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

Keil and Wilson (1993) proposed that, during partial melting of some a steroidal meteorite parent bodies, explosive pyroclastic volcanism acc elerated S-rich Fe,Ni-FeS cotectic partial melts into space. These aut hors argued that this process was responsible for the S-depletion of m any of the magmas from which the magmatic iron meteorites formed. This process only requires the presence of a few hundred to thousand ppm o f volatiles in asteroids < similar to 100 km in radius. If the precurs or materials of these magmatic iron meteorite groups were similar in c omposition to unequilibrated ordinary chondrites, then the volatile co ntents of the latter may be a measure of the potential effectiveness o f the process. Analysis of volatile contents of seven unequilibrated o rdinary 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 m elting temperature of similar to 980 degrees C. If these volatiles are largely retained in the asteroidal parent bodies until onset of parti al melting, S depletion of the residual melt might have been achieved by ejection of S-rich partial Fe,Ni-FeS melts by pyroclastic volcanism .