Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.594w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 594
Other
1
Alloys, Feni, Chondrules, Kamacite, Metal, Properties, Magnetic, Tetrataenite
Scientific paper
There is now sufficient information about the complexity of the FeNi system and the unexplained peculiarities in paleomagnetic records associated with metal bearing natural samples (Moon rocks and meteorites)to suggest that a paradigm shift in the interpretation of meteorite magnetic properties is needed. Almost nothing is known about acquisition of remanence in relatively weak magnetic fields for metals and alloys. We have no direct knowledge about whether the magnetization acquired by FeNi is preserved intact when atomic ordering effectively produces a new magnetic mineral, tetrataenite, whose magnetic anisotropy is orders of magnitude greater than the original taenite. A plausible scenario for initial chondrule magnetization is a cooling chondrule spinning and translating through a magnetic field. The physical reality in the early solar system must have been different from the geomagnetic field experience as far back in time as it can be documented. During demagnetization, either thermal or alternating field, meteorite subsamples, metal grains, and chondrules exhibit zig-zag intensity curves and vector excursions confined to a plane or else in circular excursions [1, 2]. These have never been adequately explained. Similar curves are observed in lunar samples [3, 4]. Watching a welder use a torch to cut steel pipe offered crude test specimens for an evaluation of remanence acquisition appropriate to chondrule magnetization. Most of the orange melt slag droplets cooled to black before they dropped to the cement floor, as they traveled an arced path of about 10 feet. One larger droplet was soft when it hit on the floor. These slag droplets traced a path across the geomagnetic field while they cooled, with final cooling taking place after they hit the floor. Obviously, there was little control on the relationship between magnetization acquisition and various physical parameters such as field orientation, temperature, etc. In another experiment, electropolished wires were melted by running a large current through them. The resultant metal spheres fell about 18 inches into an oil quench. The thermal and alternating field demagnetization results for the metal spheres and slag samples were quite similar to the results for meteorite and lunar samples. The new paradigm must recognize the peculiarities associated with remanence acquisition in field conditions and thermophysical circumstances more realistic for meteorite genesis and modification. The new paradigm must also explain magnetic records in terms of the FeNi phase relations which are time variable because of diffusion and specific atomic ordering. Furthermore, since saturation magnetization is large for kamacite, shape anisotropy which introduces magnetic noise, assumes importance during demagnetization of kamacite and this must be recorded. In our preliminary chondrule studies, we find tetrataenite with a broad range of magnetic hardness, which is likely related to thermophysical history. Since this is the most important magnetic mineral in chondrules, and in meteorites in general, we must understand the relationship between paleomagnetic records and tetrataenite characteristics. References: [1] Funaki M. et al. (1981) Proc. 6th Symp. Antarc. Meteorites, 300-315. [2] Sugiura N. and Strangway D. W. (1982) Proc. 7th Symp. Antarc. Meteorites, 260-280. [3] Collinson D. W. et al. (1972) Proc. LSC 3rd, 2343. [4] Hoffman K. A. and Banerjee S. K. (1975) EPSL, 25, 331.
Dickinson Tamara L.
Wasilewski Peter J.
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