Structural state of plagioclase from the Kobe CK chondrite: Implications for the thermal history of the CK parent body

We determined the structural state of six plagioclase crystals from the Kobe meteorite by measuring the separation between the 131 and 131 X-ray diffraction peaks (Δ131 parameter) using a Gandolfi camera. The formation temperature of each plagioclase crystal was then estimated by plotting the Δ131 parameter on the relation diagram between the Δ131 parameters and temperatures of plagioclase synthesis, as proposed by Smith (1972). Three of the crystals record temperatures of just under 700°C, while the other three crystals record much lower temperatures. The temperature estimates can be regarded as representing the crystallization temperature of plagioclase, and the highest recorded temperature is thought to correspond to the peak temperature of the thermal metamorphism, close to but not exceeding 700°C. A thermal model of the CK chondrites is proposed on the assumption that thermal metamorphism, with ²⁶Al decay as a heat source, began prior to accretion of the CK parent body. The model explains the homogeneous compositions of olivine and pyroxene in CK chondrites, which are inconsistent with the petrographic type suggested by their grain sizes, and explains the occurrence of silicate darkening in almost all CK chondrites.