VOLATILES IN UNEQUILIBRATED ORDINARY CHONDRITES - ABUNDANCES, SOURCES AND IMPLICATIONS FOR EXPLOSIVE VOLCANISM ON DIFFERENTIATED ASTEROIDS

Keil and Wilson (1993) proposed that, during partial melting of some asteroidal meteorite parent bodies, explosive pyroclastic volcanism accelerated S-rich Fe,Ni-FeS cotectic partial melts into space. These authors argued that this process was responsible for the S-depletion of many of the magmas from which the magmatic iron meteorites formed. This process only requires the presence of a few hundred to thousand ppm of volatiles in asteroids < similar to 100 km in radius. If the precursor materials of these magmatic iron meteorite groups were similar in composition to unequilibrated ordinary chondrites, then the volatile contents of the latter may be a measure of the potential effectiveness of the process. Analysis of volatile contents of seven unequilibrated ordinary chondrite falls by dynamic high-temperature mass spectrometry revealed that thousands of ppm of indigeneous volatiles, mostly CO, Cl, Na and S, are released at temperatures near the Fe,Ni-FeS cotectic melting temperature of similar to 980 degrees C. If these volatiles are largely retained in the asteroidal parent bodies until onset of partial melting, S depletion of the residual melt might have been achieved by ejection of S-rich partial Fe,Ni-FeS melts by pyroclastic volcanism.