Formation of ordinary chondrites

Most of the chemical and mineralogical properties of the ordinary chondrites were established by processes that occurred in the solar nebula during a short time span near the time of formation of the solar system. Four separate and distinct fractionation events appear to have been involved in their formation from more primitive material of mean solar‐system composition. In order of occurrence these were (1) a refractory‐element fractionation, which resulted in the lower Mg/Si, Ca/Si, Ai/Si and other refractory‐element/Si ratios relative to Cl chondrites; (2) a siderophilic‐element fractionation, which produced the monotonic reduction of siderophilic‐element/Si ratios as one proceeds from the H group toward the LL group and the variation in ratios of one siderophilic element to another through this sequence; (3) a fractionation of slightly volatile elements (such as Cu, Ga, and Ge), probably during chondrule formation; and (4) a fractionation of highly volatile elements, probably by thermal metamorphism while the chondritic material was stored in planetesimals of the meter‐to‐100‐meter size range. In each of the four cases the most plausible fractionation mechanism involves a separation of gases from solids. The ambient temperature in the planetary portion of the solar nebula during the formation of these chondrites appears to have been characterized by two maximums.