Paleomagnetism of Antarctic achondrites (II)

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The paleointensity (F_p) of NRM of the TRM origin can be approximately estimated by comparing the NRM-lost obtained by the stepwise AF-demagnetization of NRM with the ARM-lost obtained by the same AF-demagnetization of the saturated ARM acquired in a magnetic field h_A through a possible relation of (NRM-lost)/F_p=ƒ_0(ARM-lost)/h_A, where NRM is assumed to be identical to a total TRM acquired in F_p, and the coefficient ƒ_0 must be experimentally determined. For ALH-77302 eucrite, F_p can be determined by the method as F_p=0.110/ƒ_0 Oe. However, an experimental determination of f_0 for meteorites is associated with much difficulty caused by chemical and structural alterations of ferromagnetic Fe-Ni metals at elevated temperatures. A three step heating procedure which is a modification of the original TAYLOR's technique (1979) is adopted in the present study with a satisfactory success of not altering the magnetic microscopic coercivity spectrum by heating up to 790℃. The procedure consists of the first heat treatment to rid a meteorite sample of H_2O vapour and loosely-bound gases at 110℃ in a low vacuum space for an hour, the second one to take out tightly-bounded gases at 355℃ in a high vacuum space with the help of the Ti-metal getter to absorb gases for an hour and a half, and then heating to any higher temperature to give pTRM or total TRM on the sample in the same high vacuum system. The f_0 value of ALH-77302 thus obtained is f_0=6.80,and consequently F_p of ALH-77302 eucrite has been obtained as F_p=0.016 Oe. F_p=0.021 Oe for another test specimen of the same eucrite. The paleointensity of ALH-77302 eucrite is therefore approximately 0.02 Oe.
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Paleomagnetism of Antarctic achondrites (II)

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