 40°29′ N, 121°30′ W, elevation ~3100 m) that contains micron-sized titanomagnetites whose thermoremanence was acquired in a known geomagnetic field (54 μT). This dacitic flow was previously investigated and found to exhibit concave and S-shaped Arai diagrams along with biased Thellier-Coe paleointensities. In this study, out of 34 specimens, we successfully obtained 22 qualified RESET-corrected paleointensity estimates that range from 50.6 to 57.4 μT with an average of 54.1 ± 1.6 μT. Our results suggest that the thermoremanence recorded in non-SD dominated igneous rocks can indeed provide accurate paleointensity estimates using the RESET method. We suggest that the RESET method provides a promising approach to expanding the range of geological materials suitable for future paleointensity studies, enabling the recovery of highly reliable and precise paleointensities from samples previously considered non-ideal. This could significantly improve our understanding of the evolution of Earth's magnetic field over geological time.

Keywords: thermoremanent magnetization, geomagnetic, paleointensity, lava flow