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Pentlandite and Magnetic Anisotropy in Allende Cv3 Chondrite

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Pentlandite and Magnetic Anisotropy in Allende Cv3 Chondrite Lunar and Planetary Science XXXI 1148.pdf PENTLANDITE AND MAGNETIC ANISOTROPY IN ALLENDE CV3 CHONDRITE. M. Funaki1 and P. Wasilewski2, 1National Institute of Polar Research, 9-10 Kaga 1, Itabashi Tokyo 173-8515, Japan, 2Goddard Space Flight Center NASA, Greenbelt Maryland 20771, USA 1. Introduction anisotropy were recognized in the sample. Pentlandite The Character of the natural remanent was observed along the rim of some chondrules and in magnetization (NRM) of Allende (CV3) has been the matrix appearing as large spongy aggregates. studied intensively since its fall in Mexico because of the Pyrrhotite is present in the chondrules and in the matrix. stability of the NRM against AF demagnetization (ie. Spherical awaruite grains less than 10µm are observed Sugiura et al., 1979) and the uniformity of opinion about in the chondrules and matrix. In our sections, magnetite the paleofield estimates. From the characteristics of the is very rare. NRM behavior before and after thermal North seeking magnetotactic bacteria were demagnetization, and the results of analyses of mutually cultivated in order that they might be used to identify the oriented subsamples the NRM acquisition scenario minerals that were carrying the NRM record. A droplet evolved : some chondrules acquired magnetization containing numerous bacteria was spread on the before they were assembled into the parent body, then polished surface in a weak magnetic field (about 1µT). magnetization was acquired in a hydrothermal The surface was sequently observed with the reflected sulfidization scenario during the course of cooling light microscope. The results showed that the clusters of through 320°C in the presence of a magnetic field of bacteria were located almost exclusively on the 100µT. This event appears to have overprinted the unidentified iron-sulphide grains, no bacteria clustered Allende body ,at least that part of the body that reached on the isotropic pentlandite. Less significant bacteria the Earth. The dominant magnetic minerals responsible clusters were formed on the grains of awaruite, for the NRM were estimated to be iron-sulphide by magnetite and some pyrrhotite. When the sample was Wasilewski (1981) and Funaki (1993). As the NRM given a saturation remanent magnetization (SIRM) in a carrier is iron-sulphide, the paleomagnetic veracity of field of 1.0T these tendencies were more clearly defined data obtained by thermal demagnetization should be so that were estimate that we were able to locate the revisited since the thermal instability of iron-sulphide is magnetic minerals in Allende using this technique. notorious. From this viewpoint, we studied the Surprisingly the unidentified iron-sulphide appears to be relationship of the magnetization in the sulphide phases the most effective NRM carrier. This conclusion is in detail using magnetotactic bacteria and EMPA. We arrived at by consideration of intensity and size of the searched for the location of the magnetic sulfides. magnetotactic bacterial clusters. After thermal demagnetization at 650°C a reapplication of the bacteria 2. Experimental results revealed that none of the minerals attracted the bacteria A bulk sample was cut along the NRM direction, indicating demagnetization and as a verification that the and subsequently it was polished for microscopical bacterial technique is reliable for the intent. When the observation. Pentlandite, pyrrhotite, awaruite, magnetite SIRM was reacquired in this sample, intensive clusters and unidentified iron-sulphide phase(s) with optical appeared on areas locating the unidentified Lunar and Planetary Science XXXI 1148.pdf ANISOTROPY IN ALLENDE CV3: M. Funaki and P. Wasilewski iron-sulphide. Smaller clusters were formed on the present during formation of nonstoichiometric magnetite and awaruite grains, but no clusters appeared pentlandite that seems to be pervasive. The obvious on almost all pentlandite and pyrrhotite grains. chemical remanent magnetization and the thermal Chemical composition of the unidentified instability of the magnetic sulfides would suggest a iron-sulphide was measured by EPMA. The results reevaluation of the techniques that would be used for any indicated that the mineral is consistent with pentlandite paleointensity analyses with Allende. The magnetic composition, but it is nonstoichiometric. There is no anisotropy results suggest that the constituent particles obvious extensive chemical difference between in the Allende body were compressed before nonmagnetic pentlandite and magnetic pentlandite. solidification and then compressed or extended in a From these viewpoints, the representative NRM carrier different direction. The Allende parent body is concluded to be nonstoichiometric pentlandite. was active and the creation of the magnetic minerals Oriented cubic samples of about 0.125cc were and the impessment of the magnetic record, likely took obtained from Allende. The anisotropy of magnetic place in a scenario that had an associated hydrothermal susceptibility (AMS) and the anisotorpy of saturation sulfidization activity. magnetization were measured. These results indicated that the samples have uniaxial magnetic anisotropy. References: The AMS showed that the fabric could be represented Funaki, M. (1993) Proc NIPR Symp. Antarct. as a triaxial ellipsoid which suggests that Allende Meteorites, 6, 391-400. experienced compression along the z-axis and Sugiura N., Lanoix M., and Strangway D. W. (1979) compression or extension along x- or y-axes. Phys. Earth Planet. Inter. 20, 342-349. Wasilewski P. J. (1981a) Phys. Earth Planet. Inter., 26, 3. Conclusion 134-148. Using magnetotactic bacteria to locate the magnetic remanence carriers, and additional magnetic experiments, we can reiterate that a magnetic field was.
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