Experimental constraints on alkali condensation in chondrule formation

To assess whether the alkali behavior observed in chondrules of primitive meteorites is attributable to volatilization from the raw materials of chondrules during chondrule formation events or attributable to condensation processes from the nebular gas, we set up a new experimental device able to expose silicate melt samples to a controlled alkali partial pressure at high temperature under fixed O fugacity. Using a mixture of potassium carbonate (K2CO3) and graphite (C) as the source of the K gas (K-g), we studied the condensation kinetics of K and its solubility in CaO-MgO-Al2O3-SiO2 silicate melts, according to the reaction 2 K ((g))+ 1/2 O-2 (g) = K2O ((melt)). From these results, we show that alkali entering in chondrules from the nebular gas is a viable mechanism to explain the chondrules alkali contents and their delta K-41-isotopic signatures, at timescales relevant to chondrule formation. Finally, we also suggest that chondrules may have formed in non-canonical nebular environments and that the flash-heating scenario is not a prerequisite to chondrule formation.