Paleointensity of the Allende carbonaceous chondrite

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The natural remanent magnetization (NRM) of the Allende carbonaceous chondrite appears to be much more complicated than interpretations given by previous works (e. g. BUTLER : Earth Planet. Sci. Lett., 17,23,1972; BANERJEE and HARGRAVES : Earth Planet. Sci. Lett; 17,110,1972; LANOIX et al. : Geophys. Res. Lett., 5,73,1978; LANOIX et al. : Lunar Planet. Sci. IX, 630,1978; NAGATA : Phys. Earth Planet. Inter., 20,324,1979; SUGIURA et al. : Phys. Earth Planet. Inter., 20,342,1979) as already pointed out by WASILEWSKI and SARALKER (Proc. Lunar Planet. Sci. Conf., 12B, 1217,1981). The ferromagnetic and/or ferrimagnetic constituents in the Allende are Ni-rich taenite, pyrrhotite and magnetite, among which taenite is most dominant in spontaneous magnetization intensity. NRM of the bulk specimen is uniform and very stable in AF-demagnetization test. NRM of matrix also is as stable as that of the bulk specimen, whereas NRM of most chondrules is either unstable or faint compared with that of matrix. The NRM intensity of bulk specimen is almost same as that of matrix specimen. NRMs of the bulk and matrix specimens can be thermally demagnetized almost completely by heating to about 320℃. Their thermal demagnetization curves well resemble the thermomagnetic curve of ferrimagnetic pyrrhotite. However, the thermal demagnetization curves of chondrules having unstable NRM consist of a component thermally demagnetized by about 320℃ and the other component thermally demagnetized by about 580℃. It is concluded from these results that the stable and uniform NRM of the Allende chondrite is mostly due to the stable NRM of matrix. Experimental results of acquisition of the partial thermoremanent magnetization (PTRM) show that these specimens can acquire PTRM in a temperature range up to about 620℃, the largest PTRM acquisition rate taking place in a range of 500-600℃. It may be provisionally concluded therefore that the Allende NRM was acquired as PTRM during a course of cooling from about 320℃ in the presence of a magnetic field or as the chemical remanent magnetization (CRM) of ferrimagnetic pyrrhotite during a formation process of ferrimagnetic pyrrhotite from iron sulfides at temperatures below 320℃. Results of Konigisberger-Thellier experiments of a number of bulk and matrix specimens and of a chondrule having stable NRM, on an assumption that NRM is PTRM acquired during a course of cooling from about 320℃ in the presence of a magnetic field (F_p), lead to a conclusion that the paleointensity of the Allende chondrite is given by F_p=(1-3) Oe.
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Paleointensity of the Allende carbonaceous chondrite

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