Oriented inclusions of magnetite in clinopyroxene: Source of stable remanent magnetization in gabbros of the Messum Complex, Namibia

Abstract

Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single‐domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions. As in previous studies of similar inclusions, these are inferred to have formed by exsolution. Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations. Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin. Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal.