Preliminary investigation has been conducted on the thermal history of the Jilin meteorite with special reference to the criteria of thermal and impact metamorphisms.The characteristics of thermal metamorphism are discussed in more detail for Meteorite No. I, which weighs about 1,770 kilograms and measures approximately 100×80×50 cm3. Systimatic sampling were carried out with respect to depth (i.e., 5, 20, 35 and 50 cm) from this meteorite. Microscopy, together with statistics of chondrule content and of the proportions between the various kinds of chondrules, shows that: (1)Chondrule content or the number of chondrule per unit area (cm2) increases gradually with the distance from the core of the meteorite mass outwards. (2) The content of devitrified and fan cbondrules tends to increase from the core to the rim in spite of the fact that no obvious variation has been noticed in the proportion between various kinds of chondrules. (3) The outline of chondrules becomes increasingly clear in going from the core to the outer parts, which is probably accounted for by a higher degree of thermal metamorphism and recrystallization, and therefore a higher metamorphic temperature, in the inner portion than that in the outer parts. This temperature gradient is presumably a function of the buried depth (20km), the large difference in cooling rate or the uneven temperature distribution due to heterogeneous arrangement of radioactive elements within the meteorite parent body.
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