Rock magnetic properties of dendrites: insights from MFM imaging and implications for paleomagnetic studies

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.