Cooling history of pyroxene chondrules in the Yamato-74191 chondrite (L3)—an electron microscopic study

Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward.The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite-C followed by decomposition into pigeonite-P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.