Exsolved magnetite inclusions in silicates: Features determining their remanence behavior

Submicroscopic, needle-shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts, making them excellent candidates for paleomagnetic studies. A suite of samples containing clinopyroxene- and plagioclase-hosted magnetite inclusions from live geologically diverse sites was examined using magnetic force microscopy to image the inclusions' magnetic domain state. Alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others. The two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions. Magnetite-ulvöspinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 10³-10⁵ magnetite prisms separated by thin ulvöspinel lamellae. The conversion of multidomain-sized needles into assemblages of interacting single domains increases the coercivity (and hence relaxation time) of the inclusions, and results in a thermochemical magnetic remanence. In samples without this exsolution microstructure, the inclusions' diameters determine coercivity and their magnetization is thermoremanent. Both styles of high-coercivity inclusions successfully record paleomagnetic directions in Mesozoic rocks, and their ubiquity within silicate minerals (clinopyroxene and plagioclase) of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field, marine magnetic anomalies, and crustal kinematics.