Effect of fine-scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks, southwest Sweden: implications for crustal magnetism

Mid-Proterozoic granulites in SW Sweden, having opaque minerals hematite-ilmenite with minor magnetite, and occurring in an area with negative aeromagnetic anomalies, have strong and stable reversed natural remanent magnetization ∼9.2 A/m, with 100% remaining after demagnetization to 100 mT. Samples were characterized by optical microscopy, electron microprobe (EMP), transmission electron microscopy (TEM), and rock-magnetic measurements. Earliest oxide equilibrium was between grains of titanohematite and ferri-ilmenite at 650°-600°C. Initial contacts were modified by many exsolution cycles. Hematite and ilmenite (Ilm) hosts and lamellae by EMP are Ilm 24-25, Ilm 88-93, like R3c titanohematite, and R3 ilmenite above 520°C on Burton's diagram [1991]. Finer hosts and lamellae by TEM are Ilm16±3 and Ilm 88±4, like coexisting antiferromagnetically ordered (AF) hematite and R3 ilmenite below 520°C on Burton's diagram. This may be the first example of analytical identification, in one sample, of former R3c hematite, now finely exsolved, and AF hematite. TEM microstructures consist of gently curving semicoherent ilmenite lamellae within hematite, flanked by precipitate-free zones and abundant ilmenite disks down to unit cell scale (1-2 nm). Strain contrast of disks suggests full coherence with the host, and probable formation at the reaction R3c titanohematite → AF hematite + R3 ilmenite at 520°C. Magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite-richer than Ilm 28, that apparently exsolved without becoming magnetized, down to 520°C where R3c hematite broke down to AF hematite plus R3 ilmenite, producing abundant AF hematite below its Néel temperature. Intensity of magnetization is greater than possible with hematite alone, and TEM work suggests that ultrafine R3 ilmenite disks in AF hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces.