Shaar R, Feinberg JM
. Rock magnetic properties of dendrites: insights from MFM imaging and implications for paleomagnetic studies
. Geochemistry Geophysics Geosystems [Internet]. 2013;14 :407-421. Publisher's VersionAbstract
Dendrites are crystals that grow in branches that diverge along crystallographically defined directions. Despite the importance of dendrites in paleomagnetic research, little is known about how dendrites act as magnetic recorders, because they exhibit complicated magnetic domain structures. In this study, we experimentally examine how textures and sizes of dendrites affect their magnetic domain structure and magnetic properties. We study two basaltic glass samples and three synthetic slag samples, which collectively define a wide range of dendritic morphologies. We use electron microscopy to characterize the morphology of the dendrites and magnetic force microscopy (MFM) to observe their magnetic domain structure. We characterize the dendrites' bulk properties by firs-order reversal curve distributions, Thellier-style paleointensity experiments, anisotropy of remanence, and anisotropy of susceptibility. The samples with the thinnest dendrites have high coercivity, stable single-domain (SD) - pseudo single-domain magnetization, and yield ideal Arai plots. By contrast, the sample with the thickest dendrites has the lowest coercivity and shows the most extreme multidomain (MD) behavior. All samples except one, exhibit significant remanence and susceptibility anisotropy. MFM observations show that dendrites built from branches of interconnected octahedra, typical for basaltic glass, have a stable, high coercivity, SD-like magnetization despite the fact that their overall dimensions exceed the SD-MD threshold. Their stability is likely due to interactions between the octahedra and their narrow rod-like interconnections. Dendrites that crystallize in faster cooling environments, such as in archaeological slag, display finer branch thicknesses (< 1 mu m) and few, if any, octahedra. The tips of these dendrites consist of closely intergrown, rounded, acicular branches that behave as pseudo single-domain grains due to interactions between the branches. The largest, thickest dendrites show MD behavior and MFM images show their magnetic domain walls to be arranged in crystallographically controlled, geometrically repetitive patterns within elongated branches, which give rise to their anisotropy characteristics.
Shaar R, Tauxe L
. Thellier GUI: An integrated tool for analyzing paleointensity data from Thellier-type experiments
. Geochemistry Geophysics Geosystems [Internet]. 2013;14 :677-692. Publisher's VersionAbstract
Thellier-type experiments are a method used to estimate the intensity of the ancient geomagnetic field from samples carrying thermoremanent magnetization. The analysis of Thellier-type experimental data is conventionally done by manually interpreting data from each specimen individually. The main limitations of this approach are: (1) manual interpretation is highly subjective and can be biased by misleading concepts, (2) the procedure is time consuming, and (3) unless the measurement data are published, the final results cannot be reproduced by readers. These issues compound when trying to combine together paleointensity data from a collection of studies. Here, we address these problems by introducing the Thellier GUI: a comprehensive tool for interpreting Thellier-type experimental data. The tool presents a graphical user interface, which allows manual interpretation of the data, but also includes two new interpretation tools: (1) Thellier Auto Interpreter: an automatic interpretation procedure based on a given set of experimental requirements, and 2) Consistency Test: a self-test for the consistency of the results assuming groups of samples that should have the same paleointensity values. We apply the new tools to data from two case studies. These demonstrate that interpretation of non-ideal Arai plots is nonunique and different selection criteria can lead to significantly different conclusions. Hence, we recommend adopting the automatic interpretation approach, as it allows a more objective interpretation, which can be easily repeated or revised by others. When the analysis is combined with a Consistency Test, the credibility of the interpretations is enhanced. We also make the case that published paleointensity studies should include the measurement data (as supplementary files or as a contributions to the MagIC database) so that results based on a particular data set can be reproduced and assessed by others.