Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite

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 similar to 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 degrees 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. Plain Language Summary The strength of the magnetic field of the Earth is controlled by changes deep in the Earth. Studying the magnetization of ancient lava flows helps us determine the history of these changes because lavas become magnetized when they cool in the Earth's magnetic field. In this paper, we used artificial rocks to study how changing the size of and distance between the magnetic particles inside the rocks affects how we can determine the past strength of Earth's magnetic field. We found that both larger particles and more closely packed particles cause our estimates for the magnetic field strength to be too high. We also found that heating the rock samples to the same temperature multiple times causes their magnetization to change each time (for rocks with small, far-apart particles, only the first heating changes the magnetization). For future studies, steps must be taken to ensure that the nature of the magnetic particles in the rocks is known and that they are suitable for ancient field strength studies.