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 similar to 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 650degrees-600degreesC. Initial contacts were modified by many exsolution cycles. Hematite and ilimenite (Ilm) hosts and lamellae by EMP are Ilm 24-25, Ilm 88-93, like R (3) over barc titanohematite, and R (3) over bar ilmenite above 520degreesC 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 R (3) over bar ilmenite below 520degreesC on Burton's diagram. This may be the first example of analytical identification, in one sample, of former R (3) over barc 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 R (3) over barc titanohematite ---> AF hematite + R (3) over bar imenite at 520degreesC. 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 520degreesC where R (3) over bar hematite broke down to AF hematite plus R (3) over bar ilmenite, producing abundant AF hematite below its Neel temperature. Intensity of magnetization is greater than possible with hematite alone, and TEM work suggests that ultrafine R (3) over bar ilmenite disks in AF hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces.