It has been frequently suggested that interstellar dust grains contain clusters of metallic iron, iron sulphides or magnetic iron oxides having superparamagnetic properties. This paper presents a simple model for the origin of superparamagnetism in H2O ice mantle grains. According to this model, impurity Fe atoms in H2O ice mantles are rapidly oxidized to magnetite (Fe3O4) precipitates during mantle heating by cosmic ray impacts. These precipitates undergo ferromagnetic coupling and aggregate into single-domain Fe3O4 particles uniformly dispersed throughout the mantle like raisins in a pudding. The single-domain particles behave like an assembly of superparamagnets, greatly enhancing Davis-Greenstein alignment of core-mantle grains in dense clouds. Superparamagnetism alone, however, cannot explain the polarized far-infrared emission observed from cold dust in Orion clouds.