Magnetic characterization of synthetic titanomagnetites: Quantifying the recording fidelity of ideal synthetic analogs

A series of four synthetic basalts comprising titanomagnetite (Fe_3-xTi_xO₄) grains of varied size and titanium content have been produced by a glass-ceramic method. Complementary characterization techniques of X-ray diffractometry, secondary electron microscopy, and transmission electron microscopy (TEM) demonstrate the reaction product composition consisted of mainly Fe_3-xTi_xO₄, pyroxene hedenbergite, fayalite, and SiO₂. The samples exhibit bimodal distributions of larger (<2 µm) and smaller Fe_3-xTi_xO₄ particles (<50 nm in diameter), the latter found inside pyroxene crystals, as well as the sporadic occurance of dendritic Fe_3-xTi_xO₄ structures. Magnetic measurements show their bulk characteristics fall into two groups: Ti-rich titanomagnetite samples with varying Ti content; and near-stoichiometric magnetite. The TEM technique of off-axis electron holography allowed for visualization of the magnetic behavior of the synthetic Fe_3-xTi_xO₄ grains. Energy dispersive X-ray analysis and off-axis electron holography confirmed the small Fe_3-xTi_xO₄ grains (<50 nm) confined within glassy pyroxene regions to be Fe-rich and single domain, carrying strong magnetic signals, compared to the relatively magnetically weak larger Fe_3-xTi_xO₄ grains (x ∼ 0.6). The large grains in the pure magnetite sample are shown to be pseudo-single domain in nature. The quenching process involved in synthesis is considered similar to that of pillow basalts found at mid-ocean ridges and hence the reaction products are thought ideal in terms of characterization and understanding, for the purpose of studying natural systems.