Title: Effects of nanoscale exsolution in hematite-ilmenite on the acquisition of stable natural remanent magnetization

Author(s): Kasama T, McEnroe SA, Ozaki N, Kogure T, Putnis A

Source: EARTH AND PLANETARY SCIENCE LETTERS 224 (3-4): 461-475 AUG 15 2004

Document Type: Article

Language: English

Abstract: To investigate the acquisition mechanism of high and stable natural remanent magnetization (NRM) in rocks of the Russell Belt, Adirondack Mountains, New York, we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties. The samples contain titanohematite with ilmenite lamellae, end-member hematite without lamellae and rare magnetite as potential carriers for the NRM. Transmission electron microscopy (TEM) observations and element mapping by energy-filtered TEM (EFTEM) of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness similar to2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick. The ilmenite lamellae and titanohematite hosts, with the compositions of Ilm(90-100)Hem(10-0) and Ilm(7-16)Hem(93-84), respectively, share (001) planes, and the abundant fine ilmenite lamellae have coherent, sharp structural and compositional interfaces with their titanohematite hosts. Comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae. These results are consistent with the lamellar magnetism hypothesis, suggesting that the acquisition of high and stable NRM is related to the interfacial area between fine ilmenite lamellae and their host titanohematite. End-member hematite with a multi-domain magnetic structure only contributes a minor amount to the NRM in these samples. (C) 2004 Elsevier B.V. All rights reserved.

Author Keywords: hematite; ilmenite; titanohematite; magnetite; nanoscale exsolution lamellae; magnetism; TEM

KeyWords Plus: AEROMAGNETIC ANOMALIES; LAMELLAR MAGNETISM; ORDER-DISORDER; HEMO-ILMENITE; SELF-REVERSAL; ALLARD LAKE; MICROSTRUCTURES; SYSTEM; ROCK; TITANOHEMATITE

Addresses: Kasama T (reprint author), Univ Cambridge, Dept Mat Sci & Met, Pembroke St, Cambridge, CB2 3QZ England
Univ Munster, Inst Mineral, Munster, D-48149 Germany
Geol Survey Norway, Trondheim, N-7491 Norway
CREST, Japan Sci & Technol Corp, Kawaguchi, Saitama 3320012 Japan
Univ Tokyo, Dept Earth & Planetary Sci, Tokyo, 1130033 Japan

E-mail Addresses: tk305@cam.ac.uk

Publisher: ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS

Subject Category: GEOCHEMISTRY & GEOPHYSICS

IDS Number: 850AF