Rock magnetic and magnetic fabric variations across three thin alkaline dykes from Sunnhordland, Western Norway; influence of initial mineralogy and secondary chemical alterations

Summary Profiles of natural remanent magnetization (NRM), rock magnetic properties and anisotropy of magnetic susceptibility (AMS) across three thin (26 to 109 cm) alkaline dykes from Sunnhordland, Western Norway are presented. The magnetic remanence is carried by pseudo‐single‐domain (PSD) low‐temperature oxidized titanomagnetite, with circa 6 per cent of cations substituted by aluminium. Systematic variations in magnetic properties across the dykes are caused by differences in titanomagnetite concentration as well as grain size. Low‐temperature alteration and laboratory oxidation by heating in air to a maximum temperature of 600°C, do not homogenize the magnetic properties across the dykes. Fine‐grained regions near dyke margins carry the most stable remanent magnetization, but are also most amenable to secondary chemical remanent magnetization (CRM) resetting. Between dyke variations in magnetic properties are larger than within dyke variations. A stable primary remanence is found in two of the dykes, while the third is totally reset by a soft viscous remanent magnetization (VRM), despite similar optical grain sizes, magnetic mineralogy and oxidation state. The difference is attributed to a lower titanium content in the latter dyke, preventing effective partitioning of magnetic grains during low‐temperature alteration. Laboratory heating affects anisotropy variations across the dykes, resulting in more consistent axis orientations within each dyke, and symmetric edge‐to‐edge variations in the degree of anisotropy. AMS directions are different from those in the country rock, and vary systematically across the thinnest dyke before heating.