Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite

Abstract

Understanding Earth's geodynamo provides us a window into the evolution of the Earth's core, which requires accurate data about how its strength varies with time. Classic Thellier‐style paleointensity experiments assume that studied specimens contain only noninteracting single‐domain (SD) magnetic particles. Interacting grains commonly occur in volcanic rocks but are generally assumed to behave like equivalently sized SD grains. Multidomain (MD) grains can cause erroneous PI estimates or cause Thellier‐style experiments to fail entirely. Synthetic specimens containing naturally formed magnetite with MD grains and oxyexsolved titanomagnetite (closely packed SD grains) were subjected to various partial thermoremanent magnetization (pTRM) experiments, which tested nonideal behavior as a function of pTRM acquisition and loss inequality, thermal history, and repeated heating steps. For all grain sizes and domain states, pTRMc (heating and cooling in a nonzero field) gives larger values, compared to pTRMb (heating in a zero field and cooling in a nonzero field), by ∼5.5%. Oxyexsolved grains appear prone to the same concave‐up, nonideal Arai plots commonly observed in MD specimens, which also has potential implications for the multiple‐specimen, domain‐state corrected protocol. Repeated heatings cause additive deviations from ideality with relatively small impacts on Arai plot curvature for both grain types. Experiments with higher initial demagnetization temperatures had lower curvatures, with the most SD‐like behavior occurring in the uppermost 20 °C of the (un)blocking temperature range. Samples containing mixtures of magnetic domain sizes are likely to behave less ideally at lower temperatures but become more ideal with increasing temperature as the nonideal grains unblock.

Comparison of Thermal and Microwave Paleointensity Estimates in Specimens Displaying Non‐Ideal Behavior in Thellier‐Style Paleointensity Experiments

“…Double-heating Thellier protocols have long been known to have problems with multidomain components causing non-linear Arai plots (Levi, 1977). Hodgson et al (2018) showed that this can apply to non-single domain oxyexsolved titanomagnetite grains as well, which are common in basaltic lavas such as the Hawaiian lavas of the SOH1 drill core. These non-single domain components can lead to concave up (two-slope) Arai plots.…”