Vibrating Sample/Alternating Gradient Magnetometer (VSM/AGM) System
Final Report Abstract
The vibrating sample magnetometer (VSM) was installed in March 2014. The first published results with data generated by the instrument began to appear in 2015. The purpose of the instrument is to characterize the magnetic properties of natural materials as a function of external field (to 2 T) and temperature (4 to 1073 K) on relatively small samples. Measurements at high temperature are useful to define the Curie temperatures while those at low temperature are useful to characterize the magnetic transitions that are common in some ferromagnetic minerals. This instrument also measures hysteresis loops and first order reversal curves, which indicate the sizes of the magnetic grains, which is critical information needed to understand the quality of the magnetic recorders in the rocks. Below we give a few of the many key results generated by the VSM. Important findings were obtained from studies at two Canadian meteorite impact craters. At the Mistastin crater, magnetic hysteresis parameters of the magnetitebearing anorthositic basement rocks revealed systematic changes as a function of distance from the crater's center with an increasing prevalence of single domain-like grains toward the center. Changes with radial distance were also found in the character of the Verwey transition in magnetite. No evidence existed that shock heating of the basement rocks exceeded 200°C at distances of 6-7 km from the crater's center. At the Manicouagan crater, Eitel et al. (2016) reported rock magnetic and paleomagnetic data from 25 widely-distributed sites of impact melt and basement rocks collected at the surface as well as from bore holes drilled to depths ≤1.5 km. Titanomagnetite concentration increased more than ten-fold toward the base of the thickest impact melt that underwent fractional crystallization. The titanomagnetite-enriched zone partially contributes to a 2000 nT magnetic anomaly in the crater’s center. We found no evidence for an aberration of the geomagnetic field over the several thousands of years it took to cool the impact melt body. Hence, the energy released by the Manicouagan impact that created one of the ten largest known craters on Earth provoked no measurable disturbance of the geodynamo. This publication was highlighted by the American Geophysical Union (https:// eos.org/research-spotlights/can-meteorite-impacts-disturb-a-planets-magnetic-field). Monoclinic pyrrhotite (Fe 7S8) owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is called the Besnus transition. Proposed explanations for the Besnus transition are either due to a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and backfield curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite every 2 K from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic aaxes and only a single inflection parallel to the a-axes. Magnetization energy calculations and relative differences of the loops showed six-fold symmetry in this temperature range and fourfold below 30 K. We proposed that the inflections stem from magnetic axis switching, in a manner somewhat analogous to an isotropic point where magnetocrystalline constants change their sign. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span (28–34 K) with a maximum rate of change at 30 K. A surprising yet puzzling finding was that the coercivity ratio becomes less than unity below the transition when four-fold symmetry arises. Because the changes in magnetic parameters are linked to the crystal structure, we concluded the Besnus transition owes its origin to a distortion of the crystallographic axes below 30 K rather than an apparition of a two-phase system. Planetary scientists want to know the range in magnetic field intensities generated from the solar nebula and by dynamo action in the early planetesmals. Such knowledge is gained by paleointensity studies from meteorites. Successful paleointensity results depend on the presence of magnetic minerals within single domain particle sizes. The remanent magnetizations in most meteorites are carried by FeNi alloys, but the multi- to single domain threshold sizes of FeNi alloys are unknown. For this reason, Volk et al. (2018) used a mechanical ball mill to synthesize FeNi alloys of precise compositions as a function of grain size. Magnetic analyses show how the ball mill process works from the alloying of component metals and then subsequent grain size reduction. In combination with X-ray diffraction, the results show that magnetostatic interactions make single domain grain sizes of FeNi alloys appear multidomain.
Publications
- (2017) A compositional tipping point governing the mobilization and eruption style of rhyolitic magma. Nature 552 (7684) 235–238
Di Genova, D.; Kolzenburg, S.; Wiesmaier, S.; Dallanave, E.; Neuville, D. R.; Hess, K. U.; Dingwell, D. B.
(See online at https://doi.org/10.1038/nature24488) - Paleomagnetic and rock magnetic study of the Mistastin Lake impact structure (Labrador, Canada): Implications for geomagnetic perturbation and shock effects. Earth and Planetary Science Letters. 2015, v 417, p 151-163
Hervé, G., S.A. Gilder, C.L. Marion, G.R. Osinski, J. Pohl, N. Petersen, and P.J. Sylvester
(See online at https://doi.org/10.1016/j.epsl.2015.02.011) - A paleomagnetic and rock magnetic study of the Manicouagan impact structure: Implications for crater formation and geodynamo effects. Journal of Geophysical Research - Solid Earth. 2016, v 121
itel, M., S.A. Gilder, J. Spray, L. Thompson, and J. Pohl
(See online at https://doi.org/10.1002/2015JB012577) - Low temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition. Geophysical Journal International, 2016, v 207, p 1783-1795
Volk, M., S.A. Gilder, and J. Feinberg
(See online at https://doi.org/10.1093/gji/ggw376) - Fast geomagnetic field intensity variations between 1400 and 400 BCE: New archaeointensity data from Germany. Physics of the Earth and Planetary Interiors. 2017, v 270, p 143-156
Hervé, G., J. Fassbinder, S.A. Gilder et al.
(See online at https://doi.org/10.1016/j.pepi.2017.07.002) - Neutron depolarization measurements of magnetite in chiton teeth. Journal of Physics: Conference Series. 2017, v 862, 012024
Seifert, M., M. Schulz, G. Benka, C. Pfleiderer, and S. Gilder
(See online at https://doi.org/10.1088/1742-6596/862/1/012024) - Variability of the 0-3 Ma palaeomagnetic field observed from the Boring Volcanic Field of the Pacific Northwest. Geophysical Journal International. 2017, v 211, p 69-79
Lhuillier, F., V. Shcherbakov, S.A. Gilder, and J.T. Hagstrum
(See online at https://doi.org/10.1093/gji/ggx288) - Monitoring the alloying process of mechanically synthesized Fe80Ni20 through changes in magnetic properties. Journal of Alloys and Compounds. 2018, v 732, p 336-342
Volk, M.W.R., Wack, M.R., Maier, B.
(See online at https://doi.org/10.1016/j.jallcom.2017.10.090)
