  The bulk volatile contents of chondritic meteorites provide clues to their
origins. Matrix and chondrules carry differing abundances of moderately
volatile elements, with chondrules carrying a refractory signature. At the high
temperatures of chondrule formation and the low pressures of the solar nebula,
many elements, including Na and Fe, should have been volatile. Yet the evidence
is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO
and Fe-metal) were present in about their current abundances in molten
chondrules. This seems to require very high solid densities during chondrule
formation to prevent significant evaporation. Evaporation should also be
accompanied by isotopic mass fractionation. Evidence from a wide range of
isotopic systems indicates only slight isotopic mass fractionations of
moderately vola-tile elements, further supporting high solid densities.
However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated
outer zones that have been interpreted as the prod-ucts of late condensation of
SiO2 into chondrule melts. Late condensation of more refractory SiO2 is
inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal
in many chondrules. Despite significant recent experimental work bearing on
this problem, the conditions under which chondrules behaved as open systems
remain enigmatic.
