Distribution anisotropy: The cause of AMS in igneous rocks?

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Abstract
Understanding of AMS in pristine basic igneous rocks in terms of flow‐orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains (1) typically are irregularly equant or skeletal rather than distinctly ellipsoidal, and (2) usually crystallize interstitially and late, after magma flow has ceased. However, the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly‐oriented (by flow) silicate “template”. This constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth, but in addition, we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of AMS. Experiments involving the casting of magnetite‐epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected AMS symmetry, i.e. K3 perpendicular to glass plates or K1 parallel to glass rods. Slicing of near isotropic synthetic blank‐ and natural rock‐cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from ∼0.4% to 2%, and the natural rock from ∼0.9% to ∼2.9%. In both the K3 axis migrates to be perpendicular to the glass slides.