How microstructures control remanent magnetization: Micromagnetic models and bulk measurements of exsolved magnetite in basalts of the Stardalur volcano, Iceland
Abstract
The origin and stability of remanent magnetization is of fundamental importance for the interpretation of remanence-dominated magnetic anomalies. When the remanence direction is near the direction of the inducing field, the resulting strong anomaly is frequently misinterpreted as due to a strong induced component, which is commonly attributed to coarse MD magnetite with high magnetic susceptibility.
The Stardalur volcano, Iceland, consists of young basaltic lavas (~1.7 Ma) and stands out due to an unusually strong positive magnetic anomaly (27 μT above background at ground level; Kristjánsson, 2013). The basalts have an average Q value of 61 and NRM values range from 20-128 A/m. Yet, magneto-mineralogical studies indicate that magnetite is the dominant magnetic carrier. The magnetite grains have an unusually high abundance of exsolution lamellae. Here, we investigate how these observations are connected. Realistic geometries of magnetite particles, subdivided by oxidation exsolution lamellae of ilmenite, were obtained by FIB-SEM nano-tomography. Micromagnetic simulations in MERRILL modeled the magnetization of these geometries. The influence of exsolution microstructures was estimated by modeling the grains with and without microstructures. The micromagnetic simulations did not include stress anisotropy, and therefore provide purely for the influence of geometric differences due to exsolution lamellae on the enhancement of remanence. To quantify the internal stress in the sample due to the exsolution microstructures the modeled hysteresis loop is compared with measured hysteresis behavior of the bulk sample. The difference in approach-to-saturation behavior between the modeled and measured hysteresis loop allows for an approximation of the internal stress, which amounts to ~91 MPa. The formation of exsolution lamellae has two effects on magnetic properties. 1) the apparent grain size is geometrically reduced, which increases Mrs and Hc by up to a factor two. 2) formation of lamellae produces internal stress fields which provide additional anisotropy energy that apparently increase Mrs and Hc by up to a factor ten. Accordingly, stress dominates the remanent magnetic properties in the Stardalur basalts and may be the decisive effect explaining the origin of this unusual strong magnetic anomaly.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP33B0753T
- Keywords:
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- 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1541 Satellite magnetics: main field;
- crustal field;
- external field;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1595 Planetary magnetism: all frequencies and wavelengths;
- GEOMAGNETISM AND PALEOMAGNETISM