Nanoscale haematite-ilmenite lamellae in massive ilmenite rocks: An example of 'lamellar magnetism' with implications for planetary magnetic anomalies

Abstract

Massive, nearly 'pure', haemo-ilmenite layers from historic ore deposits in Rogaland, Norway contain very few silicates or other oxides and typically produce remanence-dominated magnetic anomalies. These rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains. A typical bulk composition, 11m 84, exsolved at high temperature to produce host ilmenite 11m 94 and micro-sized haematite lamellae 11m 23 as measured by electron microprobe (EMP). Subsequent undercooling of the ilmenite and the micron-scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit-cell scale, leaving depleted hosts between lamellae with compositions of 11m 98 and 11m 15-13 as measured by TEM-EDX. Samples have high coercivities, and average NRM values of 25 A m-1, which typically show ∼2 per cent saturation in the NRM state. The amount of magnetization in these samples is too high to be solely accounted for by a spin-canted AF moment in the haematite. Based on Monte Carlo simulations of haematite-ilmenite interfaces at the atomic scale and on measured rock-magnetic properties, we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine-scale titanohaematite and ilmenite exsolution lamellae.