Eleven iron meteorite groups show correlations between Ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas. These meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids. Many of these groups crystallized from S-depleted magmas which we propose resulted from removal of the first partial melt (the Fe,NiFeS cotectic melt) by explosive pyroclastic volcanism of the type envisaged by Wilson and Keil . We show that these dense, negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt. We also show that, in typical asteroidal materials, veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces. Since cotectic Fe,NiFeS melt consists of about 85 wt% FeS and 15 wt% Fe,Ni, removal of small volumes of eutectic melts results in major loss of S but only minor loss of Fe,Ni, thus leaving sufficient Fe,Ni to form sizeable asteroidal cores.