Origin of glass inclusions hosted in magnesian porphyritic olivines chondrules: Deciphering planetesimal compositions

Glass inclusions occur in the most primitive magnesian porphyritic olivines of chondrules formed very early in the solar system, and understanding their formation and preservation is crucial for explaining early solar system evolution. Several different and frequently opposed models have been proposed to explain the origin of these glass inclusions and their host olivines, ranging from condensation processes in the nebula to crystallization products of chondrule melts. However, none of these models fully satisfy all the petrographic, chemical and isotopic observations. In this experimental work, we show that the peculiar compositions of these inclusions that exhibit significant chemical disequilibrium between glass and host olivine result from the slow crystallization of metastable olivines from a magma. The inhibition of plagioclase nucleation resulting in metastable olivine crystallization instead of the equilibrium assemblage (anorthite + clinopyroxene) is a natural consequence of the high liquidus temperature of the magma. This result implies a restricted compositional range for the parental magmas of these melt inclusions. Olivine phenocrysts inside type IA chondrules are thus interpreted as fragments of magmatic cumulates formed in magma ocean-like environments on fully or partially molten planetesimals. In a similar manner as for terrestrial rocks, studies of melt inclusions, giving information about the earliest magma oceans, could open a new way of assessing the evolution of the earliest bodies of the Solar System.