Field- and Frequency-Dependence of Magnetic Susceptibility of Ordinary Chondrites in Low Fields

Peter Oliver1, Claire Samson1, Richard E Ernst2, Phil J A McCausland3 and Gordon Fox West4, (1)Carleton University, Earth Sciences, Ottawa, ON, Canada, (2)Ernst Geosciences, Department of Earth Sciences, Ottawa, ON, Canada, (3)University of Western Ontario, London, ON, Canada, (4)University of Toronto, Physics, Toronto, ON, Canada

Contact First Author: Peter Oliver; PeterOliver@cmail.carleton.ca

Abstract ID#: 34164

 

English Abstract:
Increasingly, magnetic susceptibility measurements are being used to rapidly and non-destructively characterize stony meteorites, and a meaningful database of measurements is already available. However, current data have been acquired by susceptibility meters that operate at only a few field strengths and frequencies.

The University of Toronto Electromagnetic Induction Spectrometer (UTEMIS) is a prototype instrument that measures AC complex susceptibility over a range of 35 frequencies between 90 Hz and 64 kHz by harmonic analysis of periodic signals at six different base frequencies. A very weak exciting field is employed (typically <10 A/m peak-to-peak) so a near linear magnetization response is expected, but the amplitude of the excitation signal can be varied by a factor of 15 to test this. With this instrument, the magnetic susceptibility of 27 ordinary chondrites from the LL, L, and H classes was measured over the entire UTEMIS spectral range and with variable exciting field strengths. The results have been analysed and compared with data measured with a SI-2B commercial susceptibility meter at 2 frequencies (825 Hz and 19 kHz). Systematic variation of susceptibility with both applied field and frequency were observed; susceptibility increasing with excitation amplitude and falling somewhat with increasing frequency.

Varying the applied field permits the apparent susceptibility to be separated into a field-dependent term, putatively contributed by the multi-domain ferromagnetic grain fraction, and a field-independent term likely contributed by the diamagnetic, paramagnetic, and single-domain ferromagnetic grains. Among the ordinary chondrites measured, a positive dependence of susceptibility on exciting field with increasing metal content is widely observed. The LL class displays little dependence with field strength, whereas the H class displays ubiquitous field dependence. Intermediate field dependence is found in the L class.

The degree of frequency dependence also varies with metal content; the H chondrites display a maximum of up to 17% measured from 810 to 64 kHz the L chondrites of up to 8%, and the LL chondrites up to 3%.

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