Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002geoji.151..890m&link_type=abstract
Geophysical Journal International, Volume 151, Issue 3, pp. 890-912.
Astronomy and Astrophysics
Astronomy
30
Exsolution Microstructures, Geomagnetism, Haematite-Ilmenite Series, Mineralogy, Planetary Anomalies, Rock Magnetism
Scientific paper
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, Ilm 84, exsolved at high temperature to produce host ilmenite Ilm 94 and micron-sized haematite lamellae Ilm 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 Ilm 98 and Ilm 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.
Harrison Richard J.
Langenhorst Falko
McEnroe Suzanne A.
Robinson Peter
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