Nonheating methods for absolute paleointensity determination: Comparison and calibration using synthetic and natural magnetite‐bearing samples

Abstract

AbstractNonheating paleointensity methods utilize an anhysteretic remanent magnetization (ARM) or a saturation isothermal remanent magnetization to model the natural thermal remanent magnetization (TRM) to avoid heating‐induced alteration. We report the results of paleointensity experiments using the ARM, pseudo‐Thellier, and ratio of equivalent magnetization (REM) methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors. The experiments on synthetic magnetite‐bearing samples representing single‐domain (SD) and pseudo‐single‐domain (PSD) magnetic states indicated that the correction factors for the ARM‐based methods depend on the magnetic grain size/domain state changing from ~6.3 (for SD grains) to ~4.1 (for ~1.5 µm PSD grains). The pseudo‐Thellier method yielded correct absolute paleointensity values when normalization by the TRM/ARM demagnetization slope was used. When applied to samples of lava flows and dikes from the ~32 kyr Lemptégy volcano (France), both the ARM and pseudo‐Thellier methods produced similar paleointensity estimates (28.0 ± 5.1 μT and 26.9 ± 4.7 μT, respectively) consistent with the available Thellier data for the 31–33 kyr time interval. The correction factors estimated from our synthetic samples for the REM (~3000) and for REMc (~1500) and REM′ (~1500) variants are consistent with the previously published estimates. However, all REM variants yielded unrealistically high estimates (>110 μT) of the paleofield strength from our natural samples. Our experimental results support ARM as a better proxy of TRM and suggest that the ARM‐based methods currently represent the best alternative to heating‐based absolute paleointensity method.