Using Preisach Theory to Evaluate Chemical Remanent Magnetization and Its Behavior During Thellier‐Thellier‐Coe Paleointensity Experiments

Abstract

The behavior of grain‐growth chemical remanent magnetizations (gCRM) are investigated for different coercivity and magnetostatic‐interaction‐field distributions and acquisition conditions using a thermally activated Preisach model for assemblages of interacting single‐domain grains. A new growth‐rate dependent equation was derived, from which it was found that gCRM intensity is over 10% more sensitive to growth rate than previously modeled. We compare the gCRM results to the behavior of thermoremanences (TRM). gCRMs are two times more sensitive to changes in coercivity distribution, whereas TRMs are four times more sensitive to changes in magnetostatic interactions. The Thellier‐Thellier‐Coe paleointensity protocol was simulated in Preisach space, and gCRMs were found to produce concave‐up Arai plots with pTRM checks which plot to the left of the Arai plot and positive partial‐TRM tail checks that increase with magnetostatic interactions. This often leads to the failure of selection criteria, but high‐temperature segments can pass the criteria for weakly interacting gCRMs; these estimates can underestimate the field by up to 66%.

Plain Language Summary

Rocks containing magnetic minerals usually magnetically record the Earth's magnetic field during their formation acquiring a remanent magnetization. It is known that many rocks record a thermoremanent magnetization during cooling from high temperatures, however, it is also common for magnetic minerals to acquire magnetic remanence during the growth of magnetic minerals at ambient temperatures (grain‐growth chemical remanent magnetization, gCRM). It is important to have an understanding of the magnetic remanence recording mechanism if we are to understand ancient magnetic recordings which can hold information about the formation of our Solar System and the habitability of the Earth. Most previous theoretical and experimental studies have focused on thermoremanence acquisition, and have ignored gCRM acquisition theory. We have undertaken a theoretical investigation of gCRM acquisition, and find gCRMs respond very differently to thermoremanences during simulated standard laboratory measurements. In particular, we find that gCRMs cannot be used to determine ancient magnetic field intensities using conventional methods based on thermoremanence acquisition, as they typically yield underestimates of up to 66%.

Key Points

• Grain‐growth chemical remanent magnetization (gCRM) acquisition is modeled for interacting single‐domain particles using Preisach theory

• gCRM intensities are less sensitive to changes in interactions and more sensitive to coercivity changes than thermal remanent magnetizations

• Arai plots for gCRM are concave‐up with the degree of curvature increasing with higher interactions and lower coercivities