A document by Brendan Cych. Click on the document to view its contents.

The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron-titanium spinels known as titanomagnetites. Despite the range of compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain-states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and “pTRM tails’ in paleointensity experiments. We find that although this SD+SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30-40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD+SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail-like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure.

A document by Brendan Cych. Click on the document to view its contents.

Some rocks contain multiple remanence “components”, each of which preserves a record of a different magnetic field. The temperature ranges over which these remanence components demagnetize can overlap, making it difficult to determine their directions. We present a data analysis tool called Thermal Resolution Of Unblocking Temperatures (TROUT) that treats the process of thermal demagnetization as a function of temperature (or alternating field demagnetization as a function of coercivity). TROUT models the unblocking temperature distributions of components in a demagnetization experiment, allowing these distributions to overlap. TROUT can be used to find the temperatures over which paleomagnetic directions change and when two directional components overlap resulting in curved demagnetization trajectories. When applied to specimens given multi-component Thermoremanent Magnetizations (TRMs) in the laboratory, the TROUT method estimates the temperature at which the partial TRMs were acquired to within one temperature step, even for specimens with significant overlap. TROUT has numerous applications: knowing the temperature at which the direction changes is useful for experiments in which the thermal history of a specimen is of interest (e.g. emplacement temperature of pyroclastic deposits, re-heating of archaeological artifacts, reconstruction of cooling rates of igneous bodies). The ability to determine whether a single component or multiple components are demagnetizing at a given temperature is useful for choosing appropriate ranges of temperatures to use in paleointensity experiments. Finally, the width of the range of temperature overlap may be useful for inferring the domain state of magnetic mineral assemblages.

The assumptions of paleointensity experiments are violated in many natural and archaeological materials, leading to Arai plots which do not appear linear and yield inaccurate paleointensity estimates, leading to bias in the result. Recently, paleomagnetists have adopted sets of “selection criteria” that exclude specimens with non linear Arai plots from the analysis, but there is little consensus in the paleomagnetic community on which set to use. In this paper, we present a statistical method we call Bias Corrected Estimation of Paleointensity (BiCEP), which assumes that the paleointensity recorded by each specimen is biased away from a true answer by an amount that is dependent a single metric of nonlinearity (the curvature parameter $\vec{k}$) on the Arai plot. We can use this empirical relationship to estimate the recorded paleointensity for a specimen where $\vec{k}=0$, i.e., a perfectly straight line. We apply the BiCEP method to a collection of 30 sites for which the true value of the original field is well constrained. Our method returns accurate estimates of paleointensity, with similar levels of accuracy and precision to restrictive sets of paleointensity criteria, but accepting as many sites as permissive criteria. The BiCEP method has a significant advantage over using these selection criteria because it achieves these accurate results without excluding large numbers of specimens from the analysis. It yields accurate, albeit imprecise estimates from sites whose specimens all fail traditional criteria. BiCEP combines the accuracy of the strictest selection criteria with the low failure rates of the less reliable ‘loose’ criteria.

The theory for recording of thermally blocked remanences predicts a quasi-linear relationship between low fields like the Earth’s in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long-standing questions concerning Earth’s magnetic field require a global paleointensity dataset, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero-field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are poorly understood. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, and exhibits an ”aging’ effect in which the unblocking temperature spectrum changes over only a few years resulting in non-ideal experimental behavior. While a fresh remanenence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant (up to 10 μT) for individual specimens, with a low field bias. Fragility of unblocking temperature spectra appears to be related to grain size and may be related to features observed in first-order reversal curves.