High efficiency of natural lamellar remanent magnetisation in single grains of ilmeno-hematite calculated using Mössbauer spectroscopy

Abstract

Rocks from large remanent magnetic anomalies on Earth have been found to contain exsolved rhombohedral oxides, which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on Mars. Theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite. Here, single grains (≈ 250 µm) of titanohematite with ferri-ilmenite exsolution lamellae from Mesoproterozoic metamorphic rock samples in southwest Sweden and the Adirondack Mountains, USA, are studied using room-temperature Mössbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism. Mössbauer spectra of synthetic samples of titanohematite (Fe 0.95Ti 0.05O 3 and Fe 0.9Ti 0.1O 3) were collected for comparison and showed a dominant six-line magnetic spectrum due to Fe 3+ in titanohematite with a weak sextet due to Fe 2 +–Fe 3+ charge transfer. Mössbauer spectra of the natural ilmeno-hematite grains are similar to those for synthetic titanohematite, but contain additionally two paramagnetic doublets corresponding to Fe 2+ and Fe 3+ in ilmenite. However, several grains also contain an additional weak, broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model. Similar to the previous Mössbauer results for natural hemo-ilmenite, there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri-ilmenite lamellae, and no evidence for single-domain or superparamagnetic magnetite. The compositions of titanohematite and ferri-ilmenite in the individual ilmeno-hematite grains calculated from the Mössbauer area ratios show that compositions are closer to end-member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains, consistent with observations that lamella thicknesses are as small as a few nm. Bulk compositions of ilmeno-hematite grains calculated from the Mössbauer data confirm that the Swedish samples are significantly more ilmenite-rich than the Adirondack samples, and agree with an estimation of bulk composition through point counting of electron backscatter images. The Mössbauer data allow a quantitative estimation of contact layer abundance, which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition. Because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies, contact layer abundance determination by Mössbauer spectroscopy provides a valuable new tool for mineral-based magnetic anomaly interpretation on Earth and other planetary bodies.