Role of gas-melt interaction during chondrule formation

Abstract

It is generally believed that most chondrules formed in the protoplanetary disk by melting of fine-grained solid precursor materials and that during these melting events chondrules behaved as closed systems, both chemically and isotopically; i.e., the observed range in chondrule bulk chemistry and modal mineralogy was entirely inherited from solid precursors that experienced melting to varying degrees followed by igneous crystallization accompanied by very limited, if any, evaporation-recondensation. Our mineralogical and chemical study of magnesium-rich porphyritic (Type I) chondrules in primitive ordinary and carbonaceous chondrites revealed that chondrule glasses are not located on the subtraction lines of olivine or low-Ca pyroxene in appropriate phase diagrams, indicating that Type I chondrules do not obey closed-system crystallization. We suggest that gas-melt interaction played a major role in evolution of mineralogy, bulk chemical and isotopic compositions of Type I chondrules. Chondrules are thus complex objects composed of an inherited solid component and a melt component that equilibrated with nebular gas, suggesting that chondrules formed in regions with high ambient gas` partial pressures. As a result, chondrule glasses (melts) are therefore thermochemical and isotopic sensors of the surrounding nebular gas in the chondrule-forming regions of the protoplanetary disk and can be used for evaluating its physico-chemical conditions.